Linearized Calculations of Urban Heat Island Convection Effects

Abstract

Steady, linearized flow calculations are carried out to estimate vertical temperature profiles over a heated area representing a city. Initially, planar flow is considered in a stable atmosphere with a constant stability and a constant effective eddy diffusivity for heat transfer. The calculations predict the main effects observed over urban areas: 1) positive temperature perturbations near the ground which will tend to cancel the early morning radiation inversion, and 2) negative temperature perturbations aloft which will tend to produce one or more weak inversions several hundred meters above the city. This idealized calculation gives better results than expected, with appropriate values for the flow parameters yielding the approximate mean magnitude and height of the main (lowest) layer of negative temperature perturbations. The computed flow field shows a downward velocity directly over the upwind portion of the heat island, similar to nonlinear calculations and observations for ocean islands, as well as recent nighttime observations over a city. Within the linearized framework the calculations are extended to include (i) different planar surface temperature distributions, (ii) the three-dimensional case of a circular heat island, (iii) a two-layer atmosphere having a change in stability at a suitable altitude, (iv) a constant eddy viscosity for the perturbed flow, and (v) the Coriolis force.