2002 Annual

Tuesday, 15 January 2002: 8:30 AM
Diagnostics of vertically integrated fluxes of energy.
Kevin E. Trenberth, NCAR, Boulder, CO; and D. P. Stepaniak
Poster PDF (344.2 kB)
We have made detailed computations of the vertically integrated heat, energy and moisture budgets using the reanalyses and compared the results from NCEP and ECMWF. The full resolution data four-times daily on model coordinates (T62, 28 levels for NCEP and T106, 31 levels for ECMWF) were used to obtain the best accuracy possible and this required processing of 3.1 Terabytes of data. With prospects of the future ECMWF reanalysis being at T159 resolution and 60 levels, there is interest in how well the results can be replicated with the pressure level analyses which are more readily available and constitute a much smaller processing task as they are on a 2.5 grid at 17 levels. Accordingly, we have developed a postprocessor of the model level data to recreate the pressure level archive at much higher vertical resolution.

We have had much greater difficulty in replicating the results with the NCEP pressure archive than from ECMWF, and differences were well in excess of 100 W m-2. The main problem is pathological in the NCEP reanalyses in the stratosphere. It is manifested most strongly as a two-delta vertical wave in the divergence of the wind field above steep topography especially where the wind increases with altitude. It is present primarily above 50 mb at the topmost four levels in the NCEP model used for data assimilation and appears to be directly related to the use of the sigma (terrain-following) coordinate system and the upper boundary condition in the assimilating model. Recommendations are suggested for addressing the problem.

We also address the question of how well model level diagnostics can be replicated with a coarser resolution pressure-level archive and recommend that future pressure archives should have higher vertical resolution to better resolve the boundary layer and the surface.

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