Dust over Alborz mountains and Caspian Sea

By Ebrahim As'adi (Iran Meteorological Organization) and HansPeter Roesli (EUMETSAT)

Frontal systems passing over Syria and Iraq often lift desert dust from the area. Usually once lifted the dust plumes drift down the Gulf region and the Arabian peninsula. On rare occasion though, the dust moves into a more north-east sector. The case presented here is even more exceptional, in that the dust reaches the eastern shore of the Caspian Sea.

During the days leading to 23 June 2010 dust moved east-northeastward into north-western Iran (see map of the area of interest), crossing the Zagros mountains and ending up against the most northern part of the Alborz range (see red and green arrows on map of north-west Iran,  source: Google Earth) that skirts the south-western corner of the Caspian Sea. Approaching the range from west the slope to the tops at 2000–2500m is rather gentle though irregular, whereas it sharply drops to just below sea level on the side of the Caspian Sea (-27m). The massive barrier is only broken by the Manjil gap (indicated by the green arrow), through which the Sefidrood (White River) drains water from the opposite side of the range into the Caspian Sea.

By 24 June a considerable amount of dust had accumulated along the windward flanks of the Alborz range (see 03:00 UTC Met-9 Dust RGB image). The 1000hPa forecast chart (WRF model) shows the typical pressure pattern for foehn and/or gap wind conditions over in the Alborz mountain range. Towards local noon dust is leaking through the Manjil gap, signalling the presence of gap winds, and is then fanning out over the Caspian Sea off the coast near Rasht, the 'City of Silver Rains' (see images below, Rasht is indicated by a black arrow).

Some more details can be gleaned from MODIS true colour RGB images*. At the same time the dust also starts to cross the Alborz range in the foehn flow. During the afternoon it fans out over the southern parts of the Caspian Sea and, by early morning of 25 June it passes over its eastern shore (see Met-9 Dust RGB images). Finally, the animation (00:00–23:45 UTC) shows the evolution of the dust event in more detail.

Technical note: this image composite shows a comparison between the Dust RGB product calculated from the IR8.7/10.8/12.0 SEVIRI channels and from similar MODIS channels. For the MODIS RGB the SEVIRI tuning has been applied. It is obvious that the MODIS RGB — apart from its higher spatial resolution — is somewhat less performant in showing the presence of dust (weaker pink colouring), in particular over water. The weaker performance of the MODIS Dust RGB over water is confirmed by a comparison for a large dust outbreak over northern Africa and the Atlantic Ocean (Tom Lee, 2010). The reason for this is not fully understood, but could be due to water vapour absorption differences in the IR channels, in particular the IR8.7 channel.

Dust blows through the Manjil gap (Iran)