Observations of surface radiation and stratospheric processes at Thule Air Base, Greenland, during the IPY

Ground-based measurements of atmospheric parameters have been carried
out for more than 20 years at the Network for the Detection of Atmospheric
Composition Change (NDACC) station at Thule Air Base
(76.5°N, 68.8°W), on the north-western coast of Greenland. Various instruments
dedicated to the study of the lower and middle polar atmosphere
are installed at Thule in the framework of a long standing
collaboration among Danish, Italian, and US research institutes and universities.
This effort aims at monitoring the composition, structure and
dynamics of the polar stratosphere, and at studying the Arctic energy
budget and the role played by different factors, such as aerosols, water
vapour, and surface albedo. During the International Polar Year (IPY),
in winter 2008-2009, an intensive measurement campaign was conducted
at Thule within the framework of the IPY project “Ozone layer and UV
radiation in a changing climate evaluated during IPY” (ORACLE-O3)
which sought to improve our understanding of the complex mechanisms
that lead to the Arctic stratospheric O3 depletion. The campaign involved
a lidar system, measuring aerosol backscatter and depolarization ratios
up to 35 km and atmospheric temperature profiles from 25 to 70 km altitude,
a ground-based millimeter-wave spectrometer (GBMS) used to derive
stratospheric mixing ratio profiles of different chemical species
involved in the stratospheric ozone depletion cycle, and then ground-based
radiometers and a Cimel sunphotometer to study the Arctic radiative
budget at the surface. The observations show that the surface radiation
budget is mainly regulated by the longwave component throughout most
of the year. Clouds have a significant impact contributing to enhance the
role of longwave radiation. Besides clouds, water vapour seasonal changes produce the largest modification in the shortwave component at the surface,
followed by changes in surface albedo and in aerosol amounts. For
what concerns the middle atmosphere, during the first part of winter
2008-2009 the cold polar vortex allowed for the formation of polar stratospheric
clouds (PSCs) which were observed above Thule by means of the
lidar. This period was also characterized by GBMS measurements of low
values of O3 due to the catalytic reactions prompted by the PSCs. In mid-
January, as the most intense Sudden Stratospheric Warming event ever
observed in the Arctic occurred, GBMS and lidar measurements of O3,
N2O, CO and temperature described its evolution as it propagated from
the upper atmosphere to the lower stratosphere.