A theoretical study of the influence of urban surfaces on the convective rolls dynamics

Abstract

Convective rolls are two-dimensional convective quasi-lines that can strongly influence phenomena associated with severe weather. They can also influence the urban micro-climate by increasing the surface wind shear. In this study, we aim to theoretically estimate the influence of the urban heat island and of the urban roughness on the dynamics of the convective rolls. To this purpose, we first derive analytical expressions for the vertical velocity of the rolls in the framework of the Monin–Obukhov similarity theory, which depend on the surface heat flux and of the friction velocity. By using the urban surface roughness, parameterized by its drag coefficient, related directly to the friction velocity, we use the obtained expressions of the vertical velocity of the rolls to visually analyze the contribution of surface heat flux and of the urban surface roughness on the rolls’ velocity. We thus prove that the increased heat fluxes associated with urban surfaces have a strong impact on the convective motion, and that the urban surface roughness can influence the rolls’ dynamics over urban heat islands. Finally, we argue that the studies investigating the urban heat island-induced sub-cloud convective flows would be much improved if, besides the surface temperature distributions and the surface heat fluxes, they would include the urban surface roughness.