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1. A Hail Growth Trajectory Model for Exploring the Environmental Controls on Hail Size: Model Physics and Idealized Tests

   https://doi.org/10.1175/Jas-D-20-0016.1  

   Kumjian, Matthew R.; Lombardo, Kelly (August 2020, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract A detailed microphysical model of hail growth is developed and applied to idealized numerical simulations of deep convective storms. Hailstone embryos of various sizes and densities may be initialized in and around the simulated convective storm updraft, and then are tracked as they are advected and grow through various microphysical processes. Application to an idealized squall line and supercell storm results in a plausibly realistic distribution of maximum hailstone sizes for each. Simulated hail growth trajectories through idealized supercell storms exhibit many consistencies with previous hail trajectory work that used observed storms. Systematic tests of uncertain model parameters and parameterizations are performed, with results highlighting the sensitivity of hail size distributions to these changes. A set of idealized simulations is performed for supercells in environments with varying vertical wind shear to extend and clarify our prior work. The trajectory calculations reveal that, with increased zonal deep-layer shear, broader updrafts lead to increased residence time and thus larger maximum hail sizes. For cases with increased meridional low-level shear, updraft width is also increased, but hailstone sizes are smaller. This is a result of decreased residence time in the updraft, owing to faster northward flow within the updraft that advects hailstones through the growth region more rapidly. The results suggest that environments leading to weakened horizontal flow within supercell updrafts may lead to larger maximum hailstone sizes. 

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2. Entrainment in a Simulated Supercell Thunderstorm. Part II: The Influence of Vertical Wind Shear and General Effects upon Precipitation

   https://doi.org/10.1175/JAS-D-21-0289.1  

   Jo, Enoch; Lasher-Trapp, Sonia (May 2022, Journal of the Atmospheric Sciences)

   Abstract Supercell thunderstorms can produce heavy precipitation, and an improved understanding of entrainment may help to explain why. In Part I of this series, various mechanisms of entrainment were identified in the rotating stage of a single simulated supercell thunderstorm. The current study examines the strength and effectiveness of these mechanisms as a function of the environmental vertical wind shear in eight different supercell simulations. Entrainment is calculated directly as fluxes of air over the surface of the storm core; tracers are used to assess the resulting dilution of the moistest air ingested by the storm. Model microphysical rates are used to compare the impacts of entrainment on the efficiency of condensation/deposition of water vapor on hydrometeors within the core, and ultimately, upon precipitation production. Results show that the ascending “ribbons” of horizontal vorticity wrapping around the updraft contribute more to entrainment with increasing vertical wind shear, while turbulent eddies on the opposite side of the updraft contribute less. The storm-relative airstream introduces more low-level air into the storm core with increasing vertical wind shear. Thus, the total entrainment increases with increasing vertical wind shear, but the fractional entrainment decreases, yielding an increase in undiluted air within the storm core. As a result, the condensation efficiency within the storm core also increases with increasing vertical wind shear. Due to the increase in hydrometeors detrained aloft and the resulting enhanced evaporation as they fall, the precipitation efficiency evaluated using surface rainfall decreases with increasing vertical wind shear, as found in past studies. 

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3. Influence of Low-Level Shear Orientation and Magnitude on the Evolution and Rotation of Idealized Squall Lines. Part II: Storm Propagation and Dynamic Feedbacks

   https://doi.org/10.1175/MWR-D-25-0240.1  

   Brown, Matthew C; Marion, Geoffrey R; Coniglio, Michael C (February 2026, Monthly Weather Review)

   Abstract Idealized simulations described in Part I investigated the importance of near-surface [low level (LL); 0–500 m] line-parallel shear, varied in both direction and magnitude, for modulation of quasi-linear convective system (QLCS) updraft and vortex attributes. Here, in Part II, we explore dynamical processes contributing to these attributes. Results indicate that LL shear strongly impacts the strength and streamwise nature of LL inflow vorticity, while free lower-tropospheric line-normal shear modulates storm propagation and LL storm-relative flow (largely through modifications to ground-relative flow). These combine to impact the storm-relative helicity (SRH) of QLCS inflow, with higher SRH for near-storm environments with increased LL hodograph curvature and length (i.e., enhanced LL line-parallel shear). Increased influx of streamwise vorticity fosters the stronger and wider rotating updrafts identified previously, which are driven by intense midlevel dynamic accelerations. When coincident with near-surface circulation, these strongly forced updrafts can support intense, long-lived mesovortices. These results demonstrate that changes to the orientation and magnitude of the ambient LL shear vector, as well as the vertical distribution of shear, can have considerable influence on the underlying dynamics governing the propagation, updrafts, and rotation within squall lines—even for a fixed line-normal environmental shear value. Altogether, this study (along with Part I) provides crucial context for understanding near-storm environmental impacts on QLCS morphology and rotation. Significance StatementResearch related to linearly organized storms has largely focused on vertical changes in winds (i.e., shear) oriented normal to squall lines, but squall lines that produce considerable rotation often have a component of low-level shear oriented parallel to storms. This study, Part II of a larger work, assesses how these shear components influence the updrafts within squall lines. Line-normal shear and ambient flow impact how storms propagate, while line-parallel shear impacts their near-surface rotation. These factors combine to influence the overall strength and rotation within updrafts of quasi-linear convective systems (QLCSs) and their ability to support near-surface vortices that often precede squall-line tornadoes. These physical relationships can help improve our understanding and prediction of QLCS evolution and associated hazards. 

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4. Composite In Situ Microphysical Analysis of All Spiral Vertical Profiles Executed within BAMEX and PECAN Mesoscale Convective Systems

   https://doi.org/DOI: https://doi.org/10.1175/JAS-D-19-0317.1  

   Stechman, D.M.; McFarquhar, G.M.; Rauber, R.M.; Jewett, B.F.; and Black, R.A. (July 2020, Journal of the atmospheric sciences)

   van den Heever, S. (Ed.)

   Vertical profiles of temperature, relative humidity, cloud particle concentration, median mass dimension, and mass content were derived using instruments on the NOAA P-3 aircraft for 37 spiral ascents/descents flown within five mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) project, and 16 spiral descents of the NOAA P-3 within 10 MCSs during the 2003 Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The statistical distribution of thermodynamic and microphysical properties within these spirals is presented in context of three primary MCS regions—the transition zone (TZ), enhanced stratiform rain region (ESR), and the anvil region (AR)—allowing deductions concerning the relative importance and nature of microphysical processes in each region. Aggregation was ubiquitous across all MCS zones at subfreezing temperatures, where the degree of ambient subsaturation, if present, moderated the effectiveness of this process via sublimation. The predominately ice-supersaturated ESR experienced the least impact of sublimation on microphysical characteristics relative to the TZ and AR. Aggregation was most limited by sublimation in the ice-subsaturated AR, where total particle number and mass concentrations decreased most rapidly with increasing temperature. Sublimation cooling at the surface of ice particles in the TZ, the driest of the three regions, allowed ice to survive to temperatures as high as +6.8°C. Two spirals executed behind a frontal squall line exhibited a high incidence of pristine ice crystals, and notably different characteristics from most other spirals. Gradual meso- to synoptic-scale ascent in this region likely contributed to the observed differences. 

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5. Aerosol interactions with deep convective clouds

   https://doi.org/10.1016/B978-0-12-819766-0.00001-8  

   Fan, Jiwen; Li, Zhanqing (January 2022, Elsevier)

   Deep convective clouds (DCCs) are associated with the vertical ascent of air from the lower to the upper atmosphere. They appear in various forms such as thunderstorms, supercells, and squall lines. These convective systems play important roles in the hydrological cycle, Earth’s radiative budget, and the general circulation of the atmosphere. Changes in aerosol (both cloud condensation nuclei and ice-nucleating particles) affect cloud microphysics and dynamics, and thereby influence convective intensity, precipitation, and the radiative effects of deep clouds and their cirrus anvils. However, the very complex dynamics and cloud microphysics of DCCs means that many of these processes are not yet accurately quantified in observations and models. This chapter outlines the main ways in which changes in aerosol affect the microphysical, dynamical, and radiative properties of DCCs. Aerosol interactions with DCCs depend on aerosol properties, storm dynamics, and meteorological conditions. When aerosol particles are light-absorbing, such as soot from industry or biomass burning, the aerosol radiative effects can alter the meteorological conditions under which DCCs form. These radiative effects modify temperature profiles and planetary boundary layer heights, thus changing atmospheric stability and circulation, and affecting the onset and development of DCCs. These large-scale effects, such as the effect of anthropogenic aerosol on the East and South Asian monsoons, can be simulated in coarse-resolution models. These processes are described in Chapter 13. This chapter is concerned with aerosol interactions with DCC systems ranging from individual clouds to mesoscale convective systems. Increases in cloud condensation nuclei (CCN) can enhance cloud droplet number concentrations and decrease droplet sizes, thereby narrowing the droplet size spectrum. For DCCs, a narrowed droplet size spectrum suppresses warm rain formation (rain derived from non-ice-phase processes), allowing the transport of more, smaller droplets to altitudes below 0°C. This may result in (i) freezing of more supercooled water, thereby enhancing latent heating from icerelated microphysical processes and invigorating storms (ice-phase invigoration); (ii) modification of ice-related microphysical processes, which changes cold pools, precipitation rates, and hailstone frequency and size; (iii) expansion of the mixed-phase zone and decreases in the cloud glaciation temperature; and (iv) slowing down of cloud dissipation, resulting in larger cloud cover and cloud depth in the stratiform and anvil regions due to numerous smaller ice particles. The increased cloud cover and cloud depth constitute an influence of aerosol on the cloud radiative effect. Reduced diurnal temperature variation has been observed and simulated as a result of enhanced daytime cooling and nighttime warming by expanded anvil cloud area in polluted environments. However, the global radiative effect of aerosol interactions with DCCs remains to be quantified. 

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