Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Evaluation of Atmosphere - Ocean Heat Fluxes over the Baltic Sea Using a
Number of Gridded Meteorological Databases
Anna Rutgersson 1 , Anders Omstedt 2 and Gabriella Nilsson 1
1 Department of Earth Science, Meteorology, Uppsala University, Uppsala, Sweden, anna.rutgersson@met.uu.se
2 Department of Earth Science, Oceanography, Göteborg University, Göteborg, Sweden.
1. Introduction
The water cycle and energy budget for the Baltic Sea have
been evaluated and analysed in a number of investigations.
Still, different methods give different results depending on
the method used and the time period of interest. Estimated
net-precipitation (precipitation minus evaporation) over the
Baltic Sea usually is between 1500 and 2000 m 3 s -1 with an
estimated bias of about 1000 m 3 s -1 (Meier and Döscher
2002; Rutgersson et al., 2002). Net heat exchange was in
Omstedt and Rutgersson (2001) shown to be 1 Wm -2 and
thus the Baltic Sea thermodynamically behaves like a lake.
As data are being more refined and accurate the estimates of
parameters like net precipitation and net heat exchange have
the possibility of being more certain.
In this investigation we are using a number of gridded
meteorological databases to evaluate differences in surface
heat fluxes and to give some errorbars in the determination
of the Baltic Sea energy budget. We are using Re-Analysis
data from the ECMWF (ERA40) and the gridded data-base
SMHI(1×1)˚, available from the BALTEX Hydrological
Data Centre. The data have been evaluated for limited
periods using in-situ measurements from the island of
Östergarnsholm in the Baltic Sea as well modelled data
based on Baltic Sea ocean modelling. Also gridded fluxes
from satellite-derived data is used.
2. Methods
In the ECMWF Re-Analysis project (ERA), observational
data is assimilated using the ECMWF operational global
model (http://www.ecmwf.int/research/era). For this project
all available observational data is used, including satellite
data, synoptic data, ship and buoy data as well as other
sources. The data used in the present investigation (ERA40)
covers the time-period 1958 to 2002. The ERA40-data is
used in a number of applications, forcing regional- and
meso-scale models as well as making climate statistics. Thus
it is of great importance to know the accuracy for various
regions.
ERA40 data is evaluated using in-situ measurements from
one site in the Baltic Sea. The meteorological measurements
are taken at the small very flat island of Östergarnsholm,
about 4 km east of Gotland. The data include slow
meteorological measurements as well as turbulence data. In
Smedman et al. (1999) further information on the
measurements is found. The analysed period is from 1995
too 2002 and all available data are used to give relatively
good coverage over different seasons and situations. Both
mean meteorological parameters (as temperature, windsspeed
and humidity) and turbulent fluxes are analysed for
this period.
Using a system with an ocean model forced with synoptic
data SMHI(1×1)˚ (see Rutgersson et al, 2001) gives a
consistent system with information of both mean
meteorological parameters, sea surface data and surface
fluxes. SMHI(1×1)˚ is a gridded synoptic database using
700-800 daily observations in the Nordic region. These are
interpolated in space using optimum interpolation and
used to force the ocean process-oriented model PROBE-
Baltic covering the Baltic Sea (Omstedt and Axell, 2003).
Also satellite-based (Graßl et al., 2000) and ship-based
(daSilva et al., 1994) gridded fluxes are used for the
evaluation. In the Hamburg Ocean Atmosphere Paramters
and Fluxes from Satellite Data (HOAPS) a number of
satellite-derived parameters covering the entire globe is
available for the period July 1987 to December 1998.
Monthly, seasonal and annual fields are available and also
possibly higher temporal and spatial resolution
(http://www.mpimet.mpg.de/Depts/Physik /HOAPS).
3. Results
The presentation will cover analyses of atmosphere-ocean
heat fluxes and implication on the BALTEX heat budget.
Evaluation of specific parameters in the ERA40 database
will be shown, using the other data-sources.
Figure 1. Latent (a) and sensible (b) heat fluxes from
direct measurements at the site Östergarnsholm (blue
crosses) and calculated with the ocean model forced with
the SMHI(1×1)˚ data base (red lines). Time period is days
of October 1998.
In Figure 1 sensible and latent heat fluxes from a Baltic
Sea ocean model forced with SMHI(1×1)˚ data base are
shown for October 1998. The agreement with the directly
measured fluxes from the Östergarnsholm site is relatively
good.
Figure 2 shows monthly averages of sensible and latent
heat flux for the period 1988 to 1997 for satellite-derived
data from HOAPS and the ocean model forced with
SMHI(1×1)˚ the data are averages over the entire Baltic
Sea.