Influence of the Great Plains Low-Level Jet on Summertime Precipitation and Moisture Transport over the Central United States

Abstract

The influence of the Great Plains low-level jet (LLJ) on summertime precipitation and moisture transport over the central United States is examined in observations and in assimilated datasets recently produced by the NCEP/NCAR and the NASA/DAO. Intercomparisons between the assimilated datasets and comparisons with station observations of precipitation, winds, and specific humidity are used to evaluate the limitations of the assimilated products for studying the diurnal cycle of rainfall 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 impact of the LLJ on the overall moisture budget of the central United States is also examined. An inspection of the diurnal cycle of precipitation in gridded hourly station observations for 1963?93 reveals a well-defined nocturnal maximum over the Great Plains region during the spring and summer months consistent with earlier observational studies. During summer in excess of 25% more precipitation falls during the nighttime hours than during the daytime hours over a large portion of the Great Plains, with a commensurate decrease in the percentage amount of nocturnal precipitation along the Gulf Coast. Inspection of the nighttime precipitation by month shows that the maximum in precipitation along the Gulf Coast slowly shifts northward from the lower Mississippi Valley to the upper Midwest during the late spring and summer months and then back again during the fall. Both reanalyses produce a Great Plains LLJ with a structure, diurnal cycle, and frequency of occurrence that compares favorably to hourly wind profiler data. Composites of observed nighttime rainfall during LLJ events show a fundamentally different pattern in the distribution of precipitation compared to nonjet events. Overall, LLJ events are associated with enhanced precipitation over the north central United States and Great Plains and decreased precipitation along the Gulf Coast and East Coast; nonjet events are associated with much weaker anomalies that are generally in the opposite sense. Inspection of the LLJ composites for each month shows a gradual shift of the region of enhanced precipitation from the northern tier of states toward the south and east in a manner consistent with the anomalous moisture transport. LLJ-related precipitation is found to be associated most closely with the strongest, least frequent LLJ events. The moisture transport in the reanalyses compares favorably to radiosonde data, although significant regional differences exist, particularly along the Gulf Coast during summer. The diurnal cycle of the low-level moisture transport is well resolved in the reanalyses with the largest and most extensive anomalies being those associated with the nocturnal inland flow of the Great Plains LLJ. Examination of the impact of the LLJ on the nighttime moisture transport shows a coherent evolution from May to August with a gradual increase in the anomalous westerly transport over the southeastern United States, consistent with the evolution of the precipitation patterns. The impact of the LLJ on the overall moisture budget during summer is considerable with low-level inflow from the Gulf of Mexico increasing by more than 45%, on average, over nocturnal mean values.