CARIBIC uses a passenger jet airliner, which means that the altitude of our research flights (apart from ascending and descending) is about 10 to 12 km. This is the altitude at which modern jet aircraft function optimally.
Interestingly just at this altitude, we cruise in the UTLS region (Upper
Troposphere-Lower Stratosphere region) for most of the aicraft movements.
In the tropics we clearly fly in the free tropical troposphere well below
Didier Hauglustaine has clearly summarized the importance of the UTLS
region in a brief article in the IGAC newsletter Issue number 26, June 2002.
. . . The upper-troposphere / lower-stratosphere (UTLS) is still considered as
a key region
of the atmosphere as far as composition and climate interactions are concerned for the
Radiative forcing by greenhouse gases such as
water vapor and ozone remains especially
sensitive in the UTLS to concentration changes, due to large temperature contrast with the
The UTLS is the layer in which stratospheric
and tropospheric air are mixed. The exchange
between these atmospheric domains controls the influx of tracers into the stratosphere
(including water vapor and long-lived greenhouse gases), and the O3 and NOx flux from
the stratospheric reservoir down into the troposphere.
Chemistry in the lower stratosphere is very
sensitive to temperature changes and,
at high latitudes, to the presence of Polar Stratospheric Clouds (PSCs). Stratospheric
ozone concentration and temperature changes are directly coupled providing an important
climate-chemistry interaction. The UTLS also coincides with the height of transition between
positive and negative trends as detected by ozone sondes.
Due to the influence of rapid convection
within cloud structures and the large-scale
vertical transport associated with convergence, the imprints of lower tropospheric events
such as biomass burning and forest fires and several regional air pollution episodes are
also imposed on the upper troposphere.
In this relatively dry region, species like
peroxides and oxygenated hydrocarbons play an important role in generating HOx radicals. Heterogeneous chemical reactions
on cirrus clouds may also affect the budget of ozone and other species at these altitudes.