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On 28 May 2019, a tornadic supercell, observed as part of Targeted Observation by UAS and Radars of Supercells (TORUS) produced an EF-2 tornado near Tipton, Kansas. The supercell was observed to interact with multiple preexisting airmass boundaries. These boundaries and attendant air masses were examined using unoccupied aircraft system (UAS), mobile mesonets, radiosondes, and dual-Doppler analyses derived from TORUS mobile radars. The cool-side air mass of one of these boundaries was found to have higher equivalent potential temperature and backed winds relative to the warm-side air mass; features associated with mesoscale air masses with high theta-e (MAHTEs). It is hypothesized that these characteristics may have facilitated tornadogenesis. The two additional boundaries were produced by a nearby supercell and appeared to weaken the tornadic supercell. This work represents the first time that UAS have been used to examine the impact of preexisting airmass boundaries on a supercell, and it provides insights into the influence environmental heterogeneities can have on the evolution of a supercell. [ABSTRACT FROM AUTHOR]

Mobile weather radars at high frequencies (C, X, K, and W bands) often collect data using staggered pulse repetition time (PRT) or dual pulse repetition frequency (PRF) modes to extend the effective Nyquist velocity and mitigate velocity aliasing while maintaining a useful maximum unambiguous range. These processing modes produce widely dispersed "processor" dealiasing errors in radial velocity estimates. The errors can also occur in clusters in high shear areas. Removing these errors prior to quantitative analysis requires tedious manual editing and often produces "holes" or regions of missing data in high signal-to-noise areas. Here, data from three mobile weather radars were used to show that the staggered PRT errors are related to a summation of the two Nyquist velocities associated with each of the PRTs. Using observations taken during a mature mesoscale convective system, a landfalling tropical cyclone, and a tornadic supercell storm, an algorithm to automatically identify and correct staggered PRT processor errors has been developed and tested. The algorithm creates a smooth profile of Doppler velocities using a Savitzky–Golay filter independently in radius and azimuth and then combined. Errors are easily identified by comparing the velocity at each range gate to its smoothed counterpart and corrected based on specific error characteristics. The method improves past dual PRF correction methods that were less successful at correcting "grouped" errors. Given the success of the technique across low, moderate, and high radial shear regimes, the new method should improve research radar analyses by affording the ability to retain as much data as possible rather than manually or objectively removing erroneous velocities. [ABSTRACT FROM AUTHOR]

Electrification and lightning are simulated for a small continental multicell storm. The results are consistent with observations and thus provide additional understanding of the charging processes and evolution of this storm. The first six observed lightning flashes were all negative cloud-to-ground (CG) flashes, after which intracloud (IC) flashes also occurred between middle and upper levels of the storm. The model simulation reproduces the basic evolution of lightning from low and middle levels to upper levels. The observed lightning indicated an initial charge structure of at least an inverted dipole (negative charge above positive). The simulations show that noninduetive charge separation higher in the storm can enhance the main negative charge sufficiently to produce negative CG flashes before upper-level IC flashes commence. The result is a 'bottom-heavy' tripole charge structure with midlevel negative charge and a lower positive charge region that is more significant than the upper positive region, in contrast to the traditional tripole structure that has a less significant lower positive charge region. Additionally, the occurrence of cloud-to-ground lightning is not necessarily a result of excess net charge carried by the storm, but it is primarily caused by the local potential imbalance between the lowest charge regions. The two-moment microphysics scheme used for this study predicted mass mixing ratio and number concentration of cloud droplets, rain, ice crystals, snow, and graupel. Bulk particle density of graupel was also predicted, which allows a single category to represent a greater range of particle characteristics. (An additional hail category is available but was not needed for the present study.) The prediction of hydrometeor number concentration is particularly critical for charge separation at higher temperatures (-5[degrees] < T < - 20[degrees]C) in the mixed phase region, where ice crystals are produced by time fracturing (Hallett--Mossop process) and by splintering of freezing drops. Cloud droplet concentration prediction also affected the rates of inductive charge separation between graupel and droplets. DOI: 10.1175/2009JAS2965.1

Results of a modeling study of the 24 May 1989 dryline are presented. A nonhydrostatic, two-dimensional version of the Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the impact of east-west variability of soil moisture and vegetation on convective boundary layer evolution and dryline formation. The effects of the initial moisture and wind fields and the impact of the Coriolis force on the model results are also examined. Model output is compared with special airborne and sounding observations of the 24 May dryline. Several findings of an earlier observational study of the 24 May dryline are supported in the present study. The modeled drylines are broadly comparable to the observed dryline with respect to the following properties: 1) width, 10 km; 2) strong horizontal moisture and virtual potential temperature gradients, [greater than]4 g [kg.sup.-1]/10 km and 2 K/10 km; 3) strong horizontal convergence, updraft, W-E shear of N-S wind component, 8 X [l0.sup.-4] [s.sup.-1], 1 m [s.sup.-1], 10 m [s.sup.-1]/10 km, respectively; 4) bulges of moisture and N-S wind component above the surface dryline location (due to combination of vertical mixing and vertical advection); and 5) elevated moist layer east of the dryline during late afternoon (due to eastward advection of the moisture bulge). A rather diffuse dryline begins to form by about noon from the combined effects of strong vertical mixing of moisture and westerly momentum to the west of the dryline. Convective boundary layers (CBLs) of differing depths develop on either side of the dryline, giving rise to horizontal airflow acceleration in low levels via a mesoscale hydrostatic pressure gradient east of the dryline. A vorticity dynamics analysis indicates that a horizontal virtual potential temperature gradient causes strong solenoidal forcing of horizontal vorticity (order [10.sup.-5] [s.sup.-2]) and a thermally direct vertical circulation that concentrates at the dryline. Low-level winds east of the dryline decelerate, generating convergence and resulting in the development of an easterly, subgeostrophic airflow component during midafternoon. Strong convergent frontogenesis contributes to rapid development of the horizontal thermal gradients at the dryline during middle to late afternoon. For example, convergent and net frontogenesis of vapor mixing ratio achieves order [10.sup.-7] g [kg.sup.-1] [m.sup.-1] [s.sup.-1] . Ageostrophic forcing helps maintain a low-level southerly jet east of the dryline during the daytime via the Coriolis term. Virtual temperature differences across the dryline during late afternoon, implying virtual density differences, are consistent with the notion that the dryline propagates relative to the ambient westerly shear according to the theoretical phase speed of a density current. The formation of the dryline and the evolution of the CBL are sensitive to the east-west profile of sensible heating, which in turn is sensitive to the soil moisture. A west-to-east change of volumetric soil moisture from 0.35 to 0.5 over 50 km (0.15/50 km), resulting in a sensible heat flux gradient in the surface layer of about 100 W [m.sup.-2]/50 km, is sufficiently large to cause a dryline to form. Drylines do not form in test cases where volumetric soil moisture is horizontally homogeneous with values of 0.5 or 0.35. A mesoscale convergence line, characterized by a strong updraft collocated with a weak moisture gradient, forms in the latter case. Horizontal variations of sensible heat flux in the surface layer are accentuated by vegetation contrasts, causing additional sensitivity of CBL and dryline evolution.

The dryline is recognized as a major factor in the initiation of severe thunderstorms in the central and southern plains of the United States during the spring. Although severe thunderstorm forecasters often use the strength and position of the dryline to help determine prime areas for convective development, relatively little is known of the exact mechanisms by which thunderstorms form in the dryline environment. In the spring of 1991 experiments were carried out to study the dryline and convective storms near the dryline as part of the Cooperative Oklahoma Profiler Studies program, which was supported by the National Oceanic and Atmospheric Administration, the National Science Foundation, and the National Aeronautics and Space Administration. Observing systems deployed in these experiments included a research aircraft equipped with both in situ instrumentation and a Doppler radar, two mobile laboratories capable of remote release of rawinsondes, a surface mesonetwork, the Profiler Demonstration Network, and several ground-based Doppler radars. Among the episodes intensively observed during the period were several in which tornadic storms formed in the dryline environment. The goals of the dryline experiments are described. The key weather events and observing strategies are summarized for four of the cases. Primary issues that can be addressed in future in-depth studies using these datasets are noted and preliminary findings from analyses done to date are included.

This case study analyzes a nocturnal mesoscale convective system (MCS) that was observed on 25-26 June 2015 in northeastern Kansas during the Plains Elevated Convection At Night (PECAN) project. Over the course of the observational period, a broken line of elevated nocturnal convective cells initiated around 0230UTCon the cool side of a stationary front and subsequently merged to form a quasi-linearMCSthat later developed strong, surface-based outflow and a trailing stratiform region. This study combines radar observations with mobile and fixed mesonet and sounding data taken during PECAN to analyze the kinematics and thermodynamics of the MCS from 0300 to 0630 UTC. This study is unique in that 38 consecutive multi- Doppler wind analyses are examined over the 3.5 h observation period, facilitating a long-duration analysis of the kinematic evolution of the nocturnal MCS. Radar analyses reveal that the initial convective cells and linear MCS are elevated and sustained by an elevated residual layer formed via weak ascent over the stationary front. During upscale growth, individual convective cells develop storm-scale cold pools due to pockets of descending rear-to-front flow that are measured by mobile mesonets. By 0500 UTC, kinematic analysis and mesonet observations show that the MCS has a surface-based cold pool and that convective line updrafts are ingesting parcels from below the stable layer. In this environment, the elevated system has become surface based since the cold pool lifting is sufficient for surface-based parcels to overcome the CIN associated with the frontal stable layer. [ABSTRACT FROM AUTHOR]

The 25-26 June 2015 nocturnal mesoscale convective system (MCS) from the Plains Elevated Convection at Night (PECAN) field project produced severe winds within an environment that might customarily be associated with elevated convection. This work incorporates both a full-physics real-world simulation and an idealized single-sounding simulation to explore the MCS's evolution. Initially, the simulated convective systems were elevated, being maintained by wavelike disturbances and lacking surface cold pools. As the systems matured, surface outflows began to appear, particularly where heavy precipitation was occurring, with air in the surface cold pools originating from up to 4-5 km AGL. Via this progression, the MCSs exhibited a degree of self-organization (i.e., structures that are dependent upon an MCS's particular history). The cold pools eventually became 1.5-3.5 km deep, by which point passive tracers revealed that the convection was at least partly surface based. Soon after becoming surface based, both simulations produced severe surface winds, the strongest of which were associated with embedded low-level mesovortices and their attendant outflow surges and bowing segments. The origin of the simulated mesovortices was likely the downward tilting of system-generated horizontal vorticity (from baroclinity, but also possibly friction) within the simulated MCSs' outflow, as has been argued in a number of previous studies. Taken altogether, it appears that severe nocturnal MCSs may often resemble their cold pool-driven, surface-based afternoon counterparts. [ABSTRACT FROM AUTHOR]