Spatio-temporal distributed water vapor retrieved from GNSS observations and its usage in monitoring a rainfall process in Hong Kong

Abstract: Usually, one can use the line-of-sight observations of GNSS (Global Navigation Satellite System) in not only positioning and navigation, but also in describing the medium that they pass through, such as ionosphere and troposphere. Particularly, for a specific station, one can further use obtained tropospheric wet delay in zenith direction to calculate integrated water vapor (IWV) or precipitable water vapor (PWV) with a transfer coefficient related to in-situ meteorological elements. Further, with IWVs or PWVs at a number of stations in a region, one can additionally discover the spatio-temporal distribution of water vapor by using the so-called tomography technology, noted as TWV hereafter. In this work, both retrieved PWVs and TWVs are analyzed and used in monitoring a rainfall process in Hong Kong ranging from 113.86°E to 114.36°E and 22.05°N to 22.40°N. A self-generated water vapor tomography package named GWATOS (GNSS Water vapor Tomography Software) is employed, and the Kalman filtering is used in the package to try to include more information that is valuable. The in-situ GNSS observations with interval of 5s and meteorological observations with interval of 60s at 18 stations in SatRef (Satellite positioning Reference station network) from 1^st to 31^st of May, 2016 are processed with BERNESE software by using the precise point positioning mode to retrieve the tropospheric delays. The temporal resolution of resulted PWVs is 30 minutes, and spatial resolution of TWVs is 0.05° in longitude, 0.07° in latitude and 15 unequal layers in height. In the experiment, the radiosonde profile data sets with temporal resolution of 12 hours at a station named Kings Park provided by University of Wyoming are used to externally assess GNSS retrieved water vapor. The retrieved PWVs show good consistency with results from radiosonde. The PWVs indicate obvious periods of high values, e.g., at 10^th and 21^st of May, and frequent variations, e.g., at 27^th and 28^th of May. As an example, the regional PWVs itself and its variation both in time and in space are analyzed to monitor the earlier-start, ongoing and end stage of rainfall process in 10^th of May, where both Red and Amber rainstorm warning are given. The results depict that abnormal period of PWVs are in good agreement with recorded rainfall period by Hong Kong Observatory. The results of retrieved TWVs show good internal and external agreements, the statistics are about 2.0mm and 1.7mm, respectively. The water vapor and its spatio-temporal variations in layers lower than 400 m and between 400m and 800m are investigated emphatically. The results show that both retrieved PWVs and TWVs with high spatio-temporal resolution could reflect rainfall process to some extent, especially the earlier-start and end stage of rainfall. That is to say, GNSS retrieved water vapor could be used to monitor the rainfall process in an auxiliary way. More importantly, no matter PWVs or TWVs could be used to trace the movement of water vapor or its structure both in time and in space, which can be further used in meteorology study for more detail information.