Remote Determination of Cloud Liquid Water Path from Bandwidth-Limited Shortwave Measurements

Abstract

A method of determining cloud liquid water path is developed using shortwave spectral measurements. The attenuation of shortwave radiant fluxes is due to scattering and absorption in the near-infrared band, but is caused only by scattering in the near-ultraviolet and visual band. The ratio of the reflectances in these two bands is defined as the reflectance ratio; the ratio of the transmittances as the transmittance ratio. Relationships between these ratios and the amount of cloud liquid water are developed. The use of reflectance ratio as a determinant of liquid water path has two advantages over the use of total shortwave reflectance alone. It minimizes the effect of droplet size distribution and the diagnostic curve is less sensitive to changes in reflectance than the curve based on total reflectance. The model is tested with data obtained by aircraft flights during the 1978 Joint Air-Sea Interaction Experiment (JASIN) and shows good agreement with measurements of liquid water path taken in the same area and with the results of other approximation formulas. Calculations show that this method could also be used to detect the extent of layering within a cloud deck.