The Climate Signal in Regional Moisture Fluxes: A Comparison of Three Global Data Assimilation Products

Abstract

This study assesses the quality of estimates of climate variability in moisture flux and convergence from threeassimilated datasets: two are reanalysis products generated at the Goddard Data Assimilation Office and theNational Centers for Environmental Prediction?National Center for Atmospheric Research, and the third consistsof the operational analyses generated at the European Centre for Medium-Range Weather Forecasts (ECMWF).The regions under study (the United States Great Plains, the Indian monsoon region, and Argentina east of theAndes) are characterized by frequent low-level jets and other interannual low-level wind variations tied to thelarge-scale flow. While the emphasis is on the reanalysis products, the comparison with the operational productis provided to help assess the improvements gained from a fixed analysis system. All three analyses capture the main moisture flux anomalies associated with selected extreme climate (droughtand flood) events during the period 1985?93. The correspondence is strongest over the Great Plains and weakestover the Indian monsoon region reflecting differences in the observational coverage. For the reanalysis products,the uncertainties in the lower tropospheric winds is by far the dominant source of the discrepancies in themoisture flux anomalies in the middle latitude regions. Only in the Indian Monsoon region, where interannualvariability in the low-level winds is comparatively small, does the moisture bias play a substantial role. Incontrast, the comparisons with the operational product show differences in moisture that are comparable to thedifferences in the wind in all three regions. Compared with the fluxes, the anomalous moisture convergences show substantially larger differences amongthe three products. The best agreement occurs over the Great Plains region where all three products showvertically integrated moisture convergence during the floods and divergence during the drought with differencesin magnitude of about 25%. The reanalysis products, in particular, show good agreement in depicting the differentroles of the mean flow and transients during the flood and drought periods. Differences between the threeproducts in the other two regions exceed 100% reflecting differences in the low-level jets and the large-scalecirculation patterns. The operational product tends to have locally larger amplitude convergence fields, whichaverage out in area-mean budgets: this appears to be at least in part due to errors in the surface pressure fieldsand aliasing from the higher resolution of the original ECMWF fields. On average, the reanalysis products show higher coherence with each other than with the operational productin the estimates of interannual variability. This result is less clear in the Indian monsoon region where differencesin the input observations appear to be an important factor. The agreement in the anomalous convergence patternsis, however, still rather poor even over relatively data-dense regions such as the United States Great Plains.These differences are attributed to deficiencies in the assimilating general circulation model?s representations ofthe planetary boundary layer and orography, and a global observing system incapable of resolving the highlyconfined low-level winds associated with the climate anomalies.