Diurnal Cycles of Surface Winds and Temperatures as Simulated by Five Boundary Layer Parameterizations

Abstract

Although most of the planetary boundary layer (PBL) parameterizations have demonstrated the capability to reproduce many meteorological phenomena in the lowest few kilometers, little attention has been paid to the prediction of the diurnal cycles of surface wind speed (VSFC) in relation to surface temperature (TSFC). In this study, the performance of five widely used PBL parameterizations in reproducing the diurnal cycles of VSFC and TSFC is evaluated using the 3-day mesoscale simulations of summertime weak-gradient flows over the central United States where little organized convection and topographical forcing were present. The time series of area-averaged VSFC and TSFC, as well as the vertical wind and thermal profiles from the five sensitivity simulations, are compared with hourly surface observations and other available data. The hourly surface observations reveal that the diurnal cycles of VSFC are in phase (but surface wind directions are 5?6 h out of phase) with those of TSFC. It is shown that both VSFC and TSFC are very sensitive to the PBL parameterizations, given the identical conditions for all of the other model parameters. It is found that all five of the PBL schemes can reproduce the diurnal phases of TSFC (and wind directions), albeit with different amplitudes. However, all of the schemes underestimate the strength of VSFC during the daytime, and most of them overestimate it at night. Moreover, some PBL schemes produce pronounced phase errors in VSFC or substantially weak VSFC all of the time, despite their well-simulated diurnal cycle of TSFC. The results indicate that a perfect simulation of the diurnal TSFC cycle (and the thermal structures above) does not guarantee the reproduction of the diurnal cycles of VSFC. The final outcome would depend on how various physical processes, such as the vertical turbulent exchanges of the mass and momentum under different stability conditions, are parameterized. Because the upper portion of the PBL flow is often nearly opposite in phase to VSFC under weak-gradient conditions, the results have significant implications for the predictability of diurnal precipitation and the studies of air quality, wind energy, and other environmental problems.