A Numerical Simulation of Seasonal Stratospheric Climate: Part I. Zonal Temperatures and Winds

Abstract

This paper examines the effects on the seasonal stratospheric circulation due to the following two longwave radiative transfer processes: 1) radiative coupling between the troposphere and lower stratosphere and 2) latitudinal and seasonal variation in the radiative response time h?1, where h is the Newtonian cooling coefficient. Two numerical experiments have been performed with a quasi-geostrophic nine-level global circulation model, in which one includes the two longwave processes mentioned above and the second experiment neglects both processes. The seasonal variations of the zonal temperatures and winds as simulated by the two experiments are compared to isolate the importance of the two longwave radiation processes. According to the model results, the troposphere-lower stratosphere radiative coupling resulting from the exchange of longwave energy contributes about 5?10 K to the latitudinal and seasonal variation of tropopause and lower stratosphere temperatures. At the model tropopause, the O3 heating, both solar and longwave, reaches maximum values in the midlatitude region during winter and hence the O3 heating acts in conjunction with the dynamical beating processes to maintain the warm midlatitude belt in the lower stratosphere. The large seasonal and latitudinal variations in h have significant effects on the latitudinal temperature gradients and on the seasonal variations of the temperature gradients in the middle and upper stratosphere. The results suggest that the effect of the seasonal and latitudinal variation of h is to cool the winter polar upper stratosphere and to enhance the winter and spring pole-to-equator temperature gradients. We also examine the effect on the stratospheric circulation due to a perturbation in O3 concentration at the model tropopause. This experiment indicates that the sharp reversal of vertical temperature gradient at the equatorial tropopause may be due to the steep vertical gradient in the O3 solar and 9.6 µm beating above the tropopause.