Low-Level Jets over the Mid-Atlantic States: Warm-Season Climatology and a Case Study

Abstract

Although considerable research has been conducted to study the characteristics of the low-level jets (LLJs) over the Great Plains states, little is known about the development of LLJs over the Mid-Atlantic states. In this study, the Mid-Atlantic LLJ and its associated characteristics during the warm seasons of 2001 and 2002 are documented with both the wind profiler data and the daily real-time model forecast products. A case study with three model sensitivity simulations is performed to gain insight into the three-dimensional structures and evolution of an LLJ and the mechanisms by which it developed. It is found that the Mid-Atlantic LLJ, ranging from 8 to 23 m s Ϫ1, appeared at an average altitude of 670 m and on 15-25 days of each month. About 90% of the 160 observed LLJ events occurred between 0000 and 0600 LST, and about 60% had southerly to westerly directions. Statistically, the real-time forecasts capture most of the LLJ events with nearly the right timing, intensity, and altitude, although individual forecasts may not correspond to those observed. For a selected southwesterly LLJ case, both the observations and the control simulation exhibit a pronounced diurnal cycle of horizontal winds in the lowest 1.5 km. The simulation shows that the Appalachian Mountains tend to produce a sloping mixed layer with northeasterly thermal winds during the daytime and reversed thermal winds after midnight. With additional thermal contrast effects associated with the Chesapeake Bay and the Atlantic Ocean, the daytime low-level winds vary significantly from the east coast to the mountainous regions. The LLJ after midnight tends to be peaked preferentially around 77.5°W near the middle portion of the sloping terrain, and it decreases eastward as a result of the opposite thermal gradient across the coastline from the mountain-generated thermal gradient. Although the Mid-Atlantic LLJ is much weaker and less extensive than that over the Great Plains states, it has a width of 300-400 km (to its half-peak value) and a length scale of more than 1500 km, following closely the orientation of the Appalachians. Sensitivity simulations show that eliminating the surface heat fluxes produces the most significant impact on the development of the LLJ, then topography and the land-sea contrast, with its area-averaged intensity reduced from 12 m s Ϫ1 to about 6, 9, and 10 m s Ϫ1, respectively.

Variability of the Great Plains Low-Level Jet: Large-Scale Circulation Context and Hydroclimate Impacts

“…The core is tightly packed at 25°N, likely, because of steep topography to the west. Topography is known to impact the jet structure from generation of shallow baroclinicity on its slopes (e.g., Zhang et al 2006). At 30°N, the core has shifted to the west and exhibits a more gradual eastward-downward slope, reflecting the underlying terrain slope and related thermal attributes.…”

“…The wet (dry) eastern (western) southern Plains region may produce a local enhancement of the east (high) to west (low) low-level pressure gradient, and thus the GPLLJ (Bosilovich and Sun 1999). An evaporation-LLJ relationship has also been advanced based on boundary layer forcing of the nocturnal jet and its modulation by the diurnally varying radiative responses of dry soil (Zhang et al 2006;Zhong et al 1996) 12 . However the modeling analysis of Giorgi et al found that evaporation and soil moisture forcing was a minor contributor to the drought of 1988 and had it been dominant would have provided a negative feedback to the drought via enhanced dynamical forcing of the low-level circulation (i.e., the GPLLJ).…”

Analytical description of a nocturnal low‐level jet

“…Terrain-associated baroclinicity likely also affects low-level jets in other areas of the world (e.g. Saulo et al, 2004;Zhang et al, 2006;Cuxart and Jiménez, 2007), as does baroclinicity associated with land/sea temperature contrasts (e.g. Zemba and Friehe, 1987;Karipot et al, 2009).…”

NASA GPM-Ground Validation: Integrated Precipitation and Hydrology Experiment 2014 Science Plan

“…Fuhrman et al 2008, Brun andBarros 2013). Although weaker than the Low Level JET (LLJ) in the Central Plains (Anderson and Arritt, 2001), it is possible that the nocturnal LLJ east of the Appalachians play an important role on the diurnal cycle of rainfall on analysis of profile observations in the Mid-Atlantic states (Zhang et al 2006). However, current evidence from the existing relatively sparse network of observations is lacking.…”