We have developed a multi-year climatology of summertime low-level jets (LLJs) over the central U.S. in order to determine the role of the LLJ in producing flood or drought years. This is the most extensive LLJ climatology to date and will ultimately span the period 1958-1997.
Water vapor transport by low-level jets (LLJs) over the central United States has long been recognized as a significant, if not the primary, source of atmospheric moisture for this region during the summer. In addition to the role of the LLJ in advecting moisture, the terminus region of LLJs is often a region of enhanced low-level water vapor flux convergence. This aspect of the region's hydrology has received relatively little attention due in part to the lack of routine observations of LLJs at the time of their most frequent occurrence.
The NCEP/NCAR Reanalysis provides output near the time of the maximum LLJ occurrence (06 UTC) and is used in our study as a proxy for observed data. Since the reanalysis at this time has a relatively large contribution from the underlying GCM, we have compared the depiction of LLJs in the reanalysis to hourly microwave wind profiler observations during the years 1992-1997. We find that the mean frequency of LLJ occurrence exhibits substantial biases in the reanalysis. However, more detailed comparison with the profiler data indicates that these biases are largely systematic, so that the reanalysis is able to capture the interannual trends in LLJ occurrence.
Preliminary analysis has shown that regional hydrological extremes are most closely related to the geographical distribution of strong LLJs. While drought in the southern and central Great Plains is associated with an anomalous northward shift of the maximum frequency of strong LLJs, there is not necessarily a decrease in overall frequency of occurrence of LLJs. During years of regional precipitation excess there is some increase in the frequency of strong LLJs, and the LLJs show a sharp gradient in their spatial extent. This is consistent with the notion that low-level moisture flux convergence at the terminus of LLJs is an important influence on regional precipitation, since the strongest momentum gradient is observed with the strongest LLJs