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Papers by Tsvetelina Dimitrova

<p>Severe weather events, such as intense precipitation, hail and thunderstorms, ar... more <p>Severe weather events, such as intense precipitation, hail and thunderstorms, are common summer phenomena in Bulgaria and are associated with large economic losses. An active hail suppression is taking place in North West and South Central Bulgaria over the dense agricultural regions from May to September. A joint venture between Sofia University "St. Kliment Ohridski'' and the Bulgarian Hail Suppression Agency contributed to the project “Balkan-Mediterranean real-time severe weather service” (BeRTISS, 2017-2020). As part of the BeRTISS project, a pilot operational service was established by exploiting GNSS tropospheric products in support of safety, quality of life and environmental protection in the region. To facilitate the service, in 2018 a GNSS network with 12 reference stations was installed and since February 2020, the Sofia University GNSS Analysis Center provides operational near-real time products. First results of combining GNSS derived Integrated Water Vapour (IWV) and Instability Indices (InI) are reported for Sofia plain for the period May-September (2010 - 2015). Based on statistical regression analysis, classification functions are obtained that contribute to the thunderstorm forecasting skill. The majority of the classification functions combining IWV and InI are reported with the best performance in May, followed by June and September. In this work, IWV and InI classification functions are presented for South Central Bulgaria. The first results for the period May-September 2020-2021 indicate the probability of detection 0.71 for IWV, 0.85 for InI, and 0.89 for IWV and InI combined. While false alarms decreased from 0.42 for IWV to 0.39 for InI and 0.28 for IWV and InI combined. These results will be implemented in the GNSS Strom Demonstrator web portal.  </p>

Advances in Space Research

Remote Sensing

Global Navigation Satellite System (GNSS) is an established atmospheric monitoring technique deli... more Global Navigation Satellite System (GNSS) is an established atmospheric monitoring technique delivering water vapour data in near-real time with a latency of 90 min for operational Numerical Weather Prediction in Europe within the GNSS water vapour service (E-GVAP). The advancement of GNSS processing made the quality of real-time GNSS tropospheric products comparable to near-real-time solutions. In addition, they can be provided with a temporal resolution of 5 min and latency of 10 min, suitable for severe weather nowcasting. This paper exploits the added value of sub-hourly real-time GNSS tropospheric products for the nowcasting of convective storms in Bulgaria. A convective Storm Demonstrator (Storm Demo) is build using real-time GNSS tropospheric products and Instability Indices to derive site-specific threshold values in support of public weather and hail suppression services. The Storm Demo targets the development of service featuring GNSS products for two regions with hail sup...

lightning characteristics and severe events such as tornado and large hail. The limited number of... more lightning characteristics and severe events such as tornado and large hail. The limited number of studies concerning lightning activity of thunderstorms developed over Bulgaria showed that there is a significant difference between lightning characteristics in thunderstorms, producing hail, heavy and weak rain.

This study aims to describe the main objectives and activities of the research project BeRTISS (B... more This study aims to describe the main objectives and activities of the research project BeRTISS (Balkan-Mediterranean Real Time Severe weather Service) funded by the European Territorial Cooperation Programme “Interreg V-B Balkan-Mediterranean 2014-2020". BeRTISS targets to establish the first transnational operational service for monitoring severe weather events in the Balkan‐Mediterranean area by exploiting Global Navigation Satellite Systems (GNSS) tropospheric products. GNSS signals transmitted from satellites to the ground reference stations are delayed by ionosphere and water vapor in troposphere. Ionospheric propagation delay can be easily removed, while tropospheric delay needs to be calculated using surface pressure and temperature variables. By knowing the tropospheric delay the Precipitable Water Vapour (PWV) which is the most abundant greenhouse gas is easily assessed. GNSS derived PWV has been proved to be a valuable data source for Numerical Weather Prediction (NWP...

The present study is directed to test whether data over Bulgaria (land) and Black Sea (sea) confi... more The present study is directed to test whether data over Bulgaria (land) and Black Sea (sea) confirms the established in other geographical regions difference in thermodynamic characteristics over land and sea, which may explain the land-ocean contrast in lightning activity. The territory of investigation includes maritime area (western part of the Black Sea – up to 36°E) and continental area (Bulgaria). The necessary temperature and humidity profiles are taken from GFS proximity sounding for the center of each grid box where at least one flash is recorded. Several surface and at higher levels atmospheric data, convective available potential energy (CAPE) and normalize CAPE (NCAPE) (calculated for different layers) are considered. The output of 1-D cloud model with parameterized microphysics is also used. The results indicate that the shapes of CAPE over land and sea are almost similar up to –40C; only CAPE (–40C to EL) is indistinguishably more ―elegant‖ over the sea in comparison...

Remote Sensing, 2019

Bulgaria is a country with a high frequency of hail and thunderstorms from May to September. For ... more Bulgaria is a country with a high frequency of hail and thunderstorms from May to September. For the May–September 2010–2015 period, statistical regression analysis was applied to identify predictors/classification functions that contribute skills to thunderstorm forecasting in the Sofia plain. The functions are based on (1) instability indices computed from radiosonde data from Sofia station F1, and (2) combination of instability indices and Integrated Water Vapor (IWV), derived from the Global Navigation Satellite System (GNSS) station Sofia-Plana, F2. Analysis of the probability of detection and the false alarm ratio scores showed the superiority of the F2 classification function, with the best performance in May, followed by June and September. F1 and F2 scores were computed for independent data samples in the period 2017–2018 and confirmed the findings for the 2010–2015 period. Analysis of IWV and lightning flash rates for a multicell and supercell thunderstorm in June and July...

Atmosphere, 2019

A long-lived supercell developed in Northwest Bulgaria on 15 May 2018 and inflicted widespread da... more A long-lived supercell developed in Northwest Bulgaria on 15 May 2018 and inflicted widespread damage along its track. The first part of this article presents a detailed overview of the observed storm evolution. Doppler radar observations reveal that the storm acquired typical supercellular signatures and maintained reflectivity values in excess of 63 dBZ for more than 4 h. The thunderstorm was also analyzed through lightning observations that highlighted important characteristics of the overall supercell dynamics. In its second part, the study investigates the predictability of the severe weather outbreak. In the medium forecast ranges, the global European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble indicated the presence of favorable conditions for the development of deep moist convection 4 days prior to the event. A set of three convection-allowing ensemble simulations also demonstrated that the practical predictability of the supercell was approximately 12 h, whic...

Időjárás, 2018

⎯ Bulgaria is situated in a geographical area with high frequency and intensity of thunderstorms ... more ⎯ Bulgaria is situated in a geographical area with high frequency and intensity of thunderstorms and hail events. Like in many other countries, an upward trend of weatherinduced damage has been observed during the last 15 years in Bulgaria. Much of it was caused by extreme weather events such as torrential precipitation associated with severe thunderstorms or/and wind storms. The series of flash floods and extreme hail storms, which hit Bulgaria in the warm half of 2014, were in line with that trend. On July 8, 2014, three supercell storms developed over West Bulgaria and heavily impacted urban areas in the afternoon. The extremely strong hail storms over Sofia and Montana were accompanied by strong wind gusts (about 85 km h-1) and torrential rain. The giant hail stones in Sofia had diameter of up to 10 cm and irregular shape. The severe hail and rain, and very strong wind caused substantial damage to infrastructure, buildings, vehicles, and agriculture. More than 40 people were injured by hail stones or collaterally by broken windows. One man was killed by a fallen tree. In Sofia alone, the reported damage was worth more than 100 million euro according to data from insurance companies. The paper presents the analysis of the evolution of the convective clouds based on radar data. The Doppler radar data revealed the existence of a mesocyclone, mesoanticyclone, microburst, and three-body scatter signature. The cloud top reached up to 17 km, and the maximum radar reflectivity factor was 69-71 dBZ. Analysis of the available radiosounding data, simulation with a numerical model, and the synoptic patterns are also presented.

Atmospheric Research, 2015

Atmospheric Research, 2009

IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium

The present article describes the principal objectives, the methodology and the main activities o... more The present article describes the principal objectives, the methodology and the main activities of the research project BeRTISS (Balkan-Mediterranean Real Time Severe weather Service) which is funded by the European Territorial Cooperation Programme "Interreg V-B Balkan-Mediterranean 2014-2020". Electron density of the ionosphere and water vapor in the troposphere affect the propagation speed of the GNSS signals transmitted from satellites to the ground reference stations, causing signal propagation delay. Ionospheric delay can be easily removed whereas tropospheric delay, which comprises one of the most significant error sources in satellite signal transmission, should be calculated using surface pressure and temperature variables. Tropospheric delay allows the assessment of Precipitable Water Vapour (PWV). For the first time, BeRTISS real-time service will offer perpetual information for forecasting of PWV over Greece, Bulgaria and Cyprus to the interested public by employing the GNSS derived tropospheric products.

<p>Severe weather events, such as intense precipitation, hail and thunderstorms, ar... more <p>Severe weather events, such as intense precipitation, hail and thunderstorms, are common summer phenomena in Bulgaria and are associated with large economic losses. An active hail suppression is taking place in North West and South Central Bulgaria over the dense agricultural regions from May to September. A joint venture between Sofia University "St. Kliment Ohridski'' and the Bulgarian Hail Suppression Agency contributed to the project “Balkan-Mediterranean real-time severe weather service” (BeRTISS, 2017-2020). As part of the BeRTISS project, a pilot operational service was established by exploiting GNSS tropospheric products in support of safety, quality of life and environmental protection in the region. To facilitate the service, in 2018 a GNSS network with 12 reference stations was installed and since February 2020, the Sofia University GNSS Analysis Center provides operational near-real time products. First results of combining GNSS derived Integrated Water Vapour (IWV) and Instability Indices (InI) are reported for Sofia plain for the period May-September (2010 - 2015). Based on statistical regression analysis, classification functions are obtained that contribute to the thunderstorm forecasting skill. The majority of the classification functions combining IWV and InI are reported with the best performance in May, followed by June and September. In this work, IWV and InI classification functions are presented for South Central Bulgaria. The first results for the period May-September 2020-2021 indicate the probability of detection 0.71 for IWV, 0.85 for InI, and 0.89 for IWV and InI combined. While false alarms decreased from 0.42 for IWV to 0.39 for InI and 0.28 for IWV and InI combined. These results will be implemented in the GNSS Strom Demonstrator web portal.  </p>

Advances in Space Research

Remote Sensing

Global Navigation Satellite System (GNSS) is an established atmospheric monitoring technique deli... more Global Navigation Satellite System (GNSS) is an established atmospheric monitoring technique delivering water vapour data in near-real time with a latency of 90 min for operational Numerical Weather Prediction in Europe within the GNSS water vapour service (E-GVAP). The advancement of GNSS processing made the quality of real-time GNSS tropospheric products comparable to near-real-time solutions. In addition, they can be provided with a temporal resolution of 5 min and latency of 10 min, suitable for severe weather nowcasting. This paper exploits the added value of sub-hourly real-time GNSS tropospheric products for the nowcasting of convective storms in Bulgaria. A convective Storm Demonstrator (Storm Demo) is build using real-time GNSS tropospheric products and Instability Indices to derive site-specific threshold values in support of public weather and hail suppression services. The Storm Demo targets the development of service featuring GNSS products for two regions with hail sup...

lightning characteristics and severe events such as tornado and large hail. The limited number of... more lightning characteristics and severe events such as tornado and large hail. The limited number of studies concerning lightning activity of thunderstorms developed over Bulgaria showed that there is a significant difference between lightning characteristics in thunderstorms, producing hail, heavy and weak rain.

This study aims to describe the main objectives and activities of the research project BeRTISS (B... more This study aims to describe the main objectives and activities of the research project BeRTISS (Balkan-Mediterranean Real Time Severe weather Service) funded by the European Territorial Cooperation Programme “Interreg V-B Balkan-Mediterranean 2014-2020". BeRTISS targets to establish the first transnational operational service for monitoring severe weather events in the Balkan‐Mediterranean area by exploiting Global Navigation Satellite Systems (GNSS) tropospheric products. GNSS signals transmitted from satellites to the ground reference stations are delayed by ionosphere and water vapor in troposphere. Ionospheric propagation delay can be easily removed, while tropospheric delay needs to be calculated using surface pressure and temperature variables. By knowing the tropospheric delay the Precipitable Water Vapour (PWV) which is the most abundant greenhouse gas is easily assessed. GNSS derived PWV has been proved to be a valuable data source for Numerical Weather Prediction (NWP...

The present study is directed to test whether data over Bulgaria (land) and Black Sea (sea) confi... more The present study is directed to test whether data over Bulgaria (land) and Black Sea (sea) confirms the established in other geographical regions difference in thermodynamic characteristics over land and sea, which may explain the land-ocean contrast in lightning activity. The territory of investigation includes maritime area (western part of the Black Sea – up to 36°E) and continental area (Bulgaria). The necessary temperature and humidity profiles are taken from GFS proximity sounding for the center of each grid box where at least one flash is recorded. Several surface and at higher levels atmospheric data, convective available potential energy (CAPE) and normalize CAPE (NCAPE) (calculated for different layers) are considered. The output of 1-D cloud model with parameterized microphysics is also used. The results indicate that the shapes of CAPE over land and sea are almost similar up to –40C; only CAPE (–40C to EL) is indistinguishably more ―elegant‖ over the sea in comparison...

Remote Sensing, 2019

Bulgaria is a country with a high frequency of hail and thunderstorms from May to September. For ... more Bulgaria is a country with a high frequency of hail and thunderstorms from May to September. For the May–September 2010–2015 period, statistical regression analysis was applied to identify predictors/classification functions that contribute skills to thunderstorm forecasting in the Sofia plain. The functions are based on (1) instability indices computed from radiosonde data from Sofia station F1, and (2) combination of instability indices and Integrated Water Vapor (IWV), derived from the Global Navigation Satellite System (GNSS) station Sofia-Plana, F2. Analysis of the probability of detection and the false alarm ratio scores showed the superiority of the F2 classification function, with the best performance in May, followed by June and September. F1 and F2 scores were computed for independent data samples in the period 2017–2018 and confirmed the findings for the 2010–2015 period. Analysis of IWV and lightning flash rates for a multicell and supercell thunderstorm in June and July...

Atmosphere, 2019

A long-lived supercell developed in Northwest Bulgaria on 15 May 2018 and inflicted widespread da... more A long-lived supercell developed in Northwest Bulgaria on 15 May 2018 and inflicted widespread damage along its track. The first part of this article presents a detailed overview of the observed storm evolution. Doppler radar observations reveal that the storm acquired typical supercellular signatures and maintained reflectivity values in excess of 63 dBZ for more than 4 h. The thunderstorm was also analyzed through lightning observations that highlighted important characteristics of the overall supercell dynamics. In its second part, the study investigates the predictability of the severe weather outbreak. In the medium forecast ranges, the global European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble indicated the presence of favorable conditions for the development of deep moist convection 4 days prior to the event. A set of three convection-allowing ensemble simulations also demonstrated that the practical predictability of the supercell was approximately 12 h, whic...

Időjárás, 2018

⎯ Bulgaria is situated in a geographical area with high frequency and intensity of thunderstorms ... more ⎯ Bulgaria is situated in a geographical area with high frequency and intensity of thunderstorms and hail events. Like in many other countries, an upward trend of weatherinduced damage has been observed during the last 15 years in Bulgaria. Much of it was caused by extreme weather events such as torrential precipitation associated with severe thunderstorms or/and wind storms. The series of flash floods and extreme hail storms, which hit Bulgaria in the warm half of 2014, were in line with that trend. On July 8, 2014, three supercell storms developed over West Bulgaria and heavily impacted urban areas in the afternoon. The extremely strong hail storms over Sofia and Montana were accompanied by strong wind gusts (about 85 km h-1) and torrential rain. The giant hail stones in Sofia had diameter of up to 10 cm and irregular shape. The severe hail and rain, and very strong wind caused substantial damage to infrastructure, buildings, vehicles, and agriculture. More than 40 people were injured by hail stones or collaterally by broken windows. One man was killed by a fallen tree. In Sofia alone, the reported damage was worth more than 100 million euro according to data from insurance companies. The paper presents the analysis of the evolution of the convective clouds based on radar data. The Doppler radar data revealed the existence of a mesocyclone, mesoanticyclone, microburst, and three-body scatter signature. The cloud top reached up to 17 km, and the maximum radar reflectivity factor was 69-71 dBZ. Analysis of the available radiosounding data, simulation with a numerical model, and the synoptic patterns are also presented.

Atmospheric Research, 2015

Atmospheric Research, 2009

IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium

The present article describes the principal objectives, the methodology and the main activities o... more The present article describes the principal objectives, the methodology and the main activities of the research project BeRTISS (Balkan-Mediterranean Real Time Severe weather Service) which is funded by the European Territorial Cooperation Programme "Interreg V-B Balkan-Mediterranean 2014-2020". Electron density of the ionosphere and water vapor in the troposphere affect the propagation speed of the GNSS signals transmitted from satellites to the ground reference stations, causing signal propagation delay. Ionospheric delay can be easily removed whereas tropospheric delay, which comprises one of the most significant error sources in satellite signal transmission, should be calculated using surface pressure and temperature variables. Tropospheric delay allows the assessment of Precipitable Water Vapour (PWV). For the first time, BeRTISS real-time service will offer perpetual information for forecasting of PWV over Greece, Bulgaria and Cyprus to the interested public by employing the GNSS derived tropospheric products.