The role of ocean roughness in regional climate modeling: 1994 East Asian summer monsoon case

To understand the role of the ocean roughness length, and find its sensitivity to the simulation of a regional climate model for the 1994 east Asian summer monsoon case, three experiments were performed providing different ocean roughness lengths. In order to analyze the role of the roughness length only, all other variables were fixed during the 92 summer days of the simulation period, except that different ocean roughness lengths in magnitude. The control experiment (CTRL) using a fixed value of 4 × 10^-4 m roughness length over the ocean did not properly simulate the extreme anomalous summer of 1994. The simulated results of the roughness length experiment using 1.2 × 10^-3 m (ZOEXP), and 4 × 10^-3 m (ZOEXP2) showed that the increased ocean roughness length could reduce the strength of southerly to southwesterly monsoon flows, and consequently counter the excessive northward movement of the summer monsoon fronts. ZOEXP reduced to some extent the model biases producing more anomalous features closer to the 1994 extreme summer climate than in CTRL. The most sensitive impacts resulted from the increased ocean roughness length took place in the area of northern China, Manchuria and east central China where summertime traveling cyclonic systems and typhoons associated with the summer monsoon fronts were active. In those regions, the ground temperature over the land areas was also considerably changed, that corresponded well to the changes of the surface heat fluxes. One of the important results in this study is that the simulated precipitation pattern in ZOEXP was closer to that of the Global Precipitation Climatology Program analysis data than in CTRL. The increased moisture supply near the surface along with the decreased low-level southwesterly flows due to the changes of ocean roughness length contributed to not only the organized monsoon fronts but also the appropriate movement of the monsoon fronts with the increased monsoon rain. The increased ocean roughness length altered the phase changes of precipitation in southern Korea and Japan, and the precipitation amount over central and southern China. However, in general, the magnitudes of difference fields between ZOEXP and CTRL did not overcome those of CTRL from the NCEP/NCAR reanalysis data. In the regional climate modeling not only sea surface temperature but also ocean roughness length should be reasonably provided.