Bug Status: Fixed

In the case when interpolation modes 4 or 5 are used and the top level of the input profile is low enough in the atmosphere that significant absorption occurs above this level for a given channel, the simulated radiance for this channel may exhibit very large errors (larger than for the other interpolation modes – see below for more details). If the input pressure profile fully spans the weighting functions of the channels being simulated this bug has no significant impact. A code fix is available: download this gzipped tar file, extract in the src/main/ directory and recompile RTTOV.

Note that even when the top pressure level is sufficiently high the updated code can result in small differences in simulated radiances and in the TL/AD/K output for some channels. The magnitude of the differences depends on the amount of absorption occurring above the top of the input profile, but where the amount of absorption above the top level is very small, the magnitudes are negligible compared to other errors. For example, if the top level is close to but not exactly 0.005hPa (the top level used in the optical depth regression) the differences in forward model radiances are of the order of 0.001% or less.

Some further explanation: in general the pressure levels of the input atmospheric profile should cover the full range of the atmosphere to which the channels being simulated are sensitive. In other words, the top of the input profile should lie above the channel weighting functions and the bottom of the profile should be at or below the surface pressure. If the top of the input profile is too low (so that there is significant absorption above the top level) then when opts%interpolation%spacetop is TRUE (the default) RTTOV will mitigate the resulting error by setting the optical depth of the top level to zero. This effectively stretches the top layer of the input profile to encompass the whole atmosphere between the second level and the space boundary, and both emission and absorption are taken into account for this layer in the integration of the RT equation (albeit without accurate information about the temperature of this layer). For input profiles with a sufficiently high top level this action of “spacetop” has a negligible impact on the simulations. If the spacetop option is FALSE (not generally recommended) then only absorption is accounted for above the top of the input profile (no emission). Naturally it is strongly recommended to supply an adequate input profile which fully spans the weighting functions of all simulated channels rather than to rely on the mitigating action of spacetop.