83rd Annual

Monday, 10 February 2003
A comparison of fractional cloud cover determinations by a sky imager with visual estimations by trained National Weather Service observers
Gary B. Hodges, CIRES/Univ. of Colorado, Boulder, CO; and J. A. Augustine
A relatively new instrument is being manufactured that determines fractional cloud cover, and it is now being deployed at several field research sites around the United States and other countries. This instrument, known as the Total Sky Imager (TSI), uses a digital camera focused on a convex mirror to capture full hemispherical images of the daytime sky. Separate images of the red and blue components are produced from the raw sky images, and ratios of these component images are used to objectively determine fractional cloud cover. While the TSI was undergoing development, a few comparisons with other methods of objectively determining fractional cloud cover were performed with promising results. However, up to the present time, what has never been done in a methodical fashion is a careful comparison of the objective TSI measurements with subjective observations taken by trained weather observers.

The primary objective of this research is to compare the objective TSI measurements with subjective observations by trained meteorological observers. In the current study, one year of cloud observations by National Weather Service (NWS) personel located at the Desert Rock, NV Weather Service Office are compared with data collected by a TSI located at the same facility. These comparisons are broken down and catagorized according to time of year, as well as by the different NWS observers. A secondary, but equally important objective of this research, is to carefully examine other scientific methods that indirectly estimate cloud properties.

The additional methods that are evaluated in this work include an empirical relationship between parameters computed from broadband shortwave component measurements (direct and diffuse irradiance) and cloud fraction. Another is based on downwelling infrared irradiance, air temperature, relative humidity, and surface pressure measurements. The latter method provides cloud fraction, cloud base height, and cloud base temperature, and because it is not dependent on shortwave measurements, it can also be applied at night, which is an added advantage. Given the number of surface radiation and meteorological observation stations worldwide, and if one or more of the indirect methods prove to be reliable, then cloud fraction and other cloud properties could be readily obtainable for atmospheric and climate research.

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