Vertical discretisation for advanced sounder fast radiative transfer models: grid refinement for RTIASI
SHERLOCK, Vanessa
Convergence and representativity error reduction tests have been performed for the current RTIASI 43 layer pressure grid. A minimum set of additional levels have been identified defining a 58 layer vertical grid for future fast radiative transfer model developments. Grid refinement has been made in the mid and upper troposphere to ensure accurate modelling of radiances in the H2O nu 2 band - the 58 layer grid reduces vertical discretisation errors to below instrumental noise levels (0.05 K for a 250 K scene temperature) for this spectral interval. The adequacy of current RTIASI vertical resolution in the stratopause/mesosphere region is also called into question for two limited spectral intervals: 665-670 cm to power -1 and 2300-2380 cm to power -1. Here discretisation errors are of the order of 0.15 K, comparable with conservative (AIRS) instrumental noise levels and significantly lower than other possible sources of representativity errors (mesospheric temperature inversions, departures from local thermodynamic equilibrium). In this case a more appropriate specification of forward model errors, rather than grid refinement, should be adopted.
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