Land-atmosphere Interactions: Successes, Problems and Prospects

After two decades of active research, a much better understanding of the broader role of biospheric processes on the local climate has emerged. A surface-albedo increase, particularly in desert border regions of the subtropics (as well as the deforested tropical regions), leads to a net surface energy deficit, which in turn leads to a relative sinking and reduced rainfall. On the other hand, studies of the influence of altered ratios of evapotranspiration and sensible fluxes, in situations where the net solar income is unchanged, show that evapotranspiration is a more desirable flux for increased precipitation and vitality of the biosphere. Besides providing water vapor and convective available potential energy (CAPE) to the lower troposphere, evapotranspiration helps in building lager CAPE before "turning on" the moist-convection. Larger CAPE in the lower troposphere enables convection to reach unto the deeper atmosphere thereby heating the upper troposphere; indeed, moist-convection is also accompanied by the evaporation of falling precipitation that cools and moistens the lower atmosphere. While convective, as opposed to stratiform, precipitation reduces the fractional cloud cover; it also allows more solar radiation to reach the surface thereby invigorating surface fluxes. These, together with moist-convection and associated downdrafts help to maintain the characteristic upper temperature limit(s) of the moist-land regions.

Regardless of the above understanding, several important problems continue to hinder the accurate simulation of a realistic land-atmosphere interaction in a numerical model (both GCM or Meso-scale). Among the unsolved problems are parameterization of sub-gird scale or small-scale variability of soil moisture, snow-cover and snow-physics, the biosphere itself, orography, local drainage, and surface water flow. A well-known non-linear response of surface fluxes to these variations makes the problem of parameterizing land-atmosphere interaction processes hard-to-address, if at all tractable. In our presentation we will discuss how orographic, snow-cover, and water table interactions can be included into a Simple Biosphere Model such as SiB/SSiB.