Abstract. We derived two observation-based global monthly mean dust aerosol optical
depth (DAOD) climatological datasets from 2007 to 2019 with a 2∘
(latitude) × 5∘ (longitude) spatial resolution, one based
on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the other
on Moderate Resolution Imaging Spectroradiometer (MODIS) observations. In addition, the CALIOP
climatological dataset also includes dust vertical extinction profiles. 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, Ångström exponent,
and single-scattering albedo) for MODIS, respectively. The two datasets
compare reasonably well with the results reported in previous studies and
the collocated Aerosol Robotic Network (AERONET) coarse-mode AOD. Based on these two datasets, we
carried out a comprehensive comparative study of the spatial and temporal
climatology of dust. On a multi-year average basis, the global
(60∘ S–60∘ N) annual mean DAOD is 0.032 and 0.067 according to CALIOP
and MODIS retrievals, respectively. In most dust-active regions, CALIOP DAOD
generally correlates well (correlation coefficient R>0.6) with
the MODIS DAOD, although the CALIOP value is significantly smaller. The CALIOP DAOD
is 18 %, 34 %, 54 %, and 31 % smaller than MODIS DAOD over
the Sahara, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea,
respectively. Applying a regional specific lidar ratio (LR) of 58 sr
instead of the 44 sr used in the CALIOP operational retrieval reduces the
difference from 18 % to 8 % over the Sahara and from 34 % to 12 %
over the tropical Atlantic Ocean. However, over eastern Asia and the
northwestern Pacific Ocean (NWP), 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 the 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 increasing vegetation and decreasing surface
wind speed in the area.