A novel approach to infer cloud optical depth from zenith radiance observations

Observations of a passive two-channel Narrow Field-Of-View Radiometer (2NFOV) have been used to successfully retrieve cloud optical depth for the Atmospheric Radiation Measurement (ARM) Program field campaign at Pt. Reyes, California. The 2NFOV measures zenith radiance at one red and one near-infrared (NIR) wavelength. While clouds have nearly identical optical properties in these two channels, vegetated surface is much brighter at the NIR than at the red region. This contrast in spectral surface reflectance leads to substantially different amounts of surface-cloud interactions in these two spectral regions. From the resulting difference in zenith radiance, information on cloud optical depth in a fully 3D situation can be extracted. We present cloud optical depths estimated from 1) zenith radiance only, and 2) zenith radiance coupled with flux measurements. Retrievals are validated against estimates from the ARM Microwave Radiometer. It will be demonstrated that these two approaches work not only for overcast clouds, but also for broken clouds.