Australian National Antarctic Research

    Expeditions1988, 1996, 2000

 

 

 

 

Results and data dissemination:

 

Ten publications resulting from our UW/ANARE collaboration:

 

The relative importance of clouds and sea ice for the solar energy budget of the Southern Ocean

Journal of Climate, Volume 20, No.6, pp. 941-957, 2007.

Melanie F. Fitzpatrick, and Stephen G. Warren

Abstract. The effects of clouds and sea ice on the solar radiation budget are determined for the Southern Ocean around Antarctica between latitudes 50o and 80o S. Distributions of cloud optical depth are used, together with distributions of surface albedo, to estimate the geographical and seasonal variations of shortwave irradiance and cloud radiative forcing at the surface, both for the present climate and for altered surface and cloud conditions. Poleward of 68oS in spring, ice causes a greater reduction of solar energy input to the surface than does cloud. However, in summer the clouds are more important than ice at all latitudes in the Southern Ocean.

In the present climate the clouds are optically thicker over open water than over sea ice, suggesting a possible negative feedback if the sea ice area shrinks with climatic warming. Compared to the present climate in spring, removing sea ice results in an increase in irradiance reaching the ocean surface, regardless of the type of cloud remaining. However, in summer the removal of ice results in higher irradiance at the surface only if clouds remain unchanged. If clouds become as thick as those presently over the ocean at 55o-60oS, irradiance reaching the ocean surface in summer decreases poleward of 65oS.

A PDF reprint of this paper is available here.
The definitive version of the paper is on the AMS Journals' website.
All materials ? Copyright 2005 AMS.



Surface Albedo of the Antarctic Sea-ice Zone

Journal of Climate, Volume 18, No.17, pp. 3606-3622,  2005.

 

Richard E. Brandt, Stephen G. Warren, Anthony P. Worby, and Thomas C. Grenfell

 

Abstract.  In three ship-based field experiments, spectral albedos were measured at ultraviolet, visible, and near-infrared wavelengths for open water, grease ice, nilas, young grey ice, young grey-white ice, and first-year ice, both with and without snow cover.  From the spectral measurements, broadband albedos are computed for clear and cloudy sky, for the total solar spectrum as well as for visible and near-infrared bands used in climate models and for AVHRR solar channels.  The allwave albedos vary from 0.07 for open water to 0.87 for thick snow-covered ice under cloud.  The frequency-distribution of ice types and snow coverage in all seasons is available from the project on Antarctic Sea Ice Processes and Climate (ASPeCt).  The ASPeCt dataset contains routine hourly visual observations of sea ice from research and supply ships of several nations using a standard protocol.  Ten thousand of these observations, separated by a minimum of 6 nautical miles along voyage tracks, are used together with the measured albedos for each ice type to assign an albedo to each visual observation, resulting in "ice-only" albedos as a function of latitude for each of five longitudinal sectors around Antarctica, for each of four seasons.  These ice albedos are combined with 13 years of ice-concentration estimates from satellite passive microwave measurements to obtain the geographical and seasonal variation of average surface albedo.  Most of the Antarctic sea ice is snow-covered, even in summer, so the main determinant of area-averaged albedo is the fraction of open water within the pack.

 

Links to selected Figures and Tables:

 

Spectral albedos of sea ice types (Tables and Figures)

 

Band albedos (allwave, AVHRR, GCM)  of sea ice types (Tables)

 

Antarctic sea ice albedo climatology by geography, season, and band (Tables)

A PDF reprint of this paper is available here.
The definitive version of the paper is on the AMS Journals' website.
All materials ? Copyright 2005 AMS.

Link to ASPeCt dataset description:

http://www.aad.gov.au/default.asp?casid=4302

Link to ASPeCt dataset request:

http://www.antcrc.utas.edu.au/aspect/siobservations/cdrom.html

 

Transmission of solar radiation by clouds over snow and ice surfaces, Part 2: Cloud optical depth and shortwave radiative forcing from pyranometer measurements in the Southern Ocean. 

Journal of Climate, Volume 18,  pp. 4637-4648,  2005.

Melanie F. Fitzpatrick, and Stephen G. Warren

Abstract.  Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of ?raw cloud transmittance? (trc), the ratio of downward irradiance of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-clouddroplet transition. This sparsely populated region of transmittances is referred to as the K?hler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60? is typically  250Wm2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. ?Effective? optical depths (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47?S, increasing to 24 in the region of maximum cloud cover at 58?S, and decreasing to 11 at 67?S near the coast of Antarctica.

A PDF preprint of this paper can be requested from: fitz@atmos.washington.edu
The definitive version of the paper will be on the AMS Journals' website.
All materials ? Copyright 2005 AMS.

 

Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces: A Parameterization in Terms of Optical Depth, Solar Zenith Angle, and Surface Albedo

Journal of Climate, Volume 17,  pp. 266-275,  2004.

Melanie F. Fitzpatrick, Richard E. Brandt, and Stephen G. Warren

Abstract.  A multilevel spectral radiative transfer model is used to develop simple but accurate parameterizations for cloud transmittance as a function of cloud optical depth, solar zenith angle, and surface albedo, for use over with different coefficients for each spectral interval. When the parameterization is applied to measurements of ??raw?? cloud transmittance (the ratio of downward irradiance under cloud to downward irradiance measured under clear sky at the same zenith angle), an ??effective?? optical depth t is inferred for the cloud field, which may be inhomogeneous and even patchy. This effective optical depth is only a convenient intermediate quantity, not an end in itself. It can then be used to compute what the transmittance of this same cloud field would be under different conditions of solar illumination and surface albedo, to obtain diurnal and seasonal cycles of cloud radiative forcing. The parameterization faithfully mimics the radiative transfer model, with rms errors of 1%?2%. Lack of knowledge of cloud droplet sizes causes little error in the inference of cloud radiative properties. The parameterization is applied to pyranometer measurements from a ship in the Antarctic sea ice zone; the largest source of error in inference of inherent cloud properties is uncertainty in surface albedo.

A PDF reprint of this paper is available here
The definitive version of the paper is on the AMS Journals' website.
All materials ? Copyright 2005 AMS.

 

Snowball Earth: Ice thickness on the tropical ocean

 Journal of Geophysical Research (Oceans), Volume 107, C10, 3167, doi:10.1029/2001JC001123,  2002.

Stephen G. Warren, Richard E. Brandt, Thomas C. Grenfell, and Chris P. McKay

On the tropical oceans of a neo-Proterozoic Snowball Earth, snow-free ice would have existed in regions of net sublimation. Photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear. The steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates: the heat flux from the ocean to the ice base, the latent heat of freezing to the ice base, and the solar energy absorbed within the ice. A preliminary study, using a broadband model for solar radiation and assuming a large freezing rate, had indicated that tropical ice might be only a few meters thick. Here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight. We use a spectral model for solar absorption, computing radiative transfer at 60 wavelengths, considering absorption by the ice, and scattering by bubbles. With the spectral model, the computed ice thickness is much greater. For a solar flux of 320 Wm2 at the equatorial surface and expected albedo of 0.5 for bare sea ice, we find that surface temperatures below 12oC generate ice layers too thick for photosynthesis (>100 m). If the albedo were as low as 0.4, thick ice would occur only for surface temperatures below 25oC, but such low temperatures would be difficult to maintain with such low albedo. For surface temperatures warmer than these limits, the ice becomes thin (<1 m) and is unlikely to represent a coherent ice layer. However, glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice-free patches.

A PDF preprint of this paper is available here
An edited version of this paper was published by AGU. Copyright 2002 American Geophysical Union.

 

Snow on Antarctic sea ice

Reviews of Geophysics, Volume 39, pp. 413-445 2001.

Massom, R.A., H. Eicken, C. Haas, M.O. Jeffries, M.R. Drinkwater, M. Sturm, A.P. Worby, X. Wu, V.I. Lytle, S. Ushio, K. Morris, P.A. Reid, S. Warren, and I. Allison

Abstract. Snow on Antarctic sea ice plays a complex and highly variable role in air-sea-ice interaction processes and the Earth?s climate system. Using data collected mostly during the past 10 years, this paper reviews the following topics: snow thickness and snow type and their geographical and seasonal variations; snow grain size, density, and salinity; frequency of occurrence of slush; thermal conductivity, snow surface temperature, and temperature gradients within snow; and the effect of snow thickness on albedo. Major findings include large regional and seasonal differences in snow properties and thicknesses; the consequences of thicker snow and thinner ice in the Antarctic relative to the Arctic (e.g., the importance of flooding and snow-ice formation); the potential impact of increasing snowfall resulting from global climate change; lower observed values of snow thermal conductivity than those typically used in models; periodic large-scale melt in winter; and the contrast in summer melt processes between the Arctic and the Antarctic. Both climate modeling and remote sensing would benefit by taking account of the differences between the two polar regions.

 

Other related publications: 

Warren, S.G., C.S. Roesler, and  R.E. Brandt:  Solar radiation processes in the East Antarctic sea ice zone. Antarctic Journal of the U.S., 32, 1999.

Brandt, R.E., C.S. Roesler, and S.G. Warren:  Spectral albedo, absorptance, and transmittance of Antarctic sea ice. Fifth Conference on Polar Meteorology and Oceanography, American Meteorological Society, Boston, 456-459, 1999.

Warren, S.G., C.S. Roesler, V.I. Morgan, R.E. Brandt, I.D. Goodwin, and I. Allison: Green icebergs formed by freezing of organic-rich seawater to the base of Antarctic ice shelves J. Geophys. Res. (Oceans), 98, 6921-6928.  1993.

Allison, I, R.E. Brandt, and S.G. Warren: East Antarctic sea ice; albedo, thickness distribution and snow cover J. Geophys. Res. (Oceans), 98, 12417-12429. 1993.