INFLUENCE OF THE GREAT-PLAINS LOW-LEVEL JET ON SUMMERTIME PRECIPITATION AND MOISTURE TRANSPORT OVER THE CENTRAL UNITED-STATES

The influence of the Great Plains low-lever jet (LLJ) on summertime pr ecipitation and moisture transport over the central United States is e xamined in observations and in assimilated datasets recently produced by the NCEP/NCAR and the NASA/DAO. Intercomparisons between the assimi lated datasets and comparisons with station observations of precipitat ion, winds, and specific humidity are used to evaluate the limitations of the assimilated products for studying the diurnal cycle of rainfal l and the Great Plains LLJ. The winds from the reanalyses are used to diagnose the impact of the LLJ on observed nocturnal precipitation and moisture transport over a multisummer (JJA 1985-89) period. The impac t of the LLJ on the overall inoisture budget of the central United Sta tes is also examined. An inspection of the diurnal cycle of precipitat ion in gridded hourly station observations for 1963-93 reveals a well- defined nocturnal maximum over the Great Plains region during the spri ng and summer months consistent with earlier observational studies. Du ring summer in excess of 25% more precipitation falls during the night time hours than during the daytime hours over a large portion of the G reat Plains, with a commensurate decrease in the percentage amount of nocturnal precipitation along the Gulf Coast Inspection of the nightti me precipitation by month shows that the maximum in precipitation alon g the Gulf Coast slowly shifts northward from the lower Mississippi Va lley to the upper Midwest during the late spring and summer months and then back again during the fall. Both reanalyses produce a Great Plai ns LLJ with a structure, diurnal cycle, and frequency of occurrence th at compares favorably to hourly wind profiler data. Composites of obse rved nighttime rainfall during LLJ events show a fundamentally differe nt pattern in the distribution of precipitation compared to nonjet eve nts. Overall, LLJ events are associated with enhanced precipitation ov er the north central United States and Great Plains and decreased prec ipitation along the Gulf Coast and East Coast; nonjet events are assoc iated with much weaker anomalies that are generally in the opposite:se nse. Inspection of the LLJ composites for each month shows a gradual s hift of the region of enhanced precipitation from the northern tier of states toward the south and east in a manner consistent with the anom alous moisture transport. LLJ-related precipitation is found to be ass ociated most closely with the strongest, least frequent LLJ events. Th e moisture transport in the reanalyses compares favorably to radiosond e data, although Significant regional differences exist, particularly along the Gulf Coast during summer. The diurnal cycle of the low-level moisture transport is well reserved in the reanalyses with the larges t and most extensive anomalies being those associated with the nocturn al inland flow of the Great Plains LLJ. Examination of the impact of t he LLJ on the:nighttime moisture transport shows a coherent evolution from May to August with a gradual increase in the anomalous westerly t ransport over the southeastern United States, consistent with the evol ution of the precipitation patterns. The impact of the LLJ on the over all moisture budget during summer is considerable with low-level inflo w from the Gulf of Mexico increasing by more than 45%, on average, ove r nocturnal mean values.