Wednesday, October 08, 2003
By the end of the class, students should be able to:
- distinguish between equivalent barotropic and baroclinic regions of the atmosphere
identify zones of warm and cold advection on a map of sea-level pressure
and 1000-500 mb thickness;
identify layers of warm an cold advection from veering and backing winds
in a sounding profile.
Thickness and Temperature
Recall: The hypsometric equation realates the vertical distance between pressure
levels in the atmosphere t the mean temperatures within the layer. The
pressure increases much more rapidly in cold, dense air as we move down
than in warm, light air. In the middle latitudes, the 1000-500 thickness
pattern generally follows that of the upper air heights, so that upper
level lows (highs) correspond to cold (warm) air.
Equivalent barotropic vs. baroclinic atmosphere
The 500 mb CMC anysis for 12Z 10 Oct 2001
shows contours of 500 mb height and 1000-500 mb thickness. Equivalent barotropic systems
(black circles) can be seen over the Pacific Ocean, where the jet stream flows paralell
to the thickness gradient, and in "cutoff lows" in the arctic, which correspond to regions
of isolated cold pools.
Height and thickness contours are more or
less parallel in these regions. This is an equivalent barotropic environment.
Where thickness and height contours intersect over North America (red circle), we have a baroclinic environment where
synoptic eddies deform the thermal field. Over Eastern North America, south westerrly winds advect a thermal
ridge westward. We have warm advection because winds blow across thermal gradients from warm to cold.
Temperature advection and the thermal wind
Figure 2 shows the thermal wind shear associated with warm and cold advection
in an extratropicl cyclone. We note warm southerly winds to the right of
the advancing cyclone, and cold northerly ones to the west. Veering winds
turn clockwise with height in the zone of warm advection, and counterclockwise
where there is cold advection. Note the deformation in the upper level
flow associated with the cyclone. Warm advection (more precisely, the Laplacian
of thermal advection) forces ascent and increased cyclonic vorticity in the warm
sector, and desceding motion and increased anticyclonic vorticity in the
cold sector of the storm and increased.
|Figure 2 |
N.B. : veering in the boundary layer below 900 mb is often associated with
Eckman friction and not thermal advection.
The GEM model initialization for 12Z 10 Oct. 2001
shows thickness and sea-level pressure in the top right panel. Dashed lines represent thickness,
while the critical thickness (540 dam) is marked by shading between 534 and 540 dam
A broad region of light
warm advection (red circle) dominates eastern North America between a ridge over the eastern U.S.
and a cyclone near the Great Lakes. Because thermal gradients are weak, the warm advection
is weak, and there is only
scattered cloud and precipitation associted with this warm advection.
A more sharply defined region of cold advection (blue circle) appears in the midwestern U.S.
The border between this zone of cold advection and the broad zone of warm advection
defines an advancing cold front.
The 850 mb analysis is probably the best chart to use to diagnose
thermal advection that forces fall and winter precipitation systems.
This level is generally above the boundary layer, so that winds are
geostrophic and unaffected by surface friction, yet low enough to reflect the stronger
thermal gradients found near surface fronts and baroclinic zones. In
the 850 mb analysis for 12Z 10 Oct. 2001,
blue and red circles
indicate regions of broad-scale cold and warm advection respectively.
A satellite loop
shows convection breaking out in the region of warm advection to the east of the front.
A map of rawinsonde sites allows us to locate soundings
in regions of warm advection
(North Platte [Nebraska],
Dodge City [Kansas]),
The similarity of soundings at Maniwaki and Nashville indicate the extent of the warm air mass over eastern North
America. The soundings are generally stable below 850 mb. Winds show Ekman veering below 925 mb
and weak veering associated with warm advection above this level all the way to the tropopause.
This indicates an environment of weak
ascent and scattered cloud.
- Soundings at North Platte and Dodge City are behind the advancing cold front, with North Platte being
the colder of the two (compare temperatures below 850 mb). The wind profiles are similar, however.
The boundary layer (925-850 mb) shows Ekman veering despite strong cold advection. However,
the 850-600 mb layer shows a dramatic backing of the winds, indicating strong cold advection and
and forcing for descent.
Soundings at Davenport and Lincoln indicate the presence of convection. Wind profiles show little
evidence of thermal advection. At Davenport, however, we see an almost adiabatic layer (880-720 mb)
that is highly condiinally unstable and saturated above 840 mb. Lifting a parcel from the bottom of this
saturated layer indicates and unstable environment, convection will soon break out in this region. At Lincoln,
the rawinsonde has already been lost to on of these storms at this level.
Real-time diagnostics for temperature advection can be found at
Quasi-geostrophioc analysis of Wednesday Oct. 8, 2003
- U.S. Radar and
U.S. Satellite analysis shows:
- precipitation and cloud along the Northwest Pacific coast of the U.S.
- a weak band of precipitation along the border of Saskatchewan and Manitoba
- a broad region of scattered showers over southern Texas.
- intense convection along the Atlantic coast of the southeastern U.S.
- affecting Montreal is a broad region of clear skies covering most of the northeastern U.S.
- The CMC surface analysis for 12Z 8 Oct. 2003 shows
- a 971 mb cyclone off the coast of British Columbia associated with the Pacific Northwest precipitation
- a weak 1002 mb cyclone over the Dakotas and with trough extending southward to Texas; this is associated with scattered precipitation over between Saskatchewan and Texas
- a stationary front off the coast of the southeastern U.S. associated with convective precipitation in this region
- an extensive anticyclone extending from the Atlantic across Montreal and the northeastern U.S. to the Great Lakes; this is associated with clear skies in these regions
- The CMC 250 mb analysis for 12Z 8 Oct. 2003
show the position of various climatological jetstreams and associated jet entrance and exit regions that contribute to the observed weather (some of these features also appear on the 300 mb ETA OO hr forecast) .
- The Pacific, subtropical, and Atlantic jets are all climatological features
that can usually be identified on any synoptic chart. Their position, intensity, and extent may vary, however.
- The precipitation over the Pacific Northwest is associated witht the left-exit region of the pacific jet.
- The clear skies over Northern California are associated with the right-exit region of the Pacific jet.
- The convection off the coast of the southeastern U.S. is associated with the left-exit region of the substropical jet.
- The broad zone of clear skies over the noertheastern U.S. and Montreal appears to be associated with the left-entrance region of the Atlantic jet.
- The 500 mb 00 hr GEM forecast for 12Z 8 Oct. 2003 and
500 mb 00 hr ETA forecast identify vorticity maxima associated with upper-level troughs and jet streaks. On the GEM analysis, regions of cyclonic or positive vorticity advection (CVA, PVA) are depicted in red; regions of anticyclonic or negative vorticity advection (AVA, NVA) are depicted in blue.
- there is CVA in the left exit region or the Pacific jet this forces the ascending motion associated with precipitation and the intense surface cyclone observed in this area.
- in the right-exit region of the pacific jet, there is AVA; note that the ETA analysis display smoothes out many of the vorticity centers depicted in the GEM analysis into a large area of NVA
- there is strong cva between a trough over Montana and a ridge over the Dakotas associated with precipitation and a surface cyclone
- Only a hint of CVA an be observed ahead of the cut-off low centered over the southeastern U.S. on the 500 mb map: troughs and jetstreaks embeddedd in the subtropical jet are sometimes not apparant on 500 mb maps. This is because the subtropical jet is higher (above 200 mb) than the polar Atlantic and Pacific jets (below 200 mb). It is more appropriate to use 200-300 mb maps to analyze the subtropical jet.
- The CMC 500 mb analysis identifies equivalent barotropic and baroclinic regions of the atmosphere. In barotropic portions of the atmosphere, the geostrophic wind flows paralel to the temperature gradient. There will be no or little thermal advection. In baroclinic portions of the atmosphere, the geostrophic flow runs across temperature or thickness gradients. Temperature advection will be significant in these regions.
- The Pacific jet is equivalent barotropic for the most part. At its exit region, it becomes more baroclinic.
- There is a 500 mb ridge over northern Caanda that deflects the polar het northward. The flow in the polar jet upstream (west) of the 500 mb ridge is baroclinic. Downstream (east) of the ridge, the flow is equivalent barotropic.
- The top right panel of the GEM 0 hr GEM forecast for 12Z 8 Oct. 2003
depicts SLP and 1000-500 mb thicknesses and can be used to analyse thermal advection in the lower troposphere.
- Along the Pacific coast, there is warm advection ahead of the surface cyclone in the warm sector before (east) and cold advection behind (west) the cold front. The front can be analyzed as the border between warm and cold advection. U.S. Satellite analysis shows intense cloud in the region of warm advection and clearing behind the front associated with cold advection.
- Over the Canadian Arctic, there is cold advection between an anticyclone located just north of Manitoba and a cyclone just north of Quebec.
- There is a broad zone of weak warm advection to the north of the large anticyclone dominating eastern north America. This warm, dry flow will bring Montreal unseasonable warm and dry conditions for at least half a week.
- The CMC 850 mb analysis and
ETA 850 mb analysis include winds and are more precise in locating baroclinic zones. On the CMC analysis, blue and red circles identify stations showing cold and warm advection, respectively.
- WMW - Maniwaki, Qc: An equivalent barotropic sounding with very little veering or backing.
- BUF - Buffalo, NY: Gentle veering between 900 and 700 mb associated with warm advection. This brings in warm , dry air without much cloud formation. The atmosphere will like be dominated by barotropic forcing (i.e. vorticity advection) in upper troposphere.
- YZT - Port Hardy, BC:
- This is in the zone of warm advection associated with the storm over the Pacific Northwest.
- Strong lower tropospheric veering between 900 mb and 600 mb.
Strength of winds (50 knots) indicates very strong forcing associated with veering.
- Backing of winds and cold advection 600 - 300 mb. Winds are weaker and there is less change in direction than in lower layer, so that forcing for descent will be overwhelmed by forcing for ascent in lower layer.
- Veering of winds and warm advection 300-150 mb forces ascent.
- WPL - Pickle Lake, ON: Strong backing of winds 925-850 mb indicating strong cold advection; rest of sounding is more or less equivalent barotropic.
- YCB - Cambridge Bay, NWT: Strong veering to 600 mb indicating warm advection; above this, sounding is equivalent barotropic; cloud layer starts at 700 mb, indicating that strongest ascent occurs in middle troposphere.
- LBF - North Platte, NE:
- LBF is located in region of shallow warm pool over midwestern U.S.
(see ETA 850 mb analysis).
Temperatures at 850 mb are warm. As a result, atmosphere is almost adiabatic above 850 mb.
- Southerly wind actually advects in cold air at 850 mb. Winds back between 850 and 800 mb, indicating cold advection. Above this, the winds veer with height, indicating warm advection.