Cloud-Aerosol Transport System (CATS) Single-Wavelength Data Products and Performance

The Cloud-Aerosol Transport System (CATS) is an elastic backscatter lidar that operated from the International Space Station (ISS) for 33 months (Feb. 2015 to Oct. 2017). CATS had two operating modes, but the primary mode of operation for 31 months (Mode 7.2) consisted of a single beam and single wavelength (1064 nm). While the CATS processing algorithms leveraged the heritage of the CALIPSO algorithms, the single wavelength mode necessitated modifications to traditional multi-wavelength algorithms. Additionally, simple near real time (data latency of <6 hours) CATS data products demonstrated improvements in aerosol and volcanic plume transport forecasts.

The CATS instrument design and ISS orbit lead to a higher 1064 nm signal to noise ratio than previous space based lidars, allowing for direct 1064 nm calibration using the molecular normalization technique. Nighttime CATS Version 3-00 data were calibrated by normalizing the signal between 22-26 km above mean sea level to modeled molecular profiles. The CATS nighttime 1064 nm calibration uncertainties are estimated to be 7-10%. Daytime CATS Version 3-00 data were calibrated through comparisons with nighttime thin, opaque, cirrus cloud layer integrated attenuated total backscatter (iATB). The CATS daytime cirrus cloud calibration transfer technique has an estimated uncertainty of 16-18%.

Level 2 CATS data products show that many of the popular lidar data products (layer heights, backscatter, depolarization, cloud phase, aerosol type) can be accurately produced using a single wavelength lidar like CATS Mode 7.2. The CATS L2O cloud-aerosol discrimination (CAD) relies on 1064 nm perpendicular backscatter intensity, the fractions of clouds detected at 350 m (horizontal) for each 5 km profile, and advanced horizontal persistence tests for both night and day data to overcome the absence of backscatter color ratio. Aerosol typing algorithms use depolarization to identify dust aerosols and employ model data to help differentiate smoke and polluted continental aerosols in the planetary boundary layer. Additionally, constrained lidar ratio and extinction retrievals for above cloud aerosols (ACA) have been added as standard products. CATS algorithms are very mature and demonstrate the utility of single wavelength space-based lidar products applications such as cloud diurnal variability, aerosol plume transport and forecasting, and ACA detection.