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Titre : Numerical Simulation of the Dynamics of Summer Shamal Dust Storms

Auteur : Alsubhi, Yazeed Hammad

Université de soutenance : University of Nevada Reno

Grade : Doctor of Philosophy (PhD) in Atmospheric Science 2020

Résumé partiel
This study is focused on synoptic, subsynoptic, and meso- ?/ ? scales observational and scale numerical simulations utilizing the WRF-Chem model to investigate the atmospheric dynamics that are responsible for the formation of summer Shamal dust storms over the Low-Elevated (LE) region at 20^∘-30^∘N and 45^∘-55^∘E of the Arabian Peninsula (AP). Ten Shamal case studies based on the most intense dust storms in June and July between 2008 and 2012 and a null case study were selected using the MODIS true-color RGB images, Aerosol Optical Depth (AOD) products from Terra and Aqua MODIS satellite retrievals, and TOMS Aerosol Index (AI) products. Rawinsonde soundings over the LE region on the east side of the Sarawat Mountains were employed to study the vertical temperature and wind speed/direction profiles. Synoptic, subsynoptic, and meso- ?/ ? scale observational analyses were performed through the use of the ERA-Interim reanalysis datasets to precisely investigate the synoptic features and evaluate how conducive they are to the development of summertime mesoscale circulation systems over the LE region. Our analyses indicate that in the Shamal cases, the larger scale structure of the atmosphere includes a massive deep subtropical anticyclone over the Sahara Desert, which subsequently extends eastwards to the AP and stretches into interior southcentral Asia. On the poleward (equatorward) side of this ridge, the Subtropical Westerly Jet (SWJ) (Tropical Easterly Jet) (TEJ) forms and propagates in time eastwards/northeastward (westwards/southwestward). The SWJ left-entrance region behind the deep mid-upper tropospheric troughing over the Mediterranean Sea and the TEJ right-exit region to the southeast of the AP influence the low-level mass fields, which in turn, help to maintain higher (lower) pressure on the poleward (equatorward) side of the LE region. A quasi-balanced anticyclonic gyre at 500 hPa develops to the west of the AP over the Red Sea, which subsequently expands and strengthens over time responding to height rises generated above a well-heated ground by surface sensible heating subsequently leading to an increase in the column’s hydrostatic thickness. This anticyclonic gyre creates regional upward motion within lower-tropospheric layers due to significant velocity divergence aloft and downstream subsidence under convergence aloft thus reinforcing itself. At the subsynoptic or meso- ? scale, one sees the local and regional reinforcement of the broader scale subtropical ridge, which expands eastwards toward the well-heated ground, such as the Sarawat Mountains.

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