Calculate Total Column Water Vapour
A large part of the atmospheric correction is provided by the split window formulation and by the use of retrieval coefficients derived by regressions against simulated data. The use of water vapour auxiliary data accounts better for swath angle dependences of the atmospheric correction factor, which is a smaller effect.
Water vapour data should be obtainable from two sources:
In the processing, an option is defined to switch between these two sources of water vapour data. The monthly climatology file is defined as the baseline for the algorithm. Water vapour fields will be interpolated to the current time of measurement.
If meteorological fields are not available, then the climatology data will be used. For water vapour, interpolation between analysis fields and forecast fields is acceptable.
The water vapour climatology is provided as filled fields with 12 monthly fields in one annual file (which can be updated from year to year if required, e.g. for re-processing). The water vapour climatology is interpolated to the required point in time, latitude and longitude.