Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 41 -
The Marine Boundary Layer – New Findings from the Östergarnsholm
Air-Sea Interaction Site in the Baltic Sea
Ann-Sofi Smedman and Ulf Högström
Department of Earth Sciences, Meteorology, Uppsala University, Villavaegen 16, 75236 Uppsala, Sweden
Email: annsofi@big.met.uu.se
1. Site and measurements
Östergarnsholm is situated 4 km east of the big island
Gotland (see Figure 1). It is a low island with no trees. In
spring 1995 a 30 m tower was erected at the southernmost
tip of the island. The base of the tower is situated at just
about 1 m above mean sea level, the actual sea level
varying within about ± half a meter of that (actual sea
level is gauged at the coast of Gotland). The distance from
the tower to the shoreline in calm conditions is only a few
tens of meters in the sector from northeast over south to
southwest. The slope of the sea floor outside
Östergarnsholm is about 1:30 at 500 m from the shore.
���
���
*RWODQG
������
�������
������
���
��� ���
�
�
��
7RZHU
gVWHUJDUQVKROP
�
�
��
� � � �
��
�
7RZHU
��
��
:DYH EXR\
Figure 1. Experimental site on Östergarnsholm (‘Tower’)
and the location for the wave buoy.
At about 10 km from the peninsula the depth is 50 m,
reaching below 100 m further out. The approximate sector
100-220 o is characterized by more than 150 km undisturbed
upwind over water fetch. At 4 km southeast of the tower a
3D Waverider Buoy (run and owned by Dr. Kimmo
Kahma, the Finnish Institute of Marine Research, FIMR,
Helsinki, Finland) is anchored in 40 m deep water. Wave
data is recorded once an hour. The directional spectrum is
calculated from 1600 s of data onboard the buoy.
The Östergarnsholm tower is instrumented with Solent R2
sonic anemometers at 9, 17 and 25 m above the tower base,
the signals being recorded at 20 Hz. Slow response
(‘profile’) sensors for wind speed and direction and
temperature (sampling rate 1 Hz) are mounted on five
levels between 7 and 29 m above the tower base. Rapid
humidity and CO2 fluctuations are recorded with an openpath
instrument, at 10 m.
A fundamental methodological question is related to the
general validity of the Östergarnsholm data in representing
open ocean conditions. A method was developed to identify
the effective upwind ‘footprint area’ for the turbulent flux
measured at each of the three heights on the tower.
From a map of the bottom topography around
Östergarnsholm it was possible to calculate the water depth
and hence, with the dispersion relation for shallow water
waves, the phase speed of the dominating waves of the
‘footprint area’ and to derive a weighted phase speed
average over the effective ‘footprint’ for any incident wave
field.
Our results so far strongly corroborate the conclusion that
the measurements at Östergarnsholm within the
undisturbed wind direction sector are in close agreement
with results obtained during many measurements in the
open ocean for young waves (see below).
2. Results
The Östergarnsholm air-sea interaction research project has
led to fundamentally new understanding of the marine
atmospheric boundary layer, MABL, and the exchange
process at the surface of the ocean:
1. The MABL is very much influenced by the state
of the sea
2. For growing sea (young waves) travelling slower
than the wind, the turbulence structure in the
MABL resembles the boundary layer over land
3. As soon as some waves are travelling faster than
the wind, mature sea or mixed sea, the MABL
starts to deviate from the boundary layer (BL)
over land
4. For swell conditions when long waves travelling
faster than the wind dominate, the MABL is quite
different from the BL over land.
3. Basic characterization of the state of the MABL and
the characteristics of the neutral MABL.
It is clear from the analysis that Monin-Obukhov similarity
theory (MO theory) is not always valid in the MABL.
As discussed later, the M-O parameter z/L is found to be a
complicated mixture of stability in a proper sense and wave
influences in the MABL
As a basic criterion of atmospheric stability, we therefore
simply adopted the sign of the flux of virtual potential
temperature at the surface in Smedman et al. (2003) and
other papers. For neutrality we adopted in that paper the
criterion that the magnitude of the heat flux is below a
certain limit and wind speed above some chosen value.
Analysis of that data subset revealed convincingly that
neutrality is not a sufficient criterion for a logarithmic wind
law to ensure. The governing factor turned out to be the
relative amount of long waves in the wave spectrum.
Therefore, we introduced the parameter E1/E 2, where E 1 is
the energy of waves travelling faster than the wind at 10 m
height, U10, and E 2 the corresponding energy of waves
travelling slower than U 10. It was shown that the E 2
component of the wave field is a strong function of U10,
whereas E 1 is unrelated to the local wind. A true
logarithmic wind law ensues only for E1/E 2 < 0.05.