Cloud liquid water and ice measurements from spectrally resolved near-infrared observations: A new technique

Abstract

A new technique is presented for the simultaneous measurement of water vapor, liquid water, and water ice in clouds using spectral observations of scattered sunlight between 7.865 and 1.065 μm. A nonlinear least squares approach is used to fit the measurements with the absorption spectral signatures of the vapor, liquid, and solid phases of water. This allows for the retrieval of the total path-integrated column abundances of vapor, liquid, and ice and directly yields the fractional absorption by all three phases of water within clouds at these wavelengths. Laboratory, ground-based, and aircraft-based observations are presented that illustrate the application of this technique and its comparison to other methods of measuring liquid water in widely varying conditions. The results suggest that the retrieval of liquid water is relatively insensitive to interference from other absorbers. The measurement of ice is less accurate and precise than that of liquid and is affected to some degree by the absorption of water vapor and liquid; uncertainty estimates are given. The method provides key information for radiative balance studies and has the potential to aid aviation safety. An extension of this technique to include scattered sunlight at longer wavelengths is likely to produce more accurate results and lower detection limits for both liquid and ice. Applying the technique to shorter wavelengths should allow for retrievals that are less sensitive to photon path distribution uncertainties. Copyright 2002 by the American Geophysical Union.