Development of High and Low Pressure SystemsMETR 100-1503 March 2015

OverviewExtratropical cyclone: low pressure system forming in midlatitudes, with counterclockwise wind in the N. Hemisphere Bring most severe weather we experienceso, how do they form?

Force Imbalances Force imbalances in horizontal airflow may cause vertical motion via Convergence Directional SpeedDivergence Directional Speed

Conservation of MassAir not very compressible Thus, convergence or divergence require vertical motion!

Conservation of MassConvergence: Air cannot flow into the surface, so it rises Air cannot easily enter the stratosphere, so it sinks Divergence: Air must fill a gap created by divergence Thus, air flows toward wherever there is divergence

Conservation of MassLow pressure (cyclone): surface convergence

High pressure (anticyclone): surface divergence

Forces in a Cyclone Start in geostrophic balanceAs it circles the low, direction of PGF has component opposite parcels motion Force imbalance! The parcel slowsso Coriolis effect weakens An inward-pointing force now exists

Forces in a CycloneCentripetal acceleration: the net inward-directed forceThe wind slowed: it is subgeostrophic in cyclonically-curved flow

In an AnticycloneStart in geostrophic balance again Now, the PGF has a component in the direction of the flow Force imbalance! The parcel speeds up An outward-pointing force now exists

In an AnticycloneThe wind sped up: it is supergeostrophic in anticyclonically-curved flow

Recall what a typical upper-air pattern looks like: a series of waves!

Putting the pieces togetherAreas of cyclonic & anticyclonic curvature aloft produce areas of subgeostrophic & supergeostrophic winds So where doyou think asurface lowwill develop?

A surface high?

Add a JetstreakA few more definitions: Entrance region: region where air flows into a jetstreak Exit region: region where air leaves a jetstreak Also split into right and left quadrants (imagine yourself moving with the wind)

Entrance RegionA jetstreak exists because there is a locally stronger gradient of heights The PGF increases, causing a force imbalance A parcel accelerates toward lower pressure Corresponding change in Coriolis force follows this acceleration Thus, parcel moves northeast in the entrance region!

Exit RegionIn the exit region, the opposite occurs PGF decreases Coriolis force now exceeds PGF Parcel accelerates southeastward

Add a Jetstreakand a pattern of convergence/divergence is set up!

Four-quadrant model of a jetstreak

Coupled JetstreaksWhat do you think would happen here?

Combine Curvature & JetstreaksNorth of the jetstream axis, surface highs and lows are favored To the south, neither is especially favored

Other EffectsFrictionRecall the force balance around a low at the surface:

Thus, airflow has a component toward lows/away from highs This effect is strongest at the surface

Other EffectsFrictionThus, friction works to: Weaken a low Weaken a high

Friction works to equalize pressure (reduce imbalances!)

Other Effects...HeatingDiabatic processes: processes in which heat energy is transferred Solar heating Recall that warmer layers are thicker:

Other Effects...HeatingHeating causes layer to deepen pressure gradient forms aloft divergence from the heated area convergence near surface a low forms! This is called a thermal low

Other Effects...CoolingCooling causes a layer to thin a pressure gradient forms aloft convergence toward the cooled area divergence at the surface high pressure forms!

Integrating EverythingAll these effects (and others) work with/against each otherthe result is weather systems!!

Integrating EverythingHighs and lows are often connected systems