The “wind of 120 days” and dust storm activity over the Sistan Basin

Mesoscale features play a critical role in creating the strong “wind of 120 days” common in eastern Iran and western Afghanistan. The Weather Research and Forecasting with Chemistry (WRF/Chem) model is used with the available observations to investigate the “wind of 120 days”, dust storm activity over the Sistan Basin, and major sources of dust influencing Iran. The winds are strong from mid-May to mid-September when a persistent high-pressure system over the high mountains of the Hindu Kush in northern Afghanistan, combined with a summertime thermal low over desert lands of eastern Iran and western Afghanistan, produce a strong pressure gradient. The winds become accelerated by the channeling effect of the surrounding orography. A northerly low level jet (LLJ) along the Iran–Afghanistan border has a peak at 300–500 m and is strongest in July with the nighttime monthly averaged wind speed of 20 m s− 1, and extends across a broad latitudinal area along the Iran–Afghanistan border. The strong near-surface wind speed along with the LLJ results in substantial dust emission from the Sistan Basin and subsequent long-range meridional transport. Dried Hamoun Lake in the Sistan Basin contains large amounts of erodible sediment that is required for dust entrainment. The LLJ is persistent throughout the night, but is weakened during the day. A pronounced diurnal cycle in the near-surface wind speed has been identified with a peak in the mid-morning in association with momentum transfer from the jet level down to the surface as the daytime mixed layer evolves.