Sorin Cheval - Academia.edu (original) (raw)

Papers by Sorin Cheval

International Journal of Climatology, Feb 2010

This article contributes to better understanding of the precipitation data, analyzing several mea... more This article contributes to better understanding of the precipitation data, analyzing several measurement errors in Romania. Based on the influence of wind speed, solid precipitation rate, and wetting losses, we adjusted the monthly amounts registered at 159 weather stations through 1961-2006. The results emphasize distinct temporal and spatial distributions of the adjustment magnitude. In general, the correction factors increase with altitude and they have high values in the cold season, as they highly depend on wind speed and solid precipitation percentage. In Romania, bias corrections increase monthly precipitation by less than 10% from June to September, by 10-20% in the transition months, and by higher values during the winter.

Journal of Hydrology, 2010

Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to ana... more Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

Climate of The Past Discussions, 2011

The COST (European Cooperation in Science and Technology) Action ES0601: Advances in homogenizati... more The COST (European Cooperation in Science and Technology) Action ES0601: Advances in homogenization methods of climate series: an integrated approach (HOME) has executed a blind intercomparison and validation study for monthly homogenization algorithms. Time series of monthly temperature and precipitation were evaluated because of their importance for climate studies and because they represent two important types of statistics (additive and multiplicative). The algorithms were validated against a realistic benchmark dataset. The benchmark contains real inhomogeneous data as well as simulated data with inserted inhomogeneities. Random break-type inhomogeneities were added to the simulated datasets modeled as a Poisson process with normally distributed breakpoint sizes. To approximate real world conditions, breaks were introduced that occur simultaneously in multiple station series within a simulated network of station data. The simulated time series also contained outliers, missing data periods and local station trends. Further, a stochastic nonlinear global (network-wide) trend was added. Participants provided 25 separate homogenized contributions as part of the blind study as well as 22 additional solutions submitted after the details of the imposed inhomogeneities were revealed. These homogenized datasets were assessed by a number of performance metrics including (i) the centered root mean square error relative to the true homogeneous value at various averaging scales, (ii) the error in linear trend estimates and (iii) traditional contingency skill scores. The metrics were computed both using the individual station series as well as the network average regional series. The performance of the contributions depends significantly on the error metric considered. Contingency scores by themselves are not very informative. Although relative homogenization algorithms typically improve the homogeneity of temperature data, only the best ones improve precipitation data. Training was found to be very important. Moreover, state-of-the-art relative homogenization algorithms developed to work with an inhomogeneous reference are shown to perform best. The study showed that currently automatic algorithms can perform as well as manual ones.

Journal of Hydrology, 2010

Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to ana... more Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

Theoretical and Applied Climatology, 2003

This paper promotes a GIS-based method that allows the visualization, the identification and the ... more This paper promotes a GIS-based method that allows the visualization, the identification and the qualitative assessment of relationships existing among a limitless number of climatological variables. We use a variety of precipitation variables from fourteen Romanian weather stations to demonstrate the techniques and discuss its benefits and limitations.

Journal of Hydrology, 2010

SummaryThe aim of this paper is to analyse the differences in the long-term regimes of extreme pr... more SummaryThe aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

and sharing with colleagues.

Heavy rainfall events have produced significant damages and casualties in the Moldavian Plateau (... more Heavy rainfall events have produced significant damages and casualties in the Moldavian Plateau (Romania) in the last decades. Such phenomena are characterized by large spatial and temporal variations, and the forecast of their occurrence is thus very challenging. This study aims to compare the radar estimations and the rain gauge measurements, in order to improve the quantitative precipitation estimation (QPE) in the area of interest. The research uses data from the WSR-98D S-band Doppler radar located in Bârnova, and from rain gauges within weather stations run by Meteo Romania (Romanian National Meteorological Administration). We have focused on daily (24 h) accumulations registered at weather stations, and the output sustains the radar calibration, fostering the hydrological modeling, including flash flood forecast. The differences between R and G were investigated based on two objectives functions -the ratio R/G (BIAS) and the Root Mean Square Factor (RMSf)- while the correlations used the Pearson scores. Considerable spatial distinctions between areas with good radar accuracy for QPE and perimeters where radar is not capable to provide robust information have been emphasized during the investigations. The validation aimed to predict the rain gauge amounts in certain spots by using the radar information and resulted adjustment parameters. It has been demonstrated that the Bârnova radar data are reliable within approx. 150 km radius, and the comparison with rain gauge measurements can foster consistently the QPE accuracy in the area. This research was completed in the framework of the EU FP6 Project HYDRATE (Hydrometeorological data resources and technologies for effective flash flood forecasting), Contract no: 037024, 2006-2009.

The scientific purpose of this study is to investigate the spatial extension and the magnitude of... more The scientific purpose of this study is to investigate the spatial extension and the magnitude of the heat islands of two significant urban agglomeration, the cities of Bucharest (Romania) and Prague (Czech Republic). In order to emphasize the urban heat island (UHI) of the two cities, rectangular areas of about 100 x 100 km were delimited around them, with the cities in the centre of the squares. These periurban areas include towns and villages, significant woodlands and aquatic surfaces, as well as extended agricultural land. The surface temperatures have been derived from Moderate Resolution Imaging Spectroradiometer (MODIS) images, MOD11_L2 product, both from Terra and Aqua satellites, ranging from 2000 to 2006. The satellite-derived temperatures have been compared with the ground-based temperatures in order to identify possible errors. We have calculated the magnitude of the UHIs of Bucharest and Prague as the difference between the highest temperature inside each city and the lowest temperature in the periurban areas surrounding them. Cross-profiles through the cities have documented the assessment of the spatial extension of the UHIs. We have approached two temporal patterns, striving to evaluate the seasonal and the diurnal variability of the UHIs in the two cities. The study retrieves that the local topography plays an important role in the characteristics of the urban heat island (i.e. Bucharest is situated in a much more flat area than Prague, with no influent watercourse passing through and therefore the UHI of Bucharest is more extended and more intense than the one of Prague). In order to achieve a consistent appearance of papers on the CD Rom, all papers should comply with these guidelines.

Extreme rainfall events triggering flash floods occur quite often over the territory of Romania, ... more Extreme rainfall events triggering flash floods occur quite often over the territory of Romania, leaving behind significant damages and casualties. This research is a contribution to the FP6 Project HYDRATE (Hydrometeorological data resources and technologies for effective flash flood forecasting). It aims at investigating the spatial patterns of the extreme rainfall events in Romania, based on the characteristics of their intensity-duration-frequency (IDF). The study uses the peak-over-threshold concept, which basically consists of analyzing all precipitation amounts above certain thresholds selected for different durations. The data come from 60 weather stations. They cover the warm interval (generally, April-October, but less extended for mountain stations), and at least 30 years-datasets have been used. The regional differences were retrieved from the IDF curves and they were also approached by GIS-based mapping the intensities corresponding to sub-daily durations (5 - 180 min.) and to different return periods (10,50, 100 years). The results highlight significant regional variations, that improve the understanding of the impact of the extreme rainfall events and the consequent flash floods on the natural and social environment. At the same time, overlapping the extreme rainfall data and land cover information, we have empahsized the hazard potential of the precipitation events.

Within the framework of the European project HYDRATE, this contribution is devoted to the underst... more Within the framework of the European project HYDRATE, this contribution is devoted to the understanding of the main synoptic ingredients that lead to flash flood producing storms in three hydrometeorology observatories representative of three different climatic European regions. In Romania, flash floods occur in any season, with highest frequency in summer and early autumn. The diversity of physical geography of Romania induces pronounced regional patterns to flash floods, raising many scientific and practical challenges. In Italy, the region is representative of the north-eastern side of the Alps. Here the flash floods occur most likely during the summer and the fall season. The latter region, in the South-East part of France, experiences flash flood mainly in autumn.. Several studies, mainly based on the analysis of simulated fields of few flash-flood events, highlight specific synoptic environments. The objective of this paper is to generalize these event-based analysis using weather-typing approaches. The Grosswetterlagen (GWL) classification is used as well as a weather classification based on the k-means clustering algorithm (Michelangeli et al., 1995). A short comparison is proposed in order to highlight the strength and the weakness of each classification. The results show that the GWL classification properly explains inter annual variability of mean annual rainfall; nevertheless the intra class variability is much lower. Then, for each weather type and based on the ERA40 fields, composite mean fields are plotted and analyzed in order to highlight the main synoptic factors. The circulation is plotted at Z-700hPa, and the vertically integrated moisture flux, wind, and potential vorticity fields are analyzed. The analysis shows that the precipitation patterns are strongly correlated to the intensity and the direction of the wind. The location of the trough and its intensity over the Great Britain are also main synoptic features. For each rainy class, composite maps of anomalies in moisture flux, wind and geopotential fields are proposed to point out the specific meteorological characteristics associated to : i) the flash-flood event; ii) the days before the events. It is found that each class has is specific synoptic environment, the differences between each class are mainly explained by the moisture flux intensity and orientation. These differences lead to different precipitation patterns as highlighted by composite maps of precipitation obtained for the corresponding precipitation events. The findings are in good agreement with the literature. It therefore improves our understanding of flash flood triggering environment highlighting the differences between different rainy weather regimes. It especially allows the analysis of the identification of critical meteorological environments for the previous days before the flash-flood event.

International Journal of Climatology, 2010

This article contributes to better understanding of the precipitation data, analyzing several mea... more This article contributes to better understanding of the precipitation data, analyzing several measurement errors in Romania. Based on the influence of wind speed, solid precipitation rate, and wetting losses, we adjusted the monthly amounts registered at 159 weather stations through 1961–2006. The results emphasize distinct temporal and spatial distributions of the adjustment magnitude. In general, the correction factors increase with altitude and they have high values in the cold season, as they highly depend on wind speed and solid precipitation percentage. In Romania, bias corrections increase monthly precipitation by less than 10% from June to September, by 10–20% in the transition months, and by higher values during the winter. Copyright © 2010 Royal Meteorological Society

Theoretical and Applied Climatology, 2009

The urban heat island (UHI) of the city of Bucharest (Romania) is analyzed in terms of its extens... more The urban heat island (UHI) of the city of Bucharest (Romania) is analyzed in terms of its extension, geometry, and magnitude using the surface thermal data provided by the moderate resolution imaging spectroradiometer (MODIS) sensors. An objective method is developed that allows to delineate the UHI. The study focuses on the months of July from the 2000–2006 time interval. The average surface temperatures obtained for each pixel (1 km resolution) were analyzed on cross-profiles that helped us to determine the outline of the UHI. The shifting points identified by the Rodionov test in the temperature series of each profile were considered as possible limits of the UHI. Seemingly, the land cover has a major influence on the extension and the geometry of the Bucharest UHI in July. The magnitude of the heat island was calculated by comparing the average temperature inside its limits and the average temperature of the 5 km (a) and of the 10 km (b) buffers around it. The thermal difference between the UHI and the surrounding area of Bucharest is higher and more variable during the daytime, and is noticeably related to the land cover.

Theoretical and Applied Climatology, 2009

This paper explores the characteristics of the air (Tair) and land surface temperature (LST) from... more This paper explores the characteristics of the air (Tair) and land surface temperature (LST) from the city of Bucharest (Romania) during the extreme high temperatures that affected the region in July 2007. The behavior of Bucharest’s Urban Heat Island (UHI) is quantitatively described following similar methodological approaches to previous studies. The analysis integrates thermal data supplied by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard the NASA satellites and meteorological data provided by the ground-based weather stations. Based on the Tair, one may claim that during extreme high summer temperatures, the UHI preserves its spatial and temporal pattern regarding the differences between the central urban perimeter and the suburban area. The investigation of the LST from July 2007 reveals that the nocturnal changes refer mainly to the magnitude and the limits of the UHI, while the shape is not changed. However, the extreme temperatures induce significant modifications of the features of the diurnal UHI, obliterating and dissipating it in certain spots, enlarging it and creating shifted heat islands in some other spots. The main explanation is the outstanding duration and intensity of the hot mass of air impacting the area. The correlations between the Tair measured at the weather stations in Bucharest and the corresponding LST retrieved significant values both under “normal” conditions and extreme temperature persistence, and offer good premises for robust validation studies. The MODIS products performed like an extremely useful instrument for analyzing the UHI.

International Journal of Climatology, Feb 2010

This article contributes to better understanding of the precipitation data, analyzing several mea... more This article contributes to better understanding of the precipitation data, analyzing several measurement errors in Romania. Based on the influence of wind speed, solid precipitation rate, and wetting losses, we adjusted the monthly amounts registered at 159 weather stations through 1961-2006. The results emphasize distinct temporal and spatial distributions of the adjustment magnitude. In general, the correction factors increase with altitude and they have high values in the cold season, as they highly depend on wind speed and solid precipitation percentage. In Romania, bias corrections increase monthly precipitation by less than 10% from June to September, by 10-20% in the transition months, and by higher values during the winter.

Journal of Hydrology, 2010

Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to ana... more Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

Climate of The Past Discussions, 2011

The COST (European Cooperation in Science and Technology) Action ES0601: Advances in homogenizati... more The COST (European Cooperation in Science and Technology) Action ES0601: Advances in homogenization methods of climate series: an integrated approach (HOME) has executed a blind intercomparison and validation study for monthly homogenization algorithms. Time series of monthly temperature and precipitation were evaluated because of their importance for climate studies and because they represent two important types of statistics (additive and multiplicative). The algorithms were validated against a realistic benchmark dataset. The benchmark contains real inhomogeneous data as well as simulated data with inserted inhomogeneities. Random break-type inhomogeneities were added to the simulated datasets modeled as a Poisson process with normally distributed breakpoint sizes. To approximate real world conditions, breaks were introduced that occur simultaneously in multiple station series within a simulated network of station data. The simulated time series also contained outliers, missing data periods and local station trends. Further, a stochastic nonlinear global (network-wide) trend was added. Participants provided 25 separate homogenized contributions as part of the blind study as well as 22 additional solutions submitted after the details of the imposed inhomogeneities were revealed. These homogenized datasets were assessed by a number of performance metrics including (i) the centered root mean square error relative to the true homogeneous value at various averaging scales, (ii) the error in linear trend estimates and (iii) traditional contingency skill scores. The metrics were computed both using the individual station series as well as the network average regional series. The performance of the contributions depends significantly on the error metric considered. Contingency scores by themselves are not very informative. Although relative homogenization algorithms typically improve the homogeneity of temperature data, only the best ones improve precipitation data. Training was found to be very important. Moreover, state-of-the-art relative homogenization algorithms developed to work with an inhomogeneous reference are shown to perform best. The study showed that currently automatic algorithms can perform as well as manual ones.

Journal of Hydrology, 2010

Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to ana... more Keywords: Seasonality Flood Alpine-Carpathian range s u m m a r y The aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

Theoretical and Applied Climatology, 2003

This paper promotes a GIS-based method that allows the visualization, the identification and the ... more This paper promotes a GIS-based method that allows the visualization, the identification and the qualitative assessment of relationships existing among a limitless number of climatological variables. We use a variety of precipitation variables from fourteen Romanian weather stations to demonstrate the techniques and discuss its benefits and limitations.

Journal of Hydrology, 2010

SummaryThe aim of this paper is to analyse the differences in the long-term regimes of extreme pr... more SummaryThe aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine-Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

and sharing with colleagues.

Heavy rainfall events have produced significant damages and casualties in the Moldavian Plateau (... more Heavy rainfall events have produced significant damages and casualties in the Moldavian Plateau (Romania) in the last decades. Such phenomena are characterized by large spatial and temporal variations, and the forecast of their occurrence is thus very challenging. This study aims to compare the radar estimations and the rain gauge measurements, in order to improve the quantitative precipitation estimation (QPE) in the area of interest. The research uses data from the WSR-98D S-band Doppler radar located in Bârnova, and from rain gauges within weather stations run by Meteo Romania (Romanian National Meteorological Administration). We have focused on daily (24 h) accumulations registered at weather stations, and the output sustains the radar calibration, fostering the hydrological modeling, including flash flood forecast. The differences between R and G were investigated based on two objectives functions -the ratio R/G (BIAS) and the Root Mean Square Factor (RMSf)- while the correlations used the Pearson scores. Considerable spatial distinctions between areas with good radar accuracy for QPE and perimeters where radar is not capable to provide robust information have been emphasized during the investigations. The validation aimed to predict the rain gauge amounts in certain spots by using the radar information and resulted adjustment parameters. It has been demonstrated that the Bârnova radar data are reliable within approx. 150 km radius, and the comparison with rain gauge measurements can foster consistently the QPE accuracy in the area. This research was completed in the framework of the EU FP6 Project HYDRATE (Hydrometeorological data resources and technologies for effective flash flood forecasting), Contract no: 037024, 2006-2009.

The scientific purpose of this study is to investigate the spatial extension and the magnitude of... more The scientific purpose of this study is to investigate the spatial extension and the magnitude of the heat islands of two significant urban agglomeration, the cities of Bucharest (Romania) and Prague (Czech Republic). In order to emphasize the urban heat island (UHI) of the two cities, rectangular areas of about 100 x 100 km were delimited around them, with the cities in the centre of the squares. These periurban areas include towns and villages, significant woodlands and aquatic surfaces, as well as extended agricultural land. The surface temperatures have been derived from Moderate Resolution Imaging Spectroradiometer (MODIS) images, MOD11_L2 product, both from Terra and Aqua satellites, ranging from 2000 to 2006. The satellite-derived temperatures have been compared with the ground-based temperatures in order to identify possible errors. We have calculated the magnitude of the UHIs of Bucharest and Prague as the difference between the highest temperature inside each city and the lowest temperature in the periurban areas surrounding them. Cross-profiles through the cities have documented the assessment of the spatial extension of the UHIs. We have approached two temporal patterns, striving to evaluate the seasonal and the diurnal variability of the UHIs in the two cities. The study retrieves that the local topography plays an important role in the characteristics of the urban heat island (i.e. Bucharest is situated in a much more flat area than Prague, with no influent watercourse passing through and therefore the UHI of Bucharest is more extended and more intense than the one of Prague). In order to achieve a consistent appearance of papers on the CD Rom, all papers should comply with these guidelines.

Extreme rainfall events triggering flash floods occur quite often over the territory of Romania, ... more Extreme rainfall events triggering flash floods occur quite often over the territory of Romania, leaving behind significant damages and casualties. This research is a contribution to the FP6 Project HYDRATE (Hydrometeorological data resources and technologies for effective flash flood forecasting). It aims at investigating the spatial patterns of the extreme rainfall events in Romania, based on the characteristics of their intensity-duration-frequency (IDF). The study uses the peak-over-threshold concept, which basically consists of analyzing all precipitation amounts above certain thresholds selected for different durations. The data come from 60 weather stations. They cover the warm interval (generally, April-October, but less extended for mountain stations), and at least 30 years-datasets have been used. The regional differences were retrieved from the IDF curves and they were also approached by GIS-based mapping the intensities corresponding to sub-daily durations (5 - 180 min.) and to different return periods (10,50, 100 years). The results highlight significant regional variations, that improve the understanding of the impact of the extreme rainfall events and the consequent flash floods on the natural and social environment. At the same time, overlapping the extreme rainfall data and land cover information, we have empahsized the hazard potential of the precipitation events.

Within the framework of the European project HYDRATE, this contribution is devoted to the underst... more Within the framework of the European project HYDRATE, this contribution is devoted to the understanding of the main synoptic ingredients that lead to flash flood producing storms in three hydrometeorology observatories representative of three different climatic European regions. In Romania, flash floods occur in any season, with highest frequency in summer and early autumn. The diversity of physical geography of Romania induces pronounced regional patterns to flash floods, raising many scientific and practical challenges. In Italy, the region is representative of the north-eastern side of the Alps. Here the flash floods occur most likely during the summer and the fall season. The latter region, in the South-East part of France, experiences flash flood mainly in autumn.. Several studies, mainly based on the analysis of simulated fields of few flash-flood events, highlight specific synoptic environments. The objective of this paper is to generalize these event-based analysis using weather-typing approaches. The Grosswetterlagen (GWL) classification is used as well as a weather classification based on the k-means clustering algorithm (Michelangeli et al., 1995). A short comparison is proposed in order to highlight the strength and the weakness of each classification. The results show that the GWL classification properly explains inter annual variability of mean annual rainfall; nevertheless the intra class variability is much lower. Then, for each weather type and based on the ERA40 fields, composite mean fields are plotted and analyzed in order to highlight the main synoptic factors. The circulation is plotted at Z-700hPa, and the vertically integrated moisture flux, wind, and potential vorticity fields are analyzed. The analysis shows that the precipitation patterns are strongly correlated to the intensity and the direction of the wind. The location of the trough and its intensity over the Great Britain are also main synoptic features. For each rainy class, composite maps of anomalies in moisture flux, wind and geopotential fields are proposed to point out the specific meteorological characteristics associated to : i) the flash-flood event; ii) the days before the events. It is found that each class has is specific synoptic environment, the differences between each class are mainly explained by the moisture flux intensity and orientation. These differences lead to different precipitation patterns as highlighted by composite maps of precipitation obtained for the corresponding precipitation events. The findings are in good agreement with the literature. It therefore improves our understanding of flash flood triggering environment highlighting the differences between different rainy weather regimes. It especially allows the analysis of the identification of critical meteorological environments for the previous days before the flash-flood event.

International Journal of Climatology, 2010

This article contributes to better understanding of the precipitation data, analyzing several mea... more This article contributes to better understanding of the precipitation data, analyzing several measurement errors in Romania. Based on the influence of wind speed, solid precipitation rate, and wetting losses, we adjusted the monthly amounts registered at 159 weather stations through 1961–2006. The results emphasize distinct temporal and spatial distributions of the adjustment magnitude. In general, the correction factors increase with altitude and they have high values in the cold season, as they highly depend on wind speed and solid precipitation percentage. In Romania, bias corrections increase monthly precipitation by less than 10% from June to September, by 10–20% in the transition months, and by higher values during the winter. Copyright © 2010 Royal Meteorological Society

Theoretical and Applied Climatology, 2009

The urban heat island (UHI) of the city of Bucharest (Romania) is analyzed in terms of its extens... more The urban heat island (UHI) of the city of Bucharest (Romania) is analyzed in terms of its extension, geometry, and magnitude using the surface thermal data provided by the moderate resolution imaging spectroradiometer (MODIS) sensors. An objective method is developed that allows to delineate the UHI. The study focuses on the months of July from the 2000–2006 time interval. The average surface temperatures obtained for each pixel (1 km resolution) were analyzed on cross-profiles that helped us to determine the outline of the UHI. The shifting points identified by the Rodionov test in the temperature series of each profile were considered as possible limits of the UHI. Seemingly, the land cover has a major influence on the extension and the geometry of the Bucharest UHI in July. The magnitude of the heat island was calculated by comparing the average temperature inside its limits and the average temperature of the 5 km (a) and of the 10 km (b) buffers around it. The thermal difference between the UHI and the surrounding area of Bucharest is higher and more variable during the daytime, and is noticeably related to the land cover.

Theoretical and Applied Climatology, 2009

This paper explores the characteristics of the air (Tair) and land surface temperature (LST) from... more This paper explores the characteristics of the air (Tair) and land surface temperature (LST) from the city of Bucharest (Romania) during the extreme high temperatures that affected the region in July 2007. The behavior of Bucharest’s Urban Heat Island (UHI) is quantitatively described following similar methodological approaches to previous studies. The analysis integrates thermal data supplied by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard the NASA satellites and meteorological data provided by the ground-based weather stations. Based on the Tair, one may claim that during extreme high summer temperatures, the UHI preserves its spatial and temporal pattern regarding the differences between the central urban perimeter and the suburban area. The investigation of the LST from July 2007 reveals that the nocturnal changes refer mainly to the magnitude and the limits of the UHI, while the shape is not changed. However, the extreme temperatures induce significant modifications of the features of the diurnal UHI, obliterating and dissipating it in certain spots, enlarging it and creating shifted heat islands in some other spots. The main explanation is the outstanding duration and intensity of the hot mass of air impacting the area. The correlations between the Tair measured at the weather stations in Bucharest and the corresponding LST retrieved significant values both under “normal” conditions and extreme temperature persistence, and offer good premises for robust validation studies. The MODIS products performed like an extremely useful instrument for analyzing the UHI.