GREENHOUSE-GAS RADIATIVE FORCING - EFFECTS OF AVERAGING AND INHOMOGENEITIES IN TRACE GAS-DISTRIBUTION

Radiative forcing is a useful diagnostic quantity for indicating the s ize of climate change mechanisms, and for interpreting the results of General Circulation Model (GCM) experiments. It is also ideal for expl oring the potential sensitivity of climate change simulations to assum ptions regarding, for example, the horizontal and vertical distributio n of changes in greenhouse gases, as a guide to whether these effects need to be incorporated into GCMs. This paper examines this issue for a range of greenhouse gases, and also examines the dependence of the f orcing on horizontal averaging and the definition of tropopause positi on. By comparison with calculations at a high horizontal resolution, i t is shown that the use of a single global mean profile results in glo bal mean radiative forcing errors of several percent for CO2 and chlor ofluorocarbon CFC-12 (CCl2F2); the error is reduced by an order of mag nitude or more if three profiles are used, one each representing the t ropics and the southern and northern extratropics. Three alternative d efinitions of tropopause position lead to a spread in forcing of up to 9%. The effect of the spatial Variation of a number of greenhouse gas es, which are often regarded as well-mixed, is then explored. For meth ane, three-dimensional distributions are used for present and pre-indu strial cases generated from chemical-transport models. The inhomogenei ties are shown to have little impact on the forcing, because they are concentrated at the surface. The assumption that methane is well-mixed in both the vertical and horizontal, results in errors in the global mean forcing of only 2% compared to the full calculation; at individua l locations the error never exceeds 4%. For CFC-11 (CCl3F) and CFC-12, recent satellite data from the Cryogenic Limb Array Etalon Spectromet er are used to define the Vertical and latitudinal variation in the st ratosphere. For the global mean forcing, the assumption that CFC-11 an d CFC-12 are well-mixed in the horizontal and vertical results in erro rs of less than 4%, which is significantly smaller than current spectr oscopic uncertainties. For CFC-II this error is a factor of two smalle r than that implied in recent work, where the forcing from a well-mixe d distribution was compared with that of an idealised vertical distrib ution. However, the zonal variation of the forcing is more seriously a ffected by the use of the satellite distributions, and errors can reac h 30% at high latitudes on assuming a well-mixed distribution. Finally , the distribution of hydrohalocarbons generated from a 2-D chemical m odel is used assuming an idealised, predominantly northern hemisphere source distribution. Five gases are included, with lifetimes ranging f rom 2 to 26 years. The error in global mean radiative forcing arising from the assumption that these gases are well-mixed is 10% for gases w ith lifetimes above 10 years, but can reach 30% for,oases with lifetim es of about 2 years. It is found that almost all of the error is due t o the vertical rather than horizontal distribution; the fall-off above the tropopause, determined by the stratospheric lifetime, is an impor tant determinant of the size of the error. The errors in the zonal dis tribution of the forcing are much larger, and can reach a factor of tw o for the shorter-lived gases, particularly at high latitudes.