Indications of stratospheric anomalies in the freezing rain and snow disaster in South China, 2008

Abstract

NCEP/NCAR reanalysis data were used to characterize stratospheric temperature and water-vapor anomalies before and after the freezing rain and snow disaster of South China in 2008, and the influence of stratospheric circulation anomalies on the troposphere. Stratospheric temperature and water-vapor anomalies provided good leading indicators of this weather event. The period from December 1st 2007 to February 28th 2008 was divided into 18 pentads. During the 6th pentad, temperature decreased significantly at 10 hPa in the near-polar stratospheric region, and the decreasing trend strengthened and extended downward and southward to middle and lower latitudes. During the 14th-18th pentads, the temperature decrease reached its maximum and extended to 30°N. This coincided with the widespread freezing rain and snow event. By the end of January 2008, the temperature decrease ended in the near-polar stratospheric region, but continued in the mid-latitude area of the troposphere as the freezing rain and snow weather persisted. Similar to the temperature variations, positive anomalies of relative humidity in the stratospheric near-polar region also strengthened and extended downward and southward, coinciding with the freezing rain and snow event. Along with the significant relationship between the freezing rain and snow disaster and stratospheric circulation anomalies, the stratospheric polar vortex changed its shape in late December, intensifying and spreading downward from the top of the stratosphere and southward to the Asian continent, resulting in a deepening of the East Asian Trough and a strengthening of meridional circulation. Before the occurrence of the freezing rain and snow event, temperature and vapor increases in the stratosphere transferred downward to the troposphere, along with a stratospheric flow in the near-polar region southward to lower latitudes.