Trends in tropospheric humidity from 1970-2008 over China from a homogenized radiosonde dataset

Radiosonde humidity data provide the longest record for assessing changes in atmospheric water vapor, but they often contain large discontinuities because of changes in instrumentation and observational practices. In this study, the variations and trends in tropospheric humidity (up to 300 hPa) over China are analyzed using a newly homogenized radiosonde dataset. It is shown that the homogenization removes the large shifts in the original records of dewpoint depression (DPD) resulting from sonde changes in recent years in China, and it improves the DPD’s correlation with precipitation and the spatial coherence of the DPD trend from 1970 to 2008. The homogenized DPD data, together with homogenized temperature, are used to compute the precipitable water (PW), whose correlation with the PW from ground-based global positioning system (GPS) measurements at three collocated stations is also improved after the homogenization. During 1970–2008 when the record is relatively complete, tropospheric specific humidity after the homogenization shows upward trends, with surface-300-hPa PW increasing by about 2%-5% decade⁻¹ over most of China and by more than 5% decade⁻¹ over northern China in winter. The PW variations and changes are highly correlated with those in lower–midtropospheric mean temperature (r = 0.83), with a dPW/dT slope of ∼7.6% K⁻¹, which is slightly higher than the 7% K⁻¹ implied by Clausius-Clapeyron equation with a constant relative humidity (RH). The radiosonde data show only small variations and weak trends in tropospheric RH over China. An empirical orthogonal function (EOF) analysis of the PW reveals several types of variability over China, with the first EOF (31.4% variance) representing an upward PW trend over most of China (mainly since 1987). The second EOF (12.0% variance) shows a dipole pattern between Southeast and Northwest China and it is associated with a similar dipole pattern in atmospheric vertical motion. This mode exhibits mostly multiyear variations that are significantly correlated with Pacific decadal oscillation (PDO) and ENSO indices.