Rw. Higgins et al., INFLUENCE OF THE GREAT-PLAINS LOW-LEVEL JET ON SUMMERTIME PRECIPITATION AND MOISTURE TRANSPORT OVER THE CENTRAL UNITED-STATES, Journal of climate, 10(3), 1997, pp. 481-507
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.