Microphysical Influences on cold pools

Cloud physics;precipitation physics

Mallinson, Holly M. ; Lasher-Trapp ; Sonia G.

关键词: Cloud physics;    precipitation physics;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101583/MALLINSON-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Downdrafts extending from within convective clouds to the ground can produce cold pools, regions of air at the surface cooled by melting or sublimating ice and/or evaporation of rain within the downdrafts.These cold pools can propagate outward, sometimes initiating new convection along their leading edges.Models operating at scales requiring convective parameterizations usually lack a representation of this detail, and thus omit this convective regeneration and fail to predict longer episodes of convective activity (e.g. severe weather outbreaks).Recent studies have begun attempting to parameterize cold pools and the associated convection they can trigger, but a lack of understanding of the most important factors for cold pool strength, depth, and propagation speed hampers these efforts.Prior studies have investigated the influence of different hydrometeor types upon the formation of the initial cold pool but have reached drastically different conclusions.This study uses CM1 (“Cloud Model 1”), a non-hydrostatic, fully compressible model, to produce a set of simulations in order to investigate the hydrometeor types and associated microphysical processes that are most important for determining cold pool initiation timing, strength, depth, and propagation speed.Idealized numerical simulations based upon deep convection observed on a single day during the MC3E field campaign are produced using the NSSL (6-class, double moment) microphysics scheme and a grid spacing of 250 meters.The simulations vary by altering the initial characteristics influencing warm-rain, ice processes, or secondary ice production, or the scaling factors in the underlying size distributions of hail.These simulations are all performed using the same environmental conditions.Time-integrated microphysical budgets are calculated to quantify the contribution of each hydrometeor type (e.g. melting of graupel or hail, sublimation of graupel or hail, or evaporation of rain) to the total latent cooling occurring in the downdraft prior to the initiation of a -2K cold pool.The melting and sublimation of graupel in the downdraft dominates the integrated latent cooling terms for some runs, while the evaporation of rain dominates in others.However, the contribution from the melting or sublimation of hail is minimal.Time-integrated microphysical budgets are also calculated to quantify the contribution of each hydrometeor type most responsible for sustaining the cold pool.Here, the latent cooling is calculated within all downdrafts that intersect it, for the 51 minutes after its initiation.Graupel sublimation always dominates the integrated latent cooling term in this case.Rain evaporation, while not dominant, is still an important contribution. Microphysical factors affecting the initiation timing, speed, strength and depth of the cold pool through the latent cooling they promote in the downdrafts are also explored.Quickening or slowing the warm rain process respectively hastens or slows cold pool initiation.Slowing the warm rain process also limits the maximum cold pool strength by altering not only rainfall but also the amount of graupel and hail.On the other hand, the average cold pool propagation speed best correlates with the amount of latent cooling due to the sublimation and melting of graupel.The total time- integrated latent cooling best correlates with the average cold pool depth, as no single phase change term dominates that relationship.