A long time ago in a galaxy far, far away…
Ok we’re not going to actually introduce a Star Wars film, but we are going to talk about forces in this episode of Weather 101. More specifically, we are discussing jet streaks.
What is a Jet Streak?
A jet streak is a location within the jet stream that contains the strongest winds. Ok, but why are they important to our weather here in Northwest Arkansas and the River Valley?
When jet streaks move into the area, you can typically expect weather conditions to deteriorate. This is because the stronger winds aloft can effectively steepen the lapse rates by moving cooler air into the upper levels of the atmosphere. This leads to an increase in t-storm fuel (CAPE).
Jet streaks can also bring large scale (synoptic) lift over an area depending on which side of the faster winds you are on. Jet streaks that occur in a zonal pattern without any curvature to the flow, can be broken into 4 different quadrants.
Balance in the Force
In order to understand the 4 different quadrants of jet streaks, we need to briefly discuss the 2 major forces at play within them. These two forces are the Coriolis and Pressure Gradient Force.
Pressure Gradient Force (PGF)– A force created by the differences in pressure across an area. The stronger the pressure difference, the stronger the force.
Coriolis Force– An apparent force created by the Earth’s rotation. In the northern hemisphere, this makes objects appear to deflect to the right.
4 Quadrant Jet Streak Model (Straight Flow)
Before we dive into a discussion on the 4 quadrants of a zonal jet streak, I want to clarify the terms entrance and exit. An entrance region is where the upper level winds enter a jet streak. The exit region is quite the opposite. The exit region is where the upper level winds exit the jet streak.
Left Entrance (1)– In quadrant 1, the air is converging. As the air enters the jet streak it must speed up. This increase in speed is matched proportionally by an increase in the pressure gradient force. The Coriolis force has to play catch up so the PGF pushes the air towards the north. This push of air collides with the air already moving through that quadrant. The converging air creates sinking motion.
Right Entrance (2)– In quadrant 2, the air is either being pulled north or pushed through the core of the jet streak. This causes a spreading out of the wind aloft or divergence. As we found out in a previous episode of Weather Word of the Week, divergence creates lift. In this case, the lift is fairly weak but still nonetheless present.
Left Exit (3)– In quadrant 3, the air diverges once again. This time the air is pulled to the south or pushed out of the jet streak. As the air departs the jet streak, it starts to slow down. The decrease in the wind speed is matched by a decrease in the PGF. The Coriolis force takes longer to adjust so it pulls the air to the south. This spreading out of the air creates lift at the surface. This quadrant of the jet streak, produces the strongest area of lift.
Right Exit (4)– In quadrant 4, the air converges together again as it exits the jet streak. The air is pulled together, and it piles up causing sinking motion at the surface. Just as the motion in the right entrance is weak, the sinking motion in the right exit region is fairly weak.
Stay tuned for part two of our jet streak series where we will explore what happens when jet streaks are curved.