Solar Radiation Processes on the
East Antarctic Plateau
Stephen Hudson
General Examination
7 June 2005
Outline
• Introduction and Motivation
۰ Antarctic geography
۰ Some solar radiation processes
۰ Satellite observations
• Data and Models
• Proposed Research
• Summary
The East Antarctic Plateau is a large,
spatially-homogenous land mass.
• Areas:
۰ 2-4 km on the Plateau
۰ Nearly flat on km scale
70
S
65
S
75
S
E
60
• Elevation:
S
12
0
E
C
W
0
12
۰ Antarctica ~14106 km2,
about the same as the US
and western Canada
۰ East Antarctic Plateau
~10106 km2, slightly
larger than the US
0
180
The surface of the Plateau is cold, dry
snow with roughness on decimeter-scales.
The surface of the Plateau is cold, dry
snow with roughness on decimeter-scales.
•
•
•
•
•
Maximum temperatures are below –10°C
Significant snowfall is rare
Clear-sky precipitation occurs almost daily
Frost is common at night
Properties of near-surface snow crystals do
not change much during the summer
Solar radiation interacts with atmospheric
gases, clouds, and the surface
Clear
Cloudy
Solar radiation interacts with atmospheric
gases, clouds, and the surface
Top of atmosphere shortwave cloud radiative forcing indicates
how much more or less sunlight is absorbed by the atmospheresnow system because of clouds: SWCRFTOA = A - B
Clear
A
B
Cloudy
Solar radiation interacts with atmospheric
gases, clouds, and the surface
Surface shortwave cloud radiative forcing indicates how much
more or less sunlight is absorbed by the snow surface because
of clouds.
Clear
Cloudy
Solar radiation interacts with atmospheric
gases, clouds, and the surface
Some wavelengths of sunlight are absorbed in a clear
atmosphere, mainly by O3, CO2, and H2O.
Clear
Cloudy
Satellites measure radiance, but energy
balance applications require fluxes.
• Sometimes users
assume snow is
Lambertian: F = pI
• Usually users
assume they know
the BRDF and use
that to calculate
Flux
Satellite identification of clouds over
Antarctica is unreliable.
• Minimal contrast between
clouds and snow in both
shortwave and longwave
regions
• They have similar
shortwave reflectance
• Antarctic clouds usually
have similar or higher
temperatures than the
surface
Newer platforms are improving cloud
detection, but deriving Antarctic SWCRF
from satellites is still not feasible.
Outline
• Introduction and Motivation
• Data and Models
۰ Description of Dome C data
۰ Discussion of DISORT and
ATRAD
• Proposed Research
• Summary
We measured the angular distribution of
radiance reflected from the Dome C snow.
• Used an ASD FR
spectroradiometer to
measure the reflected
radiance coming from
85 different angles
• Records radiance at
2150 wavelengths:
350—2500 nm; 3- to
11-nm FWHM
resolution
These measurements allow for the
determination of the BRDF of the snow.
• From the radiance
measurements we
determine R, the
equivalent-Lambertian
flux divided by the
actual reflected flux
• BRDF can be
calculated from R and
the albedo, which has
been measured
2005/01/27 22:48 o = 86.0,  = 800 nm
000
2
315
045
0.9
1
0.8
270
90
0.9
1
225
2
135
3
5
6
180
4
These BRDFs will be used as the lower
boundary in radiation models.
• I will model radiative transfer through the
atmosphere and clouds using DISORT or
ATRAD, depending on the application
• The measured BRDFs will be used as the
lower boundary condition so I will not need to
try to model radiative transfer in the snow
• I can then use the calculated TOA flux and
radiance to estimate the effects of clouds and
absorbing gases on the radiation budget
DISORT is a monochromatic radiative
transfer model.
• DISORT numerically solves the radiative
transfer equation through a plane-parallel
atmosphere with arbitrary upper and lower
boundary conditions and internal scattering
and absorption characteristics
• It uses the discrete ordinates method to handle
multiple scattering
• I will use DISORT to calculate narrow-band
TOA radiances and BRDFs for comparison
with Satellite observations
ATRAD is a spectral radiation model that
accounts for atmospheric absorption.
• ATRAD calculates fluxes and azimuthallyaveraged intensities in a plane-parallel
atmosphere over an arbitrary spectral interval
with high spectral resolution
• It uses the adding-doubling method to handle
multiple scattering and accounts for gaseous
absorption by fitting sums of exponentials
• I will use ATRAD to calculate broadband fluxes
to estimate SWCRF and the effect of absorbing
gases on the radiation budget
Outline
• Introduction and Motivation
• Data and Models
• Proposed Research
۰ Analyze and parameterize data
۰ Model TOA BRDF
۰ Estimate SWCRF
۰ Examine the effect of absorbing gases
• Summary
I will parameterize the BRDF data as a
function of o, r, f, , and mim.
• Early results are promising for o<70°, r<55°
• More anisotropic data yield poorer results
Measured
Modeled
My goal is to develop a parameterization
that will work for a wider range of the data.
• I may need to create separate functions for
large and small solar or viewing zeniths
• So far I have been fitting a Fourier-based
function using least
squares
• May try using neural
networks if this method
does not work out
TOA BRDF is modified by atmospheric
scattering and absorption of reflected light.
• With DISORT, I can
model the TOA BRDF
• The many satellite
overpasses of Dome C
allow for numerous direct
clear-sky comparisons
• If modeling works well, I can create a clear-sky
TOA BRDF parameterization for satellite users
Clouds over snow can significantly change
the TOA BRDF.
Clouds over snow can significantly change
the TOA BRDF.
• The presence of a cloud over snow enhances the
forward scattering peak of the BRDF and
reduces the nadir reflectance
• We saw this at Dome C above fog; it has also
been observed by satellite (MISR)
• It is an unexpected observation because the
small cloud particles should be less forwardscattering than snow grains
• I will try to explain these observations with
DISORT
Satellite estimates of SWCRF are least
accurate over the polar regions.
• If clear scenes cannot be accurately
identified then CRF cannot be determined
Sohn and Robertson
1993 BAMS.
Suggest TOA
SWCRF is small for
Antarctica, but show
estimates there are
uncertain
We can use surface-based data to determine
cloud properties then estimate SWCRF.
• Continuous observations of the emitted IR
spectrum were made with a PAERI in summers
2000-01 at South Pole and 2003-04 at Dome C
• From these data cloud particle size and optical
depth can be estimated (Mahesh; Turner)
• These retrievals are being performed by Walden
• I will use the distribution of cloud optical depths
along with ATRAD modeling results of surface
and TOA fluxes for various optical depths to
estimate SWCRF over the Antarctic Plateau
The atmosphere absorbs some of the solar
energy passing through it.
• This plot shows the global-mean absorption of
sunlight by atmospheric gases; the most
important are H20, O2, O3, and CO2
• The absorptivity may
be less for Antarctica
because the high
surface elevation and
cold atmosphere lead
to much lower water
vapor concentrations
Atmospheric absorption of sunlight over
Antarctica may be greater than elsewhere.
• In summer, daily-mean solar fluxes are greater
over Antarctica than they are anywhere else on
the planet at any time of year
• The East Antarctic Plateau has the highest
summertime albedo of any place on Earth
• Solar zenith angles in Antarctica are larger than
they are at lower latitudes
I will use ATRAD to investigate
atmospheric absorption over Antarctica.
• Which gases are most significant and is this
different from other locations because of the low
water-vapor concentrations?
• How much more significant is absorption by
each gas because of the large amount of
reflected light?
– May not be very important for H2O bands
• What effect does decreasing O3 concentrations
and increasing CO2 concentrations have?
Summary of project goals
• Provide an accurate and comprehensive
parameterization for snow-surface BRDF and
for clear-sky TOA BRDF for East Antarctica
• Model the effect clouds on the TOA BRDF for
the variety of clouds observed over the region
• Explain the significant enhancement of the
forward peak in the BRDF caused by clouds
Summary of project goals
• Use the modeled TOA BRDFs along with
PAERI-derived cloud data to estimate SWCRF
for East Antarctica
• Determine the effects of atmospheric gases on
the solar radiation budget of East Antarctica, see
how these effects might change and how they
are different from other regions