Representation of Summertime Low-Level Jets in the Central United States by the NCEP–NCAR Reanalysis

Abstract

Reanalysis datasets that are produced by assimilating observations into numerical forecast models may contain unrealistic features owing to the influence of the underlying model. The authors have evaluated the potential for such errors to affect the depiction of summertime low-level jets (LLJs) in the NCEP?NCAR reanalysis by comparing the incidence of LLJs over 7 yr (1992?98) in the reanalysis to hourly observations obtained from the NOAA Wind Profiler Network. The profiler observations are not included in the reanalysis, thereby providing an independent evaluation of the ability of the reanalysis to represent LLJs. LLJs in the NCEP?NCAR reanalysis exhibit realistic spatial structure, but strong LLJs are infrequent in the lee of the Rocky Mountains, causing substantial bias in LLJ frequency. In this region the forecast by the reanalysis model diminishes the ageostrophic wind, forcing the analysis scheme to restore the ageostrophic wind. The authors recommend sensitivity tests of LLJ simulations by GCMs in which terrain resolution and horizontal grid spacing are varied independently.