A Study of Several Approaches to Computing Surface Insolation over Tropical Oceans

Abstract

The goals of this study were to investigate the potential of various meteorological factors for representing the more variable insolation reductions in the tropical atmosphere, and to develop curves and formulas for those relationships which appeared to be effective. Emphasis was placed on developing approaches that depend on simple meteorological entries. The following were studied in relation to the average daily atmospheric transmission ratio (insolation recorded at the earth's surface divided by the computed insolation on a horizontal surface at the top of theatmosphere) on a monthly basis using four years of Canton Island data and considering only hours between sunrise and sunset: average total opaque sky cover, average total sky cover, average daily duration (in minutes) of precipitation, average daily amount (in inches) of precipitation and fraction of days with precipitation. Also, the average daily amount of precipitation and fraction of days with precipitation were studied on an annual basis in the above manner considering all hours of the day and seven years of Canton Island data. For monthly insolation estimates, the method using average total opaque sky cover during daylight hours should give the best results (perhaps within 8% of measured values). The method using average daily duration of precipitation during daylight hours should also give good results (perhaps within 12% of measured values). For annual insolation estimates, the approach using average daily amounts of precipitation appears to show considerable promise. The more suitable monthly approaches should be applicable at least to that part of the equatorial Pacific which experiences weather affecting the dry zone area in its more extensive state. They may also be useful over the greater part of the equatorial Pacific and that part of the tropics extending into the summer hemisphere of the western Pacific. The methods using annual averages could only be useful in the equatorial region. In the future it would be desirable to consider sites which record much larger monthly and annual amounts of precipitation.