Transcript Slide 1

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
Source Region
Continental (c)
Maritime (m)
Polar (P)
cP
cold,dry,
clear,stable
mP
cool,moist
cloudy,
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
Tropical (T)
cT
hot, dry, unstable
near surface
mT
warm, moist;
usually unstable
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
• rare to see all of these signs
• somewhat subjective
• 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
Idealized 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
A
Ideal cold front, showing:
• T
• jump in wind direction
• frontal lift (causing cloud, precip)
Fig. 9-6
Map symbol
• front itself can move rapidly (up to a nominal 50 kph)
Ideal warm front, showing:
Fig. 9-8
• T
• overrunning warm air (stable ELR)
• gently sloped frontal boundary
• progression of stratiform cloud types
Would you diagnose a front
(or fronts) associated with
the Manitoba storm? Where?
Why?
Cloud
type
Ns
(
,
St
(
)
)
auto-station
( )
mist
Sc
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. 9-4
warm air cut off from the
surface by the meeting of two
cold fronts
Fig. 9-10