Strong downdrafts preceding rapid tropopause ascent and their potential to identify cross-tropopause stratospheric intrusions

Abstract

The capability of measuring three-dimensional wind and tropopause structure with relatively high time and vertical resolution makes very-high-frequency (VHF) radars a potentially important tool for studying various processes of the atmosphere. However, at present several unanswered questions remain regarding the use of VHF radars to identify possible stratospheric intrusions. Here the potential detection of stratospheric intrusion events is discussed using the Beijing MST (mesosphere-stratosphere-troposphere) radar located at Xianghe (39.75° N, 116.96° E). During the passage of a cutoff low in late November 2014, a deep V-shaped tropopause structure and strong downdrafts (> 0.8 m s-1) immediately preceding the rapid tropopause ascent (> 0.2 km h-1) were observed. Within the height region of the downdrafts, the stability of the radar tropopause seems to be weakened. Analysis results from global reanalysis and the satellite data, as well as the trajectory model, have shown clear evidence of downward stratospheric intrusions (dry ozone-rich and depleted methane air) associated with the strong downdrafts. A total of 20 typical cases of such strong downdrafts, occurring during various synoptic processes in different seasons, have been presented, and 15 of them are exactly associated with some form of stratospheric intrusions. Four years (2012-2015) of such downdrafts are further discussed. The observations reveal that the strong downdrafts preceding the rapid tropopause ascent can be a valuable diagnostic for monitoring intrusion events, which helps us to gain a better understanding of stratospheric intrusions in VHF radar observations.