92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012: 3:45 PM
Climate Mean State and Diurnal Cycle in a General Circulation Model with Improved Representation of Convection and Clouds
Room 355 (New Orleans Convention Center )
Zachary A. Mangin, Iowa State University , Ames, IA; and X. Wu and T. C. Chen

A correct diurnal cycle simulation is essential in general circulation model (GCM) studies. If an incorrect diurnal cycle of precipitation exists, a mean climate state may be achieved but for the wrong reasons. Therefore, it is vital in climate simulations that day-to-day fluctuations in atmospheric variables are modeled correctly compared to observations. It is these parameters that alter the hydrological and energy cycles. An added complication includes that of regional mesoscale features, which are often improperly resolved in the models. This may be due to coarse model resolution or inaccurate large-scale interactions with atmospheric processes, such as convection. The experimental, uncoupled Iowa State University General Circulation Model (ISUGCM), based on a version of the National Center for Atmospheric Research GCM, is used in this study. It employs a modified Zhang-McFarlane convection scheme and updated cloud parameterizations based on cloud-resolving model research to test for improvements in the diurnal cycle and climate mean state. Global annual mean precipitation rates in the 10-year ISUGCM simulation closely resemble several observational datasets, and a double Intertropical Convergence Zone (ITCZ) precipitation feature is not apparent. To dissect such mean climate differences, diurnal analysis is performed. Our major focus is on precipitation since the major change in the experimental model involves clouds and convective processes. Precipitation in climate models tends to occur too often and peak a few hours too early at lower than observed intensities. With ISUGCM, the deep convection trigger is thought to prevent premature convection and allow for convective available potential energy buildup before its release. The revised convective closure assumption ties convection to destabilization of the troposphere above the boundary layer by large-scale advection. It is thus thought that large-scale forcing regulates the diurnal cycle of precipitation. Our research includes an attempt to understand the true mechanism that controls this diurnal cycle and then changes the global mean state, which will be presented in the meeting.

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