Convection-Permitting-WRF Assisted Investigation of Precipitation Biases in the CAM5 over the Tropical Western Pacific

Description

Tropical precipitation biases are among the most outstanding systematic biases in global climate models. Most of the biases are commonly attributed to deficiencies in the parameterizations of cloud-related physics. In this study, we focus on the investigation of precipitation biases in the Community Atmosphere Model version 5 (CAM5) simulations over a broader Tropical Western Pacific (TWP) region spanning 60 degrees in longitude and 40 degrees in latitude that also cover all the three ARM TWP sites. The CAM5 simulations are mainly short-term hindcasts using the Cloud Associated Parameterization Testbed (CAPT) framework to emphasize the influence of model physics on the precipitation biases. It is found that the occurrence of significant precipitation is more than twice as frequent and total precipitation amount is more than 30% higher than the observation during the study period in later December 2003. Similar biases also exist in long-term CAM5 simulations. The biases increase when the forecast range is extended, indicating the amplifying effect of error growth in the dynamical and thermodynamical conditions. Large-scale forcing derived from the ECMWF forecast analyses that are archived in the ARM external data center are further used to drive single-column model CAM5 experiments over the ARM TWP sites to quantify the relative contributions due to error growth in physical or dynamical processes. The majority of the model precipitation biases over the TWP region are produced by the convection scheme. Convection-permitting Weather Research and Forecasting (WRF) model simulations over the same TWP domain are then used to assist the investigation of the behavior of the precipitation physics, particularly the deep convective parameterization in the CAM5. Key elements that are essential to the development and evolution of convective processes but are parameterized in the CAM5 while explicitly calculated in the WRF simulations will be examined in detail. Eulerian composite of WRF simulations in terms of precipitation characteristics shows a similar thermodynamical structure that is observed in typical convective system over a life cycle. Such Eulerian composites are therefore useful guidance for convective parameterization at CAM5 grid scale. Some important convective and mesoscale properties that are missing in the current scheme are also derived to aid in the interpretation of the CAM5 simulated results.