Time-variable observational errors explain the spurious rainfall and snowfall proportion trend in China under global warming

It has been widely reported that precipitation tends to occur more in the form of rainfall rather than snowfall under global warming, as expected from theory. However, the observed data across China from 1961 to 2022 show that the rainfall to total precipitation rate decreased, especially in Northwest and Northeast China. This study investigates this paradox from perspective of the time-variable observational errors in the precipitation observation. The national standard gauge without a wind shield has long been used in China. There is an undercatch issue with the gauge caused by the turbulence generated when wind blows over it. This issue is more severe for snow and is exacerbated during strong winds. To improve the accuracy of snowfall measurement, new weighing gauges with one-layer wind shield have been deployed since 2009 in China. The authors conducted wind-induced error corrections for rainfall and snowfall at approximately 2300 national weather stations. It was found that the national mean annual precipitation, rainfall, snowfall were 794 mm, 763 mm, and 27 mm before correction and were 854 mm, 810 mm, and 40 mm after correction. After correction, the national mean rainfall to total precipitation rate showed an increasing trend (0.17 %/decade) from 1961 to 2022 instead of a decreasing trend (-0.04 %/decade) in the raw data. Especially, the trend of rainfall proportions in the Northwest China and the Northeast China changed from significant negative to positive. The key reason for this change is that wind-induced error decreased due to a reduction in surface wind speed, which is amplified by the instrument replacement. This is more obvious for snowfall.