CALIPSO lidar observations of the optical properties of Saharan dust: A case study of long-range transport

An extensive dust storm originating on 17 August 2006 in North Africa was observed and tracked by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar. Over the next several days, the dust layer moved westward across the Atlantic Ocean and into the Gulf of Mexico. The initial stages of the event were examined using a sequence of CALIPSO measurements. The first of these was acquired very near the source on 18 August. Successive measurements were made over the Atlantic Ocean on 19 and 20 August, at respective locations approximately similar to 1300 km and similar to 2400 km from the source region. The later stages of the event were assessed using measurements acquired by the NASA Langley Research Center airborne HSRL over the Gulf of Mexico on 28 August. Within the free troposphere, the intrinsic optical properties of the dust remain relatively unchanged for the first 3 d of transport over the Atlantic Ocean. This is consistent with previous in situ measurements that have shown that there is little change in the size distribution of dust as it crosses the Atlantic. After the 10 d journey to the Gulf of Mexico, some changes are seen in the lidar ratios, the backscatter color ratio, and the optical depth ratio. The linear depolarization ratio appears to remain essentially constant (similar to 0.32) at all four locations mentioned above, demonstrating a notable consistency in the dust particle nonsphericity. The measured 532 nm lidar ratios are 41 +/- 3, 41 +/- 4, 41 +/- 6 and 45.8 +/- 0.8 sr, respectively, at locations near the source, over the Atlantic Ocean, and in the Gulf of Mexico. The corresponding 1064 nm lidar ratios are 52 +/- 5, 55 +/- 5, 54 +/- 13 and 44 +/- 8.3 sr. The 532 nm lidar ratios are consistent with previous measurements and with CALIPSO's prelaunch models. The lidar ratios retrieved at 1064 nm are somewhat larger than would be expected on the basis of existing modeling studies. The backscatter color ratios are 0.74 +/- 0.07, 0.75 +/- 0.08, 0.72 +/- 0.04 and 0.62 +/- 0.01, and the optical depth ratios are 0.97 +/- 0.02, 1.01 +/- 0.05, 0.93 +/- 0.17 and 0.62 +/- 0.13, respectively.