Temporal solar radiation change at high elevations in Hawai‘i

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Abstract

Trends in downwelling global solar irradiance were evaluated at high-elevation sites on the island of Maui, Hawai‘i. Departures from monthly means were assessed for the 6 month Hawaiian wet and dry seasons over the period 1988 to 2012. Linear regression analysis was used to characterize trends in each season. For the dry season (May–October), statistically significant (p ≤ 0.05) positive trends of 9–18 W m−2 (3–6%) per decade were found at all four high-elevation stations tested. Wet season trends were not significant, except at the highest-elevation station, which had a significant negative trend. No consistent trends in aerosol concentrations have been observed at high elevations in Hawai‘i; therefore, the observed dry season brightening is most likely the result of decreasing cloud cover. Supporting this hypothesis, analysis of 15 years (1997–2012) of high temporal resolution Geostationary Operational Environmental Satellite (GOES) imagery over the Hawaiian Islands showed a statistically significant decrease in leeward cloud cover amounting to 5–11% per decade over the stations. In addition, analysis of Moderate Resolution Imaging Spectroradiometer data were in general agreement with the GOES trends, although statistically significant dry season trends were found at only one of the four stations.