Precipitable water vapor estimates from homogeneously reprocessed GPS data: An intertechnique comparison in Antarctica

Homogeneously reprocessed GPS data offer the possibility of an accurate, stable, and increasingly long-term record of integrated precipitable water vapor (PW) of particular value in data sparse regions. We present such a global reanalysis of GPS data, focusing on 12 Antarctic sites. We show stepwise improvements of GPS zenith total delay (ZTD) estimates upon adoption of each of (1) absolute antenna phase centre variations, (2) VMF1 tropospheric mapping functions, and (3) an accurate model of a priori zenith hydrostatic delay (ZHD) from observed surface meteorological data. The cumulative effect of these three additions to the analysis is a systematic decrease in the magnitude of GPS estimates of ZTD by an average of similar to 11 mm ZTD (similar to 1.8 mm PW). The resultant GPS PW data set for 2004 shows a mean bias to radiosonde measurements of -0.48 mm PW. Our conclusion is that, in Antarctica at least, a proportion of the widely observed bias between GPS and radiosonde measurements can be explained by earlier GPS analysis deficiencies. We also compare our GPS PW measurements with AIRS and MODIS level 2 PW products. The GPS agreements with AIRS and MODIS are comparable. Reanalyzed GPS gives typically larger measurements than AIRS with a mean site bias of 0.58 mm PW and a mean rms of 1.24 mm PW. By contrast, the GPS measurements are typically smaller than those from MODIS, with a mean site bias of -0.35 mm PW and rms of 1.42 mm PW. PW estimates from reprocessed GPS solutions using state-of-the-art models now have greater potential for assimilation into regional or global numerical weather models.