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

This study aims to investigate global, regional and seasonal temporal dust changes as well as the effect of dust particles on total aerosol loading using the ModIs Dust AeroSol (MIDAS) fine-resolution dataset. MIDAS delivers dust optical depth (DOD) at fine spatial resolution (0.1∘×0.1∘) spanning from 2003 to 2017. Within this study period, the dust burden increased across the central Sahara (up to 0.023 yr−1) and Arabian Peninsula (up to 0.024 yr−1). Both regions observed their highest seasonal trends in summer (up to 0.031 yr−1). On the other hand, declining DOD trends are encountered in the western (down to −0.015 yr−1) and eastern (down to −0.023 yr−1) Sahara, the Bodélé Depression (down to −0.021 yr−1), the Thar (down to −0.017 yr−1) and Gobi (down to −0.011 yr−1) deserts, and the Mediterranean Basin (down to −0.009 yr−1). In spring, the most negative seasonal trends are recorded in the Bodélé Depression (down to −0.038 yr−1) and Gobi Desert (down to −0.023 yr−1), whereas they are in the western (down to −0.028 yr−1) and the eastern Sahara (down to −0.020 yr−1) and the Thar Desert (down to −0.047 yr−1) in summer. Over the western and eastern sector of the Mediterranean Basin, the most negative seasonal trends are computed at summer (down to −0.010 yr−1) and spring (down to −0.006 yr−1), respectively. The effect of DOD on the total aerosol optical depth (AOD) change is determined by calculating the DOD-to-AOD trend ratio. Over the Sahara the median ratio values range from 0.83 to 0.95, whereas in other dust-affected areas (Arabian Peninsula, southern Mediterranean, Thar and Gobi deserts) the ratio value is approximately 0.6. In addition, a comprehensive analysis of the factors affecting the sign, the magnitude and the statistical significance of the calculated trends is conducted. Firstly, the implications of the implementation of the geometric mean instead of the arithmetic mean for trend calculations are discussed, revealing that the arithmetic-based trends tend to overestimate compared to the geometric-based trends over both land and ocean. Secondly, an analysis interpreting the differences in trend calculations under different spatial resolutions (fine and coarse) and time intervals is conducted.

A global analysis of diurnal variability in dust and dust mixture using CATS observations

The current study investigates the diurnal cycle of dust and dust mixture loading across the global tropics, subtropics, and mid-latitudes by analyzing aerosol extinction and typing profiles observed by the Cloud-Aerosol Transport System (CATS) lidar aboard the International Space Station. According to the comparison with ground-based and other satellite observations, CATS aerosol and dust and dust mixture loading observations exhibit reasonable quality but significant day–night inconsistency. To account for this day–night inconsistency in CATS data quality, the diurnal variability in dust and dust mixture characteristics is currently examined separately for daytime and nighttime periods. Based on an analysis of variance (ANOVA) analytical framework, pronounced diurnal variations in dust and dust mixture loading are generally uncovered during daytime periods and over terrestrial areas. The current study identifies statistically significant diurnal variability in dust and dust mixture loading over key dust sources, including the Bodélé Depression, the West African El Djouf, Rub' al-Khali desert, and western and southern North America, confirming the previous observation-based findings regarding the diurnal cycle of dust emission and underlying meteorological processes in these regions. Significant seasonal and diurnal variability in dust and dust mixture is identified over the Iraqi and Thar deserts. The identified significant diurnal cycles in dust mixture loading over the vegetated regions in the Amazon and tropical southern Africa are hypothesized to be driven by enhanced dust emission due to wildfires.

A Global Analysis of Dust Diurnal Variability Using CATS Observations

The current study investigates the diurnal cycle of dust loading across the global tropics, sub-tropics, and mid-latitudes by analyzing aerosol extinction and typing profiles observed by the Cloud–Aerosol Transport System (CATS) lidar aboard the International Space Station. According to the comparison with ground-based and other satellite observations, CATS aerosol and dust loading observations exhibits reasonable quality but significant day–night inconsistency. To account for this day–night inconsistency in CATS data quality, the diurnal variability in dust characteristics are currently examined separately for daytime and nighttime periods. Based on an analysis of variance analytical framework, pronounced diurnal variations in dust loading are generally uncovered during daytime periods and over terrestrial areas. The current study identifies statistically significant diurnal variability in dust loading over key dust sources, including the Bodélé Depression, the West African El Djouf, Rub-al Khali Desert, and western and southern North America, confirming the previous observation-based findings regarding the diurnal cycle of dust emission and underlying meteorological processes in these regions. Significant seasonal dust diurnal variability is identified over the Iraqi and Thar deserts. The identified significant diurnal cycles in dust loading over the rainforests in Amazon and tropical southern Africa are hypothesized to be driven by enhanced dust emission due to wildfires.

A Global Analysis of Dust Diurnal Variability Using CATS Observations

The current study investigates the diurnal cycle of dust loading across the global tropics, sub-tropics, and mid-latitudes by analyzing aerosol extinction and typing profiles observed by the Cloud-Aerosol Transport System (CATS) lidar aboard the International Space Station. According to the comparison with ground-based and other satellite observations, CATS aerosol and dust loading observations exhibits reasonable quality and insignificant day-night inconsistency, thereby supporting the current analysis of dust diurnal cycle using CATS data. Based on an analysis of variance analytical framework, statistically significant diurnal variability in dust loading is identified over key dust sources, including the Bodélé depression, West African El Djouf, Rub-al Khali desert, and western and southern North America, confirming the previous observation-based findings regarding the diurnal cycle of dust emission and underlying meteorological processes in these regions. Insignificant annual mean dust diurnal variability is identified over the Iraqi, Thar, and Taklamakan deserts. The currently identified significant diurnal cycles in dust loading over the rainforests in Amazon and tropical southern Africa, and drylands in South America and the central Australia, are hypothesized to be driven by enhanced dust emission due to wildfires and enhanced katabatic and frontal winds, respectively.

Organization of dust storms and synoptic-scale transport of dust by Kelvin waves

Based on the large-scale transport of dust driven by the winds parallel to the mountains in the Harmattan, Saudi Arabian, and Bodélé Depression dust storm cases, a detailed study of the generation of Kelvin waves and its possible role in organizing these dust storms and large-scale dust transport was accomplished. For this study, observational and numerical model analyses were done in an in depth manner. For this, MERRA reanalysis data sets; WRF-simulated high-resolution variables; MODIS Aqua and Terra images; EUMETSAT images; NAAPS aerosol modeling plots; and MERRA-2 dust scattering aerosol optical depth (AOD) modeling plots, surface observations, and rawinsonde soundings were analyzed for each of these three case studies. We found that there were meso-β-scale (horizontal length scale of 20–200 km) adjustment processes resulting in Kelvin waves only in the Harmattan and the Bodélé Depression cases. The Kelvin wave preceded a cold pool accompanying the air behind the large-scale cold front instrumental in the major dust storm. We find that this Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic).

Measuring Sand Dune Migration Rates with COSI-Corr and Landsat: Opportunities and Challenges

It has been over a decade since COSI-Corr, the Co-Registration of Optically Sensed Images and Correlation, was first used to produce a raster map of sand dune movement, however, no studies have yet applied it to the full Landsat archive. The orthorectified and geolocated Landsat Level-1 Precision Terrain (L1TP) products offer the opportunity to simplify the COSI-Corr pre-processing steps, allowing an automated workflow to be devised. In the Bodélé Depression, Chad, this automated workflow has calculated average dune speeds of 15.83 m/year and an increase in dune movement of 2.56 m/year ±12.58 m/year from 1987 to 2009. However, this increase does not stem from a systematic increase in dune mobility. The fastest 25% of dunes from 1987 to 1998 reduced their speed by 18.16%. The overall increase stems from the acceleration of features previously moving under 13.30 m/year. While successfully applied to the Bodélé Depression, the automated workflow produces highly variable outputs when applied to the Grand Erg Oriental, Algeria. Variations within path/row scene pairings are caused by the use of mobile features, such as dune crests, as ground control points (GCPs). This has the potential to warp Landsat scenes during the L1TP processing, potentially obfuscating dune migration. Two factors appear to be crucial in determining whether a Landsat scene is suitable for COSI-Corr analysis. Firstly, dune mobility must exceed the misregistration criteria. Secondly, GPCs should be located on static features such as bedrock outcrops.

Organization of Dust Storms and Synoptic Scale Transport of Dust by Kelvin Waves

Based on the large scale transport of dust driven by the winds parallel to the mountains in the Harmattan, Saudi Arabian, and Bodélé Depression dust storms cases, a detailed study of the generation of Kelvin Waves and its possible role in organizing these dust storms and large scale dust transport was accomplished. For this study, observational and numerical model analyses were done in an in depth manner. For this, MERRA reanalysis datasets, WRF simulated high resolution variables, MODIS/Aqua and Terra images, EUMETSAT images, NAAPS aerosol modelling plots, and MERRA-2 dust scattering AOD modelling plots, surface observations, and rawinsonde soundings were analyzed for each of these three case studies. We found there were meso-β scale (horizontal length scale of 20–200 km) adjustment processes resulting in Kelvin waves only in the Harmattan and the Bodélé Depression cases. The Kelvin wave preceded a cold pool accompanying the air behind the large scale cold front instrumental in the major dust storm. We find that this Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic).