Microphysical Influences on Cold Pools

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 strength, depth, and propagation speed. Numerical simulations based upon deep convection observed 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 primarily by altering the initial characteristics influencing warm-rain, ice processes, or secondary ice production, or the scaling factors in the underlying size distributions of graupel and 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 10 minutes prior to the initiation of a -2K cold pool. The melting and sublimation of graupel in the downdraft dominates the integrated latent cooling terms. The evaporation of rain in the downdraft, while not dominant, is still an important contribution, however that from the melting or sublimation of hail is minimal. Interestingly, the latent cooling of the air in the downdraft due to rain evaporation is nearly identical between the different simulations, suggesting that a specific amount may be required in order for cold pools to form in a given environment. Speeding or slowing of the warm rain process accelerated or slowed cold pool development, respectively (the latter also limiting the maximum cold pool strength) while also altering the amount of graupel and hail produced in the storms. On the other hand, the cold pool propagation speed shows a strong positive relationship with the time- integrated latent cooling due to the sublimation and melting of graupel. Similar analysis investigating the importance of these latent cooling terms upon cold pool depth will also be presented. Future work will entail analysis of simulations conducted within a variety of environments where the vertical wind shear and thermodynamic profiles differ.