Abstract
Atmospheric Water vapor is an important greenhouse gas and contributes greatly in maintaining the Earth’senergy balance. This critical meteorological parameter is not being sensed by any of the 22 synoptic weatherstations in Ghana. This study presents a highly precise tool for water vapor sensing based on the concept GlobalNavigation Satellite Systems (GNSS) meteorology and tests the computed results against global reanalysis data.Conventional approaches used to sense the atmospheric water vapor or precipitable water (PW) such as such asradiosondes, hygrometers, microwave radiometers or sun photometers are affected by meteorological conditionsand expensive. Whereas GNSS meteorological concept offers an easier, inexpensive and all-weather technique toretrieve Precipitable Water (PW) or Integrated water vapor (IWV) from zenith tropospheric delays (ZTD) overa reference station. This study employed precise point positioning (PPP) techniques to quantify the extendof delays on the signal due to the troposphere and stratosphere where the atmospheric water vapor resides.Stringent processing criteria were set using an elevation cut-off of 5 degrees, precise orbital and clock products,nominal tropospheric correction and mapping functions. The delays which are originally slanted are mappedunto the zenith direction and integrated with surface meteorological parameters to retrieve PW or IWV. ThegLAB software, Canadian Spatial Reference System (CSRS) and Automatic Precise Positioning Service (APPS)online PPP services were the approaches used to compute ZTD. PW values obtained were compared with JRA,ERA-interim and NCEP global reanalysis data. Correlation analysis were run on the logged station data usingthe three approaches and global reanalysis data. The obtained results show stronger correlation between theretrieved PW values and those provided by the ERA-interim. Finally, the study results indicate that with amore densified network of GNSS base stations the retrieved PW or IWV will greatly improve numerical weatherpredictions in Ghana.
Highlights
As a major constituent in the atmospheric greenhouse gases, water vapour concentration is not significantly influenced by direct anthropogenic activities (USGS-Editors, 2011; Seidel, 2002)
Further analyses were conducted to compute the correlation coefficients between gLAB zenith tropospheric delays (ZTD) values and ZTDs obtained from Automatic Precise Positioning Service (APPS) and Canadian Spatial Reference System (CSRS) servers
Global Navigation Satellite Systems (GNSS) signals coupled with surface meteorological data can be utilized to derive tropospheric delays and Precipitable Water (PW) with high accuracy
Summary
As a major constituent in the atmospheric greenhouse gases, water vapour concentration is not significantly influenced by direct anthropogenic activities (USGS-Editors, 2011; Seidel, 2002). Sensing and measurement of water vapour by conventional methods such as radiosondes, hygrometers, microwave radiometers, sun photometers are affected by meteorological conditions (Maghrabi and Al Dajani, 2013; Pérez‐Ramírez et al, 2012; England et al, 1992) They are expensive and have coverage limitations. GMet instead uses most products from the European Center for Medium-Range Weather Forecasts (ECMWF) and other global numerical weather prediction models for their activities These NWP server products are very good but there still exhibits substantial data gaps with respect to ground-based GNSS observational networks for the African region (refer Figure 1). Seidel et al, 2009 and Teunissen, 2003 have reported of densification of stations of the Global Climate Observing System (GCOS) Upper-Air Network (GUAN) with greater availability and improved data accessed for weather forecasting. With these modernization efforts being carried-out, are aimed at improving coverage and data availability which will go a long way to improve and enhance global weather forecasting
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