An operational hydrodynamic model of the Gulf of Gdańsk

Transcription

1 An operational hydrodynamic of the Gulf of Gdańsk Marek Kowalewski University of Gdańsk, Institute of Oceanography, Al. Marszałk a Piłsudskiego 46, Gdynia ocemk@univ.gda.pl Abstract A lately set three-dimensional hydrodynamic of the Gulf of Gdańsk has calculated a daily 48-hours forecast of temperature, salinity as well as a free surface elevation and its flow fields. The includes the Baltic Sea and the Danish Straits, and is based on the numerical weather forecast of the ICM (the Warsaw University Interdisciplinary Centre for Mathematical and Computer Modelling). The results of the ling simulations compare sufficiently with the measurements and they allow for a practical application. The analysis points to the imperfection of the coastal conditions, namely, erroneous meteorological forecast, simplified conditions on the open boundary of the as well as an exclusion of the actual river inflows. If these inadequacies are eliminated then, the ling results reliability can be improved. A further improvement could be achieved by modifying the so that the current sea level, salinity, and temperature data from the coastal stations, measurement buoys and satellite images are assimilated. Introduction There is lately an increased interest in the mathematical ling methods of the sea hydrodynamic processes. As far as the Gulf of Gdansk is concerned this interest was related to the problem of dispersing substances of a land origin (Jędrasik and Kowalewski, 1993; Van der Vat et al., 1994; Ołdakowski et al., 1994; Robakiewicz and Karelse, 1994). In the Oceanography Institute of the Gdansk University there are studies carried out that target operational s describing both the hydrodynamic as well as biogeochemical processes occurring in the Baltic Sea, and in the Gulf of Gdańsk specifically. A three dimensional hydrodynamic was worked out in 1995 to 1997 (Kowalewski, 1997). The was used to create an ecological of the Gulf of Gdańsk (Jędrasik, 1997), and to study the upwelling in the Baltic Sea (Kowalewski, 1998). In 1998, the hydrodynamic was adapted to take advantage of the digital 48 hourly meteorological ICM forecast (the Warsaw University Interdisciplinary Centre for Mathematical and Computer Modelling) that originated from a mesoscale weather UMPL. This enabled in June1999 setting up a trial operational version of the that could calculate 48 hourly hydrodynamic forecast. The results as maps of the temperature, salinity, currents, and sea level deflections are shown on the Internet ( The potential users of the marine operational forecast could be following: marine life saving services and marine oil pollution combating services environmental protection services and institutions ports, fishing companies and seafarers marine scientific institutes sea-bed exploring companies public media (water temperature predict etc.) beach-goers, yachtsmen, surfers, etc.

2 2 Model characteristics The theoretical and numerical solution was based on a Blumberg and Mellor (1997) POM (Princeton Ocean Model) that was modified for the Baltic Sea application (Kowalewski, 1997). Since the water exchange between the Gulf of Gdansk and the Baltic Sea is substantial, it is necessary to include the whole Baltic Sea in the ling of the Gulf dynamics. Therefore, an open boundary was set up between the Kattegat and Skagerrak, through which waters are exchanged with the North Sea. The radiation boundary condition was applied to the flows averaged vertically assuming a constant sea level at the Skagerrak. When the momentum value of the free surface deflection is higher than the accepted constant value, there is the Baltic Sea waters outflow that is proportional to the differences between the two values. In a reverse case, when a sea level in the Kattegat is lower there is an inflow of waters from Skagerrak. To decrease a number of calculations a locally condensed calculating grid was applied. The includes two areas of different spatial steps i. e. the Baltic Sea of 5NM step, and the Gulf of Gdańsk of 1NM step (Fig. 1). The calculations in both areas are simultaneous, and information exchange on the common boundary occurs on each time step. All the variables from a border of one area serve as a boundary condition for the other area. The algorithm making a connection between the two assures retaining the mass and energy. Władysławowo Hel Gulf of Gdańsk Sopot Gdańsk Vistula Baltic Sea Fig. 1. The led areas the Baltic Sea and the Gulf of Gdańsk The applied sigma-transformation produces in every point in the sea a vertical profile that could be evenly stratified regardless of its depth (Fig. 2). Thus, both a better representations of a near-bottom layer as well as a simplified calculation numerical scheme can be achieved. However, particular layers are not exactly horizontal resulting in some discrepancies in calculations of the horizontal pressure gradients (Haney, 1991) and the horizontal diffusion. Thus, the discrepancies cause some errors in the calculated flows. To minimize such errors, a technique of subtraction of the area-averaged density before density gradient

3 3 evaluation (Gary, 1973; Mellor et al. 1994) was applied. In the operational version of the, an unequal thickness 18-layers division was applied. For a better representation of the surface and nearbottom layers, their thicknesses were smaller than the rest. δ = H δ =-1 Fig. 2. A scheme of the area division using layers with sigma co-ordinates The inflows of the 49 rivers were included together with the 1 rivers flowing to the Gulf of Gdansk. The flow values are calculated using the trigonometric series that describes a seasonal variation of rivers outflows based on the long-term data (Cyberski, 1997). The also allows for a solar energy inflow as well as a thermal exchange of the sea surface (Jędrasik, 1997). The meteorological data of all - a wind field, an air temperature and pressure, and a vapour pressure were taken from a mesoscale weather UMPL. The initial conditions for hydrological fields were based on the February distributions of temperature and salinity. After activating the, the variations of the temperature and salinity acted only upon the influence of the variable in time meteorological conditions and rivers inflows, i. e. no hydrological data assimilation was done. The efforts to set up an operational hydrodynamic for the Gulf of Gdańsk resulted in1996 with initiated by the Marine Institute a research experiment POLRODEX 99 (Gajewski J. and Gajewski L. 1997). The experiment aimed at collecting data necessary for calibrations and verifications of s. A big interest generated by the experiment resulted in its repeating in the consecutive years (1997, 1998, and 1999). Many scientific institutions participated in it as well. The obtained data were used to verify the in relation to the water temperature, variation of the sea level and the distribution of the passive substance (rodamin). Verification of the operational prognosis A comparison between the surface waters temperatures at the coastal stations and the results of the hydrodynamic showed (Fig. 3) their general compatibility with each other (correlation coefficients ranged from.65 to.86). The measurements in Sopot were done on hourly basis over the whole summer season. For stations Hel and Gdańsk-Port Północny, the measurements were done once a day (data from the Institute of meteorology and Water Management, Marine Section). The sea temperatures from July to mid-august were very high ( 24 C) and they were related to the very high air temperatures of the time. The sea temperatures for the relevant period calculated by the, were regularly lower. This was caused by the lowered sea air temperature data from the meteorological forecast of the weather UMPL that allowed for the climatic sea temperature (Fig. 4). In the second half of

4 4 August and in September the differences between the measurements and the results were insignificant. a) temperature [ C] Sopot R=.65 b) 12 temperature [ C] VII 15.VII 1.VIII 15.VIII 1.IX HEL R = VIII 6.VIII 11.VIII 16.VIII 21.VIII 26.VIII 31.VIII c) temperature [ C] Gdańsk-Port Północny R = VIII 6.VIII 11.VIII 16.VIII 21.VIII 26.VIII 31.VIII Fig. 3. The comparison of the and led temperature of the surface water in the Gulf of Gdańsk in summer 1999 (R a linear correlation coefficient) Based on the data from the POLRODEX 99 experiment from August16 to 25, 1999 a comparison was made between the water changes in the vertical profile (Fig. 5), and the hydrological results. The measurements of the vertical distributions of water temperature were taken hourly aboard the ORP

5 5 Kopernik at the Vistula River mouth ( N, 19.9E). During the measurement period, there were two distinct increases of the thermocline (August 16 to 17 and to 21) related to the coastal upwelling. These were also shown in the, albeit the vertical changes in the temperature were less clear. Probably this was due to insufficient number of the layers of the numerical grid. The temperatures of the surface layer were approximately 1 to 2 C lower than the ones, and these could be related to the lower air temperature data from the weather UMPL (Fig. 5a) Tempetature [ C] UMPL 1 15.VI 1.VII 15.VII 1.VIII 15.VII 1.IX Fig. 4. The comparison of the and led (UMPL) air temperature on the station Hel in summer 1999 The ling data of the sea level variations (Fig. 6) and those in August 1999 on IMGW stations (Władysławowo, Hel, Gdańsk-Port Północny) were showing a substantial compatibility with the correlation coefficient approximately.9. However, the data are relative since it is difficult to relate them to the average sea level. The reason being in imperfection of the coastal conditions applied to the open Baltic Sea boundary at the Skagerrak and Kattegat. If the constant of the sea level was replaced by the water changes on the coastal stations in a vicinity of the open boundary, then the prognostic accuracy of the sea level deflection would be improved, thus allowing to calculate absolute sea levels, for example in relation to NN Amsterdam1955. The measurements performed by the Marine Institute of Gdańsk during the POLRODEX 99 that dealt with dispersion of a rodamin spot in the Gulf of Gdańsk, allowed to verify the for its prognostic application to the spreading of pollutants and other substances. On August 17, an alcohol rodamin solution was thrown into the water in the Vistula River estuary (Fig. 7). The spot concentration and changes in its locations were monitored over 24 hours with a fluorometer (a spot route on Fig. 7). For a purpose of a ling simulation a data of August 17, hour (thus before the actual rodamin experiment started) were used as provided by a 48 hours meteorological forecast of the UMPL.

6 6 a) b) Temperature [ C] VIII 19.VIII 21.VIII 23.VIII 25VIII UMPL 17.VIII 19.VIII 21.VIII 23.VIII 25.VIII Measured Depth [m] c) -5-1 Model Depth [m] Temperature [ C] Fig. 5. The comparison of the air temperature: and obtained by the UMPL (a), water temperature (b) and obtained by the hydrodynamic (c) in Vistula River estuary ( N, 19.9 E) in August 1999

7 7 sea level [cm] sea level [cm] sea level [cm] Władysławowo R = VIII 6.VIII 11.VIII 16.VIII 21.VIII 26.VIII 31.VIII Hel -15 R = VIII 6.VIII 11.VIII 16.VIII 21.VIII 26.VIII 31.VIII Gdańsk-Port Północny R =.87 1.VIII 6.VIII 11.VIII 16.VIII 21.VIII 26.VIII 31.VIII Fig. 6. The comparison of the led and sea levels on stations Władysławowo, Hel and Gdańsk- Port Północny (R a linear correlation coefficient) The comparison of the monitored and the led rodamin routes showed their compatibility. However, there was a difference in a speed movement the led one was much slower. One of the reasons for

8 8 that could be exclusion of data of the actual Vistula River flows that are influencing the water dynamics in the area. The statistical description of river inflows employed by the present version of the takes into account only seasonal changes. However, short-term changes caused for instance by the atmospheric precipitation are not taken into account, and by so doing can increase an error margin. Another reason could be a lower wind speed given by the meteorological forecast of the UMPL. The actual aboard wind speeds were higher than the prognostic ones (Fig. 8). Starting position : 21:15 14:24 Gulf ofgdańsk 4:36 1: 4:36 1:3 21:15 17:46 7:11 1: Vistula led monitored Fig. 7. The monitored (POLRODEX 99) and led routes of the rodamin spot wind speed [m s -1 ] UMPL Fig. 8. The comparison of the and led (UMPL) wind speeds The verification results of the trial version under operational conditions showed less compatibility to the measurements than to calculations based on the archive data (Kowalewski, 1997). This is a result of the limited access to the hydrological and meteorological data such as river flows, air temperature etc. Effectively, ling water temperature errors reached 6 C (Fig. 4a), whereas those based on the actual air temperature were below 2 C (Kowalewski, 1997), thus confirming a possibility of minimising errors by taking into account actual measurements. The decreased air temperature above the sea given by the

9 9 weather UMPL caused a regular ling decrease of the water temperature in summer. Those errors could be eliminated either by improvements to the weather or inclusion in the hydrodynamic the air temperature data from the IMGW, from coastal and buoy stations as well as current satellite thermal photographic data. Adding the actual data on river flows and their temperatures could achieve a further improvement in accuracy of the hydrodynamic, particularly in the estuarine regions. As far as the Gulf of Gdańsk is concerned, data on the Vistula River take priority since the great impact of the river on the water in the area. Conclusions 1. The verification results of the indicate its use for forecast of flows, water temperature, and changes of the sea level. A practical use of the in the coastal zone and in the open sea is encouraged by a good compatibility between the simulations and the measurements. 2. To improve a prognostic accuracy it is necessary to include actual data on coastal conditions (meteorological ones, sea level on the open boundary, flows, and river waters temperatures). 3. Data assimilation of the satellite thermal photographs and the automatic measuring station data can further improve prognostic reliability. References: Blumberg A.F., Mellor G.L., 1987, A description of the three-dimensional coastal ocean circulation, [w] Three-Dimensional Coastal Ocean Models, N. Heaps, American Geophysical Union, 1-16 Cyberski J., 1997, Riverine water outflow into the Gulf of Gdańsk, Oceanol. Stud., 26 (4), Gary J.M., 1973, Estimate of truncation errors in transformed coordinate, primitive equation atmospheric s, J. Atmos. Sci., 3, Gajewski J., Gajewski L., 1997, Planned and realised POLRODEX 96 activities, Bulletin of the Maritime Institute, 24 (1), 7-12 Haney R.L., 1991, On the pressure gradient force over steep topography in sigma coordinate ocean s, J. Phys. Oceanogr., 21, Jędrasik J., 1997, A of matter exchange and flow of energy in the Gulf of Gdańsk ecosystem - overview, Oceanol. Stud., 26 (4), 3 Jędrasik J., Kowalewski M., 1993, Transport of pollutants in Gdansk Bay, Stud. i Mater. Oceanol., 64 (3), Kowalewski M., 1997, A three-dimensional, hydrodynamic of the Gulf of Gdańsk, Oceanol. Stud., 26 (4), Kowalewski M., 1998, Upwellingi przybrzeżne w płytkim morzu stratyfikowanym na przykładzie Bałtyku, rozprawa doktorska, maszynopis, Gdynia, Biblioteka Wydziału BGiO Uniwersytetu Gdańskiego, 85 Krężel A., 1997, A of solar energy input to the sea surface, Oceanol. Stud., 26 (4), Mellor, G. L., Ezer T., Oey L.Y., 1994, The pressure gradient conundrum of sigma coordinate ocean s, J. Atmos. Oceanic. Technol., 11 (4), Part 2, Ołdakowski B., Kowalewski M., Jędrasik J., 1994, Nutrient dynamics for the Gulf of Gdansk, Proc. of the 19th Conference of the Baltic oceanographers, Sopot - Poland,

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