Simultaneous occurrence of polar stratospheric clouds and upper-tropospheric clouds caused by blocking anticyclones in the Southern Hemisphere M. Kohma and K. Sato Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
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.
Citation: Kohma, M. and Sato, K.: Simultaneous occurrence of polar stratospheric clouds and upper-tropospheric clouds caused by blocking anticyclones in the Southern Hemisphere, Atmos. Chem. Phys., 13, 3849-3864, doi:10.5194/acp-13-3849-2013, 2013.