Simultaneous occurrence of polar stratospheric clouds and upper-tropospheric clouds caused by blocking anticyclones in the Southern Hemisphere

Abstract

Previous studies have reported that polar stratospheric clouds (PSCs) are frequently observed simultaneously with upper-tropospheric clouds (UCs) in the Southern Hemisphere. However, it has not yet been examined whether the UCs that simultaneously occur with PSCs are actually located below the height of the tropopause, which is modified by tropospheric disturbances. Furthermore, the mechanism of this simultaneous occurrence has not yet been clarified. This study statistically examines the simultaneous appearance of PSCs and UCs using the Cloud-Aerosol Lidar and Pathfinder Satellite Observation (CALIPSO) for the five austral winters of 2007–2011. From correlation analyses and statistical dependence tests, it is shown that the simultaneous occurrence frequencies of clouds with an altitude range of 15–25 km and 9–11 km are significant. The analyses based on tropopause-relative altitude suggest that the occurrence frequency of clouds at altitudes higher than 6 km above the local tropopause (i.e., PSCs) is significantly correlated with that of clouds around and slightly above the tropopause. These results indicate that the UCs observed simultaneously with PSCs reported in previous case studies are likely located around and slightly above the tropopause rather than in the troposphere. It is also shown that the simultaneous occurrence of PSCs and UCs is frequently associated with blocking highs that have large horizontal scales (several thousand kilometers) and tall structure (up to a height of ~15 km). The longitudinal variation of blocking high frequency accords well with that of the simultaneous occurrence frequency of PSCs and UCs. This fact suggests that the blocking highs provide a preferable condition for such simultaneous occurrences. Moreover, the composition of PSCs is investigated as a function of relative longitude of the anticyclones including blocking highs. It was discovered that relatively high proportions of STS (super-cooled ternary solutions), Ice, and Mix2 (mixture of nitric acid trihydrate and STS) types are distributed towards the windward, near, and leeward side of anticyclones in westerly background flows, respectively

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006 to 2017

“…This zonal asymmetry in PSC occurrence is especially pronounced in JulySeptember with the maximum occurrence frequency at 0-90 • W longitude near the base of the Antarctic Peninsula. The enhancement in PSC occurrence at longitudes near the Antarctic Peninsula is due to frequent mountain wave activity in this region (Alexander et al, 2011(Alexander et al, , 2013Hoffmann et al, 2017b) and the large-scale upper tropospheric forcing events which are more frequent at these longitudes (Kohma and Sato, 2013).…”

“…An interesting feature seen in most years is the apparent merging of the upper tropospheric and lower stratospheric cloud layers in July and August associated with CALIOP observations of deep synoptic-scale clouds extending from the troposphere into the stratosphere to altitudes well above 20 km. These episodic events are likely produced by largescale adiabatic cooling along upwardly displaced isentropic surfaces above upper tropospheric anticyclones (e.g., Teitelbaum and Sadourny, 1998;Teitelbaum et al, 2001;Kohma and Sato, 2013). Distinctive tilted cloud layers formed in the cold phases of strong orographic gravity waves (e.g., Cariolle et al, 1989;Höpfner et al, 2006;Orr et al, 2015) are also occasionally observed to extend from the troposphere well into the stratosphere, primarily over the Antarctic Peninsula.…”

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006–2017

“…These episodic events are likely produced by large-scale adiabatic cooling along upwardly displaced isentropic surfaces above upper tropospheric anticyclones (e.g. Teitelbaum and 30 Sadourny, 1998;Teitelbaum et al, 2001;Kohma and Sato, 2013). Distinctive tilted cloud layers formed in the cold phases of strong orographic gravity waves (e.g.…”

“…The enhancement in PSC occurrence at longitudes near the Antarctic Peninsula is due to frequent mountain wave activity in this region (Alexander et al, 2011;Alexander et al, 2013;Hoffman et al, 2017) and the large-scale upper tropospheric forcing events which are more frequent at these longitudes (Kohma and Sato, 2013).…”

First quasi-Lagrangian in situ measurements of Antarctic Polar springtime ozone: observed ozone loss rates from the Concordiasi long-duration balloon campaign

“…Kohma and Sato (2013) demonstrate that, in 2010, 33 % of PSCs between 15 and 20 km were associated with the radiative cooling resulting from blocking anticyclones and clouds in the troposphere. Thus, the changing climate, which affects the presence of tropospheric cloud systems, also modulates stratospheric ozone loss.…”