Influence of surface characteristics on sensible and latent heat fluxes and boundary-layer mesoscale circulations

Data from the Cooperative Atmosphere-Surface Exchange Study 1997 field program CASES-97 and the international H2O Project (IHOP_2002) are used to illustrate how surface-modulated thermodynamic fluxes influence the development of mesoscale (>50 km) circulations. Surface properties (vegetative cover, soil moisture, terrain) all influence the horizontal distribution of surface heat and moisture fluxes, and terrain influences the horizontal and vertical distribution of temperature and the resulting convergence-divergence patterns. We compare the evolution of boundary layer structure in CASES-97 (Spring, growing winter wheat, dormant grass) and IHOP_2002 (Summer, senescent or harvested winter wheat, mature grass) to highlight the differences and similarities of the roles of the individual surface properties, and discuss what distinguishes days with apparent mesoscale circulations from days on which synoptic patterns seem to dominate the wind field. The data for the case studies of interest were collected on fair-weather days, but the processes should apply to days on which convection develops.