Surface Radiative Energy Budget and Cloud Forcing: Results from Toga Coare and Techniques for Identifying and Calculating Clear Sky Irradiance.

Surface measurements are needed for radiative transfer model and satellite retrieval algorithm development. The tropical western Pacific is an area of particular importance in global circulation, which until the recent Combined Ocean-Atmosphere Response Experiment (COARE) suffered a lack of high-quality measurements. The shortwave irradiance dominates both the magnitude and variability of the COARE surface radiative energy budget. The COARE averages of 194 and -57 Wm^{-2 } for the net shortwave and infrared, respectively, result in a net energy deposition of 137 Wm^ {-2} into the ocean. Contrasted with the corresponding 286 and rm -76 Wm ^{-2} clear sky irradiances, the presence of clouds causes a net loss of -79 Wm^{-2} at the surface. Using corresponding satellite derived top-of-atmosphere irradiances, the tropospheric cooling rates of 0.66 K/day for all-sky and 1.0 K/day for clear sky suggest that clouds cause heating of the tropical atmosphere at the expense of the surface. These cooling rates are about half the previous estimates for the tropical eastern Atlantic. Identification of clear sky is difficult in the complex and persistently cloudy atmosphere of the tropical western Pacific. Yet clear sky measurements are necessary to verify model calculations and satellite clear sky pixel identification, or to empirically fit a clear sky irradiance function. Clear sky periods are identified using only measurements of downwelling total and diffuse solar irradiance. This identification method works for all locations tested, including mid-latitudes. The clear measurements are used in an empirical fitting algorithm to calculate solar cloud forcing. The resultant cloud forcing estimates are mostly unbiased by calibration offsets. To facilitate the measurement of the solar components, an inexpensive shading arm attachment for standard pyranometers that requires minimal maintenance has been designed. This design is ideal for remote deployment at many locations and, for a moderate cost, can be affixed to any existing radiometer installation currently collecting only total hemispheric data. Design features include automated data quality control. Thus, the availability of accurate measured data needed for model and satellite retrieval refinement can be greatly enhanced.