Solar Radiation Processes on the East Antarctic Plateau

Stephen Hudson

General Examination

7 June 2005

Outline

• Introduction and Motivation۰ Antarctic geography۰ Some solar radiation processes۰ Satellite observations

• Data and Models

• Proposed Research

• Summary

The East Antarctic Plateau is a large, spatially-homogenous land mass.

• Areas:۰ Antarctica ~14106 km2,

about the same as the US and western Canada

۰ East Antarctic Plateau ~10106 km2, slightly larger than the US

• Elevation:۰ 2-4 km on the Plateau۰ Nearly flat on km scale

0

180

S

C

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

Clear Cloudy

Top of atmosphere shortwave cloud radiative forcing indicateshow much more or less sunlight is absorbed by the atmosphere-snow system because of clouds: SWCRFTOA = A - B

A B

Solar radiation interacts with atmospheric gases, clouds, and the surface

Clear Cloudy

Surface shortwave cloud radiative forcing indicates how much more or less sunlight is absorbed by the snow surface because of clouds.

Solar radiation interacts with atmospheric gases, clouds, and the surface

Clear Cloudy

Some wavelengths of sunlight are absorbed in a clearatmosphere, mainly by O3, CO2, and H2O.

Satellites measure radiance, but energy balance applications require fluxes.

• Sometimes users assume snow is Lambertian: F = I

• 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 Models۰ Description of Dome C data۰ 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.

045

225

000

180

315

135

270 90

2

0.9

1

3

2005/01/27 22:48 o = 86.0 , = 800 nm• 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

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 azimuthally-averaged 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 Research۰ Analyze and parameterize data۰ Model TOA BRDF۰ Estimate SWCRF۰ Examine the effect of absorbing gases

• Summary

I will parameterize the BRDF data as a function of orand mim

Measured Modeled

• Early results are promising for o<70°, r<55°• More anisotropic data yield poorer results

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 leastsquares

• May try using neuralnetworks if this methoddoes 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.

• 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 forward-scattering than snow grains

• I will try to explain these observations with DISORT

Clouds over snow can significantly change the TOA BRDF.

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