Assessment of Calipso Satellite Aerosol Optical Depth Observations Over The UAE

Abstract

This study provides an intercomparison of aerosol optical depth (AOD) retrieved from CALIOP (cloud aerosol lidar infrared observation pathfinder) aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite over the UAE (United Arab Emirates) between 2012 and 2017, with validation against the ground-based AERONET (Aerosol Robotic Network) and space-based SEVIRI (spinning enhanced visible infrared radiometer instrument) AOD observations. We used overpass coincidence criteria of ± 30 min and within 120 km radius, and found 43 and 57 suitable overpasses over Mezairra (24.442N, 54.617E) and Masdar (23.105N, 53.755E) AERONET stations respectively for the specified study period. The intercomparison revealed good degree of correlation (68.4% and 74.5%) over Mezairra and an acceptable correlation over Masdar (50.4% and 58.3%) at the 532 nm and 1064 nm wavelength channels respectively. It has been observed that the correlation at both sites was better in the near infrared channel (1064 nm). This can be attributed to the limitation of the CALIPSO satellite to collect data from near the ground during the day hours in the visible channel (532 nm) as a result of low SNR (signal to noise ratio) expected due to high solar back ground[1]. Significant difference in the correlation between the two stations was also observed and this is mainly due to proximity of the CALIPSO overpass to the Mezairra station. Moreover, we have compared the aerosol subtypes from CALIPSO with aerosol types derived from AERONET. For the Mezairra station, we found 23 suitable days for comparison and the measurements agreed 69.5% (16 days out of 23) if only most common subtypes are considered and 78% (18 days out of 23) if the second most dominant subtypes are included in the comparison. Likewise, out of 9 suitable days for the Masdar station the measurement agreed in 66.7% (6 days out of total 9 days) for the most common subtypes. It was observed that majority of the retrieved aerosol subtype from both measurements was dominated by dust and polluted dust. Furthermore, we assessed spatial variability of AOD over the UAE by using the CALIPSO and SEVIRI data. We used mean monthly average CALIOP level 3 (L3) and SEVIRI products for the intercomparison and found poor correlation between them. This is mainly due to the difference with which both satellites operate, where the CALIPSO's repeat cycle of 16 days is much coarser as compared to SEVIRI's data acquisition frequency 15 mins.

Date of Award	May 2020
Original language	American English