National Institute of Meteorology and Hydrology

Sofia, Bulgaria

National Institute of Meteorology and Hydrology

Sofia, Bulgaria
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Denev T.,National Institute of Meteorology and Hydrology | Alexandrov V.,National Institute of Meteorology and Hydrology
9th International Workshop on Precipitation in Urban Areas: Urban Challenges in Rainfall Analysis, UrbanRain 2012 | Year: 2017

Urban precipitation is affected by the alteration of industrial processes such as the appearance of urban heat islands. Urban heat islands depend on various socio-economic factors involved in urban development and are a typical phenom-enon nowadays. A decrease in precipitation is observed for the warm half year. The aim of this study was to assess pre-cipitation with a particular emphasis on peculiarities in the Sofia urban agglomeration. Changes in the urban environ-ment such as natural disasters-droughts and floods, water availability and human health, both under the current and especially future climate conditions-are of major importance for the human population and sustainable development of the Sofia region. Data from a network of 10 weather stations were analysed for a 50 year period, namely 1961-2010. Some of the challenges in this study were related to the impacts and adaptation measures with respect to increases in excessive precipitation, monitoring and early warning systems. Another aspect is the development of a model based on meteorological phenomena which can be used in an operational regime for the prediction, monitoring and visualisation of spatial maps in areas with potential risk of floods, droughts and heat waves. Various governmental institutions can apply the obtained results to mitigate damages to infrastructure and socio-economic systems. Finally the presented study is a part of a pilot project to address the distribution of precipitation in urban areas. Future research will include the effect of changes in temperature and green house gasses as well as extend the analysis in other representative parts of the climate system which influence the precipitation regime.

Hirschi M.,ETH Zurich | Seneviratne S.I.,ETH Zurich | Alexandrov V.,National Institute of Meteorology and Hydrology | Boberg F.,Danish Meteorological Institute | And 5 more authors.
Nature Geoscience | Year: 2011

Climate change is expected to affect not only the means of climatic variables, but also their variabilities and extremes such as heat waves. In particular, modelling studies have postulated a possible impact of soil-moisture deficit and drought on hot extremes. Such effects could be responsible for impending changes in the occurrence of heat waves in Europe. Here we analyse observational indices based on measurements at 275 meteorological stations in central and southeastern Europe, and on publicly available gridded observations. We find a relationship between soil-moisture deficit, as expressed by the standardized precipitation index, and summer hot extremes in southeastern Europe. This relationship is stronger for the high end of the distribution of temperature extremes. We compare our results with simulations of current climate models and find that the models correctly represent the soil-moisture impacts on temperature extremes in southeastern Europe, but overestimate them in central Europe. Given the memory associated with soil moisture storage, our findings may help with climate-change-adaptation measures, such as early-warning and prediction tools for extreme heat waves. © 2011 Macmillan Publishers Limited. All rights reserved.

Farda A.,Czech Hydrometeorological Institute | Deue M.,French National Center of Weather Research | Somot S.,French National Center of Weather Research | Horanyi A.,Hungarian Meteorological Service | And 2 more authors.
Studia Geophysica et Geodaetica | Year: 2010

Results obtained with two versions of the Limited Area Model (LAM) ALADIN over differently sized integration domains (large, intermediate and small) in the European area are presented in order to investigate both the general model performance and the influence of domain choice on the quality of obtained results. The aim is also to illustrate the issues related to the strategy of selection of the optimal integration domain. Each of these studies has been performed with two versions of the ALADIN model: the first one is ALADIN-CLIMATE developed at CNRM/Météo-France, the second one is ALADIN-CLIMATE/CZ prepared at the Czech Hydrometeorological Institute (CHMI). This leaves us with total of six experiments forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 reanalysis data. The west Balkan domain covering Bulgaria is used as an evaluation region for investigation of the temporal and spatial properties of simulated precipitation and temperature fields. This region has been selected for its challenging orography making the results obtained here a valuable source for studies leading to further developments in climate modeling. It was found that size of the domain strongly affects the quality of obtained results. We have found that the largest domain reproduces the spatial characteristics of climate (such as bias) very well, but its use results in a poor representation of temporal aspects, which are however captured very well in experiments over both smaller domains. Our findings suggest that there is no optimal choice of domain size, securing the best results for both spatial and temporal evaluation. Our study also proves that model ALADIN can be efficiently used for climate research purposes, which together with its modest computational demands should make it as an attractive modeling choice for the Central and Eastern European climate research community. © 2010 Institute of Geophysics of the ASCR, v.v.i.

Denkova D.,National Institute of Meteorology and Hydrology
International Multidisciplinary Scientific GeoConference Surveying Geology and Mining Ecology Management, SGEM | Year: 2014

Monitoring is fundamental for meteorology .The quality of the data and profitability depends on optimal design of monitoring network. There are different approaches and methods for optimizing the network. Geostatistical estimation variance reduction is one of the unbiased ways of optimizing a meteorological network with a desired accuracy. The existing meteorological network in Bulgaria is consisting of 126 climate monitoring stations. The air temperature data received from them were analyzed geostatistically to reason the spatial variability oh the parameter. The threshold on the maximum standard deviation of the estimation error was defined by comparing with the standard deviation of the data to maintain the order of magnitude. The calculation of the geostatistical estimation error is independent of the measured values so the variance of the estimation error of an uniform grid was calculated to be compared with the threshold. Using this procedure for a given network, is shown an way for its optimizing with a fixed accuracy in the form of maximum permitted variance of the estimation error. ©, 2014, SGEM2014.

Atanassov D.,National Institute of Meteorology and Hydrology | Galeriu D.,Horia Hulubei National Institute of Physics and Nuclear Engineering
Journal of Environmental Radioactivity | Year: 2011

The tradition in tritium washout modeling is to unite the washout model with a Gaussian plume model describing dispersion of tritium vapour in the atmosphere. In the present study, an alternative approach is proposed. A numerical Eulerian model that describes washout independently of dispersion is developed. The sensitivity analysis to model parameters has shown that the washout process is influenced most significantly by rainfall parameters and air temperature: different raindrop size distributions cause differences of up to about 70% in the washout outputs; a change of 15 °C in the air temperature causes an effect of about 50%.Results are presented showing calculated values of washout outputs (tritium concentration in rain, tritium downward flux, washout coefficient) for different tritium vapour profiles, rainfall rates and air temperatures. The general conclusion is that the washout process is too complex to be described comprehensively by the simple washout coefficient concept. We suggest the approach proposed here for directly calculating the tritium downward flux and concentration in the rainwater is preferable. © 2010 Elsevier Ltd.

Gryning S.-E.,Technical University of Denmark | Batchvarova E.,Technical University of Denmark | Batchvarova E.,National Institute of Meteorology and Hydrology | Floors R.,Technical University of Denmark | And 4 more authors.
Boundary-Layer Meteorology | Year: 2014

An investigation of the long-term variability of wind profiles for wind energy applications is presented. The observations consists of wind measurements obtained from a ground-based wind lidar at heights between 100 and 600 m, in combination with measurements from tall meteorological towers at a flat rural coastal site in western Denmark and at an inland suburban area near Hamburg in Germany. Simulations with the weather research and forecasting numerical model were carried out in both forecast and analysis configurations. The scatter between measured and modelled wind speeds expressed by the root-mean-square error was about 10 % lower for the analysis compared to the forecast simulations. At the rural coastal site, the observed mean wind speeds above 60 m were underestimated by both the analysis and forecast model runs. For the inland suburban area, the mean wind speed is overestimated by both types of the simulations below 500 m. When studying the wind-speed variability with the Weibull distribution, the shape parameter was always underestimated by the forecast compared to both analysis simulations and measurements. At the rural coastal site although the measured and modelled Weibull distributions are different their variances are nearly the same. It is suggested to use the shape parameter for climatological mesoscale model evaluation. Based on the new measurements, a parametrization of the shape parameter for practical applications is suggested. © 2013 Springer Science+Business Media Dordrecht.

Floors R.,Technical University of Denmark | Vincent C.L.,Technical University of Denmark | Gryning S.-,Technical University of Denmark | Pena A.,Technical University of Denmark | Batchvarova E.,National Institute of Meteorology and Hydrology
Boundary-Layer Meteorology | Year: 2013

Traditionally it has been difficult to verify mesoscale model wind predictions against observations in the planetary boundary layer (PBL). Here we used measurements from a wind lidar to study the PBL up to 800 m above the surface at a flat coastal site in Denmark during a one month period in autumn. We ran the Weather Research and Forecasting numerical model with two different roughness descriptions over land, two different synoptic forcings and two different PBL schemes at two vertical resolutions and evaluated the wind profile against observations from the wind lidar. The simulated wind profile did not have enough vertical shear in the lower part of the PBL and also had a negative bias higher up in the boundary layer. Near the surface the internal boundary layer and the surface roughness influenced the wind speed, while higher up it was only influenced by the choice of PBL scheme and the synoptic forcing. By replacing the roughness value for the land-use category in the model with a more representative mesoscale roughness, the observed bias in friction velocity was reduced. A higher-order PBL scheme simulated the wind profile from the west with a lower wind-speed bias at the top of the PBL. For easterly winds low-level jets contributed to a negative wind-speed bias around 300 m and were better simulated by the first-order scheme. In all simulations, the wind-profile shape, wind speed and turbulent fluxes were not improved when a higher vertical resolution or different synoptic forcing were used. © 2013 Springer Science+Business Media Dordrecht.

Bocheva L.,National Institute of Meteorology and Hydrology
International Multidisciplinary Scientific GeoConference Surveying Geology and Mining Ecology Management, SGEM | Year: 2015

The main objective of this study is variability and trends in extreme daily precipitation over non-mountainous regions of Bulgaria during the period 1961 – 2010. The regime of potential dangerous events (totals over 30 mm/24 h are considered as possibly risky for floods) and torrential precipitation (totals ≥60 mm/24 h) is summarized and compared for different regions for two periods: 1961 – 1990 (basic for climatological normals) and 1991 – 2010. Statistically significant increase (about 40%) of torrential 24-hours precipitation is revealed during the second period in central and east parts of the country, while in regions from West Bulgaria these dangerous events decreases with about 10%. The differences in the monthly distribution of extreme precipitation during the investigated periods are also presented. Statistically not significant decrease in the number of precipitation days has also been observed for most of the regions in Bulgaria while the number of extreme-precipitation days increases. Changes in daily precipitation totals exceeding various thresholds such as 5, 15, 30 and 60 mm/24 h have also been observed. The statistical significance of annual variations of extreme precipitation is calculated by the Mann-Kendall test (for the whole period 1961-2010) and ANOVA with Poisson distribution (for the comparison of the two periods 1961-1990 and 1991-2010). © SGEM2015.

Simeonov P.,National Institute of Meteorology and Hydrology | Bocheva L.,National Institute of Meteorology and Hydrology | Gospodinov I.,National Institute of Meteorology and Hydrology
Atmospheric Research | Year: 2013

The first part of this study deals with the available information (date, time, location) of 57 thunderstorms with tornado or with waterspout (9 of which have evolved near the Black Sea coast and 4 are formed over dams). These are about all cases that have been reported in Bulgaria between 1956 and 2010. About half of all tornadoes (49%) that occurred in land are formed over mountainous or hilly terrain and usually move over the near river valleys. Almost all tornado events occur during the warm half of the year with maximum in June, when about 25% of all tornado storms take place. There are only 4 cases from the cold half of the year - 1 in December, 1 in February, and 2 in March. The second part of the paper gives some basic analysis of two severe thunderstorms, connected with the development of tornado events. The typical synoptic situation associated with the formation of a tornado is a slow-moving cold atmospheric front with meridional stretch. It should have bigger than usual temperature gradient between the cold and the warm air masses. A less common synoptic scale structure that leads to the formation of tornado is an upper-air summer low centred over the Black Sea. The first analysed storm is a "winter" case. The tornado occurred in South-central Bulgaria on 24 March 2004. The other one is a typical "summer" case. It occurred in the north-west of the country on 2 June 2009. The data and information that are used in these analyses come from classic meteorological observations, radar images, records from post-event in-situ field investigation as well as various media sources. The characteristics of these tornadoes give us reason to classify them as moderate (class F2 according to the Fujita scale). © 2012 Elsevier B.V.

Gryning S.-E.,Technical University of Denmark | Batchvarova E.,Technical University of Denmark | Batchvarova E.,National Institute of Meteorology and Hydrology | Floors R.,Technical University of Denmark
Journal of Applied Meteorology and Climatology | Year: 2013

By use of 1 yr of measurements performed with a wind lidar up to 600-m height, in combination with a tall meteorological tower, the impact of nudging on the simulated wind profile at a flat coastal site (Høvsøre) in western Denmark using the Advanced Research version of the Weather Research and Forecasting model (WRF) is studied. It was found that the mean wind speed, the wind direction change with height, and the wind power density profiles are underestimated with the configuration ofWRFused and that the impact of nudging on the simulated mean values was minor. Nudging was found to reduce the scatter between the simulated and measured wind speeds, expressed by the root-mean-square error, by about 20% between altitudes of 100 and 500 m. The root-mean-square error was nearly constant with height for the nudged case (∼2.2 m s1) and slightly increased with height for the nonnudged one, reaching 2.8 m s1 at 300 and 500 m. In studying the long-term wind speed variability with the Weibull distribution, it was found that nudging had a minor effect on the scale parameter profile, which is closely connected to the mean wind speed. Improvement by nudging was seen on the profile of the shape parameter. Without nudging, the shape parameter was underestimated at all heights; with nudging, the agreement was good up to about 100 m and above that height the shape parameter was underestimated. © 2013 American Meteorological Society.

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