Evaluation Of An Improved Convection Triggering Mechanism In The Ncar Community Atmosphere Model Cam2 Under Capt Framework PDF Download

The problem that convection over land is overactive during warm-season daytime in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM2 and its previous version (CCM3) has been found both in its single-column model (SCM) simulations (Xie and Zhang 2000; Ghan et al. 2000; Xie et al. 2002) and in its full general circulation model (GCM) short-range weather forecasts (Phillips et al. 2003) and climate simulations (Dai and Trenberth 2003). These studies showed that this problem is closely related to the convection triggering mechanism used in its deep convection scheme (Zhang and McFarlane 1995), which assumes that convection is triggered whenever there is positive convective available potential energy (CAPE). The positive CAPE triggering mechanism initiates model convection too often during the day because of the strong diurnal variations in the surface isolation and the induced CAPE diurnal change over land in the warm season. To reduce the problem, Xie and Zhang (2000) introduced a dynamic constraint, i.e., a dynamic CAPE generation rate (DCAPE) determined by the large-scale advective tendencies of temperature and moisture, to control the onset of deep convection. They showed that positive DCAPE is closely associated with convection in observations and the dynamic constraint could largely reduce the effect of the strong diurnal variations in the surface fluxes on the initiation of convection. Using the SCM version of CCM3, which has the same deep convection scheme as CAM2, Xie and Zhang (2000) showed that considerable improvements can be obtained in the model simulation of precipitation and other thermodynamic fields when the dynamic constraint was applied to the model triggering function. However, the performance of the improved convection triggering mechanism in the full GCM has not been tested. In this study, we will test the improved convection trigger mechanism in CAM2 under the U.S. Department of Energy's Climate Change Prediction Program (CCPP)--Atmospheric Radiation Measurement Program (ARM) Parameterization Testbed (CAPT) framework, which provides a flexible environment for running climate models in Numerical Weather Prediction (NWP) mode. In comparison with testing physical parameterizations in climate simulations, the CAPT strategy uses more available observations and high-frequency NWP analyses to evaluate model performance in short-range weather forecasts. This allows specific parameterization deficiencies to be identified before the compensation of multiple errors masks the deficiencies, as can occur in model climate simulation. Another advantage of the CAPT approach is its capability to link model deficiencies directly with atmospheric processes through case studies using data collected from major field programs (e.g., ARM).