Advection of lee-side shed vortices over Reykjavík Hálfdán Ágústsson (1,2) and Haraldur Ólafsson (2,3,4) (1) Institute for Meteorological Research, Iceland, (2) University of Iceland, (3) The Icelandic Meteorological Office, (4)Bergen School of Meteorology, Geophysical Institute, University of Bergen, Norway On 12 August 2009 satellite images showed the asymmetric shedding of lee-side vortices from Mount Snæfellsjökull on the Snæfellsnes peninsula in West-Iceland. The vortices were advected to the southeast with the ambient flow, across the Faxaflói bay. They passed over Reykjavík 3-4 hours after their generation. Here the vortices are explored using satellite images, upper-air and surface observations as well as mesoscale modeling at high horizontal resolution. Satellite observations of location of vortex Snæfellsnes peninsula Upper air observations from Keflavík (K) show a strong inversion at 900 hPa with winds approx. 8 m/s in the boundary layer. A thin layer of stratus clouds tops the boundary layer. Faxaflói Bay Keflavík upper-air station Top of Mt. A series of vortices are shed at Mt. Snæfellsjökull (M) in northwesterly flow. The shedding period is on the order of 2 hours. A series of satellite images record the advection of the same vortex across Faxaflói bay and towards Reykjavík (R). Its estimated velocity is approx. 8 m/s, same as the ambient flow. High spatial and temporal surface based observations the passage of a vortex above Reykjavík, starting at 14:10 UTC. Observations record the path of the vortex as it is carried further inland by the flow aloft. The passage of the vortex is associated with a short and local clearing in the cloud cover as well as transient changes in the wind speed and direction. Observed wind direction in Reykjavík Time of passage of vortex Atmospheric simulations with horizontal resolution of up to 1 km reproduce asymmetric vortex shedding in the lee of Mt. Snæfellsjökull. Further to the east there is gravity wave activity and downslope accelerated flow on the Snæfellsnes peninsula. Simulated winds at 300 m at a resolution of 1 km at 12 UTC Simulated surface temperature and pressure at a resolution of 1 km at 12 UTC B A Observed and simulated wind dir. near Reykjavík The simulated disturbances are advected with the mean flow across Faxaflói bay and make landfall near Reykjavík at approx 14:30 UTC.. Simulated wind speed and isentropes from NW to SE along section A, from Mt. Snæfellsjökull to Reykjavík, at a resolution of 1 km at 12 UTC. Simulated wind speed and isentropes from NW to SE along section B, across Mt. Snæfellsjökull, at a resolution of 1 km at 12 UTC The upstream flow at Mt. Snæfellsjökull is blocked. and there are indications of the baroclinic generation of the vortices. Downstream from Mt. Snæfellsjökull the disturbances are limited to the boundary layer and they are strongest near its top at 900 hPa. Contact: Hálfdán Ágústsson, halfdana@gmail.com