A NARR-Derived Climatology of Southerly and Northerly Low-Level Jets over North America and Coastal Environs

The North American Regional Reanalysis (NARR) was used to develop an expanded, long-term (1979-2009) climatology of meridional (southerly and northerly) low-level jets over North America and surrounding coastal environs. NARR has greater spatial coverage and finer temporal (3-hourly) and horizontal (32 km) resolutions compared to routine rawinsonde wind measurements. The NARR climatology focuses on jet frequency and average speed and elevation by month and 3-hourly time step. To evaluate the plausibility of the climatology, jet characteristics were compared to those obtained from prior climatological analyses, case studies, field campaigns, and numerical simulations.

Strong agreement was found with many of the previously documented characteristics of well-known jets, including the northerly Pacific coast jet and southerly Great Plains jet. The NARR climatology provides additional insights on the spatial extent and seasonal shifts of the locations with large jet frequencies, and on diurnal fluctuations in frequency, speed and elevation. Weaker and/or less spatially extensive jets are also well depicted in the NARR climatology, including the southerly Gulf of California jet, summertime southerly jets and autumn northerly jets off the mid-Atlantic coast, northerly jets in the High Plains, and northerly jets associated with cold air incursions into the southwestern Gulf of Mexico. Furthermore, several previously unreported areas of relatively frequent jet occurrence, most of which align with shallow thermal gradients, are seen in the NARR climatology. The NARR climatology supplements and enhances the understanding of North American low-level jets, and points to the need for additional research on both the climatological characteristics of these jets and on the processes contributing to their formation.