Developments in the ECMWF humidity background errors
3 Oct, Noon, in 2155
The background error used in 4D-Var and the EDA has unbalanced relative
humidity as control variable, with analytic balance operator between
humidity and temperature in cloudy conditions. Recent developments have
added humidity background error variances taken directly from the
Ensemble of Data Assimilations (EDA), like for all other variables.
This replaces earlier background and level dependent statistical
estimates of the humidity variances. This change improved forecast
scores and background fit to observations, including winds, and went
operational 11 July 2017. I will show some selected results from this
change and discuss some further developments. These include single
observation experiments with a new analytic balance operator coupling
humidity and dynamic background errors and the sensitivity of the
analysis to using stratospheric humidity variances from the EDA versus
tapering them to very small values, as done operationally.