A real-time cloud optical depth sensor

Poster PDF

Description

While working for the Air Force Research Laboratory, Aerodyne Research, Inc. has developed and successfully deployed a unique, real-time cloud optical depth sensor, dubbed the Three-Waveband Spectrally-agile Technique (TWST). It was developed to serve as a ground-based, portable, and reliable means for remote cloud monitoring. It uses the spectral radiance of scattered sunshine from a small area of overhead cloud and the MODTRAN5 model of atmospheric radiation transport to determine cloud optical depths (CODs). A real-time output assists with monitoring dynamic situations, and continuous data logging permits detailed post processing. The basic phenomenology of TWST is similar to that employed in Aerosol Robotic Network (AERONET) sensors running in cloud-mode, which provided direct validation of TWST during simultaneous TWST-AERONET data collections at Harvard Forest in Petersham, Massachusetts, during October 2012. Unlike AERONET, however, TWST is a dedicated, continuous cloud monitor with the ability to work in vegetated and non-vegetated terrains, in addition to being portable rather than a fixed installation like AERONET.

Numerous investigators have shown success in inferring cloud optical depth from passive optical measurements of solar-scattered radiance, either from below or above. The algorithms in these techniques are built around the ability to do forward modeling of the cloud radiance for various scenarios, so that one can search for the best fit to the data. One of the underlying problems with this approach is that the downwelling cloud radiance is a two-valued function of the COD. Therefore, the first problem with COD retrieval is to distinguish whether to use the “thin cloud” or “thick cloud” branch of the radiance. For TWST COD retrieval we adopted the O2 A-band equivalent width as the means to overcome this ambiguity. Unlike water vapor, O2 concentration in the atmosphere is quite stable. The strength of the O2 absorption depends on the total photon path length and increases with more scattering events before reaching the sensor. Thus, the relative absorption from traversing a thin cloud (single scatter only) differs from that through a thick cloud. A number of other researchers have utilized this feature.

TWST's unique combination of features makes it ideal for ground truth measurements in support of airborne or satellite sensors. It has performed quite successfully in field tests covering a wide range of locations and diverse background conditions.