Cloud property estimates from zenith spectral measurements of scattered sunlight between 0.9 and 1.7 μm

Abstract

A theoretical approach is used to quantify the information available to retrieve cloud physical properties from data taken by a ground-based spectrometer measuring scattered sunlight in the near-infrared wavelength region. Three wavelength regions between 0.9 and 1.7 μm, each containing water vapor, liquid, and ice absorption features, are examined using a differential optical absorption spectroscopy optimal estimation retrieval technique. Cloud properties that can be retrieved include path-integrated liquid water path and path-integrated ice water path (PLWP and PIWP), cloud liquid and ice temperatures, and the second moment of the photon path distribution. The accuracy of these cloud property retrievals is estimated for a variety of simulated conditions, with key analysis assumptions identified. The sensitivity of the measurements in the longest wavelength region to liquid water and ice is high, allowing for accurate estimates of PLWP and PIWP under optically thin clouds, while the shorter two wavelength bands provide more information under optically thicker clouds. Observations of mixed-phase clouds over Barrow, Alaska, are used to illustrate the practicality of the technique. Retrieved LWP values (inferred from PLWP) are compared to LWP estimates from a microwave radiometer and an atmospheric emitted radiance interferometer; PIWP estimates are compared to IWP estimates from a millimeter-wave cloud radar. Cloud liquid temperature and photon path distribution information retrieved from these data are also presented. Furthermore, we suggest a technique for combining near-infrared spectral PLWP measurements with microwave radiometer observations to estimate cloud droplet effective radius. Copyright 2006 by the American Geophysical Union.