The problem that convection is too active over land during summertime in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM2 has been documented in many studies. This problem is closely related to the convection triggering mechanism used in its deep convection scheme, 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 sensible- and latent-heat fluxes, which can greatly affect the generation of CAPE over the land surface in summer. To reduce the problem, this study implements an improved triggering mechanism into the CAM2 deep convection scheme. The new triggering function introduces a dynamic CAPE generation rate to link the convective triggering condition to the large-scale dynamic processes, in order to reduce the effect of the strong diurnal variations in the surface fluxes on the initiation of convection. We evaluate the new triggering function in CAM2 under the U.S. Department of Energy~{!/~}s Climate Change Prediction Program (CCPP) - Atmospheric Radiation Measurement Program (ARM) Parameterization Testbed (CAPT) framework. Numerical weather prediction (NWP) reanalysis data and observations from ARM have been used to evaluate model simulations in this study. It is shown that the new triggering mechanism can considerably improve the model-predicted precipitation, in comparison with data collected during the ARM summer 1997 Intensive Operation Period (IOP). Impacts of the new triggering mechanism on other important simulated meteorological fields and simulations in other important climate regions are also discussed.

This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.