An Investigation of Hydrometeor Latent Cooling upon Cold Pool Formation, Sustainment, and Properties

Downdrafts extending from convective clouds can produce cold pools, regions of air at the surface cooled by the melting, sublimation, or evaporation of hydrometeors within the downdrafts. These cold pools can propagate outward, sometimes initiating new convection along their leading edges. Models operating at scales requiring convective parameterizations typically lack a representation of this detail and thus omit convective regeneration and fail to predict longer episodes of convective activity (e.g. severe weather outbreaks). Recent studies have attempted to parameterize cold pools and the associated convection they can trigger, but a lack of understanding of the most important factors influencing cold pool properties hampers these efforts. Prior studies have investigated the influence of different hydrometeor types upon the formation of the initial cold pool but have reached different conclusions.

This study uses CM1 (“Cloud Model 1”), a non-hydrostatic, fully compressible model, to produce a set of 12 simulations within a single convective environment. Numerical simulations based upon deep convection observed during the MC3E field campaign are produced using the NSSL (6-class, double moment) microphysics scheme with a grid spacing of 250 meters. The simulations vary by altering the initial characteristics influencing warm-rain or ice processes, or the scaling factors in the underlying size distributions of graupel and hail, in order to investigate the hydrometeor type(s) and associated microphysical cooling that are most important for: (i) influencing the initial cold pool formation, (ii) sustaining the cold pool, and (iii) determining cold pool expansion rates, depth, and strength.

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 downdrafts forming and sustaining the cold pool. The cold pool onset time is earlier in cases with a strong warm-rain process, but both rain and graupel were equally likely to be the dominant hydrometeor in the downdraft contributing to cold pool formation. Graupel sublimation is the dominant term in sustaining the cold pool in all simulations, but the evaporation of rain has the strongest correlation to the cold pool expansion rate, depth, and intensity. The melting or sublimation of hail is minimal in all simulations in this particular environment, for both cold pool formation and sustainment. This study suggests that the consideration of microphysics of deep convection is necessary to accurately represent cold pools and their effects in larger-scale weather and climate parameterizations.