RFM Spectral Grids | ||
Contents
| 15JAN24 |
It is emphasised that RFM performs monochromatic radiative transfer calculations at a set of discrete spectral points, whereas the 'real' spectrum is a continuous function.
If the RFM grid is fine enough to capture the underlying spectral structure of the real atmosphere, then e.g., joining up the RFM output points with straight line segments will be a reasonable approximation to the 'real' spectrum. But if the RFM calculations are on too coarse a grid, complete spectral lines may be missed.
Other options are:
that for the Earth's atmosphere, in the mid-infrared (around 10μm, or 1000
cm-1, pressure broadening dominates in the troposphere, Doppler
broadening in the mesosphere, with a transition in the stratosphere.
In terms of actual line width, pressure broadening is approximately 0.1
cm-1 at surface pressure. Since the width in wavenumber (or
frequency) space corresponds directly to energy uncertainty, set by collisionaly
frequency, this number approximately holds for all molecules at all wavelengths,
and varies proportional to pressure (with some additional
temperature dependence).
Doppler broadening, on the other hand, depends on the average speed of the
molecules, which is of the order of 10-6 the speed
of light (i.e. 300 m/s compared to 3x108 m/s), consequently
the Doppler width is approximately 10-6 x the line centre frequency
(it also depends on temperature and molecular mass but these generally
vary by less than an order of magnitude). Thus, for the mid-infrared
(1000 cm-1) Doppler widths are of the order of 0.001 cm-1,
hence take over from pressure broadening for pressures less than 10 hPa.
However, for microwave spectra (10~cm-1) the Doppler width is
10-5 cm-1 and so Pressure broadening dominates up to
0.1 hPa, i.e. well into the mesosphere.
There are three basic ways of specifying the RFM spectral grid:
Cases (2) and (3) are actually very similar, the difference
being that for (2) the RFM calculates its own triangular ILS shape.
In these two cases the RFM performs calculations on a fine grid
set by default to 0.0005 cm-1 (2000 pts per cm-1), which
is suitable for limb-viewing in the mid-infrared (for Earth) but may need to
be increased for longer wavelengths and/or cooler atmospheres
(FIN flag +
*FIN section).
Since
Doppler width scales as wavenumber
(set by 1/DEFFIN set in rfmcon_dat.f90),
Spectral Structure
How fine a grid is required to capture atmospheric spectral structure?
An atmospheric absorption/emission lines represents transition between
rotational/vibrational energy levels within a molecule. Since these energy
levels are well defined, these spectral lines should be infinitely narrow.
However, there are two mechanisms which contribute to broadening of lines
Lorentz broadening is proportional to pressure, but Doppler
broadening is independent of pressure, so at the upper levels of any atmosphere
the line widths will be limited by Doppler broadening. Assuming molecular
velocities of the order of 300 m/s, a factor 106 smaller than the
speed of light, this suggests Doppler widths of the order of
10-6
ν0,
where ν0 is the wavenumber at the line centre. For
ν0=1000 cm-1 (i.e. 10μm, mid-infrared) this
gives a Doppler width of 0.001 cm-1.
Fine Grid
By default
this is 0.0005 cm-1 (or 2000 pts/cm-1, which
is determined by the typical
mid-infrared Doppler half-width of 0.001 cm-1
in the Earth's atmosphere.