UCLA

Abstract: Stable cold pools in California’s Central Valley (CV) are conducive to freezing temperatures, high relative humidity, and, in some cases, fog. In this study it will be shown that the Weather Research and Forecasting (WRF) Model as commonly configured cannot reproduce such conditions because of a persistent warm and dry bias near the surface. It was found that removing horizontal diffusion, which by default operates on model levels and thus up and down the valley’s sides, can reduce but not entirely fix the problem. Other improvements include enhancing the near-surface vertical resolution and the surface–air coupling, as both directly control the surface fluxes, especially evaporation. However, these alterations actually have the largest impact in the forested region surrounding the Central Valley, and influence the nighttime relative humidity in the CV only indirectly via nocturnal drainage flows. While it is not clear how realistic are the increased evaporation in the forest or the drainage flows, how and why these alterations result in significantly improved relative humidity reconstructions within the Central Valley are shown.