Response of a Stably Stratified Atmosphere to Low-Level Heating— An Application to the Heat Island Problem

Abstract

Two-dimensional airflow characteristics past a heat island are investigated using both a linear analytic model and a numerical model in the context of the response of a stably stratified atmosphere to specified low-level heating in a constant shear flow. Results from the steady-state, linear, analytic solutions exhibit typical flow response fields that gravity waves produce in response to the local heat source in the presence of environmental flow. The magnitude of the perturbation vertical velocity is shown to be much larger in the shear-flow case than in the uniform-flow case. This is because the basic-state wind shear is a source of the perturbation wave energy. Nonlinear numerical model experiments over a wide range of heating amplitudes are performed to examine nonlinear effects on the simulated flow field. For smaller heating amplitude (hence, smaller nonlinearity factor), the flow response field is similar to that produced by the linear gravity waves. On the other hand, two distinct flow features are observed for larger heating amplitude (hence, larger nonlinearity factor): the gravity-wave-type response field on the upstream side of the heat island and the strong updraft circulation cell located on the downstream side. As the heating amplitude increases, the updraft circulation cell strengthens and shifts farther downwind. The strong updraft cell is believed to be partly responsible for precipitation enhancement observed on the downstream side of the heat island. It is found that the continuing downwind propagation of the updraft circulation cell is related to basic-state wind speed.