The impact of geometrical effects on calculated value of albedo on the heterogeneous environmental interfaces

Abstract

The albedo of the interface is an important parameter for the calculation of the radiation fluxes in environmental studies. The problem arises when the surface is a heterogenous one. There have been developed various manners to calculate the aggregated albedo. Our previous research has indicated, however, existence of a geometrical effect related to the fact that different parts of the interface may have different heights. In these studies we have offered the general approach to the calculation of the flux that is lost due to the absorption on the vertical lateral boundaries. We derived analytically the expressions for this loss coefficient which for some ideal urban geometries coincides with Oke's skyview factor. The aim of this paper is to elaborate the effect for more complex geometry. The number of geometries allowing easy analytic solutions is rather restricted, so it was necessary to develop an efficient numerical procedure. In this paper we expose so called ray-trace MC approach, then we use it to test the known analytical solutions and combine it with a parameterisation scheme (LAPS). An example of central geometry is considered in this study. It is assumed that the region consists of two-patch grid-cell with a square geometrical distribution and different heights of its parts. Simulations were done for several patch areas, with different horizontal dimensions and heights ("propagating building") as well as for different surface types. The multiple scattering effect and the dependence of the albedo on the zenithal angle of the incident radiation were neglected. The derived expression for the albedo is compared with the conventional approach. Changes in albedo lead to a significant change in partitioning of the energy at environmental interface. The most remarkable changes are in values of sensible and latent heat fluxes, and surface temperature. Their values are calculated using LAPS parameterisation scheme and then compared to the values obtained with a conventional parameterisation of the albedo.