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This study uses high-resolution, convection-permitting, dynamically downscaled regional climate simulation output to assess how long-lived, convectively induced, extratropical windstorms known as derechos may change across the CONUS during the twenty-first century. Three 15-yr epochs including a historical period (1990–2005) and two separate late-twenty-first-century periods (2085–2100) employing intermediate (RCP4.5) and pessimistic (RCP8.5) greenhouse gas concentration scenarios are evaluated. A mesoscale convective system (MCS) identification and tracking tool catalogs derecho candidates across epochs using simulated radar reflectivity and maximum 10-m wind speed as a proxy for near-surface severe wind gusts. Results indicate that MCS-based windstorms, including derechos, are more frequent, widespread, and intense in both future climate scenarios examined for most regions of the central and eastern CONUS. Increases are suggested across all parts of the year, with significant changes in populations concentrated during the early spring and summer months, suggesting the potential for a longer, more extreme MCS windstorm season. This research provides insights for forecasters, emergency managers, and wind-vulnerable stakeholders on how these events may change across the twenty-first century so that they may mitigate, adapt to, and become resilient against severe convective storm perils. Significance Statement: Long-lived, thunderstorm-induced, damaging wind events known as derechos may increase across most portions of the central and eastern United States in the future, with projections indicating a near doubling or tripling of annual cases in the Midwest, eastern Great Plains, and Mississippi and Ohio Valley regions by the end of the twenty-first century. Modeling projections suggest that future derechos could generally be longer-lived, more expansive, and capable of producing more severe wind gusts and damage, which will ultimately increase risk to life, infrastructure, and wind-sensitive industries affected by these extreme thunderstorm events. [ABSTRACT FROM AUTHOR]

Quasi-linear convective systems (QLCSs) are responsible for approximately a quarter of all tornado events in the United States, but no field campaigns have focused specifically on collecting data to understand QLCS tornadogenesis. The Propagation, Evolution, and Rotation in Linear Storms (PERiLS) project was the first observational study of tornadoes associated with QLCSs ever undertaken. Participants were drawn from more than 10 universities, laboratories, and institutes, with over 100 students participating in field activities. The PERiLS field phases spanned 2 years, late winters and early springs of 2022 and 2023, to increase the probability of intercepting significant tornadic QLCS events in a range of large-scale and local environments. The field phases of PERiLS collected data in nine tornadic and nontornadic QLCSs with unprecedented detail and diversity of measurements. The design and execution of the PERiLS field phase and preliminary data and ongoing analyses are shown. [ABSTRACT FROM AUTHOR]

This study employs 3D idealized numerical experiments to investigate the physical processes associated with coastal convection initiation (CI) as an offshore-moving squall line traverses a mountainous coastal region. A squall line can propagate discretely as convection initiates over the lee slope downstream of the primary storm as the cold pool collides with a sea breeze. Intensity of the initiating convection, thus the downstream squall line, is sensitive to the sea-breeze numerical initialization method, since it influences sea-breeze and cold pool characteristics, instability and vertical wind shear in the sea-breeze environment, and ultimately the vertical acceleration of air parcels during CI. Here, sea breezes are generated through four commonly used numerical methods: a cold-block marine atmospheric boundary layer (MABL), a prescribed surface sensible heat flux function, a prescribed surface sensible plus latent heat flux functions, and radiation plus surface-layer parameterization schemes. For MABL-initialized sea breezes, shallow weak sea-breeze flow in a relatively low instability environment results in weak CI. For the remainder, deeper stronger sea-breeze flow in an environment of enhanced instability supports more robust CI. In a subset of experiments, however, the vertical trajectory of air parcels is suppressed leading to weaker convection. Downward acceleration forms due to the horizontal rotation of the sea-breeze flow. Accurate simulations of coastal convective storms rely on an accurate representation of sea breezes. For idealized experiments such as the present simulations, a combination of initialization methods likely produces a more realistic representation of sea breeze and the associated physical processes. [ABSTRACT FROM AUTHOR]

Quasi-linear convective systems (QLCSs) can produce multiple hazards (e.g., straight-line winds, flash flooding, and mesovortex tornadoes) that pose a significant threat to life and property, and are often difficult to accurately forecast. The NSSL Warn-on-Forecast System (WoFS) is a convection-allowing ensemble system developed to provide short-term, probabilistic forecasting guidance for severe convective events. Examination of WoFS's capability to predict QLCSs has yet to be systematically assessed across a large number of cases for 0–6-h forecast times. In this study, the quality of WoFS QLCS forecasts for 50 QLCS days occurring between 2017 and 2020 is evaluated using object-based verification techniques. First, a storm mode identification and classification algorithm is tuned to identify high-reflectivity, linear convective structures. The algorithm is used to identify convective line objects in WoFS forecasts and Multi-Radar Multi-Sensor system (MRMS) gridded observations. WoFS QLCS objects are matched with MRMS observed objects to generate bulk verification statistics. Results suggest WoFS's QLCS forecasts are skillful with the 3- and 6-h forecasts having similar probability of detection and false alarm ratio values near 0.59 and 0.34, respectively. The WoFS objects are larger, more intense, and less eccentric than those in MRMS. A novel centerline analysis is performed to evaluate orientation, length, and tortuosity (i.e., curvature) differences, and spatial displacements between observed and predicted convective lines. While no systematic propagation biases are found, WoFS typically has centerlines that are more tortuous and displaced to the northwest of MRMS centerlines, suggesting WoFS may be overforecasting the intensity of the QLCS's rear-inflow jet and northern bookend vortex. Significance Statement: Quasi-linear convective systems (QLCSs), also known as squall lines, can be very destructive to life and property as they produce multiple hazards such as hail, severe straight-line winds, flash flooding, and tornadoes that typically form quickly and may be difficult to observe on radar. These storms can occur year-round and have the propensity to develop overnight or into the early morning hours, potentially catching the public off-guard. An ensemble prediction system called the Warn-on-Forecast System (WoFS), created by the National Severe Storms Laboratory, has shown promise in accurately forecasting a variety of severe weather events. This research evaluates the quality of the WoFS's QLCS forecasts. Results show WoFS can accurately predict these systems for forecast times out to 6 h. [ABSTRACT FROM AUTHOR]

Squall lines are substantially influenced by the interaction of low‐level shear with cold pools associated with convective downdrafts. Beyond an optimal shear amplitude, squall lines tend to orient themselves at an angle with respect to the low‐level shear. While the mechanisms behind squall line orientation seem to be increasingly well understood, uncertainties remain on the implications of this orientation. Roca and Fiolleau (2020, https://doi.org/10.1038/s43247-020-00015-4) show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. This article investigates the influence of the interaction between low‐level shear and squall line outflow on squall line generated precipitation extrema in the tropics. Using a cloud resolving model, simulated squall lines in radiative convective equilibrium amid a shear‐dominated regime (super optimal), a balanced regime (optimal), and an outflow dominated regime (suboptimal). Our results show that precipitation extremes in squall lines are 40% more intense in the case of optimal shear and remain 30% superior in the superoptimal regime relative to a disorganized case. With a theoretical scaling of precipitation extremes (C. Muller & Takayabu, 2020, https://doi.org/10.1088/1748-9326/ab7130), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its change in vertical mass flux (dynamic component). The reduction of dilution by entrainment explains half of this change, consistent with Mulholland et al. (2021, https://doi.org/10.1175/jas-d-20-0299.1). The other half is explained by increased cloud‐base velocity intensity in optimal and superoptimal squall lines. Plain Language Summary: Squall lines are bands of clouds and thunderstorms spanning hundreds of kilometers, also called quasi‐linear mesoscale convective systems. These systems are associated with extreme weather conditions, including extreme rainfall rates. To better understand and therefore predict this high impact phenomenon, this study investigates the physical processes leading to enhanced precipitation rates when clouds are organized into squall lines, using idealized high‐resolution simulations. Interestingly, the dynamics of squall lines, notably their wind structures, are found to play a key role in setting the intensity of extreme rainfall rates. Key Points: Precipitation extremes are enhanced by about 30%–40% in optimal and superoptimal squall lines compared to random convectionThe enhancement of extremes is due to reduced dilution by entrainment and enhanced initial vertical velocity of updrafts in optimal and superoptimal regimesThe enhanced vertical velocity in convective updrafts does not depend on the orientation of squall lines in the superoptimal regime [ABSTRACT FROM AUTHOR]

Forecasting tornadogenesis remains a difficult problem in meteorology, especially for short-lived, predominantly nonsupercellular tornadic storms embedded within mesoscale convective systems (MCSs). This study compares populations of tornadic nonsupercellular MCS storm cells with their nontornadic counterparts, focusing on nontornadic storms that have similar radar characteristics to tornadic storms. Comparisons of single-polarization radar variables during storm lifetimes show that median values of low-level, midlevel, and column-maximum azimuthal shear, as well as low-level radial divergence, enable the highest degree of separation between tornadic and nontornadic storms. Focusing on low-level azimuthal shear values, null storms were randomly selected such that the distribution of null low-level azimuthal shear values matched the distribution of tornadic values. After isolating the null cases from the nontornadic population, signatures emerge in single-polarization data that enable discrimination between nontornadic and tornadic storms. In comparison, dual-polarization variables show little deviation between storm types. Tornadic storms both at tornadogenesis and at a 20-min lead time show collocation of the primary storm updraft with enhanced near-surface rotation and convergence, facilitating the nonmesocyclonic tornadogenesis processes. [ABSTRACT FROM AUTHOR]

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]

Severe wind gusts produced by squall lines are difficult to monitor and forecast. This paper assessed and improved the physics-based Brasseur WGE (wind gust estimate) method for diagnosing wind gust of squall lines by coupling the WGE methods with the WRF (Weather Research and Forecasting) model. The simulation results show that the Brasseur WGE method accurately captured the strong gust feature with 32 m·s−1 maximum wind speed during the disastering Shipwreck event occurred over Yangtze River on 1 June 2015, but overestimated the extended area of severe gust speeds. Analysis of the kinematic structure and boundary-layer conditions of the squall line confirmed the theoretical applicability of the Brasseur WGE method for squall lines. A novel gust-front-area limiting method was introduced to modify the Brasseur WGE method, which effectively reduces its gust wind overestimation area. Furthermore, five squall line events occurred in the middle China during 2021 were simulated to test the modified WGE method and the results exhibit significant improvements to the wind gust forecasts, with an average false alarm rate decreased from 0.89 to 0.54, and the critical success index(CSI) increased from 0.1 to 0.4.

Squall lines observed over the sahelo-sudanese regions are responsible for most of the annual rainfall recorded in these regions. They originate from the development of convective processes embedded in the Mesoscale Convective Systems (MCS). The impact assessment of the West African Monsoon (WAM) on the development of squall lines in the sahelo-sudanese regions, particularly the far north region of Cameroon is studied. In this study, the Tropical Rainfall Measuring Mission data (TRMM 3A12 and 3B43) and ERA5 reanalysis data from 1998 to 2008 have been used. The technique used is based on the use of the Climate Data Operator (CDO) software and the NCAR command language (NCL). Analysis of these data shows that the seasonal distribution of winds from the saharan heat-low and the Saint Helena high is the dominant atmospheric dynamics associated with the disturbances behind the development of squall lines. This distribution makes it possible to specifically draw up the evolution of the InterTropical Discontinuity (ITD) on the African continent, and thus, highlight the contribution of the WAM on precipitation in the far north region of Cameroon. This study is performed to contribute to the prediction of hydro-meteorological phenomena to improve disaster risks management associated with floods and landslides in this region, the most vulnerable in Cameroon. [ABSTRACT FROM AUTHOR]

The Maritime Continent experiences some of the world’s most severe convective rainfall, with an intense diurnal cycle. A key feature is offshore propagation of convection overnight, having peaked over land during the evening. Existing hypotheses suggest this propagation is due to the nocturnal land breeze and environmental wind causing low-level convergence; and/or gravity waves triggering convection as they propagate. We use a convection-permitting configuration of the Met Office Unified Model over Sumatra to test these hypotheses, verifying against observations from the Japanese Years of the Maritime Continent field campaign. In selected case studies there is an organized squall line propagating with the land-breeze density current, possibly reinforced by convective cold pools, at ~3ms-1 to around 150–300 km offshore. Propagation at these speeds is also seen in a composite mean diurnal cycle. The density current is verified by observations, with offshore low-level wind and virtual potential temperature showing a rapid decrease consistent with a density current front, accompanied by rainfall. Gravity waves are identified in the model with a typical phase speed of 16 m s-1. They trigger isolated cells of convection, usually farther offshore and with much weaker precipitation than the squall line. Occasionally, the isolated convection may deepen and the rainfall intensify, if the gravity wave interacts with a substantial preexisting perturbation such as shallow cloud. The localized convection triggered by gravity waves does not generally propagate at the wave’s own speed, but this phenomenon may appear as propagation along a wave trajectory in a composite that averages over many days of the diurnal cycle. [ABSTRACT FROM AUTHOR]

Identifying the coupling relationships between gusts at different altitudes of weather systems over land is essential to recognize the inherent energy transfer patterns of different types of windy weather systems and design civil structures and infrastructures. Using the time‐lagged convergent cross‐mapping (LCCM) and data obtained from the 356‐m‐high Shenzhen Meteorological Gradient Tower, we present an observational study of the gusts and momentum flux of two super typhoons (STs) and two squall lines (SLs). Compared to STs, the gusts in SLs are characterized by high amplitude bursts and first occur near the surface. Bivariate toy models and observational data showed that LCCM reliably identifies coupling relationships in the nonlinear system. Subsequently, we systematically demonstrated that LCCM can identify long‐distance interactions of multilevel gusts in different types of gale weather systems. Finally, the downward transportation of energy in ST was verified from the perspective of wind structure and turbulent vertical flux. The upper‐level STs gusts varied ahead of the lower‐level gusts, whereas the SLs showed the opposite result, which is consistent with the coupling relationship obtained by the LCCM method. The results of the study are expected to enhance the understanding of the characteristics of multilevel energy transfer of TC and SL boundary layers over land, provide useful information for validating numerical weather prediction models of damaging weather systems, and facilitate gale hazard prevention in coastal regions. Plain Language Summary: Typhoon and squall line are two different weather systems. Although they both produce strong gusts, the characteristics of gusts induced by them are quite different. Understanding the difference of energy transportation in vertical direction in the different weather systems is of great significance for the application of wind energy, the wind resistance design of buildings and the development of early warning systems of gusts. An advanced mathematical method is used to capture the interaction between gusts at different altitudes. It is found that during the typhoon, the variation of upper gust is generally earlier than that of lower gust, and the energy is generally transferred from top to bottom. While during the squall line, the result is opposite. The difference in the internal structure of the two weather systems may be the reason for this result. Key Points: Gusts in squall lines exerts abrupt changes, whereas gusts changes in typhoons are relatively steadyThe occurrence of gusts in the high level (320 m) is prior to that in lower level (10 m) in typhoons, while the opposite in squall linesThe coupling relationships between gusts at different heights are determined by the vertical structure of typhoons and squall lines [ABSTRACT FROM AUTHOR]

Water isotopes are tracers of convective processes and are often used as proxies for past precipitation. These applications require a better understanding of the impact of convective processes on the isotopic composition of water vapor and precipitation. One way to advance this understanding is to analyze the isotopic mesoscale variations during organized convective systems such as tropical cyclones or squall lines. The goal of this study is to understand these isotopic mesoscale variations with particular attention to isotopic signals in near‐surface vapor and precipitation that may be present in observations and in paleoclimate proxies. With this aim, we run cloud resolving model simulations in radiative‐convective equilibrium in which rotation or wind shear is added, allowing us to simulate tropical cyclones or squall lines. The simulations capture the robust aspects of mesoscale isotopic variations in observed tropical cyclones and squall lines. We interpret these variations using a simple water budget model for the sub‐cloud layer of different parts of the domain. We find that rain evaporation and rain‐vapor diffusive exchanges are the main drivers of isotopic depletion within tropical cyclones and squall lines. Horizontal advection spreads isotopic anomalies, thus reshaping the mesoscale isotopic pattern. This study contributes to our understanding of mesoscale isotopic variability and provides physical arguments supporting the interpretation of paleoclimate isotopic archives in tropical regions in terms of past cyclonic activity. Plain Language Summary: Naturally available, stable water molecules can be light (one oxygen atom and two hydrogen atoms) or heavy (one hydrogen atom is replaced by a deuterium atom). These different molecules are called water isotopes. In large, long‐lived, severe storms such as tropical cyclones or squall lines (thunderstorms that organize into lines), the rain is observed to be more depleted in heavy isotopes. Several studies have exploited this property to reconstruct the past variations in the frequency of occurrence of tropical cyclones or severe thunderstorms based on isotope variations observed in speleothems. The aim of this study is to understand what controls the depletion in heavy isotopes of the rain in tropical cyclones and squall lines. With this aim, for the first time we use high‐resolution simulations (2–4 km in horizontal) to simulate the internal dynamics of tropical cyclones and squall lines and their isotope composition. We design a simple model to interpret the results. We show that the rain evaporation and rain‐vapor exchanges deplete the water vapor, and the subsequent rainfall, in heavy isotopes. Key Points: Cloud resolving model simulations capture robust aspects of mesoscale isotopic variations observed in tropical cyclones and squall linesRain evaporation and rain‐vapor diffusive exchanges are the main drivers of isotopic depletion within tropical cyclones and squall linesHorizontal advection spreads isotopic anomalies, thus reshaping the stable isotope patterns at the mesoscale [ABSTRACT FROM AUTHOR]

Over half of the total rainfall and more than 70% of heavy and extreme rainfall in the Melbourne, Australia, region occurs on days with linearly organized precipitation. These systems are typically convective in nature and frequently associated with cold fronts. It is useful to understand the processes that support extreme rainfall in organized convection, for prediction of both near-term and future extreme rainfall, and the topography and climate of Melbourne are different from many of the regions where QLCSs have been studied more extensively (e.g., the U. S. Great Plains region). On both 7 and 8 December 2010, a QLCS passed through the Melbourne region. Both QLCSs resembled classic systems on radar, but heavy rainfall was much more widespread on the second day. The goals of this work are to 1) understand the processes that drive these seemingly similar QLCSs; 2) explore the relationship between the convective inflow layer and moisture sources; and 3) to better understand the characteristics of rain bearing systems in the Melbourne region, which have received little attention to date. A convection-permitting WRF-ARW simulation captures both events. The mesoscale structure is different in each case, but generally is explainable by the existing theory. The development of a mesoscale downdraft, along with more moisture (and CAPE) over a deeper layer, contributed to higher rainfall totals on the second day. Low-level moisture in the QLCS region comes from the east, and parcel trajectories become increasingly westerly with height. On the second day some parcels originate in the tropics; these tend to have the most moisture. Significance Statement: A lot of the rain that falls in Melbourne, Australia, occurs in storms that are grouped or "organized" in the shape of a line. Many studies have looked at how lines of storms work in other places in the world. In southeast Australia, only one study in the 1980s looked at observations of a line of storms. Since then, our understanding of storms and our ability to use computer models to simulate them has improved considerably. In this study we simulate two lines of storms that happened two days in a row. We found that even though they looked similar on weather radar, they had many differences including how air flows through the storm, the role of rain-cooled air, and where moisture comes from. [ABSTRACT FROM AUTHOR]

This study examines how organized lines of deep convective storms can be impacted by a large city with a prominent urban heat island and how low‐level environmental vertical wind shear may influence the outcomes of that interaction. Idealized simulations of squall lines are conducted in which a simplified urban area—defined by perturbations to skin temperature and surface roughness length—is placed in the center of an otherwise horizontally homogeneous domain. Simulations are conducted with three different magnitudes of low‐level vertical wind shear representing "weak," "medium," and "strong" shear environments. Results show that storms experience noticeable modification—including enhanced downwind precipitation—after interacting with a prominent urban heat island in all three shear configurations. However, the details of the modification are a function of the shear magnitude. In the medium and strong shear simulations, updrafts are enhanced via increased buoyancy after passing over a prominent urban heat island. In contrast, little updraft strengthening is evident in the weak‐shear simulations. Instead, near‐surface winds are enhanced downwind of the urban heat island due to a more prominent descending rear‐inflow jet. Plain Language Summary: Idealized computer model simulations are conducted in which a line of thunderstorms pass over a simplified representation of a large urban area. Two main characteristics of the urban area (skin temperature excess, and surface roughness length) are systematically altered. The storm environment is also altered by varying the change in wind speed with height, or "vertical wind shear." Results show that the presence of a city can modify the structure of the thunderstorms, but the type of modification varies depending on the vertical wind shear strength. Key Points: Organized lines of convective storms can strengthen after passing over a large city with an urban heat islandHeat island‐induced intensification to updrafts is more prominent in environments with stronger low‐level vertical wind shearStorms in weak‐shear environments experience an enhancement to the rear‐inflow jet after passing over an urban heat island [ABSTRACT FROM AUTHOR]

A West Sumatra squall line occurred on 10 January 2016, with a clear offshore propagation of convection. Satellite-derived products from Himawari-8 Advanced Himawari Imager and the Geostationary Cloud Algorithm Testbed Geocat are used to investigate the westward propagation of cloudiness from Sumatra to the Indian Ocean with a lifetime of 1.5 days. A convective mask based on deep convective cell detection and a cell-tracking algorithm are used to estimate the propagation speed of the cloud system. Two distinct mesoscale convective responses are identified: 1) a rapid development in South Sumatra is influenced by the convective environment over the Indian Ocean. The propagation speed is estimated to be ∼5 m s−1 within the first 200 km from the coast. This speed is consistent with density currents. In contrast, 2) the coupling to the inertia–gravity wave is only evident for the northwest of Sumatra with speeds of ∼12 m s−1. The analysis of brightness temperature from the 10.4-μm spectral band and cloud-top temperature showed that the lifetime of the squall line is approximately 30 h with a propagating distance of ∼1000 km. Retrieved cloud properties and tracking of the offshore propagation indicated that the cloud structure consisted of multiple types of cells, propagating as envelopes of convection, and revealed the influence of large-scale variability of the Indian Ocean. Filtered OLR anomalies, satellite-derived rainfall, moisture flux convergence, and background winds flow around Sumatra are used to explore the effects of Kelvin wave activity that likely influenced the lifetime of the squall line. [ABSTRACT FROM AUTHOR]

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Upper tropospheric outflow is analysed in cloud resolving large eddy simulations. Thereby, the role of convective organisation, latent heating and other factors in upper tropospheric divergent outflow variability from deep convection is diagnosed using a set of about 100 large eddy simulations, because the outflows are thought to be an important feedback from (organised) to large scale atmospheric flows: perturbations in those outflows may sometimes propagate into larger scale perturbations. Upper tropospheric divergence is found to be controlled by net latent heating and convective organisation. At low precipitation rates isolated convective cells have a stronger mass divergence than squall lines. The squall line divergence is the weakest (relative to the net latent heating) when the outflow is purely 2D, in case of an infinite length squall line. At high precipitation rates the mass divergence discrepancy between the various modes of convection reduces. Hence, overall the magnitude of divergent outflow is explained by the latent heating and the dimensionality of the outflow, which together create a non-linear relation. [ABSTRACT FROM AUTHOR]

Abstract Mesoscale convective systems (MCS) contribute about half of the world's precipitation and create flash flooding as well as other extreme weather events. Despite steady progress in research of these systems in the last several decades, better theoretical understanding is still needed to understand their dynamical organizations and to improve their forecasts in numerical models. By using the Moncrieff‐Green horizontal vorticity equation, this paper presents an analytical model of one type of MCS, the steady‐state squall lines. It describes the organization, propagation, and properties of mesoscale circulations of two‐dimensional steady‐state convective systems under sheared environment. Far‐side solutions are formulated to illustrate the underlying physical processes of squall line flows. Numerical procedures are described to solve the model under general environmental conditions. The model leads to the following prediction of squall line properties: Given the environmental profiles of wind and convective available potential energy (CAPE), the propagation speed, the depths and mass fluxes of the tilted ascending front‐to‐rear flow, the overturning updraft, and the descending rear inflow of the squall line can be all determined from the cold pool buoyancy or CAPE. The squall lines are therefore self‐organized dynamical systems that have limited degrees of freedom in their properties. The model advances the theoretical understanding of how mesoscale flow components interact to sustain organized convection. It provides a new tool to interpret squall line systems in high resolution models and to parameterize them in climate models.

This study investigates the characteristics of mesoscale convective systems (MCSs) as a function of MCS organizational mode over China, using long-term precipitation radar observations from the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement Mission (GPM). The spaceborne radar-based MCS climatology shows maximum population over low-elevation regions, marked decrease over the foothills, and minimum frequency over the Tibetan Plateau. Linear and nonlinear MCSs account for 17% and 83% of the MCSs over China, respectively. Linear MCSs have much stronger convective intensity and heavier precipitation than nonlinear MCSs, as indicated by TRMM convective proxies. Interestingly, though broad-stratiform MCSs have the weakest convection, they produce the heaviest (maximum) rain rate and the largest amount of heavy rainfall among nonlinear MCSs. Among various types of linear MCSs, bow echoes (BEs) and no-stratiform (NS) systems exhibit the strongest convective intensity, embedded lines exhibit the weakest, and convective lines with trailing/leading stratiform in between. BEs and NSs share the most vertically extended structure, strongest microwave ice scattering, and highest lightning flash rates, but NS systems have a much lower surface rain rate likely due to a drier environment. Vertical radar reflectivity profiles suggest that both ice-based and warm-rain processes play an important role in the precipitation processes of linear MCSs over China, including the most intense BE storms. In short, this study helps to better understand the convective organization, precipitation structure, and ensemble microphysical properties of MCSs over China, and potentially provides guidelines for evaluating high-resolution model simulations and satellite rainfall retrievals for monsoonal MCSs. [ABSTRACT FROM AUTHOR]

Windthrows are a recurrent disturbance in Amazonia and are an important driver of forest dynamics and carbon storage. In this study, we present for the first time the seasonal and interannual variability of windthrows, focusing on Central Amazonia, and discuss the potential meteorological factors associated with this variability. Landsat images over the 1998-2010 time period were used to detect the occurrence of windthrows, which were identified based on their spectral characteristics and shape. Here, we found that windthrows occurred every year but were more frequent between September and February. Organized convective activity associated with multicell storms embedded in mesoscale convective systems, such as northerly squall lines (that move from northeast to southwest) and southerly squall lines (that move from southwest to northeast) can cause windthrows. We also found that southerly squall lines occurred more frequently than their previously reported ~50 year interval. At the interannual scale, we did not find an association between El Niño-Southern Oscillation (ENSO) and windthrows.

To examine the utility of smartphone pressure observations (SPOs), a climatology of mesoscale pressure features was developed to evaluate whether SPOs could better resolve mesoscale phenomena than existing surface pressure networks (MADIS). A comparison between MADIS and smartphone pressure analyses was performed by tracking and characterizing bandpass-filtered, mesoscale pressure features. Over the year 2018, nearly 3000 pressure features were tracked across the central and eastern United States. Pressure features identified by smartphone observations lasted, on average, 25 min longer, traveled 25 km farther, and exhibited larger amplitudes than features observed by MADIS. An examination of smartphone pressure features tracks by season and location found that almost all pressure features propagated eastward. With over 87% of observed pressure features associated with convection, the climatology of surface pressure features largely reflects the geographic and seasonal variation of mesoscale convection. Phase relationships between pressure features and other surface variables were consistent with those expected for mesohighs and wake lows. These results suggest that SPOs could enhance convective analyses and forecasts compared to existing surface networks like MADIS by better resolving mesoscale structures and features, such as wake lows and mesohighs. Significance Statement: While smartphone pressure networks provide unprecedented observation coverage and density, it was unclear whether they can add value to existing surface pressure networks. This study addresses this question by developing a yearlong record of mesoscale pressure features over the eastern and central United States. Analysis of this record revealed that smartphone analyses better resolved mesoscale pressure features, especially across the central United States where existing surface pressure networks are sparser. Nearly all observed pressure features were observed near precipitation, with five in six associated with convection. Relationships between mesoscale pressure features and other surface state variables were consistent with those expected for mesohighs and wake lows. [ABSTRACT FROM AUTHOR]

A convective system was initiated under weak forcing on the plains near Beijing on the afternoon of 26 June 2009 and developed into a squall line that resulted in heavy precipitation. Prediction of the convective initiation (CI) of the system was challenging due to the lack of precursors detectable by conventional observation networks. In this study, we investigated the CI mechanism using high‐resolution analyses obtained from the Variational Doppler Radar Analysis System via the assimilation of radar and dense surface observations. It was found that the CI was resulted from the interactions of a local cold air mass and the outflows from a weakened storm propagating from the nearby mountains. The cold air mass built up an unstable condition and veered air flow to form a local convergence zone in which moisture was accumulated. New convection was triggered as a result of strengthened convergence when the outflows from the dissipating storm approached the existing convergence zone. To confirm the critical role played by the local cold air mass, sensitivity experiments were conducted using WRF‐FDDA data assimilation and forecast system. The results suggested that the local cold air mass along with the southerly environmental wind and storm outflows exerted a dominant influence on the CI. Key Points: A squall line was initiated on the plains near Beijing under weak forcing, and developed into that resulted in heavy precipitationHigh‐resolution reanalysis and nudging systems show the local‐scale thermal and dynamical features associated with the convective initiation (CI)Interactions of a local cold air mass and the outflows from a weakened storm to trigger CI under weak forcing [ABSTRACT FROM AUTHOR]

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that culminated with the development of numerous long-track, violent tornadoes over the southeastern United States during the afternoon of 27 April. This notorious afternoon supercell outbreak was preceded by two quasi-linear convective systems (hereinafter QLCS1 and QLCS2), the first of which was an anomalously severe nocturnal system that rapidly grew upscale during the previous evening. Here in Part II, we use a series of RUC 1-h forecasts and output from convection-permitting WRF-ARW simulations configured both with and without latent heat release to investigate how environmental modifications and upscale feedbacks produced by the two QLCSs contributed to the evolution and exceptional severity of this multiepisode outbreak. QLCS1 was primarily responsible for amplifying the large-scale flow pattern, inducing two upper-level jet streaks, and promoting secondary surface cyclogenesis downstream from the primary baroclinic system. Upper-level divergence markedly increased after QLCS1 developed, which yielded strong isallobaric forcing that rapidly strengthened the low-level jet (LLJ) and vertical wind shear over the warm sector and contributed to the system's upscale growth and notable severity. Moreover, QLCS2 modified the mesoscale environment prior to the supercell outbreak by promoting the downstream formation of a pronounced upper-level jet streak, altering the midlevel jet structure, and furthering the development of a highly ageostrophic LLJ over the Southeast. Collectively, the flow modifications produced by both QLCSs contributed to the notably favorable shear profiles present during the afternoon supercell outbreak. Significance Statement: The tornado outbreak that impacted the United States on 26–27 April 2011 was part of an extended outbreak that produced 343 tornadoes and numerous fatalities. This paper is Part II of a study that describes the meteorological factors supporting such a prolific event. Herein we investigate the convectively forced environmental modifications that occurred during a 36-h period encompassing three successive convective episodes. The first two episodes collectively altered the upper-level flow pattern and markedly enhanced low-level winds throughout the warm sector. These modifications served as upscale feedbacks that contributed to the first episode's exceptional severity and to the remarkable vertical shear profiles that supported numerous long-track and violent tornadoes during the final episode on the afternoon of 27 April. [ABSTRACT FROM AUTHOR]

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that primarily impacted the southeastern United States, including two quasi-linear convective systems (hereinafter QLCS1 and QLCS2) that preceded the notorious outbreak of long-track, violent tornadoes spawned by numerous supercells on the afternoon of 27 April. The ∼36-h period encompassing these three episodes was part of a longer multiday outbreak that occurred ahead of a slowly moving upper-level trough over the Rocky Mountains. Here in Part I, we detail how the environment evolved to support this extended outbreak, with particular attention given to the three successive systems that each exhibited a different morphology and severity. The amplifying upper-level trough and attendant jet streak resulted from a Rossby wave–breaking event that yielded a complex tropopause structure and supported three prominent shortwave troughs that sequentially moved into the south-central United States. QLCS1 formed ahead of the second shortwave and was accompanied by rapid flow modifications, including considerable low-level jet (LLJ) intensification. The third shortwave moved into the lee of the Rockies early on 27 April to yield destabilization behind QLCS1 and support the formation of QLCS2, which was followed by further LLJ intensification and helped to establish favorable deep-layer shear profiles over the warm sector. The afternoon supercell outbreak commenced following the movement of this shortwave into the Mississippi Valley, which was attended by a deep tropopause fold, cold front aloft, and dryline that promoted two prominent bands of tornadic supercells over the Southeast. Significance Statement: The tornado outbreak that impacted the United States on 26–27 April 2011 was part of an extended outbreak that produced 343 tornadoes and numerous fatalities. This paper is Part I of a study that describes the meteorological factors supporting such a prolific event. We focus on a 36-h period encompassing three convective episodes that impacted the Southeast and culminated in the widespread formation of long-track, violent tornadoes. Flow enhancements accompanying the first two episodes and the movement of a cold front aloft into the Southeast were found to contribute to the evolution of the final and most devastating episode. Part II describes how the first two episodes directly modified the environment to enhance the severity of this outbreak. [ABSTRACT FROM AUTHOR]

We explore whether and how the land characteristics, especially surface drag, affect the squall line, in an idealized modeling framework. We find that the balance between the ambient low‐level shear and cold‐pool‐induced shear in the squall line can be modified by surface drag: it not only changes the ambient shear if near‐surface environmental wind is nonzero, but also reduces the cold‐pool‐induced shear by both weakening the low‐level wind and deepening the cold pool. The land surface impact on squall lines is larger with lower convective available potential energy (CAPE), partly because of stronger convection with higher CAPE. The cold pool intensity (vertically integrated negative buoyancy) could not explain the reduced cold pool shear induced by the land. Furthermore, land characteristics—such as roughness lengths—help maintain the shear balance and potentially make the squall line long‐lived, suggesting that they be included in convective (or cold pool) parameterizations to improve climate and Earth system models. Key Points: Surface drag affects squall lines by weakening near‐surface wind and deepening the cold poolThe widely used cold pool intensity no longer characterizes the cold‐pool‐induced shear when surface drag is includedDynamical effects of the land surface are missed in thermodynamic‐based cold pool parameterizations [ABSTRACT FROM AUTHOR]

Squall lines are known to be the consequence of the interaction of low‐level shear with cold pools associated with convective downdrafts. Also, as the magnitude of the shear increases beyond a critical shear, squall lines tend to orient themselves. The existing literature suggests that this orientation reduces incoming wind shear to the squall line, and maintains equilibrium between wind shear and cold pool spreading. Although this theory is widely accepted, very few quantitative studies have been conducted on supercritical regime especially. Here, we test this hypothesis with tropical squall lines obtained by imposing a vertical wind shear in cloud resolving simulations in radiative convective equilibrium. In the sub‐critical regime, squall lines are perpendicular to the shear. In the super‐critical regime, their orientation maintain the equilibrium, supporting existing theories. We also find that as shear increases, cold pools become more intense. However, this intensification has little impact on squall line orientation. Plain Language Summary: A squall line is a line of thunderstorms associated with heavy precipitation and only a well‐informed meteorologist would notice that these convective bands are sometimes oriented with respect to the wind direction. Yet this is the case, and in this study we seek to understand what sets tropical squall lines orientation, and why. Using numerical simulations, we show quantitatively that the orientation of the squall lines restores the equilibrium between the wind and the cold pool spreading, providing for the first time a quantitative validation of existing theories. Key Points: In the subcritical regime, squall lines are perpendicular to the wind shear, until equilibrium between wind shear and cold pool spreadingIn the supercritical regime, the orientation of the squall lines reduces the incoming wind shear, and maintains the equilibriumChanging properties of cold pools with background shear, notably their deepening, has little effect on the squall line orientation [ABSTRACT FROM AUTHOR]

The severity, duration, and spatial extent of thunderstorm impacts are related to convective storm mode. This study assesses the skill of the High-Resolution Rapid Refresh Ensemble (HRRR-E) and its deterministic counterpart (HRRRv4) at predicting convective mode and storm macrophysical properties using 35 convective events observed during the 2020 warm season across the eastern United States. Seven cases were selected from each of five subjectively determined convective organization modes: tropical cyclones, mesoscale convective systems (MCSs), quasi-linear convective systems, clusters, and cellular convection. These storm events were assessed using an object-based approach to identify convective storms and determine their individual size. Averaged across all 35 cases, both the HRRR-E and HRRRv4 predicted storm areas were generally larger than observed, with this bias being a function of storm lifetime and convective mode. Both modeling systems also underpredicted the rapid increase in storm counts during the initiation period, particularly for the smaller-scale storm modes. Interestingly, performance of the HRRRv4 differed from that of the HRRR-E, with the HRRRv4 generally having a larger bias in total storm area than the HRRR-E due to HRRRv4 predicting up to 66% more storm objects than the HRRR-E. The HRRR-E accurately predicted the convective mode 65% of the time, with complete misses being very rare (<5% of the time overall). However, an evaluation of rank histograms across all 35 cases revealed that the HRRR-E tended to be underdispersive when predicting storm size for all but the MCS mode. [ABSTRACT FROM AUTHOR]

Mesoscale convective cloud clusters develop and organize in the form of squall lines along the coastal Amazon in the afternoon hours and propagate inland during the evening hours. The frequency, location, organization into lines, and movement of the convective systems are determined by analyzing the "precipitation features" obtained from the TRMM satellite for the period 1998–2014. The convective clusters and their alignments into Amazon coastal squall lines are more frequent from December to July, and they mostly stay within 170 km of the coastline. Their development and movement in the afternoon and evening hours of about 14 m s−1 are helped by the sea breeze. Negative phase of Atlantic dipole and La Niña combined increase the frequency of convective clusters over the coastal Amazon. Composite environmental conditions of 13 large Amazon coastal squall-line cases in April show that conditional instability increases from 0900 to 1200 LT and the wind profiles show a jet-like structure at low levels of the atmosphere. The differences in the vertical profiles of temperature and humidity between the large-squall-line composites and no-squall-line composites are weak. However, appreciable increase in the mean value of CAPE from 0900 to 1500 LT is found in the large-squall-line composite. The mean mixing ratio of the mixed layer at 0900 LT in La Niña situations is significantly larger in the large-squall-line composite. Thus, CAPE and mixed-layer mixing ratio are considered to be promising indicators of the convective activity over the coastal belt of the Amazon basin. [ABSTRACT FROM AUTHOR]

We analyse how the representation of deep convection affects the characteristics of convective precipitation over tropical Africa in global storm-resolving simulations with the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). The simulations consist in 48 hr forecasts initiated daily at 0000 UTC for a 40-day period in August and September 2016, at ∼9 and 4 km resolution. We find that results agree best with the satellite retrieval Global Precipitation Measurement (GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG) when a term for the integrated and scaled total advective moisture tendency is added to the convective instability closure. This allows for a better coupling of the convective parametrization to the mesoscale dynamics and improves precipitation intensity (increased), precipitating area fraction (decreased), the size of individual deep convective systems (decreased) and their propagation (westward), even though the duration of rain events remains underestimated. With explicit deep convection, the duration of rain events is more realistic, but precipitation intensity is significantly overestimated, and extreme rain events (>400mm⋅day-1) are three times more likely than in GPM IMERG, causing biases in the organization of deep convection. In particular, individual convective systems are too small, hindering the formation of mesoscale convective systems. These findings are also valid for two other regions: the Maritime Continent and the contiguous United States. Our results suggest that deep convection is not completely resolved at a resolution of 9 or even 4 km, and that some scale-aware parametrization is still needed for global numerical weather prediction. [ABSTRACT FROM AUTHOR]

A mechanism for precipitation enhancement in squall lines moving over mountainous coastal regions is quantified through idealized numerical simulations. Storm intensity and precipitation peak over the sloping terrain as storms descend from an elevated plateau toward the coastline and encounter the marine atmospheric boundary layer (MABL). Storms are most intense as they encounter the deepest MABLs. As the descending storm outflow collides with a moving MABL (sea breeze), surface and low-level air parcels initially accelerate upward, though their ultimate trajectory is governed by the magnitude of the negative nonhydrostatic inertial pressure perturbation behind the cold pool leading edge. For shallow MABLs, the baroclinic gradient across the gust front generates large horizontal vorticity, a low-level negative pressure perturbation, and thus a downward acceleration of air parcels following their initial ascent. A deep MABL reduces the baroclinically generated vorticity, leading to a weaker pressure perturbation and minimal downward acceleration, allowing air to accelerate into a storm's updraft. Once storms move away from the terrain base and over the full depth of the MABLs, storms over the deepest MABLs decay most rapidly, while those over the shallowest MABLs initially intensify. Though elevated ascent exists above all MABLs, the deepest MABLs substantially reduce the depth of the high-θe layer above the MABLs and limit instability. This relationship is insensitive to MABL temperature, even though surface-based ascent is present for the less cold MABLs, the MABL thermal deficit is smaller, and convective available potential energy (CAPE) is higher. [ABSTRACT FROM AUTHOR]

In the lee of major mountain chains worldwide, diurnal physics of organized propagating convection project onto seasonal and climate time scales of the hydrologic cycle, but this phenomenon is not represented in conventional global climate models (GCMs). Analysis of an experimental version of the superparameterized (SP) Community Atmosphere Model (CAM) demonstrates that propagating orogenic nocturnal convection in the central U.S. warm season is, however, representable in GCMs that use the embedded explicit convection model approach [i.e., multiscale modeling frameworks (MMFs)]. SP-CAM admits propagating organized convective systems in the lee of the Rockies during synoptic conditions similar to those that generate mesoscale convective systems in nature. The simulated convective systems exhibit spatial scales, phase speeds, and propagation speeds comparable to radar observations, and the genesis mechanism in the model agrees qualitatively with established conceptual models. Convective heating and condensate structures are examined on both resolved scales in SP-CAM, and coherently propagating cloud “metastructures” are shown to transcend individual cloud-resolving model arrays. In reconciling how this new mode of diurnal convective variability is admitted in SP-CAM despite the severe idealizations in the cloud-resolving model configuration, an updated discussion is presented of what physics may transcend the re-engineered scale interface in MMFs. The authors suggest that the improved diurnal propagation physics in SP-CAM are mediated by large-scale first-baroclinic gravity wave interactions with a prognostic organization life cycle, emphasizing the physical importance of preserving “memory” at the inner resolved scale.

In this study, we estimate bulk entrainment rates for deep convection in convection-permitting simulations, conducted over the tropical Atlantic Ocean, encompassing parts of Africa and South America. We find that, even though entrainment rates decrease with height in all regions, they are, when averaging between 600 and 800 hPa, generally higher over land than over ocean. This is so because, over Amazonia, shallow convection causes an increase of bulk entrainment rates at lower levels and because, over West Africa, where entrainment rates are highest, convection is organized in squall lines. These squall lines are associated with strong mesoscale convergence, causing more intense updrafts and stronger turbulence generation in the vicinity of updrafts, increasing the entrainment rates. With the exception of West Africa, entrainment rates differ less across regions than across different environments within the regions. In contrast to what is usually assumed in convective parameterizations, entrainment rates increase with environmental humidity. Furthermore, over ocean, they increase with static stability, while over land, they decrease. In addition, confirming the results of a recent idealized study, entrainment rates increase with convective aggregation, except in regions dominated by squall lines, like over West Africa. [ABSTRACT FROM AUTHOR]

The purpose of this study is to examine the influences of mesoscale thermal and dynamical processes near land‐sea boundary on the formation and development of a squall line across the Hangzhou Bay (HZB) of East China Sea that resulted in strong surface winds and heavy rainfall. High‐resolution spatiotemporal analyses obtained from Variational Doppler Radar Analysis System via the assimilation of radar and surface observations are used for this study. We focus on meso‐scale factors leading to the initiation of the squall line and the development of a bow echo that caused damaging winds and heavy rainfall. It was found that a local‐scale sea breeze associated with the distinct coastline curvature of the HZB was pivotal to the formation of the squall line. The convective initiation occurred near the HZB's western coastline when the leading edge of the northeasterly sea breeze was met by a westerly outflow from a previous precipitation system to the west of HZB. After the formation of the squall line, a rear inflow jet descending from mid‐level to surface contributed to the formation of the bow echo and resulted in damaging local winds. Key Points: High‐resolution spatiotemporal analyses provided local‐scale flow features for the study of a squall line crossing Hangzhou BayInteraction of a sea breeze and an outflow from a preexisting convective system was pivotal to the formation of the squall lineDescent rear inflow jet contributed to the formation of a bow echo resulting in strong local wind [ABSTRACT FROM AUTHOR]

The Storm Prediction Center (SPC) has developed a database of damage-surveyed tornadoes in the contiguous United States (2009–17) that relates environmental and radar-derived storm attributes to damage ratings that change during a tornado life cycle. Damage indicators (DIs), and the associated wind speed estimates from tornado damage surveys compiled in the Damage Assessment Toolkit (DAT) dataset, were linked to the nearest manual calculations of 0.5° tilt angle maximum rotational velocity Vrot from single-site WSR-88D data. For each radar scan, the maximum wind speed from the highest-rated DI, Vrot, and the significant tornado parameter (STP) from the SPC hourly objective mesoscale analysis archive were recorded and analyzed. Results from examining Vrot and STP data indicate an increasing conditional probability for higher-rated DIs (i.e., EF-scale wind speed estimate) as both STP and Vrot increase. This work suggests that tornadic wind speed exceedance probabilities can be estimated in real time, on a scan-by-scan basis, via Vrot and STP for ongoing tornadoes. [ABSTRACT FROM AUTHOR]

Despite their structural differences, supercells and quasi-linear convective systems (QLCS) are both capable of producing severe weather, including tornadoes. Previous research has highlighted multiple potential mechanisms by which horizontal vorticity may be reoriented into the vertical at low levels, but it is not clear in which situation what mechanism dominates. In this study, we use the CM1 model to simulate three different storm modes, each of which developed relatively large near-surface vertical vorticity. Using forward-integrated parcel trajectories, we analyze vorticity budgets and demonstrate that there seems to be a common mechanism for maintaining the near-surface vortices across storm structures. The parcels do not acquire vertical vorticity until they reach the base of the vortices. The vertical vorticity results from vigorous upward tilting of horizontal vorticity and simultaneous vertical stretching. While the parcels analyzed in our simulations do have a history of descent, they do not acquire appreciable vertical vorticity during their descent. Rather, during the analysis period relatively large horizontal vorticity develops as a result of horizontal stretching, and therefore this vorticity can be effectively tilted into the vertical. [ABSTRACT FROM AUTHOR]

This paper is the first to document the development of a type of bow echo, termed merger‐formation bow echo (MFBE), in southeast China evolving from a subtropical squall line (SL) merging with a preline convective cell (CC). Although this MFBE did not produce damaging surface winds, its intense rain rate resulted in local flooding. The evolution of the kinematic, thermodynamic, and microphysical structures is investigated using the variational Doppler radar analysis system (VDRAS) analysis and polarimetric radar observations. Key factors of this MFBE event including the rear‐inflow jet (RIJ) and cold pool exhibited different characteristics from those in classical bow echoes and limited MFBE cases. As the SL propagated towards the coast, a CC was triggered along a sea breeze front. The SL did possess a RIJ, but a bow‐shaped reflectivity did not appear until the SL‐CC merger. The RIJ weakened and became elevated during the merger, while the leading edge of SL cold pool accompanied by a weak diverging outflow did not advance with the reflectivity field. The updraft was strengthened due to the merger, resulting in enhanced precipitation falling ahead of the original SL. The subcloud evaporation locally cooled the air ahead of the SL and merged with the original SL cold pool. This combined cold pool advanced forward rapidly and caught up with the bowing radar reflectivity to form this MFBE. This study illustrates the processes of a SL‐CC merger leading to the formation of a different type of MFBE that did not produce damaging surface winds. Key Points: The first VDRAS analysis combines polarimetric radar to document microphysical and dynamic processes of a merger‐formation bow echo in ChinaRear‐inflow jet weakens during the formation of this nonclassical bow echoEvaporative cooling from intensified rainfall leads to the surge of cold pool [ABSTRACT FROM AUTHOR]

Sumatra squalls are important rain-bearing weather systems that affect Singapore and southern Peninsular Malaysia. The performance of forecasts for 63 past squall events is evaluated using a subjective evaluation by forecasters and an objective evaluation based on the fractions skill score (FSS). The purpose of this study is to investigate whether an objective procedure can reproduce the main results of the subjective evaluation. A convection permitting version of the Met Office (UKMO) Unified Model (UM), configured for a limited domain in the southern region of the South China Sea, is used with two driving global deterministic models: the UM and the European Centre for Medium-Range Weather Forecasts (ECMWF) model. Subjective and objective evaluation scoring methods for the two limited-area forecasts of the UM are compared, and it is shown that the objective procedure can reasonably emulate the scores produced by the forecasters in the context of parameters that are of direct relevance to the forecast process. This indicates that automated objective verification methods may be a reasonable alternative to resource intensive subjective evaluations for some cases. The robustness of the objective results is investigated using 7 months of data, and issues of statistical significance are considered. [ABSTRACT FROM AUTHOR]

A modified Fast Spectral Bin Microphysics scheme (FSBM‐2) embedded into the Weather Research and Forecasting (WRF) model is used to simulate a mesoscale deep convective system observed during the Midlatitude Continental Convective Clouds Experiment (MC3E). FSBM‐2 uses modified source codes as compared to the current FSBM (FSBM‐1). In contrast to FSBM‐1, FSBM‐2 can simulate hail of several centimeters in diameter and includes additional processes such as spontaneous breakup of raindrops and aerosol regeneration by drop evaporation. It is shown that allowing large hail particles of diameters exceeding about 1 cm substantially increases the agreement between the simulated and observed squall‐line structures in both the convective and stratiform regions. In contrast, if graupel particles are used to represent high‐density hydrometeors in convective areas, the ratio of convective‐to‐stratiform areas diverges from the ratio seen in observations and maximum radar reflectivities are substantially underestimated. Analysis of snow size distributions in the stratiform area shows an important link between the ice crystals formed by homogeneous freezing in the convective area to ice particle number concentration in the stratiform region. Simulated raindrop size distributions in the stratiform area below the melting level from FSBM‐2 show a good agreement with observations. The regeneration of cloud condensational nuclei (CCN) by droplet evaporation and detrainment of these CCN to the stratiform region increases the concentration of CCN there up to 8‐ to 9‐km altitude; the additional CCN penetrate clouds and produce new droplets, leading to some convection intensification. Key Points: A modified Fast Spectral Bin Microphysics scheme (FSBM‐2) embedded into the Weather Research and Forecasting Model (WRF) is developed and used to simulate a mesoscale deep convective system MC3EAllowing large hail particles substantially increases the agreement between the simulated and observed squall‐line structures in both the convective and stratiform regionsThere is an important link between the ice crystals formed by homogeneous freezing in the convective area to ice particle number concentration in the stratiform region Plain Language Summary: The new bin‐microphysical scheme included into Weather Research and Forecasting (WRF) allows simulation of hail storms with hail with diameters up to 6 cm, as well as large snowflakes. The model is tested by simulating a well‐measured squall line observed in the Midlatitude Continental Convective Clouds Experiment (MC3E). It is shown that allowing large hail and snowflake particles substantially increases the agreement between the simulated and observed squall‐line structures in both the convective and stratiform regions. It is also shown that concentration of ice particles in the stratiform region is closely related to huge ice crystal concentration at the upper levels formed by homogeneous freezing of droplets, which in turn are formed by nucleation of smallest aerosols. So, the microphysical structure of squall lines depends of the existence of small aerosol particles in the aerosol size distribution. This is a new result showing close relationship between the structure of Mesoscale Deep Convective Systems and aerosols. [ABSTRACT FROM AUTHOR]

Accurate representation of cloud vertical overlap in climate models is particularly significant for predicting the total cloud fraction (TCF) and calculating radiative budget. It refers to the parameterization of overlap parameter—decorrelation length scale L—but the potential of dynamical factors in developing parameterization of L has still received far less attention. Using ground‐based radar observation over Atmospheric Radiation Measurement Southern Great Plains site, here long‐term seasonal‐averaged L is retrieved and shows a very high anticorrelation with TCF from different data sets, indicating that TCF is sensitive to the way of cloud overlap. Therefore, combined with meteorological reanalysis data set, a robust multiple regression model between L and dynamical factors is built and exhibits smaller TCF bias compared with previous parameterization of L. Contribution calculation further verifies that atmospheric instability contributes 70% of L variation, indicating that it dominates the long‐term variation of L over Southern Great Plains site. This finding implies that dynamical factors should be taken into account in the parameterization of L. Plain Language Summary: The total cloud fraction plays a key role in modulating the Earth's radiation budget. Its calculation in atmospheric models involves many microphysical/dynamical processes and related parameterizations. One of the outstanding but less concerned challenges involves how to reasonably parameterize the cloud overlap parameter—decorrelation length scale L in these models. In this study, we demonstrate the potential of dynamical variables in developing the adjustable parameterization of L. By using the ground‐based radar observations and meteorological reanalysis data set, the long‐term climatology of L and a robust multiple regression model between L and dynamical factors are built. Statistical results indicate that new parameterization of L effectively reduces the bias of total cloud fraction compared with previous one. Such a long‐term climatology of L is also enough to interpret the key issue of what factor dominates the long‐term temporal variability of L over a fixed location. Here, we find that the atmospheric instability contributes 70% of L variation over SGP site. This finding implies that dynamical factors should be considered in the parameterization of L. Key Points: Cloud overlap parameterization used to calculate the total cloud fraction (TCF) in climate models has still received far less attentionDynamical factors are proven able to develop overlap parameterization and reduce the TCF bias over SGP site compared with previous oneContribution calculation verifies that atmospheric instability contributes 70% of long‐term variation of cloud overlap over SGP site [ABSTRACT FROM AUTHOR]

A comparison between retrieved properties of the rain drop size distributions (DSDs) from multifrequency cloud radar observations and WRF Model results using either the Morrison or the Thompson bulk microphysics scheme is performed in order to evaluate the model's ability to predict the rain microphysics. This comparison reveals discrepancies in the vertical profile of the rain DSDs for the stratiform region of the squall-line system observed on 12 June 2011 over Oklahoma. Based on numerical sensitivity analyses, this study addresses the bias at the top of the rain layer and the vertical evolution of the DSD properties (i.e., of Dm and N 0 * ). In this way, the Thompson scheme is used to explore the sensitivity to the melting process. Moreover, using the Thompson and Morrison schemes, the sensitivity of the DSD vertical evolution to different breakup and self-collection parameterizations is studied. Results show that the DSDs are strongly dependent on the representation of the melting process in the Thompson scheme. In the Morrison scheme, the simulations with more efficient breakup reproduce the DSD properties with better fidelity. This study highlights how the inaccuracies in simulated Dm and N 0 * for both microphysics schemes can impact the evaporation rate, which is systematically underestimated in the model. [ABSTRACT FROM AUTHOR]

Abstract The main objective of this work is to better understand the increase of ozone ( O 3 ) in the surface by the influence of density currents formed by downdrafts from mesoscale convective systems, using as tool the JULES-CCATT-BRAMS model. Initially, the superficial increases of O 3 were investigated with the ECMWF Era-Interim reanalysis that showed a plume rich in O 3 located at middle troposphere along with the increase of this gas' concentration in the surface during the rainy period in Central Amazon. GOES satellite images and surface synoptic charts showed the formation of a squall line (SL) in the interior of the Continent by the influence of a Frontal System that reached the southeast of South America in this period. The numerical simulation results provided an understanding of the three-dimensional structure of the chemistry and thermodynamics of the atmosphere during the passage of this SL at dawn on April 14, 2014. The downdrafts from SL bring cooler, drier, dense air to near the surface. This downdrafts air column passed by the O 3 "plume" in the middle troposphere, formed rich gas density currents and low level jets were induced near the surface and spread the O 3 . Finally, this cleaner air column decreased the surface carbon monoxide (CO) levels, and as a result of the increase in O 3 concentration resulted in an increase in surface nitrogen dioxide ( NO 2 ). Highlights • Continental squall line change structure of the lower troposphere. • Downdrafts induced the formation of density currents. • Density currents spread out in the form of low-level jets. • Low-level jets can transport gaseous chemical compounds over large distances. [ABSTRACT FROM AUTHOR]

This study documents atmospheric conditions, development, and evolution of a severe weather outbreak that occurred on 11 August 2017 in Poland. The emphasis is on analyzing system morphology and highlighting the importance of a mesovortex in producing the most significant wind damages. A derecho-producing mesoscale convective system (MCS) had a remarkable intensity and was one of the most impactful convective storms in the history of Poland. It destroyed and partially damaged 79 700 ha of forest (9.8 million m3 of wood), 6 people lost their lives, and 58 were injured. The MCS developed in an environment of high 0–3-km wind shear (20–25 m s−1), strong 0–3-km storm relative helicity (200–600 m2 s−2), moderate most-unstable convective available potential energy (1000–2500 J kg−1), and high precipitable water (40–46 mm). Within the support of a midtropospheric jet, the MCS moved northeast with a simultaneous northeastward inflow of warm and very moist air, which contributed to strong downdrafts. A mesocyclone embedded in the convective line induced the rear inflow jet (RIJ) to descend and develop a bow echo. In the mature stage, a supercell evolved into a bookend vortex and later into a mesoscale convective vortex. Swaths of the most significant wind damage followed the aforementioned vortex features. A high-resolution simulation performed with initial conditions derived from GFS and ECMWF global models predicted the possibility of a linear MCS with widespread damaging wind gusts and embedded supercells. Simulations highlighted the importance of cloud cover in the preconvective environment, which influenced the placement and propagation of the resulting MCS. [ABSTRACT FROM AUTHOR]

Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells. [ABSTRACT FROM AUTHOR]

Abstract A new lightning data assimilation (LDA) scheme comprehensively nudging water contents in the WRF model is developed at cloud-resolving scale, which takes the dynamical and thermodynamic conditions into consideration and nudges the low-level water vapor and graupel mass within the mixed-phase region according to the detected total lightning flash rate and model environments (here named C18). The main nudging functions of the LDA scheme in this work are modified based on the work of Fierro et al. (2012, hereafter F12) and Qie et al. (2014, hereafter Q14). To evaluate the application of LDA in high-impact weather simulations and quantitative precipitation forecasts, the three LDA schemes, including F12, Q14 and C18, are tested and compared for two severe squall line cases occurred over the Beijing metropolitan region (BMR). Benefiting from assimilation of lightning data, the simulated storm structure and surface cold pool are improved and closer to the observations for all the three schemes than the control run without LDA. For the F12 scheme, although the simulated thunderstorms are characterized by wide spread stratiform clouds and relatively weaker cold pool intensity and coverage, the positive impacts on both storm evolution and precipitation endures most prominently. For the Q14 scheme, intense downdrafts lead to the strongest cold pool and earlier dissipation of storms comparing with the observations. For the C18 scheme, increased graupel mass affect the cold-cloud processes leading to an earlier onset of rainfall, and the simulated surface cold pool is in better agreement with the observations. Profiting from more moisture added into the lower layers in C18, the storm develops more intensively toward higher altitude and sustains its development over a longer period after LDA, resulting in the best quantitative precipitation forecasts consequently. Highlights • A total lightning data assimilation (LDA) scheme was developed that adjusts comprehensively both water vapor and ice-phase particles. • The new LDA scheme was compared with two existed LDA methods, and two severe squall line cases were simulated to illustrate the positive impacts from LDA and different features of each LDA scheme. • The new scheme improved surface cold pool simulations and the positive impacts from LDA endured longer in the subsequent storm evolution, which eventually led to increased capacity of quantitative precipitation forecasts. [ABSTRACT FROM AUTHOR]

An elevated convection initiation (CI) of a quasi-linear mesoscale convective system (MCS) that occurred in a weak-lifting environment in the early morning on 23 June 2016 in central-eastern China was investigated using observational analysis and convection-permitting numerical simulations. This MCS gradually developed into a surface-based MCS and eventually produced a strong supercell that spawned an EF4 tornado in Yancheng City of Jiangsu Province and killed 98 people. This elevated MCS was initiated ahead of a surface front without identifiable boundaries at the surface. An elevated moist absolutely unstable layer (MAUL) was found to be conducive to the CI. The MAUL provided negligible convective inhibition and contributed to CI without strong-lifting mechanisms. Numerical simulation results showed that the formation of the elevated MAUL was mainly attributed to adiabatic cooling by weak vertical ascent and sufficient horizontal moisture transport near the terminus of a low-level jet. The weak vertical ascent before the CI was sloping and was likely to be relevant to the layer-lifting process associated with the realization of potential instability. The results showed that the MAUL in this weak-lifting environment was characterized by a shallower depth, a weaker lapse rate, and a longer sustaining period than the conditions in a strong-lifting environment. The predictability of this elevated CI case was examined using a 10-member ensemble forecast. A total of 80% of the ensemble members captured the CI. Rather than a difference in lifting, whether having an elevated MAUL or not was the major difference between CI and non-CI members in the present case. [ABSTRACT FROM AUTHOR]

Extreme mesoscale weather, including tropical cyclones, squall lines, and floods, can be enormously damaging and yet challenging to simulate; hence, there is a pressing need for more efficient simulation strategies. Here, we present a new rare event sampling algorithm called quantile diffusion Monte Carlo (quantile DMC). Quantile DMC is a simple-to-use algorithm that can sample extreme tail behavior for a wide class of processes. We demonstrate the advantages of quantile DMC compared to other sampling methods and discuss practical aspects of implementing quantile DMC. To test the feasibility of quantile DMC for extreme mesoscale weather, we sample extremely intense realizations of two historical tropical cyclones, 2010 Hurricane Earl and 2015 Hurricane Joaquin. Our results demonstrate quantile DMC's potential to provide low-variance extreme weather statistics while highlighting the work that is necessary for quantile DMC to attain greater efficiency in future applications. [ABSTRACT FROM AUTHOR]

Dual-Doppler radar observations of two cold-season, wave-propagating quasi-linear convective systems (QLCS), which evolved in high-shear, low-CAPE (HSLC) environments, are analyzed to determine the role that horizontal shearing instability (HSI) plays in the formation of mesovortices. One QLCS occurred on 4 January 2015 and produced two mesovortices within the dual-Doppler region, one of which was associated with an EF-1 tornado with a pathlength of 10 km. The second QLCS occurred on 28 November 2016 and did not produce any mesovortices. Storm characteristics such as the low-level wind speed and wind shift angle are investigated. Rayleigh and Fjørtoft instability criteria, which are required but insufficient for HSI, are also examined. The Rayleigh and Fjørtoft instability criteria are satisfied for the 4 January 2015 QLCS and the 28 November 2016 QLCS, highlighting one of the issues of the "required, but insufficient" characteristic of the criteria. Analysis of the wind shift angle and wind speed agree with previous studies that pronounced wind shifts close to 90° and strong wind speeds were conducive to the formation of mesovortices, while weak wind shift angles and weaker wind speeds were not. It was found that for the 4 January 2015 case, HSI was the likely formation mechanism of the vortices as other features associated with preexisting mesovortexgenesis theories were not observed. [ABSTRACT FROM AUTHOR]