A specific stratospheric precursory signal of overshooting convective systems over the southern slope of the Tibetan Plateau

Abstract

Using the TRMM PR observations, ERA5 reanalysis and Outgoing Longwave Radiation (OLR) datasets, a specific stratospheric precursory signal to the occurrence of overshooting convective systems (OCSs) over the southern slope of the Tibetan Plateau (SSTP) is investigated. We compared two types of OCS events: OCSs associated with stratospheric intrusions and other OCSs. The results indicate that there is a notable decline in the tropopause height starting two days before to on the day of the OCS onset for OCSs associated with stratospheric intrusions, but the tropopause height for other OCSs increases with the occurrence of OCS. It is found that the descending of the tropopause height is accompanied by downward intrusion of stratospheric high potential vorticity (PV) air into the troposphere for the OCSs associated with stratospheric intrusions. The increase in PV near the tropopause can cause an increase in temperature and relative vorticity near the tropopause. The OCSs associated with stratospheric intrusions are closely related to strong synoptic variability. As the stratospheric intrusion deepens on the day of the OCS onset, increased relative vorticity near the tropopause, stronger upward motion in the troposphere associated with divergence and convergence, more specific humidity in the troposphere, and weakened static stability in the middle troposphere are found within the OCS region, which are conducive to the development of deep convection that penetrates into the stratospheric intrusion. Compared with OCSs associated with stratospheric intrusions, other OCS events are characterized by stronger high cloud cover (HCC) and negative Outgoing Longwave Radiation (OLR) anomaly, as well as higher upward movement in the troposphere. This is conducive to other OCSs occurring in a more barotropic convective environment characteristic of the Asian monsoon, the vertical development height of weakened static stability and upward motion are higher, promoting the development of convective activity to higher altitudes and penetrating into the tropopause, thereby causing the uplift of the tropopause.