Studies with the Biosphere-Atmosphere Transfer Scheme

Abstract

In order to better model the climate system, land-surface models are continuously being improved. Several studies using the Biosphere-Atmosphere Transfer Scheme (BATS) are presented. One study compares simulations with the new ten-layer soil model (TLSM) and the previous BATS soil model at global and regional scales. TLSM tends to have much higher bare soil evaporation than the previous soil model. Soil in regions with high precipitation became wetter, while soil in regions with less precipitation became much drier. Potential errors in TLSM included underpredicted runoff and high-latitude transpiration. Corrections for these errors were incorporated and tested at six points. Surface runoff is increased by extracting water from the upper three TLSM layers rather than only the top layer. Bare soil evaporation is limited to the water present in the surface soil layer. A diurnal temporal error in the downward longwave radiation forcing data did not appear to significantly affect simulated long-term or large-scale averages. However, the assumption of uniform hourly distribution of 6-hour total precipitation did impact the partitioning of precipitation into evaporation, transpiration and runoff. A new method for modeling vertical water flow in heterogeneous porous media using the water-content based form of Richards equation is described, then used with BATS/TLSM to simulate the boreal forest energy and water exchanges at a black spruce site, where a thick moss layer covers a peat/loam soil, and at an aspen site with a homogenous clay soil. The moss is treated as a type of porous media, so its unique hydraulic and thermal properties can be modeled directly. Simulated net radiation is very similar to that observed over the summer months at both sites, but latent heat is greatly overestimated and simulated sensible heat fluxes are not well correlated with the observations. Observed soil temperature profiles and soil water content profiles are well captured at the black spruce site, as is the ability of moss to keep the underlying soil layers moist and cool. Despite the successful modifications made to TLSM during this study, the overestimation of evaporation remains a problem that should be addressed before widespread use of this model occurs.