A comparative study of atmospheric humidity over China between radiosonde and the third generation reanalysis datasets

Based on quality controlled (RAW) and homogenized (ADJ) radiosonde humidity collected at 118 stations in China and monthly mean specific humidity and relative humidity from four reanalysis datasets (REA) including ERA-Interim, JRA-55, MERRA and CFSR, a preliminary comparison of upper air specific humidity and relative humidity over China between radiosonde data and the 3rd generation reanalysis datasets is conducted. The relative differences, correlations, standard deviations and linear trends during 1979-2015 derived from RAW, ADJ and REA are analyzed. The results reveal significant discontinuities in the time series of RAW radiosonde humidity in China. Homogenization has improved the continuity of time series of humidity but introduced significant dry biases, which explains why ADJ is 5%-43% lower than RAW. The averaged specific humidity and relative humidity over China from REA are generally higher than those from radiosonde and the differences in relative humidity (7%-48%) are higher than that in specific humidity (4%-13%). Meanwhile, the differences in the upper troposphere are larger than those in the lower troposphere. The differences in spring and autumn are more significant than those in summer. The specific humidity and relative humidity from the four reanalysis datasets are highly consistent, whereas that from the JRA-55 are relatively more similar to radiosonde with lower humidity than other three reanalysis products. Correlations and standard deviation ratios of specific humidity and relative humidity demonstrate a higher consistency regarding annual variability and dispersion in the lower troposphere between REA and ADJ, and the dispersion in REA is obviously higher than that in ADJ in the middle and upper troposphere. The linear trends of specific humidity are consistent between REA and ADJ in the lower troposphere, where both show a wetting trend, but different in the middle troposphere, where a wetting trend is found in ADJ and a drying trend is found in REA. The linear trends of relative humidity in the troposphere are obviously different between REA and ADJ, i.e., a drying trend is found in REA in the troposphere while a wetting trend and an even more significant wetting trend in the middle and upper troposphere from ADJ.