The accuracy of stratospheric analyses in the northern hemisphere inferred from long-duration balloon flights

摘要

In January and February 2002, six superprcssure balloons (SPBs) were launched from Kiruna, Sweden (69°N, 21°E). The balloons drifted in the polar lower stratosphere for up to 45 days. Temperature, wind and altitude observations wore collected every15 minutes during the Mights. This dataset scries as a reference to assess the accuracy of the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyzes in the lower stratosphere during the 2002 winter. In particular, it is found that both models succeeded in simulatinsi the large temperature fluctuations observed during the SPB flights, which were induced by a ideographical shift between the stratospheric vortex and the low temperature area. Furthermore, the model biases are found to be small for all variables (temperature, wind, height). Nevertheless, it is found that NCEP/NCAR reanalvses tend to be slightly wanner (0.8 K) than the observations, while the converse is true for ECMWF analyses (--0.3 K). The observations also exhibit small-scale fluctuations that are presumably produced by (inertia-) gravity waves and which induce some scatter in the analysis/observation comparisons. Finally, trajectory comparisons are performed. It is found that trajectories built with ECMWF vv inds are more accurate than those built with NCEP/NCAR winds. With both models, the trajectory errors increase with lime upto about 15 days and thentend to fluctuate. Typical errors after 15 days are 1000 ± l200 km for F.CMWF and 2300 ± 1300 km for NCHP trajectories.