Smoothing the application of CH4 surface constraints between different months
Past analysis of the CH4 emission estimates available in the chemical budget files shows that for e.g. 50-70S there can be large fluctations between the monthly addition/reduction of CH4, which represents either a pseudo-emission or sink term. This is decoupled from the seasonal varibility in temperatures and photochemical activity (i.e. the strength of the chemical sink term). This is potentially related to differences in the monthly mean measurements at the 6 stations used to construct the yearly-specific surface fields. Therefore so as to introduce less erratic variability the smoothing term introduced for nudging the stratospheric mixing ratios is applied at the surface.
Currently this is coded for the recent past pre-2015.
There needs to be some modifications when moving to future years which apply growth in yearly emission fluxes according the RCP scenarios.