Underestimation of GCM-Calculated Short-Wave Atmospheric Absorption in Areas Affected by Biomass burning

Abstract

Listen

Translate article

Many current General Circulation Models (GCMs) exhibit a common problem, namely that their atmosphere is too transparent to solar radiation. The underestimation of atmospheric short-wave absorption by these models is particularly large in areas and seasons where extensive biomass burning takes place. This is shown using surface radiation measurements combined with co-located satellite observations at sites affected by biomass burning in Equatorial Africa. The observed absorption of solar radiation in the atmosphere is significantly larger during the dry season where biomass burning takes place than during the wet season. In contrast, GCMs, which do not account explicitly for aerosols arising from biomass burning, calculate a maximum absorption in the wet season and a minimum absorption in the dry season, as a result of the dominance of water vapour absorption in the calculations. This indicates that the inadequate representation of absorbing aerosols in GCMs significantly contributes to the underestimated short-wave atmospheric absorption in regions affected by biomass burning. This can reach up to 30–40 Wm-2 regionally and seasonally, and more so locally. For a realistic simulation of the radiation budget in these regions, an adequate spatial and temporal representation of aerosols is therefore crucial. Thus, in addition to possible underestimates in water vapour and cloud absorption, the lack of absorbing aerosols should be accounted for in the ongoing discussion of the problem of underestimated short-wave absorption in the GCM atmospheres. Evidence is presented that an appropriate representation of aerosol and water vapour absorption may be sufficient to close the gaps between model-calculated and observed estimates of short-wave atmospheric absorption. Hence, a substantial enhancement of cloud absorption in GCMs, as claimed in other studies, seems not immediately necessary from the present investigation.