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The work that will be
carried out
at South Pole is described in the following proposals, Longwave
Radiation Processes on the Antarctic Plateau and The
Surface of the Antarctic Plateau as a Natural Laboratory for Radiation
Processes.
Longwave Radiation Processes on The
Antarctic Plateau
Climatic processes involving longwave radiation will be studied at
South
Pole Station during the austral summer of 1999-2000 and a full year
2000-2001,
expanding on an experiment carried out in 1992. A Fourier transform
interferometer
(FTIR) will be aquired and adapted to measure atmospheric infrared
emission
at 1 cm-1 resolution over the spectral range 3-25 um, providing better
radiometric calibration and a wider spectral range than were available
in the 1992 experiment. The interferometer will also be configured to
measure
atmospheric transmission along horizontal paths over the spectral range
2-25 um.
Humidity and "Diamond Dust" in the Inversion Layer.
A tethered balloon carrying a frost-point hygrometer will be used to
measure
temperature and humidity from the surface, through the inversion layer
(~300-400m), up to 1 km. A balloon-borne sampler will also collect
diamond-dust
ice crystals froming at each level. The conditions under which diamond
dust forms, and the variation of its properties with altitude, will be
determine. A similar experiment will be carried out in blowing snow to
determine its vertical distribution. This information will be useful in
assessing the effects of diamond dust and blowing snow on the climate
of
the Plateau, as well as for choosing the apporpriate height for
astronomical
telescopes.
Spectral Emissivity and Surface Temperature of Snow.
These will be determine by an adaptation of a ground-based remote
sensing
technique developed for determining sea surface temperature and
emissivity.
The snow temperature will also be measured directly using a vertical
string
of closely space thermistors. Spectral emissivity will be related to
snow
grain size, density and surface structure (frost or crust), to test
conflicting
theoretical models.
Spectral Longwave Climatology.
The downward infrared radiance spectrum will be measured throughout the
year. Temperature and humidity retrievals will be performed using the
spectra
to provide information on boundary layer structure. Radiation fluxes
will
be integrated across several spectral bands to isolate the separate
contribution
of CO2, H2O, O3, and CH4/N2O. These values can be compared to results
of
GCM radiation codes. For the two years 1992 and 2001, a seasonal
climatology
of contributions to the downward longwave flux by each greenhouse gas,
and the surface longwave cloud radiative forcing in each spectral band
by each cloud type, will be developed.
Climatology of Cloud Properties.
Ground-based remote sensing techniques developed using the infrared
spectral
radiances for 1992 will be used to determine cloud base heights,
optical
thicknesses, and crystal sizes during 2001. A micropulse lidar will be
operated to evaluate the infrared-emission method developed for
retrieval
of cloud base height. Frequency distributions of cloud heights and
optical
thicknesses will be obtained for each season.
Radiation Modeling of the Antarctic Atmosphere.
The comparison of downward infrared radiance measurements with
line-by-line
radiative transfer calculations done for 1992 will be repeated in 2001,
with improved humidity measurement and radiometric calibration. The
results
may lead to improved representation of the foreign-broadened
water-vapor
continuum in atmospheric radiative transfer models.
The Surface of the Antarctic Plateau as a
Natural Laboratory
for Radiation Processes. in the Global Upper Troposphere: Water Vapor
and
Ice Clouds
Processes involving atmospheric longwave radiation, which occur
worldwide
in the upper troposphere and are important for global climate, can be
studied
at the surface of the Antarctic Plateau. Such studies offer some
advantages
over aircraft and laboratory experiments. A Fourier-transform
interferometer,
designed to measure atmospheric infrared emission with a spectral
resolution
of 1 cm-1, will be modified to measure atmospheric transmission along
horizontal
paths over the spectral range 2-40 um. Fieldwork will be carried out at
South Pole Station during the austral summer of 1999-2000 and a full
year
2000-2001, expanding on an experiment carried out in 1992.
Laboratory for Upper Tropospheric Water Vapor.
Temperatures and water vapor pressures near the surface at the South
Pole
are similar to those in the upper troposphere at lower latitudes, where
cooling to space by the water vapor continuum is important for climate.
Continuum absorption coefficients for water vapor have not been
accurately
quantified at low temperatures, because of the difficulty of long-path
laboratory experiments. Atmospheric transmission will be measured
through
clear horizontal optical paths of up to 1 km, using a reflector at half
the distance. Using measurements of temperature and humidity along this
path, the continuum absorption will be inferred by taking the ratio of
the signal to that measured over a very short path, in clear infrared
"mini-windows"
between spectral lines after removing the absorption from nearby
spectral
lines. Measurements in summer and winter will permit investigation of
the
temperature dependence of the continuum from -20 dgrees C to -70
degrees
C. The results may lead to improved representation of the
foreign-broadened
water vapor continuum absorption in atmospheric radiative transfer
models.
Laboratory for Ice Clouds.
The near-surface atmosphere over the Antarctic Plateau often contains
numerous
ice particles, either "diamond-dust" ice crystals (long hexagonal
columns
of flat hexagonal plates) or nearly spherical grains of blowing snow.
These
two types of ice crystals occur under distinct meteorological
conditions
and offer the possibility of studying the radiative properties of ice
clouds,
particularly the effect of particle shape, at the surface rather than
from
aircraft. The numbers, sizes and shapes of ice crystals will be
measured
along the same horizontal optical path over which spectral transmission
is measured. The transmission measurements will be compared to model
calculations,
examining the radiative effects of spherical versus nonspherical ice
particles.
For computation of radiation absorption and scattering, the accuracy of
rpresenting a long column or a thin plate by an assembly of spheres,
rather
than by a single "equivalent" sphere, will be assessed.
The proposed work is complementary to a field experiment with
the same
PIs, recently approved by NSF's Office of Polar Programs (OPP), Longwave
Radiation processes on the Antarctic Plateau. Under the OPP grant,
we will study spectral emissivity of snow and cloud radiative forcing,
construct climatologies of spectral longwave radiation and cloud
optical
depths and particle sizes, compare atmospheric longwave emission to
line-by-line
claculations, and measure humidity in the boundary layer. The proposed
work will take place during the same field seasons as the OPP work and
will use some of the same instrumentation.
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