Marine atmospheric boundary layer divergence and clouds along California in June 1996

The authors have performed a numerical experiment using Mesoscale Model 5 ( MM5) with a horizontal resolution of 9 km to simulate hourly atmospheric dy namics and thermodynamics along the U.S. California coast for all of June 1 996. The MM5 results were evaluated using more than 18 000 data points from wind profilers, radiosondes, buoys, and land stations; the results support the use of modeled dynamics for reliable monthly statistics and calculatio n of diurnal variations. Month-long mesoscale simulations of the marine atm ospheric boundary layer (MABL) and satellite observations have been used to investigate the diurnal variation of near-shore and farther offshore cloud s along the U.S. California coast. The authors extended the usual model eva luation with respect to time series and power spectrum analysis to investig ate a link between the evaluated dynamics and satellite-derived cloudiness. Two distinct types of cloudiness variation were revealed. One is in the ne ar-shore zone, extending approximately 100 km in the offshore direction, wh ere the diurnal variation of cloudiness develops in response to the formati on of MABL wind divergence and convergence fields. Each of the five major c apes between southern Oregon and southern California has a satellite-derive d, low-cloud maximum albedo on the leeward side and a minimum on the windwa rd side that closely corresponds to "expansion fans" and "compression bulge s." The expansion fan is associated with a divergence field of fast horizon tal winds, shallow MABL, and high Froude number. The compression bulge is a ssociated mainly with relatively weak winds (convergent or slightly diverge nt), a deeper MABL, and smaller Froude number. Simulated divergence in the expansion fan areas shows a significant diurnal trend with the maximum duri ng the late morning through early afternoon. In the compression bulge, eith er the divergence is an order of magnitude less, or the flow becomes conver gent. Going westward, the MABL divergence becomes an order of magnitude les s at distances of 30-40 km from the coastline. Since the expansion fan is c haracteristic of the MABL, the effect of the divergence field decays rapidl y in the vertical and, due to mass continuity, reverses into a convergent f low above the MABL. Farther offshore, the cloudiness variation is at a minimum around midday as well, but that is mainly a consequence of radiative heat transfer effects within the cloud. Marine atmospheric boundary layer divergence does not hav e a significant diurnal trend in that area. Daytime offshore cloud clearing begins first in the northern domain, where the marine layer and clouds are shallower. The clearing propagates southward until the marine layer and cl ouds are too deep; generally the clouds persist throughout the entire day. The study shows the importance of dynamics on the evolution of observed clo udiness and constitutes an approach to indirectly evaluate modeled dynamics using satellite-derived cloudiness.