CLEAR-SKY ATMOSPHERIC SOLAR TRANSMISSION - AN ANALYSIS BASED ON FIRE-1991 FIELD EXPERIMENT DATA

We consider explanations for disagreements between models and measurem ents of clear-sky solar transmission. Data from four cloud-free days d uring the FIRE 1991 field experiment are studied. Our model simulation s of solar broadband fluxes exceed measurements by about 10%. About ha lf of this discrepancy occurs in the near-infrared spectral region. Ab out half of this discrepancy can be linked to the direct irradiance. T hese deviations suggest (1) model under estimates in near-infrared tra ce-gas absorption and (2) errors in visible diffuse fluxes. The near-i nfrared error in other models is generally less than the one associate d with our model. Other models suggest more trace-gas absorption. Howe ver, variations among models are so large that they swamp current meas urement errors such as uncertainties of the water vapor column. The vi sible diffuse flux error cannot be easily explained. We illustrate tha t a poor cosine response of solar flux instruments could have contribu ted. Also, underestimates of aerosol absorption could be a factor. The re is clearly a need for measurements and comparisons at high spectral resolution to better quantify the sources of these errors. However, t hese comparisons will only be fruitful if we have confidence in the ac curacy of our models and the accuracy of the data.