Based on satellite-observed precipitation data and reanalysis data, this paper identifies the asymmetric characteristics of Typhoon "Mangkhut" precipitation and its correlation with vertical wind shear, and further uses WRF for numerical simulation to explore the feedback effect of asymmetric precipitation induced internal dynamic process on environmental flow. The main conclusions are as follows: (1) The precipitation distribution of Typhoon "Mangkhut" shows obvious asymmetry. It is mainly concentrated in the south, while the average wind shear of the typhoon points to the southwest, and the precipitation is always on the left side of the downshear. (2) At the initial stage of typhoon development, the wind shear points to the direction of heavy precipitation, rotates counterclockwise and increases continuously. It reaches the maximum when turning to the south, and then decreases gradually. (3) The change of asymmetric precipitation location is accompanied by the change of asymmetric wind field, and it affects the change of asymmetric temperature and pressure field; the change of temperature and pressure field further changes the asymmetric wind and precipitation. Therefore, there is an interactive feedback between Typhoon "Mangkhut" and the environmental flow field. [ABSTRACT FROM AUTHOR]
Mesoscale convective systems (MCSs) bring large amounts of rainfall and strong wind gusts to the midlatitude land regions, with significant impacts on local weather and hydrologic cycle. However, weather and climate models face a huge challenge in accurately modeling the MCS life cycle and the associated precipitation, highlighting an urgent need for a better understanding of the underlying mechanisms of MCS initiation and propagation. From a theoretical perspective, a suitable model to capture the realistic properties of MCSs and isolate the bare-bones mechanisms for their initiation, intensification, and eastward propagation is still lacking. To simulate midlatitude MCSs over land, we develop a simple moist potential vorticity (PV) model that readily describes the interactions among PV perturbations, air moisture, and soil moisture. Multiple experiments with or without various environmental factors and external forcing are used to investigate their impacts on MCS dynamics and mesoscale circulation vertical structures. The result shows that mechanical forcing can induce lower-level updraft and cooling, providing favorable conditions for MCS initiation. A positive feedback among surface winds, evaporation rate, and air moisture similar to the wind-induced surface heat exchange over tropical ocean is found to support MCS intensification. Both background surface westerlies and vertical westerly wind shear are shown to provide favorable conditions for the eastward propagation of MCSs. Last, our result highlights the crucial role of stratiform heating in shaping mesoscale circulation response. The model should serve as a useful tool for understanding the fundamental mechanisms of MCS dynamics. [ABSTRACT FROM AUTHOR]
Tropical weather phenomena—including tropical cyclones (TCs) and equatorial waves—are influenced by planetary‐to‐convective‐scale processes; yet, existing data sets and tools can only capture a subset of those processes. This study introduces a convection‐permitting aquaplanet simulation that can be used as a laboratory to study TCs, equatorial waves, and their interactions. The simulation was produced with the Model for Prediction Across Scales‐Atmosphere (MPAS‐A) using a variable resolution mesh with convection‐permitting resolution (i.e., 3‐km cell spacing) between 10°S and 30°N. The underlying sea‐surface temperature is given by a zonally symmetric profile with a peak at 10°N, which allows for the formation of TCs. A comparison between the simulation and satellite, reanalysis, and airborne dropsonde data is presented to determine the realism of the simulated phenomena. The simulation captures a realistic TC intensity distribution, including major hurricanes, but their lifetime maximum intensities may be limited by the stronger vertical wind shear in the simulation compared to the observed tropical Pacific region. The simulation also captures convectively coupled equatorial waves, including Kelvin waves and easterly waves. Despite the idealization of the aquaplanet setup, the simulated three‐dimensional structure of both groups of waves is consistent with their observed structure as deduced from satellite and reanalysis data. Easterly waves, however, have peak rotation and meridional winds at a slightly higher altitude than in the reanalysis. Future studies may use this simulation to understand how convectively coupled equatorial waves influence the multi‐scale processes leading to tropical cyclogenesis. Plain Language Summary: Despite many advancements in the science and prediction of tropical cyclones (TCs), scientists are still trying to explain the most important processes governing the evolution and characteristics of TCs. An emerging area of focus is how atmospheric oscillations, known as Kelvin waves, may increase the likelihood that a disturbance (often times referred to as an easterly wave) may become a TC. However, available atmospheric data sets are unable to capture all the fine details of TCs, disturbances, and Kelvin waves. To alleviate this challenge, this study presents a computer simulation of an Earth‐like atmosphere except without continents or seasons. The simplicity of the simulation allows scientists to study the full life cycle of TCs—from their formation to their evolution into powerful hurricanes. Results of this study show that the simulated TCs, Kelvin waves, and easterly waves resemble those that happen in nature. Therefore, the simulation can be used to advance our understanding of how TCs form and of how multi‐scale phenomena, such as Kelvin waves, affect the chances of TC formation at a particular location and time. Key Points: An aquaplanet simulation with convection‐permitting resolution in the tropics is presented as a tool to study tropical weather phenomenaThe structure of tropical cyclones (TCs) and equatorial waves is realistically captured despite the idealized configurationThe simulation may be used for fundamental process‐based studies of TCs, equatorial waves, and their interactions [ABSTRACT FROM AUTHOR]
It has been widely recognized that tropical cyclone (TC) genesis requires favorable large‐scale environmental conditions. Based on these linkages, numerous efforts have been made to establish an empirical relationship between seasonal TC activities and large‐scale environmental favorability in a quantitative way, which lead to conceptual functions such as the TC genesis index. However, due to the limited amount of reliable TC observations and complexity of the climate system, a simple analytic function may not be an accurate portrait of the empirical relationship between TCs and their ambiences. In this research, we use convolution neural networks (CNNs) to disentangle this complex relationship. To circumvent the limited amount of seasonal TC observation records, we implement transfer‐learning technique to train ensemble of CNNs first on suites of high‐resolution climate model simulations with realistic seasonal TC activities and large‐scale environmental conditions, and then on a state‐of‐the‐art reanalysis from 1950 to 2019. The trained CNNs can well reproduce the historical TC records and yields significant seasonal prediction skills when the large‐scale environmental inputs are provided by operational climate forecasts. Furthermore, by inputting the ensemble CNNs with 20th century reanalysis products and Phase 6 of the Coupled Model Intercomparison Project (CMIP6) simulations, we investigated TC variability and its changes in the past and future climates. Specifically, our ensemble CNNs project a decreasing trend of global mean TC activity in the future warming scenario, which is consistent with our future projections using high‐resolution climate model. Plain Language Summary: Tropical cyclones (TCs) require favorable large‐scale environmental conditions to form. Pioneer studies show that these conducive conditions include warm sea surface temperatures (SSTs), sufficient low‐level vorticities and mid‐level humilities, as well as weak‐to‐moderate vertical wind shears. Several follow‐up studies have focused on improving the empirical linkage between number of TC and environmental conditions and developed sets of TC genesis index based on conventional statistical methods. Although these indices can capture climatology of TC genesis spatial distributions and seasonal variation reasonably well, their representations of interannual TC variability are degraded. With the aim to better represent TC interannual variability and long‐term trend using large‐scale environmental conditions, we trained ensembles of convolution neural networks (CNNs) based on the combination of observations and large sets of high‐resolution dynamical climate simulations. The trained CNNs perform significantly well in capturing observed TC interannual‐to‐multidecadal variability and are broadly applicable to many areas of seasonal TC activities. Using a deep learning technique, this paper introduces a new potential avenue to improve our understanding of TC variability and future changes. Key Points: Ensemble convolutional neural networks (CNNs) are trained to emulate seasonal tropical cyclone (TC) activity using environmental factorsThe trained CNNs can be utilized to study seasonal TC variability, and their changes in the past, current and future climatesSkillful seasonal TC predictions can be made using CNN‐based statistical‐dynamical hybrid framework [ABSTRACT FROM AUTHOR]
One of the factors that influence the complexity of the weather and climate phenomena in Indonesia is the diverse topographical conditions. The Mountain waves are quite common in Indonesia, especially on the island of Java, which has several mountains with quite high peaks. The existence of mountain waves can be known from the presence of lenticularis clouds both on top of the mountain are regularly reported by residents in the area of Mount Ungaran and Mount Lawu in Centra Java. This study utilized the Weather Reasearch Forecast (WRF), a numerical weather model to identify mountain waves and their characteristics in Mount Ungaran and Mount Lawu. The results of the study are that there were 2 mountain wave events in Mount Ungaran on 9 September 2018 with an average duration of 5 h, and horizontal wavelengths reaches 28 km. During that mountain wave events, there was also a type 1 rotor with dominant positive vorticity and a breaking pattern that indicates the potential for strong turbulence which is supported by critical Ri value at a distance of 30 - 90 m on the leeward side. Meanwhile, in Mount Lawu case on 9 March 2019, the mountain wave onset occurs during the day is in the afternoon, its duration is 2 h, wavelengths ranging from 5.9 to 6.1 km, where in the leeward side has no strong turbulence potential. In general, WRF can simulate both mountain waves and rotors, but has not been able to properly simulate the downslope windstorms. [ABSTRACT FROM AUTHOR]
The conditions associated with tropical cyclones undergoing downshear reformation are explored for the North Atlantic basin from 1998 to 2020. These storms were compared to analog tropical cyclones with similar intensity, vertical wind shear, and maximum potential intensity, but did not undergo downshear reformation. Storm-centered, shear-relative composites were generated using ERA5 and GridSat-B1 data. Downshear reformation predominately occurs for tropical cyclones of tropical storm intensity embedded in moderate vertical wind shear. A comparison between composites suggests that reformed storms are characterized by greater low-level and midtropospheric relative humidity downshear, larger surface latent heat fluxes downshear and left of shear, and larger low-level equivalent potential temperatures and CAPE right of shear. These factors increase thermodynamic favorability, building a reservoir of potential energy and decreasing dry air entrainment, promoting sustained convection downshear, and favoring the development of a new center. Significance Statement: The development of a new low-level circulation center in tropical cyclones that replaces the original center, called downshear reformation, can affect the structure and intensity of storms, representing a challenge in forecasting tropical cyclones. While there have been a handful of case studies on downshear reformation, this study aims to more comprehensively understand the conditions that favor downshear reformation by comparing a large set of North Atlantic tropical cyclones that underwent reformation with a similar set of tropical cyclones that did not undergo reformation. Tropical cyclones that undergo reformation have a moister environment, larger surface evaporation, and higher low-level instability in specific regions that help sustain deep, downshear convection that favors the development of a new center. [ABSTRACT FROM AUTHOR]
This study examines the impact of the winter Arctic sea ice concentration (ASIC) anomaly on the succedent summer tropical cyclone genesis frequency (TCGF) over the western North Pacific (WNP) and provides a new insight into the underlying physical mechanisms. There is a significant time-lagged relation between winter ASIC anomalies over Greenland-Barents-Kara (GBK) seas and the following summer TCGF over the southeastern part of the WNP. This delayed association is attributable to large-scale circulation anomalies and the air-sea interaction processes over the North Pacific induced by the winter ASIC anomalies. Specifically, a higher winter ASIC over the GBK seas induces an atmospheric wave train that propagates southeastward to the North Pacific. The associated cyclonic anomaly over the mid-latitude North Pacific is accompanied by southwesterly wind anomalies over the subtropics and results in sea surface temperature (SST) warming by reducing upward surface heat fluxes. This SST warming is maintained and further extends southward to the tropical Pacific in the following summer via a wind-evaporation-SST feedback, which in turn forces overlying atmospheric circulation via a Gill-type atmospheric response, including a pair of cyclonic and anticyclonic anomalies in the low- and upper-level troposphere, respectively, over the WNP. These atmospheric anomalies favor TC genesis over the southeastern part of the WNP by decreasing the vertical wind shear and increasing the convection, low-level vorticity and humidity. The above processes apply to the years when lower ASIC winters are followed by decreased TC genesis over the southeastern part of the WNP except for opposite signs of SST and atmospheric circulation anomalies. This study suggests that the winter ASIC anomaly over the GBK seas is a potential predictor for the prediction of the WNP TCGF in the following summer. [ABSTRACT FROM AUTHOR]
RAINFALL, WATER vapor transport, RAINSTORMS, VERTICAL wind shear, AUTOMATIC meteorological stations, DEW point
Abstract
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The generation mechanism of stratospheric gravity waves (SGWs) in tropical cyclones (TCs) is investigated with an idealized experiment using numerical model. The results show that there are two peaks of SGW amplitude during the mature period of TC, one above the eyewall and the other above the rainband. The SGWs can only exist in unbalanced flow, so a simplified balance equation suitable for the special structure of the TC is proposed to investigate the SGWs. Diagnosis of this equation shows there is significant unbalanced flow around the eyewall and rainband caused by the convection and the vertical shear between outflow and compensating inflow. Diagnosis of the source function indicates that diabatic heating and mechanical oscillations are the main mechanisms generating the SGWs above the eyewall and rainband. The Kelvin–Helmholtz instability induced by vertical shear also makes a non‐negligible contribution to the generation of SGWs above the rainband. Plain Language Summary: As intense convective systems, tropical cyclones (TCs) can generate intense stratospheric gravity waves (SGWs). However, TCs contain many unique structures such as the eyewall, spiral rainband, outflow layer, and inflow layer. How are the SGWs distributed in TCs and what effect does TC structure have on the generation of SGWs? Here we find there are two peaks of SGW amplitude during the mature period of TCs, one above the eyewall and one above the rainband. They appear mainly in the unbalanced flow regions, and the unbalanced flow in TCs is generated by convection around the eyewall and rainband and the vertical shear of the radial wind caused by outflow above the rainband. Thus, the convection and vertical shear play key roles in the generation of SGWs in TCs. Diabatic heating is the main mechanism generating SGWs above the eyewall and rainband by convection. Meanwhile, mechanical oscillation also generates convective SGWs above the eyewall. The Kelvin–Helmholtz instability induced by the vertical shear also makes a non‐negligible contribution to the generation of SGWs above the rainband. Key Points: There is significant unbalanced flow that can generate stratospheric gravity waves around the eyewall and rainband of tropical cyclonesThe unbalanced flow is generated by convection and vertical shear of the radial wind around the eyewall and outflow above the rainbandDiabatic heating, mechanical oscillation, and Kelvin–Helmholtz instability contribute to the generation of stratospheric gravity waves [ABSTRACT FROM AUTHOR]
Initial conditions of current numerical weather prediction systems insufficiently represent the sharp vertical gradients across the midlatitude tropopause. Data assimilation may provide a means to improve the tropopause structure by correcting the erroneous background forecast towards the observations. In this paper, the influence of assimilating radiosonde observations on the tropopause structure, i.e., the sharpness and altitude, is investigated in the ECMWF IFS. We evaluate 9729 midlatitude radiosondes launched during one month in autumn 2016. About 500 of these radiosondes, launched on request during the North Atlantic Waveguide Downstream impact Experiment (NAWDEX) field campaign, were used to set up an observing system experiment (OSE). The OSE comprises two cycled assimilation forecast experiments, one with and one without the non-operational soundings. The influence on the tropopause is assessed in a statistical, tropopause–relative evaluation of observation departures of temperature, static stability (N2), wind speed and wind shear from the background forecast and the analysis. The background temperature is overestimated at the tropopause (warm bias, ~1 K) and underestimated in the lower stratosphere (cold bias, −0.3 K) leading to an underestimation of the abrupt vertical increase of N2 at the tropopause. We show that the increments (differences of analysis and background) reduce background biases and improve the tropopause sharpness. Profiles with sharper tropopause exhibit stronger background biases and, in turn, an increased positive influence of the observations on temperature and N2 in the analysis. Wind speed is underestimated in the background, especially in the upper troposphere (~1 m s−1), but the assimilation improves the wind profile. For the strongest winds the background bias is roughly halved. The positive influence on the analysis wind distribution is associated with an increase of vertical wind speed shear, which is underestimated above the tropopause. In addition to the tropopause sharpening, we detect a shift of the analysis tropopause altitude towards the observations. The comparison of the OSE runs highlights that the main contribution to the tropopause sharpening can be attributed to the radiosondes. This study shows that data assimilation improves wind and temperature gradients across the tropopause, but the sharpening is small compared to the model biases. Hence, the analysis still systematically underestimates the tropopause sharpness which may negatively impact weather and climate forecasts. [ABSTRACT FROM AUTHOR]
The present study examines the drivers of the observed increasing trend in the Genesis Potential Index (GPI) of the post-monsoon season (October-November-December) tropical cyclones in the Arabian Sea (AS) during the period, 1998–2021. The increase in atmospheric moisture loading, ocean heat content in the upper 300 m (OHC300) and reduction in vertical wind shear are the major factors which caused the intensification in cyclone GPI in the recent decades. The increase in atmospheric moisture loading and OHC300 are consistent with the overall observed ocean warming trend of the region. However, the reduction in vertical wind shear has resulted from an anomalous large-scale upper atmospheric anticyclonic circulation over central India. Further investigation shows a concurrent transition of the Warm Arctic Cold Eurasia (WACE) pattern to its positive phase which strengthened and shifted the Subtropical Jet (STJ) poleward. This resulted in the anticyclonic circulation anomaly and altered the upper tropospheric zonal winds over the AS cyclone genesis region, weakening the vertical wind shear. The study demonstrates a possible physical mechanism through which remote forcing due to changes in the northern high-latitude climate can influence the AS cyclone genesis. [ABSTRACT FROM AUTHOR]
Stall-Induced Vibrations (SIV) are an important design consideration for wind turbine blade design, especially for large, modern, wind turbines with highly flexible blades. Their severity depends on both the inflow and the structural characteristics of the blades. Studying SIV has a high computational cost, because it requires high-fidelity aeroelastic simulations, and potentially a large number of input variables. In an effort to reduce the computational cost of the domain exploration, in this work we have adopted a Surrogate-Based Optimization (SBO) framework. This way, the combination of input variables that lead to SIV can be explored with the minimum number of aeroelastic simulations. The proposed SBO framework can use any type of surrogate model, and leverages Delaunay triangulation to iteratively select samples to refine the surrogate model. The occurrence and severity of SIV on the IEA 10MW turbine is studied in a five variables space consisting of: wind speed, yaw angle, vertical wind shear, wind veer, and atmospheric temperature. A well-trained surrogate model is developed and used to predict the damping ratio of the first blade edgewise mode in the entire inflow space at a reduced computational cost. Sensitivity analysis of the predicted damping ratio shows that yaw angle is the most influential variable, while temperature is the least influential variable in terms of inflow conditions that can lead to the occurrence of SIV. Inflow conditions with a moderate yaw angle (around 10–25 deg), high wind speeds, and moderate to high negative veer are found to lead to severe SIV. This study should serve as a guiding tool to decide the scope of the more computationally expensive simulations such as high-fidelity CFD-based aeroelastic simulations which can provide a more accurate description of SIV. • Effect of multiple inflow variables on SIV, which has limited literature is examined. • For the IEA 10 MW turbine, yaw angle has a significant effect on occurrence of SIV. • Negative wind veer lowers the yaw angle at which SIV occur. • Air Temperature has an insignificant effect on SIV. • Surrogate models offer huge cost savings to study effect of inflow variables on SIV. [ABSTRACT FROM AUTHOR]
Extreme precipitation events have been occurring frequently worldwide, and their causative factors and convection initiation (CI) mechanisms have been attracting more and more attention in recent years. As a comprehensive study on the CI mechanisms of extreme rainstorms over the northern slope of the Kunlun Mountains (KLM), Xinjiang, based on both observational and high tempo-spatial numerical simulation, the major findings of this work are as follows: A cold pool (CP) was formed in the northwestern Tarim Basin under the influence of early precipitation evaporation, and it moved towards the northern slope of the KLM several hours before the CI. With the movement of the CP, a significant vertical temperature gradient was formed close to the leading edge of the CP, thereby enhancing local convective instability (up to ~10 PVU). In addition, the vertical shear of the horizontal winds at the leading edge of the CP led to a notable increase in the baroclinic component of moist potential vorticity, thus reinforcing the local conditional symmetric instability (up to ~8 PVU), providing another important unstable energy for the CI. In addition, the combined effect of the convergent lifting of a boundary layer jet (BLJ, the maximum wind speed below 1 km exceeding 10 m s−1) and the significant frontogenetical forcing (up to ~100 × 10−8 K m−1 s−1) at the leading edge of the CP were the causes of the release of the unstable energies. Further analysis of the frontogenetical forcing associated with the CP indicates that the convergence (up to ~2 × 10−3 s−1), diabatic heating and slantwise terms (indicates the baroclinicity and inhomogeneity of the vertical momentum in horizontal direction) were the major contributors, whereas the deformation term at the leading edge of the CP provided a relatively weaker contribution. [ABSTRACT FROM AUTHOR]
Tropical cyclogenesis in the Atlantic is influenced by environmental parameters including vertical wind shear, which is sensitive to forcing from the tropical Pacific. Reliable projections of the response of such parameters to radiative forcing are key to understanding the future of hurricanes and coastal risk. One of the least certain aspects of future climate is the warming of the eastern tropical Pacific Ocean. Using climate model experiments isolating the warming of the eastern Pacific and controlling for other factors including El Niño‐Southern Oscillation (ENSO), changes in Atlantic tropical cyclogenesis potential by the end of this century are ∼20% lower with enhanced eastern Pacific warming. The ENSO signal in Atlantic tropical cyclogenesis potential amplifies with global warming, and that amplification is larger with enhanced eastern Pacific warming. The largest changes and dependencies on eastern Pacific warming are found in the south‐central main development region, attributable to changes in zonal overturning. Plain Language Summary: Today, there are about 15 tropical storms per year in the Atlantic. That number varies considerably from year to year, with El Niño being one major factor. When the eastern Pacific Ocean warms temporarily, Atlantic hurricanes tend to be suppressed (and vice versa for La Niña). As the climate warms due to greenhouse gas emissions, hurricanes are expected to change. Such changes could include the average number of tropical storms per year, where they tend to form, how strong they become, how far and fast they travel, how much rain they produce, and how El Niño affects them. This study investigates how the formation regions of Atlantic hurricanes may change in the future, particularly as a function of how much the eastern Pacific Ocean warms in the future, which is one of the most uncertain aspects of climate change. We find that the warming of the eastern Pacific strongly influences predictions of future changes in Atlantic hurricanes, including how El Niño affects them. Specifically, a strong eastern Pacific warming causes a change in the winds over the tropical Atlantic, which shifts where hurricanes will tend to form in the future, and increases the effect of El Niño. Key Points: Enhanced surface warming in the eastern equatorial Pacific Ocean impacts the response of Atlantic hurricanes to global warmingGenesis potential decreases in the south‐central part of the main development region, but only with enhanced eastern Pacific warmingThe El Niño/La Niña signal in Atlantic genesis potential amplifies with global warming—more so with enhanced eastern Pacific warming [ABSTRACT FROM AUTHOR]
Using 42 years of reanalysis data, we investigate regional, storm‐relative characteristics of three groups of Atlantic tropical cyclone intensification: slightly, moderately, and rapidly intensifying. Probability density functions are distinct between these groups for vertical wind shear, sea surface temperature (SST), and radius of maximum winds (RMW), but less so for relative humidity (RH). In the Gulf of Mexico and southern North Atlantic, shear and RMW are good predictors. In the open Atlantic, north of 22°N, shear and SST are the best predictors. In the Caribbean, weaker relationships suggest low statistical predictability in a region where RI cases increased between 1980–2000 and 2001–2021. Of our storm‐relative variables tested, increasing SST appears to be most closely connected to the 36% increase in rapidly intensifying events between the two periods, whereas shear and RH are not significantly more favorable. The variability across regions, periods, and variables motivates further investigation. Plain Language Summary: Although many damaging Atlantic tropical cyclones intensify rapidly during their lifetime, forecasting how rapidly they will intensify remains difficult. To address this, we need to better understand whether the conditions in a storm's environment can help identify whether it will intensify rapidly or more slowly, and whether these results change in different regions. Using 42 years of data, we find that wind shear (change in wind speed and direction with height) and ocean temperature are useful to discriminate whether a storm may intensify rapidly versus slowly across the basin, although there is variability in the results. Humidity is less useful. Our results vary by region. The Caribbean shows lower discrimination between intensification rates, highlighting potential difficulties in predicting intensification in this region. This is concerning since the number of rapidly intensifying events has risen sharply in the Caribbean between 1980–2000 and 2001–2021. We also find that the trends in intensifying storms during these 21‐year periods vary by region. The most consistent signal related to the rise in intensifying events is the increasing ocean temperature. Key Points: Atlantic basin rapid intensification trends vary by region, with the greatest increases between 1980–2000 and 2001–2021 south of 22°NWind shear, sea surface temperature, and radius of maximum winds are useful discriminators between intensification ratesStorm‐relative environmental variable trends vary by region, with sea surface temperatures often related to regional intensity change trends [ABSTRACT FROM AUTHOR]
The variation of tropical cyclone (TC) genesis over the North Indian Ocean (NIO) and South China Sea (SCS) during the post‐monsoon season is explored during 1979–2020. A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and Bay of Bengal (BOB) –SCS (BOBSCS for brevity). More (less) TCs over the AS tend to concur with fewer (more) TCs over the BOBSCS. Analyses of both the observations and the Coupled Model Intercomparison Project Phase 6 outputs show that tripole sea surface temperature (SST) anomalies in the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesis. Positive western Indian (WI) Ocean and tropical central and eastern Pacific (CEP) Ocean SST anomalies and negative western Pacific (WP) SST anomalies induce anomalous ascending and descending motions and high and low humidity transportation to the middle troposphere over the AS and BOBSCS, respectively. Meantime, anticyclonic vorticity anomalies appear in the BOBSCS region and negative anomalies of vertical wind shear are induced over the NIO and SCS. The combined positive contributions of upward motion, mid‐level humidity increase, and low vertical wind shear favor higher TC genesis over the AS, whereas the joint negative effects of downward motion, mid‐level water vapor decrease, and lower‐level anticyclonic vorticity anomalies result in less TC genesis over the BOBSCS. Numerical experiments show a critical role of the WI SST anomalies in the dipole pattern of vertical motion, which is crucial for the zonal dipole variation of TC genesis. Plain Language Summary: Although the tropical cyclones (TCs) frequency number that form over the South China Sea (SCS), Bay of Bengal (BOB) and Arabian Sea (AS) are relatively small, these regions are densely populated, which means that the impacts of TCs can be significant and widespread. The study presents observational and modeling evidence for a see‐saw variation of tropical cyclogenesis between the AS and BOB‐SCS during the post‐monsoon season (September–December). Composite analyses display that the zonal see‐saw connection of tropical cyclogenesis frequency over the AS and BOB‐SCS is closely linked to the zonal tripole distribution of the sea surface temperature anomalies over the tropics of the Indian and Pacific Oceans, which exert a significant influence on the vertical–zonal overturning circulation and the environmental parameters for TC formation over the AS and BOB‐SCS. We conduct an evaluation of 19 CMIP6/AMIP (CMIP6/CMIP) simulations and reveal that 53% (26%) models are able to successfully simulate the zonal dipole pattern of TC‐related variables between AS and BOB‐SCS regions. The present findings may have the potential to enhance the seasonal prediction for tropical cyclogenesis over the SCS, AS and BOB regions. Key Points: A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and the Bay of Bengal‐South China Sea (BOBSCS)The tripole SST anomalies over the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesisNumerical experiments show a critical role of SST anomalies in the western Indian Ocean in the dipole pattern of vertical motion [ABSTRACT FROM AUTHOR]
Cold pools that form from existing convection can generate new clouds nearby, a process that has been suggested to contribute to cloud organization. However, some disagreement exists on the role of cold pools in cloud organization over tropical oceans, and much remains to be understood on how this role depends on the state of large‐scale disturbances such as the Madden‐Julian Oscillation (MJO). This study addresses this question by examining the intraseasonal variability in the properties of cold pools and their effects on convection triggering using observations. The unique set of surface meteorology data and ground‐based radars deployed during the DYNAMO/AMIE field campaign are used to identify cold pools and the associated spatiotemporal evolution of rainfall. The results suggest that cold pools enhance rainfall at their expected speed of 3–8 m s−1, which includes the Doppler‐shifting effect by the background wind. The cold pools also enhance rainfall mainly within a 20 km radius, especially in the direction of boundary‐layer vertical wind shear. However, gravity waves appear to contribute predominantly to the propagation of convection over a wider range of distances due to their faster propagation speeds. The interface of convection triggering and propagation at different speeds of cold pools and gravity waves seems to contribute to cloud organization. The effectiveness of cold pools and gravity waves on cloud organization also depends highly on the MJO due to the combined effects of changes in environmental humidity and boundary to lower‐tropospheric wind. Plain Language Summary: High‐density air that forms due to evaporative cooling of rainfall and downward injection of colder air aloft spreads horizontally, a phenomenon known as a cold pool. The spreading edges of cold pools initiate new cumulus clouds by lifting humid air, which is thought to cluster cumulus clouds that can merge and form larger clouds. However, the importance of cold pools in cloud clustering remains debated. This study uses a unique combination of observational data collected during a field campaign deployed over the Indian Ocean to examine the effect of cold pools on cloud clustering. The results provide observational evidence that cold pools support cloud formation within a 20 km radius of the identified cold pools, helping the clustering of clouds within a limited distance. In addition to cold pools, gravity waves that form from cumulus clouds mainly lead to the propagation and formation of cumulus clouds over longer distances due to their faster propagation speeds. The formation of cumulus clouds over a varying range of distances due to different propagation speeds of cold pools and gravity waves seems critical to the clustering of clouds. However, their effectiveness on cloud formation depends on atmospheric conditions set by intraseasonal variability. Key Points: Precipitation‐induced cold pools are observed to enhance rainfall within 20 km through interactions with boundary‐layer vertical wind shearWhile cold pools trigger convection within a limited distance, gravity waves mainly propagate convection over a wider range of distancesThe cold pools and gravity waves together lead to cloud organization, but their effectiveness depends on the large‐scale environment [ABSTRACT FROM AUTHOR]
In this study, the characteristics of azimuthally asymmetric equivalent potential temperature (θe) distributions in the outer core of tropical cyclones (TCs) encountering weak and strong vertical wind shear are examined using a Lagrangian trajectory method. Evaporatively forced downdrafts in the outer rainbands can transport low-entropy air downward, resulting in the lowest θe in the downshear-left boundary layer. Quantitative estimations of θe recovery indicate that air parcels, especially those originating from the downshear-left outer core, can gradually revive from a low entropy state through surface enthalpy fluxes as the parcels move cyclonically. As a result, the maximum θe is observed in the downshear-right quadrant of a highly sheared TC. The trajectory analyses also indicate that parcels that move upward in the outer rainbands and those that travel through the inner core due to shear make a dominant contribution to the midlevel enhancement of θe in the downshear-left outer core. In particular, the former plays a leading role in such θe enhancements, while the latter plays a secondary role. As a result, moist potential stability occurs in the middle-to-lower troposphere in the downshear-left outer core. [ABSTRACT FROM AUTHOR]
On 13 November 2019, seven commercial aircraft of China Eastern Airlines encountered nine severe-or-greater clear-air turbulence (CAT) events over central and eastern China within 12 h (0000–1200 UTC). These events mainly occurred at altitudes between 6.0 and 6.7 km. A high-resolution nested numerical simulation is carried out using the Weather Research and Forecasting (WRF) Model to investigate the generation mechanism of these CAT events, with a horizontal resolution of 1 km over the inner domain. In addition, seven CAT diagnostics with outstanding performances are employed for the mechanism analysis. The WRF Model can reasonably reproduce both synoptic-scale systems (Siberian high and upper-level jet stream) and local vertical structures (temperature, dewpoint temperature, and wind field). The simulation indicates that an upper-level front–jet system with a remarkable meridional temperature gradient intensifies over central and eastern China, with the maximum wind speed increasing from 59.0 to 67.3 m s−1. The intensification of the front–jet system induces the tropopause folding, and nine localized CAT events occur in the region with large vertical wind shear (VWS) (1.55 × 10−2–2.53 × 10−2 s−1) and small Richardson numbers (Ri) (0.42–0.85) below the cyclonic side of the jet stream. Diagnostic analysis reveals that Kelvin–Helmholtz instability plays an important role in CAT generation, while convective and inertial instability is not directly associated with CAT generation in this study. A typical flight case with continuous CAT events also suggests that large VWS (greater than 1.3 × 10−2 s−1) accompanied with small Ri (less than 1) favors CAT generation in a front–jet system environment. Significance Statement: A high-resolution nested numerical simulation is carried out using the Weather Research and Forecasting (WRF) Model to investigate the generation mechanism of nine severe-or-greater clear-air turbulence (CAT) events over central and eastern China. Intensification of a front–jet system induces tropopause folding, and nine CAT events occur in the region with large vertical wind shear (greater than 1.55 × 10−2 s−1) and small Richardson numbers (less than 0.85) below the cyclonic side of the jet stream. Kelvin–Helmholtz instability plays an important role in the CAT generation, rather than convective and inertial instability. [ABSTRACT FROM AUTHOR]
To assess the performance and scalability of the Unified Forecast System (UFS) Short-Range Weather (SRW) application, case studies are chosen to cover a wide variety of forecast applications. Here, model forecasts of Hurricane Barry (July 2019) are examined and analyzed. Several versions of the Global Forecast System (GFS) and Rapid Refresh Forecast System (RRFS) physics suites are run in the UFS-SRW at grid spacings of 25 km, 13 km, and 3 km. All model configurations produce significant track errors of up to 350 km at landfall. The track errors are investigated, and several commonalities are seen between model configurations. A westerly bias in the environmental steering flow surrounding the tropical cyclone (TC) is seen across forecasts, and this bias is coincident with a warm sea surface temperature (SST) bias and overactive convection on the eastern side of the forecasted TC. Positive feedback between the surface winds, latent heating, moisture, convection, and TC intensification is initiated by this SST bias. The asymmetric divergent flow induced by the excess convection results in all model TC tracks being diverted to the east as compared to the track derived from reanalysis. The large differences between runs using the same physics packages at different grid spacing suggest a deficiency in the scalability of these packages with respect to hurricane forecasting in vertical wind shear. [ABSTRACT FROM AUTHOR]
The combination of moderate vertical wind shear (VWS) and dry environments can produce the most uncertain scenarios for tropical cyclone (TC) genesis and intensification. We investigated the sources of increased uncertainty of TC development under moderate VWS and dry environments using a set of Weather Research and Forecasting (WRF) ensemble simulations. Statistical analysis of ensemble members for precursor events and time-lagged correlations indicates that successful TC development is dependent on a specific set of precursor events. A deficiency in any of these precursor events leads to a failure of TC intensification. The uncertainty of TC intensification can be largely attributed to the probabilistic characteristics of precursor events lining up together before TC intensification. The critical bifurcation point between successful and failed trials in these idealized simulations is the sustained vortex alignment process. Even for the failed intensification cases, most simulations showed deep organized convection, which reformed a midlevel vortex. However, for the failed cycles, the new midlevel vortex could not sustain vertical alignment with the low-level center and was carried away by VWS shortly. Under the most uncertain setup (VWS 5 7.5 m s-1 and 50% moisture), the latest-developing ensemble member had seven events of tilt decreasing and increasing again that occurred during the 8 days before genesis. Some unsuccessful precursor events looked very close to the successful ones, implying limits on the intrinsic predictability for TC genesis and intensification in moderately sheared and dry environments. [ABSTRACT FROM AUTHOR]
During September 2021, a tropical cyclone (TC) named Gulab formed in Bay of Bengal (BoB) region of North Indian Ocean (NIO) and by nature, it was quite an unusual one as it crossed the Indian subcontinent and re-emerged as TC Shaheen in Arabian Sea (AS) which made landfall at the coast of Oman. These two cyclones were unique from two aspects: (i) formed in active southwest monsoon period in the month of September, which is a very rare event in NIO and (ii) formation of TCs in BoB and its re-emergence in AS, in the form of a cyclonic storm after crossing the Indian continent is uncommon. Along with these two, another exception was landfall at Oman coast, which is again very rare. The different large scale atmospheric and oceanic parameters, during development of TC Gulab in BoB and its re-emergence as TC Shaheen in AS are analysed using the reanalysis data and satellite derived products. The results suggest that vertical wind shear (VWS) during genesis of TC Gulab was unusually low and favourable for cyclonic storm development in BoB. The middle level relative humidity over central India was also high (positive anomaly), which supported remnants of TC Gulab to survive as a low-pressure weather system in land region. Later, it evolves into as TC Shaheen in AS, and due to favourable Sea Surface Temperature and Oceanic Heat Content it further intensifies to a very severe category cyclonic storm. Research highlights: The best track data analysis in the north Indian Ocean reveals that TC Gulab and Shaheen were the unique systems in terms of their genesis, track and landfall. During the September month due to high vertical wind shear in the BoB, in general the low- pressure systems do not attain the intensity of a tropical storm. However, TC Gulab could turn into a TC in the September month due to favourable SST and low vertical wind shear. The atmospheric moisture content over the land was high due to active monsoon month, which helps the survival of remnant of TC Gulab, re-emerges and evolves as TC Shaheen in the Arabian Sea due to high ocean heat potential and sufficiently warm sea water, along with the favourable atmospheric conditions like low wind shear and high relative humidity. The behaviour of TC Gulab and Shaheen may have some linkage to the climate change, which is influencing the occurrence of more intense cyclonic system in the Arabian Sea that is affecting the Middle East costal region. The recent trend of rising SST signals the more frequent occurrence of such TCs in the Arabian Sea, creating an alarm situation for the cyclone vulnerability of the Arabian Sea basin countries. [ABSTRACT FROM AUTHOR]
*GLOBAL warming, *HURRICANES, *OCEAN temperature, *VERTICAL wind shear, *ATMOSPHERIC temperature
Abstract
Although Miller extended his maximum wind speeds to a sea temperature of about 90°F, Table 1 includes sea surface temperature only to 86°F which is about the highest temperature I've ever observed in the Gulf of Mexico or the Caribbean. HT
Maximum wind speed and kinetic destructiveness of hurricane dependent on sea surface temperature (necessary minimum sea surface temperature is 80°F)
Kinetic Energy Index V2 X 1000
Sea SFC Temp (F)
Max Wind Speed (MPH)
0 Prior to 1990, areas where sea surface temperatures were between 78° and 80°F would now have temperatures at or above the 80°F threshold for hurricane formation, thus adding considerable area favorable for hurricane formation and longer storm trajectories. [Extracted from the article]
Multiple parallel rainbands (MPRBs) involve the organization of mesoscale convective systems (MCSs) characterized by multiple parallel convective rainbands, which may produce high rainfall accumulation. A total of 178 MPRBs were identified from 2016 to 2020 in China, which were classified into the initiation type (∼40%), where rainbands initiate individually, and differentiation type (∼60%), where rainbands form through the splitting of large rainbands or merging of smaller cells. Results showed that the occurrence frequency of MPRBs peaks in July with a midnight major peak and a morning minor peak. The highest occurrence frequency is observed in the northern Beibu Gulf and its coastal areas, with minor high frequencies in Guangdong, northern Jiangxi, and southern Shandong provinces, typically in a southwesterly low-level jet to the west of the subtropical high. MPRBs mainly contain 3–4 rainbands with a spacing distance of 30–50 km and an orientation generally consistent with the direction of 850-hPa winds and 0–1-km vertical wind shear. MPRBs generally move slower than that of squall lines in East China ranging from 4 to 8 m s−1 with 16% being quasi-stationary, which is mainly due to the occurrence of band back building mainly associated with cold pool. Most MPRBs have training effects with band training as the dominant mode. Because of the band training effect and slower movement of MPRBs mainly due to band back building, 71% of MPRBs are associated with enhanced maximum hourly rainfall. Rainfall severity may be alleviated somewhat by the generally short duration of MPRBs with 78% being shorter than 2 h. Significance Statement: The purpose of this study is to document the general features of mesoscale convective systems (MCSs) with a specific organization of multiple parallel rainbands (MPRBs). MCSs with this unique organization tend to produce extremely heavy rainfall partly due to the training of multiple rainbands as well as their slow movement because of back building. The organization pattern of MPRBs was previously found in a case study. The possible formation mechanism was also previously examined based on case studies. As a complement to these studies, this work aims to reveal the temporal and spatial distributions, movement and duration, morphology, precipitation patterns, and environmental features of MPRBs in China based on statistics using 5-yr radar reflectivity data. [ABSTRACT FROM AUTHOR]
Kim, Taehyung, Kim, Eunji, Lee, Minkyu, Cha, Dong-Hyun, Lee, Sang-Min, Lee, Johan, and Boo, Kyung-On
Subjects
*TROPICAL cyclones, *CLIMATE extremes, *VERTICAL wind shear, *OCEAN temperature, EL Nino, LA Nina
Abstract
The characteristics of tropical cyclones (TCs) in sub-seasonal forecasting with the Global Seasonal Forecast System 5 (GloSea5) of the Korea Meteorological Administration (KMA) were assessed for June–September (JJAS) from 1991 to 2010 over the western North Pacific (WNP). The performance of GloSea5 was examined for its ability to reproduce observed TC climatology as well as changes in TC genesis with the El Niño-Southern Oscillation (ENSO) and a 1998/1999 climate regime shift (e.g., frequency, genesis spatial distribution). GloSea5 showed skillful performance in predicting the frequency and genesis spatial distribution of TCs in climatology and both ENSO phases; this performance was best during periods of La Niña. Environmental fields related to TC genesis (e.g., sea surface temperature [SST], vertical wind shear [VWS], 850-hPa wind and relative vorticity) were also reasonably captured, despite some systematic biases in SST, low-level circulation, relative vorticity, and VWS. GloSea5 performed well in terms of characteristic of changes in TC genesis before and after the regime shift. However, there were biases in TC frequency before the regime shift and changes in TC-related environmental fields. Our results imply that GloSea5 with a high predictive skill for TC genesis over the WNP can be used as an operational model for sub-seasonal TC forecasting, although it requires continuous improvements to reduce systematic errors. [ABSTRACT FROM AUTHOR]
The multi-year simulation of tropical cyclones (TCs) over the Western North Pacific (WNP) in the variable resolution (VR) CAM-MPAS model is studied. Experiments with the global quasi-uniform low resolution of 120 km (MPAS-UR) and the variable resolution mesh of 30–120 km refined over East Asia (MPAS-VR) are integrated from 1980 to 2005 following the Atmospheric Model Intercomparison Project protocol. By utilizing an objective detection method, TCs in ERA5 reanalysis and model simulations are tracked and compared against observations. MPAS-VR shows significant advantages over MPAS-UR as indicated by more realistic TC counts, intensities, lifetime distribution, and seasonal variation. The large-scale circulation and precipitation patterns associated with TCs are also improved in MPAS-VR relative to MPAS-UR. Based on the theory of Dynamic Genesis Potential Index, the multi-year TC records are further used to quantify the dependence of TC genesis on various dynamical environmental factors from the perspective of seasonal variation. We find that in ERA5, the relative contribution of the 500 hPa vertical pressure velocity term to TC genesis exceeds that of the 200–850 hPa vertical wind shear term, which is responsible for the August peak and strong seasonal variation of TC genesis. MPAS-UR fails to capture such relationship while MPAS-VR performs much better in this regard, suggesting that the higher skills in simulating the relative contributions from different dynamical environmental factors to the simulated seasonal cycle of TC genesis may explain the improvements from MPAS-UR to MPAS-VR. [ABSTRACT FROM AUTHOR]
VERTICAL wind shear, ION migration & velocity, ELECTRIC field effects, WIND shear, ELECTRIC fields, LATITUDE
Abstract
In previous studies, various physical parameters, such as vertical wind shear null, vertical ion velocity null (IVN), and maximum gradient of vertical ion velocity, have been used as an indicator of Es layer height in light of the wind shear theory. Using a numerical Es layer model, we investigated which parameter represents the height of Es layer over Wuhan (114.4°E, 30.5°N) most precisely. The simulation results show that the height of Es layer above 110 km depends most strongly on the height of vertical IVN. However, below 110 km, the height of vertical IVN does not always agree with the height of Es layer. The analysis of simulation results suggests that the vertical gradient of the ratio of the ion‐neutral collision frequency (νi) to the ion gyrofrequency (ωi) affects the Es layer formation at lower altitudes (below 110 km). We also examined the effect of electric fields on the height and intensity of Es layers over Wuhan. It is found that eastward/upward electric fields can lift the Es layer height and reduce the Es layer intensity. Key Points: Above 110 km, the height of Es layer agrees well with the height of the vertical ion velocity nullMid‐latitude Es layer density and height can be modulated by the electric fieldThe vertical gradient of the ratio of the ion‐neutral collision frequency (νi ${\nu }_{i}$) to the ion gyrofrequency (ωi ${\omega }_{i}$) also promotes Es layer formation [ABSTRACT FROM AUTHOR]
The recent 2020 Atlantic Hurricane Season was the most active, with 31 storms. September was the most active month of the season, with a simultaneous occurrence of five storms. This study probed into the meteorological and oceanographic conditions prevailing in the Atlantic Main Development Region (MDR) during the high activity months of August, September, and October of 2020. The mean sea surface temperature (SST) for the month of September 2020 was around 0.2 °C higher than the 30 years climatological average. Vertical wind shear (WSH) was well below the threshold for cyclogenesis, with a mean of ~ 5 m/s. Such conditions favoured the consecutive storm formations in the basin. Statistical sensitivity analysis was extended for the above three months of 1991–2020, using SST, WSH, and low-level relative vorticity (VOR) as predictors. The analysis showed mean difference between MDR and tropical region SST (SSTDIFF) to be a better influencer of hurricane count (HC) variability, with r2 values of 0.43 and 0.35 for the months of August and October of 1991–2020 period, respectively. VOR was found to be the dominant influencer of hurricane activity in the month of September, with r2 value of 0.47. Wavelet local multiple correlation technique showed correlation values to be higher (~ 0.75) for a SSTDIFF–HC pair for the months of August and October. However, VOR–HC pair had the highest correlation (~ 0.8) for the month of September. The WSH condition of the region, although favourable, was not found to be influencing hurricane activity significantly for this period. [ABSTRACT FROM AUTHOR]
The diagonal squall line that passed through the Korean Peninsula on the 18 May 2020 was examined using wind data retrieved from multiple Doppler radar synthesis focusing on its kinematic and dynamic aspects. The low-level jet, along with warm and moist air in the lower level, served as the primary source of moisture supply during the initiation and formation process. The presence of a cold pool accompanying the squall line played a role in retaining moisture at the surface. As the squall line approached the Korean Peninsula, the convective bands in the northern segment (NS) and southern segment (SS) of the squall line exhibited distinct evolutionary patterns. The vertical wind shear in the NS area was more pronounced compared to that in the SS. The ascending inflow associated with the tilted updraft in the NS reached an altitude of 7 km, whereas it was only up to 4 km in the SS. The difference was caused by the strong descending rear flow, which obstructed the ascending inflow and let to significant updraft in the SS.
Previous studies have investigated how the environmental vertical wind shear (VWS) may trigger the asymmetric structure in an initially axisymmetric tropical cyclone (TC) vortex and how TC intensity changes in response. In this study, the possible effect of the initial vortex asymmetric structure on the TC intensity change in response to an imposed environmental VWS is investigated based on idealized full‐physics model simulations. Results show that the effect of the asymmetric structure in the initial TC vortex can either enhance or suppress the initial weakening of the TC in response to the imposed environmental VWS. When the initial asymmetric structure is in phase of the VWS‐induced asymmetric structure, the TC weakening will be enhanced and vice versa. Our finding calls for realistic representation of initial TC asymmetric structure in numerical weather prediction models and observations to better resolve the asymmetric structure in TCs. Plain Language Summary: Although a strong tropical cyclone (TC) is often treated as an axisymmetric vortex in most theoretical studies, asymmetric structure always exists in a TC in nature, such as that generated by the environmental vertical wind shear (VWS). However, it is unclear whether and how the asymmetric structure in the initial TC vortex may affect the TC intensity change in response to an imposed environmental VWS. This has been addressed in this study by conducting idealized high‐resolution numerical simulations. Results show that the asymmetric structure in the initial TC vortex can either enhance or suppress the initial weakening of the TC in response to an imposed environmental VWS depending on how the initial asymmetry is aligned with the asymmetry induced by the VWS. If they are in phase, the TC weakening would be enhanced and vice versa. Our finding highlights the importance of realistically representing the asymmetric structure in the initial TC vortex in numerical weather prediction models for TC forecasts and also the need to better resolve the TC asymmetric structure in observations. Key Points: The initial asymmetric structure in a tropical cyclone (TC) vortex can either enhance or suppress the TC weakening induced by an imposed environmental vertical wind shear (VWS)If the initial asymmetric structure is in phase with the VWS‐induced asymmetric structure, the TC weakening would be enhanced and vice versaThe TC asymmetric structure should be better observed in real time and realistically represented in numerical weather prediction models [ABSTRACT FROM AUTHOR]
Tropical cyclones do not form easily near the equator but can intensify rapidly, leaving little time for preparation. We investigate the number of near-equatorial (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon season (October to December) over the past 60 years. The study reveals a marked 43% decline in the number of such cyclones in recent decades (1981–2010) compared to earlier (1951–1980). Here, we show this decline in tropical cyclone frequency is primarily due to the weakened low-level vorticity modulated by the Pacific Decadal Oscillation (PDO) and increased vertical wind shear. In the presence of low-latitude basin-wide warming and a favorable phase of the PDO, both the intensity and frequency of such cyclones are expected to increase. Such dramatic and unique changes in tropical cyclonic activity due to the interplay between natural variability and climate change call for appropriate planning and mitigation strategies. The north Indian Ocean is a hotbed for Low Latitude Cyclones (LLCs; originating between 5°N and 11°N). This study finds a remarkable decline in the frequency of LLCs in recent decades modulated by the remote influence of Pacific Decadal Oscillation. [ABSTRACT FROM AUTHOR]
This study analyzes microphysical signatures relevant to the short‐cycle lightning activity in the inner core of Typhoon Hato (2017) before landfall in China. Observations reveal that the lightning bursts were accompanied by enhanced inner‐core convection with a behavior cycle of about 3 hr. The coupling between wavenumber‐2 vortex Rossby waves (VRWs) and shear‐forced convective asymmetries resulted in a local updraft enhancement. Furthermore, supercooled liquid water droplets invigorated a striking enhancement of graupel via riming immediately above the freezing level, further enhancing charge separation and lightning generation outside the eyewall. Furthermore, when similar phase‐locking was associated with other propagating VRWs, graupel and updraft volumes were significantly boosted, leading to short‐cycle lightning outbreaks in the inner core. Plain Language Summary: Lightning behavior is one of the well‐defined indicators of convection within the Tropical cyclone (TC) circulation. Before the landfall of Typhoon Hato (2017), we found short‐cycle lightning outbreaks in the inner core of this typhoon. The interaction between the internal structures of the storm and changes in wind speed and direction with height could strikingly reinforce convection, thereby favorable for the short‐cycle electrification enhancement. This scenario is distinct from diurnal lightning bursts within TCs, which were found in previous studies. The diurnal lightning activity is thermodynamically governed by radiation, while vortex‐environment interactions in the inner core of Hato dynamically force the short‐cycle lightning outbreaks revealed here. Therefore, the findings presented in this case study suggest that interactions between the internal structures of a TC and the environment warrant an adequate focus in the short‐range forecasting of inner‐core convection in landfalling TCs. Key Points: Short‐cycle lightning outbreaks were found in the inner core of Typhoon Hato (2017)Interactions between vortex Rossby waves and the shear‐induced convective enhancement likely led to the quasi‐periodic lightning behaviorStrengthened updrafts and the significant growth of graupel enhanced the charge separation and lightning outbreaks [ABSTRACT FROM AUTHOR]
Turbulence is ubiquitous in the planetary boundary layer (PBL), which is of great importance to the prediction of weather and air quality. Nevertheless, the profiles of turbulence in the whole PBL as observed by radar wind profilers (RWPs) are rarely reported. In this communication, the purpose was to investigate the vertical structures of turbulence dissipation rate (ε) obtained from the Doppler spectrum width measurements from two RWPs at plateau (Zhangbei) and plain (Baoding) stations in the North China Plain for the year 2021, and to tease out the underlying mechanism for the difference of ε between Zhangbei and Baoding. Under clear-sky conditions, the annual mean value of ε in the PBL over the plateau station was found to be higher than that over the plain station throughout the daytime from 0900 to 1700 local standard time. The magnitude of ε at both stations showed significant seasonal variation, with the strongest ε in summer but the weakest in winter. If a larger difference between the 2 m air temperature and surface temperature (Ta−Ts), as a surrogate of sensible heat flux, is observed, the turbulence intensity tends to become stronger. The influence of vertical wind shear on turbulence was also analyzed. Comparison analyses showed that the plateau station of Zhangbei was characterized by larger sensible heat flux and stronger wind shear compared with the plain station of Baoding. This may account for the more intense ε within the PBL of Zhangbei. Moreover, the magnitude of ε in the PBL was positively correlated with the values of both Ta−Ts and vertical wind shear. The findings highlight the urgent need to characterize the vertical turbulence structure in the PBL over a variety of surfaces in China. [ABSTRACT FROM AUTHOR]
MACHINE learning, VERTICAL wind shear, AEROSOLS, STRATOCUMULUS clouds, OCEAN temperature, SURFACE of the earth
Abstract
Aerosol-cloud interactions (ACI) have a pronounced influence on the Earth's radiation budget but continue to pose one of the most substantial uncertainties in the climate system. Marine boundary-layer clouds (MBLCs) are particularly important since they cover a large portion of the Earth's surface. One of the biggest challenges in quantifying ACI from observations lies in isolating adjustments of cloud fraction (CLF) to aerosol perturbations from the covariability and influence of the local meteorological conditions. In this study, this isolation is attempted using nine years (2011–2019) of near-global daily satellite cloud products in combination with reanalysis data of meteorological parameters. With cloud-droplet number concentration (Nd) as a proxy for aerosol, MBLC CLF is predicted by region-specific gradient boosting machine learning models. By means of SHapley Additive exPlanation (SHAP) regression values, CLF sensitivity to Nd and meteorological factors as well as meteorological influences on the Nd –CLF sensitivity are quantified. The regional ML models are able to capture on average 45 % of the CLF variability. Global patterns of CLF sensitivity show that CLF is positively associated with Nd , in particular in the stratocumulus-to-cumulus transition regions and in the Southern Ocean. CLF sensitivity to estimated inversion strength (EIS) is ubiquitously positive and strongest in tropical and subtropical regions topped by stratocumulus and within the midlatitudes. Globally, increased sea surface temperature (SST) reduces CLF, particularly in stratocumulus regions. The spatial patterns of CLF sensitivity to horizontal wind components in the free troposphere point to the impact of synoptic-scale weather systems and vertical wind shear on MBLCs. The Nd –CLF relationship is found to depend more on the selected thermodynamical variables than dynamical variables, and in particular on EIS and SST. In the midlatitudes, a stronger inversion is found to amplify the Nd –CLF relationship, while this is not observed in the stratocumulus regions. In the stratocumulus-to-cumulus transition regions, the Nd –CLF sensitivity is found to be amplified by higher SSTs, potentially pointing to Nd more frequently delaying this transition in these conditions. The expected climatic changes of EIS and SST may thus influence future forcings from ACIs. The near-global ML framework introduced in this study produces a better quantification of the response of MBLC CLF to aerosols taking into account the covariations with meteorology. [ABSTRACT FROM AUTHOR]
The planetary boundary layer (PBL) structure and its evolution can significantly affect air pollution. Here, the PBL's characteristics and their association with air pollution were analyzed in Hefei, east China, using ERA5 reanalysis data, weather observations and air pollutant measurements from 2016 to 2021. In the near-surface level, air pollution was directly influenced by ground meteorological conditions, and high PM2.5 was normally related to weak wind speed, northwest wind anomalies, low temperature and high relative humidity. Moreover, in the trajectory analysis, air masses from the north and the northwest with short length played an important role in the high PM2.5 with pollutant transport within the PBL. Furthermore, high PM2.5 showed a tight dependence on PBL stratification. There was high temperature and relative humidity and low wind speed and PBL height within all PBL altitudes in the polluted condition. Notably, vertical wind shear (VWS) and temperature gradient tended to be much weaker below 900 hPa, which created a deeply stable stratification that acted as a cap to upward-moving air. Such a PBL structure facilitated more stable stratification and enhanced the generation of air pollution. Finally, the stable stratification in the PBL was related to the special synoptic configuration for the high PM2.5 conditions, which included the block situation at the high level, the southerly wind anomalies at the middle level and the wild range of the uniform pressure field at the near-ground level. Therefore, air pollutant concentrations were regulated by ground factors, PBL structure and the synoptic situation. Our results provide a precise understanding of the role of PBL features in air pollution, which contributes to improving the assimilation method of the atmospheric chemistry model in east China. [ABSTRACT FROM AUTHOR]
The WPR-LQ-7 is a UHF (1.3575 GHz) wind profiler radar used for routine measurements of the lower troposphere at Shigaraki MU Observatory (34.85 ∘ N, 136.10 ∘ E; Japan) at a vertical resolution of 100 m and a time resolution of 10 min. Following studies carried out with the 46.5 MHz middle and upper atmosphere (MU) radar (Luce et al., 2018), we tested models used to estimate the rate of turbulence kinetic energy (TKE) dissipation ε from the Doppler spectral width in the altitude range ∼ 0.7 to 4.0 km above sea level (a.s.l.). For this purpose, we compared LQ-7-derived ε using processed data available online (http://www.rish.kyoto-u.ac.jp/radar-group/blr/shigaraki/data/ , last access: 24 July 2023) with direct estimates of ε (εU) from DataHawk UAVs. The statistical results reveal the same trends as reported by Luce et al. (2018) with the MU radar, namely (1) the simple formulation based on dimensional analysis εLout=σ3/Lout , with Lout∼70 m, provides the best statistical agreement with εU. (2) The model εN predicting a σ2N law (N is Brunt–Vaïsälä frequency) for stably stratified conditions tends to overestimate for εU≲5×10-4 m 2 s -3 and to underestimate for εU≳5×10-4 m 2 s -3. We also tested a model εS predicting a σ2S law (S is the vertical shear of horizontal wind) supposed to be valid for low Richardson numbers (Ri=N2/S2). From the case study of a turbulent layer produced by a Kelvin–Helmholtz (K–H) instability, we found that εS and εLout are both very consistent with εU , while εN underestimates εU in the core of the turbulent layer where N is minimum. We also applied the Thorpe method from data collected from a nearly simultaneous radiosonde and tested an alternative interpretation of the Thorpe length in terms of the Corrsin length scale defined for weakly stratified turbulence. A statistical analysis showed that εS also provides better statistical agreement with εU and is much less biased than εN. Combining estimates of N and shear from DataHawk and radar data, respectively, a rough estimate of the Richardson number at a vertical resolution of 100 m (Ri100) was obtained. We performed a statistical analysis on the Ri dependence of the models. The main outcome is that εS compares well with εU for low Ri100 (Ri100≲1), while εN fails. εLout varies as εS with Ri100 , so that εLout remains the best (and simplest) model in the absence of information on Ri. Also, σ appears to vary as Ri100-1/2 when Ri100≳0.4 and shows a degree of dependence on S100 (vertical shear at a vertical resolution of 100 m) otherwise. [ABSTRACT FROM AUTHOR]
A rare large-scale hail process in Yantai and Weihai City in the autumn of 2021 was analyzed based on the surface meteorology, high-altitude observation and S-band radar detection data. The results showed that this process was influenced by shallow trough, and strong upper-level jet stream in the middle and upper levels guided the intrusion of mid-level cold air into the warm and humid environment at the lower level. The shear lines and trunk lines in the lower level also provided certain triggering conditions for uplift, brewing conditions suitable for the occurrence and development of strong convection. The vertical wind shear was unusually large, and vertical wind shear was up to 10 ml s at 0 - 1 km, 20 ml s at 0 - 3 km, and 32 ml s at 0 - 6 km, which was conducive to the growth of the storm. In this process, isolated multi-cell thunderstorm was generated at the beginning, and then it developed and strengthened continuously, moving to the southeast. In the later stage, a convection cell was born at the boundary between Yantai and Weihai, and developed into supercell storm. The storm lasted for a long time, and continued to develop and strengthen as it moved eastward, affecting most areas of Weihai from northwest to southeast. In the development stage of the storm, the radar reflectivity factor had obvious characteristics of hook echo and three-body scatter spike, and there was echo overhang and other hail echo features in the middle layer. Weak mesocyclone appeared at a low elevation, and there was the potential for the occurrence of tornado. [ABSTRACT FROM AUTHOR]
The purpose of this study is to diagnose mesoscale factors responsible for the formation and development of an extreme rainstorm that occurred on 20 July 2021 in Zhengzhou, China. The rainstorm produced 201.9 mm of rainfall in 1 h, breaking the record of mainland China for 1-h rainfall accumulation in the past 73 years. Using 2-km continuously cycled analyses with 6-min updates that were produced by assimilating observations from radar and dense surface networks with a four-dimensional variational (4DVar) data assimilation system, we illustrate that the modification of environmental easterlies by three mesoscale disturbances played a critical role in the development of the rainstorm. Among the three systems, a mesobeta-scale low pressure system (mesolow) that developed from an inverted trough southwest of Zhengzhou was key to the formation and intensification of the rainstorm. We show that the rainstorm formed via sequential merging of three convective cells, which initiated along the convergence bands in the mesolow. Further, we present evidence to suggest that the mesolow and two terrain-influenced flows near the Taihang Mountains north of Zhengzhou, including a barrier jet and a downslope flow, contributed to the local intensification of the rainstorm and the intense 1-h rainfall. The three mesoscale features coexisted near Zhengzhou in the several hours before the extreme 1-h rainfall and enhanced local wind convergence and moisture transport synergistically. Our analysis also indicated that the strong midlevel south/southwesterly winds from the mesolow along with the gravity-current-modified low-level northeasterly barrier jet enhanced the vertical wind shear, which provided favorable local environment supporting the severe rainstorm. [ABSTRACT FROM AUTHOR]
Several of the best track datasets reveal the earlier end of the tropical cyclone (TC) season over the main WNP (the east of the Philippine Islands) from 1980 to 2019, especially by the date of the upper decile of the storm days (constructed by adding one to each date if any TC exists). The enhanced vertical wind shear (VWS) during the late season (October and November) is consistent with this earlier end of the TC season. In October, the enhanced VWS is the result of the increased zonal and meridional shears, which are affected by the earlier circulation transition of the East Asian summer monsoon recessed and winter monsoon onset during late October. During November, the zonal wind shear dominates the enhanced VWS, which vertically is the opposite trend of air temperature below and above the tropopause and the meridionally opposite trend of air temperature in the north and south of 30°N. The opposite air temperature trends induce the thermal wind balance, which causes enhanced westerlies in the mid-latitudes and strong zonal shear. Three pathways increase VWS in the late season over the WNP, leading to the earlier end of the TC season. The strong impact of VWS limits TC formation, although the ambient warming underlying the surface (supports a broader and stronger potential intensity). The effect of the thermodynamical parameter, depending on relative humidity at the low and middle troposphere, is uncertain because of the inconsistent and weak trends in October between ERA5 and MERRA-2, but they both become unfavorable to TC genesis during November. The practically important meaning of the VWS for the TC season provides a possibility for future projections of the TC season. [ABSTRACT FROM AUTHOR]
Tropical cyclones – TCs – affecting the South Pacific region are studied using coupled atmosphere-ocean earth system models and (offline) storm tracking software which tracks the position of simulated pressure lows through time. The models used are the United Kingdom Earth System Model, version 1 – UKESM1 – and the related New Zealand Earth System Model, the NZESM. The model pair considered here differ only in their treatment of the ocean and the NZESM has a nominal resolution of 0.2° in the region surrounding New Zealand and 1° elsewhere; UKESM1 has a a uniform 1° resolution everywhere. After validating the storm tracking algorithm against the track of cyclone Giselle from 1968 and cyclone Gabrielle from 2023 we use the Saffir-Simpson scale to split the tracked systems into categories based on their severity. For systems formed in the vicinity of New Zealand (and globally) the overall number is overestimated but stronger (category 2 and 3) storms are underestimated. We also see a general decrease in the total number of storms as radiative forcing, F , increases although there is some evidence of a small increase at extreme levels of warming. In the metrics studied here we find no difference between the ensembles of UKESM1 and NZESM simulations and going forward use the UKESM1, which has larger available ensembles. The power dissipation index, PDI, gives a first order measure of TC strength and we find that the average PDI per storm increases with F by up to 26 % under a 'fossil-fuelled development' scenario. Although the physical mechanisms behind the increase in average PDI with F are relatively simple to understand, those governing the frequency of occurrence are not. In the results shown here, vertical wind shear increases with F which tends to reduce TC numbers but the effect of the tropospheric relative humidity is much less clear. The increase in the area of the tropics bounded by the 26.5° isotherm should, on its own, increase the number of TCs, in opposition to the general behaviour observed, except perhaps at extreme levels of future warming. [ABSTRACT FROM AUTHOR]
The article presents a study conducted to analyze whether the Hadley cell (HC) edge's shift towards poles could be reversed through removal of carbon dioxide (CO2). It states that reducing CO2 concentrations does not return the poleward-shifted HC edge to its original state. It mentions that hemispherically asymmetric changes are linked to variations in vertical wind shear due to the prolonged ocean response to CO2 removal.
Observations of vertical wind in Chongqing, a typical mountainous city in China, are important, but sparse and have low resolution. To obtain more wind profile data, this study matched the Aeolus track with ground-based wind observation sites in Chongqing in 2021. Based on the obtained results, verification and quality control studies were conducted on the wind observations of a wind profile radar (WPR) with radiosonde (RS) data, and a comparison of the Aeolus Mie-cloudy and Rayleigh-clear wind products with WPR data was then performed. The conclusions can be summarized as follows: (1) A clear correlation between the wind observations of WPR and RS was found, with a correlation coefficient (R) of 69.92%. Their root-mean-square deviation increased with height but decreased by 3 - 4 km. (2) After quality control of Gaussian filtering (GF) and empirical orthogonal function construction (EOFc, G = 87.23%) of the WPR data, the R between the WPR and RS reached 76.00% and 95.44%, respectively. The vertical distribution showed that GF could better retain the characteristics of WPR wind observations, but with limited improvement in decreasing deviations, whereas EOFc performed better in decreasing deviations, but considerably modified the original characteristics of the wind field, especially regarding intensive vertical wind shear in strong convective weather processes. (3) In terms of the differences between the Aeolus and WPR data, 56.0% and 67.8% deviations were observed between ± 5 m/s for Rayleigh-clear and Mie-cloudy winds vs. WPR winds, respectively. Vertically, the mean differences of both Rayleigh-clean and Mie-cloudy winds versus WPR winds appeared below 1.5 km, which is attributed to the prevailing quiet and small winds within the boundary layer in Chongqing, such that the movement of molecules and aerosols is mostly affected by irregular turbulence. Additionally, large mean differences of 4-8 km for Mie-cloudy versus WPR winds may be related to the high content of cloud liquid water in the middle troposphere, influenced by the topography of the Tibetan Plateau. (4) The differences in both Rayleigh-clear and Mie-cloudy versus WPR winds had changed. Deviations of 58.9% and 59.6% were concentrated between ±5m/s for Rayleigh-clear versus WPR winds with GF and EOFc quality control, respectively. In contrast, 69.1% and 70.2% of deviations appeared between ± 5 m/s for Rayleigh-clear versus WPR and EOFc WPR winds, respectively. These results shed light on the comprehensive applications of multi-source wind profile data in mountainous cities or areas with sparse ground-based wind observations. [ABSTRACT FROM AUTHOR]
How future multiple tropical cyclone events (MTCEs) could change is crucial for effective risk management and ensuring human safety, however, it remains unclear. This study projects changes in MTCEs by 2050 in the major basins of the Northern Hemisphere using high‐resolution climate models. Results show a significant increase in the frequency and duration of MTCEs over the North Atlantic (NA), a notable decrease over the western North Pacific (WNP), and little change over the eastern North Pacific (ENP). The increase in MTCEs over the NA is concentrated in August–September, while the decrease over the WNP occurs in most months. In contrast, the ENP exhibits large yet insignificant seasonal variation, suggesting considerable uncertainty in this basin. Further analysis shows that mid‐level vertical motion dominates the MTCE changes over the WNP, while vertical wind shear contributes the most to the NA, which may be linked to future changes in tropical convection. Plain Language Summary: Multiple tropical cyclone (TC) events (MTCEs), that is two or more TCs simultaneously occurring in the same basin, pose great risks to human society. This study projects future changes in the MTCEs by 2050, showing a significant increase over the North Atlantic (NA) while a robust reduction over the western North Pacific (WNP). The future MTCEs over the eastern North Pacific (ENP) show little change relative to the present climate. The increase of MTCEs over the NA is concentrated in August–September, while the decrease over the WNP occurs nearly from April to November. In contrast, there is large yet insignificant seasonal variation over the ENP, which could lead to little change in annual MTCEs. Furthermore, these changes are primarily attributed to the changes in local large‐scale dynamic conditions associated with tropical convection in future decades. Key Points: The projection of future multiple tropical cyclone events (MTCEs) shows a decline in the western North Pacific (WNP) but a significant increase in the North Atlantic (NA)The mid‐level vertical motion dominates the MTCE changes over the WNP, while vertical wind shear is important for the NAFuture monthly MTCEs show large yet insignificant seasonal variation over the eastern North Pacific, causing little trend in annual MTCEs [ABSTRACT FROM AUTHOR]
Understanding and quantifying the large‐scale environmental control of spatio‐temporal tropical cyclone (TC) variability beyond a few weeks remains a challenge with significant implications for societal impacts. This study focuses on the relationship between the North Atlantic Oscillation (NAO) and year‐to‐year changes in TC activity and rainfall using observational data and reanalysis products. We use Poisson regression models to show that low‐frequency NAO (LF‐NAO) variability is associated with a distinct pattern of TC activity, which extends across the western North Atlantic, the Caribbean Basin, and the Gulf of Mexico. The negative LF‐NAO phase is characterized by enhanced TC activity: an interquartile range decrease in the NAO corresponds to a 30%–40% increase in TC track density. While the NAO is known to affect the weather regimes of the mid‐latitudes, we show that its low‐frequency component has a strong correlation to the large‐scale environment across the Main Development Region of TCs. The negative LF‐NAO phase is associated with two favorable environmental conditions for TCs: significantly higher sea‐surface temperature and weaker deep‐tropospheric wind shear. The LF‐NAO relationship to TC activity is strongest during El Niño Southern Oscillation positive or neutral conditions and during the negative Atlantic Multidecadal Oscillation phase, and it can also be detected in the basin‐scale variations of TC rainfall. By developing annual TC rainfall composites from satellite data and reanalysis products, we show that TC rainfall is strongly enhanced in the Caribbean and in the Gulf of Mexico during the negative LF‐NAO phase. Plain Language Summary: Better understanding high‐impact weather events like tropical cyclones (TCs), in particular their variability in a changing climate and rising sealevel, is key to improving our preparedness and minimizing their impact. This study advances our knowledge of what controls TC numbers, locations, and heavy rainfall. In this study, we investigate how changes in atmospheric and oceanic conditions associated with the North Atlantic Oscillation (NAO) modulate North Atlantic TC tracks and rainfall. We show that year‐to‐year changes in the number of TCs across the Western Tropical Atlantic, the Caribbean basin and the Gulf of Mexico reflect slow‐evolving variations in the NAO. By comparing opposite phases of the NAO we show that the number of TCs increases by 30%–40% when the NAO is in the negative phase. Observations and reanalysis products show that the negative phase of the NAO is associated with two significant changes in the environmental conditions that affect TC activity across the tropical North Atlantic: (a) above‐average sea‐surface temperature and (b) below‐average vertical wind shear, which are favorable for TC formation and intensification. The influence of the NAO is strongest during otherwise less favorable large‐scale conditions (El Niño Southern Oscillation positive/neutral and negative Atlantic Multidecadal Oscillation). Both satellite‐ and reanalysis‐derived rainfall data show that an NAO‐induced increase in TC activity and a shift of TC tracks from the western Atlantic to the Caribbean and the Gulf of Mexico have direct impacts on TC rainfall. Key Points: Year‐to‐year changes in North Atlantic tropical cyclone (TC) activity are negatively correlated to low‐frequency North Atlantic Oscillation (NAO) variabilityLow‐frequency NAO variability is associated with sea surface temperature and wind shear anomalies across the tropical North AtlanticAs a result of the NAO modulation TC activity, enhanced annual TC rainfall is observed across the Caribbean Basin and the Gulf of Mexico [ABSTRACT FROM AUTHOR]
Tropical cyclone (TC) intensification is a process depending on many factors related to the thermodynamical state and environmental influences. It remains a challenge to accurately model TC intensity due to the role of unsteady features like deep convective bursts, boundary layer dynamics and eddy processes. The impermeability theorem for potential vorticity substance, PVS , on isentropic surfaces provides a way to analyze the absolute vorticity structure and tendency in TC s. We will examine this theorem in a numerical simulation of hurricane Irma (2017) near lifetime-peak intensity. Hurricane Irma was a very intense hurricane that persisted as a category five hurricane on the Saffir-Simpson intensity scale for three consecutive days, the longest for any Atlantic hurricane since satellite observations. During this period the intensity of Irma was remarkably constant. According to the impermeability theorem, the radially outward vorticity flux due to divergence above the atmospheric boundary layer must be compensated by an equally strong radially inward vorticity flux due to the effect of diabatic heating in the presence of vertical wind shear. The model results agree with this theorem and we find a strong anticorrelation between the advective and diabatic components of the radial vorticity flux. The impact of parametrized turbulence on the vorticity balance is found to be weak and does not explain the residual flux that would otherwise close the vorticity balance. [ABSTRACT FROM AUTHOR]
The usually short lifetime of convective storms and their rapid development during unstable weather conditions makes forecasting these storms challenging. It is necessary, therefore, to improve the procedures for estimating the storms' expected life cycles, including the storms' lifetime, size, and intensity development. We present an analysis of the life cycles of convective cells in Germany, focusing on the relevance of the prevailing atmospheric conditions. Using data from the radar‐based cell detection and tracking algorithm KONRAD of the German Weather Service, the life cycles of isolated convective storms are analysed for the summer half‐years from 2011 to 2016. In addition, numerous convection‐relevant atmospheric ambient variables (e.g., deep‐layer shear, convective available potential energy, lifted index), which were calculated using high‐resolution COSMO‐EU assimilation analyses (0.0625°), are combined with the life cycles. The statistical analyses of the life cycles reveal that rapid initial area growth supports wider horizontal expansion of a cell in the subsequent development and, indirectly, a longer lifetime. Specifically, the information about the initial horizontal cell area is the most important predictor for the lifetime and expected maximum cell area during the life cycle. However, its predictive skill turns out to be moderate at most, but still considerably higher than the skill of any ambient variable is. Of the latter, measures of midtropospheric mean wind and vertical wind shear are most suitable for distinguishing between convective cells with short lifetime and those with long lifetime. Higher thermal instability is associated with faster initial growth, thus favouring larger and longer living cells. A detailed objective correlation analysis between ambient variables, coupled with analyses discriminating groups of different lifetime and maximum cell area, makes it possible to gain new insights into their statistical connections. The results of this study provide guidance for predictor selection and advancements of nowcasting applications. [ABSTRACT FROM AUTHOR]
Cloud-resolving modeling has been implemented to investigate a lifecycle of two quasi-tropical cyclones (QTCs) over the Black and Mediterranean seas in September 2005 and 2018. The formation of the Mediterranean QTC under the influence of the PV streamer and the problems of predicting this phenomenon are considered. The influence of the mesocyclone wind system on cumulus convection is studied, while mesocyclone winds are assumed to be axisymmetric for simplicity. It has been found for the largest and most intense QTCs that vertical wind shear may lead to the formation of supercellular storms, but to a rather limited extent and as a result of deviations of mesocyclone currents from the axial symmetry. In small intense QTCs, horizontal wind shear can suppress convection over a certain range of distances from the center of a cyclone, leading to a release of latent heat closer to it. [ABSTRACT FROM AUTHOR]
Four-dimensional COAMPS dynamic initialization (FCDI) analyses with high temporal and spatial resolution GOES-16 atmospheric motion vectors (AMVs) are utilized to analyze the development and rapid intensification of a mesovortex about 150 km to the south of the center of the subtropical cyclone, Cyclone Henri (2021). During the period of the unusual Henri westward track along 30°N, the FCDI z = 300-m wind vector analyses demonstrate highly asymmetric wind fields and a horseshoe-shaped isotach maximum that is about 75 km from the center, which are characteristics more consistent with the definition of a subtropical cyclone than of a tropical cyclone. Furthermore, the Henri westward track and the vertical wind shear have characteristics resembling a Rossby wave breaking conceptual model. The GOES-16 mesodomain AMVs allow the visualization of a series of outflow bursts in space and time in association with the southern mesovortex development and intensification. Then the FCDI analyses forced by those thousands of AMVs each 15 min depict the z = 13 910-m wind field responses and the subsequent z = 300-m wind field adjustments in the southern mesovortex. A second northern outflow burst displaced to the southeast of the main Henri vortex also led to a strong low-level mesovortex. It was when the two outflow bursts joined to create an eastward radial outflow all along the line between them that the southern mesovortex reached maximum intensity and maximum size. In contrast to the numerical model predictions of intensification, outflow from the mesovortex directed over the main Henri vortex led to a decrease in intensity. [ABSTRACT FROM AUTHOR]
Using ground-based remote sensing equipment (wind profile radar and microwave radiometer) data and ground automatic station data, a ground-based remote sensing sounding profile system (FAS) is constructed, which aims to make use of its advantages of high resolution, high accuracy, and low cost to make up for the lack of space–time density in existing conventional sounding layer information. The retrieval results of remote sensing sounding profiles in Beijing from May 2021 to September 2022 were tested and evaluated. The results show that the correlation coefficient between FAS and conventional sounding specific humidity is 0.89, the root–mean–square deviation is 1.53 g/kg, and the evolution trends of different data sources of convective available potential energy (CAPE) and vertical wind shear are synchronous. A case study was conducted to evaluate the effectiveness of 40 severe convective processes in the Beijing Plain area. The results show that, due to the minute-level time resolution of FAS, the retrieved convective parameters could track the evolution trend of the atmospheric state with high timeliness, dynamically describe the configuration of thermodynamic parameters, and indicate the time-varying local convective potential and instability level. Therefore, it has certain short-term forecasting significance for the occurrence time, intensity, and convection type. [ABSTRACT FROM AUTHOR]
Comprehensive understanding of the spatial characteristics of tropical cyclone (TC) precipitation is essential for effective socioeconomic planning and scientific research. The present study examines the spatial asymmetry in TC precipitation over the western North Pacific (WNP) concerning various factors, including latitudes, sea surface temperature (SST), TC intensity, and translation speed, based on satellite observations. The results reveal a significant poleward migration of TC precipitation asymmetry particularly above 15° N. Furthermore, the asymmetry exhibits considerable sensitivity to changes in TC center latitudes and SSTs, characterized by anticlockwise and northeastward migration, respectively. The poleward migration of TC precipitation spatial asymmetry is primarily due to the poleward decreasing SST, increasing vertical wind shear and increasing TC translation speed. These findings contribute to a comprehensive understanding of TC behavior over the WNP and provide valuable insights for disaster preparedness and mitigation efforts. [ABSTRACT FROM AUTHOR]