Solar Heating Rates: The Importance of Spherical Geometry

Abstract

A crucial component of any GCM is a scheme for calculating atmospheric heating rates. Since a detailed treatment of all processes involved is time consuming, many approximations are usually made. An approximation used in virtually all GCM radiation codes that extend into the middle atmosphere is that the atmosphere can be treated as plane parallel. This approximation breaks down when the sun is close to the horizon and does not apply for large solar zenith angles. This paper shows that this approximation leads to a very serious underestimate of solar heating rates in the polar regions. Ignoring spherical effects, and in particular the heating due to absorption at zenith angles greater than 90°, gives rise to a very different latitudinal gradient in the diurnally averaged heating rates calculated at the equinox. Such a change in the latitudinal gradient in the heating rate is of significance for the general circulation. Accounting for the heating that occurs at zenith angles greater than 90° is shown to be important.