RFM Numerical Optimisations

30SEP17

The principle calculation unit is a 'path', defined as the segment of a tangent ray bounded by an atmospheric profile layer for a single gas. For example, if an atmosphere is defined at 0, 10, 20 .... 60 km, and a single CO2 radiance spectrum from tangent height 35 km is required, there will be 3 paths corresponding to the altitudes 35-40, 40-50 and 50-60 km. If H2O is added, this will double the number of paths to 6. Adding horizontal structure (GRA Flag) will also double the number of paths since the upward and downward parts of the ray path are no longer identical. If a FOV convolution is required the number of paths is again increased by a factor corresponding to the number of tangent rays required to specify the FOV.

If processing speed rather than accuracy is a requirement, several optimisations are available which either reduce the number of calculated paths, or speed up the spectral calculation for each path.

Starting with the simplest, the following options are suggested:

.atm File
The number of paths is also determined by the height and resolution of the atmospheric profile, i.e. the altitude grid specified in the first file named in the *ATM section of the Driver Table. It is generally adequate to set an upper limit of 120km for the earth's atmosphere (unless performing specialist high-altitude calculations) and to increase the profile spacing in the upper levels. Rather than reformat the standard atmosphere files, the suggested method is to prepare a single height profile in the .atm file format which contains the required subset of the US Std atmosphere altitudes (file hgt_std.atm) and insert this as the first file in the *ATM section. Even this is probably too fine a resolution at high altitudes for nadir-viewing.

*TAN Section
Compute as many tangent heights as possible at the same time. This is because many path sections in the upper atmosphere have similar Curtis-Godson p,T values so the absorption coefficients only need to be calculated for one path segment and will be automatically scaled by the absorber amount for similar segments (the CLC Flag switches off this scaling).
For plane-parallel atmospheres (NAD and ZEN flags) this makes no difference as the RFM already uses the fact that all slant paths have mathematically identical CG parameters.

.fov File
The details of the FOV tabulation critically affect the number of calculated paths. For single tangent heights, the number of paths will be proportional to the number of points required to specify the FOV. For multiple, regularly-spaced tangent heights, efficiency can be gained by using a FOV tabulation on the same spacing as the tangent heights which allows individual tangent paths to be reused and adding the FVZ Flag. For example, the MIPAS trapezium-shape response (0,1,1,1,0) at relative altitude coordinates (-2,-1.4,0,+1.4,+2) can be replaced by an 'equivalent' shape (0,y,1,y,0) where y=0.2798 for (-3,-1.5,0,+1.5,+3), or y=0.2302 for (-2,-1,0,+1,+2). For the 1.5km FOV spacing, a series of n limb views at 3 km increments will require 2n+3 tangent paths instead of 5n.

REJ Flag
Sets lower limits on the path-adjusted strength of lines which are included in the spectral calculation. Different criteria will be appropriate for different spectral regions and tangent heights so it is advisable to compare results with and without this option for specific cases.

WNG Flag
Excludes all line wings beyond 200 line-widths where this is narrower than the normal fine-mesh calculation limit of 1-2 cm-1. This reduces the number of line-shape calcuations for weak lines or high altitudes.

SHP Flag
By default the RFM uses the Voigt lineshape throughout (=convolution of Lorentz and Doppler lineshapes). However if it is known that either Doppler or Lorentz are appropriate, the calculation is quicker if these lineshapes are used explicitly. (Warning: although the Lorentz line shape may describe the lineshape at low tangent altitudes it is often necessary to allow for the Doppler 'core' of lines when performing calculations using the limb-viewing geometry)

FIN Flag
If an ILS convolution is required (set by the ILS or AVG Flags) the default is to perform the calculation at 2000 pts/cm-1 (i.e. 0.0005 cm-1 grid) which is adequate to resolve Doppler lines at high altitudes (required for limb-viewing calculations), but for nadir-viewing calculations 1000 pts/cm-1 is usually adequate. The user can specify a coarser resolution to speed up the calculation. A warning message is printed in the rfm.runlog file if the user-specified resolution seems too coarse.

GRD Flag
The user can supply irregular grids (.grd files) to specify the spectral points at which the lineshapes, absorption coefficients, radiative transfer etc are calculated instead of using every point on the regular fine grid. The path radiances, absorptions and transmissions are then interpolated to the regular grid before output (also prior to any ILS convolution). Alternatively, the GRD flag can be used in conjunction with .tab files to calculate radiances only at the grid points contained in the files.

LUT Flag
The user can supply Look-Up Tables (.tab files) to supply pretabulated absorption coefficients for specific absorbers and spectral ranges, instead of using a line-by-line calculation.