## RFM Viewing Geometries |

## IntroductionThe definition of viewing geometry depends on which of three possible representations of the atmosphere are used. In order of increasing complexity these are:- Homogeneous
- Plane-parallel (1D)
- Circular (2D)
The homogeneous case, suitable for representing lab measurements or horizontal paths in the free atmosphere, is trivial and, apart from temperature, pressure and composition, the only additional parameter required to define the 'view' is the path length. For such cases, usually the only calculation required is of path transmittance. The plane-parallel representation treats the atmosphere as a set of horizontal layers. The viewing direction is specified by secθ (equivalent to an 'air mass factor'), where θ is the angle with respect to the vertical. Provided that θ represents the viewing angle at the surface rather than the satellite, this generally provides an acceptable simulation for nadir-viewing instruments, at least for secθ≲2 (or θ≲60°) The circular atmosphere is the most general case, allowing for the curvature of the earth, refraction and field-of-view convolution. The curvature is assumed fixed over the path length, i.e., profile levels forming concentric circles in the viewing plane. Viewing directions can be specified either as tangent heights (for limb-viewing) or elevation angles α relative to the observer horizontal (suitable for geometries which intersect the surface or upward views from within the atmosphere), in which case the observation altitude is also required. In principle, nadir-views for the circular (α=-90°) and plane-parallel (θ=0°) atmospheres are identical, refraction and curvature having no effect; however small differences arise from the different path integration methods.
When running the RFM, the basic viewing geometry is specified in the
driver table by the appropriate flag
in the ## Homogeneous Path
***FLG**section- Add
**HOM**flag, which tells the RFM that there is a single path segment to be modelled. ***TAN**(or***LEN**) section- Length of path, in km. In fact multiple path lengths can be calculated
at the same time, although I doubt you'll need this feature. The path
length is specified in km for consistency with the units used in the tangent
height definition for limb-viewing, but for
RFM v5.10 onwards you can also specify
alternative units, eg
**UNITS=CM**.
*ATM section of the driver table, either
using a single profile level
.atm file, and/or directly within the
*ATM section itself using a series
of PARAMETER=VALUE pairs, e.g. TEM=296, CO2=400
It is assumed that most homogeneous path calculations will be for transmittance
but radiance can also be calculated. Then it is necessary to consider the
boundary condition at the remote end of the path. By default, the RFM assumes
a space view, but by using the ## Plane Parallel
An advantage of this assumption is that Curtis-Godson equivalent paths can be calculated analytically from the atmospheric profile values, and are the same for all paths through a layer (for a given absorber) at any angle. In principle effects such as refraction and field-of-view convolution could be applied to such an atmosphere, but the RFM only applies these for the circular geometry. For basic ray calculations through a plane-parallel atmosphere it is necessary to specify the following in the driver table -
***FLG** - Either
**ZEN**(upward viewing) or**NAD**(downward viewing) flags. -
***TAN**(or***SEC**section) - secθ value of path(s), where θ is the angle to the vertical. Thus only values secθ ≥ 1 are allowed and secθ=1 specifies a vertical path.
NAD flag), then the
surface flag (SFC) is also required.
The RFM assumes, by default, that the surface has an emissivity of 1
(i.e., blackbody) and a temperature equal to the temperature at the base of
the atmosphere. These values may be altered using the
*SFC section of the driver table.
If an observer altitude is specified within the atmosphere
(
Flux calculations ( ## Circular Geometry
HOM,
NAD,
ZEN or
FLX flags) is for viewing a
circular atmosphere. This includes refraction and allows for
field-of-view convolution. It is necessary to include an altitude
profile in the *ATM section of
the
driver file since this is required to establish
the local curvature of the atmosphere for ray-tracing.
The ray path is specified
according to
how the -
***TAN**(usual case) - Path specified by
*refracted*, or actual, tangent height [km] -
***GEO** - Path specified by
*geometric*, or projected tangent height [km]. That is: the tangent height that would be expected in the absence of any refraction effects -
***ELE** - Path specified by elevation angle [deg] above horizontal from the
observer. Viewing from above the atmosphere (ie satellites)
this would be a negative number. This also requires the observer
altitude to be set (
**OBS**flag and***OBS**Section).
radius of curvature
for the atmosphere is defined for the altitude 0 km.
By default this is set to a typical earth value (6367.421 km, in
phyadj_dat.f90), but can be altered using the
RADCRV parameter in the
*PHY section of the
driver file.
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