Column Water Vapor from Diffuse Irradiance

A possibility of measuring water vapor column from diffuse irradiance, and thus the extension of optical retrievals to cloudy days, was investigated. The data from the Rotating Shadowband Spectroradiometer (RSS) during its winter deployment at the North Slope of Alaska (NSA) site is used. The initial analysis covers 20 days in March 1999 that include clear, partly cloudy, and overcast days. During these days, water vapor column according to the NSA site’s Microwave Radiometer (MWR) varied between 1 and 5 mm. The diffuse irradiances in the 820 and 940-nm water vapor absorption bands are compared with water vapor column obtained from the MWR. While these irradiances do not correlate well with the water vapor column, we found that by using the diffuse transmittance in the 760-nm oxygen absorption band to obtain an effective air mass, we could derive a method that greatly increases the correlation. The end result is a correlation of 0.97 and 0.95 between MWR and diffuse RSS retrievals using 820 and 940-nm transmittances, respectively. This encouraging result is based on empirical data analysis only. It implies that the diffuse irradiance may contain sufficient information to retrieve water vapor column. However, we must analyze broader data sets to ensure that the resulting high correlation in these data is not predicated on the limited climatological conditions experienced in March 1999 at the NSA site.