An Evaluation and Intercomparison of Global Analyses from the National Meteorological Center and the European Centre for Medium Range Weather Forecasts

Abstract

In order to help establish a global climate record data sets of global analyses from the U.S. National Meteorological Center (NMC) and the European Centre for Medium Range Weather Forecasts (ECMWF) have been comprehensively evaluated. A detailed chronology of the changes in the analysis-forecast system at NMC and ECMWF has been compiled and the main impacts on the analyses have been identified. Discontinuities have been found in certain characteristics of the analyses when major changes occur. Ale main quantities so affected are the divergent wind component and associated vertical motion fields, and the moisture fields. A detailed intercomparison of the two data sets and statistical results show fairly widespread agreement between the analyses from the two centers over the Northern Hemisphere extratropics. In general, the quality of the analyses is much lower in the tropics and Southern Hemisphere. This is reflected in much greater differences in wind fields south of 20°N, with root-mean-square differences in the north-south and east-west components often exceeding 5 m s?1 above ?500 mb throughout most of this region. Much greater differences in geopotential height south of ?3°S exist. Major problems at NMC prior to May 1986 occur south of ?50°S. ECMWF has also experienced difficulties over and around Antarctica, especially prior to 1982. In the tropics, there are major disagreements between the analyses of the divergent wind field and associated vertical motions which have become more intense and more realistic with time, but still appear to be poorly known. The relative humility field is the poorest known and has undergone major changes with time at both centers. Possible reasons for these results am discussed and some implications and recommendations are given.