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

Abstract

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.

Evaluation of the Horizontal Winds Simulated by IAP-HAGCM through Comparison with Beijing MST Radar Observations

“…The detailed technical specifications of the Beijing MST radar can be found in the study of Tian and Lu [56][57][58]. It has been used to investigate the horizontal wind, tropopause height [59,60], gravity waves, and turbulence [61,62]. The observational performance and data accuracy of the Beijing MST radar in terms of horizontal wind in the altitudinal range of 3-25 km have been validated by comparing with the nearest radiosonde data, proving that there is perfect agreement between the two types of measurements based on 427 profiles and 15,210 data pairs [57].…”

Wuhan MST Radar Observations of a Tropopause Descent Event during Heavy Rain on 1–2 June 2015

“…According to the radial velocities recorded by the five beams, the velocities of three-dimensional wind fields can be estimated. As a result of partial specular reflection from the atmospheric stable layer, the tropopause can be simply and directly detected by very-high-frequency (VHF) radar by the vertical beam [24,25]. During the heavy rain on 2 June 2015, the Wuhan MST radar was used to investigate the atmospheric responses at troposphere and low-stratosphere heights.…”

High-resolution Beijing mesosphere–stratosphere–troposphere (MST) radar detection of tropopause structure and variability over Xianghe (39.75° N, 116.96° E), China

“…As a result, the time resolutions of the low-, middle-, and high-mode measurements are all 30 min. A more detailed review of the radar system is given by Chen et al (2016).…”

“…Using this characteristic, the RT height can be determined effectively. Here, the RT is defined as the altitude (above 500 hPa) where the maximum vertical gradient of echo power is located (Vaughan et al, 1995;Alexander et al, 2013;Ravindrababu et al, 2014;Chen et al, 2018). Considering the occasional and random noise to which the derived RT is sensitive, the echo-power profiles are smoothed by a three-point running mean.…”