A new approach to measure low liquid water path using solar radiance in an infrared window

Clouds with low liquid water paths (below 100 g/m²) are frequently observed.  These clouds are more important from a climate sensitivity point of view than thicker clouds, whose albedoes saturate, but their optical depths and liquid water paths are poorly measured at present (different methods often differ by more than 100%).  We present a new approach to measure low liquid water path using a customized Infrared Thermometer (IRT).  This IRT tracks the Sun and measures solar radiation with a field of view of 1.1 degree in the wavelength window 10.5 – 11.5 microns.  This window is in the most transparent part of the 8 – 12 micron window, thus minimizing emission from the atmosphere.  Laboratory experiments and theoretical calculations have demonstrated that liquid water path can be derived simply from transmission of 11 micron radiation, with errors no larger than 10 – 20%.  Unlike microwave radiometers, which are sensitive only to liquid water, our approach works for mixed-phase and ice clouds.