The Carolina Nocturnal Low-Level Jet: Synoptic Climatology and a Case Study

Abstract

The nocturnal low-level jet (LLJ), a common Phenomenon in Many Part of the world, also occurs over the Carolinas in the eastern United States. The Carolina LLJ was studied using pilot balloon (PIBAL) observations taken from 1978 to 1982 at Poinsett Range in South Carolina and Dare County Range and Pope AFB in North Carolina. It was discovered that the jet, found in 5% of the PIBAL observations, usually forms between midnight and sunrise, after which time it dissipates. The Carolina jet can develop in any season. The average altitude of the jet is 600?700 m above ground level, and the primary wind directions of the jet are northeast and southwest. Most of the 50 LLJ cases inspected in detail were preceded in time by the nearby passage of a 300-mb jet streak, such that the LLJ region was on the anticyclonic shear side of the upper-level jet (ULJ) at 0000 UTC before LLJ formation. Local(sea-level isobars were anticyclonically curved in these cases and 0000 UTC surface winds in the LLJ region were directed toward a maximum in 12-h sea-level pressure falls. A coupling of the LLJ and ULJ in these cases is hypothesized and supported by a calculation of the isallobaric wind in one case. On the basis of the findings a forecasting rule for the formation of the Carolina nocturnal LLJ is proposed.