Signatures of northeast monsoon activity and passage of tropical cyclones in the integrated precipitable water vapour estimated through GPS technique

Abstract

Water vapour represents a key variable in the atmospheric processes. The importance of assessing water vapour availability in the atmosphere is indicated by the currently prevalent use of vast number of observing systems, both of in-situ and remote sensing types, designed to measure its distribution accurately over wide ranges of space and time scales. One of the widely used techniques world over is use of ground based GPS receivers for measurement of total precipitable water vapour in the atmosphere over the station. One such system is being operated at Chennai since 2007.  An analysis of hourly Integrated Precipitable Water Vapour (IWV) data received from this system during Northeast Monsoon (NEM) season of 2008 shows the signatures of NEM activity and the passage of tropical disturbances like cyclonic storms and depressions in the vicinity of the GPS observation site. The GPS based IWV values are found to agree fairly well with radiosonde based IWV values and a good correlation exists between them. The IWV values obtained from GPS based system are found to be consistent with activity of Northeast monsoon with increase (decrease) of IWV during active (weak) phase of NEM 2008. The general expected trend of increase in IWV with approach of tropical systems in the vicinity of GPS station, reaching maximum during closest approach and again its decrease with increase of distance from the station is noticed. The diurnal variation of GPS based IWV estimates during NEM 2008 does not appear to be significant.