Top: Time series of observed (dark blue circles, "OBS"), assimilated (red circles, "SIM") and prior (green circles, "PRIOR") average methane mole fraction. For the "PRIOR" simulations, prior fluxes were used to calculate CH4 mole fractions, while for the "SIM" simulations CH4 was calculated using fluxes that were adjusted for optimal agreement with atmosphereic observations. Note that the "PRIOR" simulations diverge from observations over time, while the "SIM" CH4 mole fractions lie between the "PRIOR" and "OBS" CH4 mole fractions. The location of the "SIM" between the "PRIOR" and "OBS" curves is determined by the relative weighting of the prior flux estimates and observations in the assimilation scheme. Units are ppb (10-9 mol mol-1).
Bottom: Derived growth rate of CH4 mole fraction for observed and assimilated CH4 mole fraction. The growth rate is computed by taking the first derivative of the average mole fractions shown in the top figure, and it is directly related to the emissions and sinks of CH4 mole fraction. The assimilation should be able to match the observed growth rate closely, otherwise emissions or sinks may not be captured by the assimilation (as may happen when not enough data exist to adequately constrain the assimilation).Units are ppb/yr (10-9 mol mol-1 yr-1).