Investigation of sky radiance in tropical climate of Thailand

Abstract

This study aims to develop two empirical models related to sky radiance in the tropical climate of Thailand. The first objective is to create an empirical model for calculating relative sky radiance under various sky and climatic conditions. The model identifies sky conditions through a cloud index derived from geostationary satellite imagery and employs empirical adjustable coefficients established from five years of averaged sky radiance data at Nakhon Pathom station. Validation using measurement data from four main regions: Chiang Mai, Ubon Ratchathani, Songkhla, and Nakhon Pathom, under clear, intermediate and overcast sky conditions showed that the model achieved root mean square error (RMSE) values of approximately 30% across the different conditions. Although the coefficients were derived from a single station, comparisons across regions with diverse climatic characteristics confirmed the broader applicability of the model. The second objective is to develop an empirical model for calculating hourly zenith sky radiance as a function of solar altitude under clear sky conditions, focusing on a case study of Chiang Mai and Nakhon Pathom. Hourly zenith sky radiance measurement data from the Nakhon Pathom station were used for model development. Data collected at both Chiang Mai and Nakhon Pathom stations were employed to validate the accuracy of the model. The results demonstrated reasonable accuracy, with an RMSE of 30.17% and a mean bias error (MBE) of 7.48%.

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