Outline. Jet Streams I (without the math) Definitions. Polar Jet Stream. Polar Jet Stream

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1 Jet Streams I (without the math) Outline A few definitions, types, etc Thermal wind and the polar jet Ageostrophic circulations, entrance and exit regions Coupled Jets Definitions Jet Stream An intense, narrow horizontal current of air associated with strong vertical wind shear Mesoscale in the cross-flow direction, synoptic scale in the along-flow direction Jet Streak An isotach maximum embedded within a jet stream Polar Jet Stream Polar Jet Stream Found about mb at the transition between tropical (or mid-latitude) and polar air Occurs in the vicinity of the surface polar front Location of jet there explained by the thermal wind relation High short term variability, thus does not show up well on zonal averages (mean wind maps) But the day to day position in highly variable! 1

2 Subtropical Jet Located near 200 mb, primarily in winter ear latitudes of Usually weaker than polar jet Sometimes indistinguishable (merged) Influenced/caused by temp gradients along with upper divergence and conservation of angular momentum from deep tropical convection eakens and or disappears in the summer ith GARP satellite imagery to help visualize. Review: Thermal ind Review: Thermal ind The (vector) difference between the wind at two levels (pressure levels). sider, first, the simple case of no wind near the surface (the 1000 mb surface is horizontal) and a temperature pattern with a uniform north-to-south gradient up to the 500 mb level. z V 2 V 1 =0 V T VT =V 2 -V 1 V 2 : geostrophic wind at upper level V 1 : geostrophic wind at lower level Since V 1 =0, V T =V mb west wind X Δn 500 mb Δz z cold warm no wind 1000 mb 1000 mb S Cold air V T arm air S The difference in wind between the two levels (the thermal wind) relates to the temperature pattern (low temperature on the left) the same way that the geostrophic wind relates to the pressure pattern (low pressure on the left). Thermal ind Jet Stream Basics From the thermal wind balance point of view: cold V 1-10 o C -5 o C V 2 0 o C V T =V 2 -V 1 V T 5 o C warm Veering or backing here z Cold air arm air quator S- temperature gradient results from the imbalance between the long-wave and short-wave radiation at different latitudes. esterly wind must increase with height in order to maintain the thermal wind balance. Maximum wind occurs below the level where temperature gradient changes the sign (tropopause). Low level temperature gradients tend to concentrate along the polar front and this gives rise to the polar jet. 2

3 *xample for a Straight Jet Important component of mid-latitude weather Responsible for much synoptic scale weather phenomena Let s look at the Four Quadrant Model P 1 < P 2 < P 3 P 1 P 2 P 3 As air parcels enter the stronger pressure gradient, they accelerate. However, they decelerate as they leave the jet maxima. *xample for a Straight Jet *xample for a Straight Jet P 1 V ag Sub-geostrophic (2) (1) (2) (3) (4) (5) sider the force balances: Locations 1, 3, and 5 are in geostrophic balance, 2 and 4 are not. P 2 P 3 To account for the increased PGF (the coriolis force is not in balance), an ageostrophic wind will yield a northward displacement of the parcels. V ag (4) To account for the decreased PGF (the coriolis force is not in balance), an ageostrophic wind will yield a southward displacement of the parcels. Super-geostrophic sider only the Ageostrophic Motions So, for a straight jet P 1 (A) (B) P 2 ntrance Region left rear entrance vergence xit Region Left front exit ergence (C) (D) Ageostrophic vergence at A and D, and Ageostrophic ergence at C and B. P 3 ergence right rear entrance These are the four quadrants vergence right front exit 3

4 Let s look at a Cross Section through the xit Region J Let s look at a Cross Section through the ntrance Region J Cold arm Cold arm Thermally Indirect Circulation: - Strengthens the lower-level thermal gradient. Thermally Direct Circulation: - eakens the lower-level thermal gradient. and Vorticity Generation of cyclonic shear vorticity on the cold side of the jet. ( cyclonic shear side of jet) Jet Streak Circulations and Troughs The principles of jet streaks still hold true, even though the flow is cyclonically curved. ind velocities decrease away from the jet streak. Generation of anticyclonic shear vorticity on the warm side of the jet.( anticyclonic shear side of jet) However, typically downstream of the trough axis, the advection of cyclonic vorticity (increasing w/height) yields increased ergence aloft and rising motion. Similarly, upstream of the trough axis, the advection of anticyclonic vorticity (increasing w/height) yields increased vergence aloft and subsidence motion. 4

5 Jet Streak Circulations and Troughs Jet Streak Circulations and Ridges CO DIV Thus, ergence aloft occurs in the left exit region of the jet and vergence occurs in the left entrance region of the jet. Due to the conflicting physical processes opposing the divergence and convergence patterns aloft, it is difficult to determine rising or sinking motion on the warm side of the jet without explicitly computing the values. Again, the principles of jet streaks still hold true, even though the flow is anticyclonically curved. However, typically downstream of the ridge axis, the advection of anticyclonic vorticity (increasing w/height) yields increased vergence aloft and subsidence. Similarly, upstream of the ridge axis, the advection of cyclonic vorticity (increasing w/height) yields increased ergence aloft and rising motion. Jet Streak Circulations and Ridges Thus, vergence aloft occurs in the right exit region of the jet and ergence occurs in the right entrance region of the jet. DIV CO Due to the conflicting physical processes opposing the divergence and convergence patterns aloft, it is difficult to determine rising or sinking motion on the cold side of the jet without explicitly computing the values. Overlapping regions of ergence aloft! ntrance Region vergence xit Region ergence ergence J 2 vergence vergence ergence J 1 ergence vergence ntrance Region xit Region Due to the nature of low-level advection, coupled jet streaks can occur and have a significant impact on the development of synoptic systems. 5

6 Let s look at a Cross Section through the coupled jets ergence xample.. J 2 J 1 Known as the Q II Storm ntrance Region vergence xit Region *Isotachs are in m/s here on this slide do you see jet streak coupling 6