Papers by Guergana Guerova
Journal of Atmospheric and Oceanic Technology, May 1, 2022
Monitoring atmospheric conditions that lead to severe weather events is critical to their timely ... more Monitoring atmospheric conditions that lead to severe weather events is critical to their timely and accurate prediction and can help prevent of large economic losses. Bulgaria, located in southeastern Europe, has the highest mean number of thunderstorms and hailstorms. These events generally occur between April and September with a peak in July. In this study, both radio occultation (RO) and ground-based observations from the Global Navigation Satellite Systems (GNSS) were used to study two severe hailstorms that occurred in 2014 and 2019. In both storms, a cold upper-air pool was detected in addition to a large specific humidity anomaly between 2 and 6 km. In the hailstorm that occurred in July 2014, there was an RO temperature anomaly between 10 and 14 km as well as a positive specific humidity anomaly between 4 and 6 km. The integrated vapor transport (IVT) reanalysis from ERA5, indicated that the high specific humidity over the Mediterranean could be tracked to an atmospheric river over the North Atlantic, which was connected to a tropical cyclone. In the hailstorm that occurred in May 2019, elevated IVT values were observed before the storm. During this storm, a negative temperature anomaly peak was observed in the RO profile at 11.3 km as well as a positive specific humidity anomaly between 2 and 4.5 km. The WRF Model and the ERA5 dataset could reproduce the temperature profiles for both storms relatively well; however, they tended to underestimate specific humidity. The RO profiles were complemented by ground-based GNSS tropospheric delays with high temporal resolution. The evaluation of the WRF with ground-based GNSS tropospheric products revealed a time delay between the modeled and observed developments of both hailstorms.
<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>
Remote Sensing, Aug 4, 2022
The European summer of 2003 was characterised by intense heat, prolonged isolation and suppressed... more The European summer of 2003 was characterised by intense heat, prolonged isolation and suppressed ventilation of the boundary layer which, combined with large anthropogenic emissions and strong fires, resulted in a build up of an unprecedentedly high and long-lasting photochemical smog over large parts of the continent. In this work, a global chemistry and transport model GEOS-Chem is compared with surface O3 concentrations observed in 2003 in order to examine the extent to which the model is capable of reproducing such an extreme event. The GEOS-Chem reproduces the temporal variation of O3 at the Jungfraujoch mountain site, Switzerland, including the enhanced concentrations associated with the August 2003 heat wave (r = 0.84). The spatial distribution of the enhanced surface O3 over Spain, France, Germany and Italy is also captured to some extent (r = 0.63), although the largest concentrations appear to be located over the Italian Peninsula in the model rather than over Central Europe as suggested by the surface O3 observations. In general, the observed differences between the European averaged O3 concentrations in the summer of 2003 to those in 2004 are larger in the observations than in the model, as the model reproduces relatively well the enhanced levels in 2003 but overestimates those observed in 2004. Preliminary contributions of various sources to the O3 surface concentrations over Europe during the heat wave indicate that anthropogenic emissions from Europe contribute the most to the O3 build up near the surface (40 to 50%, i.e. 30 ppb). The contribution from anthropogenic emissions from the other major source regions of the northern hemisphere, in particular North America, tends to be smaller than those of other years. The model indicates that the large fires that occurred in that year contributed up to 5% (3 ppb) to surface O3 in close proximity to the fire regions and less elsewhere in Europe. Biogenic volatile organic compounds (VOCs) emitted by grass and forest areas contributed up to 10% (56 ppb) of surface O3 over France, Germany and northern Italy, which represents a contribution that is twice as large than that found in 2004. These results in terms of contributions from various sources, particularly biogenic emissions, should be seen as preliminary, as the response of vegetation to such extreme events may not be well represented in the model
<p>The Knowledge of the development of the severe thunderstorms is crucial for impr... more <p>The Knowledge of the development of the severe thunderstorms is crucial for improvement of weather forecasts which are provided to air traffic control (ATC) units. Intense thunderstorms, strong winds and hazards such as severe turbulence and icing, lightning activity, microbursts, and hail are very important for the aviation safety.</p> <p>The present work focuses on the study case of atmospheric conditions preceding and during the development of unusual severe convection over southeast Bulgaria on 17<sup>th</sup> September 2022. when the squall line passed through the airport of Burgas bringing a lot of damage to the airport and surrounding towns.</p> <p>On this days the weather was dominated by the presence of a very unstable air mass over Southeast Bulgaria, ahead of the atmospheric frontal zone. As convection continued its development, it moved east/northeast forward Burgas with overshooting cloud top height up to 14 km, cloud top temperature of -70C and maximum radar reflectivity of 60 dBz. In the late afternoon the process led to formation of gust front that reached the Burgas airport with the record for the past 50 years wind speed exceeding 45 m/s that damaged the mast of the airport's instrument landing system. At the same time heavy rain and intense lighting activity are reported. For analysis of the severe weather conditions are used weather satellite, radar and lightning data, Mode-S aircraft wind data and surface observations. In addition, information provided by the ground based Global Navigation Satellite System (GNSS) network is used. Integrated water vapor shows a sharp increase several hours before the convection developed. As moisture is one of the main ingredients for intense thunderstorms formation, GNSS tropospheric products are used together with other surface data to help the process of deep convection monitoring and forecasting.</p>
Remote Sensing
In this study, we conducted a global assessment of the sensitivity of vegetation greenness (VGS) ... more In this study, we conducted a global assessment of the sensitivity of vegetation greenness (VGS) to precipitation and to the estimated Lagrangian precipitation time series of oceanic (PLO) and terrestrial (PLT) origin. The study was carried out for terrestrial ecosystems consisting of 9 biomes and 139 ecoregions during the period of 2001–2018. This analysis aimed to diagnose the vegetative response of vegetation to the dominant component of precipitation, which is of particular interest considering the hydroclimatic characteristics of each ecoregion, climate variability, and changes in the origin of precipitation that may occur in the context of climate change. The enhanced vegetation index (EVI) was used as an indicator of vegetation greenness. Without consideration of semi-arid and arid regions and removing the role of temperature and radiation, the results show the maximum VGS to precipitation in boreal high-latitude ecoregions that belong to boreal forest/taiga: temperate grassl...
Atmosphere
Low visibility caused by fog events can lead to disruption of every type of public transportation... more Low visibility caused by fog events can lead to disruption of every type of public transportation, and even loss of life. The focus of this study is the synoptic conditions associated with fog formation. The data used in this study was collected over the course of ten years (2010–2019) in Sofia, Bulgaria. The forecast skills of the Fog Stability Index (FSI) and the local Sofia Stability Index (SSI), as well as the relation between the Integrated Water Vapor (IWV) and fog from the Global Navigation Satellite System (GNSS), were tested. Both fog indices are used for fog nowcasting as their lead times are short and unclear. The Jenkinson–Collison Type method was used for extracting the predominant synoptic-scale pressure systems which provide suitable weather conditions for fog formation. Surface observations from two synoptic stations were used to calculate and evaluate the performance of the two fog indices and of the ground-based GNSS receiver for the IWV. The forecast skills provid...
<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>
Dokladi na Bʺlgarskata akademiâ na naukite, Aug 31, 2022
EGU General Assembly Conference Abstracts, Apr 1, 2019
Advanced GNSS Tropospheric Products for Monitoring Severe Weather Events and Climate, 2019
Over the last decade, near real-time analysis of GPS data has become a well-established atmospher... more Over the last decade, near real-time analysis of GPS data has become a well-established atmospheric observing tool, primarily coordinated by the EIG EUMETNET GPS Water Vapour Programme (E-GVAP) in Europe. In the near future, four operational GNSS will be available for commercial and scientific applications with atmospheric science benefiting from new signals from up to 60 satellites observed at any one place and time, however, many challenges remain regarding their optimal combined utilization. Besides raw data streaming, recent availability of precise real-time orbit and clock corrections enable wide utilization of autonomous Precise Point Positioning (PPP), which is particularly efficient for high-rate, real-time and multi-GNSS analyses.
EGU General Assembly Conference Abstracts, Apr 1, 2017
Journal of Atmospheric and Oceanic Technology, 2022
Monitoring atmospheric conditions that lead to severe weather events is critical to their timely ... more Monitoring atmospheric conditions that lead to severe weather events is critical to their timely and accurate prediction and can help prevent of large economic losses. Bulgaria, located in southeastern Europe, has the highest mean number of thunderstorms and hailstorms. These events generally occur between April and September with a peak in July. In this study, both radio occultation (RO) and ground-based observations from the Global Navigation Satellite Systems (GNSS) were used to study two severe hailstorms that occurred in 2014 and 2019. In both storms, a cold upper-air pool was detected in addition to a large specific humidity anomaly between 2 and 6 km. In the hailstorm that occurred in July 2014, there was an RO temperature anomaly between 10 and 14 km as well as a positive specific humidity anomaly between 4 and 6 km. The integrated vapor transport (IVT) reanalysis from ERA5, indicated that the high specific humidity over the Mediterranean could be tracked to an atmospheric r...
The Sofia University GNSS Analysis Centre (SUGAC, suada.phys.uni-sofia.bg) is a new analysis cent... more The Sofia University GNSS Analysis Centre (SUGAC, suada.phys.uni-sofia.bg) is a new analysis centre established via collaboration between the Department of Meteorology and Geophysics of Sofia University, the IPOS BuliPOS GNSS network in Bulgaria and the University of Luxembourg. In April 2014, the first processing campaign took place. One year GNSS data from 7 stations of the BuliPOS network are processed in collaboration with the University of Luxembourg. Tropospheric products (Zenith Total Delay and gradients) with 5 min temporal resolution are obtained using the NAPEOS software, developed by ESA. The tropospheric products from this campaign will be used for validation of the Weather Research and Forecasting (WRF) model as well as for case studies during intense precipitation events and fog. In this work the WRF model validation for Bulgaria will be presented. Future work will be the establishment of autonomous near real-time processing of the regional ground-based GNSS network in...
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Papers by Guergana Guerova