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Wolski T.,University Of Szczecin | Wisniewski B.,Maritime University Of Szczecin | Giza A.,University Of Szczecin | Kowalewska-Kalkowska H.,University Of Szczecin | And 5 more authors.
Oceanologia | Year: 2014

The purpose of this article is to analyse and describe the extreme characteristics of the water levels and illustrate them as the topography of the sea surface along the whole Baltic Sea coast. The general pattern is to show the maxima and minima of Baltic Sea water levels and the extent of their variations in the period from 1960 to 2010. A probability analysis is carried out on the annual sea level maxima and minima for 31 water level gauges in order to define the probability of occurrence of theoretical sea levels once in a specific number of years. The spatial distribution of sea levels for hundred-year maximum and minimum water levels is illustrated. Then, the number of storm surges for the accepted criteria are presented: these numbers increased in the 50-year period analysed. The final part of the work analyses some extreme storm events and calculates the static value and dynamic deformation of the sea surface by mesoscale, deep low-pressure systems. © by Polish Academy of Sciences, Institute of Oceanology, 2014.

Changes in the location, instrumentation and measurement times are documented for three Estonian meteorological stations during the last century. These metadata were used to check if such changes have introduced significant discontinuities into the time series of daily and monthly mean temperature, average wind speed and daily and monthly precipitation sums. For this purpose, time periods of the length of at least ten years were separated before and after each change and average values of meteorological elements during these periods were compared by means of the t-test at the significance level of 0.05. On the daily basis, such changes introduced an increase in all parameters under consideration. On the monthly basis, only wind speed and precipitation sums were affected in some cases. Earlier climatological analyses have shown that in Estonia the temperature has risen and the precipitation sums have increased. Therefore, it is very difficult to separate natural trends from the artificial changes.

Keevallik S.,Tallinn University of Technology | Mannik A.,University of Tartu | Hinnov J.,Estonian Meteorological and Hydrological Institute
Estonian Journal of Earth Sciences | Year: 2010

The possibilities of using High Resolution Limited Area Model (HIRLAM) version 6.4.0 outputs to describe wind parameters in the coastal zone of Estonia were investigated. For this purpose output from 3-dimensional variational (3DVAR) analysis and 24 h forecast files were compared with measurements at nine coastal sites during January and April-December 2007. Special attention was paid to moderate and strong winds (wind speed > 5 m/s) that are responsible for sea level changes and high wave heights in the coastal area. It is shown that HIRLAM overestimates the wind speed. This overestimation is stronger in cases where HIRLAM uses an inadequate land-sea fraction in the respective cells. The model describes the angular distribution of moderate and strong winds better than that of weak winds. Except for one station, approximately 90% of HIRLAM estimates of the direction of moderate and strong winds differ less than ± 22.5° from the measured values; in approximately 60% of cases the direction differs less than ± 10°. The HIRLAM system approximates best the winds at the westernmost stations on the Estonian islands and in Pärnu, whereas a 24 h forecast gives somewhat better results than the winds diagnosed from 3DVAR analysis.

Eerme K.,Tartu Observatory | Kallis A.,Estonian Meteorological and Hydrological Institute | Kallis A.,Tallinn University of Technology | Veismann U.,Tartu Observatory | And 2 more authors.
Theoretical and Applied Climatology | Year: 2010

The time series of the daily sums of global and direct irradiance recorded at Tartu-Tõravere Meteorological Station site (58°16′N, 26°28′E, 70 m a. s. l.) in 1955-2006 have been analyzed in seasonal timescales. The average daily ratio G/G clear of available global irradiance to its local climatic clear-sky value in the summer half-year corresponds to 65. 5%, while that of the direct irradiance on the horizontal surface I′/I′ clear was 41% of the climatic clear-sky value. In the case of dry Rayleigh atmosphere as a reference, these ratios are 53.5% and 28%, respectively. The time series of the summer season totals reveal a longer interval of reduced values in 1976-1993 as well as two periods of frequent sunny summers in 1967-1975 and since 1994. The probability density distribution of the summer season totals during the observed period is strongly asymmetric; in spring, however, it is close to the normal distribution. In winter, there is a moderate negative correlation between the G/G clear and the North Atlantic Oscillation as well as the Arctic Oscillation indices. © 2009 Springer-Verlag.

Sluzenikina J.,Estonian Meteorological and Hydrological Institute | Keevallik S.,Tallinn University of Technology
International Journal of Remote Sensing | Year: 2013

This article presents an overview of marine winds in the Gulf of Riga, measured by the SeaWinds instrument on the Quick Scatterometer (QuikSCAT) satellite during the whole lifetime period of the satellite, i.e. 1999-2009. The data were collected with a resolution of 12.5 km during the satellite overflights at 02-04 UTC and 16-18 UTC and referenced to the height of 10 m. The quality of the data was carefully checked, and necessary adjustment was applied to remove the contaminated recordings. Wind speed and direction were compared with those registered on the islands of Kihnu and Ruhnu. It has been shown that allowing lenient filtering of rain-contaminated data derives larger wind speed estimates but increases considerably the quantity of data, allowing separate analysis of the northern and southern parts of the gulf. Wind speed in the northern part is slightly higher, the wind roses for the early morning measurements are similar, but those for the evening measurements show that in spring and summer, the most frequent winds in the southern part are northwesterly and in the northern part are westerly. Wind speed measured on the islands is less than that estimated from the satellite even in the case when rain contamination is removed through application of strict criteria. Wind roses measured at Kihnu are practically similar to those estimated from satellites for the northern part of the gulf in the evening but show some differences during the early morning. In winter, ground-based measurements show maximal frequency of southerly winds, and satellite measurements show southwesterly winds. In spring, the secondary maximum in the wind rose shows northwesterly winds in ground-based records and easterly winds in satellite measurements. Ground-based wind directions are well correlated with those measured by the satellite showing correlation coefficients of over 0.9. For wind speed, this quantity is somewhat lower, i.e. around 0.6. © 2013 Copyright Taylor and Francis Group, LLC.

Keevallik S.,Tallinn University of Technology | Krabbi M.,Estonian Meteorological and Hydrological Institute
Estonian Journal of Engineering | Year: 2011

Radiosonde observations of temperature, relative humidity and wind properties are compared at two neighbouring stations - Tallinn-Harku in Estonia and Jokioinen in Finland - with the aim to optimize radiosonde network and measurement times. The comparison is carried out for the period of 1993-2009 when both stations used similar equipment. Midnight and noon soundings are compared separately. It is concluded that the profiles of the temperature and wind speed at these two stations are similar, but those of the relative humidity differ significantly, showing coefficient of correlation over 0.7 only near the tropopause. Wind roses are similar in summer, but somewhat different in winter, especially in the stratosphere during the daytime.

Kannel M.,University of Tartu | Ohvril H.,University of Tartu | Okulov O.,Estonian Meteorological and Hydrological Institute
Proceedings of the Estonian Academy of Sciences | Year: 2012

The concept behind the shortcut idea is a close correlation between column broadband aerosol optical depth (BAOD) and aerosol optical depth at 500 nm (AOD500). The method uses only two input parameters: (a) the Bouguer broadband coefficient of column transparency for optical mass m = 2 (solar elevation about 30°) and (b) integrated column precipitable water vapour which can be roughly estimated using surface water vapour pressure. In creating the method, a large database, including almost 20 000 complex, spectral and broadband direct solar beam observations at Tõravere, Estonia, during all seasons of a 8-year period, 2002-2009, was used. The AOD500 observations were performed by the NASA project AERONET and the broadband direct beam ones by the Estonian Meteorological and Hydrological Institute. Analysis of this database revealed a high correlation between BAOD and AOD500 which enabled transition from broadband to spectral AOD. Almost 82% of the observations in the database belonged to lower turbidities when AOD500 < 0.2. The root mean square deviation (RMSD) for AOD500 prediction in this range was 0.022. For AOD500 = 0.2-0.4, the RMSD was 0.035, for 0.4-0.6, the RMSD was 0.042. Relative RMSD for these ranges was about 22%, 12%, and 9%, respectively. For AOD500 > 0.6, relative RMSD remained 9%. For comparison, the same database was used to test Gueymard's broadband parameterization based on his SMARTS2 classic model. The last one, apparently due to problems with circumsolar radiation, slightly but systematically underestimated the AOD500. However, there was a close correlation between our shortcut results and Gueymard's broadband parameterization.

Luhamaa A.,University of Tartu | Luhamaa A.,Estonian Meteorological and Hydrological Institute | Kimmel K.,University of Tartu | Mannik A.,University of Tartu | And 2 more authors.
Climate Dynamics | Year: 2011

Regional reanalysis database BaltAn65+ comprising meteorological data for Baltic Sea region for the time period 1965-2005 is described. For data assimilation and hindcasts, the numerical weather prediction model HIRLAM 7.1.4 is applied, with 11 km horizontal and 60-layer vertical resolution. Reanalysis includes three-dimensional weather analysis data. Standard surface observations and meteorological soundings together with ship and buoy measurements from WMO observational network are used in analysis. Boundary fields are obtained from ECMWF ERA-40 global re-analysis. The BaltAn65+ can be considered as a regional refinement of ERA-40 for Baltic Sea region, providing the historical weather and climate data with enhanced spatial resolution, which is main motivation for creation of this novel reanalysis database. © 2010 Springer-Verlag.

Tammets T.,Estonian Meteorological and Hydrological Institute | Jaagus J.,University of Tartu
Theoretical and Applied Climatology | Year: 2013

A method of moving precipitation totals is described and applied for the analysis of precipitation extremes in Estonia. Numbers of extremely wet and extremely dry days and other indices of precipitation extremes were calculated using the daily precipitation data measured at 51 stations over Estonia during 1957-2009. Mean regularities of spatial and seasonal distribution were determined. Long-term changes were detected using Sen's method and Mann-Kendall test. The highest risk of heavy precipitation is in the regions of higher mean precipitation on the uplands and on the belt of higher precipitation in the western part of continental Estonia. Wet spells have their sharp maxima in July and August. The highest risk of droughts is observed in the coastal regions of West Estonia. In the coastal area, droughts appear mostly in the first half of summer, while in the eastern Estonia, they are usually observed during the second half of summer. Extreme precipitation events have become more frequent and intense. Statistically significant increasing trends were, first of all, found in the time series of winter extreme precipitation indices. In summer and autumn, trends existed in some indices, but in spring, there were no trends at all. There were no trends in time series of dryness indices in Estonia in 1957-2009. © 2012 Springer-Verlag.

Jakobson L.,University of Tartu | Vihma T.,Finnish Meteorological Institute | Jakobson E.,Tartu Observatory | Jakobson E.,University of Tartu | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2013

Low-level jets (LLJ) are important for turbulence in the stably stratified atmospheric boundary layer, but their occurrence, properties, and generation mechanisms in the Arctic are not well known. We analysed LLJs over the central Arctic Ocean in spring and summer 2007 on the basis of data collected in the drifting ice station Tara. Instead of traditional radiosonde soundings, data from tethersonde soundings with a high vertical resolution were used. The Tara results showed a lower occurrence of LLJs (46 ± 8%) than many previous studies over polar sea ice. Strong jet core winds contributed to growth of the turbulent layer. Complex relationships between the jet core height and the temperature inversion top height were detected: substantial correlation (r Combining double low line 0.72; p < 0.01) occurred when the jet core was above the turbulent layer, but when inside the turbulent layer there was no correlation. The most important forcing mechanism for LLJs was baroclinicity, which was responsible for the generation of strong and warm LLJs, which on average occurred at lower altitudes than other jets. Baroclinic jets were mostly associated with transient cyclones instead of the climatological air temperature gradients. Besides baroclinicity, cases related to inertial oscillations and gusts were detected. As many as 49% of the LLJs observed were associated with a frontal passage, which provides favourable conditions for baroclinicity, inertial oscillations, and gusts. Further research needs on LLJs in the Arctic include investigation of low-level jet streams and their effects on the sea ice drift and atmospheric moisture transport. © 2013 Author(s).

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