Abstract
This study describes the process of deriving integrated water vapor (IWV) from (a) a set of 18 GPS receivers that were installed at different airports across India and (b) a pair of GPS receivers located in Ahmedabad situated around 8 km apart. The Zenith Tropospheric Delay was estimated from the GPS observations using the GAMIT software. Further, IWV was estimated from the ZTD values using surface temperature and pressure from ERA-I reanalysis as additional inputs. The IWV estimates for 1 year—March 2013 to February 2014—were compared with ECMWF Reanalysis Interim (ERA-I) reanalysis as well as radiosonde soundings. The Root Mean Squared Error (RMSE) was ≈6 mm or better for most stations. The IWV estimates for July 2013 were assimilated into the WRF model and had a positive impact on model analysis of IWV. The forecasted rain improved by up to 3–4 mm/day in some regions as a result of GPS-derived IWV estimates. For the Ahmedabad receivers, the GPS-derived IWV was compared with IWV from ERA-I reanalysis and was found to have a RMSE of ≈7.7 mm which is <20% of the mean value. The study demonstrates that the observed IWV variation is consistent with rainfall patterns over Ahmedabad. The rise and dips in the IWV correlate well with the active-break cycle in the monsoon rain. The study demonstrates the value of local measurements of IWV with high temporal frequency, as they are more likely to respond to fast-moving weather phenomena such as rainfall. Thus, the GPS-derived IWV measurements are likely to have significant value in the short-term forecasts of precipitation.
Highlights
Atmospheric water vapour is a critical atmospheric parameter that plays an important role in the transfer of energy in the atmosphere and to our understanding of weather phenomena [1]
For the Ahmedabad receivers, the GPS-derived integrated water vapor (IWV) was compared with IWV from ECMWF Reanalysis Interim (ERA-I) reanalysis and was found to have a Root Mean Squared Error (RMSE) of ≈7.7 mm which is
The main steps in the procedure are described below: The zenith tropospheric delay (ZTD) is modelled as a combination of a zenith hydrostatic delay (ZHD) and a zenith wet delay (ZWD) which is a function of the IWV
Summary
Atmospheric water vapour is a critical atmospheric parameter that plays an important role in the transfer of energy in the atmosphere and to our understanding of weather phenomena [1]. Estimation of IWV has been done conventionally with radiosonde balloon sounding at globally distributed network of stations at 00Z and 12Z UTC While still continuing, these observations, owing to their low temporal sampling (only twice a day) and issues of instrument calibration, are deemed to have limited usefulness in studies related to short-term prediction of IWV. The GPS-based technique is relatively unaffected by instrument calibration, unlike radiosondes and MWR, ensuring signal fidelity and measurement reliability This is significant when it comes to up-scaling the location-based methodology to a wider network of GPS ground-tracking stations. Authors found neutral impact in general, but for a severe rain event, assimilation of GPS improved the forecast skill and suggested that the GPS data has excellent potential for improving numerical models in rapidly developing, high moisture flux situations. We describe the procedure used to derive IWV from the GPS data
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