Hagedorn, Renate

Jacob, Daniel

Lehmann, Andreas

English

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It is prohibited to take money for copies or printed versions of the free online version. - 66 -A Coupled High Resolution Atmosphere - Ocean Model for the BALTEX Region Renate Hagedorn, Institut fur Meereskunde, Dilsternbrooker Weg 20, D-24105 Kiel, Germany, e-mail: rhagedorn@ifm.uni-kiel.deDaniela Jacob, Max-Planck-Institut fiir Meteorologie, BundesstraBe 55, D-20146 Hamburg, Germany Andreas Lehmann, Institut fur Meereskunde, Dilsternbrooker Weg 20, D-24105 Kiel, Germany The major objective of the BALTEX program is to provide validated, coupled modelling tools for explaining and predicting the energy and water processes in the climate system, consisting of the atmosphere, continental surfaces and the ocean ( International BALTEX Secretariat, 1997). To contribute to this aim, the atmospheric regional model (REMO) from Max-Planck-Institute for Meteorology has been coupled to the Kiel Baltic Sea model (BSMO). The 3-dimensional atmospheric model REMO is based on the Europa-Modell (EM), the main weather forecast modelof the German Weather Service (DWD). It is used in the so-called climate mode, i.e. during the whole integrationonly the lateral boundary fields were updated every 6 hours (Jacob et al., 1997). The horizontal resolution is 1/6° on the rotated longitude/latitude grid, this is equivalent to approximately 18 x 18 km2. The vertical distributionof the 20 model levels is the same as in the EM. The Kiel Baltic Sea Model is a 3-dimensional eddy-resolvingbaroclinic model with a horizontal resolution of approximately 5 x 5 km2 and a discretization of 28 levels in thevertical (Lehmann, 1995). Until now, these two models have been run seperately and both were forced with DWDanalyses or forecasts, respectively.As a first step towards a fully coupled system some sensitivity studies have been done varying the surface boundary conditions in both models. The simulations were performed for the PIDCAP period from August to October 1995. Firstly, REMO was run by using DWD analyses as boundary conditions. Atmospheric parameters produced by this run were used as forcing for the Kiel Baltic Sea Model. For a further run of REMO we replaced the DWD-SSTs in the BALTEX region with BSMO-SSTs achieved from the previous run. These BSMO-SSTs are in some situations, when rapid cooling occurs, quite different from DWD-SSTs and closer to satellite observed SSTs (Fig. 1, lower panel). As a result of these changed lower boundary condition, among other things, REMO calculated heat fluxes are different (Fig. 1, upper panel). From the end of August to the beginning of September major changes occured in the eastern Gotland Basin. For instance on August 31st a difference of 3°C in SST causes a reduced latent heat flux of 200 wm-2 . According to that, also precipitation is reduced in this region. Whereas the different SSTs have only a slight effect on the dynamic variables. The next step was the development of a fully coupled model system. Therefore the Baltic Sea model was incorpora­ted into REMO as an optional subroutine. The interface between both models organizes the exchange of the mutual forcing at the common boundary. That means the interface has to provide REMO calculated heat fluxes, radiation fluxes, wind stresses, precipitation and mean sea level pressure on the BSMO grid, as well as the BSMO calculated SSTs on the REMO grid. The frequency of information exchange can freely be chosen and was set to 6 hours in the first coupled run. The SSTs from the coupled run show a similar behaviour compared to the BSMO-SSTs from the experiment with no feedback to the forcing fields (Fig. 1, lower panel). On the one hand this indicates that - in this experiment - mainly minor changes in the dynamic variables of the atmospheric model occur due to different SSTs. On the other hand this points out that the Baltic Sea model responses reasonably to the REMO calculated fluxes. This is not obvious, because until now the BSMO was forced by atmospheric variables and the flux calcu­lation routines in the Baltic Sea model are not exactly the same as in REMO. Thus it is a good result that the Baltic Sea model has no drift when it's forced by REMO fluxes instead of atmospheric variables and the coupling was realized without any flux corrections. Finally, we have to remark that another reason for the non existing drift in the coupled system could be the strong constraint of lateral boundary conditions for the atmospheric model. Further we don't know whether the coupled model shows a drift during longer simulation periods. Therefore we have to extent our experiments to longer runs with different atmospheric conditions. Hagedorn, Renate, Jacob, Daniel and Lehmann, Andreas (1998) A Coupled High Resolution Atmosphere - Ocean Model for the BALTEX Region. Open Access [Paper] In: Second study conference on the Baltic Sea Experiment. , 25.-29.05.1998, Geesthacht, Germany . Conference Proceedings of the Second Study Conference on BALTEX. ; pp. 66-67- 67 -0 ' '_d .'l·�IMI�· \I -100) �1./"F\' � .... r� , .• · -i i .. · \i - .1f·� -i-� , Ill rfY �r\ l��>< -200 ::, " :t -300c: 8 ..J -400 -50020 .... � 18 e " c. § 16f-" u ..::: 14 -� <I) " v \{ I ' I I ' / �I-· r··. I I I I I I �JI ?( � \ llfi ' .. \ i \ �-v . :=, t v \ � .Ji Ii:1 V � I I , _\ {·)(..."·.,� ... "\� I I I ··) \ I "\ \. ,•. l '\ J.•••. I \-0+:�-;.� 1- ... , .... l � V··.········ .. '""'·· " �-� .. _ ..1 --y1 ············ ·-·-�. l I<ll 12 I 10 �- X r----' ,,: _:····, ... I4'�� I 7 14 21 28 4 11 18 25 2 9 16 23 30 Aur,ust ISeotember !October1995 Figure 1: ( Lower panel) Sea Surface Temperatures (SST), crosses: satellite observed SST from weekly charts, solid line: SST from DWD analyses (0.5° EM), dotted line: SST from BSMO (forced by REMO) dashed line: SST from the coupled run. (Upper panel) REMO calculated latent heat flux (LHF) mean over 6:00 to 12:00, solid line: LHF from the first run with DWD-SST, dotted line: LHF from the second run with BSMO-SST, dashed line: LHF from the coupled run. All values are means over a 1 ° x 2° wide region in the eastern Gotland Basin. References: Int. BALTEX Secretariat, 1997: The Main BALTEX Experiment 1999-2001 - BRIDGE. Strategic Plan. Pub. No. 9, 78 pages Jacob, D., Podzun, R., 1997: Sensitivity Studies with the Regional Climate Model REMO. METEOROL. ATMOS. PHYS., 63, 119 - 129 Lehmann, A., 1995: A three-dimensional baroclinic eddy-resolving model of the Baltic Sea. TELLUS, 47A, 1013 - 1031 

A Coupled High Resolution Atmosphere - Ocean Model for the BALTEX Region

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A Coupled High Resolution Atmosphere - Ocean Model for the BALTEX Region

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