This chapter describes the unique work carried out for this thesis in GNSS Meteorology.
It is a first effort to derive high sampling frequency GNSS tropospheric products with the PPP processing strategy for validation of NWP models. The work is a Bulgarian contribution to Working Group 1 of the COST Action ES1206 GNSS4SWEC (Dousa et al., 2020) with SUGAC being among the newly established Analysis Centers (AC’s) within the project (Simeonov and Guerova, 2020).
Firstly, in section 4.2, a database for meteorological and GNSS observations is de-veloped to comprise tropospheric products from leading GNSS Analysis Centers (AC’s), meteorological observations and Numerical Weather Prediction (NWP) models simula-tions. In section 4.2.1 the effects of the 2007 heat wave over the Balkan peninsula are discussed in the context of GNSS station Sofia. Additionally, in section 4.2.2, a climato-logical study for station Sofia is carried out with comparisons between the tropospheric products from 5 different GNSS processing AC’s.
Secondly, in section 4.3, a GNSS processing of a network of 7 stations in Bulgaria is performed with the NAPEOS software and the data is converted and analysed both for seasonal variations, as well as extreme meteorological events. The results of the data processing are compared to the results from a processing with the Bernese software.
4.1 State-of-the-art of GNSS meteorology in Bulgaria
Remote sensing of the troposphere with GNSS is a well-established method in western Europe (Guerova et al., 2016a). The use of this method in South-Eastern Europe started in 2011 at Sofia University "St. Kliment Ohridski" with the FP7 project "Exploitation of ground-based global navigation satellite systems (GNSS) for meteorology and climate studies in Bulgaria/South-East Europe" https://cordis.europa.eu/result/rcn/1640 29_en.html. As a part of the project, a regional database Sofia University Atmospheric Data Archive (SUADA) is established. Within this thesis the first paper (Guerova et al., 2014), presenting the SUADA database and two case studies of short- and long-term
variation of IWV are conducted and presented in section 4.2. The work continues with the establishment of the Sofia University GNSS Analysis Center (SUGAC). Its first data processing campaign (2014) involves 7 Bulgarian GNSS stations in PPP mode with a temporal resolution of the tropospheric products of 5 minutes (see section 4.3). The resulting tropospheric products are used to evaluate a numerical weather forecasting model (WRF Weather Research and Forecasting) for 2013 (Simeonov and Guerova, 2020).
GNSS water vapour measurements in Bulgaria is a recently established topic of re-search. The only GNSS station, which has continuously been operated for water vapour retrievals is the IGS station SOFI. It is located in the Plana mountain, 1120m asl, about 20km from Sofia. The station is equipped with Leica antenna and multi-frequency re-ceiver. It is established by the Bundesamt für Kartographie und Geodäsie - the German Federal Agency for Cartography and Geodesy (BKG) and is in operation since 1998 for their global GNSS network positioning products. As of 2019, SOFI is also the only station on the territory of Bulgaria, which has been included in the EUMETNET EIG GNSS wa-ter vapour programme (EGVAP). EGVAP is a network, providing IWV measurements to the European national operational meteorological services. The station was not functional between the springs of 2013 and 2014, so all GNSS datasets have gaps in this period. Ad-ditionally the data density in 2018 is reduced due to communication problems. The Sofia meteorological station (WMO 15614) is located 15km away from the GNSS site at an altitude of 595m asl Surface observations are carried out in all synoptic periods (every 3 hours, starting 00 UTC), together with radiosoundings, performed daily at 12 UTC. The station is established in the 1950’s, then outside the city borders. Nowadays the station is located within one of the large residential districts of the city. A study of data from the 2007 heat wave for Sofia is presented in section 4.2.1, followed by an analysis of the historical datasets since 1999 in section 4.2.2.
There are more then 4 operational private GNSS networks in Bulgaria, operating in total more than 100 GNSS sites. BuliPOS is one of them, part of the European EUPOS network. Data from 7 stations for a period of 1 year are provided from this network for the study of the water vapour over the territory of Bulgaria. This study is summarized in section 4.3.
The work in this thesis has served as basis for several peer-reviewed scientific articles:
Mircheva et al.(2017),Stoycheva et al.(2017), conference proceeding and research reports:
Haralambous et al. (2018), Guerova et al. (2016b), Simeonov and Guerova (2020) and more.
4.2 Sofia University Atmospheric Data Archive 49
4.2 Sofia University Atmospheric Data Archive
Part of this section is published in AMT, 7/p2683–2694, 2014 (Guerova et al., 2014).
Currently SUADA has 5 GNSS datasets processed with different software and strategies.
As seen in table 4.1 the GNSS data offer high temporal resolution from 5 min to 6 hours and the IGS station in Sofia Bulgaria (SOFI, marked by red pointer in figure B.2) is available since 1997. The GNSS datasets are discussed bellow.
The SUADA is a regional database aiming at: 1) achieving atmospheric water vapour observations from different techniques and 2) using the data for meteorological and cli-matic studies in Bulgaria/South-East Europe. Similar to the STARTWAVE database (Morland et al., 2006), SUADA is designed to enhance and facilitate the atmospheric re-search at the Sofia University, but also to provide online data access, via a web portal, for interested researchers in Bulgaria and the neighbouring countries. SUADA is developed using the My Structured Query Language (MySQL) for a relational database management system (Codd, 1970).
dataset tropos. available number of observation number of name product yyyy - yyyy stations frequency observations
IGS repro 1 ZTD 1995 - 2019 9 5 min 8 262 322
ZenitGEO ZTD 2011 - 2013 30 5 min 26 043 846
ZenitGEO IWV 2012 30 3 hours 299 598
SUGAC ZTD 2013 7 5 min 581 003
SUGAC IWV 2013 7 30 min 104 812
Balkan ZTD 2007 - 2014 23 5 min 21 607
Balkan IWV 2011 - 2014 10 3 hours 6 480
GFZ ZTD 2010 - 2019 1 5 min 217 748
GFZ IWV 2010 - 2019 1 3 hours 16 665
Radiosonde Profiles 1980 - 2019 1 1 day 18 989
SYNOP surface
P, T 1999 - 2019 26 3 hours 437 865
WRF Profiles 2010 - 2019 139 30 min 6 086 961
WRF IWV 2011 - 2019 139 30 min 640 245
Table 4.1: A summary of the GNSS, meteorological and NWP datasets present in the SUADA database as of 1.09.2019. More information on the datasets can be found in appendix B.1.2.
Datasets from multiple sources are incorporated into SUADA, including GNSS rou-tine and post-processed data (as seen in table 4.2), as well as data from meteorological
name scale method strategy source
IGS repro 1 Global PPP Post-processed 1
CODE repro 2 Global Network Solution,
Table 4.2: Used datasets processing strategies and methods, as described in (Guerova et al., 2016a). 1. ftp://cddis.gsfc.nasa.gov/pub/gps/products/troposphere/zpd/
2. ftp://gssc.esa.int/gnss/products/ 3. http://igs.bkg.bund.de/root_ftp/EUR EF/products/ 4. ftp://ftp.gfz-potsdam.de/GNSS/products/nrttrop/product_COS T_EPOS8/ 5. described in section 3.4. 6. described in appendix B.1.2.
surface observations, radiosoundings and NWP models (see table 4.1). A comprehensive description of the datasets is provided in appendix B.1.2.