Subtropical Climatology of Direct Beam Solar Radiation

Abstract

A climatology of direct beam irradiance has been compiled for Mauna Loa Observatory. A broadband transmittance, calculated from the direct-beam data, has been stratified into clear sky and optically thin and thick cloud regimes; statistics of this stratification may represent a means of monitoring possible climate change. We have shown how direct beam irradiance observations may be used to infer a climatology of optically thin cloudiness. These techniques have been applied to a five-year data set from Mauna Loa Observatory. Clear sky frequency of occurrence was 56% with a mean clear sky transmittance of 0.72. Clouds influenced the direct solar beam irradiance 44% of the time; this resulted in a mean transmittance of 0.49. Clear skies exhibited an annual cycle with less direct beam attenuation in the winter months. Although no trends in optically thin cloudiness were apparent in the five-year data set, the variability of the data allows one to estimate the length of record required to detect a given magnitude trend. For example, trends of 0.1 and 1.0% per year would require monitoring times of 212 and 46 months, respectively. After removing zenith-angle dependence from the data through a vertical transformation, thin cloud occurrence is uniformly distributed between the 0.12-0.60 transmittance range.