Speaker: Susan C. van den Heever, Department of Atmospheric Science, Colorado State University, Fort Collins, CO

Seminar Title: Diving into Cold Pools

Abstract: Cold pools are produced by most precipitating convective storms and are frequent features in both the tropics and mid-latitudes. They form from the evaporation and/or melting of liquid and ice species within clouds which produce cold, dense air. Being less buoyant that its environment, this dense air sinks to Earth’s surface where it spreads radially outwards from the parent storm, thus forming a pool of near-surface cold, dense air referred to as the cold pool. Cold pools subsequently dissipate through the entrainment of environmental air, as well as through the impacts of surface fluxes. Cold pools play an important role in storm organization, storm longevity and the initiation of new convective storms. Given their significant role in convective storm life cycles, recent efforts are focused on improving their representation within weather forecast models, as well as developing cold pool parameterizations to better represent their impacts within global climate models. Understanding the physical processes governing cold pool formation and dissipation is therefore critical.

Cold pool development is influenced by the microphysical characteristics of storms, including the amount of precipitation being produced and the size distributions of cloud drops and ice crystals. These, in turn, are impacted by environmental characteristics including the relative humidity, dry layers, vertical wind shear and aerosol characteristics. Similar environmental characteristics impact the thermodynamic state of the air being entrained into cold pools and hence their dissipation rates. Cold pool dissipation rates are also determined by the sensible and latent heat fluxes of the surface over which cold pools are flowing, which are in turn a function of the soil and vegetation characteristics. In this seminar, the relative importance of various processes and environmental factors on cold pool formation and dissipation will be presented. The manner in which such factors are represented in cloud-resolving models will also be analyzed. Finally, the results from a recent field campaign in which UAVs were used to measure cold pool characteristics over northern Colorado will be discussed.