Near-surface flow in complex terrain with coastal and urban influence

Abstract

A simple conceptual model is presented to describe the near-surface flow of a long, partially urbanized valley of slope \(\beta \) located normal to a coastline, considering forcing due to differential surface temperatures between the sea, undeveloped (rural) land and urban area. Accordingly, under weak synoptic conditions and when the coastal and urban (thermally induced pressure-gradient) forcing are in phase with that of the valley thermal circulation, the mean flow velocity \(U\) is parameterized by the cumulative effects of multiple forcing: \(U = \varGamma w_*\beta ^{1/3} +C(g\alpha \varDelta TL)^{1/2}\). This accounts for the coastal/urban forcing due to surface-air buoyancy difference \(g\alpha \Delta T\) over a distance \(L\). Here \(\varGamma \) and \(C\) are constants and \(w_*\) the convective velocity. Comparisons with data of the Meteo-diffusion field experiment conducted in a coastal semi-urbanized valley of Italy (Biferno Valley) reveal that the inferences of the model are consistent with observed valley flow velocities as well as sharp morning and prolonged evening transitions. While the experimental dataset is limited, the agreement with observations suggests that the model captures essential dynamics of valley circulation subjected to multiple forcing. Further observations are necessary to investigate the general efficacy of the model.