Atmospheric conservation properties in ERA-Interim
Berrisford, P. ; Kallberg, P. ; Kobayashi, S. ; Dee, D. ; Uppala, S. ; Simmons, A.J. ; Poli, P. ; Sato, H.
We study the global atmospheric budgets of mass, moisture, energy and angular momentum in the latest reanalysis from The European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim, for the period 1989-2008 and compare with ERA-40. Most of the measures we use indicate that the ERA-Interim reanalysis is superior in quality to ERA-40. In ERA-Interim the standard deviation of the monthly mean global dry mass of 0.7 kgm-2 (0.007%) is slightly worse than in ERA-40, and long time scale variations in dry mass originate predominately in the surface pressure field. The divergent winds are improved in ERA-Interim: the global standard deviation of the time averaged dry mass budget residual is 10 kgm-2day-1 and the quality of the cross equatorial mass fluxes is improved. The temporal variations in the global evaporation minus precipitation (E-P) are too large but the global moisture budget residual is 0.003 kgm-2day-1 with a spatial standard deviation of 0.3 kgm-2day-1. Both the E-P over ocean and P-E over land are about 15% larger than the 1.1 Tgs-1 transport of water from ocean to land. The top of atmosphere (TOA) net energy losses are improved, with a value of 1 Wm-2 but the meridional gradient of the TOA net energy flux is smaller than that from the Clouds and the Earth's Radiant Energy System (CERES) data. At the surface the global energy losses are worse, with a value of 7 Wm-2. Over land however, the energy loss is only 0.5 Wm-2. The downwelling thermal radiation at the surface in ERA-Interim of 341 Wm-2 is towards the higher end of previous estimates. The global mass adjusted energy budget residual is 8 Wm-2 with a spatial standard deviation of 11 Wm-2 and the mass adjusted atmospheric energy transport from low to high latitudes (the sum for the two hemispheres) is 9.5 PW.
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