Algorithms to Account for Topographic Shading Effects and Surface Temperature Dependence on Terrain Elevation in Diagnostic Meteorological Models

Abstract

Modern diagnostic meteorological models such as CALMET parameterise slope flows, kinematic terrain effects, terrain blocking effects, and include a divergence minimisation procedure. They also contain a micrometeorological model for the calculation of the planetary boundary-layer parameters both over-land and over-water. In CALMET, in particular, the energy balance equation gives the total contribution of direct and diffuse solar radiation reaching the ground, and topography shading is not accounted for. However, especially in the case of very complex terrain, topography shading generally has important effects on the energy balance, and as a consequence, on the flow field. Also, air temperature at the ground depends on the altitude above sea level, and interpolation of sparse measurements on a regular domain grid should account for this. We present algorithms that should improve the description of the physical effects mentioned above, through the independent calculation of direct, diffuse and reflected components of shortwave radiation, the consideration of the air temperature vertical gradient and vegetation cover for the interpolation of surface temperature observations, and the evaluation of the position of the Sun and topography slope and aspect. These algorithms, of general application, were implemented into CALMET to evaluate, qualitatively and against measurements, their performance on a very complex Alpine domain in northern Italy. CALMET was chosen since it is the diagnostic meteorological model of the CALPUFF modelling system. CALPUFF is recommended by the United States Environmental Protection Agency (US-EPA) as the preferred model for long-range transport in general, and for shorter distances on a case-by-case basis.