| Obtaining an accurate estimate of the vertically-integrated liquid cloud water, which is directly related to shortwave cloud optical depth, is an essential part of a complete cloud observation system. Current operational methods to obtain these observations either fail when the cloud field changes from overcast to broken or scattered cloud coverage or have limitations on the range of cloud liquid water for which the retrievals work properly. In this study, several algorithms that take advantage of a difference for most mid-latitude or tropical regions in surface albedo from solar to near-infrared wavelengths due to the presence of vegetation are explored and analyzed. These theoretical retrievals require an estimate of the regional shortwave surface albedo, downward zenith radiance and downward hemispheric flux at wavelengths of 0.67 mum and 0.87 mum at high temporal resolution (1 Hz). Surface albedoes are provided using satellite estimates. The flux and radiance measurements are obtained using a new instrument, the Penn State Zenith-Looking, Narrow Field-of-View and Hemispheric Flux Spectrometer, developed specifically for this work. Both the cloud optical depth retrieval algorithms and the Pennsylvania State University Zenith-Looking, Narrow Field-of-View and Hemispheric Flux Spectrometer are tested at the Atmopsheric Radiation Measurement Program Southern Great Plains Central Facility in north-central Oklahoma for four months starting in July 2001 and ending in October 2001. Results from the experiment show that cloud optical depths retrieved in stratiform clouds track well with similar estimated made by either tradition flux methods or estimations made using microwave radiometers. This new cloud optical depth retrieval system also showed promise to fill the observation gap for all liquid water clouds with optical depths less than 20 and greater than 1. |
