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Chapter 2. Radiation
IFS documentation Front Page
Chapter 1. Overview
Chapter 2. Radiation
Chapter 3. Turbulent diffusion and interactions with the surface
Chapter 4. Subgrid-scale orographic drag
Chapter 5. Convection
Chapter 6. Clouds and large-scale precipitation
Chapter 7. Land suface parametrization
Chapter 8. Methane oxidation
Chapter 9. Climatological data
REFERENCES
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2.5 Input to the radiation scheme
2.5.1 Model variables
Temperature values are needed at the boundaries of the layers, where the fluxes are computed. They are derived from the full level temperatures with a pressure weighted interpolation
|
(2.59) |
At the bottom of the atmosphere, either the surface temperature or the temperature at 2 m is used, while at the top of the atmosphere the temperature is extrapolated from the first full level and second half level temperatures.
2.5.2 Clouds
Cloud fraction, and liquid/ice water content is provided in all layers by the cloud scheme.
2.5.3 Aerosols
Horizontal distributions for four climatological types of aerosols (oceanic, desert, urban, and stratospheric background) are defined from T5 spectral coefficients, with fixed vertical distributions following Tanre et al. (1984).
2.5.4 Carbon dioxide, ozone and trace gases
Carbon dioxide, methane, nitrous oxide, CFC-11 and CFC-12 have constant volume concentrations of 353 ppm, 1.72 ppm, 0.31 ppm, 280 ppt, and 484 ppt respectively (IPCC/SACC, 1990).
Two climatologies are available for the ozone distribution. In the first one (NOZOCL = 0), the ozone mixing ratio
depends on height,
latitude, longitude and season. Its vertical distribution is assumed to
be such that its integral from 0 to the pressure 
is
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(2.60) |
The constants
and 
are related to the total amount of ozone
and the height of its maximum mixing ratio. They are imposed in terms of
a limited series of spherical harmonics (T10) for the geographical distribution
and a Fourier series for the seasonal variation. The total amount of ozone
was taken from London
et al. (1976) and the altitude of the maximum concentration was derived
from Wilcox and
Belmont (1977). Plots of these values can be found in the Appendix.
In the second climatology (NOZOCL = 1), the ozone mixing ratio 
depends on height, latitude and month, and is taken
from Fortuin and Langematz (1994).2.5.5 Ground albedo and emissivity
The background land albedo,
, is interpolated to the model grid from the
monthly mean values of a snow-free albedo produced for the combined 1982-1990
years. The albedo for that dataset was computed using the method of Sellers et al. (1996), but with new maps
of soil reflectance, new values of vegetation reflectance and the biophysical
parameters described in Los et al. (2000). More information on the
original data and plots of the monthly mean albedo are shown in
Chapter 9.Spectral albedos for parallel and diffuse radiation are needed by the radiative code. In addition, the surface energy balance equation (see Chapter 3 on vertical diffusion) needs a spectrally integrated parallel+diffused albedo, specified for each independent surface functional unit, tile. The procedure is summarized in Table 2.1. Over open water, the surface albedo for direct parallel radiation is a fit to low-flying aircraft measurements over the ocean given by Taylor et al. (1996)
|
(2.61) |
For sea ice, monthly values based on Ebert and Curry (1993) albedos for the Arctic Ocean are interpolated to the forecast time. The bare sea ice albedo value in Ebert and Curry is taken as a representative value for summer, and the dry snow albedo value is used for the winter months. Values for the Antarctic are shifted by six months. Separate values for visible and near-infrared spectral bands are used. The time-varying snow albedo (
, see Chapter
7), is used for the exposed snow tile only. Finally, the average of
the diffuse and parallel albedos are spectrally integrated for each tile.| Tile |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|---|---|---|---|---|---|---|---|---|
| Description |
Open sea |
Sea ice |
Interception layer |
Low vegetation |
Exposed snow |
High vegetation |
Shaded snow |
Bare ground |
| Diffuse albedo |
0.06 |
Ebert and Curry (1993) |
 |
 |
 |
 |
0.15 |
 |
| Parallel albedo |
Taylor et al. (1996) |
Ebert and Curry (1993) |
 |
 |
 |
 |
0.15 |
 |
| Window emissivity |
0.99 |
0.98 |
0.96 |
0.93-0.96 |
0.98 |
0.93-0.96 |
0.93-0.96 |
0.93-0.96 |
The thermal emissivity of the surface outside the 800-1250
spectral region is assumed to be 0.99 everywhere. In the window region,
the spectral emissivity is constant for open water, sea ice, the interception
layer and exposed snow tiles. For low and high vegetation and for shaded
snow the emissivity depends on the water content in the top soil layer.
Emissivity decreases linearly from 0.96 for soils at or above field capacity
to 0.93 for soils at or below permanent wilting point. The same formulation
is used for bare ground, except for desert areas (
), where a
value of 0.93 is used independently of the soil water content. Finally,
a broadband emissivity is obtained by convolution of the spectral emissivity
and the Planck function at the skin temperature.2.5.6 Solar zenith angle
Equations to compute the annual variation of the solar constant
, the solar declination 
and the difference
between solar time and official time can be found in Paltridge and Platt
(1976). These equations are used to give the cosine of the solar angle at
the ground. Because of the curvature of the earth, the zenith angle is not
quite constant along the path of a sun ray. Hence a correction is applied
to 
to give an average 
for the atmosphere:
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(2.62) |
where
is the earth radius and 
is the atmospheric
equivalent height. 
is fixed at
0.001277.Next Section
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05.04.2002 |
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