Dynamics of Reversed Urban Breeze Circulation

Abstract

he urban breeze circulation (UBC) is a thermally forced mesoscale circulation that is characterized by low-level inward flow toward the urban center, updrafts near the urban center, upper-level outward flows, and weak downdrafts outside the urban area. Previous numerical modeling studies indicate that in the early morning the direction of the UBC can be reversed. Here, the dynamics of a reversed UBC is studied in the context of the response of the atmosphere to specified thermal forcing, which represents diurnally varying urban heating. For this, a linearized, two-dimensional, hydrostatic, Boussinesq airflow system in a rotating frame with specified thermal forcing is solved using the Fourier transform method. The occurrence of a reversed UBC in the early morning is confirmed. The Coriolis parameter affects the strength and vertical structure of the UBC, whose role is similar to that of the coefficient of Rayleigh friction and Newtonian cooling. The occurrence condition, strength, and vertical structure of a reversed UBC are examined. The Coriolis force as well as urban heating alters the occurrence time of the reversed UBC. For a strongly viscous system, a reversed UBC occurs only in high latitudes with low occurrence possibility. A simple oscillation-type model for the horizontal velocity is constructed to get some dynamical insights into a reversed UBC. The analysis results also show that the Coriolis force alters the occurrence time of the reversed UBC.