Forcing Mechanisms of Summertime PBL Depths in the Los Angeles Basin and Central Valley of California

Four months of observations from a network of eight 915-MHz boundary-layer radar wind profilers equipped with radio acoustic sounding systems operating in California's Central Valley and Los Angeles basin during the CalNex experiment of 2010 are used to investigate the variability of convective boundary-layer depth and its correlation to meteorological parameters and conditions. Surface meteorological variables such as pressure, temperature, relative humidity, wind speed, wind direction, solar radiation, net radiation, and precipitation, were measured by surface sensors at the same sites, or nearby. A previous study conducted over the entire year 2008 for five of the eight sites examined here showed that the boundary-layer height reaches its maximum in the late-spring months and then unexpectedly markedly decreases during the summer months, with July PBL depths nearly equal to those in December. Similar results are found for all the eight sites, including those in the LA basin, over the late-spring to summer period of year 2010. Forcing mechanisms driving the seasonal behavior of boundary-layer depth are investigated, including the effect of marine layer intrusion in the LA basin, and the reduction of solar radiation due to marine stratus. To find out if the numerical model predictions are able to mimic the observed behavior of the PBL depth and associated forcing mechanisms, the first hour forecast fields of the High-Resolution Rapid Refresh (HRRR) Model, run at 3 km resolution over continental USA, are evaluated.