|Title||Diagnostics of linear and incremental approximations in 4D-Var revisited for higher resolution analysis|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Radnóti, G, Trémolet, Y, Andersson, E, Isaksen, L, Hólm, EV, Janiskova, M|
|Place Published||Shinfield Park, Reading|
Diagnostic experiments on the validity of the tangent linear approximation, essential in the context of ECMWF 4D-Var, are examined using the most recent versions of the assimilation system. The results are compared with those obtained with a three years earlier model version. The tests are extended to T799 horizontal resolution for the full non-linear model and to T319 for the tangent-linear, in view of planned resolution upgrades to the ECMWF forecast system. The role of various components of the physics is investigated, as is the impact of a new conserving interpolation scheme, and a newly developed grid-point representation for humidity in the tangent-linear model. Our results show that improved accuracy can be obtained from increased analysis resolution, and in particular that the T799/T255 system is viable, and should be pursued for pre-operational testing. It has been affirmed that a rapid initial growth of error near the surface is present in ECMWF 4D-Var. The tests show that the discrepancies between the inner- and outer-loop resolution, non-linearities in the vertical diffusion and in other physical processes equally contribute to this. Tests with the new conserving interpolation being under development and to be used in the near future within 4D-Var indicate its potential positive impact. The grid-point representation for humidity in the tangent-linear model, according to first tests, seems to better represent humidity increments associated with smaller scale phenomena, such as organized convection.