Idealised numerical simulations of diurnal sea breeze characteristics over a step change in roughness

Abstract

A two-dimensional hydrostatic meso-scale model is used to study possible effects of a step change in roughness on the daytime sea breeze (SB). Two approaches are employed for the calm synoptic condition with a clear sky. First, idealised numerical simulations are made with the length of aerodynamical roughness (z0) ranging from 0.05m to 0.6 m and by an increase of 0.05 m. The magnitudes are changed in all ground surface grid points. The specific features examined are the maxima of the wind velocity field, the turbulent fluxes and the turbulent kinetic energy (TKE), the height of the mixed layer, the inland penetration of the SB and the slope of the SB front. Two z0 effects are pointed out: an increase in the turbulent mixing and a reduction in the wind speeds in the surface layer. Results show that a relationship between these two effects is crucial for the SB inland penetration. A moderate z0 change increases both turbulence intensity and a vertical extent of the boundary layer. An increase of both heights and magnitudes of the SB circulation, caused by a vertical extension of the turbulent field, what followed by a faster inland penetration of the SB. For larger z0 change, the effect of dissipation overcomes the effect of the SB strengthening and the inland penetration is slower. Second, an urban area is simulated only by a locally increased z0. The rough area intensifies both TKE and the SB during its growing stage. An increase of z0 also reduces the wind speed in the vicinity of the urban area and it augments the slope of the SB front.