Application of Oxygen A-band Equivalent Width for Cloud Optical Depth Measurement
Because clouds are dynamic as well as non-uniform, both the spectral radiance and the equivalent width are always changing. For optically thin clouds these quantities are positively correlated; for optically thick clouds they are negatively correlated. By examining the sign of the correlation over a short time window (about 5 seconds) the brightness ambiguity can be resolved. For those times when the equivalent width variation is not dominated by the optical depth, a suitable nonlinear filter is able still to resolve the ambiguity. This filter, while based on the well-known statistical method of maximum likelihood (ML), is not strictly a ML estimate of the optical depth. Rather, it assigns a likelihood of the observed portion of sky being in one of five mutually exclusive states based on the spectral measurements, their past values and physics-based heuristics. This technique appears capable of identifying the thick and thin states with a notably low error rate and thus is an effective means of retrieving the correct optical depth.
We have applied this algorithm to more than 30 days of the recent DOE Two-Column Aerosol Project at the ARM Mobile Facility on Cape Cod using the Aerodyne Three-Waveband Spectrally-agile Technique (TWST) sensor. Samples and a summary of this data will be presented. Other applications of the A-band equivalent width to monitoring cloud properties will also be discussed.
Supplementary URL: www.aerodyne.com