Abstract
During the Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment a persistent, diurnally varying, northeasterly low-level jet (LLJ) was observed along the Carolina coastal plain. Nocturnal maxima of over 20 m s−1 were observed near 960 mb. The daytime speed reduction varied considerably from a 2–5 m s−1 decrease in extreme eastern North Carolina to a 10–16 m s−1 decrease at locations to the west along the coastal plain. An intense coastal baroclinic zone, associated with cold air dammed to the east of the Appalachian Mountains and the warm marine-atmospheric boundary layer over the Gulf Stream, resulted in a northeasterly low-level geostrophic wind maximum near the surface almost parallel to the coast.
A simulation of the LLJ evolution using a one-dimensional planetary boundary layer model (Zhang and Anthes 1982) indicates that the initial acceleration of the LLJ was controlled by the increasing low-level geostrophic wind speed. The large daytime speed reduction resulted from a rapid increase in the frictional stress at the jet level. The LLJ was reestablished the following evening when the nocturnal inversion developed and the ageostrophic component, which increased substantially during the daytime, rotated to a direction nearly parallel to the jet.
Sensitivity experiments indicate that a specific geostrophic wind velocity profile was necessary to produce many of the observed Carolina LLJ characteristics. The LLJ was insensitive to separate reasonable changes in the roughness length, moisture availability, albedo, and thermal inertia, however, when the surface parameters were simultaneously changed to correspond to a surface covered by snow, a temporally continuous LLJ structure resulted. A maritime source region influenced the boundary layer in extreme eastern North Carolina; however, locations to the west had more of a continental source region. Thus, the observed spatial variations in the daytime Carolina LLJ structure may have been a result of upwind differences in the roughness length and diurnal mixing effects.
