  Ground-based and satellite observations have hinted at the existence of polar
stratospheric clouds (PSCs) with relatively high optical depths, even if
optical depth values are hard to come by. This study documents a type II PSC
observed from spaceborne lidar, with visible optical depths up to 0.8.
Comparisons with multiple temperature fields, including reanalyses and results
from mesoscale simulations, suggest that intense small-scale temperature
fluctuations due to gravity waves play an important role in its formation,
while nearby observations show the presence of a potentially related type Ia
PSC farther downstream inside the polar vortex. Following this first case, the
geographic distribution and microphysical properties of PSCs with optical
depths above 0.3 are explored over Antarctica during the 2006 and 2007 austral
winters. These clouds are rare (less than 1% of profiles) and concentrated over
areas where strong winds hit steep ground slopes in the Western Hemisphere,
especially over the peninsula. Such PSCs are colder than the general PSC
population, and their detection is correlated with daily temperature minima
across Antarctica. Lidar and depolarization ratios within these clouds suggest
they are most likely ice-based (type II). Similarities between the case study
and other PSCs suggest they might share the same formation mechanisms.
