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Day-night Differences and Long-term Trends in Dust Activities Over the Dust Belt of North Africa, the Middle East, and Asia in the Early 21st Century
Tindan, Jacob Zora-Oni
Tindan, Jacob Zora-Oni
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Abstract
Mineral dust is one of the key variables affecting the climate system, yet constraining the diurnal cycle of dust from current visible satellite products and sparse in situ observations presents a challenge, which contributes to the sources of uncertainties in estimating the total radiative forcing of aerosols and climate change projections. Utilizing the well-calibrated, high spectral resolution, and equal-quality-performance for day and night observations of the Infrared Atmospheric Sounder Interferometer (IASI) products, this study investigates the day-night differences and the interannual variations in dust activities over the dust belt of North Africa, the Middle East, and Asia. IASI reveals significant (95% confidence level) day-night differences in dust activities over the major dust sources within the dust belt. Annual mean daytime dust optical depth (DOD) at 9:30 a.m. is significantly higher than that of nighttime at 9:30 p.m. in the central to northern Sahara Desert, the central to eastern Arabian Peninsula and dust source regions in South and East Asia including the Taklamakan Desert, but lower over the southern Sahel to the Guinea Coast, and the central to southern Indian Subcontinent. The magnitude of the day-night difference in DOD is larger and more significant in boreal winter (December, January and February (DJF)) and spring (March, April and May (MAM)) than other seasons. An analysis of 10 m wind speed and dust uplift potential suggests that the positive day-night differences in DOD over the central Sahara, the Middle East, and Asia are associated with enhanced dust emissions driven by stronger wind speed. Dust layer heights demonstrate negative day-night differences over dust source regions in the central Sahara, central Arabian Peninsula, and Asia, and positive height differences in the southern Sahel to the Guinea Coast, southern parts of the Arabian Peninsula, and large parts of the Indian Subcontinent. The higher dust layer height over the Guinea Coast and the Indian Subcontinent during daytime is associated with a deeper planetary boundary layer height and greater convective instability around 9:30 a.m. than that during 9:30 p.m., which promotes vertical transport and mixing of dust. The corresponding lower daytime DOD over these downwind regions indicates a possible dilution of dust aerosols when they are transported to higher altitude by convections and are more susceptible to horizontal transport. Surface observations of dust aerosols exhibit a spatially varying diurnal cycle across the dust belt, generally consistent with day-night differences in dust activities revealed by IASI. In terms of the interannual variations, there is significantly positive trend in DOD over the central to the Western Sahara during the nighttime in summer (June, July and August (JJA)), significantly decreasing trends in DOD in MAM over the central to the northwestern Indian Peninsula at nighttime, and significantly decreasing trends over the Arabian Peninsula in MAM and JJA at daytime. In terms of the day-night differences in DOD, there is a decreasing trend over a small area of the central to the Western Sahara in JJA. Over the Indian Peninsula, a significantly increasing trend is observed at the central to the northwestern parts in MAM, yet decreasing significantly over the Arabian Peninsula in MAM and JJA. The interannual variability of IASI dust layer height revealed a significant decreasing trends of large magnitude over the central to the Western Sahara during both daytime and nighttime in JJA with slightly higher magnitude at nighttime. Over the Arabian Peninsula, a significantly decreasing trend in dust layer height is observed at nighttime around the southern to the central parts. Using the random forest regression model, it is found that the nighttime increasing trends in DOD over the central to the Western Sahara in JJA is associated with increasing trends in convective available potential energy (CAPE) and surface temperature, and a decreasing trend in vegetation. Over the Indian Peninsula, the decreasing trend in DOD in MAM is found to be related to increasing vegetation, soil moisture, and decreasing wind speed. Results of DOD from reanalysis products revealed that they largely capture the temporal and spatial variability of DOD on the seasonal scale, but failed to capture the day-night differences in DOD in large parts of the dust belt. Overall, this thesis provides a detailed and comprehensive analysis on the day-night differences and the interannual variations of dust over the dust belt, which could improve our current understanding of physical mechanisms of dust cycle at the diurnal timescale.
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Date
2022-08-31
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University of Kansas
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Keywords
Atmospheric sciences, concentration, course-mode AOD, dust layer height, dust optical depth, Infrared Atmospheric Sounder Interferometer (IASI), satellite