Global Dust Optical Depth Climatology Derived from CALIOP and MODIS Aerosol Retrievals on Decadal Time Scales: Regional and Interannual Variability

Abstract

Abstract. We present a satellite-derived global dust climatological record over the last two decades, including the monthly mean visible dust optical depth (DAOD) and vertical distribution of dust extinction coefficient at a 2º (latitude) × 5º (longitude) spatial resolution derived from CALIOP and MODIS. Dust is distinguished from non-dust aerosols based on particle shape information (e.g., lidar depolarization ratio) for CALIOP, and on dust size and absorption information (e.g., fine-mode fraction, Angstrom exponent, and single-scattering albedo) for MODIS, respectively. On multi-year average basis, the global (60° S–60° N) and annual mean DAOD is 0.029 and 0.063 derived from CALIOP and MODIS retrievals, respectively. In most dust active regions, CALIOP DAOD generally correlates well with the MODIS DAOD, with CALIOP DAOD being significantly smaller. CALIOP DAOD is 18 %, 34 %, 54 % and 31 % smaller than MODIS DAOD over Sahara Deserts, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea, respectively. Over East Asia and the northwestern Pacific Ocean (NWP), however, the two datasets show weak correlation. Despite these discrepancies, CALIOP and MODIS show similar seasonal and interannual variations in regional DAOD. For dust aerosol over NWP, both CALIOP and MODIS show a declining trend of DAOD at a rate of about 2 % yr−1. This decreasing trend is consistent with the observed declining trend of DAOD in the southern Gobi Desert at a rate of −3 % yr−1 and −5 % yr−1 according to CALIOP and MODIS, respectively. The decreasing trend of DAOD in the southern Gobi Desert is in turn found to be significantly correlated with an increasing trend of vegetation and a decreasing trend of surface wind speed in the area.

Comment on acp-2021-1

“…Even if the episodic dust events are sampled by CALIOP, 370 the monthly averaging would diminish the sparse daily DAOD retrievals in those regions. Indeed, Prospero (1999) reported that dust signals were shown in the daily TOMS aerosol index (AI) product in those regions but were not captured in TOMS monthly-mean AI product. The difference also is very large in open oceans, suggesting that dust aerosols are present at a very low frequency.…”

“…As a result, CALIOP shape-based DAOD derivation method could not capture the dust particles contained in the mixture, while those dust particles can be captured by MODIS size-based method. Another potential reason could be associated with that dust plumes in this region are vertically dispersed (Yu et al, 2010;Su and Toon, 2011). These 390 tenuous dust layers are likely to go undetected by CALIOP because of its relatively low sensitivity.…”

15-year variability of desert dust optical depth on global and regional scales

“…Across South Asia, a negative shift in DOD interannual variation is recorded during the pre-monsoon season between , based on CALIOP observations (Lakshmi et al, 2019. In the southern sector of the Gobi Desert, declining DOD trends are observed from MODIS and CALIOP DOD datasets during 2007-2019 (Song et al, 2021).…”

“…One more advantage of the high resolution DOD analysis is the flexibility on the final grid size selection depending on data availability, a critical aspect when satellite observations are used. MIDAS data can be easily upscaled at coarser spatial resolutions in order to match spaceborne observations which have been commonly used in trend analyses available in literature (Hsu et al, 2012;Yoon et al, 2014;Notaro et al, 2015;Pozzer et al, 2015;Klingmüller et al, 2016;Alfaro-Contreras et al, 2017;Che et al, 2019;Guo et al, 2019;Voss and Evan 2020;Song et al, 2021). Nevertheless, relying on fine spatial resolution data it is ensured a more realistic collocation with ground-based measurements for validating the obtained DOD trends.…”

“…Nevertheless, relying on fine spatial resolution data it is ensured a more realistic collocation with ground-based measurements for validating the obtained DOD trends. Another interesting point is that few studies have concentrated on pure DOD (Xi and Sokolik 2015;Guo et al, 2019;Lakshmi et al, 2019;Voss and Evan 2020;Song et al, 2021) rather than AOD to analyze trends of mineral particles' load. Even though the consideration of the latter parameter is quite reasonable across deserts, its representativeness over downwind areas it is questionable due to the coexistence of other aerosol types.…”