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Homogenization of long-term solar radiation time series using a clear-sky radiation model and assessment of solar radiation variability at upper elevations on Maui, Hawaiʻi

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Item Summary

Title: Homogenization of long-term solar radiation time series using a clear-sky radiation model and assessment of solar radiation variability at upper elevations on Maui, Hawaiʻi
Authors: Longman, Ryan James
Keywords: solar radiation
Issue Date: Dec 2011
Publisher: [Honolulu] : [University of Hawaii at Manoa], [December 2011]
Abstract: The amount of solar radiation that reaches the Earth's surface is modulated by fluctuations in atmospheric gases including water vapor, the density of aerosols and cloud optical depth. Regional trends in solar radiation have been observed in various places around the Earth, but little research has been done on solar radiation trends in Hawaiʻi. To address this, data obtained from upper elevation climate stations within the HaleNet climate network on the Island of Maui, Hawaiʻi were evaluated for trends. First, solar radiation was modeled using the SPECTRAL2 clear day radiation model and tested against observations. Solar radiation was modeled using a series of model parameterization scenarios at different temporal resolutions, to determine the scenario that most accurately represented clear sky radiation at the points of observations. Second, the modeled solar radiation was compared with observations to identify and correct instrument response changes over time. Thirdly, a long-term record of solar radiation was evaluated for trends at three upper elevation stations.
Solar radiation was modeled at three HaleNet locations along a 1630-m elevation gradient on the slopes of Haleakalā. The model was parameterized using daily, monthly, and annual mean input scenarios. The model tended to overestimate clear sky solar radiation at all stations under three input scenarios for a 1-year test period, but nevertheless produced satisfactory results with regression slopes all within 0.03 of unity and RMSE values in the range of 18.9--38.0 W m-2. The model performed best at these stations when input parameters were held constant at their annual mean values.
Clear day solar radiation was modeled using a fixed parameterization scenario that was determined to give the best simulation of clear sky radiation at the HaleNet stations. A clear day ratio (CDR) of the observed to modeled clear day radiation was established for the ~21 year record at each station. Historical calibration records were used to assess time periods where sensors were either recalibrated or replaced. Apparent shifts in sensor response were assessed using a linear regression analysis including the affects of autocorrelation. Using the mean CDR for a given time period or a linear regression equation (when a significant trend in CDR was identified), solar radiation was adjusted to give the CDR during a time when instrument calibration was deemed to be the most reliable.
A long-term record of solar radiation at three HaleNet stations was analyzed for trends. Annual anomalies were calculated as departures from the respective long-term mean and a clear day index was established as the ratio of monthly averaged solar radiation to calculated monthly extraterrestrial radiation at the top of the atmosphere. Trends were assessed for all available monthly data, and two 6-month seasonal periods, which roughly correspond to the wet and dry seasons in Hawaiʻi. Positive trends in annual anomalies were found at all three experimental stations (0.29 to 0.9 W m-2 per yr.), however, these trends were not statistically significant. During the June to November (dry) seasonal period, statistically significant positive trends were shown (1.1 to 1.7 W m-2 per yr.), and maximum positive anomaly was observed in 2010. Clear day index results were consistent with these findings, indicating that cloud cover has decreased over time at high elevations on Maui.
Description: M.A. University of Hawaii at Manoa 2011.
Includes bibliographical references.
URI/DOI: http://hdl.handle.net/10125/101378
Appears in Collections:M.A. - Geography



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