Storm Structures Research Articles

On a Modified Definition of a Derecho. Part II: An Updated Spatial Climatology of Derechos across the Contiguous United States

Abstract Derechos are known as widespread severe windstorms that accompany extratropical MCSs. However, questions remain whether the term derecho should apply to widespread windstorms from all linear storm structures, or just MCSs that evolve from a progressive cold pool, which support concentrated swaths of damaging winds. Part I of this manuscript focused on the derecho definition to windstorms with hurricane-force gusts that originate from cold-pool-driven MCSs. While Part I modified the derecho definition and formulated quantitative criteria for identifying future events, a derecho spatial climatology across the contiguous United States was derived for 2000–22 using this more focused definition in Part II. The derecho climatology was created from cases identified with the same algorithm employed in Part I of this manuscript, which ingested a combination of datasets to objectively detect derechos. For the 2000–22 climatology, derechos were found to be primarily warm-season events, with corridors of derecho occurrence noted across the Midwest and southern plains, in agreement with earlier studies, as well as the northern plains. Seventy derechos were identified over a period of 23 years. Derechos occur, on average, three times per year nationally, and once every 1–2 years at given points within the plains and Midwest corridors, with greater frequencies noted in earlier studies, likely due to stricter criteria requiring hurricane-force gusts and supporting cold-pool-driven MCS structures in the present study. Derechos frequently form in the afternoon and dissipate later at night but peak in intensity just a few hours after development.

Implementation of storm-following nest for the next-generation Hurricane Analysis and Forecast System (HAFS)

Tropical cyclones models have long used nesting to achieve higher resolution of the inner core than was feasible for entire model domains. These high resolution nests have been shown to better capture storm structures and improve forecast accuracy. The Hurricane Analysis and Forecast System (HAFS) is the new-generation numerical model embedded within NOAA’s Unified Forecast System (UFS). The document highlights the importance of high horizontal resolution (2 km or finer) in accurately simulating the small-scale features of tropical cyclones, such as the eyewall and eye. To meet this need, HAFS was developed by NOAA leveraging a high-resolution, storm-following nest. This nest moves with the cyclone, allowing better representation of small-scale features and more accurate feedback between the cyclone’s inner core and the larger environment. This hurricane following nest capability, implemented in the Finite-Volume Cubed-Sphere (FV3) dynamical core within the UFS framework, can be run both within the regional as well as global forecast systems. A regional version of HAFS with a single moving nest went into operations in 2023. HAFS also includes the first ever moving nest implemented within a global model which is currently being used for research. In this document we provide details of the implementation of moving nests and provide some of the results from both global and regional simulations. For the first time NOAA P3 flight data was used to evaluate the inner core structure from the global run.

Comparison of Two-Moment and Three-Moment Bulk Microphysics Schemes in Thunderstorm Simulations over Indian Subcontinent

We simulated three different thunderstorm events over India using a regional model with two different multi-moments cloud microphysical schemes: (i) Morrison double moment (Morrison-2 M) and (ii) National Taiwan University (NTU) triple moment (NTU-3 M). The results show that the choice of microphysical scheme significantly impacts storm structures, cloud formations, lightning intensity, and rainfall patterns. The lightning flash counts from the in-situ lightning detection network (LDN) are also used to compare the simulated storms. The Lightning Potential Index (LPI) was computed for both Morrison-2 M and NTU-3 M microphysics schemes and compared it with the LDN observation. Differences in simulations are attributed to the simulation of ice crystals, snow, and graupel by the microphysical schemes. The effects on the size distributions of cloud hydrometeors between the two microphysical schemes are also found to be important. The inclusions of ice crystal shapes likely accounted for the key differences between the two microphysics schemes. The results demonstrate that the simulation of lightning event characteristics is sensitive to the choice of microphysical parameterization schemes in numerical weather prediction models. In comparison to both the microphysical schemes, some aspects are better in the 2 M microphysics scheme, and some are better in the 3 M microphysics scheme. Morrison-2 M simulates more mixing ratio for the majority of cloud hydrometeors across all cases than the NTU-3 M whereas the Morrison-2 M has less rainfall bias over the ocean for the majority of the cases. Overall, the present study suggests that the choice of microphysical scheme can greatly influence the accuracy of thunderstorm and lightning event predictions in operational weather forecasting models.

Runoff generation in a semiarid environment: The role of rainstorm intra-event temporal variability and antecedent soil moisture

Rainfall intensity and antecedent soil moisture are key variables affecting the initiation and accumulation of runoff, and therefore, impact on the occurence flood events. We used time series of rainfall intensities characterizing rainstorms with different mean, standard deviation, and skewness to simulate runoff response of a sealed loamy soil profile under various initial soil moisture conditions. We found that shifting from wet to dry initial soil moisture leads to a 300 % increase in ponding time and a 350 % decrease in total runoff. Intense rainfall events catalyze runoff development and runoff occurs earlier with increasing mean, maximum intensity, and variance of rainfall. Higher variance in intra-event storm-intensity temporal distribution yields more runoff. Earlier runoff formation aligned with negatively skewed storm structures, but largest cumulative runoff resulted from storms displaying near-zero skewness values. The combination between rainfall intensification and extended dry periods between storms might reduce potential runoff. These insights improve our understanding of anticipated impacts of climate variations on floods and may also contribute to ecosystem health evaluations, and bolster predictive capabilities of surface processes.

Comparison of Spatiotemporal Distribution and Occurrence Conditions of Large and Small Hail Events in the Beijing–Tianjin–Hebei Region

Abstract Using hail records at national meteorological stations for 2014–18, ERA-Interim reanalysis data, and Doppler weather radar data, the spatiotemporal distribution of hail events (HEs) in the Beijing–Tianjin–Hebei region is revealed, and the environmental conditions and hailstorm structures corresponding to large hail (diameter ≥ 20 mm) events (LHEs) and small hail (2 ≤ diameter < 20 mm) events (SHEs) are compared. It is found that, although HEs may be more frequent in mountainous areas, most LHEs occur in the plains and near the foot of the mountains. The HE frequency peaks in June, and the average hailstone size is larger during May and June. According to daytime records, the HEs predominantly occur in the afternoon and evening, whereas LHE tends to be more in the evening. Comparison of environmental parameters suggests that, relative to SHEs, LHEs tend to correspond to higher 2-m temperature, a wetter lower layer, a larger difference in relative humidity between 925 and 500 hPa, greater unstable energy, and stronger wind shear. Hailstorms associated with LHEs tend to feature greater mesoscale rotation velocity than those associated with SHEs. Hailstorms usually show rapid increase (RI) in vertically integrated liquid (VIL) before hailstones are observed. A significant difference between the hailstorms associated with LHEs and SHEs is that the former has an obviously longer time interval between the end of VIL RI and the occurrence of hailfall, indicating that the large hail size benefits from the constant supply of liquid water and the hail can be lifted by updrafts for a long time. Significance Statement Whereas previous studies have predominantly focused on large hail (diameter ≥ 20 mm) events (LHEs) and their yielding conditions, this study was devoted to examining the difference between the LHEs and small hail (2 ≤ diameter < 20 mm) events in their associated atmospheric environments and storm structures. The interesting new insight is that the hailstorms yielding LHEs tend to feature a significantly longer time interval after the rapid increase of vertically integrated liquid and before hailfall. This study can provide a reference for the early warning of the scale of hail, which is one of the difficulties of weather services.

Combining CloudSat/CALIPSO and MODIS measurements to reconstruct tropical convective cloud structure

Tropical convective clouds are a crucial component of the Earth's weather and climate system. However, there are still large uncertainties in model simulations of tropical convective clouds and precipitation. In this study, CloudSat/CALIPSO and MODIS measurements are combined to provide vertical convective cloud structures under horizontal storm context and structures of convective cores. Convective cells are identified with MODIS brightness temperature at 11 μm (BT11) and convective centers are determined for each cell by finding the minimum BT11 (BT11min). The difference between BT11 and BT11min is calculated to provide the distance to the convective center as in brightness temperature. Therefore, detailed convective cloud structures from centers to the stratiform regions could be shown as in the contoured frequency by temperature diagram (CFTD) of radar reflectivity factor (Ze). A group of strongly attenuated Ze profiles is identified near the convective center in Africa during daytime, and the occurrence of this group of profiles is much smaller in the west Pacific during daytime. The out-of-phase day/night variations of convective system structures over ocean and over land are also analyzed. It is shown that over ocean regions, rather than land regions, nighttime convective clouds have larger Ze than daytime ones between 210 K to 260 K, but they have smaller Ze at temperatures colder than 210 K near the cloud top. The convective cores are analyzed by separating and classifying those strongly attenuated Ze profiles as group one convective cores representing the developing stage, and the rest regular profiles as group two convective cores representing the later stage. As BT11min decreases for group one, the cores grow stronger with the maximum Ze shifts into lower temperature; and as BT11min decreases for group two, the CFTD for the cores gradually shifts to larger Ze for temperatures colder than 273.15 K and smaller Ze for temperatures warmer than 273.15 K. These observed composite structures of convective clouds and land/ocean and day/night variations provide crucial information for model evaluations.

Estimates of Near-Inertial Wind Power Input Using Novel In Situ Wind Measurements from Minimet Surface Drifters in the Iceland Basin

Abstract The Minimet is a Lagrangian surface drifter measuring near-surface winds in situ. Ten Minimets were deployed in the Iceland Basin over the course of two field seasons in 2018 and 2019. We compared Minimet wind measurements to coincident ship winds from the R/V Armstrong meteorology package and to hourly ERA5 reanalysis winds and found that the Minimets accurately captured wind variability across a variety of time scales. Comparisons between the ship, Minimets, and ERA5 winds point to significant discrepancies between the in situ wind measurements and ERA5, with the most reasonable explanation being related to spatial offsets of small-scale storm structures in the reanalysis model. After a general assessment of the Minimet performance, we compare estimates of wind power input in the near-inertial band using the Minimet winds and their measured drift to those using ERA5 winds and the Minimet drift. Minimet-derived near-inertial wind power estimates exceed those from Minimet drift combined with ERA5 winds by about 42%. The results highlight the importance of accurately capturing small-scale, high-frequency wind events and suggest that in situ Minimet measurements are beneficial for accurately quantifying near-inertial wind work on the ocean. Significance Statement In this study we introduce a novel, freely drifting wind measurement platform, the Minimet. After an initial validation of Minimet sea surface wind measurements against independent wind measurements from a nearby research vessel, we investigate their utility in context of the near-inertial work done by the wind on the ocean, which is important for the ocean’s energy budget. We find Minimet near-inertial wind work estimates exceed those estimated using winds from a state-of-the-art wind product by 42%. Our results indicate that capturing storm events happening on time scales less than 12 h is crucial for accurately quantifying near-inertial wind work on the ocean, making wind measurements from platforms such as the Minimet invaluable for these analyses.

Evaluating an Ice Crystal Trajectory Growth (ICTG) Model on a Quasi‐Idealized Simulation of a Squall Line

AbstractA 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.

Contribution to the study of thunderstorms in the Congo Basin: Analysis of periods with intense activity

Lightning activity, storm structures, meteorological and environmental conditions are analyzed within a large area of Central Africa (25° × 25°) for two periods of one month (March and December) with high thunderstorm activity in 2013. The lightning flashes issued from WorldWide Lightning Location Network (WWLLN) allow to contribute in highlighting some characteristics in agreement with the literature. Some new findings, that can be useful for future storm monitoring thanks to lightning data issued from the Lightning Imager (LI) onboard Meteosat Third Generation (MTG), have been noted and have to be confirmed with more case studies. Thus, a seasonal correlation is found between the daily flash number and the extent of large CAPE values. The lightning flash density exhibits the same location of its maximum, for December and March with about 3 and 2 flashes km−2, respectively. This location is also reported in the literature for the annual activity in the study area. The storm developments often occur in the regions that combine the presence of lakes and mountains. The flash clusters allow to localize the strong convective activity and to follow the displacement of the storm systems when they move. The flash rate density (FRD) is larger in the convective systems well organized, as the typical MCS, during the development of the storms and when several cells merge. The daily cycle shows a lightning flash rate maximum between 14:00 and 15:00 UT that precedes the maximum development of the storm by one hour. It also shows that the storms that occur within the region of the main maximum of lightning activity, are more frequent and produce larger values of FRD.

Scale Analysis of Infrared Water Vapor Brightness Temperatures for Tropical Cyclone All‐Sky Radiance Assimilation

AbstractWe analyzed the scale features of satellite infrared (IR) water vapor (WV) brightness temperature observations of tropical cyclones (TCs). This is to characterize the storm information at dominate scales in all‐sky radiance assimilation for TC numerical weather prediction. This paper presents the results from the study of Hurricane Patricia (2015). Our study shows that IR WV brightness temperatures have the ability to observe multiscale structures of TCs, ranging from a size of above 1,000 km that covers the entire storm and its surrounding areas to a scale resolving individual convective clouds embedded in the TC. The atmospheric moisture for TC development is mainly represented by large scales covering the storm and surrounding areas while the storm structures are characterized basically by all scales. The large‐scale moisture and small‐scale convection demonstrate strong correlation and are closely related to the TC development, suggesting the need for all‐sky radiance assimilation at multiple scales.

Evaluating Simulated Raindrop Size Distributions and Ice Microphysical Processes With Polarimetric Radar Observations in a Meiyu Front Event Over Eastern China

AbstractTo better understand the characteristics of simulated raindrop size distributions (DSDs) and ice microphysical processes for convective systems in the East Asia monsoon region, a typical Meiyu event is simulated with the Weather Research and Forecasting (WRF) model using three two‐moment bulk microphysics schemes in this study. The simulated microphysical characteristics are then evaluated using polarimetric radar observations and retrievals. Although the observed linear storm structures are well simulated in terms of their location, there are significant deviations regarding simulated polarimetric radar variables and DSD parameters when compared to the observations. Compared to radar retrievals, all the simulated low‐level raindrops are found to have lower number concentration and larger mean sizes. To investigate the sources of the simulated DSD biases, vertical distributions of radar reflectivity and specific ice hydrometers (snow, graupel), as well as profiles of liquid/ice water contents from radar retrievals and simulations are further compared. In addition, variations about the occurrence frequency and transfer rate for four categories of ice processes (deposition, aggregation, riming, and melting) among the three schemes are analyzed. Results indicate that the overprediction of snow and graupel from riming processes is likely to be responsible for the production of extremely large raindrops at low levels.

Strength and Challenges of global model MPAS with regional mesh refinement for mid-latitude storm forecasting: a case study

Abstract. With the rising share of renewable energy sources like wind energy in the energy mix, high-impact weather events like mid-latitude storms increasingly affect energy production, grid stability and safety and reliable forecasting becomes very relevant for e.g. transmission system operators to allow for actions to reduce imbalances. Traditionally, meteorological forecasts are provided by limited-area weather prediction models (LAMs), which can use high enough model resolution to represent the range of atmospheric scales of motions associated with such storm structures. While generally satisfactory, deterioration and insufficient deepening of large-scale storm structures are observed when they are introduced near the lateral boundaries of the LAM due to inadequate spatial and temporal interpolation. Global models with regional mesh refinement capabilities like the Model for Prediction Across Scales (MPAS) have the potential to provide an alternative, while avoiding sharp resolution jumps and lateral boundaries. In this study, MPAS' capabilities of simulating key evaluation metrics like storm intensity, storm location and storm duration are investigated based on a case study and assessed in comparison with buoy measurements, forecast products from the Climate Forecast System (CFSv2) and simulations with the Weather Research and Forecasting (WRF) LAM. Quasi-uniform and variable-resolution MPAS mesh configurations with different model physics settings are designed to analyze the impact of the mesh refinement and model physics on the model performance. MPAS shows good performance in predicting storm intensity based on the local minimum sea level pressure, while time of local minimum sea level pressure (storm duration) was generally estimated too late (too long) in comparison with the buoy measurements in part due to an early west-wards shift of the storm center in MPAS. The variable-resolution configurations showed a combination of an additional south-westwards shift and deviations in the sea level pressure field south-west of the storm center that introduced additional bias to the time of local minimum sea level pressure at some locations. The study highlights the need for a more detailed analysis of applied mesh refinements for particular applications and emphasizes the importance of methods like data assimilation techniques to prevent model drifts.

Examining Conditions Supporting the Development of Anomalous Charge Structures in Supercell Thunderstorms in the Southeastern United States

AbstractHypotheses regarding favorable conditions for anomalous charging have primarily resulted from studies within the Great Plains region of the United States, where the efficiency of warm precipitation processes is thought to be fundamental. Rare observations of anomalous charge structures in the Southeastern region challenge existing conceptual models used to explain anomalous charging. As a rigorous evaluation of conditions that support anomalous charge structures, environmental characteristics and bulk kinematic and microphysical properties of two normal and two anomalous supercell thunderstorms observed in the Southeast were compared. Within the anomalous supercells, greater quantities of precipitation ice were identified at higher altitudes and colder temperatures, suggesting a greater depth of riming growth and increased vertical transport of rimed hydrometeors. Deeper anomalous supercell updrafts were larger and stronger in the upper mixed‐phase and glaciated regions. However, normal supercells were characterized by more robust low‐level updrafts, resulting in comparable warm cloud residence times that suggested warm precipitation processes were not necessarily less efficient in the anomalous supercells. Indications of enhanced mixed‐phase liquid water content in favor of anomalous charging were observed in the anomalous supercells, though contrasts in related environmental parameters were not as large as observed in other comparative studies. Anomalous supercell environments were characterized by increased instability, shallower warm cloud depth, as well as lower relative humidity in the 700–500 mb layer. Evidence of impacts from dry air in anomalous storm structures suggested that water vapor content may have affected particle‐scale charge transfer in support of anomalous charge structure development.

A 4-Year Climatological Analysis Based on GPM Observations of Deep Convective Events in the Mediterranean Region

Since early March 2014, the NASA/JAXA Global Precipitation Measurement Core- Observatory (GPM-CO) satellite has allowed analysis of precipitation systems around the globe, thanks to the capabilities of the GPM Microwave Imager (GMI) and Dual-Frequency Precipitation Radar (DPR). In this work, we demonstrate how GPM-CO measurements obtained from 4 years of observations over the Mediterranean area can be used as an extremely effective tool to study the main climatological characteristics of the most intense Mediterranean storm structures. DPR and GMI-based Precipitation Features (PFs) parameters are used as proxies of the vertical structure and microphysical properties of these events, and their statistical distribution is analyzed to identify extremes. The analysis of annual, seasonal and geographical distribution of the identified deep convective systems highlights substantial differences in their diurnal cycle and in the distribution between land-sea and summer-winter. There is a general shift of the convective systems from the south (mostly over the sea) in the cold season, to the north (mostly over land) in the warm season. The analysis shows also that the inferred convective intensity is not always related to heavy precipitation. Known DPR and GMI-based criteria were adopted to identify overshooting top events and potential hailstorms, identify extreme deep convection signatures, like those observed for tropical and subtropical systems, and the most intense occur mostly over the sea. Although the analysis is limited to four years, the results show that the GPM-CO offers unprecedented measurements to identify and characterize extreme weather events in the Mediterranean region, with unique potentials for future long-term climatology and interannual variability analysis.

Assimilating All-sky Infrared Radiances from Himawari-8 Using the 3DVar Method for the Prediction of a Severe Storm over North China

Although radar observations capture storm structures with high spatiotemporal resolutions, they are limited within the storm region after the precipitation formed. Geostationary satellites data cover the gaps in the radar network prior to the formation of the precipitation for the storms and their environment. The study explores the effects of assimilating the water vapor channel radiances from Himawari-8 data with Weather Research and Forecasting model data assimilation system (WRFDA) for a severe storm case over north China. A fast cloud detection scheme for Advanced Himawari imager (AHI) radiance is enhanced in the framework of the WRFDA system initially in this study. The bias corrections, the cloud detection for the clear-sky AHI radiance, and the observation error modeling for cloudy radiance are conducted before the data assimilation. All AHI radiance observations are fully applied without any quality control for all-sky AHI radiance data assimilation. Results show that the simulated all-sky AHI radiance fits the observations better by using the cloud dependent observation error model, further improving the cloud heights. The all-sky AHI radiance assimilation adjusts all types of hydrometeor variables, especially cloud water and precipitation snow. It is proven that assimilating all-sky AHI data improves hydrometeor specifications when verified against the radar reflectivity. Consequently, the assimilation of AHI observations under the all-sky condition has an overall improved impact on both the precipitation locations and intensity compared to the experiment with only conventional and AHI clear-sky radiance data.

Land-Surface Diurnal Effects on the Asymmetric Structures of a Postlandfall Tropical Storm.

After a tropical storm makes landfall, its vortex interacts with the surrounding environment and the underlying surface. It is expected that diurnal variation over land will affect storm structures. However, this has not yet been explored in previous studies. In this paper, numerical simulation of postlandfall Tropical Storm Bill (2015) is conducted using a research version of the NCEP Hurricane Weather Research and Forecasting (HWRF) model. Results indicate that during the storm's interaction with midlatitude westerlies over the Great Plains, the simulated storm with the SLAB land‐surface scheme is stronger, with faster eastward movement and attenuation, and more asymmetric structures than that with the NOAH land‐surface scheme. More symmetric structures correspond with a slower weakening and slower eastward movement of the storm over land. Further diagnoses suggest an obvious response of the storm's asymmetric structures to diurnal effects over land. Surface diabatic heating in the storm environment is important for the storm's symmetric structures and intensity over land. Specifically, during the transition from nighttime to daytime, the evident strengthening of convective instability, atmospheric baroclinicity, and the lateral advection of high θe air in the storm environment, associated with the rapid increase in surface diabatic heating, are conducive to the development of vertical vorticity and storm‐relative helicity, thus contributing to the maintenance of the storm's symmetric structures and intensity after landfall.

Are Storm Characteristics the Same When Viewed Using Merged Surface Radars or a Merged Satellite Product?

AbstractHigh-resolution datasets offer the potential to improve our understanding of spatial and temporal precipitation patterns and storm structures. The goal of this study is to evaluate the similarities and differences of object-based storm characteristics as observed using space- or land-based sensors. The Method of Object-based Diagnostic Evaluation (MODE) Time Domain (MTD) is used to identify and track storm objects in two high-resolution merged datasets: the Integrated Multisatellite Retrievals for Global Precipitation Measurement (IMERG) final product V06B and gauge-corrected ground-radar-based Multi-Radar Multi-Sensor (MRMS) quantitative precipitation estimations. Characteristics associated with landfalling hurricanes were also examined as a separate category of storm. The results reveal that IMERG and MRMS agree reasonably well across many object-based storm characteristics. However, there are some discrepancies that are statistically significant. MRMS storms are more concentrated, with smaller areas and higher peak intensities, which implies higher flash flood risks associated with the storms. On the other hand, IMERG storms can travel longer distances with a higher volume of precipitation, which implies higher risk of riverine flooding. Agreement between the datasets is higher for faster-moving hurricanes in terms of the averaged intensity. Finally, MRMS indicates a higher average precipitation intensity during the hurricane’s lifetime. However, in non-hurricanes, the opposite result was observed. This is likely related to MRMS having higher resolution; monitoring the hurricanes from many viewing angles, leading to different signal saturation properties compared to IMERG; and/or the dominance of droplet aggregation effects over evaporation effects at lower altitudes.

Analysis of a Record-Breaking Rainfall Event Associated With a Monsoon Coastal Megacity of South China Using Multisource Data

Monsoon coastal cities often suffer from extreme rain-induced flooding and severe hazard. However, the associated physical mechanisms and detailed storm structures are poorly understood due to the lack of high-resolution data. This study presents an analysis of a thunderstorm that produces extreme hourly rainfall (EXHR) of 219 mm over the Guangzhou megacity on the southern coast of China using integrated multiplatform observations and a four-dimensional variational Doppler radar analysis system. Results indicate that weak environmental flows and convectively generated weak cold pool facilitate the formation of a quasi-stationary storm, while onshore warm and moist flows in the boundary layer (BL) provide the needed moisture supply. The 219-mm EXHR is attendant by a shallow meso- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -scale vortex due to stretching of intense latent heating-induced convergence, which, in turn, helps organize convective updrafts into its core region. Lightning and dual-polarization radar observations reveal active warm-rain (but weak mixed-phase) microphysical processes, with raindrop size distribution (RSD) closer to marine convection. In contrast, another storm develops about 4 h earlier and only 35 km to the northwest, but with more lightning, higher cloud tops, more graupel and supercooled liquid water content, more continental RSD, little evidence of rotation, and much less rainfall; they are attributable to the presence of larger convective available potential energy resulting from the urban heat island effects and less moisture supply in the BL. These results highlight the importance of using multisource remote sensing data sets in understanding the microphysical and kinematic structures of EXHR-producing storms.

The Mechanisms Responsible for Large Near-Surface Vertical Vorticity within Simulated Supercells and Quasi-Linear Storms

AbstractDespite 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.

Simultaneous Assimilation of Radar and All-Sky Satellite Infrared Radiance Observations for Convection-Allowing Ensemble Analysis and Prediction of Severe Thunderstorms

Abstract This study explores the benefits of assimilating infrared (IR) brightness temperature (BT) observations from geostationary satellites jointly with radial velocity (Vr) and reflectivity (Z) observations from Doppler weather radars within an ensemble Kalman filter (EnKF) data assimilation system to the convection-allowing ensemble analysis and prediction of a tornadic supercell thunderstorm event on 12 June 2017 across Wyoming and Nebraska. While radar observations sample the three-dimensional storm structures with high fidelity, BT observations provide information about clouds prior to the formation of precipitation particles when in-storm radar observations are not yet available and also provide information on the environment outside the thunderstorms. To better understand the strengths and limitations of each observation type, the satellite and Doppler radar observations are assimilated separately and jointly, and the ensemble analyses and forecasts are compared with available observations. Results show that assimilating BT observations has the potential to increase the forecast and warning lead times of severe weather events compared with radar observations and may also potentially complement the sparse surface observations in some regions as revealed by the probabilistic prediction of mesocyclone tracks initialized from EnKF analyses as various times. Additionally, the assimilation of both BT and Vr observations yields the best ensemble forecasts, providing higher confidence, improved accuracy, and longer lead times on the probabilistic prediction of midlevel mesocyclones.