[A-AS21_30AM1] Stratospheric Processes And their Role in Climate

[AAS21-02] Dynamical mechanism of multiple tropopause structure observed over Syowa Station

*Ryosuke SHIBUYA¹, Kaoru SATO¹ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)

Keywords:Tropopause, Multiple tropopause, Gravity wave

Multiple tropopauses which are determined following the definition by the World Meteorological Organization (WMO) were detected in winter at Syowa Station (69.0S, 39.6E). It is shown that the multiple tropopause structures were observed along with a descent of the first (i.e., lowest) tropopause five times in the autumn period from 1 April and 16 May 2013. A detailed analysis using data from the PANSY radar and radiosonde observations was performed for a typical case in 8-11.The mechanism of the multiple tropopause structure was analyzed using the PANSY radar and radiosonde observations. It is shown that the multiple tropopause structure was regarded as strong temperature fluctuations with a vertical length of about 3 km. Moreover, it is seen that the temperature fluctuations were out of phase with vertical wind fluctuations observed by the PANSY radar by 90o. This feature is consistent with the linear inertia-gravity wave theory. Thus, it is likely that the multiple tropopause structure above the first tropopause was due to the temperature fluctuations associated with an inertia-gravity waves (IGW) having a vertical length of about 3 km. The hodograph analysis also indicates that the multiple tropopause structure above the first tropopause is due to a monochromatic IGW.To examine the dynamical mechanism and there-dimensional structure of this phenomenon, a numerical simulation was performed by NICAM without using any gravity wave parameterization. The model simulation period is from 0000 UTC 7 April 2013 to 0000 UTC 12 April 2013.A close look at the time-height cross section of the zonal wind velocity and the static stability over Syowa Station indicates that the multiple tropopause structures together with the descent of the first tropopauses and associated wind disturbances were successfully simulated. A polar front jet strongly meanders in the time period from 8 April to 10 April and a tropopause folding structure is developed near Syowa Station. This means that the descent of the first tropopause was likely caused by the passage of a developing tropopause folding over Syowa Station. The IGW parameters were also consistent with those estimated by the hodograph analysis using the PANSY radar data.Next, possible sources of the IGWs observed over Syowa Station were examined using data from the NICAM simulation. As a result, it was shown that wave packets observed over Syowa Station include gravity waves both excited by the steep topographic effect and the spontaneous adjustment process.This mechanism is quite different from mechanisms which previous studies examined in the monsoon region or midlatitude, which is closely related to stratosphere-troposphere exchange (e.g. Randel et al. 2007). It is suggested this enable us to interpret a part of a significant seasonal sensitivity in the poles discussed by Anel et al. (2008). The static stability in the winter lower stratosphere in the Antarctic is particularly weaker than in other latitudes (Gettleman et al., 2011). It is likely because ozone heating is absent due to polar night. Based on the radiosonde observations, Tomikawa et al. (2009) also shows that the static stability in the lower stratosphere over Syowa Station is minimized in April through July. Temperature fluctuations associated with gravity waves are observed as fluctuations of the static stability. Thus, when the background static stability is sufficiently weak such as in the polar lower stratosphere, the temperature fluctuations associated with gravity waves can make local minima of the static stability which are detected as thermal tropopauses. Therefore, it is likely that multiple tropopause events due to IGWs are considered to occur frequently in the Arctic / Ataractic region in winter.