Modelling regional scale surface energy exchanges and cbl growth in a heterogeneous, urban-rural landscape

Over the last decade, simple models of the convective boundary layer (CBL) have been suggested as an approach to inferring regionally averaged land-air exchanges of heat, water and trace gases, because the properties of the CBL respond to an average of the underlying small-scale heterogeneity. This paper explores the use of an integral CBL method to infer regionally averaged fluxes in a landscape that has at least three major sources of heterogeneity - irrigated and non-irrigated rural land use and a large urban area (Sacramento region, California). The first part of the paper assesses the validity of the simple slab model of the CBL - this is integrated forwards in time using local-scale measured heat and water vapour fluxes, to predict mixed-layer depth, temperature and humidity. Of the four different CBL growth schemes used, the Tennekes and Driedonks model is found to give the best performance. Evaluation of the model performance with different weightings of heat and water vapour fluxes based on the land use characteristics in the region suggest that the source area for the boundary-layer sonde measurements is larger than physically-based estimates would suggest. Finally, measured time series of potential temperature are used to infer regionally averaged sensible heat fluxes using an integral CBL (ICBL) method. These ICBL fluxes are compared with those measured at the local scale over the three land use types that comprise the region of interest. They are found to be closest to the heat fluxes calculated by appropriately weighting the measured heat fluxes in the source area calculated for the ICBL. We conclude that the integral CBL budget method provides adequate estimates of regionally-averaged surface heat fluxes in a landscape that is characterised by surface types with distinctly different surface energy budgets.