03-08-2009
Supplement to Pinker write-up
(First part dealt with shortwave (SW) radiative fluxes only)
Definitions of Surface Radiative Fluxes
Shortwave fluxes (SW)
Solar radiation (or shortwave radiation) from the sun in the spectral interval of
(0.3-4.0 µm) that reaches the Earth’s surface is the major source of energy for heating our
planet. At the mean distance of the earth from the sun (1.50 × 108 km) the incident
radiant flux density (irradiance) on a surface perpendicular to the solar beam is known as
the Solar Constant (about 1366 Wm2). At various locations of the earth, it is determined
by the earth–sun distance, and the solar zenith angle. It arrives at the surface as direct
solar and diffuse sky radiation due to scattering in the atmosphere.
Long-wave fluxes (LW)
The absorbed energy in the atmosphere re-radiates back to the surface and to
outer space in the form of long-wave (terrestrial) radiation (4.0-100.0 µm); part o the
absorbed energy at the surface is re-radiated back to the atmosphere and space.
Absorption of solar radiation in the atmosphere is mostly by ozone, water vapor, carbon
dioxide, oxygen and clouds.
The earth climate is a result of the balance maintained between the solar radiation
absorbed by the atmosphere/earth system (gain) and the emission of the terrestrial
radiation back to space (loss). This balance is also referred to as net radiation or
radiation balance, composed of net solar radiation and net long-wave radiation.
1.
Longwave (LW) Radiation-General Information
Downwelling long-wave radiation (DLW) at the surface originates from
radiatively active gases in the atmosphere and depends on the vertical profiles of
temperature, gaseous absorbers and clouds. Gaseous emission/absorption is present in
specific wavebands in the range 0.3–30μm, whereas emission from clouds corresponds to
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black body radiation at the temperature of the cloud base. The intensity of the thermal
emission from a cloud varies with its temperature and the optical thickness of the cloud.
From cloudless skies, more than half the long-wave flux received at the ground
comes from gases in the lowest 100 m, and roughly 90% from the lowest kilometer.
Cloud contributions are mainly from the atmospheric window region (8.0-13.0 µm), and
the relevant cloud parameters are cloud base height location and temperature, emittance,
and cloud amount. Relative importance of cloud contribution decreases with moister
atmospheres since the transparency of the window decreases due to water vapor
continuum absorption. Consequently, DLW is often estimated as a function of a bulk
atmospheric temperature (in Kelvin) approximated by air temperature at the ground and
an estimated broadband atmospheric emissivity. The upwelling LW flux may be
calculated from information on surface temperature and emissivity. Long-wave
techniques could be characterized as being of two distinct types: flux calculation
techniques, which use radiative transfer models to calculate the DLW directly from
retrieved profiles of temperature and water vapor and estimates of cloud fraction and
cloud base altitude (e.g., Gupta et al.,1992) and flux inference techniques which use
direct inference from TOVS or HIRS radiance observations, and statistical inference
scheme based on radiance-flux relationship developed with detailed radiative transfer
models and large number of observed atmospheric soundings (for clear sky and overcast
conditions) with some built in cloud height and base information. It is often assumed that
the long-wave emissivity of most natural surfaces is unity, so that upwelling LW can be
estimated from knowledge of the surface temperature.
2.
Specific issues to High Latitudes
Most of the information on LW fluxes at high latitudes comes from measurements
over the Arctic and Antarctic. There are almost no observations of radiative fluxes (SW
or LW) over high latitude water bodies. Therefore, this section will highlight issues
unique to high latitudes as identified from the available observations. Not all of these
findings would be applicable over water; yet, issues related to LW radiation at high
latitudes over all type of surfaces have some commonality.
In Polar Regions long wave radiation is a very important component of the
surface radiation balance. It is the primary mechanism through which the loss of energy
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occurs, resulting in strong and frequent surface inversions. Water vapor plays a large role
in heating the Earth but at high latitudes, water vapor amounts are small and not well
known. The high albedo of the snow- and ice-covered polar regions together with the
large loss of long-wave radiation through the clear and dry atmosphere, result in a net
loss of radiation in most months of the year.
Less is known about high latitudes clouds than on clouds at lower latitudes. Most
are ice clouds with low optical thickness. It is difficult to quantify them from space due to
issues related to low solar zenith angles and lack of contrast over snow and ice. Even less
is known on aerosol effects on high latitude clouds. Recent satellite measurements on
aerosol changes in clouds of the Northern Slope of Alaska reveal that enhanced aerosol
concentrations alter the micro-physical properties of Arctic clouds in a process known as
the "first indirect effect", (Lubin and Vogelmann, 2006). The Arctic region experiences
significant influx of anthropogenic aerosols from industrial regions of lower latitudes.
The study of Lubin and Vogelmann (2006) found that anthropogenic aerosols lead to an
increase of about 3.4 W/m2 in the earth surface long-wave radiation. Therefore, inference
schemes to derive radiative fluxes from satellite observations should account for aerosol
effects.
3.
Selected results from observations on LW fluxes at high latitudes
Pietroni et al. (2008) analyzed sets of long-wave radiation data (incoming and
outgoing) from two Antarctic sites at same latitude but at different altitudes. At a coastal
site (Halley) for years 2003 and 2004 and at a continental site (Concordia) for 2005.
At Halley, during the summer, the net long-wave radiation ranges between -100
W/m2 and 40 W/m2, while in winter it ranges between -40 W/m2 and 30 W/m2.
In both cases the peak of the occurrence distribution is around 0 W/m2.
At Concordia the long wave radiation distribution of the fluxes
Shows a plateau between -75 W/m2 and -50 W/m2 in summer and a
Peak in the occurrence distribution at -25 W/m2 in winters. They conclude that
the differences in long wave radiation and long wave occurrence distribution is
strongly related to the type and amount of clouds cover observed at each station both
in summer and winter.
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Since March 1982 surface radiation balance measurements have been carried out
at the German Antarctic Georg-von-Neumayer Station of the Alfred Wegener
Institute for Polar and Marine Research (AWI). Daily averaged radiation data are
available from 13 March 1982 till 28 February 1994. They can be retrieved directly via
World Wide Web: http://www.bremerhaven.de/sixcms/detail.php?id=13742
4.
Selected attempts to parameterize DLW at high latitudes
At low latitudes, many attempts have been made to propose simple
parameterization of LW in terms of readily observable parameters (Idso and Jackson
(1969); Brutsaert (1975); Prata (1996)). This is feasible since most of the moisture is
concentrated close to the ground and surface temperature is a good indicator of moisture
conditions. Such an approach is not valid at high latitudes. Pirazzini et al. (2000) used
ground-based data collected at Ny-Ålesund (Svalbard islands) during the ARTIST (Arctic
Radiation and Turbulence Interaction Study) experiment to study the effect of clouds on
the downward long-wave radiation, carried out during the spring of 1998 around the
Spitsbergen Island (78o-79o N). Various parameterizations including screen-level air
temperature, water vapor pressure and cloud cover index as independent variables were
tested for clear sky and all-sky conditions. Two parameterization schemes of downward
longwave radiation have been derived and their performances were found to be equally
good.
Several authors have recently evaluated the accuracy of the most used
parameterizations of DLW radiation data in Polar Regions. The focus of their studies was
the verification of the formulas with year-round data (König-Langlo and Augstein, 1994),
polar summer data (Key et al., 1996) or polar winter/late-autumn data (Guest, 1998,
Makshtas et al., 1999). The parameterization of König-Langlo and Augstein (1994)
reproduced the observations with root mean square (RMS) deviations of less than 16
W/m2
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5.
Satellite estimates of DLW at high latitudes
Need to complete
6.
Results from numerical models
Observations of LW fluxes at high latitudes are also needed to verify climate and
NWP models. As shown in Figure 1 (Wild et al. 2005), there is a large discrepancy
between model estimates of DLW and ground observations over the poles.
Downward longwave radiation (DLW) climatologies have been assessed against ground
observations in four GCMs with four independent radiation schemes and in the ERA
using direct observations. Based on a total of 45 worldwide distributed observation sites
from the Global Energy Budget Archive (GEBA) and Baseline Surface Radiation
Network (BSRN) databases, a tendency was found in the GCMs to underestimate the
DLW. The best estimate given for global mean DLW is 344 W/m2, a value higher than
typically found in GCMs. However, it has been shown that this underestimation is not
uniform over the globe, but depends systematically on the prevailing climatic conditions.
Significant underestimates are found at observation sites in cold and dry climates with
low DLW emission. This underestimation gradually diminishes at sites with more
moderate climates, while at sites with warm and humid atmospheres and high DLW
emission the biases are small or even reversed in some of the models. This implies that
the meridional gradient of DLW in the GCMs is excessively strong. There is some
evidence that the underestimate of DLR for cold, dry climates is less serious for the more
recent radiation codes because the Edwards and Slingo (1996) radiation scheme includes
the CKD formulation of the water vapor continuum, which leads to an increase in the
DLR for such conditions. This result is consistent with Iacono et al. (2000), who found a
substantial increase in the DLR at high latitudes when they included the Rapid Radiative
Transfer Model radiation code, which uses the CKD continuum, in the National Center
for Atmospheric Research Community Climate Model. Further comparisons between
models and observations, particularly at high latitudes, are needed in order to determine
the source of the errors discussed here.
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Figure 1.
Annual cycles of model-calculated and observed DLR (W/m2) at some of
the most reliable high-latitude sites, mid-latitude sites,
and low-latitude sites: model calculations by ECHAM3 (dotted) and
ECHAM4 (dashed), and observed (solid).
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REFERENCES
Brutsaert, W., 1975: On derivable formula for long-wave radiation from clear skies.
Water Resour. Res., 11, 742-744.
Idso, S. B., and R. D. Jackson, 1969: Thermal radiation from the atmosphere. J Geophys.
Res., 74, 5397-5403.
Idso, S. B., 1981: A set of equations for full spectrum and
-5 to 12.5
radiation from cloudless skies. Water Resour. Res., 17 (2), 295-304.
Key, J. R., R. A. Silicox, R. S. Stone, 1996: Evaluation of the surface radiative flux
parameterizations for use in sea ice models. J Geophys. Res., 101, 3839-3849.
König-Langlo, G., E. Augstein, 1994: Parameterization of the downward longwave
radiation at the Earth’s surface in polar regions. Meteorol. Z., 3, 343-347.
Lubin, D. And A. M. Vogelmann, 2006. A climatologically significant aerosol longwave
indirect effect in the Arctic. Nature Magazine, 439 (7075), 453-456.
Prata, A. J., 1996. A new long-wave formula estimating downward clear-sky radiation at
the surface. Quart. J. R. Meteor. Soc., 122, 1127-1151.
Pietroni, I., P. Anderson, S. Argentini, J. King, 2008. Long wave radiation behaviour at
Halley and Concordia stations, Antarctica. SRef-ID: 1607-7962/gra/EGU2008-A03254 EGU General Assembly 2008.
Pirazzini, R., M. Nardino, A. Orsini, F. Calzolari, T. Georgiadis and V. Levizzani, 2000.
Parameterization of the downward longwave radiation from clear and cloudy
skies at NY Alesund (Svalbard). IRS 2000, International Radiation Symposium,
St. Petersburg, Russia, 24-29 July, 2000.
Wild, M., A. Ohmura, H. Gilgen, Jean-Jacques Morcrette, and A. Sling, 2001. Evaluation
of Downward Longwave Radiation in General Circulation Models. J. Of Climate,
14, 3227-3239.
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