Trends in the Frequency of High Relative Humidity over China: 1979 to 2012

Abstract:

High relative humidity (HRH) is defined as a relative humidity of at least 80%, which is often associated with the occurrence of cloud layers. Thus, the frequency of HRH and its changes in the troposphere may be related to the occurrence frequency of cloud layers and their changes. In this study, trends in the frequency of HRH (defined as days with relative humidity ≥80%) over China from the surface to the mid-troposphere (≥400 hPa) from 1979 to 2012 were analyzed using a homogenized humidity dataset for spring (March－May), summer (June－August), autumn (September－November), and winter (December－February). The results for the ground level indicate decreasing trends at most stations in southeastern China in spring and in northern China in summer. In the lower troposphere (850 hPa and 700 hPa), most stations over China exhibited positive trends in summer, autumn, and winter. For the mid-troposphere (500 hPa to 400 hPa), increasing trends dominated over China in spring, summer, and autumn. Finally, six reanalysis datasets, including the NCEP/NCAR I, NCEP/DOE II, CFSR, ERA-Interim, MERRA, and JRA-55 datasets, were compared with the observed increasing trends in HRH frequency in the low-mid troposphere. Similar vertical profiles of HRH trends in the reanalysis datasets and the homogenized humidity dataset were observed in certain seasons and for certain regions, with positive or small negative trends at high levels and large negative trends at 850 hPa, respectively. These results are consistent with the increasing low-mid cloud amounts in recent decades.