Summer Land–Atmosphere Coupling Strength over the United States: Results from the Regional Climate Model RegCM4–CLM3.5

Abstract

his study investigates the land?atmosphere coupling strength during summer over the United States using the Regional Climate Model version 4 (RegCM4)?Community Land Model version 3.5 (CLM3.5). First, a 10-yr simulation driven with reanalysis lateral boundary conditions (LBCs) is conducted to evaluate the model performance. The model is then used to quantify the land?atmosphere coupling strength, predictability, and added forecast skill (for precipitation and 2-m air temperature) attributed to realistic land surface initialization following the Global Land?Atmosphere Coupling Experiment (GLACE) approaches. Similar to previous GLACE results using global climate models (GCMs), GLACE-type experiments with RegCM4 identify the central United States as a region of strong land?atmosphere coupling, with soil moisture?temperature coupling being stronger than soil moisture?precipitation coupling, and confirm that realistic soil moisture initialization is more promising in improving temperature forecasts than precipitation forecasts. At a 1?15-day lead, the added forecast skill reflects predictability (or land?atmosphere coupling strength) indicating that that model can capture the realistic land?atmosphere coupling at a short time scale. However, at a 16?30-day lead, predictability cannot translate to added forecast skill, implying that the coupling at the longer time scale may not be represented well in the model. In addition, comparison of results from GLACE2-type experiments with RegCM4 driven by reanalysis LBCs and those driven by GCM LBCs suggest that the intrinsic land?atmosphere coupling strength within the regional model is the dominant factor for the added forecast skill at a 1?15-day lead, while the impact of LBCs from the GCM may play a dominant role in determining the signal of added forecast skill in the regional model at a 16?30-day lead. It demonstrates the complexities of using regional climate model for GLACE-type studies.