Model simulations of coastal flow in the
southeast of Sweden

In the south-east of Sweden lies a coastal area to the Baltic Sea that has an open bay, the Hanö Bay, and the surrounding coast forms several pieces of semi-straight coast separated by nearly orthogonal changes in coastline orientation.
 

The flow along these streches is margedly different and dependent on background flow and on the temperature difference between the land and the sea surface. During spring and most of summer, the see surface is typically much cooler than land, in particular during the day, while in autumn the water is warmer than land.

Jets

During nighttime conditions in spring with north-easterly flow, the low-level flow along the Kalmar coast (se above) is enhanced first by downslope flow (se the top panel below) as the land surface cools and later by a thermal wind that accelerates along the Kalmar coast (lower panel below)and continues southwest out over the Bay of Hanö.
 

The figure below illustrate the thermal wind over the sloping Kalmar coast. This figure shows vertical cross-sections of scalar wind speed (upper left), wind direction (upper right), potential temperature (lower left) and specific humidity (lower right); east is to the right in the plot. The jet is cleary seen as a wind maximum less than 100 m above the surface. Below the jet there is a slight drainage flow. The geometry of the jet follows closely the sloping invesrion, that follows the terrain, except for right at the coastline where a convergence zone between the drier air over land and the moister marine air is clearly seen.
 


 
Sea-breezes
As time goes, the land area is heated and a see-breeze forms. In the plot below the wind field at different times is shown. One can see that a convergence is formed along the Blekinge coast while the jet flow is still predominant along the Kalmar coast. The latter is weakened gradually through mid-day but the convergence at the Blekinge coast remains until the jet at the Kalmar coast collapse in the afternoon (lower left to upper right panel). After this, the Blekinge sea-breeze front propagates inland, leaving a weak mesoscale anticyclone over the southeast coast.
 

The figure below shows a cross-section through the stagnant sea-breeze at the Blekinge coast, before it starts to propagate. This figure shows alongcoast wind (upper left), across coast wind (upper right), potential temperature (lower left) and specific humidity (lower right). North is to the right in the panels and west is into the plane. The actual sea-breeze cisrulation is quite shallow, less than 500 m, and stationary at this time. It is capped off-shore by a elevated weak wind maximum in the alongcoast wind. As the jet eventually propagates, the latter is enhanced and lowered. It is noteworthy to see that the temperature gradient is much stronger along the surface than at the coast. Note also the plune of moist air that is lifted by the circulation; this reaches over 2km as long as the sea-breeze remains stationary.
 

The above results come from a numerical modeling study, however, the case is based on aircraft measurements carried out in this area, and the background conditions are taken from real observations. For practical reasons, the jet was not observed; it occured at night and no automatic observations were made in this area. The stagnant sea-breeze was however measured during two flights with a research aircraft. In the subsequent numerical simulations it was found that the jet was a necessary preconditioning for the observed sea-breeze.