Computer Simulation of Irradiance Measurements from Aircraft

Abstract

A computer simulation has been developed to optimize the use of the aircraft platform for the measurement of shortwave irradiances. This model simulates the measurement of radiative fluxes in order to determine the approximate sample sizes required under various conditions of cloudiness. The simulated required sampling length or averaging distance was found to be inversely proportional to the height of the sensor above or below the cloud field. The magnitude of the averaging distance and the rate of its decrease with height are the result of signal variations on two scales. Near the cloud surface the data have a high variance due to small-scale, large-amplitude variations in the irradiance. These fluctuations are rapidly smoothed as the aircraft-cloud separation increases. The longer period oscillations are not as easily smoothed. When the aircraft is farther from the cloud, large-scale effects become the primary control on the averaging distance.