Wednesday, 16 January 2002: 4:30 PM
Convection and Large-scale Quasi-equilibrium in Midlatitude Continental Environment
The quasi-equilibrium assumption proposed by Arakawa and Schubert (1974) is the cornerstone of the modern convective parameterization development for global climate modeling. It assumes that statistically there is a quasi-equilibrium between the large-scale forcing, which builds convective instability, and convection, which consumes convective instability. The assumption was developed largely based on the observations in the tropical maritime environment and has not been evaluated in midlatitude environment. This study examines the quasi-equilibrium assumption in the midlatitude continental convection environment using summertime observations from the Southern Great Plains of the United States. Two complementary approaches are taken for this purpose. The first one compares the net time rate of change of convective available potential energy to that due to the large-scale forcing. The second one examines the contributions to the net change of convective available potential energy from the boundary layer air and the free tropospheric air above the boundary layer. Results from both the approaches show that the quasi-equilibrium assumption is not well suited for midlatitude continental convection. The main reason for this is the large contribution to the net change of convective instability from the thermodynamic changes of the boundary layer air. Based on these observations, a modification to the quasi-equilibrium assumption is suggested, and its implications for convective parameterization in climate models are discussed.