CLIMATOLOGY OF THE SIMULATED GREAT-PLAINS LOW-LEVEL JET AND ITS CONTRIBUTION TO THE CONTINENTAL MOISTURE BUDGET OF THE UNITED-STATES

The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest kilometer of the atmosphere, it may cover a substantial region of the Great Pla ins, and typically reaches maximum amplitudes of more than 20 m s(-1). A two-month, springtime simulation with the Goddard Earth Observing S ystem (GEOS-1) atmospheric general circulation model (AGCM) has produc ed a Great Plains LLJ with a vertical and temporal structure, directio nality, and climatological distribution that compare favorably with ob servations. The diurnal cycle of the low-level flow is dramatic and co herent over a subcontinental area that includes much of the western Un ited States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated wi th the Bermuda high) into the North American continent as surface fric tion decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called ''Baja Jet.'' Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations. The time-averaged climatological picture of the lo w-level flow is dominated over land by the nocturnal phase of the diur nal cycle, in which surface friction is minimal and wind speeds are st rongest. This pattern, with its zones of strong convergence, is charac teristic of an unsteady, strongly forced flow. Over the open ocean, th e mean low-level now is more reminiscent of a smooth, climatological p attern. Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined; region of strong southe rly moisture transport with a strong diurnal cycle in the region of th e Great Plains LLJ, as has been found in observations of water vapor t ransport. The LLJ plays a key role in that budget by transporting almo st one-third of all the moisture that enters the continental United St ates with most of the influx from the LLJ (slightly less than two-thir ds of it) entering during the 12 nighttime hours. However, it is the m ean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture tr ansport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in t he jet region, and covariances on the diurnal timescale are negative a nd negligible despite the strong diurnal signal in the wind.