Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 90 -
Atypical Coastal Gradients in the Wind Speed and Air Humidity over the
Baltic Sea
Timo Vihma
Finnish Institute of Marine Research, P.O. Box 33, 00931 Helsinki, Finland. vihma@fimr.fi
1. Introduction
A major part of the Baltic Sea locates within 100 km from
the nearest coast. In this scale, the structure of the
atmospheric boundary layer (ABL) is usually affected by the
fetch from coast. Quantitative knowledge on the coastal
influence on the ABL is important for example for the
estimation of regional evaporation over the Baltic Sea (e.g.
Bumke et al., 1998).
During off-shore winds, the wind speed typically increases
with fetch from the coast. This is due to the smaller
aerodynamic roughness of the sea compared to the land
surface. Sometimes, however, the wind speed decreases with
fetch. The decrease may be related to mesoscale
circulations, baroclinicity, or a coastal change in ABL
stratification.
The air specific and relative humidity usually increase with
fetch over the sea. In cold temperatures, however, the
surface sensible heat flux may so much exceed the latent
heat flux that the relative humidity decreases with fetch over
the sea. In rare occassions, the air specific humidity may
also exceed the saturation specific humidity corresponding
to the sea surface temperature. Then the latent heat flux is
from air to sea, and also the air specific humidity decreases
with fetch.
The monthly mean coastal gradients of various marine
meteorological quantities are illustrated in Mietus (1998).
The objective of the present study is to quantify the
conditions in which the above-mentioned atypical coastal
gradients may occur.
2. Model Simulations
We apply a two-dimensional mesoscale model (Alestalo and
Savijärvi, 1985) to simulate off-shore flow over the Baltic
Sea under various combinations of boundary conditions. The
model is forced by a large-scale pressure gradient. The
model has a 2-km grid resolution with 100 grid points in the
horizontal and 50 in the vertical. To study the combined
effect of a coastal change in surface roughness and surface
temperature on the wind speed, we make a set of steadystate
model runs with the sea surface temperature prescribed
to 0°C (spring conditions). The land surface temperature is
modelled, and the solar radiation is calculated for 1 May at
60°N. The initial 2-m air temperature at the inflow boundary
ranges from 0 to +15°C (11 different values) in the various
simulations, and the geostrophic wind speed ranges from 2
to 20 m/s (10 different values), yielding a matrix of 110
steady-state simulations. Further, the sensitivity of the model
results to the parameterizations of surface roughness was
studied by running the set of 110 simulations with various
com-binations of roughness length z0 for land (z0 = 0.01 – 1
m) and sea (z0 according to Wu (1980), Taylor (2002), and
the Charnock formula). In addition, model runs were made
with a prescribed neutral stratification over the land surface.
To study the effect of fetch on the air relative and specific
humidity, we made another group of steady-state simulations
with the inflow air temperature and relative humidity
ranging from -15 to 15°C and from 70 to 100%,
respectively. The geostrophic wind speed ranges from 5 to
15 m/s.
3. Results
The model results suggest that, if the stratification over the
land surface is neutral, the 10-m wind speed (U10m) is
higher over the sea than over the land even in cases of
very stable stratification over the cold sea. Further, U10m
increases with fetch over the sea.
10 m wind speed (m/s)
Initial T(2m) over land
4.5
4
Ug = 6 m/s
0 o C
15 o C
3.5
−20 0 20 40
distance from the coast (km)
Increase in U(10m) over 10 km (m / s)
15
10
5
0
5 10 15 20
Geostrophic wind speed (m / s)
1.5
1
0.5
Figure 1. Model results for the coastal gradients in the 10m
wind speed: cross-section of the wind speed in model
runs with UG = 6 m/s and initial T2m over land ranging
from 0 to 15°C (11 lines for various values of T2m), and
(below) increase in the wind speed over a 10 km fetch
over the sea in model runs with various boundary
conditions with respect to UG and initial T2m over land.
The results are calculated for 1 May at 60°N at 3 p.m.
0