Chapter summary

The atmospheric boundary layer is the layer of the atmosphere above Earth's surface that is directly affected over the course of a day by surface fluxes such as sensible heat, latent heat, water vapor, and CO2. Sensible heat from the surface warms the boundary layer as heat is carried vertically by turbulent motion. Evaporated water moistens the boundary layer, releasing latent heat during condensation and forming clouds. The diurnal cycle of surface fluxes imparts a diurnal cycle to the boundary layer. At night, the boundary layer is typically stable with weak turbulent motion. Surface heating during the day warms the boundary layer and it becomes unstable. Soil water greatly influences the boundary layer by its effect on the partitioning of net radiation into sensible and latent heat. The Bowen ratio is typically smaller where soil water does not limit evapotranspiration, and the boundary layer is cooler, moister, and shallower than in the absence of evapotranspiration. Dry sites have lower latent heat flux and a warmer, drier, and deeper boundary layer. The changes in surface fluxes and boundary layer characteristics associated with wet soil may create conditions that favor precipitation. Surface heterogeneity in soil moisture can also generate mesoscale atmospheric circulations. Large patches of wet soil interspersed in a comparatively dry landscape create a contrast in surface fluxes.