Precipitable Water Vapor Variation in the Clear-Cloud Transition Zone From the ARM Shortwave Spectrometer

A new technique to retrieve precipitable water vapor (PWV) amount in the clear-cloud transition zone using ground-based zenith spectral radiance is developed. The method uses zenith radiances at the water vapor band at 720 nm and at the adjacent nonadsorbing band at 750 nm. Radiative transfer calculations show that the relative difference in zenith radiance between the two bands depends on PWV and the variations in cloud optical depth introduce a small change in the relative difference which is independent of PWV amount. This allows us to retrieve PWV variations in the clear-cloud transition zone for clouds over dark ocean surface. We applied this method to a Cu cloud case with zenith radiance observations by Shortwave Array Spectroradiometer-Zenith (SASZe) during the Marine ARM GPCI Investigation of Clouds (MAGIC) field campaign. We found that there is about 10% change in PWV amount from the known-cloudy region to known-clear sky in the cloud edges.