Analysis of the Multi-scale Organization of a Winter Shamal Dust Storm over the Middle East

A winter Shamal dust storm that affected much of the Middle East in February 2017 is studied using multi-scale observations and WRF-CHEM numerical simulations. The organization of a strong cross-mountain wind surge across the eastern Taurus Mountains into Syria and Iraq by the downscale forcing of a Rossby wave break in the baroclinic northern branch of the polar jet stream and its subsequent leeside expansion is the key driver of surface winds that organize this event. Strong mid-winter blocking across the northern hemisphere forces the polar jet stream to circumvent massive positively tilted ridges and troughs. As this jet's two split branches subsequently merge in association with the amplification and thinning of an adjacent trough, the result is upstream low-level pressure rises and downstream falls, creating a cross-mountain and leeside pressure gradient. This results in a strong hydraulically-driven surge of momentum which propagates across the mountains into northern Syria and Iraq. High pressure builds poleward of these mountains sustaining cross-mountain flow and creating a density current-like cold front. This front and its weak precursor overrun surface-heated air, causing substantial boundary layer buoyancy accompanying strong low-level winds and turbulence kinetic energy that ablates dust and transports it with the leeside cold surge from near the Iraq-Syria border across the lowlands of Iraq and Iran, and northern Saudi Arabia, to the Persian Gulf. This serves as an example of how winter Shamal dust storms are driven by continental-scale adjustments accompanying the polar jet. The Anatolian Mountains act as the key orographic feature.