Effects of Spatial and Temporal Variations in PBL Depth on a GCM

Abstract

The effect of variations in planetary boundary layer (PBL) height on a GCM was investigated using the Yonsei University (YONU) AGCM. This GCM assumes a convective boundary layer constrained to the two lowest model levels. As a consequence of this fixed PBL height, the model tends to produce excessive mixing. In this study the authors focuse on the impact of spatially and temporally varying PBL height. In addition to stability-dependent eddy diffusivity, the model adopts both bulk mixing due to the surface heat flux and z-less mixing under stable conditions. The variable-depth PBL strongly suppressed excessive mixing so that the model bias was reduced over stable regions such as the Antarctic continent. On the other hand, vertical mixing over continents in the summer hemisphere was stronger than for the fixed-PBL version, resulting in greater rising motion and warming effects in the lower atmosphere. The variable-PBL height reduces excess precipitation over the western Pacific and East Asian monsoon region. Based on the improved PBL parameterization, widespread decreases in surface sensible heat flux and rising motion in the lower troposphere occurred simultaneously over the East Asian continent. This implies that the precipitation simulation is very sensitive to PBL processes.