SOLAR HEATING RATES - THE IMPORTANCE OF SPHERICAL GEOMETRY

A crucial component of any GCM is a scheme for calculating atmospheric heating rates. Since a detailed treatment of all processes involved i s time consuming, many approximations are usually made. An approximati on used in virtually all GCM radiation codes that extend into the midd le atmosphere is that the atmosphere can be treated as plane parallel. This approximation breaks down when the sun is close to the horizon a nd does not apply for large solar zenith angles. This paper shows that this approximation leads to a very serious underestimate of solar hea ting rates in the polar regions. Ignoring spherical effects, and in pa rticular the heating due to absorption at zenith angles greater than 9 0-degrees, gives rise to a very different latitudinal gradient in the diurnally averaged heating rates calculated at the equinox. Such a cha nge in the latitudinal gradient in the heating rate is of significance for the general circulation. Accounting for the heating that occurs a t zenith angles greater than 90-degrees is shown to be important.