Broadband water vapor absorption of solar radiation tested using ARM data

A high quality data set collected at the ARM SGP central facility is analyzed to identify the water vapor-related anomalous absorption of solar radiation. We focus on the direct solar broadband surface irradiances under cloud-free conditions in the April 1994 CAGEX and summer 1996 periods. A radiation model without including the H2O continuum in the solar spectrum is compared with observations. This comparison reveals a mean bias of 4.5 W m(-2) that is roughly equivalent to a globally averaged bias of similar to 2 W m(-2). It is therefore unlikely that the excess absorption suggested by recent studies can be attributed to the H2O continuum. We also show that the difference between calculated and observed irradiances has a very weak correlation with water vapor. It is suggested that the discrepancy between model and observations is largely due to the uncertainties associated with aerosol optical properties.