A steady plane-parallel isothermal turbulent flow of a viscous incompressible liquid above a surface with large-scale roughness is considered. The numerical model includes the Reynolds equations with the Boussinesq closing relations. Turbulent parameters are predicted on the basis of the k-e model of turbulence and Launder-Spalding's method of wall functions for calculation of near-wall flows. The problem is solved numerically by the finite-volume method. The influence of atmospheric parameters on airflows in a street canyon is investigated. Unfavorable meteorology conditions leading to accumulation of pollutants in urban blocks are revealed.