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Convective initiation (CI) nowcasting in subtropical regions often faces challenges, such as complex physical processes and imbalanced samples of CI events, resulting in a high false alarm ratio (FAR). In this paper, we propose a Storm Warning System with Physics‐Augmentation (SWASP) based on the random forest algorithm and cloud physical conditions, using Himawari‐8 Advanced Himawari Imager data from April to September 2019 in South China. The cloud physical conditions (e.g., cloud‐top cooling rates) were investigated to establish regional thresholds for convection occurrence. Ancillary information, including elevation, satellite zenith angle, and latitude, was also incorporated into the SWASP model. Compared to conventional methods, the SWASP model exhibits an improved probability of detection by 0.11 and 0.08 and a decreased FAR by 0.38 and 0.44 for daytime and nighttime forecasts. Moreover, the SWASP model enables the detection of local convective storm systems about 30 min to 1 hr ahead of radar detection in typical convective storm cases. This study contributes to further advancements of the SWASP model by incorporating physical conditions and emphasizes the potential application of geostationary satellites in convective early warnings. Key Points: A Storm Warning System with Physics‐Augmentation (SWASP) has been proposed, which utilizes the random forest algorithmSWASP enhances the probability of detection and reduces false alarm ratios in storm nowcastingChanges in cloud‐top height are identified as the critical factor for accurate storm nowcasting [ABSTRACT FROM AUTHOR]

This study aims to understand and enhance the indicative significance of convective initiation (CI) products from Fengyun-4A (FY-4A) satellite for summer precipitation over the Qinghai-Tibet Plateau. Using the convective initiation (CI) products of FY-4A and precipitation data from the Global Precipitation Measurement Program (GPM), and based on the correspondence between the CI samples identified by the FY-4A CI product and the actual observed precipitation one hour after identifying the CI in the Qinghai-Tibet Plateau region from June to August 2022, three categories of the CI samples, including no precipitation CI, weak precipitation CI, and strong precipitation CI, were divided. Then a CI class recognition model was established and the model performance testing was conducted by combining atmospheric convective parameters and geographic location information, two machine learning methods, decision tree and random forest. The results show that there are significant regional differences in the precipitation situation within one hour after the occurrence of CI in the Qinghai Tibet Plateau region, with a higher proportion of no precipitation in the northwest region and a higher proportion of precipitation in the southeast region. By utilizing atmospheric convective parameters such as lift index, total cloud water, wind shear, middle and low level humidity, cloud bottom height, zero degree layer height and so on, it is possible to better distinguish whether there is precipitation and the strength of precipitation after the appearance of CI in the Qinghai Tibet Plateau. The random forests identify model have better performance for CI classification than decision tree, and the use of random forests identify model can more effectively classify summer CI on the Qinghai Tibet Plateau according to precipitation intensity.

Abstract Convective initiation (CI) nowcasting in subtropical regions often faces challenges, such as complex physical processes and imbalanced samples of CI events, resulting in a high false alarm ratio (FAR). In this paper, we propose a Storm Warning System with Physics‐Augmentation (SWASP) based on the random forest algorithm and cloud physical conditions, using Himawari‐8 Advanced Himawari Imager data from April to September 2019 in South China. The cloud physical conditions (e.g., cloud‐top cooling rates) were investigated to establish regional thresholds for convection occurrence. Ancillary information, including elevation, satellite zenith angle, and latitude, was also incorporated into the SWASP model. Compared to conventional methods, the SWASP model exhibits an improved probability of detection by 0.11 and 0.08 and a decreased FAR by 0.38 and 0.44 for daytime and nighttime forecasts. Moreover, the SWASP model enables the detection of local convective storm systems about 30 min to 1 hr ahead of radar detection in typical convective storm cases. This study contributes to further advancements of the SWASP model by incorporating physical conditions and emphasizes the potential application of geostationary satellites in convective early warnings.

In order to enhance the understanding of the environmental field characteristics of the convective regeneration triggered by thunderstorm gust fronts.Based on Beijing sounding observation， automatic stations， S-band Doppler radar and new detection data to analyse environmental field characteristics of two typical cases in which convection initiation triggered by thunderstorm gust fronts.The results show that： （1） One type is convection triggered by collision of multiple gust fronts or the encounter of gust fronts with other boundary layer convergence lines in the region with high Convective Available Potential Energy （CAPE） and sufficient water vapor （collision triggering）.In this type， the vertical velocity of two collision gust fronts is much larger than that of a single gust front.Strong upward movement is easy to provide good lifting conditions for the triggering of convection， and convection is easy to trigger.（2） The other type is the gust front triggers convection in the unstable region （non-collision trigger）.In this type， the low-level wind direction is perpendicular to the gust front， which is conducive to the strong convergence between the ambient wind near the ground and the gust front.Meanwhile， the updraft of the convection triggered by the gust front tends to be vertical， which is beneficial to convective initiation.（3） By comparing the environmental field of gust fronts trigger and non-trigger Convective Initiation （CI） cases， the results show that there are deep wet convection potential with high CAPE and low Convective Inhibition （CIN） of the environment field in these cases which CI are triggered by gust fronts.The wind profile and microwave radiometer observations indicate that there are cold dry advection at the upper level and warm wet advection at the lower level in most cases CI triggered by the gust fronts.In most cases which gust fronts trigger CI occur at the wind speed or wind direction convergence areas with high CAPE.（4） In cases which non-triggered CI， vertical distribution of environmental field is not conducive to the regeneration and development of convection， and gust fronts pass the area which are divergence or there are low CAPE and high CIN.These conditions are adverse to convection triggering.

The convection initiation (CI) mechanisms of severe storms have received increasing attention because severe storms have been occurring more frequently around the globe in recent years. In this work, the CI mechanisms of severe convective weather associated with a gust front (GF) which occurred on 9 July 2016, near the Korla at the northern edge of the Tarim Basin, Xinjiang, is investigated using observational data including Doppler weather radar data and automatic weather stations data, and high-resolution numerical simulation data. The results showed that, during the eastward movement of the GF, a number of convective cells were successively triggered in the vicinity of the GF, which developed rapidly and continuously merged with the convective system from behind, resulting in the further development and maintenance of this convective system. According to the diagnostic analysis of vertical acceleration which can be decomposed into dynamic acceleration ( a d ) and buoyant acceleration ( a b ), it was found that both a d (up to ~4 × 10−3 m s−2) and a b (up to ~7 × 10−3 m s−2) made positive contributions to the CI. Further analyses based on the decompositions of the a d and a b revealed that the extension term was the main contributor for the a d , while the warming of the dry air due to the release of latent heat from the precipitation condensate made the major contribution to the a b . The extension term indicates the elevated convergence jointly induced by the airflow of mid-level horizontal convective rolls (MHCRs) and updraft flow near the leading edge of the GF. The jointly induced elevated convergent updraft can be considered to be an important contributor for the CI mechanisms. [ABSTRACT FROM AUTHOR]

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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]

The convection initiation (CI) mechanisms of severe storms have received increasing attention because severe storms have been occurring more frequently around the globe in recent years. In this work, the CI mechanisms of severe convective weather associated with a gust front (GF) which occurred on 9 July 2016, near the Korla at the northern edge of the Tarim Basin, Xinjiang, is investigated using observational data including Doppler weather radar data and automatic weather stations data, and high-resolution numerical simulation data. The results showed that, during the eastward movement of the GF, a number of convective cells were successively triggered in the vicinity of the GF, which developed rapidly and continuously merged with the convective system from behind, resulting in the further development and maintenance of this convective system. According to the diagnostic analysis of vertical acceleration which can be decomposed into dynamic acceleration (ad) and buoyant acceleration (ab), it was found that both ad (up to ~4 × 10−3 m s−2) and ab (up to ~7 × 10−3 m s−2) made positive contributions to the CI. Further analyses based on the decompositions of the ad and ab revealed that the extension term was the main contributor for the ad, while the warming of the dry air due to the release of latent heat from the precipitation condensate made the major contribution to the ab. The extension term indicates the elevated convergence jointly induced by the airflow of mid-level horizontal convective rolls (MHCRs) and updraft flow near the leading edge of the GF. The jointly induced elevated convergent updraft can be considered to be an important contributor for the CI mechanisms.

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.

The dependence of the feedback signal on atmospheric conditions is still poorly understood and may lead to an underestimation of feedback strength due to opposite responses of precipitation to soil moisture. Based on data from observation stations on the Tibetan Plateau and from satellites, this study evaluated the response of convective clouds to the change of evaporation fraction (EF) and analyzed how surface conditions affect the initiation and development of convection in different coupling regimes. After considering coupling regimes classified by the tropospheric state, the afternoon convective cloud showed a strong response to EF, which is negative feedback in the dry coupling regime and positive feedback in the wet coupling regime. Organized deep convection defined by both the cloud properties and the afternoon precipitation displayed this phenomenon. This shows that the lack of a strong response of convection to EF is due to the dependence of this response on the coupling regime, rather than on the method of defining convection. We also found that the difference in surface heat flux between the two regimes is more significant in the afternoon, while the difference of meteorological elements is most significant before noon. These results provide support for the use of ground‐based meteorological data to determine coupling regimes. We also used a regression tree to decompose the effects of coupling regimes and EF into basic near‐surface meteorological elements, the results of which provide support for some of our conclusions. Key Points: Considering coupling regimes classified by the tropospheric condition, the convection showed a strong response to evaporation fraction (EF)Two convection definitions, depending on cloud properties and afternoon rainfall, showed regime‐dependent responses to EFIt is possible to determine the regime without profiles, surface meteorological elements are significantly different in the morning [ABSTRACT FROM AUTHOR]

The Florida peninsula in the USA has a frequent occurrence of sea breeze (SB) thunderstorms. In this study, the numerical simulation of a Florida SB and its associated convective initiation (CI) is simulated using the mesoscale community Weather Research and Forecasting (WRF) model in one-way nested domains at different horizontal resolutions. Results are compared with observations to examine the accuracy of model-simulated SB convection and factors that influence SB CI within the simulation. It is found that the WRF model can realistically reproduce the observed SB CI. Differences are found in the timing, location, and intensity of the convective cells at different domains with various spatial resolutions. With increasing spatial resolution, the simulation improvements are manifested mainly in the timing of CI and the orientation of the convection after the sea breeze front (SBF) merger into the squall line over the peninsula. Diagnoses indicate that accurate representation of geophysical variables (e.g., coastline and bay shape, small lakes measuring 10–30 km2), better resolved by the high resolution, play a significant role in improving the simulations. The geophysical variables, together with the high resolution, impact the location and timing of SB CI due to changes in low-level atmospheric convergence and surface sensible heating. More importantly, they enable Florida lakes (30 km2 and larger) to produce noticeable lake breezes (LBs) that collide with the SBFs to produce CI. Furthermore, they also help the model reproduce a stronger convective squall line caused by merging SBs, leading to more accurate locations of postfrontal convective systems. [ABSTRACT FROM AUTHOR]

The present study examines mechanisms by which deep moist convection may be initiated through deepening of a well‐mixed convective boundary layer under large‐scale low‐level convergence. The process is examined using a standard formulation for the well‐mixed boundary layer, in which the depth increases exponentially with time under a constant convergence with height. Consideration is also given to a case with a more realistic mean vertical velocity profile which has a maximum in the middle troposphere and attenuates to zero at the tropopause. In the latter case, the unstable well‐mixed layer under convergence grows to a troposphere‐deep mixed layer. Importantly, under these scenarios, the so‐called convective inhibition (CIN) does not inhibit deepening of a well‐mixed convective boundary layer, nor does deep moist convection need to overcome CIN to initiate. [ABSTRACT FROM AUTHOR]

On June 15, 2021, the Hotan region of southern Xinjiang was hit by an extreme rainfall event. A numerical simulation of the storm process, with a resolution of 3 km, was conducted to analyze the initiation mechanism of the rainfall. The results show that lifting conditions play a crucial role in triggering convection, compared to moisture and stratification conditions. The easterly jet changes the direction in the western part of the Tarim Basin and speeds up, creating a zone of high wind speeds in front of Kunlun Mountain's northern slope, which provides favorable conditions for convection initiation. A remarkable feature during this process is the effect of a preexisting distant cold pool on the southern slope of Tianshan Mountain. The cold pool impedes the easterly development of the easterly airflow in the basin, causing it to turn northward, and accelerates the mountain front airflow in front of Kunlun Mountain. The inner dynamic mechanism is a strengthening of the low-level horizontal pressure gradient between the cold pool and the low-pressure belt along Kunlun Mountain. The numerical sensitivity experiments also highlight the key role the distant cold pool plays in the development of the high-speed wind belt ahead of the mountain and causes convection to start 4 h earlier. These results will serve as a valuable reference for future studies on the formation process of convection and improve the understanding of precipitation forecast in arid and semi-arid regions. • The extreme rainfall in southern Xinjiang is affected by a preexisting distant cold pool. • The distant cold pool functions like a topographical barrier and blocks airflow. • The existence of the cold pool makes the deep convection occur earlier. [ABSTRACT FROM AUTHOR]

The presence of deep moist convection in a large part of Mexico is a condition typical of summer; the effects of land surface processes and mesoscale circulations are usually not included in the diagnostic and forecasting of deep moist convection, mainly because of the scale at which they occur. This study examines the effect of the soil moisture fluxes on convective initiation in the central region of Mexico during the summer periods of 2010–2014. Other aspects considered are the function of the average flux of the boundary layer in the transport of humidity towards the initiation zone. Remote sensing satellite data used are the cloud-top temperature, soil moisture, land surface temperature and precipitation. The results show over the initiation zone an increase in the soil moisture and a decrease in the land surface temperature in the direction of the mean flux at low levels. The soil moisture anomaly (SMA) presents a temporal variability being May, the composite more similar to previous studies, initiation zone drier and warmer than surroundings. The background low-level winds have an important role in the displacement of convection core and creating different features of SMA and land surface temperature anomaly.

Parallel back-building convective lines are often observed extending to the southwest of some mesoscale convective systems (MCSs) embedded in the mei-yu front in China. The convective lines with echo training behavior can quickly develop into a stronger convective group of echoes, resulting in locally heavy rainfall within the mei-yu front rainband. The initiation mechanism of the back-building convective lines is still unclear and is studied based on high-resolution numerical simulation of a case that occurred during 27–28 June 2013. In the present case, the new convection along the convective lines was found to be forced by nonuniform interaction between the cold outflow associated with the mei-yu front MCSs and the warm southerly airflow on the south side of the mei-yu front, which both are modified by local terrain. The mei-yu front MCSs evolved from the western to the eastern side of a basin surrounded by several mesoscale mountains and induced cold outflow centered over the eastern part of the basin. The strong southwest airflow ahead of the mei-yu front passed the Nanling Mountains and impacted the cold outflow within the basin. The nonuniform interaction led to the first stage of parallel convective line formation, in which the low mountains along the boundary of the two airflows enhanced the heterogeneity of their interaction. Subsequently, the convective group quickly developed from the first stage convective lines resulted in apparent precipitation cooling that enhanced the cold outflow and made the cold outflow a sharp southward windshift. The enhanced cold outflow pushed the warm southerly airflow southward and impacted the mountains on the southeast side of the basin, where the roughly parallel mountain valleys or gaps play a controlling role in a second stage formation of parallel convective lines. [ABSTRACT FROM AUTHOR]

Among the densely-populated coastal areas of China, the southeastern coast has received less attention in convective development despite having been suffering from significantly increasing thunderstorm activities. The convective complexity under such a region with extremely complex underlying and convective conditions deserves in-depth observational surveys. This present study examined a high-impact convection outbreak event with over 40 hail reports in the southeastern coast of China on 6 May 2020 by focusing on contrasting the convective development (from convective initiation to supercell occurrences) among three adjacent convection-active zones (north (N), middle (M), and south (S)). The areas from N to S featured overall flatter terrain, higher levels of free convection, lower relative humidity, larger convective inhibition, more convective available potential energy, and greater vertical wind shears. With these mesoscale environmental variations, distinct inter-zone differences in the convective development were observed with the region’s surveillance radar network and the Himawari-8 geostationary satellite. Convection initiated in succession from N to S and began with more warm-rain processes in N and M and more ice-phase processes in S. The subsequent convection underwent more vigorous vertical growth from N to S. The extremely deep convection in S was characterized by the considerably strong precipitation above the freezing level, echo tops of up to 18 km, and a great amount of deep (even overshooting) and thick convective clouds with significant cloud-top glaciation. Horizontal anvil expansion in convective clouds was uniquely apparent over S. From N to S, more pronounced mesocyclone and weak-echo region signatures indicated high risks of severe supercell hailstorms. These results demonstrate the strong linkage between the occurrence likelihood of severe convection and associated weather (such as supercells and hailstones) and the early-stage convective development that can be well-captured by high-resolution observations and may facilitate fine-scale convection nowcasting.

Land‐atmosphere interactions and boundary layer processes often control the formation of shallow clouds and subsequently deep convective precipitation over the U.S. Southern Great Plains. In this study, we examine the impacts of large‐scale advection on the cloud populations and land‐atmospheric coupling observed during the Holistic Interactions of Shallow Clouds, Aerosols, and Land‐Ecosystems field campaign in 2016. We performed two large eddy simulations using the Weather Research and Forecasting Model for a day in which the transition from clear‐sky to shallow precipitating clouds and isolated deep convection was observed. The control simulation reproduced the overall distribution of cloud populations by using realistic soil conditions with an interactive land model. In the sensitivity simulation where large‐scale advection is removed, a strong relationship between the land and boundary layer is found. To study the timing, location, and intensity of convective initiation and the relationship of clouds with land surface properties, a cluster analysis of equivalent potential temperature is performed for the simulation without large‐scale advection. That analysis shows that convective clouds first form over regions with higher surface sensible heat flux. Precipitation from those convective clouds likely triggers new updrafts nearby about 2 hr later through the lifting associated with cold pools. The cluster analysis also shows that in addition to the spatial pattern of soil moisture, land use and soil texture in western Oklahoma also influence the location of convective initiation. Plain Language Summary: Accurately predicting the timing, location, and intensities of convection over midlatitude land is extremely important. Yet, complete answers to the related questions such as how the land properties impact the atmosphere and cloud properties remain lacking. Models that resolve boundary layer turbulence and clouds have been used extensively to understand processes controlling land‐atmosphere interactions, but computational expense limits the employment of realistic land properties. In this study, we employ a more realistic modeling approach to study abovementioned question. First, we examined the relative role of large‐scale advection and the local effects of land properties in convection and cloud formation. A comparison of two sensitivity studies shows that large‐scale advection is vital in the characterization of land‐atmosphere interactions not only in terms of the magnitude but also the types of relationships. Then, we analyzed the simulation with only local effects by cluster analysis. We found that convection initiates over the regions with high surface sensible heat flux. Precipitation from the earliest convection likely triggers new convection around about 2 hr later by forming a cold surface area due to evaporating cooling. Moreover, soil moisture, land use type, and soil texture play roles in determining the surface heat fluxes. Key Points: Large‐scale advection is vital in characterizing land‐atmosphere interactions not only in magnitude but also types of relationshipsConvection initiates over regions with high sensible heat flux; precipitation likely triggers new updrafts around through cold poolsSoil moisture, land use type, and soil texture play roles in the timing, location, and type of convective clouds [ABSTRACT FROM AUTHOR]

High-resolution numerical simulation results of a squall line initiated along a convergence zone in northeast China on 26 June 2014 were presented in this study. The simulation was performed by a convection-permitting model with coarse and fine grids of 4 and 1.33 km, respectively, and the simulation results were validated against the observation. Results showed that the simulation adequately reproduced the life cycle of the squall line, which allowed detailed investigation of the mechanism of convective initiation in this case. The synoptic condition was favorable for convective initiation and the convection was triggered in a convergence zone, where a branch of dry and cold air and a branch of moist and warm air collided. The water vapor flux divergence was inhomogeneous and some cores of water vapor convergence existed in the convergence zone. These cores were the spots where water vapor converged intensely and the air there was forced to rise, creating favorable spots where the convection was initially triggered. A series of quasi-equally spaced vortices near the surface, which themselves were the result of horizontal shear instability, were accountable for the inhomogeneity of the surface water vapor flux divergence. These vortices rotated the moist air into their north and dry air into their south, thus creating more favorable spots for convective initiation in their north. After initiation, the updraft turned the horizontal vorticity into vertical vorticity in the mid-level. The vortices near the surface collaborated with the vorticity maxima in the mid-level and enhanced the development of convection by providing water vapor. [ABSTRACT FROM AUTHOR]

The detection of soil moisture‐precipitation feedback is complicated by the dependence of feedback sign on atmospheric state and cloud types. We evaluated responses of convective clouds to changes in surface evaporative fraction (EF) from satellite and ground‐based measurements over the Southern Great Plains in summer months. Strong responses to EF were inferred only after accounting for tropospheric states supporting systematically positive or negative responses. State dependence (inferred from controlled model experiments) explains the weak response found in previous studies. The probability of transitions from shallow to deep cumulus, and subsequent precipitation onset, increases by 58% over drier surfaces in weakly stratified environments having deeper boundary layers. Boundary layer dynamics play a more important role than low‐level relative humidity in these environments. Wetter surfaces yield increased deep convection in moist and thermally stratified environments. The distinct pathways of convective initiation over dry and wet land surfaces can play important roles in drought onset and termination. Plain Language Summary: Connections between cumulus clouds and the land surface are critical in determining the impacts of climate and land surface change on drought and extreme precipitation events. In particular, the transition from shallow (non‐precipitating) to deep (precipitating) cumulus clouds depends on the state of the land surface and that of the overlying atmosphere. In this study, we differentiated between atmospheric states that allow either positive or negative responses of the shallow‐to‐deep transition to increasing evaporation (soil moisture). After distinguishing the atmospheric states, we found that the transition is more frequent over a dry surface (a negative feedback) in a dry but less stratified atmosphere, where clouds can be formed by lifting and cooling air to saturation. In contrast, there are more shallow‐to‐deep transition and precipitation events over a wetter surface (a positive feedback) in a more stratified and humid atmosphere. The strong negative feedback of a dry surface on the transition from shallow to deep convection has not been reported previously for this region and may play a key role in ending droughts, while the positive feedback of a wet surface on precipitation occurrence may act to prolong periods of heavy rainfall and flooding. Key Points: A strong negative feedback of dry soil on convective initiation was found in atmospheric states supporting deep boundary layersConvection and precipitation show positive relationships to surface evapotranspiration only in moist, thermally stratified atmospheresState dependence explains why weak or no relationship was found between soil moisture and precipitation over the SGP in previous studies [ABSTRACT FROM AUTHOR]

The spatiotemporal statistical characteristics of warm-season deep convective systems, particularly deep convective systems initiation (DCSI), over China and its vicinity are investigated using Himawari-8 geostationary satellite measurements collected during April-September from 2016 to 2020. Based on a satellite brightness temperature multiple-threshold convection identification and tracking method, a total of 47593 deep convective systems with lifetimes of at least 3 h were identified in the region. There are three outstanding local maxima in the region, located in the southwestern, central and eastern Tibetan Plateau and Yunnan-Guizhou Plateau, followed by a region of high convective activities in South China. Most convective systems are developed over the Tibetan Plateau, predominantly eastward-moving, while those developed in Yunnan-Guizhou Plateau and South China mostly move westward and southwestward. The DSCI occurrences become extremely active after the onset of the summer monsoon and tend to reach a maximum in July and August, with a diurnal peak at 11–13 LST in response to the enhanced solar heating and monsoon flows. Several DCSI hotspots are identified in the regions of inland mountains, tropical islands and coastal mountains during daytime, but in basins, plains and coastal areas during nighttime. DCSI over land and oceans exhibits significantly different sub-seasonal and diurnal variations. Oceanic DCSI has an ambiguous diurnal variation, although its sub-seasonal variation is similar to that over land. It is demonstrated that the high spatiotemporal resolution satellite dataset provides rich information for understanding the convective systems over China and vicinity, particularly the complex terrain and oceans where radar observations are sparse or none, which will help to improve the convective systems and initiation nowcasting.

The relations between lightning occurrence over Europe from the EUCLID network (2008–2013) and parameters derived from ERA-Interim reanalysis data were studied to increase the understanding of the conditions under which thunderstorms form. The objective was to identify relevant factors beyond instability and convective inhibition, in order to better model thunderstorms using numerical weather prediction or climate model data. It was found that latent instability is only required up to a certain amount of approximately 200–400 J kg-1 CAPE. For higher values of CAPE (~800$\sim800$–2800 J kg-1), the relative frequency of lightning is rather constant. Relative humidity in the low to mid-troposphere has a major influence on storm occurrence with low relative humidity strongly suppressing thunderstorm development. For an average 850–500 hPa relative humidity below 50 %, the frequency of lightning decreases to below 15 %, even when CIN is negligible and CAPE sufficient. A subtle dependency on wind shear was found in which two regimes of higher frequency of lightning were identified. For very weak and for high shear the probability was higher than for intermediate values of both deep-layer and low-level shear.

Weather is by far the most important factor in air traffic delays in the United States' National Airspace System (NAS) according to the Federal Aviation Administration (FAA). Geostationary satellites have been an effective tool for the monitoring of meteorological conditions that affect aviation operations since the launch of the first Synchronous Meteorological Satellite (SMS) in the United States in 1974. This paper will review the global use of geostationary satellites in support of aviation weather since their inception, with an emphasis on the latest generation of satellites, such as Geostationary Operational Environmental Satellite (GOES)-R (16) with its Advanced Baseline Imager (ABI) and Geostationary Lightning Mapper (GLM). Specific applications discussed in this paper include monitoring of convective storms and their associated hazards, fog and low stratus, turbulence, volcanic hazards, and aircraft icing. [ABSTRACT FROM AUTHOR]

Convective initiation is a challenge for convection‐permitting models due to its sensitivity to sub‐km processes. We evaluate the representation of convective storms and their initiation over South Africa during four summer months in Met Office Unified Model simulations at a 1.5‐km horizontal grid length. Storm size distributions from the model compare well with radar observations, but rainfall in the model is predominantly produced by large storms (50 km in diameter or larger) in the evening, whereas radar observations show that most rainfall occurs throughout the afternoon, from storms 10–50 km in diameter. In all months, the modelled maximum number of storm initiations occurs at least 2 hr prior to the radar‐observed maximum. However, the diurnal cycle of rainfall between the model and observations compares well, suggesting that the numerous storm initiations in the simulations do not produce much rainfall. Modelled storms are generally less intense than those in radar observations, especially in early summer. In February, when tropical influences dominate, the simulated storms are of similar intensity to observed storms. Simulated storms tend to reach their peak intensity in the first 15 min after initiation, then gradually become less intense as they grow. In radar observations, storms reach their peak intensity 15 min into their life cycle, stay intense as they grow larger, then gradually weaken after they have reached their maximum diameter. Two November case studies of severe convection are analysed in detail. A higher resolution grid length initiates convection slightly earlier (300 m as opposed to 1.5 km) with the same scientific settings. Two 1.5 km simulations that apply more subgrid mixing have delayed convective initiation. Analysis of soundings indicates little difference in the convective indices, suggesting that differences in convection may be attributed to the choice of subgrid mixing parameters. [ABSTRACT FROM AUTHOR]

The presence of deep moist convection in a large part of Mexico is a condition typical of summer; the effects of land surface processes and mesoscale circulations are usually not included in the diagnostic and forecasting of deep moist convection, mainly because of the scale at which they occur. This study examines the effect of the soil moisture fluxes on convective initiation in the central region of Mexico during the summer periods of 2010–2014. Other aspects considered are the function of the average flux of the boundary layer in the transport of humidity towards the initiation zone. Remote sensing satellite data used are the cloud-top temperature, soil moisture, land surface temperature and precipitation. The results show over the initiation zone an increase in the soil moisture and a decrease in the land surface temperature in the direction of the mean flux at low levels. The soil moisture anomaly (SMA) presents a temporal variability being May, the composite more similar to previous studies, initiation zone drier and warmer than surroundings. The background low-level winds have an important role in the displacement of convection core and creating different features of SMA and land surface temperature anomaly. [ABSTRACT FROM AUTHOR]

The Great Plains low-level jet (LLJ) is a contributing factor to the initiation and evolution of nocturnal Mesoscale Convective Systems (MCSs) in the central United States by supplying moisture, warm air advection, and a source of convergence. Thus, the ability of models to correctly depict thermodynamics in the LLJ likely influences how accurately they forecast MCSs. In this study, the Weather Research and Forecasting (WRF) model was used to examine the relationship between spatial displacement errors for initiating simulated MCSs, and errors in forecast thermodynamic variables up to three hours before downstream MCS initiation in 18 cases. Rapid Update Cycle (RUC) analyses in 3 layers below 1500 m above ground level were used to represent observations. Correlations between simulated MCS initiation spatial displacements and errors in the magnitude of forecast thermodynamic variables were examined in regions near and upstream of both observed and simulated MCSs, and were found to vary depending on the synoptic environment. In strongly-forced cases, large negative moisture errors resulted in simulated MCSs initiating further downstream with respect to the low-level flow from those observed. For weakly-forced cases, correlations were weaker, with a tendency for smaller negative moisture errors to be associated with larger displacement errors to the right of the inflow direction for initiating MCSs.

A torrential rainfall that occurred in Beijing during the period of 21–22 July 2012 is simulated by the Weather Research and Forecasting Model in order to investigate the probable mechanisms for the initiation and organization of warm-sector mesoscale convective systems (MCSs). The simulated results show that the cyclone, which formed in Hetao area, Inner Mongolia and moved eastward slowly, played a key role in the formation and development of warm-sector precipitation, although the favorable atmospheric environment and the configuration of weather systems are also important, which caused the trigger and organization of convective cells along Taihang Mountains. It is the interaction of the local terrain convergence line and the southerly airflows of Hetao cyclone that cause the continuous trigger of convective cells along Taihang Mountains. While, the triggers of convective cells in the plains are caused by the gravity waves, which is related to the development and eastward movement of Hetao cyclone. It must be pointed out that the merging and coupling between the cells that triggered in Taihang Mountains and moved southwesterly and the cells that triggered in plains and moved northeasterly are the key factors for the formation and development of MCSs during the warm-sector precipitation. In addition, the back-building processes and the cold pool forcing are also important for the formation and development of MCSs in this study.

Thunderstorms in southeastern South America (SESA) stand out in satellite observations as being among the strongest on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, the prevalence for extremely tall, wide convective cores and broad stratiform regions. Accurately quantifying when and where strong convection is initiated presents great interest in operational forecasting and convective system process studies due to the relationship between convective storms and severe weather phenomena. This paper generates a novel methodology to determine convective initiation (CI) signatures associated with extreme convective systems, including extreme events. Based on the well-established area-overlapping technique, an adaptive brightness temperature threshold for identification and backward tracking with infrared data is introduced in order to better identify areas of deep convection associated with and embedded within larger cloud clusters. This is particularly important over SESA because ground-based weather radar observations are currently limited to particular areas. Extreme rain precipitation features (ERPFs) from Tropical Rainfall Measurement Mission are examined to quantify the full satellite-observed life cycle of extreme convective events, although this technique allows examination of other intense convection proxies such as the identification of overshooting tops. CI annual and diurnal cycles are analyzed and distinctive behaviors are observed for different regions over SESA. It is found that near principal mountain barriers, a bimodal diurnal CI distribution is observed denoting the existence of multiple CI triggers, while convective initiation over flat terrain has a maximum frequency in the afternoon.

The initiation processes of one of the initial convective cells near and on the east side of a dryline on 19 June 2002 during the IHOP_2002 field experiment in the central United States is analyzed in detail based on a high-resolution numerical simulation. Prominent horizontal convective rolls and associated near-surface moisture convergence bands [called roll convergence bands (RCBs) here] develop within the convective boundary layer (CBL) due to surface heating, in the hours leading to convective initiation (CI). The RCBs east of the dryline are advected toward the primary dryline convergence boundary (PDCB) by the southerly moist flow as the CBL deepens with time. Backward trajectories of air parcels forming the initial precipitating updraft of the convective cell are found to primarily originate at about 1-1.5 km above ground, within the upper portion of the shallower CBL earlier on. The representative air parcel is found to follow and stay on top of a surface RCB as the RCB moves toward the PDCB, but the RCB forcing alone is not enough to initiate convection. As this RCB gets close to the PDCB, it moves into a zone of mesoscale convergence and a deeper CBL that exhibits an upward moisture bulge associated with the PDCB. The combined upward forcing of the RCB and the mesoscale PDCB convergence quickly lifts the representative air parcel above its level of free convection to initiate convection. A conceptual model summarizing the CI processes is proposed. [ABSTRACT FROM AUTHOR]

The tendency of convective rainfall to initiate over a wetter or drier land surface is a critical feedback process in the climate system, influencing the hydrological cycle on a variety of spatial scales, especially in parts of the world where water is limited. A simple algebraic solution is derived from fundamental physical equations, to predict the sign of this convective rainfall feedback with the surface. The tendency for convection to occur is evaluated by the rate at which the convective boundary‐layer top approaches the level of free convection. Well‐known integral models predict the rate of ascent of the boundary‐layer top, which tends to be faster over a dry surface. The associated changes in equivalent potential temperature in the boundary layer determine the rate at which the level of free convection descends, typically faster over a wet surface, as a function of the ambient profile, the thermodynamic forcing and the surface Bowen ratio. The resulting system is controlled by three parameters. Two nondimensional parameters determine whether there is wet or dry "advantage"; the Bowen ratio at the boundary‐layer top and a "convective instability parameter," defined as the ratio of the vertical gradient of saturated equivalent potential temperature at the level of free convection to the profile stability just above the boundary layer. A dimensional function, dependent on the surface fluxes, the boundary‐layer depth, and the profile stability, provides the magnitude of the response. In comparison with previous work, the solution is both rigorously derived from physical principles and encapsulated in a simple algebraic form. A first evaluation of the theoretical framework has been made using data from a convection‐permitting numerical model simulation over India, and this indicates that the equations successfully determine the conditions under which convection is triggered over dry surfaces. We analyse the evolution of a convective boundary layer and the conditions for initiation of deep convection. Considering the rate at which the boundary‐layer top and the level of free convection converge, we find an equation determining whether deep convection will occur preferentially over a wet or a dry surface. The nondimensional inversion Bowen ratio and a "convective instability parameter" determine whether there is wet or dry "advantage," but a third, dimensional parameter gives the amplitude of the advantage. [ABSTRACT FROM AUTHOR]

Convective initiation (CI) nowcasting often has a low probability of detection (POD) and a high false-alarm ratio (FAR) at subtropical regions where the warm-rain processes often occur. Using the high spatial and temporal resolution and multispectral data from the Advanced Himawari Imager (AHI) on board Japanese new-generation geostationary satellite Himawari-8, a standalone CI nowcasting algorithm is developed in this study. The AHIbased CI algorithm utilizes the reflectance observations from channels 1 (0.47 µm) and 7 (3.9 µm); brightness temperature observations from infrared window channel 13 (10.4 µm), the dual-spectral differences between channels 10 (7.3 µm) and 13, 13, and 15 (12.4 µm), and a tri-spectral combination of channels 11, 15, and 13, as CI predictors without relying on any dynamic ancillary data (e.g., cloud type and atmospheric motion vector products). The proposed AHI-based algorithm is applied to CI cases over Fujian province in the Southeastern China. When validated by S-band radar observations, the CI algorithm produced a POD as high as 93.33 %, and an FAR as low as 33.33 % for a CI case day that occurred on August 1, 2015, over Northern Fujian. For over 216 CI events that occurred in a 3-month period from July to September 2015, the CI nowcasting lead time has a mean value of ~ 64 min, with a longest lead time over 120 min. It is suggested that false-alarm nowcasts that occur in the presence of capping inversion require further investigation and algorithm enhancements. [ABSTRACT FROM AUTHOR]

Geostationary satellites are able to nowcast Convective Initiation (CI) for the next 0-6 h. Compared to using satellite predictors only, the incorporation of satellite and Numerical Weather Prediction (NWP) predictors can provide the possibility to reduce false alarm rates in 0-1:30 Convective Initiation Nowcasting (COIN). However, the correlation among these predictors not only can cause error in COIN, but also increases the runtime. In this study for the first time, all effective predictors in Satellite Convection Analysis and Tracking version 2 (SATCASTv2) and NWP were applied over Iran from 22nd March 2015 to 9th January 2016. In applying SATCASTv2 over Iran, it was necessary to make some modifications to the algorithm, such as removing case specific thresholds of satellite predictors and rearranging COIN predictors. Then, SATCASTv2 was tested and evaluated with both the full and reduced set of predictors. The results suggested that using fixed thresholds for temporal difference predictors could miss COIN in some cases. To investigate the possibility of improving computational efficiency, a dimension reduction was conducted by Factor Analysis (FA) and the number of predictors was reduced from 22 to 11. The NWP-satellite, reduced NWP-satellite, and satellite predictors were used as input in Random Forest (RF), as a parametric machine learning method, for COIN evaluation. The Combination of NWP model and satellite predictors had lower false alarm rates in contrast with satellite predictors. This is in agreement with previous studies. The results from statistical metrics showed that the reduced NWP-satellite predictors had comparable performance to the NWP-satellite predictors over study area, but decreased the run time by almost 50%. The results indicated that Convective Inhibition (CIN) was the most significant predictor when the reduced set of predictors was used. [ABSTRACT FROM AUTHOR]

To gain insight into the processes leading to the convective initiation ( CI) of an isolated thunderstorm near the central urban area ( CUA) of the Beijing metropolitan region ( BMR) on 9 August 2011, a series of numerical sensitivity experiments by removing or modifying the CUA, terrain morphology, and convectively generated cold outflows is performed. Results show that the CUA accounts for the formation of cloud condensation near the top of the mixed layer at the CI site as a result of the growing boundary-layer depth through its urban heat island effects and urban-induced convergence between a warm and moist southerly flow and an easterly flow on the downstream side. The northerly approaching cold outflows from the mesoscale convective system ( MCS) facilitate the generation of northerly flows through 'a northwestward-concave valley' such that the enhanced low-level convergence with the southerly and easterly flows near the CUA could determine the timing of the outbreak of shallow clouds near the mixed-layer top into an intense thunderstorm. If the concave valley to the northwest of the CUA is eliminated, little northerly flow ahead of the cold outflow boundaries into the CI site would be present. The results obtained herein appear to have significant implications for understanding and forecasting the development of afternoon thunderstorms in the BMR under weak-gradient environments. [ABSTRACT FROM AUTHOR]

A widely recognized issue found in most Earth System Models is that of convective precipitation being triggered too early during the day over land, often resulting in persistent light rainfall. In an effort to address this issue, a model-consistent subgrid convective trigger is demonstrated in the Community Earth System Model (CESM). Subgrid convective initiation is achieved by first calculating a convective threshold within the Community Atmosphere Model (version 5.3). The convective threshold is then passed to each subgrid land cover type within the Community Land Model (version 4.0) and convection is considered to be active if the convective threshold is exceeded and the convective available potential energy (CAPE) exceeds 70 J kg−1 (the default trigger). In total, three simulations are performed: (1) a Control (CTL) with the default CAPE trigger, (2) a simulation with the default CAPE trigger as well as the subgrid trigger (HCF trigger) applied over land and ocean, and (3) a simulation the same as the HCF trigger but only applied over land (HCF land). By including this subgrid trigger, convective onset time is delayed by 1-4 h; however, the time of peak precipitation is unchanged. This diurnal shift improves the number of hours the deep convective scheme is invoked from more than 75% to less than 50%, alleviating the overactivity problem. The delayed onset time of deep convection allows for greater CAPE accumulation and a greater proportion of precipitation generated outside the convective scheme. More CAPE consequently produces an overall increase in intense precipitation events. [ABSTRACT FROM AUTHOR]

The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region, China, is studied through numerical simulations that include local data assimilation. A primary convergence line is found over the ridge of the Dabie Mountains, and along the ridge line several locally enhanced convergence centers preferentially initiate convection. Three processes responsible for creating the overall convergence pattern are identified. First, thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge, which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front. Second, flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks. Third, upslope winds develop along the roughly north-south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects, and create additional convergence between the peaks along the ridge. The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains. Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge. These conclusions are supported by two sensitivity experiments in which the environmental wind (dynamic forcing) or radiative and land surface thermal forcing are removed, respectively. Overall, the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow. [ABSTRACT FROM AUTHOR]

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This study assesses the impact of urban land use on the climatological distribution of thunderstorm initiation occurrences in the humid subtropical region of the southeast United States, which includes the Atlanta, Georgia metropolitan area. Initially, an automated technique is developed to extract the locations of isolated convective initiation (ICI) events from 17 years (1997-2013) of composite reflectivity radar data for the study area. Nearly 26 000 ICI points were detected during 85 warm-season months, providing the foundation for first long-term, systematic assessment of the influence of urban land use on thunderstorm development. Results reveal that ICI events occur more often over the urban area compared to its surrounding rural counterparts, confirming that anthropogenic-induced changes in land cover in moist tropical environments lead to more initiation events, resulting thunderstorms and affiliated hazards over the developed area. The ICI risk for Atlanta is greatest during the late afternoon and early evening in July and August in synoptically benign conditions. Greater ICI counts downwind of Atlanta suggest that prevailing wind direction also influences the location of these events. Moreover, ICI occurrences over the city were significantly higher on weekdays compared to weekend days-a result that was not apparent in a rural control region located west of the city. This suggests that the weekly commuting cycle and associated aerosol levels of Atlanta may amplify ICI rates. The investigation provides a methodological framework for future studies that examine the effect of land use, land cover, and terrain discontinuities on the spatio-temporal character of ICI events. [ABSTRACT FROM AUTHOR]

Characteristics of convective initiation (CI) in the Beijing-Tianjin region during the warm season of 2008-2013 are examined. A total of 38877 CI cases are identified by a thunderstorm identification, tracking, analysis, and nowcasting algorithm. CI cases are evaluated in the context of associated terrain, weather systems, and land cover properties. The spatial distribution of all CI cases shows that there are dense CI activities around the 200-m elevation, which means that convective storms are more easily triggered over foothills. From 1500-1800 to 0300-0600 BT (Beijing Time), the high-occurrence CI region tends to propagate southeastward (i.e., from mountains to plains, then to ocean). Among the four local weather systems, the Mongolian cold vortex has the highest CI frequency while the after-trough system has the lowest CI frequency. For the land cover relationships with CI, the urban land cover has the highest CI density and the forest-type land cover has the second highest CI density; these two types of land cover are more conducive to CI formation. [ABSTRACT FROM AUTHOR]

Refractivity changes (Δ N) derived from radar ground clutter returns serve as a proxy for near-surface humidity changes (1 N unit ≡ 1% relative humidity at 20 °C). Previous studies have indicated that better humidity observations should improve forecasts of convection initiation. A preliminary assessment of the potential of refractivity retrievals from an operational magnetron-based C-band radar is presented. The increased phase noise at shorter wavelengths, exacerbated by the unknown position of the target within the 300 m gate, make it difficult to obtain absolute refractivity values, so we consider the information in 1 h changes. These have been derived to a range of 30 km with a spatial resolution of ∼4 km; the consistency of the individual estimates (within each 4 km × 4 km area) indicates that Δ N errors are about 1 N unit, in agreement with in situ observations. Measurements from an instrumented tower on summer days show that the 1 h refractivity changes up to a height of 100 m remain well correlated with near-surface values. The analysis of refractivity as represented in the operational Met Office Unified Model at 1.5, 4 and 12 km grid lengths demonstrates that, as model resolution increases, the spatial scales of the refractivity structures improve. It is shown that the magnitude of refractivity changes is progressively underestimated at larger grid lengths during summer. However, the daily time series of 1 h refractivity changes reveal that, whereas the radar-derived values are very well correlated with the in situ observations, the high-resolution model runs have little skill in getting the right values of Δ N in the right place at the right time. This suggests that the assimilation of these radar refractivity observations could benefit forecasts of the initiation of convection. [ABSTRACT FROM AUTHOR]

Abstract: This study demonstrates a simple procedure for sharpening the infrared (IR) field data for 3km sampling distance Meteosat Second Generation (MSG) pixels that are “partly cloudy,” and hence are not completely cloud filled. Without a sharpened IR pixel, signals of clouds are blurred, whereas with sharpening, both partly cloudy (pixels with 1/9th to 8/9th cloud cover) and completely cloudy pixels possess similar IR channel and trend values, making for more consistent use of these data within algorithms that diagnose and predict aspects of clouds (i.e. growth, vertical development, microphysical characteristics). For this method, 1km sampling distance MSG High Resolution Visible (HRV) and IR data are first used to determine a cloud type, and then a fraction of pixels within an IR pixel is determined. Each IR pixel contains 3×3 HRV pixels. The focus here is on more dynamically growing convective clouds, and data for 187 cumulus cloud scenes are processed over regions of Europe on 25 May 2009 and 15 August 2010. Results show that the IR fields are normalized from partly cloudy to completely cloudy pixels. Sharpened IR channel data are subsequently used within channel differences and time trends to show the applicability of this exercise. [Copyright &y& Elsevier]

Abstract: The accurate prediction of convective cloud development in advance of thunderstorm formation (so‐called “convective initiation,” CI) is a challenging forecast problem, one in which the processing of 5–15min interval imagery from geostationary satellites (e.g., Meteosat Second Generation) offers considerable promise. A present drawback to using sequences of visible or infrared (IR) satellite images to monitor growing cumulus clouds is that higher altitude cirrus clouds often obscure the view of the low‐level cumulus in the pre‐convective environment. In particular, cirrus anvils from pre‐existing convection, and cirrus caused by deep layer quasi‐geostrophic ascent, are very common in pre‐CI environments. Cloud derived parameters from GOES are used here to demonstrate how quantities like visible optical depth (τ), emittance, liquid water path, and effective particle size can be used to quantify cumulus cloud growth in advance of CI. Time rates of change of these derived quantities, as well as IR interest fields that describe cumulus cloud development rates beneath cirrus, are analyzed as τ of the cirrus are binned from 1 to >50. Results confirm that if cirrus possess τ <20, up to >90% of the “signal” in the IR interest field remains, compared to clear‐sky values, and it is proposed that CI can still be adequately nowcasted using IR channel data similar to what is done in the absence of cirrus. Similarly, cloud derived parameters become valuable as their time rates of change measure cumulus cloud growth beneath the higher clouds. In contrast, once τ values increase beyond ∼20, cumulus cloud growth signals decrease significantly through cirrus, and as τ becomes >40, little information from the cumulus remains. [Copyright &y& Elsevier]

When the National Weather Radar Testbed (NWRT) was installed in 2004, a single-channel digital receiver was implemented so that the radar could mimic typical Weather Surveillance Radar (WSR) version 1988 Doppler (WSR-88D) capability. This, however, left unused eight other channels, built into the antenna. This paper describes the hardware instrumentation of a recently completed project that digitizes the radar signals produced by these channels. The NWRT is the nation's first phased array devoted to weather observations, and this testbed serves as an evaluation platform to test new hardware and signal processing concepts. The multichannel digital data will foster a new generation of adaptive/fast scanning techniques and space-antenna/interferometry measurements, which will then be used for improved weather forecasting via data assimilation. The multichannel receiver collects signals from the sum, azimuth-difference, elevation-difference, and five broad-beamed auxiliary channels. One of the major advantages of the NWRT is the capability to adaptively scan weather phenomena at a higher temporal resolution than is possible with the WSR-88D. Access to the auxiliary channels will enable clutter mitigation and advanced array processing for higher data quality with shorter dwell times. Potential benefits of higher quality and higher resolution data include: better understanding of storm dynamics and convective initiation; better detection of small-scale phenomena, including tornadoes and microbursts; and crossbeam wind, shear, and turbulence estimates. These items have the distinct possibility to ultimately render increased lead time for warnings and improved weather prediction. Finally, samples of recently collected data are presented in the results section of this paper. [ABSTRACT FROM PUBLISHER]

Abstract: This study develops an understanding on how retrieved cloud parameter fields from the Optimal Cloud Analysis (OCA) algorithm, operating on Meteosat Second Generation (MSG), Spinning Enhanced Visible and Infrared Imager (SEVIRI) data, behave at 5-min time resolutions for growing cumulus clouds. Fields retrieved by the OCA algorithm include cloud optical thickness (τ), cloud-top particle effective radius (re), cloud-top pressure (pc), and cloud-top phase. OCA is based on a one-dimensional optimal estimation methodology, and a measure of radiance fit, the cost function (Jm), is a quantity developed as part of the retrieval process and is shown to be useful in delineating mixed phase clouds; it too is evaluated (at 5-min intervals) for the information it provides. Data for 94 growing cumulus cloud events are processed. An “event” is defined as a cumulus cloud that is monitored at 5-min intervals with OCA, as it grows from the “fair weather” or “towering cumulus” stage to near the cumulonimbus stage when precipitation begins. The hypothesis is that OCA products are of high-enough quality to provide unique information about microphysical processes occurring at and near cloud top. The goal through analysis of the 94 events is to identify consistent, repeating patterns in OCA fields during cloud growth that can be in turn used to infer physical processes. Data from the Convective and Orographically-induced Precipitation Study (June and July 2007) and in four regions of Europe on 25 May 2009 are used. The validity of the OCA data is presented with a comparison to CloudSat Precipitation Radar and MODerate resolution Imaging Spectroradiometer retrieved cloud properties, showing good statistical agreements. Subsequently, results from the analysis of OCA fields for all events show that as cumuli deepen, re values tend to increase, and then decrease in size as cloud tops glaciate and particle settling begins. The τ magnitudes generally increase as clouds deepen, while pc values and cloud-top temperatures fall as expected. The Jm values exhibit the pattern of spiking in magnitude (over a 5–10-min period), which indicates the increase “misfit” within OCA during the mixed phase, at about the time τ values increase substantially as clouds deepen. [Copyright &y& Elsevier]

Among the densely-populated coastal areas of China, the southeastern coast has received less attention in convective development despite having been suffering from significantly increasing thunderstorm activities. The convective complexity under such a region with extremely complex underlying and convective conditions deserves in-depth observational surveys. This present study examined a high-impact convection outbreak event with over 40 hail reports in the southeastern coast of China on 6 May 2020 by focusing on contrasting the convective development (from convective initiation to supercell occurrences) among three adjacent convection-active zones (north (N), middle (M), and south (S)). The areas from N to S featured overall flatter terrain, higher levels of free convection, lower relative humidity, larger convective inhibition, more convective available potential energy, and greater vertical wind shears. With these mesoscale environmental variations, distinct inter-zone differences in the convective development were observed with the region's surveillance radar network and the Himawari-8 geostationary satellite. Convection initiated in succession from N to S and began with more warm-rain processes in N and M and more ice-phase processes in S. The subsequent convection underwent more vigorous vertical growth from N to S. The extremely deep convection in S was characterized by the considerably strong precipitation above the freezing level, echo tops of up to 18 km, and a great amount of deep (even overshooting) and thick convective clouds with significant cloud-top glaciation. Horizontal anvil expansion in convective clouds was uniquely apparent over S. From N to S, more pronounced mesocyclone and weak-echo region signatures indicated high risks of severe supercell hailstorms. These results demonstrate the strong linkage between the occurrence likelihood of severe convection and associated weather (such as supercells and hailstones) and the early-stage convective development that can be well-captured by high-resolution observations and may facilitate fine-scale convection nowcasting. [ABSTRACT FROM AUTHOR]

Abstract: Water vapor (WV) and infrared (IR) imageries from METEOSAT-5 are used to study the mid-upper and lower atmospheric water vapor patterns pertinent to the initiation of convective cells of Nor''wester, a severe thunderstorm during pre-monsoon season (March, April, and May) in northeastern part of Indian subcontinent. WV images are used to diagnose the mid-upper atmospheric dryness, while low-level water vapor patterns are monitored using IR images. Six important combinations of mid-upper and lower atmospheric moisture patterns are identified and their role in convective initiation for a variety of Nor''westers (A, B, and D type Nor''westers) is investigated. The detailed image analysis reveals that, in general, convective cells of Nor''wester have a strong tendency to be initiated along/near the edges of mid-upper atmospheric dryness passing over a low-level moist layer of atmosphere. Moisture patterns associated with dryline activity play a crucial role in convective initiation for A type Nor''wester while for B and D type Nor''westers they are totally absent. The features of combinations of moisture patterns are discussed in the context of a case study. The conceptual considerations describe the possible physical processes behind the phenomena. WV and IR images provide indications of convective instability and mesoscale lifting processes. These observations, at a higher temporal and spatial resolutions, could be very useful for forecasting Nor''westers. [Copyright &y& Elsevier]