Special observations were made over the southwest island area of the East China Sea from 12 to 27 January 1991 as part of the World Climate Research Program in Japan (WENPEXWestern North Pacific CloudRadiation Experiment). Two aircraft were used to determine the air truth of the total vertical liquid water path (LWP) using a microwave radiometer. One airplane was fitted with a 37-GHz radiometer and flew above planetary boundary layer clouds. The other flew inside the clouds with a cloud droplet spectrometer. These aircraft flew simultaneously along the same flight path when planetary boundary layer clouds were formed over the warm sea during an outbreak of cold air.
The result of the air truth of the LWPradiometer indicates that the 37-GHZ microwave radiometer gives an estimation of the LWP accurate to 100 mg cm−2. The shortwave cloud albedo was related to the LWPradiometer. The albedo increases with the LWP, independent of cloud type, when measured just above the cloud tops. The measured albedo is nearly the same as the calculated albedo when the LWPradiometer is larger than 60 mg cm−2 but much smaller than the calculated albedo when the LWPradiometer is less than 40 mg cm−2. Cloud-top irregularity is suggested to be the primary cause of this discrepancy. The degree of inhomogeneity of the horizontal distribution of liquid water appears to be correlated with the amount of precipitable water in the planetary boundary layer.