Convective Cold Pool Responses to Changes in Soil Moisture

Cold pools produced by convective storms play an important role in convective organization and development. It is therefore important to understand the factors governing cold pool properties and evolution. Motivated by studies pointing out the role of surface heat fluxes in cold pool dissipation, this study investigates the question, how does soil moisture affect cold pool properties? In particular, how does soil moisture affect cold pool strength, cold pool size, and cold pool thermodynamic structure, and why? This study addresses these questions through the application of a cold pool identification and tracking algorithm to simulations conducted using the open-source Regional Atmospheric Modeling System (RAMS), which is coupled to the LEAF-3 interactive land surface model.

The RAMS model was used to conduct three simulations of tropical sea-breeze convection. The domain dimensions were 550 km × 200 km × 26 km, and the horizontal grid spacing was 1 km, with 57 vertical levels. The model was run for 16 hours and simulated the development of two types of convection: continental convection associated with diurnal heating, which is the convection analyzed in this study, and convection generated by the sea breeze, which is not analyzed here. Each of the three simulations was initialized with a different amount of soil moisture. The moist, intermediate, and dry simulations were initialized at 75%, 50%, and 25% of soil water saturation, respectively, and the soil moisture and surface fluxes were allowed to evolve over time.

Within each of the simulations, an ensemble of convective cold pools develops. A novel cold pool analysis algorithm identifies and tracks the cold pool "objects". For each simulation, the thermodynamic, kinetic, and microphysical fields associated with each cold pool are averaged together. This forms a "composite cold pool" with properties that reflect the full ensemble of cold pools in that simulation.

When soil moisture is decreased, stronger cold pools result even though the parent convection produces less rain. These stronger cold pools are also smaller because increased environmental sensible heat fluxes in the reduced soil-moisture simulations cause the cold pools to dissipate more quickly as they expand. Finally, the rings of enhanced water vapor that have been documented in previous studies of tropical oceanic cold pools largely disappear when soil moisture is reduced. These results serve to emphasize the role that land surface properties can have in modulating cold pool properties.