Numerical simulations of the postsunrise reorganization of a nocturnal mesoscale convective system during 13 june IHOP_2002

A convection-permitting numerical model is used to simulate the postsunrise reorganization of a nocturnal mesoscale convective system (MCS) observed over western and central Oklahoma on 13 June 2002 during the International H ₂O (IHOP_2002) Field Experiment. The MCS reorganization consists of a transition from northwest-southeast-oriented convective rainbands near sunrise to a single northeast-southwest (NE-SW)- oriented convective rainband with trailing stratiform precipitation later in the morning. Results indicate the importance of environmental preconditioning on MCS reorganization. In particular, the development of the NE-SW rainband that redefines the MCS organization is facilitated by a similarly oriented zone of antecedent mesoscale upward motion, which increases the depth of large water vapor mixing ratios. This allows convective updrafts to be fed primarily by moist and conditionally unstable air from1 to 2.5 kmAGL in the NE-SW-oriented rainband, which lacks a surface cold pool during its incipient postsunrise stage. The MCS develops a strong surface cold pool from latent cooling-induced downdrafts by midmorning and evolves into an upshear-tilted squall-type system. These downdrafts and the resulting cold pool are not necessary for the overall reorganization and maintenance of the MCS in this environment where earlier mesoscale ascent has occurred. However, the latent cooling from downdrafts does influence the MCS strength, vertical structure, and horizontal motion by early in the postsunrise stage. In contrast, surface heating of the preconvective environment has little effect on the strength and structural characteristics of the MCS until midday, by which time the convection has become primarily surface based.