The 13th Symposium on Boundary Layers and Turbulence

9A.8
MESO SCALE CIRCULATION OBSERVED IN A COASTAL AREA DURING WINTER CONDITIONS

Mikael Magnusson, Uppsala Univ, Uppsala, Sweden

Intensive field measurements were performed in and around the area of the southern part of the Gulf of Bothnia, SPGB, during February 1998 in an EU-project, BASIS.
The aim with the BASIS project is to study the exchange processes between air, sea and ice in and above the SPGB area. Tower measurements at three land stations, one on the Swedish East Coast and two on the Finnish West Coast, with profile and turbulence instruments were performed. At every station radiosoundings were released every sixth hour. Around a research vessel, RV Arranda that was stationed in the SPGB, similar measurements were conducted on the ice. Aircraft measurements were also performed in the area at six occasions.

The site operated by the department of Earth Sciences / Meteorology Uppsala University, MIUU, was situated at Loevoeudden (63deg 40,5' N, 20deg 24,0' E) at Holmsund 25 km outside the town of Umeaa at the Swedish East Coast. Loevoeudden is small peninsula at the Umeå river mouth. The site has an open ice/water fetch between 50 and 230 deg, disturbed only by some small islands. Wind direction and wind speed in the lower boundary layer were measured with a pibal tracking technique. A mesoscale circulation phenomenon perpendicular to the coastline was observed in these measurements, an ice breeze. This event occurred when the temperature difference
between the snow covered land and the open sea was large enough. The distance between the two areas was frozen ice surface that did not reduce the velocity of the generated winds. For some cases the circulation was observed for more than twelve hours, although the geostrophic wind was parallel to the coast in these events.

The data collected at the site during these events have been analysed together with aircraft measurements, when available. Model simulation of the events has also been performed. The conclusion is that the driving force of the phenomena is the temperature difference between land and open water, the ice covered area in-between
contributes to the high wind speeds due to the low friction. The event seem to be independent of the time of the day since the criteria for the event to occur is that the temperature different is large enough and this can happen at any time of the day depending on the synoptic weather conditions in combination with the cloud cover.

The 13th Symposium on Boundary Layers and Turbulence