Lecture 19: Air masses & fronts (Ch 9)

• answers to numerical problems are now on the web

• definition & classification of airmasses

• airmass formation/transformation

• fronts & their recognition

AirmassAirmass

• an idealisation

• body of air with rather uniform T, Td over huge horizontal distance; airmasses separated by narrow boundary zones, ie. “fronts”

• originate by having stagnated (light winds, anticyclonic conditions) in a source region - generally uniform

• in mid latitudes there is strong spatial variation in T, p (etc.) and (thus) strong winds. In mid-latitudes therefore we have a transition zone: air masses invade, confront each other across fronts, are modified… producing “weather”

• concept of “airmass weather” – static, because one is in the interior of an airmass: diurnal changes only

• passage of a front is a significant weather event – large sudden change

Airmass classificationAirmass classification

Source Region Polar (P) Tropical (T) Continental (c) cP

cold,dry, clear,stable

cT hot, dry, unstable near surface

Maritime (m) mP cool,moist cloudy, unstable

mT warm, moist; usually unstable

• extremely cold cP air is called continental arctic (cA)

• though uniform horizontally, an airmass cannot be uniform in the vertical… necessarily there are vertical gradients, affecting airmass stability

Airmass source regionsAirmass source regionsFig. 9-1

• cP by far our most common airmass in Ab.

• eg. classic Ab. summer day - dry, cool, light Cu

• hi latitude winter

• long night, low sun

• snowcover? – high albedo

• therefore daily totalized Q* likely to be negative

• airmass cooled from the base implies inversion (poor mixing – bad air quality), no convection

• which may deepen day after day

• cold, dry air + subsidence, few clouds

Winter-timeWinter-time

• In summer, less extreme

• not so dry• daytime heating erases inversion, permits Cu

(p256-7)

cA airmasses and the arctic frontcA airmasses and the arctic front

• extremely cold airmasses are usually shallow

• sometimes one may distinguish a sharp boundary with less extremely cold and dry air, ie. the “arctic front”

• little or no “weather” associated with such fronts (too dry)

Fig. 9-2

(p258)

Airmass modification – exemplified here as mechanism to form Airmass modification – exemplified here as mechanism to form mP airmass…mP airmass…

*

* Barely appropriate to name a particular storm the “Aleutian low”… latter is a climatological feature

Criteria to locate fronts (ie. airmass boundaries)Criteria to locate fronts (ie. airmass boundaries)

• large T over short distance (packed isotherms)

• large Td over short distance

• sudden change in wind direction

• sudden change in sign or magnitude of pressure trend p /t

• clouds and precipitation

• front located along troughline (ie. along kink or bend in isobars)

As a front sweeps by, the above noted spatial changes are experienced as a rapid temporal change

Classic signs of cold front passage in Alberta: suddenly gusting wind, turns from SE or S or SW towards W or NW; rapid cooling; clearing follows

• rare to see rare to see allall of these signs of these signs• somewhat subjectivesomewhat subjective

Idealized frontal structure of a mid-latitude cycloneIdealized frontal structure of a mid-latitude cyclone

• fronts along/near isobar kinks• low level cross-isobar wind• wind direction change across

front• imagine whole picture moves eastward… observer at A sees initially falling, then steady, then finally rising pressure

Fig. 9-5

AA

Fig. 9-6

Ideal cold front, showing:Ideal cold front, showing: • T• jump in wind direction• frontal lift (causing cloud, precip)

Map symbol• front itself can move rapidly (up to a nominal 50 kph)

Fig. 9-8

• T• overrunning warm air (stable ELR)• gently sloped frontal boundary• progression of stratiform cloud types

Ideal warm front, showing:Ideal warm front, showing:

Would you diagnose a front (or fronts) associated with the Manitoba storm? Where?Why?

Cloudtype

Ns( , )

St( )

auto-station ( )

mist

Sc

The 850 mb isotherms help

CMC 850 mb analysis, 12Z Sep 20, 2004.

CMC surface analysis, 12Z Nov 28, 2003. Storm trough through C. and NE. Ab, plus wind induced lee trough in the SW; wind warning for SW Ab.

Would you diagnose a front (or fronts) associated with this N. Alberta storm?

very coldvery cold

coldcold

mildmild

CMC 700 mb analysis, 12Z Nov 28, 2003. SW current aloft across Rockies

warm air cut off from the surface by the meeting of two cold fronts

Fig. 9-10Fig. 9-4

Fig. 9-4