Boreal environment research 16 (suppl. a): 27–41 © 2011 issn 1239-6095 (print) issn 1797-2469 (online) helsinki 31 march 2011 Performance of operationally calculated hydrodynamic forecasts during storm surges in the Pomeranian Bay and the szczecin lagoon marek Kowalewski1) and halina Kowalewska-Kalkowska2) 1) Institute of Oceanography, University of Gdańsk, al. Marszałka J. Piłsudskiego 46, PL-81-378 Gdynia, Poland (corresponding author‘s e-mail: [email protected]) 2) Institute of Marine Sciences, University of Szczecin, Mickiewicza 18, PL-70-383 Szczecin, Poland Received 23 Nov. 2009, accepted 31 Aug. 2010 (Editor in charge of this article: Kai Myrberg) Kowalewski, m. & Kowalewska-Kalkowska, h. 2011: Performance of operationally calculated hydrody- namic forecasts during storm surges in the Pomeranian Bay and the szczecin lagoon. Boreal Env. Res. 16 (suppl. a): 27–41. The modified hydrodynamic model of the Baltic Sea, developed at the Institute of Ocea- nography, University of Gdańsk was validated using storm-related sea level fluctuations as well as water temperature and salinity variations in the Pomeranian Bay and the Szczecin Lagoon (southern Baltic). The high resolution (about 300 m) applied to the model for the Szczecin Lagoon area resulted in a much better description of the area’s bathymetry, and in an improved fit between the modelled and the observed distributions of the data sets both in the Bay and in the Lagoon. Model quality tests involving 2002–2007 storm surge events showed a better representation of events characterised by rapid water level fluctuations and fast changes of physical water properties. The numerical model’s high quality of simula- tions allows for applying the higher resolution of spatial spacing to the Szczecin Lagoon area also in the operational version of the model. Introduction Sea from N to W sector, or less frequently from W to SW sector. Moreover, strong northerly air Storm surges represent a particular threat for flow occurs when lows travel outside the Baltic low-lying coastal areas of the Pomeranian Bay Sea, i.e., north of the Gulf of Bothnia or over the and Szczecin Lagoon (southern Baltic) by pro- central and eastern parts of Europe. Sztobryn et ducing flooding events, resulting in coastal ero- al. (2005) identified several types of pressure pat- sion and causing many problems to inhabitants terns leading to storm surges at the German and of the coastal areas. These rapid, non-periodic, Polish Baltic Sea coast, i.e., northerly air flow short-term sea level fluctuations are associated over Scandinavia and the Baltic Sea, stormy low with cyclonic circulation having its centre within pressure systems moving over the central and or close to the Baltic Sea, when strong north- southern Baltic Sea and storms from the eastern westerly to northeasterly onshore winds blow sector. The authors also showed the amount of onto land (Zeidler et al. 1995). Majewski et al. water in the Baltic Sea (‘fill-up’) to be of great (1983) showed that those winds are associated importance in generating particularly dangerous mainly with passages of lows entering the Baltic storm surges. In turn, Jensen and Müller-Navarra 28 Marek Kowalewski & Kowalewska-Kalkowska • Boreal env. res. Vol. 16 (suppl. a) Fig. 1. the modelled regions (Baltic sea, Po me ranian Bay, szcze- cin lagoon) with station locations (stars). (2008) pointed out that seiches involving the contribute 15%–20% to the water exchange. The whole water body of the Baltic Sea can influ- water exchange between the Szczecin Lagoon ence the storm water level in the western Baltic. and the Pomeranian Bay occurs as pulse-like In a recent study, Kowalewska-Kalkowska and in- and outflow events. Water circulation within Wiśniewski (2009) demonstrated that the most the Odra mouth is greatly affected by the ship- dangerous storm surges at the Pomeranian Bay ping channel, 66 km long, 10–11 m deep, and coasts occur during the passages of deep and 250-m wide, extending from the Szczecin Har- intensive low pressure systems near the coast of bour along the western Odra to intersect the the southern Baltic, with an extensive system of eastern part of the Szczecin Lagoon, the Great north-westerly to north-easterly winds. Lagoon and along the Świna Strait — to reach The shallow, almost non-tidal Pomeranian the Pomeranian Bay (Majewski 1980). Bay, with a mean depth of about 13 m and an The hydrodynamic conditions of the Szc- area of about 6000 km2, is affected by the inten- zecin Lagoon are driven by wind action, water sive wind-induced mixing of fresh and brackish level differences between the Lagoon and the waters (Fig. 1). It receives annually on average Pomeranian Bay, fresh water inflow from the 17.6 km3 freshwater input mainly from the Odra Odra, and salt water intrusions through the straits River (Mikulski 1970). In its downstream reach, connecting the Lagoon with the Pomeranian Bay the Odra opens first into the Szczecin Lagoon, (Jasińska et al. 2003). As a result of a very low a coastal water body of about 687 km2 surface slope of water surface within the whole Odra area, 2.5825 km3 water volume and 3.8 m in mouth area, water levels in the Szczecin Lagoon mean depth (Majewski 1980). Then, it drains and in the Lower Odra channels are strongly into the Pomeranian Bay through three narrow affected by changes in the sea level. During straits: the Świna, the Dziwna and the Peen- heavy storm surges associated with strong north- estrom. The Świna consists of both natural and erly winds, when the sea level in the Bay is man-made canals and serves as the most impor- higher than that in the Lagoon, the Bay’s brack- tant conduit of the water exchange between the ish water enters the Lagoon and raises the water Lagoon and the Bay. According to Mohrholz level both there and in the Lower Odra chan- and Lass (1998), it covers 60%–70% of the nels (Buchholz 2009, Kowalewska-Kalkowska mass transport. The Peenestrom and the Dziwna and Wiśniewski 2009). The influx affects the Boreal env. res. Vol. 16 (suppl. a) • Performance of numerical forecasts in the Pomeranian Bay 29 Lagoon’s physical and chemical characteristics most advanced model describing the hydrody- as well. Jasińska and Massel (2007) reported that namic regime of the Lower Odra River was during storm surge events the brackish Pomera- developed by Ewertowski (1988). nian Bay water, with a salinity up to 8‰, was Over the recent years, operational meteoro- advected through the Świna and spread into the logical and hydrodynamic forecasting within the Szczecin Lagoon. On the other hand, effect of Baltic Sea region has been a target of many the instantaneous Odra discharge on water levels numerical studies. The High Resolution Oper- in the river’s mouth area is of less importance ational Model of the Baltic Sea (HIROMB) because, even during Odra flood events, the was developed at the Bundesamt für Seeschiff- water level increases by only a few centimetres fahrt und Hydrographie (BSH) in Hamburg as the flood wave enters the Szczecin Lagoon. and subsequently extended in cooperation with However, the increased Odra discharge results in the Swedish Meteorological and Hydrologi- a drop of salinity there. As reported by Mohrholz cal Institute (SMHI) in Norrköping (Eigenheer et al. (1998), during the Odra flood event in 1999, Funkquist 2001). Kałas et al. (2001) and August 1997 the salinity of the eastern part of Stanisławczyk (2002) validated the forecasts for the Szczecin Lagoon decreased to 0.15‰. the Polish coastal zone. The description and vali- Due to complex nature of meteorological dation of the Bundesamt für Seeschifffahrt und and hydrological factors affecting hydrodynam- Hydrographie, circulation model (BSHcmod) ics of Baltic coastal waters, numerical model- was presented in detail by Dick et al. (2001). ling has become an essential tool in offshore Filinkowa et al. (2002) applied the model in zone management and flood protection there. the eastern Gulf of Finland. Gästgifvars et al. Suursaar et al. (2002) applied two-dimensional (2008) proved that the Baltic Sea forecast models hydrodynamic model for the Gulf of Riga and HIROMB, BSHcmod, and DMI-BSHcmod run the Väinameri Sea for examining sea level vari- by the three institutes SMHI, BSH, and DMI in ations there. The model was then used in stud- their daily routine services are well suited to fore- ies on the extreme sea level events (Suursaar et cast water level changes in the Gulf of Finland. al. 2003, 2006). Andrejev et al. (2004) used a The three-dimensional operational hydrody- three-dimensional baroclinic prognostic model namic model of the Baltic Sea (M3D_UG) built to study mean circulation and water exchange in on the coastal ocean circulation model known as the Gulf of Finland, and recently Averkiev and the Princeton Ocean Model (POM) was devel- Klevanny (2007) applied successfully the CAR- oped in 1995–1997 at the Institute of Oceanog- DINAL numerical model in analyses of extreme raphy, University of Gdańsk (Kowalewski 1997). sea levels in St. Petersburg (Russia). At first, the model was generating hydrodynamic The hydrodynamic regimes of the Szczecin forecasts for two areas: the southern Baltic and Lagoon and Pomeranian Bay have been mostly the Gulf of Gdańsk (Kowalewski 2002). Further described by three-dimensional models, such as modification of the model resulting in develop- ESTURO (Jasińska and Massel 2007) and the ment of the Odra discharge model (Kowalew- Warnemünder Ostsee Model (WOM) (Lass et ska-Kalkowska and Kowalewski 2006) allowed al. 2001). Siegel et al. (2005) analysed discharge to issue 60-h hydrodynamic forecasts of water and transport processes in the western Baltic Sea levels, currents, water temperature and salinity for and in the Szczecin Lagoon using three-dimen- the Pomeranian Bay and Szczecin Lagoon (http:// sional MOM-3 model and two-dimensional model.ocean.univ.gda.pl).