A Study of Convection Initiation in a Mesoscale Model Using High-Resolution Land Surface Initial Conditions

Abstract

A coupled convection-resolving mesoscale atmosphere?land surface model (LSM) is used to investigate land surface?planetary boundary layer (PBL) interactions responsible for the initiation of deep, moist convection over the southern Great Plains of the United States on 19 June 1998. A high-resolution land data assimilation system provides initial conditions to the LSM, facilitating examination of soil moisture effects on forecasts of deep convection. During the late morning and early afternoon, the southwestern portion of a simulated southwest?northeast (SW?NE)-oriented surface water vapor gradient zone evolves into an intense dryline, unlike the northeastern portion, which remains relatively weak. Despite these regional differences, midafternoon convection initiation occurs within a ?100-km-wide region of enhanced PBL depth along much of the SW?NE extent of the water vapor gradient zone. The afternoon PBL depth maximum results from a midmorning-to-early afternoon surface sensible heat flux maximum of similar horizontal scale, and is reinforced by an ensuing mesoscale (L ? 100 km) vertical circulation. Finescale (L ? 10 km) PBL circulations that directly trigger deep convection are confined within this mesoscale region that contains the deeper and more unstable PBL. Comparisons among different simulations reveal that thermodynamic stability and simulated convection initiation are affected by details in the initial soil moisture distribution, through differences in the partitioning of the surface heat and moisture fluxes. These differences in convection initiation among simulations occur despite only minor differences in the overall structure of the afternoon surface moisture gradient zone, which has potentially important implications for operational forecasts of deep convection.