ATMS 310 Jet Streams

Transcription

1 ATMS 310 Jet Streams Jet Streams A jet stream is an intense (30+ m/s in upper troposphere, 15+ m/s lower troposphere), narrow (width at least ½ order magnitude less than the length) horizontal current of air associated with strong (at least 5-10 ms -1 km -1 ) vertical wind shear. A Jet Streak is an isotach maximum embedded within a jet stream. Jet streams are mesoscale in the cross-flow direction and synoptic-scale in the along-flow direction. They were discovered in the 1940s at the University of Chicago. Polar Jet A jet stream found at about mb at the transition from tropical (or mid-latitude) to polar air. Occurs in the vicinity of the surface polar front. The relationship between the jet location and temperature gradient is a consequence of the thermal wind relation. The polar jet for the most part does not show up on mean wind maps, mostly because it meanders so much. However, there are two climatological polar jets that do show up on mean maps:

2 There is a big difference in the zonal wind along about 30 latitude. For example, the jet core over the western Pacific has wind speeds about 50 m/s higher than similar latitudes off the west coast of California. It is believed that the land/sea contrasts off the east coasts of Asia and North America, particularly during the winter season, enhance the pressure gradients and thus the jetstreams as well. Subtropical Jet Located near 200 mb in winter at latitudes between 20 and 35. In winter it is hard to distinguish between the polar and subtropical jets as the polar jet penetrates far south toward the equator. In summer, the subtropical jet disappears. Low-level Jet This is a jet stream found typically below 850 mb, usually in the southern plains of the U.S. east of the Rocky mountains. It may extend eastward to the Mississippi valley. Also occurs east of a low-level cyclone or south/southwest of a low-level cyclone. See the figure below, which highlight low-level jets.

3 Jet streams are important source regions for storm development. To see how, examine the zonal component of the momentum equation (Eq in Holton): D g u Dt (a) g o ( v vg ) f ova = f (1) (b) where a) is the total rate of change of the geostrophic zonal wind with time, which is a function of b), the Coriolis parameter multiplied by the ageostrophic component of the meridional wind ( v = v ). In the entrance region of the jet stream (upstream from a v g the core) the zonal wind is accelerating. In order for that to happen, va > 0 (poleward D g u g wind). Conversely, in the exit region (downstream from the core) < 0 and thus v a Dt < 0 (equatorward wind). The figure below illustrates these effects:

4 The secondary circulation induced by the jet stream creates two different environments, depending on where one is in relation to the jet core. If you are north of the jet core in the entrance region, where there is a poleward flow at upper levels, high pressure is created as the upper level flow sinks at higher latitudes, and the return flow is relatively weak. If you are located downstream and north of the jet core, you will encounter strong westerly winds and low-level convergence (stormy weather). There is a strong eddy heat flux (warm air advection) in that region as well. See the figure below (Fig in Holton):

5 Note that the convergence on the poleward side of the jet is thermally indirect that is, cool air is rising and warm air is sinking. Typical synoptic-scale disturbances develop under the jet in the entrance region, grow as the move under the jet core, and then decay in the jet exit region. Jet Stream Development/Propagation The change in kinetic energy of a parcel of air can be found by dot multiplying the horizontal equation of motion in isobaric coordinates by the 3-dimensional wind to yield: dk dt v = gv z a p v v 2 2 V V u + v v 2 where k = kinetic energy = V / 2 and V a is the ageostrophic wind. If 2 2 the ageostrophic component of the flow flows toward lower heights ( z < 0 ), then the RHS of (2) is positive and the kinetic energy increases with time. Qualitatively, acceleration will occur as the air parcel tries to restore geostrophic balance. Thus, jet streams are formed or intensified by mechanisms that increase the pressure (or height) gradient in a concentrated area. There are four processes that can do this: 1) Differential heating 2) Differential temperature advection 3) Differential adiabatic cooling 4) Differential vorticity advection For example, the following scenario would intensify this jet: (2) since the processes north of the jet core will lower the heights there and processes south of the jet core will raise the heights. This will increase the height gradient and thus through (2) increase the kinetic energy of the jet.

6 Alternatively, you can think of the process in terms of the thermal wind. The geostrophic wind must increase with height if the temperature gradient increases on a pressure level. The same process works to propagate the jet stream. If the forcing mechanisms occur ahead of the jet, the jet will move in that direction. Coupled Jets If two jet streaks are sufficiently close to one another and placed such that the right entrance region of one is superimposed on the left exit region of the other, their interaction will enhance the upward vertical motion in that area. These are called coupled jets (see Uccellini and Kocin, Weather and Forecasting, 1987, ). The figure below is from the Uccellini and Kocin paper note the location of the jets and the enhanced secondary circulation. This situation may contribute to bomb development.