An empirical model for estimating hourly solar radiation over the Indian seas during summer monsoon season

Performance of the widely used Octa model [Quart. J. R. Met. Soc., 114 (198 8) 165-182] for the estimation of hourly solar radiation over the Indian se as during summer monsoon season is evaluated. This model was found to overe stimate radiation for all sky conditions, in particular for 7 and 8 octa cl oud cover by 300 W.m(-2). For overcast sky conditions, observed radiation w as found to be independent of the altitude of the sun and dependent only on cloud thickness. The dominant cloud types observed during this period are thick cumulus and cumulonimbus with an average base height of 300-1000 m. W ith a view to improve hourly estimation of radiation for all cloud categori es, a new set of regression equations was developed based on transmission o f radiation in the atmosphere. and the altitude of the sun. With the new mo del, estimation has considerably improved for all cloud amounts as indicate d by the reduction in the standard error estimates, in particular for 7 and 8 octa cloud cover. Application of these two models to three independent d ata sets also suggests the suitability of the new model for the hourly esti mation of radiation for monsoon cloud conditions over the Indian Seas.