Surface Solar Irradiance in the Central Pacific during Tropic Heat: Comparisons between in Situ Measurements and Satellite Estimates

Catherine Gautier California Space Institute, Scripps Institution Of Oceanography, University of California, San Diego

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Abstract

We present the first results concerning solar radiation at the ocean surface during the Tropic Heat experiment. Using calibrated GOES visible brightness measurements, a simple radiative transfer model calculates hourly and daily surface solar irradiance values. To validate the satellite-estimated solar irradiance, surface solar irradiance measurements are taken from three sources; the Tropic Heat buoy 3, the R/V Weeama, and the small tropical Pacific island of Hiva Oa. The comparison with the limited set of ocean measurements demonstrates that the method's accuracy is about 12 W m−2 on a daily (for the range of observed values: 240 to 310 W m−2), which meets the requirements of the TOGA program. These results, however, am not yet statistically significant. In comparing model estimates to island data, both satellite observations and measurements indicate that the island's topography influences the oceanic environment by causing local, daily arographic cloud formation over the island's highest mountain. In partly clear conditions these clouds have a twofold effect: 1) to reduce solar irradiance under the mountain by shadowing, and 2) to increase surface solar irradiance near the mountain (instrument location) by cloud side reflection. Because of these effects new-noon surface measurements are often larger than the model estimates (up to 80 W m−2). Comparisons between the model results and oceanic measurements (buoy and the R/V Wecoma) suggest that the satellite-based estimates represent the oceanic conditions better than the island measurements.

Abstract

We present the first results concerning solar radiation at the ocean surface during the Tropic Heat experiment. Using calibrated GOES visible brightness measurements, a simple radiative transfer model calculates hourly and daily surface solar irradiance values. To validate the satellite-estimated solar irradiance, surface solar irradiance measurements are taken from three sources; the Tropic Heat buoy 3, the R/V Weeama, and the small tropical Pacific island of Hiva Oa. The comparison with the limited set of ocean measurements demonstrates that the method's accuracy is about 12 W m−2 on a daily (for the range of observed values: 240 to 310 W m−2), which meets the requirements of the TOGA program. These results, however, am not yet statistically significant. In comparing model estimates to island data, both satellite observations and measurements indicate that the island's topography influences the oceanic environment by causing local, daily arographic cloud formation over the island's highest mountain. In partly clear conditions these clouds have a twofold effect: 1) to reduce solar irradiance under the mountain by shadowing, and 2) to increase surface solar irradiance near the mountain (instrument location) by cloud side reflection. Because of these effects new-noon surface measurements are often larger than the model estimates (up to 80 W m−2). Comparisons between the model results and oceanic measurements (buoy and the R/V Wecoma) suggest that the satellite-based estimates represent the oceanic conditions better than the island measurements.

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