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

Abstract

he 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 than do 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 with 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 into the spatial extent and seasonal shifts of large jet frequencies and into 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, and northerly jets in the high plains. Furthermore, several new 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 our 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.