THE CLIMATE SIGNAL IN REGIONAL MOISTURE FLUXES - A COMPARISON OF 3 GLOBAL DATA ASSIMILATION PRODUCTS

This study assesses the quality of estimates of climate variability in moisture flux and convergence from three assimilated datasets: two ar e reanalysis products generated at the Goddard Data Assimilation Offic e and the National Centers for Environmental Prediction-National Cente r for Atmospheric Research, and the third consists of the operational analyses generated at the European Centre for Medium-Range Weather For ecasts (ECMWF). The regions under study (the United States Great Plain s, the Indian monsoon region, and Argentina east of the Andes) are cha racterized by frequent low-level jets and other interannual low-level wind variations tied to the large-scale flow. While the emphasis is on the reanalysis products, the comparison with the operational product is provided to help assess the improvements gained from a fixed analys is system. All three analyses capture the main moisture flux anomalies associated with selected extreme climate (drought and flood) events d uring the period 1985-93. The correspondence is strongest over the Gre at Plains and weakest over the Indian monsoon region reflecting differ ences in the observational coverage. For the reanalysis products, the uncertainties in the lower tropospheric winds is by far the dominant s ource of the discrepancies in the moisture flux anomalies in the middl e latitude regions. Only in the Indian Monsoon region, where interannu al variability in the low-level winds is comparatively small, does the moisture bias play a substantial role. In contrast, the comparisons w ith the operational product show differences in moisture that are comp arable to the differences in the wind in all three regions. Compared w ith the fluxes, the anomalous moisture convergences show substantially larger differences among the three products. The best agreement occur s over the Great Plains region where all three products show verticall y integrated moisture convergence during the floods and divergence dur ing the drought with differences in magnitude of about 25%. The reanal ysis products, in particular, show good agreement in depicting the dif ferent roles of the mean how and transients during the flood and droug ht periods. Differences between the three products in the other two re gions exceed 100% reflecting differences in the low-level jets and the large-scale circulation patterns. The operational product tends to ha ve locally larger amplitude convergence fields, which average out in a rea-mean budgets: this appears to be at least in part due to errors in the surface pressure fields and aliasing from the higher resolution o f the original ECMWF fields. On average, the reanalysis products show higher coherence with each other than with the operational product in the estimates of interannual variability. This result is less clear in the Indian monsoon region where differences in the input observations appear to be an important factor. The agreement in the anomalous conv ergence patterns is, however, still rather poor even over relatively d ata-dense regions such as the United States Great Plains. These differ ences are attributed to deficiencies in the assimilating general circu lation model's representations of the planetary boundary layer and oro graphy, and a global observing system incapable of resolving the highl y confined low-level winds associated with the climate anomalies.