4-D Var Data Assimilation Method
Find optimal fluxes u and initial CO2 field xo to minimize





subject to the dynamical constraint


where
x  are state variables (CO2 concentrations),
h  is a vector of measurement operators
z  are the observations,
R  is the covariance matrix for z,
uo is an a priori estimate of the fluxes,
Puo is the covariance matrix for uo,
xo is an a priori estimate of the initial concentrations,
Pxo  is the covariance matrix for xo,
Fis the transition matrix (representing the transport model), and
G distributes the current fluxes into the bottom layer of the model