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Water vapour estimates over Antarctica from 12 years ofglobally reprocessed GPS solutions Ian Thomas Matt King Peter Clarke Newcastle University UK email ian thomas ncl ac uk Introduction Stability of reprocessed time seriesAtmospheric water vapour a natural greenhouse gas is often poorly To confirm stability of the reprocessed GPS daily solutions were.monitored in spatial and temporal resolution particularly in remote and mapped to ITRF2005 by a 7 parameter Helmert Transformation inhospitable regions such as Antarctica GPS is known to be able to Helmert parameters show GPS solutions to have good long termprovide accurate estimates of precipitable water PW and is wellstability with respect to ITRF2005 particularly scale parameter bias ofsuited to contribute useful information in such inhospitable regions . 0 4 ppb and scale drift 0 002 ppb yr 1 left plot Continual advances in GPS modelling and processing strategies over Drift in X Y and Z translation parameters of 0 25 0 40 0 02 mm yr 1the last decade have meant that GPS time series of geodeticrespectively right plot .parameters e g those derived using IGS final orbits are most ofteninhomogeneous Such inconsistencies limit the usefulness of GPS for A strong near annual signal is evident in the scale of GPS relative tothe reliable detection of small secular trends e g climatic signals ITRF2005 vertical surface velocities Here we present the results of a homogenous global reprocessing of.12 years of GPS data 1995 2006 from which we derive a 2 hourlyprecipitable water PW data set at 12 Antarctic locations BELG SYOG Plots left to right and top to bottom 1 Radiosonde versus GPS PW scatterplots for 6 Antarctic sites .Antarctic water vapour results 2 Monthly mean of GPS PW blue radiosonde PW green andtemperature red right scale DUM1 We compare our two hourly GPS PW estimates with measurements 3 1995 2004 GPS radiosonde PW derived from radiosonde and from instruments aboard NASA s AQUA and temperature at site DAV1 .satellite AIRS MODIS AMSR E 4 As 3 for site AMUN 5 Spectral analysis for site MAW1 Plot below shows measurements for 2004 for site CAS1 Global reprocessing strategy Results and inter technique agreement at CAS1 are typical of East black all red summer blue winter .Signals are typical of E Antarctica Daily 60 station global GPS networks e g 1st Jan 2004 above Antarctic coastal locations processed using GAMIT 10 3 software Most sites are in the IGS Parameters estimated station coordinates satellite orbital Strong annual signal in PW 1 mm winter to 9 mm summer .Conclusions The Antarctic Peninsula is more humid 12 mm PW in summer Theparameters Earth orientation parameters tropospheric zenith South Pole AMUN is drier 2mm PW in summer Reprocessed GPS provides a temporally stable PW measurementdelays and horizontal gradients technique that compare favourably with radiosonde and AIRS satellite Observation models measurements for the dry Antarctic climate . Absolute satellite and receiver antenna phase centre variations GPS PW agrees best with radiosonde sub millimetre followed by igs05 1421 atx AIRS MODIS and AMSR E VMF1 wet and dry mapping functions GPS captures the dry climate of Antarctica and the dominant annualsignal plus a semi annual signal at East Antarctic sites Elevation cut off angle of 7 5 10 for Antarctic sites . A summer time diurnal S1 signal is observed at many sites OTL displacements computed from FES2004 model Possible secular increases in PW in East Antarctica and at the South Atmospheric loading displacements computed from 6 hourly NCEPPole although even with reprocessing longer time series are.reanalysis data required for GPS to be useful as climate tool Ambiguities resolved GAMIT LC HELP LC AUTCLN Acknowledgement We thank Merrit N Deeter of NCAR Boulder for computing and providing the AMSR E PW estimates 3 hourly surface temperature pressure data used to compute a Reference Bevis M S Businger and Chiswell S R Herring T A Anthes A Rocken C Ware R 1994 GPS.priori ZHD and for ZWD PW conversion Bevis et al 1994 Meteorology Mapping zenith wet delays onto precipitable water J Appl Met 33 379 386Absolute satellite and receiver antenna phase centre variations (igs05_1421.atx). VMF1 wet and dry mapping functions. Elevation cut-off angle of 7.5Â° (10Â° for Antarctic sites). OTL displacements computed from FES2004 model. Atmospheric loading displacements computed from 6-hourly NCEP reanalysis data.