Cyclogenesis and Upper-Level Jet Streaks and their Influence on the

Low-Level Jet

Keith Wagner, Lance F. Bosart, and Daniel Keyser

Department of Earth and Atmospheric Sciences

University at Albany/SUNY

1400 Washington Avenue

Albany, NY 12222

Background on the Low-Level Jet (LLJ)

• Great Plains LLJ tied to increased thunderstorm activity

• Narrow core of winds of at least 12 m/s found at or below 850mb

• High theta-e air is pumped northward

• Convergence occurs on the downstream side of the LLJ

• Enhanced lift, moisture, and instability

How does the LLJ form?• LLJ typically considered to form due to boundary layer

processes

• Classic LLJ is a combination of diurnally varying differential heating and diurnally varying boundary layer frictional processes

• These processes combine to maximize the strength of the LLJ at night and into the early morning

• Implies the LLJ is tied to the terrain and to the boundary layer

How does the LLJ form?• Uccellini (1980) reexamined 15 LLJ cases previously

studied by Bonner (1966), Izumi (1964), Hoecker (1963), and Newton (1956)

• Only 3 of the 15 cases studied had the classic diurnal pattern for LLJ formation

• The other 12 cases had a progressive trough over the Rockies

• With these cases there was also a general pattern of leeside cyclogenesis north of the LLJ

Method• The 15 LLJ cases were broken into 2 groups as identified

by Uccellini (1980)

• Composite maps were produced using gridded reanalysis data from NCEP/NCAR

• 850mb and 300mb winds, 300mb heights, and sea level pressure

• 23 April 1961 upper-tropospheric influenced LLJ

LLJ Data

“Coupled LLJ”

18 November 194814 August 195920 August 195919 April 196022 April 196023 April 196010 July 196023 August 19602 December 196023 April 196117 May 196130 May 1961

“Classic LLJ”

14 July 1959

15 March 1961

28 May 1961

Fig. 1: Type 1 850mb winds (m/s)

Fig. 2: Type 2 850mb winds (m/s)

Fig. 3: Type 1 300mb winds (m/s)

Fig. 4: Type 2 300mb winds (m/s)

Fig. 5: Type 1 300mb heights (m)

Fig. 6: Type 2 300mb heights (m)

Fig. 7: Type 1 Sea Level pressure (mb)

Fig. 8: Type 2 Sea Level Pressure (mb)

Coupled Composite Results

• 300mb height and wind composites for the Coupled LLJ show a positively tilted trough with an upper-level jet streak propagating around the base

• The Coupled setup also shows a fairly strong cyclone to the north

• A solid base for leeside cyclogenesis is occurring

Classic Composite Results

• 300mb height and wind composites for the Classic LLJ show a strong ridge over the Rockies with weak upper-level flow

• The Classic setup shows a broad, but weaker low pressure to the north

• Further intensification seems unlikely

• Here the LLJ begins to break down by late morning

Fig. 9: 23 April 1961 6Z 850mb wind

Fig. 10: 23 April 1961 18Z 850mb wind

Fig. 11: 23 April 1961 6Z 300mb wind

Fig. 13: 23 April 1961 18Z 300mb wind

Fig. 14: 23 April 1961 6Z Sea Level Pressure (mb)

Fig. 15: 23 April 1961 18Z Sea Level Pressure (mb)

23 April 1961 LLJ Results

• Previous studies show that a Classic LLJ decreases in magnitude and organization by late morning, and that it does not move eastward

• The 23 April 1961 LLJ maintains its strength and organization while propagating eastward

• A cyclone north of the LLJ is causing a pressure gradient across the Plains

• By 18Z, there is a southeastward expansion of the 1000mb isobar, which tightens the pressure gradient

23 April 1961 LLJ Results

• Strong upper-level jet streak propagating eastward

• Upper-level jet streaks are critical for leeside cyclogenesis

• Maintenance of the LLJ is most likely caused by the increased pressure gradient due to cyclogenesis

• Suggests that the LLJ is tied to the dynamics in this case, not the terrain

Conclusions

• The LLJ is a key player in precipitation development

• NCEP/NCAR reanalysis shows that 2 different upper-tropospheric flow regimes can exist during a LLJ event

• The Classic LLJ features strong ridging aloft with weak winds

• It breaks down by late morning without propagating eastward due to changes taking place in the boundary layer

• The Coupled LLJ features a progressive trough with eastward propagating jet streaks leading to cyclogenesis

Conclusions

• Cyclogenesis leads to a tightening of the pressure gradient across the Plains

• Not all LLJ’s are tied to boundary layer processes

• Most LLJ’s that we experience in the Northeast are of the dynamically tied variety

• It is a big mistake to treat the LLJ as being strictly a boundary layer phenomena without first looking at the upper-tropospheric flow