New soil physical properties implemented in the Unified Model at PS18

Abstract

This note describes the improvements to the Unified Model (UM) soil hydraulic and thermal properties implemented in Parallel Suite 18 (PS18) that became operational in April 2008. Also described are trials of a two-stream multi-layer canopy radiation scheme that has been implemented in PS20 and became operational in November 2008. The soil hydraulic properties affect the soils ability to hold water and the rate at which water moves through the soil. The soil moisture together with the soil hydraulic properties control transpiration from plants and direct evaporation from bare soil. The UM soil hydraulic properties are derived using the Cosby et al (1984) equations from information about soil texture; fractions of sand, silt and clay particles. Also, the soil thermal conductivity and heat capacity both depend on soil moisture and soil texture, so that these properties in turn influence the land surface temperature. Thus, soil moisture and the soil physical properties control the partitioning of net surface