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Numerical Simulation of the Dynamics of Summer Shamal Dust Storms
AuthorAlsubhi, Yazeed Hammad
AdvisorKaplan, Michael L.
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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. These adjustments indicate a direct and synergistic relationship between the intensity of this mid-tropospheric ridge and the area of well-heated air. Most important, however, is the low-level cool air mass over the Mediterranean coastal region which develops due to the sea breeze ahead of the broader scale front under the SWJ accompanied by the westerly/northwesterly wind. This feature, in turn, causes the rise of the low-level geopotential heights to the northwest of the LE. There are decreasing geopotential heights under the 500-700 hPa layer to the southeast of the LE in response to the relatively hot air in the boundary layer due to the surface sensible heating. In time, the westerly/northwesterly cool Mediterranean airflow over the LE region turns south-southwestward in response to the rightward deflection of the Coriolis force (CF) until it becomes perpendicular to the pressure gradient resulting in a balance between the PGF and the CF, producing near geostrophic balance with the exception of frictional influences. Additionally, large value near-surface isallobaric-ageostrophic flow develops over the LE as part of the meso-? scale adjustment processes accompanied by large magnitude turbulence kinetic energy (TKE) in the hot PBL and then the ablation of dust storms. As a result of significant vertical motion and the TKE formation, there are additional meso-?/? scale adjustment processes leading to the thermally-driven mountain-plain solenoid (MPS) circulation in response to the surface sensible heating gradient between the Sarawat Mountains and the LE. As the day evolves, the eastward propagation of the solenoidal circulation plays an important role in minimizing the equatorward pressure as the sensible heat flux increases. Consequently, the Shamal dust storms are comparatively more intense than the earlier time period, i.e., wind velocity magnitude as well as areal extent, due to stronger buoyancy and downward mixing process of kinetic energy, which results in strong magnitudes of TKE and low-level dust ablation. In the null case, the accompanying SWJ is adjacent to a blocking ridge that develops in the upper tropospheric levels to the northwest of the AP over the Mediterranean region and the deep trough occurs on the poleward side of the AP over Iraq, Syria, and Iran. In this case, the massive ridge blocks the cool marine air from penetrating into the area of study and shifts the trajectory of this transport mechanism towards the west-southwest of the AP, resulting in some weak cool Mediterranean air advection in the lower tropospheric levels to the northwest of the AP propagating to Syria and Iraq as well as to the southwest. There is a development of a strong pressure gradient along the western part of the AP in response to the juxtaposition of the high pressure within the cool Mediterranean air to the northwest of the AP and the relative low pressure to the southwest. These two systems create a significant pressure gradient far west of the LE because of the ridging aloft, which neglects the impact of the offshore troughing.