06 Oct 2022
 | 06 Oct 2022

Preconditioning of block onset in the Southern Hemisphere: a perspective from static stability

Li Dong, Hairu Ding, Guochun Shi, and Stephen J. Colucci

Abstract. The horizontal and temporal variations of static stability prior to blocking onset are characterized through composite analysis of twenty blocking events in the Southern Hemisphere. It is found that, along with a low potential vorticity (PV) anomaly formation, a local minimum of static stability in the upper troposphere and on the tropopause is achieved over the block-onset region when blocking onset takes place. By partitioning the isentropic PV into the absolute vorticity and static stability contributions, it is found that they account for roughly 70 % and 30 % of low-PV anomaly formation over the block-onset region, respectively. A static stability budget analysis revealed that the decrease of static stability in the upper troposphere and on the tropopuase prior to blocking onset is attributable to horizontal advection of low static stability from subtropics to midlatitude as well as the stretching effect associated with upper-level convergence over the block-onset region, with the horizontal advection forcing being the primary contributor. On the other hand, the vertical advection of static stability tends to oppose the decreasing static stability through advecting more stable air downward such that it stabilizes the local air over the block-onset region. Furthermore, the direct effect of diabatic heating is negligible as its magnitude is generally an order of magnitude smaller than other effects in the static stability budget. Nevertheless, the indirect effect of diabatic heating, manifested as the advection of low static stability by diabatically forced upper-tropospheric outflow, greatly favors blocking onsets by destabilizing the air upstream block-onset region.

Li Dong et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1038', Anonymous Referee #1, 26 Oct 2022
    • CC1: 'Reply on RC1', Li Dong, 29 Oct 2022
    • CC2: 'Reply on RC1', Li Dong, 17 Nov 2022
    • AC2: 'Reply on RC1', Hairu Ding, 31 Jan 2023
  • RC2: 'Comment on egusphere-2022-1038', Anonymous Referee #2, 23 Nov 2022
    • CC3: 'Reply on RC2', Li Dong, 23 Nov 2022
    • AC1: 'Reply on RC2', Hairu Ding, 31 Jan 2023

Li Dong et al.

Li Dong et al.


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Short summary
This paper investigated the spatial and temporal variations of static stability prior to blocking onsets. It quantified the relative contributions of static stability versus absolute vorticity in that processes. Based on the budget analysis, the decreasing static stability is found to be taken into account by the roles of horizontal advection, vertical advection, stretching effect and diabatic heating. In particular, the indirect effect of diabatic heating assumes an important role.