Lecture 19: Air Masses & Fronts (Ch.9)

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

Airmass • 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 classification • 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 regions Fig. 9-1 • cP by far our most common airmass in Ab. • eg. classic Ab. summer day - dry, cool, light Cu

(p256-7) Winter-time • 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 • In summer, less extreme • not so dry • daytime heating erases inversion, permits Cu

cA 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

Airmass modification – exemplified here as mechanism to form mP airmass… (p258) * * Barely appropriate to name a particular storm the “Aleutian low”… latter is a climatological feature

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 all of these signs • somewhat subjective

Idealized frontal structure of a mid-latitude cyclone Fig. 9-5 A • 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

Ideal cold front, showing: Map symbol • T • jump in wind direction • frontal lift (causing cloud, precip) Fig. 9-6 • front itself can move rapidly (up to a nominal 50 kph)

Ideal warm front, showing: • T • overrunning warm air (stable ELR) • gently sloped frontal boundary • progression of stratiform cloud types Fig. 9-8

Would you diagnose a front (or fronts) associated with the Manitoba storm? Where? Why? Ns ( , ) St ( ) auto-station ( ) mist Sc Cloud type

The 850 mb isotherms help CMC 850 mb analysis, 12Z Sep 20, 2004.

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

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

Fig. 9-4 Fig. 9-10 Fig. 9-4 warm air cut off from the surface by the meeting of two cold fronts

Lecture 19: Air Masses & Fronts (Ch.9)