Influence of Land Surface Fluxes on Precipitation: Inferences from Simulations Forced with Four ARM–CART SCM Datasets

Abstract

Four different Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed (ARM–CART) Single-Column Model (SCM) datasets were used to force an SCM in a number of simulations performed to study the influence of land surface fluxes on precipitation. The SCM employed Goddard Earth Observing System (GEOS-2) GCM physics, which includes a recent version of prognostic cloud scheme (Microphysics of Clouds with Relaxed Arakawa–Schubert), and a land model (Simplified Simple Biosphere Model) coupled to a highly resolved soil hydrological description in the vertical. The four ARM–CART datasets employed in these studies are referred as case 1, case 3, case 4, and case 8. The SCM simulation results broadly confirm the previous findings that an increase in the solar absorption and surface evaporation helps to increase the local rainfall, but they also reveal that the magnitude of the rainfall increase is strongly affected by the ability of the background circulation to promote moist convection. The simulated precipitation increase was as large as 50% of the evapotranspiration increase for case 1 that covered a relatively wet period. It was substantially reduced for cases 3 and 4 covering a normal rainfall period and became negligible for case 8, a dry case. A part of evaporation increase became horizontal divergence of water vapor; this would have the potential of increasing the precipitation downstream of the test region. For a particular background circulation, it was found that the evaporation–precipitation relationship, often defined as recycling ratio, is remarkably robust even for a large range of vegetation covers, soil types, and initial soil moistures. Notwithstanding the limitations of only one-way interaction (i.e., the large scale influencing the regional physics and not vice versa), the current SCM simulations show that recycling ratio is a function of the background circulation and not a regional and/or seasonal feature. Indeed, a vigorous biosphere can help to produce more rainfall under wet conditions but may do little to dislodge a large-scale drought. It is pointed out that even though these inferences are robust, they are prone to weaknesses of the SCM physics as well as the assumption of the large scale remaining unaffected by changes of moist processes.