Atmospheric dynamics of the Harmattan surge in March 2, 2004


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Ashok Kumar Pokharel, Michael L. Kaplan, Stephanie Fiedler

Pp 84-93

DOI:10.14644/dust.2016.014

Meso-α/β scale observational and meso-β/γ scale numerical model analyses were performed to study the atmospheric dynamics responsible for generating Harmattan dust storm. For this dust storm case study, MERRA reanalysis datasets, WRF simulated very high resolution datasets, MODIS/Aqua and Terra images, EUMETSAT images, NAAPS aerosol modelling plots, surface observations, and rawinsonde soundings were analyzed. The analysis of this dust storm shows (1) the presence of a well-organized baroclinic synoptic scale system, (2) this dust storm composed of two dust storm events, (3) small scale dust emission events (first dust storm event) which occurred prior to the formation of the primary large-scale dust storms (second dust storm event), (4) cross Atlas Mountain flows which produced a strong leeside inversion layer prior to the large scale dust storm, (5) the presence of thermal wind imbalance in the exit region of the mid-tropospheric jet streak in the lee of the Atlas Mountains shortly after the time of the inversion formation, (6) major dust storm formation was accompanied by large magnitude ageostrophic isallobaric low-level winds as part of the meso-β scale adjustment process, (7) substantial low-level turbulence kinetic energy (TKE), and (8) the emission of the dust occurred initially in narrow meso-β scale zones parallel to the mountains, and later reached the meso-α scale when suspended dust was transported away from the mountains. In addition to this there were additional meso-β and meso-γ scale adjustment processes resulting in Kelvin waves and the thermally-forced MPS circulation, respectively. The Kelvin wave preceded a cold pool accompanying the air behind the large scale cold front instrumental in the major dust storm. The Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic scale). The thermally-forced meso-γ scale adjustment processes, which occurred in the canyons, resulted in the numerous dust streaks leading to the entry of the dust into the atmosphere due to the presence of significant vertical motion and the TKE generation. This indicates that there were meso-β to meso-γ scale adjustment processes at the lower levels after the imbalance within the exit region of the upper level jet streak and these processes were responsible for causing this large scale dust storm (synoptic scale).

Keywords: Jetlet; Thermal ridge; Jet adjustment; Ageostrophic/isallobaric wind; Vorticity; TKE