Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 44 -
Sensitivity in Calculation of Turbulent Fluxes over Sea to the State of the
Surface Waves
Anna Rutgersson, Ann-Sofi Smedman and Björn Carlsson
Department of Earth Sciences, Meteorology, Uppsala University, Uppsala, Sweden.
anna.rutgersson@geo.uu.se
1. Introduction
It has been shown in numerous investigations, from the
Baltic Sea as well as other regions that the state of the waves
influences the overlying atmosphere. Relatively recently it
was found that also the situation with long waves (swell)
changes the structure of the atmosphere. This has been seen
and shown for many atmospheric parameters in Smedman et
al. (1999), Rutgersson et al. (2001) and other investigations.
In numerical wave models the process of including also
swell effects is now starting and has been shown to be of
importance for surface fluxes (Kudryavtsev and Makin,
2003). To find out if the effect of swell is so important that it
should be included in atmospheric and ocean models we are
investigating the sensitivity of turbulent fluxes in different
models to wave effects.
2. Methods
The specific situation of interest here is the situation where
we have long waves (or swell). Then the waves are not
locally generated by the wind, but have travelled from other
regions. These situations have been shown to occur
frequently over the Baltic Sea (Rutgersson et al., 2001). In
Guo Larsén et al., (2003a and b) and Rutgersson et al.,
(2001) new values for the dimensionless gradients as well as
transfer coefficients for heat and momentum have been
determined for the situation when we have long waves. In
Figure 1 the non-dimensional wind gradient (Φm)
φ
m
∂U
zκ
=
∂z
u
*
at 10 and 26 meters is shown when swell is present. Data is
from the Östergarnsholm-site in the Baltic Sea. As is clear
from the figure the gradients differ significantly from the
expected value (represented by the solid line). Effect of
changed in transfer coefficients and gradients are
investigated when calculating the surface fluxes from bulkformulations.
Bulk-formulations are frequently used when
estimating surface fluxes in different types of databases
(ship data, satellite data and also in gridded synoptic
databases). Thus, if waves influence surface fluxes
calculated by bulk-formulations is of great importance to
know when determining for example heat and water budgets
over the Baltic Sea.
In most models transfer coefficients are calculated with
assumptions based on previous knowledge of air-sea
interaction. It is possible to assume that the wave effect is
smaller in a model, since models tend to adjust themselves
to a changed parameterisation. The sensitivity is investigated
in a ocean process-oriented model, PROBE-Baltic covering
the Baltic Sea (Omstedt and Axell, 2003) and forced with
gridded meteorological data.
The regional climate model RCA (Rummukainen et al.,
2001) is used for limited periods to investigate the possible
results of introducing wave-effects in surface fluxes in a
regional climate model-system covering the Baltic Sea
region.
Figure 1. Non-dimensional wind gradient at 10 and 26
metres for the situation when swell is present (filled
symbols). The solid lines represent the expected gradients
for different values of the atmospheric stability (z/L).
3. Results
Since there generally exist no information about the state
of the waves in most models it is not possible to include a
full description of the wave effects, but we are instead
investigating the sensitivity of the models to possible wave
effects. This is important to know before starting to couple
to an ocean wave model. In Figure 2 the surface stress is
calculated using a bulk-formulation developed for the
situation with swell (red-dashed curve) and a state-of-theart
bulk-formulation not including wave effects (blue-solid
curve). The period is one with very well developed swell
and the red curve thus agrees well with the measured
surface stress (green pluses). It is here interesting to note
that the blue curve is about 40% larger that the red and
would thus give an overestimation of the flux using
ordinary bulk methods with about 40%. This is expected
to be larger than for sensible and latent heat fluxes.