Effects of bias correction on precipitation trend over China

Long-term monthly climatic data during 1951-1998 at 627 meteorological stations over China have been analysed in this study to examine the impact of bias corrections on monthly and yearly precipitation trends. The impacts of bias corrections on trends depended on the gauge catch efficiency and changes of wind speed, temperature, snow percentage, and number of trace and measurable precipitation days. Trends of corrected and measured monthly and yearly precipitation have similar regional patterns over China, although the trends are different quantitatively over some regions. Precipitation trends in summer months are generally higher than winter season because of high precipitation in summer and low in winter. For instance, trends in January and July are -8 to + 20 mm/10a and -37 to + 55 mm/10a over China for the measured data during 1951-1998, respectively. Bias corrections enhance monthly precipitation trends in every month by about 7 to 18%, leading to an 8% overall enhancement of absolute yearly precipitation trends over China. For the individual stations, the difference in corrected and measured annual precipitation trends varies from -4.1 to + 19.0 mm/10a (-5.0 to + 1.7%/10a) across China. It is important to note that bias corrections can change precipitation trend. Our analyses show that bias corrections have changed monthly and yearly precipitation trend direction from positive to negative over some regions in China. The trend changes reported here are large enough to affect regional climate and hydrology analyses, particularly over cold regions. More efforts are needed to quantify the impact of bias corrections on precipitation change analyses over other large countries and regions.