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In this paper, the characteristics of a squall line that occurred in central and northern Zhejiang on 21 July 2022 are analyzed by using S-band dual-polarization radar, X-band phased array radar data, wind retrieval, lightning location and ERA5 reanalysis data. The results show that: The squall line affected the area for up to 6 hours, causing large-scale winds of 8-10 levels along the way. The pressure surge ahead of the strong winds and precipitation at the extremely strong wind station served as a precursor to extreme winds. The squall line occurred under the control of the subtropical high. The dry and cold air at the bottom of the high-level cold vortex invaded, with the high-energy and high humidity in the low-level forming an unstable layer structure, which provided favourable environmental conditions for the generation and maintenance of the squall line. The observation of the S-band dual polarisation radar showed that the evolution of squall lines went through processes such as initiation, merging, growth, maturity, and disappearance. The initial convection was formed in the mesoscale convergence lines in the northwest and east of Hangzhou. Strong storms in the northwest of Hangzhou moved faster than those in the east, leading to the rapid merging and development of the two into squall lines. After the squall line entered the south bank of the Hangzhou Bay, the storm outflow overlapped with the mesoscale convergence centre on the south bank of the Hangzhou Bay, leading to the rapid development of the squall line to its mature stage. In addition, the growth of vertical wind shear was also conducive to the rapid development of the squall line. Strong winds often occurred in areas with dense cloud flashes. The X-band phased array radar could finely detect the vertical structure of strong cells before and after extreme winds occurred. The development of ZDR and KDP columns in mature strong cells represented storm intensification, and the rapid decline of the echo centroid could indicate surface winds. The falling and dragging of precipitation particles intensified the sinking motion, which further enhanced the strong winds. The configuration of inclined upward and downward airflow in strong squall lines ensured that the falling of precipitation particles did not affect the upward airflow, which was beneficial for the development and maintenance of storms. The oblique sinking airflow met the near-surface environmental wind, triggering the development of frontal convection, which was conducive to the propagation of squall lines downstream.

Deciphering the physical origin of transient crustal deformation is a difficult challenge in the field of continuous borehole observations of crustal movement because the relevant influencing factors of the phenomenon are extremely multifarious. In the Shanxi Rift, squall lines are a frequently occurring mesoscale convective system. Consequently, strain transients are always observed by the borehole dilatometer network within the Shanxi Rift; however, a fundamental understanding of the transient phenomena induced by squall lines remains elusive. To address this challenge, we adopted a second-order Butterworth band-pass filters (0.5–2 h) to retrieve the borehole volumetric deformations excited by a "dry" squall line from September 21, 2017, that occurred in the Shanxi Rift. The elastic loading of Ricker-wavelet-shaped atmospheric waves induced by squall line, which can cause the main signature of transient deformation to adopt a highly similar shape; this pressure-induced deformation process can last approximately 148–164 min and has a maximum magnitude of approximately 19 nstrain. This work of signal processing clearly identifies the crustal transient induced by a squall line and provides new insights into the origin of some short-lasting crustal deformations in the Shanxi Rift and elsewhere. [ABSTRACT FROM AUTHOR]

In the present study, a severe squall line (SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front (SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of lowlevel mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation. [ABSTRACT FROM AUTHOR]

Squall lines often produce extremely disastrous gales and play a crucial role in the safety of hytropower stations.This article explores a squall line event over the lower reaches of Jinsha River on June 4， 2016， which brought 13-level wind at the canyon of Baihetan Hytropower Station.This research detect the formation mechanism of this squall line from environmental conditions and medium-scale meteorology.The following conclusions are obtained： （1） Before the squall line arrived， the ground pressure dropped steadily by 9 hPa because of the low pressure， and the temperature of the canyon rises up above 14 ℃.As the squall line arrived， the pressure rises sharply with thunderstorm high pressure， and the temperature dropped more than 10 ℃.At the same time， the relative humidity reached as higher as 70%， accompanied by short-term precipitation.After the squall line left， the aforementioned meteorological elements restored to their original values.（2） Analysis on the synoptic scale circulation， we finds that Baihetan hytropower station located in the south of southwest vortex， which induced a wind convergence background at the low level.Meanwhile， the station lied in right side of the jet stream inlet at upper level， and strong vertical motion formed by the wind shear.As a result， the squall line to come into being.Furthermore， in the troposphere before the squall line， the atmospheric circulation displayed a dry-cold advection in the middle-upper level superimposed over a warm-wet advection and higher ground temperature in the lower levels， increasing the temperature lapse rate.t As a result， the strong static instability and convection accumulated， and the conditions were favorable for the squall line to live a long time.The analyses from vertical wind speed showed that the subsidence movement made the momentum spread down， and promoted the uplift in the front side of line， and a positive feedback effect developed， promoting a stable mechanism for the strong storm.（3） Radar of Zhaotong detection indicates that echo cell that greater than 30 dBZ aggregated costly into a band and embed with the strong echo cores of greater than 50 dBZ， as a cylindrical convective cloud of higher than 10 km.On the radical velocity field， the meso-scale convergence line coinciding with the banded strong echo， as well as MARC （Mid-Altitude Radial Convergence） characteristics of the mid-altitude radial convergence， were the trigger mechanism for the squall line.As the cloud structure of the storm， strong upward movement in front of the squall line leaded to the dome.As the squall line arrived， the echo core dropped rapidly， and the extreme gale of downburst formed， resulted the new convective monomers come in being along the canyon.

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The multi-cell storm stage and squall line stage of a severe convective weather process over the Dongting Lake on 15 May 2021 in this study are analyzed mainly based on S-band dual-polarized doppler weather radar data. The study focuses on the analysis of the supercell storm I2 in the multi-cell storm stage. The results are as follows. (1) During the initial period of the I2, the strong echo area (the horizontal reflectivity factor, ZH > 55 dBz) and the differential reflectivity factor (ZDR) column (ZDR > 2.5 dB) extend to Wet Blub Zero (WBZ) level, corresponding to the areas with large specific differential phase (KDP) (> 1.7 °·km-1) and large correlation coefficient (CC) (0.9-0.99). It indicates that during this period, the precipitation phase is dominated by water condensate mingled with large raindrops, and these supercooled raindrops provide hail embryos for the later hail development. (2) During the hail colliding-growth period of the I2, ZH intensity and height increase rapidly, and vertically integrated liquid water content (VIL) shows an obvious leap increase. The strong-echo center (ZH > 60 dBz) extends to the level of above -10 ℃. In the corresponding regions, the ZDR drops below 0, the ZDR column (ZDR> 2.5 dB) extends to the level of -10 ℃, CC drops, and a "hole" appears in the KDP value area. These indicate that the precipitation during this period is dominated by solid particles and these solid particles are in an increasing period. (3) During the mature hailfall period of the I2, the bottom of the ZH intensity center (ZH > 60 dBz) drops below the WBZ level and the CC is locally as low as 0.8. There are ZDR negative areas and KDP voids in the corresponding area. These means that the dragging effect of the falling hail further weakens the strength of the updraft, which means that the hail is about to land. Then comes the squall line stage. (4) Unlike the multi-cell storm stage, the squall line stage has anomalously large KDP values and ZDR values greater than 1. (5) Before the occurrence of extreme gales of the squall line stage, the areas with strong echo greater than 55 dBz and velocity larger than 27 m·s-1 extend below 1 km, and the KDP in the strong echo area is obviously large, corresponding to the strong downdraft. Besides, the strong precipitation dragging effect caused by the melting of descending precipitation particles intensifies the generation of extreme gales.

During the spring and summer seasons, in the South China region where abundant water vapor is present, squall lines can rapidly develop into larger scales within a short period of time. In order to explore the influence of water vapor content on the process of squall line scale growth in South China, using the WRF model, a numerical simulation was conducted for a squall line system in South China on 11 May 2020. We investigated the effect of the variation of water vapor at different levels on its intensity and structure, and discussed the growth mechanism of the squall line system. This squall line occurred with the presence of high-level jet and low-level wind shear complementing each other, within an unstable layer of "dry at the high level and wet at the low level". The simulation showed that, in the early stage of the squall line development, a large maximum convective available potential energy (MCAPE) was observed in the southern part of the convection and coastal warm areas, which is beneficial for the accumulation of unstable energy here. Meanwhile, with the high low-level wind shear, the linear structure of the convection was well maintained. Subsequently, the squall line propagated southward and merged with warm region convection, resulting a further scale growth. Water vapor experiments showed that the MCAPE values are primarily influenced by the moisture content in the low-level atmosphere. More low-level moisture content causes stronger thunderstorm high pressure. Additionally, the presence of higher MCAPE values and larger low-level vertical wind shear contribute to the growth of convective cells in the post-convective stage, prolonging their existence. Reducing the mid-level water vapor content results in a decrease in intense surface precipitation, a weakening of convective intensity, and a quick dissipation into individual convective cells. But when the squall line moves into the area with high MCAPE values, it once again develops into a linear structure. Therefore, an increase in low-level moisture or a decrease in mid-level moisture favors the genesis of convection. However, reducing mid-level moisture results in relatively drier air at mid-levels, making it difficult for the linear structure to be sustained. Further investigation into the internal structure of the squall line reveals that vertical motion and rear inflow also influence the scale growth of the squall line. In the convective system analyzed in this study, the strong rearward inflow enhances the upward motion and generates forward outflow, leading to severe surface wind. Strengthening low-level moisture not only increases the size of the stratiform cloud region at the rear of the convection, but also leads to a stronger upward motion sustaining vertically, which promotes prolonged convective activity. On the other hand, reducing mid-level moisture weakens convective intensity and lowers the height of the echo tops. During the development stage, the rearward inflow intensifies, and dry cold air descends rapidly, leading to the strengthening of the surface cold pool, and causing strong winds due to the forward outflow.

Secondary ice productions (SIPs) can produce ice crystals with a number concentration much higher than that of ice nucleating particles in mixed-phase clouds and therefore influence cloud glaciation and precipitation. For midlatitude continental mesoscale convective systems (MCSs), how SIPs affect the microphysical properties and precipitation is still not clear. There are few studies of SIPs in midlatitude continental MCSs. This study investigates the roles of three SIPs (rime splintering, freezing drop shattering, and ice-ice collisional breakup) on a squall line case in North China on 18 August 2020 using the WRF model with a modified Morrison double-moment bulk microphysical scheme. Including SIPs, especially ice-ice collisional breakup, in the model simulations markedly improves the simulated convective area and convective precipitation rate of the squall line, while slightly improving the area and precipitation of the stratiform region. Within the mixed-phase layer in both the convective and stratiform regions of the squall line, ice-ice collisional breakup is the dominant process to generate ice crystals. In contrast, rime splintering generates an order of magnitude fewer ice crystals than ice-ice collisional breakup, while freezing drop shattering plays a negligible role due to the lack of large drops. Ice multiplication through ice-ice collisional breakup and rime splintering produces numerous snowflakes and graupel. This leads to enhanced depositional growth and weaker riming, which in turn weakens rime splintering. It is recommended to add SIP parameterization to the model. [ABSTRACT FROM AUTHOR]

Using the high-resolution numerical weather research and forecasting (WRF) model, study the squall line process that occurred on Hainan Island on 22 April 2020. The findings indicate that high terrain blocks the swift accumulation of water vapor carried by the sea breeze and aids in preserving the accumulated water vapor. According to the sensitivity experiment, terrain height has minimal impact on the macroscopic effects of mesoscale weather processes. However, it does influence where the sea breeze converges. During this process, the ocean-land thermal contrast not only takes the main responsibility for the sea breeze but also leads to uplift motion, which affects the formation, intensity, and duration of the squall line. Additionally, the unstable conditions suggest that a thermal and dynamic environment promote the scale of this squall line. Utilizing the Rotunno–Klemp–Weisman theory (RKW), this study analyzes the effects of the cold pool and vertical wind shear. The analysis reveals that significant vertical wind shear at lower levels and the ground-cold pool contribute to the sustenance and growth of the squall line system. This squall line process has had the greatest impact on the Haikou area due to the strong low-level vertical wind shear and prolonged interaction with the cold pool. When the interaction between the cold pool and the vertical wind shear weakens, the squall dissipates. [ABSTRACT FROM AUTHOR]

Based on three-dimensional lightning data and an S-band Doppler radar, a strong relationship was identified between lightning activity and the radar volume of squall lines. A detailed analysis of the squall line investigates the relationship following an exponential relationship. According to the correlation between lightning and the radar volume, three radar-volume-based lightning parameterization schemes, named the V30dBZ, V35dBZ, and V40dBZ lightning schemes, have been established and introduced into the weather research and forecasting (WRF) model. The performance of lightning precondition by different lightning parameterization schemes was evaluated, including the radar-volume-based schemes (V30dBZ, V35dBZ, and V40dBZ), as well as existing lightning schemes (PR92_1, PR92_2, and the Lightning Potential Index (LPI)). The evaluation shows that the simulated spatial lightning density and temporal lightning frequency by the radar-volume-based lightning schemes are more consistent with the observations. While the two PR_92 lightning schemes significantly underestimated the magnitude of lightning density. The radar-volume-based lightning parameterization schemes are proven to be more reliable in estimating lightning activity than other lightning schemes. [ABSTRACT FROM AUTHOR]

Highlights: A deep learning dataset of squall lines with over 49,920 samples was constructed based on RADAR-base data by means of manual classification and data augment. Three squall lines automatic recognition modes are trained according to the distance of label data away from RADARs. The models have good generalization ability which can effectively capture the characteristics of squall lines from RADAR-base data to realize its automatic recognition well. Squall line (SL) is a convective weather process that often causes disasters. The automatic recognition and early warning of SL are important objectives in the field of meteorology. By collecting the new-generation weather RADARs (CINRAD/SA and CINRAD/SAD) base data during 12 SL weather events occurred in Jiangsu, Shanghai, Shandong, Hebei, and other regions of China from 2019 to 2021, the dataset has a total of 49,920 samples with a window size of 40 km. The 40 km area was labeled by employing manual classification and data augmentation to construct the deep learning dataset with a positive and negative sample ratio of 1:1, of which 80% and 20% are separated as the training and test set, respectively. Based on the echo height of each elevation beam at different distances, three deep learning-based models are trained for SL automatic recognition, which include a near-distance model (M1) trained by the data in nine RADAR elevation angles within 45 km from RADARs, a mid-distance model (M2) by the data in six elevations from 45 to 135 km, and a far-distance model (M3) by the data in three elevations from 135 to 230 km. A confusion matrix and its derived metrics including receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) are introduced as the indicators to evaluate the models by the test dataset. The results indicate that the accuracy of models are over 86% with the hit rates over 87%, the false alarm rates less than 21%, and the critical success indexes (CSI) surpass 78%. All the optimal critical points on the ROC curves are close to (0, 1), and the AUC values are above 0.95, so the three models have high hit rates and low false alarm rates for ensuring SL discrimination. Finally, the effectiveness of the models is further demonstrated through two SL events detected with Nanjing, Yancheng and Qingpu RADARs. [ABSTRACT FROM AUTHOR]

A long-lived squall line system caused strong winds and heavy rainfall in the central and northern parts of Hainan Island on April 22, 2020. The present study analyzed the evolution of the squall line system by comparing conventional meteorological data with dual-polarization radar detection data. The results indicated that the squall line occurred on the northwestern edge of the subtropical high-pressure zone, with a quasi-stationary front controlling the ground. Mainly affected by the warm and humid southwest airflow, the intensity of cold air was generally weak. The ground convergence line and suitable thermal conditions acted as environmental triggers. Throughout the evolution of the squall line, the radar combined reflectivity consistently maintained a strong value with large areas exceeding 55 dBZ. However, the differential reflectance (ZDR) of the cloud system exhibited a large area exceeding 5 dB and extended up to 8-9 km vertically during the initial formation of the squall line (16:20-16:43). During the squall line process, the ZDR over Haikou and Wenchang stations peaked approximately 12 minutes before ground precipitation and decreased as precipitation increased. The KDP was closely correlated with precipitation intensity. Additionally, the frequency of ground lightning increased in the early stage of precipitation, peaking approximately 6 to 12 minutes before ZDR and about 24 to 48 minutes before KDP. [ABSTRACT FROM AUTHOR]

The effect of radar data assimilation under different horizontal resolutions on the numerical simulation of a squall line that occurred in Chongqing on 5 June 2019 was studied in this paper. The Weather Research and Forecasting (WRF) model and the three-dimensional variational assimilation (3DVAR) system of the advanced regional prediction system (ARPS) and its data assimilation system (ADAS) were used. Our major conclusions are as follows: (1) Without radar data assimilation, the simulation results are still not significantly improved with the horizontal resolution increasing from 900 m to 300 m. (2) In comparison, with radar assimilation, the simulations under different horizontal resolutions have a certain improvement in the shape, intensity and falling area of the radar echo. (3) Combining an adequate higher resolution with the initial radar data assimilation can further optimize and adjust the dynamic thermal water vapor conditions of the simulation, making the simulated characteristics of evolution process and organizational structure of this squall line closer to the observations. Therefore, improving the horizontal resolution and using radar data assimilation at the same time might significantly improve the model prediction capability.

The short-lived tropical squall lines could trigger weather-related hazards to the northern part of the Indonesia Maritime Continent (IMC), such as Sumatra and Kalimantan. Herein, we investigated the rare propagation event of the long-lived Sumatra squall line associated with a severe storm surge that induced coastal inundation in Java-Bali with devastating impacts from 22 May–2 June 2020. With a comprehensive approach combining observational, numerical, and analytical studies, for the first time, we proposed the possible mechanism related to the long-lived squall line over the IMC, which represents the largest equatorial tropical region with the most complicated air-sea interaction area in the world. Our findings suggest that the long-lived squall line related to the supercell-like thunderstorm initiated from multicell over central Sumatra on May 20, 2020, continuously propagated southeastward until several days later reached Bali. The near-quasi steady convective line has 6 hours of time travel from central Sumatra to west Java. The supercell-like rapidly develops from multicell with a deep convective updraft under the strong and fast cold pool (∼13.8 m s−1). The further southeastward propagation of squall line with broken line type seems reinforced by low-level moist transport from the Java Sea. This study also suggested that this unusual event of a long-lived squall line might occur more frequently in the warming upper ocean in the IMC.

Based on several kinds of observational data and numerical simulation, the mesoscale features of a strong squall line on March 19, 2013, in southern China were investigated. The results show that: (1) The squall line occurred in front of a 500-hPa trough and was near a cold front on the ground. The ambient wind was mainly southwestern, and its cross-line component was smaller than the along-line component. (2) There were significant differences between the east and the west of the squall line. The eastern part was dry, and the DCAPE was large; as a result, short-term heavy rainfall was weaker than that in the western part, while thunderstorm gales were stronger. Convective cells in the eastern part initially distributed in a fragmented manner at the cold side of the frontal zone. The western part was humid, with short-term heavy rainfall and concentrated hail. Convective cells appeared within the mesoscale convergence line near the surface front area, and the convergence line was persistent. There are constantly new cells growing at the convergence center above them. (3) The gravity waves affected the maintenance of the squall line, and their effect in the western part was obviously stronger than that in the eastern part. When the updraft of the jet intersected with the convergent line, the lower-level updraft at the intersection strengthened and led to convective initiation (CI). Finally, the squall line formed along the amplitude of the gravity waves. (4) New convective cells emerged in both the north and the south of older cells. The upper and lower gravity waves of those new cells in the north were out of phase, restraining the convection; however, they were in phase in the southern new cells, which positively contributed to CI. New cells continued to emerge and replaced old ones, and the squall line propagated to the southeast. [ABSTRACT FROM AUTHOR]

On 30 June 2013, 19 Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire, driven by erratic winds, blocked their escape route and overran their location. A previous study is extended to simulate and analyze the downscale organization of convective circulations that redirected the wildfire, which started from the scale of the Rossby Wave Breaking over North America to a convective gust front that redirected the wildfire, trapping the firefighters. Five stages are found: Stage I, the initial deep prolonged gust front; Stage II, a front-to-rear jet and its ascending motions that organized high-based convection; Stage III, high-based dry microburst-induced downdrafts organized initially by ascending flow in Stage II that transported mass and entropy to the surface; Stage IV; multiple meso-γ-scale high centers and confluence zones formed that encompassed the firefighters' location, which established a favorable environment leading to Stage V, canyon-scale circulations formed surrounding the fire. The atmosphere thus transitioned from supporting a deep and long-lived convective density current to elevated dry microbursts with mass and wind outflow into a canyon, redirecting the ongoing wildfire. [ABSTRACT FROM AUTHOR]

Dynamical physical processes associated with an onshore moving marine atmospheric boundary layer (MABL, i.e., sea breeze) over sloping terrain, sensitivity of these processes to MABL characteristics, and flow modifications induced by an offshore‐moving squall line are investigated using idealized simulations. The moving MABL gradually advances inland, exhibiting farther advancement and greater upslope wind speed for deeper and cooler MABLs. The local acceleration is primarily driven by a MABL‐generated perturbation pressure gradient force (PPGF). As the moving MABL air accumulates onshore over time, an opposing force associated with the increasing negative buoyancy eventually balances the PPGF and results in a quasi‐steady upslope flow. The approaching squall line disrupts this flow in two distinct ways; Initially the storm's cold pool enhances the ambient downslope winds which diminishes the upslope wind speeds, and subsequently the storm‐generated high‐frequency waves and the associated surface pressure low enhances the upslope‐directed PPGF which reintensifies the upslope flows. Plain Language Summary: This study uses numerical simulations to investigate the physical processes driving sea breezes over mountainous coastal regions, the sensitivity of the sea breeze evolution to its density and depth, and changes to the sea breeze by approaching organized deep convective storms (i.e., squall lines). Deeper and cooler marine atmospheric boundary layers move farther inland and have stronger wind speeds. The onshore upslope sea breeze is driven by an onshore‐directed pressure gradient force (PGF) induced by the denser marine air. However, as the marine air accumulates onshore over time, its negative buoyancy drives an opposing offshore downslope force that helps to prevent any additional inland advancement of the sea breeze front. An offshore‐moving squall line influences this flow in two ways; Squall line outflow initially weakens the sea breeze but then enhances the sea breeze by decreasing the surface pressure ahead of the sea breeze front through storm‐induced gravity waves. Key Points: Upslope flow is intensified by a moving marine atmospheric boundary layer (MABL), with its features relying on a MABL's depth and densityUpslope acceleration is enhanced by the MABL‐induced perturbation pressure gradient force but is reduced over time by the negative buoyancyThe passage of storm‐generated high‐frequency gravity waves and the associated surface pressure lows temporarily enhance the upslope flow [ABSTRACT FROM AUTHOR]

At the end of June 2020, an explosive extratropical cyclone was responsible for an environment in which a squall line developed and caused life and economic losses in Santa Catarina state, southern Brazil. The aims of this case study are the following: (a) to describe the drivers of the cyclogenesis; (b) to investigate through numerical simulations the contribution of sea–air interaction to the development of the cyclone as an explosive system; and (c) to present the physical properties of the clouds associated with the squall line. The cyclogenesis started at 1200 UTC on 30 June 2020 on the border of southern Brazil and Uruguay, having a trough at middle-upper levels as a forcing, which is a common driver of cyclogenesis in the studied region. In addition, the cyclone's lifecycle followed Bjerknes and Solberg's conceptual model of cyclone development. A special feature of this cyclone was its fast deepening, reaching the explosive status 12 h after its genesis. A comparison between numerical experiments with sensible and latent turbulent heat fluxes switched on and off showed that the sea–air interaction (turbulent heat fluxes) contributed to the cyclone's deepening leading it to the explosive status. The cold front, which is a component of the cyclone, favored the development of a pre-frontal squall line, responsible for the rough weather conditions in Santa Catarina state. While satellite images do not clearly show the squall line located ahead of the cold front in the cyclone wave due to their coarse resolution, radar reflectivity data represent the propagation of the squall line over southern Brazil. On 30 June 2020, the clouds in the squall line had more than 10 km of vertical extension and a reflectivity higher than 40 dBZ in some parts of the storm; this is an indicator of hail and, consequently, is a required condition for storm electrification. In fact, electrical activity was registered on this day. [ABSTRACT FROM AUTHOR]

Abstract Dynamical physical processes associated with an onshore moving marine atmospheric boundary layer (MABL, i.e., sea breeze) over sloping terrain, sensitivity of these processes to MABL characteristics, and flow modifications induced by an offshore‐moving squall line are investigated using idealized simulations. The moving MABL gradually advances inland, exhibiting farther advancement and greater upslope wind speed for deeper and cooler MABLs. The local acceleration is primarily driven by a MABL‐generated perturbation pressure gradient force (PPGF). As the moving MABL air accumulates onshore over time, an opposing force associated with the increasing negative buoyancy eventually balances the PPGF and results in a quasi‐steady upslope flow. The approaching squall line disrupts this flow in two distinct ways; Initially the storm's cold pool enhances the ambient downslope winds which diminishes the upslope wind speeds, and subsequently the storm‐generated high‐frequency waves and the associated surface pressure low enhances the upslope‐directed PPGF which reintensifies the upslope flows.

On June 30 and July 1, 2020, the southern region of Brazil was the scene of an explosive cyclogenesis that directly impacted society, causing, material, economic and social damage and human fatalities. In the present work, the objective is to characterize the genesis of the absolute maximum gusts (=20 m/s) that occurred during that extreme meteorological event. From this, it was possible to differentiate two main groups with distinct gusts: one of convective origin and the other, nonconvective. In addition to the different genesis, the geographic coverage and the period in which the absolute maximum gusts occurred in each group, attest that although affected by the extratropical cyclone, the relative position to it was preponderant. A cold front system caused a squall line that was responsible for the absolute maximum gusts belonging to the first group, which, given the wide-ranging conditions (from the southern portion of the Paraná State to the north of the Rio Grande do Sul), could be treated by using the meteorological term derecho. In the second group, that is, nonconvective gusts, the intense pressure gradient given by the deepening of the extratropical (explosive) cyclone over the ocean, conditioned absolute maximum gusts in the coastal strip, especially in Rio Grande do Sul State. [ABSTRACT FROM AUTHOR]

Secondary ice productions (SIPs) can produce ice crystals with a number concentration much higher than that of ice nucleating particles in mixed-phase clouds and therefore influence cloud glaciation and precipitation. For midlatitude continental mesoscale convective systems (MCSs), how SIPs affect the microphysical properties and precipitation is still not clear. There are few studies of SIPs in midlatitude continental MCSs. This study investigates the roles of three SIPs (rime splintering, freezing drop shattering, and ice-ice collisional breakup) on a squall line case in North China on 18 August 2020 using the WRF model with a modified Morrison double-moment bulk microphysical scheme. Including SIPs, especially ice-ice collisional breakup, in the model simulations markedly improves the simulated convective area and convective precipitation rate of the squall line, while slightly improving the area and precipitation of the stratiform region. Within the mixed-phase layer in both the convective and stratiform regions of the squall line, ice-ice collisional breakup is the dominant process to generate ice crystals. In contrast, rime splintering generates an order of magnitude fewer ice crystals than ice-ice collisional breakup, while freezing drop shattering plays a negligible role due to the lack of large drops. Ice multiplication through ice-ice collisional breakup and rime splintering produces numerous snowflakes and graupel. This leads to enhanced depositional growth and weaker riming, which in turn weakens rime splintering. It is recommended to add SIP parameterization to the model.

Using the high-resolution numerical weather research and forecasting (WRF) model, study the squall line process that occurred on Hainan Island on 22 April 2020. The findings indicate that high terrain blocks the swift accumulation of water vapor carried by the sea breeze and aids in preserving the accumulated water vapor. According to the sensitivity experiment, terrain height has minimal impact on the macroscopic effects of mesoscale weather processes. However, it does influence where the sea breeze converges. During this process, the ocean-land thermal contrast not only takes the main responsibility for the sea breeze but also leads to uplift motion, which affects the formation, intensity, and duration of the squall line. Additionally, the unstable conditions suggest that a thermal and dynamic environment promote the scale of this squall line. Utilizing the Rotunno–Klemp–Weisman theory (RKW), this study analyzes the effects of the cold pool and vertical wind shear. The analysis reveals that significant vertical wind shear at lower levels and the ground-cold pool contribute to the sustenance and growth of the squall line system. This squall line process has had the greatest impact on the Haikou area due to the strong low-level vertical wind shear and prolonged interaction with the cold pool. When the interaction between the cold pool and the vertical wind shear weakens, the squall dissipates.

Based on three-dimensional lightning data and an S-band Doppler radar, a strong relationship was identified between lightning activity and the radar volume of squall lines. A detailed analysis of the squall line investigates the relationship following an exponential relationship. According to the correlation between lightning and the radar volume, three radar-volume-based lightning parameterization schemes, named the V30dBZ, V35dBZ, and V40dBZ lightning schemes, have been established and introduced into the weather research and forecasting (WRF) model. The performance of lightning precondition by different lightning parameterization schemes was evaluated, including the radar-volume-based schemes (V30dBZ, V35dBZ, and V40dBZ), as well as existing lightning schemes (PR92_1, PR92_2, and the Lightning Potential Index (LPI)). The evaluation shows that the simulated spatial lightning density and temporal lightning frequency by the radar-volume-based lightning schemes are more consistent with the observations. While the two PR_92 lightning schemes significantly underestimated the magnitude of lightning density. The radar-volume-based lightning parameterization schemes are proven to be more reliable in estimating lightning activity than other lightning schemes.

Squall line (SL) is a convective weather process that often causes disasters. The automatic recognition and early warning of SL are important objectives in the field of meteorology. By collecting the new-generation weather RADARs (CINRAD/SA and CINRAD/SAD) base data during 12 SL weather events occurred in Jiangsu, Shanghai, Shandong, Hebei, and other regions of China from 2019 to 2021, the dataset has a total of 49,920 samples with a window size of 40 km. The 40 km area was labeled by employing manual classification and data augmentation to construct the deep learning dataset with a positive and negative sample ratio of 1:1, of which 80% and 20% are separated as the training and test set, respectively. Based on the echo height of each elevation beam at different distances, three deep learning-based models are trained for SL automatic recognition, which include a near-distance model (M1) trained by the data in nine RADAR elevation angles within 45 km from RADARs, a mid-distance model (M2) by the data in six elevations from 45 to 135 km, and a far-distance model (M3) by the data in three elevations from 135 to 230 km. A confusion matrix and its derived metrics including receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) are introduced as the indicators to evaluate the models by the test dataset. The results indicate that the accuracy of models are over 86% with the hit rates over 87%, the false alarm rates less than 21%, and the critical success indexes (CSI) surpass 78%. All the optimal critical points on the ROC curves are close to (0, 1), and the AUC values are above 0.95, so the three models have high hit rates and low false alarm rates for ensuring SL discrimination. Finally, the effectiveness of the models is further demonstrated through two SL events detected with Nanjing, Yancheng and Qingpu RADARs.

A squall line event in northern Jiangsu in midsummer is simulated and analyzed by adopting regional WRF model with triple nesting, and the conceptual model diagram in the radial section of squall line is given. The main results are as follows. The simulated and actual squall lines are very similar, they both have the same nature and characteristics. Thus, it is feasible to determine the location and intensity of squall lines using the range and intensity of the simulated linear strong precipitation zone. There are areas of strong convergence, strong ascending motion and high value of pseudo equivalent temperature in the squall line during its mature stage. These three all have the columnar structure and vertical extent. There is an inflow of high value of pseudo equivalent potential temperature in the lower layer in front of the squall line (the moving direction of squall line), which brings a large amount of water vapor and energy to the squall line. There is a shallow inflow in the lower layer behind. The direction of surface wind changes sharply when the squall line passes through. The potential temperature is almost constant and appears as a neutral stratification in the middle layer of squall line, which is related to the release of convective condensation latent heat. The squall line event can be regarded as pseudo adiabatic process and has the unbalanced property of gravity wave. Thus, there are multi-scale interactions in its generation and evolution.

In this study, multi‐source data, especially weather radar data, were used in the analysis of structural features and evolution of a squall line‐like rainband (SLLR) of a linear mesoscale convective system (MCS) in the eastern slope of the Taihang Mountains on 12 and 13 August 2018 and reached the following conclusions: (a) The SLLR, located in north of the linear MCS, exhibited a significant horizontal gradient of reflectivity and large vertical extension. The front‐to‐rear flow that appeared between the surface and the stratosphere was lifted upward immediately in front of a cold pool that formed rearward‐tilting updrafts. (b) As the SLLR passed over, surface meteorological quantities changed drastically, coinciding with the heavy precipitation. (c) The SLLR, when propagating northeastward, showed a complex convective development of sequential intensification, weakening, and re‐intensification within a 2.5‐hr period. The temporal intensification was related not only to the development of a low‐level rear‐inflow jet but also to the dynamic interaction between the vertical environmental wind shear and the cold pool. The lifted rear‐inflow jet, which formed over a vertically stacked horizontal vorticity couplet behind the leading edge, provides a reasonable explanation for the intensification of the SLLR, resulting in the reduction of the cold pool circulation and enhancement of the convective updraft. Topographic forcing might play a crucial role in the re‐intensification of the SLLR, which suggests the importance of the cold pool and topography in structural features and convective evolution of the squall line‐like rain bands. Key Points: The squall line‐like rainband (SLLR) in a linear mesoscale convective system exhibited the characteristics of the typical squall lineThe SLLR was associated with a complex convective development of sequential intensification, weakening, and re‐intensificationThe cold pool and topography play crucial roles in structural features and convective evolution of the SLLR [ABSTRACT FROM AUTHOR]

Squall line often leads to heavy rain, gale and hail, which is a difficult key problem in nowcasting. In order to explore the feasibility of deep learning for squall line identification, the training, validation and test set sample sets are established based on the radar data of Zhengzhou and Zhumadian in Henan Province during 2008-2020. The convolutional neural network (CNN) algorithm is used to construct a squall line identification model. The critical success index (CSI), equitable threat score (ETS), hit rate (POD) and false positive rate (FAR) are used to quantitatively evaluate the identification effect of the model. The influence of different sample composition and network structure on squall line identification effect are compared. The results show that the composition ratio of sample is imbalanced, because squall line accounts for very small proportion in all kinds of weather processes. This imbalance will degrade the classification performance of the identification model to squall line samples. The imbalance of sample composition can be improved by changing sampling mode and optimizing network structure, both can improve the identification efficiency, especially the latter. However, the combination of the two methods does not bring further improvement. The over fitting problem in network training can be alleviated by increasing the sparsity and randomness of the network structure. The validation set shows that CSI is 0.87, ETS is 0.82, POD is 0.96, and FAR is 0.10. Based on the test set, the echo can be correctly identified by network as non-squall line in the weak stage of convection development, and as squall line in the strong stage of squall line development. The echo intensity and spatial distribution of squall line cases differ greatly, and the samples in the test set have the image features which are not included in the training set, and therefore the identification effect reduces. The test set show that CSI is 0.66, ETS is 0.58, POD is 0.86, and FAR is 0.24. The research reveals that CNN can extract and learn the image features of squall line echo, and it has a certain ability to identify squall line.

Abstract In this study, multi‐source data, especially weather radar data, were used in the analysis of structural features and evolution of a squall line‐like rainband (SLLR) of a linear mesoscale convective system (MCS) in the eastern slope of the Taihang Mountains on 12 and 13 August 2018 and reached the following conclusions: (a) The SLLR, located in north of the linear MCS, exhibited a significant horizontal gradient of reflectivity and large vertical extension. The front‐to‐rear flow that appeared between the surface and the stratosphere was lifted upward immediately in front of a cold pool that formed rearward‐tilting updrafts. (b) As the SLLR passed over, surface meteorological quantities changed drastically, coinciding with the heavy precipitation. (c) The SLLR, when propagating northeastward, showed a complex convective development of sequential intensification, weakening, and re‐intensification within a 2.5‐hr period. The temporal intensification was related not only to the development of a low‐level rear‐inflow jet but also to the dynamic interaction between the vertical environmental wind shear and the cold pool. The lifted rear‐inflow jet, which formed over a vertically stacked horizontal vorticity couplet behind the leading edge, provides a reasonable explanation for the intensification of the SLLR, resulting in the reduction of the cold pool circulation and enhancement of the convective updraft. Topographic forcing might play a crucial role in the re‐intensification of the SLLR, which suggests the importance of the cold pool and topography in structural features and convective evolution of the squall line‐like rain bands.

Squall line is a kind of common mesoscale disaster weather. At present, there are few studies on the elaborate detection of squall line by dual polarization radar. With the dual polarization upgrade of weather radar network, we need to study the relationship between squall line echoes of base data and polarization data to reveal new echo phenomena and formation mechanisms. The relationship between radar parameters and atmospheric physical processes also need to be examined. Based on the NUIST CDP radar, a squall line in the Yangtze and Huaihe River basin that occurred from July 30 to 31, 2014 is analyzed. The results show that polarization parameters have obvious advantages in the characteristics analysis of size, phase state, shape and orientation of the water condensate particles. The phase states of water condensate particles in convection cell can be distinguished through comparative discussion. Several phase states exist in the squall line, including small, medium and large raindrops, melting hails, dry hails and ice crystal particles and the ZDR column can be used to identify the location of the main updraft. In addition, the polarization parameters are more sensitive to the melting layer. The gust front is presented as a narrow linear echo in Z affected by strong turbulence. It is an obvious velocity convergence line in V and approximately 0.70 in ρHV. The ZDR can be used as a criterion to distinguish the horizontal and vertical scale of turbulence. The deforming turbulence, which is affected by environmental airflow, will cause an abnormally high ZDR in the gust front and a negative ZDR before and after the gust front. The variation of ZDR depends on the turbulence arrangement, orientation and relative position between turbulence and radar. These dual polarization parameter characteristics offer insights into understanding the structure and evolution of the squall line. [ABSTRACT FROM AUTHOR]

A major challenge in numerical weather prediction models is the ability to accurately simulate the microphysical properties and growth of ice hydrometeors in clouds. Eulerian bulk microphysics schemes in these models tend to obscure the properties and evolution of individual ice crystals, often resulting in inaccurate simulations of storm structures. To address this issue, this study presents a novel ice crystal trajectory growth (ICTG) model that simultaneously grows and advects individual ice crystals while tracking their evolving properties along their trajectories. The model is evaluated on a 3D quasi‐idealized leading‐convective, trailing‐stratiform squall line simulation. The ICTG model successfully produced a spatial distribution of ice crystal trajectories consistent with the simulated reflectivity structure of the storm above the melting level. Smaller initialized crystals (d ≤ 0.1 mm) were largely transported to the anvil and the trailing stratiform region. One primary trajectory involved sustained growth in the stratiform mesoscale updraft for ∼1.5 hr, resulting in a density reduction down to 600 kg m−3, a final particle size greater than 0.9 mm, and potential branching. In contrast, larger initialized crystals (d ≥ 0.5 mm) collected more rime and fell out primarily in the leading convective line. The ICTG model's realistic production of varied crystal growth properties owing to differences in transport and initial size suggests that it can be a valuable tool for learning about ice microphysical processes in a variety of cold cloud systems. Plain Language Summary: Numerical weather prediction models are often challenged by their ability to accurately simulate the structure and evolution of ice in clouds. The specific trajectories that ice crystals follow in a cloud are known to determine the growth evolution of the crystals, which can greatly impact which regions of a cloud the crystals are transported to. This study develops and evaluates a novel ice crystal trajectory growth model that simultaneously grows and transports ice crystals according to the spatially varying temperature, humidity, and wind in a simulated thunderstorm. The model tracks the growth evolution of individual ice crystals along their trajectories, which traditional precipitation models are not capable of. The model was able to produce a crystal trajectory pattern that was consistent with the precipitation structure of the storm. Furthermore, the model successfully produced differences in evolving crystal properties based on the crystal's initial size and its transport through the varied growth conditions throughout the storm. This finding is important as it demonstrates the model's consistency with past observational studies and its ability to produce realistic results, which lends to the model's future use to study the ice crystal growth trajectories in other storm types. Key Points: The ice crystal trajectory growth model tracks the evolution of individual crystals using laboratory‐based algorithms for crystal growthThe novel model successfully produced particle trajectories consistent with the structure of a simulated squall line and past observationsVariations in initial crystal size and initial position in the convective line yielded diverse trajectories and final crystal properties [ABSTRACT FROM AUTHOR]

Abstract A major challenge in numerical weather prediction models is the ability to accurately simulate the microphysical properties and growth of ice hydrometeors in clouds. Eulerian bulk microphysics schemes in these models tend to obscure the properties and evolution of individual ice crystals, often resulting in inaccurate simulations of storm structures. To address this issue, this study presents a novel ice crystal trajectory growth (ICTG) model that simultaneously grows and advects individual ice crystals while tracking their evolving properties along their trajectories. The model is evaluated on a 3D quasi‐idealized leading‐convective, trailing‐stratiform squall line simulation. The ICTG model successfully produced a spatial distribution of ice crystal trajectories consistent with the simulated reflectivity structure of the storm above the melting level. Smaller initialized crystals (d ≤ 0.1 mm) were largely transported to the anvil and the trailing stratiform region. One primary trajectory involved sustained growth in the stratiform mesoscale updraft for ∼1.5 hr, resulting in a density reduction down to 600 kg m−3, a final particle size greater than 0.9 mm, and potential branching. In contrast, larger initialized crystals (d ≥ 0.5 mm) collected more rime and fell out primarily in the leading convective line. The ICTG model's realistic production of varied crystal growth properties owing to differences in transport and initial size suggests that it can be a valuable tool for learning about ice microphysical processes in a variety of cold cloud systems.

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]

Particle size distribution (PSD) characteristics of mature squall lines are investigated through global precipitation measurement (GPM) dual-frequency precipitation radar (DPR) measurements. These squall lines consist of a leading convective (LC) line, a weak-echo transition (WT) region and a trailing stratiform (TS) region. Their PSD characteristics are quite different from the existing conceptual models of mature squall line, given that many small raindrops/ice particles are found in the WT region while in the TS region raindrops/ice particles are sparse. Analysis shows that it is likely due to the short distance from LC to WT, where more particles may be dispersed from LC region and fall into WT region but barely have time to grow in size. In the TS region further behind LC, the particles have more time to get larger. Analysis also reveals that the mesoscale updraft generally occurs at mid-to-high levels in the TS region so that aggregations and collisions-coalescences could be promoted to increase the particle size but decrease the particle number. Through the GPM PSD data analysis, a refined conceptual model of a mesoscale convective system (MCS) with squall line is presented in this study. [ABSTRACT FROM AUTHOR]

A regional thunderstorm gale weather event occurred in central and southern Hebei on 9 July 2017. It is a typical severe convective weather with the cold advection forcing in upper air. The convective cloud clusters successively affected the south area (I) and north area (II) of central and southern Hebei during this event. Based on the routine upper-air and surface observations, satellite cloud images, Doppler weather radar data, observations from regional automatic weather stations and NCEP reanalysis data with 1°×1° resolution, we have conducted an analysis of the environmental conditions and the evolution characteristics of the convective storm when this event occurred. The main results are as follow. (1) This event occurs under the background of the Mongolian cold vortex weather. The cold air at the rear of the cold vortex and the warm and wet air in the low-level jointly form an unstable stratification with the dry and cold air in high level and the warm and humid air in low level, in which vertical wind shear is strong in southern Hebei Province. The convection in area I is triggered by topographic uplift, and moves southeastwards under the action of northwest airflow in the high level. Nevertheless, the convection in area II is directly triggered by the cold front, and moves eastwards and northwards under the combined action of advection and propagation. (2) The convective systems causing the gale in area I include the squall line and the supercell associated with mesocyclone. The rear inflow jet below 1 km exceeds 31 m·s-1 in mature stage of the squall line. The surface gale appears at the front of the center of high wind speed, the front of the thunderstorm high and the vicinity of hourly positive pressure center. There are various types of convective systems causing the gale in area II, including supercell, block-shaped echo and squall line. The gale squall line appears between the rear side of gust front and the hourly positive pressure center. After the echo intensity of the squall line is decreased, the combined effect of the cold pool density flow, the downward momentum transportation and the allobaric wind can still cause surface gale. (3) The fact that high wind speed core exceeding 30 m·s-1 in the radar radial velocities at the low elevation angle corresponded to the hourly positive surface pressure change of over 5 hPa, and the descending velocity of 7-10 m·s-1 below 5 km followed by the strong northwest wind below 1 km in the wind profiler radar product can be used as a reference indicator for the 0-2 h nowcasting and warning of the thunderstorm gale above level 8 on the ground.

At the end of June 2020, an explosive extratropical cyclone was responsible for an environment in which a squall line developed and caused life and economic losses in Santa Catarina state, southern Brazil. The aims of this case study are the following: (a) to describe the drivers of the cyclogenesis; (b) to investigate through numerical simulations the contribution of sea–air interaction to the development of the cyclone as an explosive system; and (c) to present the physical properties of the clouds associated with the squall line. The cyclogenesis started at 1200 UTC on 30 June 2020 on the border of southern Brazil and Uruguay, having a trough at middle-upper levels as a forcing, which is a common driver of cyclogenesis in the studied region. In addition, the cyclone’s lifecycle followed Bjerknes and Solberg’s conceptual model of cyclone development. A special feature of this cyclone was its fast deepening, reaching the explosive status 12 h after its genesis. A comparison between numerical experiments with sensible and latent turbulent heat fluxes switched on and off showed that the sea–air interaction (turbulent heat fluxes) contributed to the cyclone’s deepening leading it to the explosive status. The cold front, which is a component of the cyclone, favored the development of a pre-frontal squall line, responsible for the rough weather conditions in Santa Catarina state. While satellite images do not clearly show the squall line located ahead of the cold front in the cyclone wave due to their coarse resolution, radar reflectivity data represent the propagation of the squall line over southern Brazil. On 30 June 2020, the clouds in the squall line had more than 10 km of vertical extension and a reflectivity higher than 40 dBZ in some parts of the storm; this is an indicator of hail and, consequently, is a required condition for storm electrification. In fact, electrical activity was registered on this day.

A double‐moment version of the SBU‐YLIN cloud microphysical scheme in WRF is introduced. It predicts the mass and number mixing ratios of cloud droplet, rain, cloud ice, and precipitating ice. In addition, a number of physical processes, like rain evaporation, collection between rain and snow are also optimized in the new scheme. The scheme is evaluated and compared with the original one‐moment scheme for a squall line case. We found that the double‐moment approach gives a better representation of rain evaporation, which is critical for the development, morphology, and evolution of the simulated squall line, especially for the enhanced trailing stratiform cloud and leading convective line. The relationship between key microphysical processes and squall line dynamics is investigated to identify the driving mechanisms of the descending rear inflow, cold pool, and slantwise updraft. Furthermore, formation of the transition zone in the simulated squall line strongly depends on the flexible description of ice particle properties, such as size, degree of riming and fall speed. Plain Language Summary: A double‐moment version of the SBU‐YLIN cloud microphysical scheme in WRF is introduced and evaluated. The scheme improves a squall line simulation in terms of the shape and structure of the squall line with enhanced stratiform precipitation. The improvement is mainly related to more realistic rain evaporation and flexible description of ice particle properties. Close interactions between cloud and precipitation physics and dynamics is key for the development, maintenance and movement of squall lines. Key Points: A double‐moment version of the SBU‐YLIN scheme is introducedThe scheme improves the simulation of a squall line in terms of the transition zone and stratiform precipitationRain evaporation and flexible description of ice particles are critical for a realistic simulation [ABSTRACT FROM AUTHOR]

Abstract A double‐moment version of the SBU‐YLIN cloud microphysical scheme in WRF is introduced. It predicts the mass and number mixing ratios of cloud droplet, rain, cloud ice, and precipitating ice. In addition, a number of physical processes, like rain evaporation, collection between rain and snow are also optimized in the new scheme. The scheme is evaluated and compared with the original one‐moment scheme for a squall line case. We found that the double‐moment approach gives a better representation of rain evaporation, which is critical for the development, morphology, and evolution of the simulated squall line, especially for the enhanced trailing stratiform cloud and leading convective line. The relationship between key microphysical processes and squall line dynamics is investigated to identify the driving mechanisms of the descending rear inflow, cold pool, and slantwise updraft. Furthermore, formation of the transition zone in the simulated squall line strongly depends on the flexible description of ice particle properties, such as size, degree of riming and fall speed.

A squall line event that occurred in the south of the Yangtze River in China on 4 May 2020 was simulated using the WRF model and 3DVAR data assimilation. The radar radial wind, reflectivity, and conventional observation data were assimilated in a cycling interval of 15 min. The main findings from the examination of the development mechanisms of the squall line are as follows. The simulation effectively reproduced the development and evolution of the squall line. The squall line strengthened from the Wuling Mountains to the Dongting Lake Plain, moved eastwards and developed to its strongest stage over the plains, before weakening from thereon to the Mulianjiu Mountains. When the squall line moved from the mountains to the plains, the convergence of the airflow in front of the squall line increased because of the strengthening of the near-surface easterly airflow over the plains. Concurrently, the intensity of the cold pool behind the squall line decreased, while the low-level vertical wind shear over the plains increased. This reduced the gap between the cold pool and vertical wind shear. The low-level upward airflow in front of the cold pool tended to be vertical, which was conducive to the development and maintenance of the squall line. When the squall line moved eastwards from the plains to the mountains, the convergence of the airflow in front of the squall line weakened because of the weakening of the surface easterly airflow over the mountains. Concurrently, the intensity of the cold pool behind the squall line increased, while the low-level vertical wind shear in front of the squall line significantly declined. The intensity of the cold pool significantly exceeded the low-level vertical wind shear, inducing a tendency for the low-level upward airflow in front of the cold pool to tilt towards the cold pool, which was not conducive to the development and maintenance of the squall line. This was a reason why the squall line moved eastwards faster after going up the mountains. Based on these observational and simulation results, physical conceptual models for the enhancement of squall lines from the mountains to the plains and their weakening from the plains to the mountains have been preliminarily developed. • The differences in mesosclae features (cold pool, vertical shear) could explain the evolution of the squall line. • The strong wind and vertical shear over the plains are conductive to strengthening of the mountain-to-plain squall line. • The weak vertical wind shear over the mountains could be a key reason of weakening of the plain-to-mountain squall line. [ABSTRACT FROM AUTHOR]

Using the products from Doppler weather radars at Yichun, Nanchang, Jingdezhen and other places, as well as conventional observations and observational data from regional automatic weather stations, we have conducted an analysis of the cause and radar echo characteristics of the rare regional force 10 gale during a strong squall line event in Jiangxi on 4 March 2018. Results are as follows. (1) The squall lines in this event occur in the warm zone in front of a trough, and strong southwest warm and wet airflow in the low-level, dry and cold air in the middle level, and the maintenance of strong vertical wind shear between middle and low-level provide the favorable environmental conditions for the stability and continuance of squall lines. (2) The downward transport of momentum by the inflow jet on the rear side of the squall lines and the evaporation of the dry air wrapping up storm result in the strong descending airflow, then lead to the cold pool moving rapidly and cause surface gale. (3) The radar echo of regional force 10 thunderstorm gale is characterized by bow-shaped echo being 200-450 km in scale, 55-60 dBz in centre intensity and 100-120 km·h-1 in moving speed. Its low-level radial velocity is much large and appears aliasing. The radial convergence at front side of high wind core is strong. The maximum radial velocity shear value at the lower level is large, and strong echo centre, high radial velocity core and radial convergence zone all have a forward tilting feature. (4) High velocity cores above 31 m·s-1 and radial velocity shears above 30 m·s-1·(10 km)-1 in the radial velocities of low elevation angles, bow-shaped echoes with moving speed more than 100 km·h-1 in the reflectivity factor chart, and the descending of high velocity cores of 20 m·s-1 in the VWP production can be used as the reference indicators for early warning of the force 10 gale.

Based on conventional observations, Doppler weather radar date, NCEP/NCAR reanalysis data with 1°×1° resolution and other data, we have performed a comparative analysis between two squall line events (hereinafter referred to as the "5·11" event and the "6·5" event, respectively) occurred in Hunan on 11 May and 5 June of 2017. Results are as follows. (1) Both events occur under the condition of low-level warm convection forcing, in which the cold advection is obvious before the "5·11" event occurred, and the warm and wet airflow prevails before the "6·5" event occurred. The difference in the position of the subtropical high results in that the latter event has better water vapor condition than the former one. (2) During the "5·11" event, atmosphere in the middle layer is drier, and the height of 0℃ layer is lower (0.6 km), which is conductive to the formation of thunderstorm gale and small hail over a broad area. Meanwhile, during the "6·5" event, the free convection level (LFC) is relatively low, and humidity in the low-level is greater, which is beneficial to produce greater short-term severe precipitation. (3) The environmental conditions in which a widespread thunderstorm gale occurred in the "5·11" event is better than that in "6.5" event, while in the latter case, the strengthen of atmospheric convective instability is due to the development of surface inverted trough, the intensive warm and wet airflow and the eastward-moving of low vortex, which is more conductive to the emergence of local disastrous gale. (4) In the "6·5" event, there is only one mesocyclone which lasted only for a short time, and the characteristics of thunderstorm gale in the vertical wind profile (VWP) and radar radial velocity chart are not typical enough, which increases the difficulty of early warning to it.

Using datasets of conventional observations, regional automatic weather station (AWS), Doppler weather radar, wind profile radar and GFS reanalysis with 0.5°×0.5° resolution, we have conducted analysis on the mesoscale characteristics and cause of an extreme thunderstorm gale event in Jiangxi on 4 March 2018, and discussed its extremeness. The results indicate that (1) this event is caused by a strong squall line, and pre-squall depression, squall front, rear-flow depression as well as the thunderstorm high are very clear in it. (2) The squall line developed intensely after it entered Jiangxi under a favorable environment, including the unstable stratification of dry and cold air in the high level and warm and humid air in the low level which was created by strong low-level warm and wet air advection, middle-level dry air, ground warming and low-level wetting, the strong lifting conditions formed by middle-level trough moving eastward, multi-layer jet superposition and convergence line in ground level, and the strong vertical wind shear in the mid- and low- level prolonging its life. (3) The increased angle between the squall line and the steering flow makes it move speedily, and both the flat topography around the Poyang Lake and the rich water vapor enhanced surface gale. (4) Extremely unusual values of physical variables at Nanchang station and their differences with respect to corresponding years of dekad norms exceeding 2σ (σ is root mean square), including surface depression, downdraft convective available potential energy (DCAPE), the temperature difference (T850-500) between 850 hPa and 500 hPa and the vertical wind shear from 925 hPa to 1000 hPa etc., show a good indication to the potential prediction of extreme thunderstorm gale. In addition, high wind zone below 1 km in wind-profiling radar products appears 30 min ahead, and a substantial jump of refractivity structure constant (Cn2) in the upper-level appears 10-20 min ahead. Both can be used as reference in the early warning of thunderstorm gale.

The present paper aims a comparative analysis of the cases when the convective wind gusts reached and exceeded 25 m/s in the warm season (April-September) of the years 2012-2017. In the range studied were identified 6 cases: 2 cases in 2012, one case in 2015, and 3 cases in 2017. The 90 km/h limit registers on level 10 on the Beaufort scale, which may cause uprooting trees and affecting the structure of the buildings. identifies the synoptic situations that led to the emergence of convective systems by analyzing the temperature and geopotential parameters at standard levels. The degree of instability of the atmosphere is assessed on the basis of instability indices (CAPE, LI, VTI). The mesoscale analysis was realized and the evolution of the systems that generated the wind was monitored by satellite and radar imaging.

Based on the conventional observation data, automatic weather station data, Doppler radar data and NCEP(0.25°×0.25°) reanalysis data, a strong convective weather event occurred in the southeastern Inner Mongolia on 24 September 2016 caused by the disaster squall line was analyzed. The main conclusions are as follows. In the high and middle levels, cold and dry air masses spread to the southeast with the convergence of the low southwestern jet drastically strengthened, which provides this strong squall line process a very good large-scale circulation background. The CAPE has a significant increase before the strong convective weather occurred. The middle and low level distribution of θse presents like inverted funnel, and decreases with the increase of the height. This kind of dry and wet structure is favorable for the occurrence of strong convective weather such as thunderstorms and hails. The direct trigger and maintenance mechanism of this strong convective weather are due to the longtime maintaining, coupling and increasing between ground scale dew point front(dry line) and mesoscale convergence line. The thunderstorm high pressure stayed after the squall line indicating that there is a surface strong wind. The characteristic of radar echo appears like bow echo. The reflectivity profile of the bow echo front shows that there is a boundary weak echo region(BWER) in the lower layer. In the high-level, it has obvious echo drape. Strong echo zone achieve 50-55 dBz extends to 7.5 km, indicating that a strong upward flow is in the convective storm which is conducive to the formation of short-term heavy rainfall and the large hail. The arcuate velocity profile of the bow echo represents a strong sinking airflow and the middle ground radial convergence (MARC) of the ground wind warning index.

The squall line is a linear mesoscale convective system often seen in summer, which could bring severe disasters, such as thunderstorms, hails and tornadoes. So, the identification and forecast of squall lines are the important and difficult problems in operational weather nowcasting. In this paper, based on weather radar data an automatic identifying method of the squall line is presented. After image de-noising, extraction of the central axis of strong echo areas, and the Hough Transform, the spatial form and intensity variation characteristics of the reflectivity factors in the radar image are analyzed. On this basis, the automatic identification of squall line could be achieved. This method can overcome the adverse effects of the discontinuity of strong echo areas on the automatic identification of squall lines. The verification of squall line cases shows that the successful identification rate of squall lines is over 95%. Especially, when the boundary of the strong echo area is clear and is in a straight line or minor arc, the identification rate is higher. Overall, this new method has realized the automatic identification of squall lines on radar images, which could greatly improve the accuracy and time-effectiveness of squall line identification, and could provide a solid basis for the automatic forecast of squall lines in operational weather nowcasting. [ABSTRACT FROM AUTHOR]

Combining multiple observational data and ERA5 time-by-time reanalysis data, combined with a limited area wind field analysis method, the process of controlling the next fast-developing bow-shaped squall line of Hainan Island's subtropical high on April 22, 2020 was analyzed. The results show that under the background of the weaker environmental wind shear in Hainan Island, the interaction of gust fronts, ground β-mesoscale convergence lines, and small-scale thunderstorms quickly developed into a highly organized bow-shaped squall line, and a strong cold pool near the ground. Drive the storm to deviate from the average guided airflow and move quickly to the southeast. The divergent wind field obtained by the decomposition of the wind field more intuitively reflects the diagnostic information of the weather system than the original wind field. The high-altitude divergent wind field more clearly shows the convective development and the dissipated early high-altitude divergence on the original wind field. The ground mesoscale convergence line reflects the characteristics of strong ground convergence at the bow-shaped echo bulge and rapid advancement of the cold pool. The next 1h precipitation will be on the warm and wet inflow side near the convergence line. [ABSTRACT FROM AUTHOR]

Several strong sea-level oscillations in the southern region of Brazil with periods in the range of tsunamis, but attributed to atmospheric disturbances, have been publicized in the media or in scientific articles in recent years. However, only a few were registered by instruments. On October 29, 2019, a squall line, linked to an atmospheric storm and a positive jump in atmospheric pressure, spread from south to north along the coast of Santa Catarina. The atmospheric disturbance generated, through Proudman's resonance mechanism, at least two tsunami-like waves with heights greater than 0.70 m and periods around 15 min. These waves were recorded by a network of tide gauges, allowing to monitor their evolution and transformations for more than 200 km. Although there were no reported fatalities, there were significant material losses linked to this occurrence. The objective of the present work is to analyze and connect atmospheric conditions and wave signals in order to further understand the circumstances leading to the formation of this type of event in the southern coast of Brazil. [ABSTRACT FROM AUTHOR]

This study documents the atmospheric system driving the observed meteotsunami waves that hit the northern Persian Gulf on 19 March 2017 during high tide. This destructive meteotsunami event resulted in coastal inundations that reached several hundred metres inland along the 100-km coastline between the cities of Dayyer and Asaluyeh and caused the death or injury of 27 persons. Based on previously published research, eyewitness reports, oceanic and atmospheric observations, including synoptic station and weather radar data, and available reanalysis ERA5 products, this study provides new insights into destructive events, particularly mesoscale atmospheric systems conjoined with observed meteotsunami waves. Precipitation intensity, maximum reflectivity and echo top height images provided by the weather radar covering the affected area and the area over which the meteotsunamigenic disturbance travelled revealed that a strong convective system that encompassed the mid- and upper troposphere entered the northern Persian Gulf approximately 4 h before the event and moved eastward. Two hours before reaching the affected coastline, this convective system was reshaped to an elongated and narrow squall line varying between 70 and 130 km in length with a width of less than 10 km and travelled at an average speed of approximately 24 m/s over the sea. The peak maximum reflectivity of the squall line always surpassed 40 dBZ, while it increased to 60 dBZ near the Dayyer area. As such, intense atmospheric disturbances are known to be associated with sharp air pressure increases, and these disturbances were found to resonantly pump energy to the ocean through Proudman resonance for over 12 or more disturbance wavelengths (i.e. up to 120 km in this study). A half-metre meteotsunami wave was presumably created in the open sea and then amplified while travelling towards the shore where it broke as a meteotsunami bore and inundated the coastal areas. Further research on the physical mechanisms driving the interactions between meteotsunamigenic disturbances and ocean responses, the recurrence of such events and meteotsunami hazard assessments along the affected coastline is envisaged. [ABSTRACT FROM AUTHOR]

Abstract This study examines the impacts of humidity adjustment in a cloud analysis system on the analysis and forecast of a squall line that occurred in southeast China on 23–24 April 2007. Radial velocity data are assimilated using the ARPS three‐dimensional variational system while reflectivity data are assimilated by a cloud analysis system. Experiments with two different humidity adjustment schemes are performed, with the original and enhanced versions. Another experiment does not adjust moisture. Both schemes generally decrease the humidity in front of the convective line and increase the humidity within the convective and stratiform regions of squall line compared to no humidity adjustment, and the original scheme produces the higher humidity within precipitation regions, especially the stratiform region. Both schemes improve the forecast of squall line structure, including the leading convective line, a transition zone, and a trailing stratiform region. Among the three experiments, the enhanced scheme produces the highest precipitation forecast skill. The latent heating rates are also diagnosed to investigate the microphysical responses to the humidity adjustment. The cooling outside of the observed precipitation regions corresponding to the humidity reduction also acts to suppress spurious precipitation. Water vapor condensation into cloud water and cloud water evaporation generally dominate the latent heating/cooling below the freezing level. Compared to the enhanced scheme, the original scheme releases much more latent heat in the middle troposphere, causing more warming. This is linked to the higher cloud water condensation rate, due to the higher amount of moisture addition/adjustment by the original scheme.