Fourth Study Conference on BALTEX Scala Cinema Gudhjem

On the other hand, if the stratification over the land surface
is convective, a sea-breeze circulation is generated, and in
the surface layer it opposes the off-shore wind. In such
conditions, the wind speed over the sea may remain
practically constant or slightly decrease with fetch. The
latter holds especially if the stratification over the sea is
stable. An example is shown in Figure 1. In conditions of a
very weak geostrophic wind (UG ≤ 4 m/s), the off-shore
component of U10m becomes negative, but an along-shore
component develops keeping the wind speed almost constant
with fetch over the first 10 km off the coast. In all
stratification conditions over the sea, the wind speed starts to
increase more after a fetch of approximately 30 km (Figure
1), which is beyond the influence of the mesoscale
circulation cell. From the point of view of the wind field
over the sea, the mesoscale conditions, such as the land-sea
temperature difference, were more important than the
parameterization method for the sea surface roughness.
Considering the coastal gradients in air humidity, the initial
model results suggest that in mid-winter conditions the
relative humidity often decreases with fetch over the sea.
This is qualitatively in agreement with Mietus (1988).
According to the model results, a decrease of specific
humidity with fetch is much more rare, but it may take place
in conditions realistic for spring and early summer. This is in
agreement with certain cases reported in Niros et al. (2002).
A more detailed comparison of the model results with
coastal, archipelago, and lighthouse stations is under work.
References
Alestalo, M., and Savijärvi, H., Mesoscale circu-lations in a
hydrostatic model: coastal convergence and orographic
lifting, Tellus 37A, 156-162, 1985.
Bumke, K, U. Karger, L. Hasse, and K. Niekamp, Evaporation
over the Baltic Sea as an example of a semienclosed
sea, Contr. Atmos. Phys., 71, 249-261, 1998.
Mietus, M., The climate of the Baltic Sea basin, Marine
Meteorology and Related Activities, Report No. 41,
WMO/TD No. 933, World Meteorological Organization,
Geneve, 62 + 134 p., 1998.
Niros, A., T. Vihma and J. Launiainen, Marine meteorological
conditions and air-sea exchange processes over
the Baltic Sea in 1990s, Geophysica, 38, 59-87, 2002.
Taylor, P. K., Air-sea interaction / momentum, heat and
vapour fluxes, In: Holton, J. P., Curry, J. A. ja Pyle, J.
(eds.), Encyclopedia of Atmospheric Sciences. Academic
Press, London, p. 93-102, 2002.
Wu, J., Wind-stress coefficients over sea surface in nearneutral
conditions, a revisit, J. Phys. Oceanogr., 10, 727-
240, 1980.
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