Transcript Lecture 13 - University of Alberta

Lecture 13: Map discussion + Cloud development and forms (Ch 6)
• Quiz 2 next Friday 13 Oct covers to end of Ch. 6
• map discussion (situation of Thurs 5 Oct)
• mechanisms that cause lift
• classifying the “static stability” of atmospheric layers
Thurs 5 Oct. Fine, calm, sunny day… frost on rooves near university; patchy on lawns
(we hit 20oC by late afternoon)
City Centre Airport
MDT
T
Td
12 Sunny
13
2
11 Sunny
10
3
10 Sunny
7
2
09 Sunny
4
2
08 Light Drizzle
3
2
07 Clear
4
3
06 Clear
4
3
05 Clear
5
3
04 Clear
6
3
03 Clear
6
3
02 Clear
7
3
01 Clear
8
3
Nocturnal cooling
• dry atmosphere
• light winds
• strong nocturnal inversion
ENVIRONMENT CANADA AT 7:00 AM CDT THURSDAY OCTOBER 5 2006.
DISCUSSION... UNSEASONABLY WARM TEMPERATURES FOR NEXT TWO
DAYS BEFORE COOLING COMES TO PRAIRIES AS UPPER RIDGE COLLAPSES
AND LONG WAVE TROUGH BUILDS OVER ALBERTA.
2:00 PM CDT THURSDAY … AB…UPPER TROUGH DEVELOPING OVER THE
PROVINCE TONIGHT WILL GENERATE BROAD AREA OF PRECIPITATION OVER
CENTRAL AND SOUTHERN ALBERTA. COLDER AIR FROM THE NORTH WILL
MOVE INTO ALBERTA ON FRIDAY RESULTING IN THE POTENTIAL FOR SNOW
OVER MOUNTAIN REGION ON FRIDAY NIGHT.
• warm & dry over Edmonton
• Edmntn on edge of main stream,
which is carrying cloud to our N
• ridge over Saskatchewan
CMC 500 mb analysis,12Z Thurs 5 Oct
Weak cold advection
• warm & dry over Edmonton
• weak flow
• identify temperature advection
Stronger warm
advection
CMC 850 mb analysis,12Z Thurs 5 Oct
• temperature advection is
accomplished by the velocity
component that is perpendicular
to the isotherms
• at a rate proportional to that
velocity component and
proportional to the temperature
gradient
• thus the smallest rectangles
with corner angles closest to 90o
are zones of strongest advection
• warm advection where warm
isotherm blows towards cold
isotherm, and vice versa
• advection best viewed on the
850 mb chart
• anticlockwise winds
• crossing the isobars
• p falling on east side
• p rising on west side
Cloud development and forms (Ch6) – “This chapter discusses the
processes and conditions associated with the formation of clouds due to upward
motions”
• the four mechanisms that cause lift:
Fig. 6-2
Fig. 6-5
Frontal lift
Orographic lift
• slow lift
• large area
• fast lift
• small area
Weather system convergence
(cross-isobar flow into a Low)
Localized
convection
Static Stability and the ELR in relation to clouds
“… clouds form as air rises, expands, and cools. But why does the
air rise on some occasions, and not on others? And why does the size
and shape of clouds vary so much when the air does rise?” (Ahrens,
“Essentials of Meteo.”)
These “whys” can be explored by considering the causal origin of
the lift (spontaneous?, eg. Cumulus due to thermals in an unstable
boundary layer; or forced?, eg. wind forced to blow over a
mountain) and the inherent susceptibility of the atmosphere to
permit (or limit/attenuate/forbid) ascent of parcels
Assessing Static Stability of the Atmosphere
We compare the ELR (environmental lapse rate) against “benchmark”
lapse rates (DALR, SALR) that are defined by appeal to an idealized
conceptual process of the adiabatic lift of a parcel.
ie. the dry adiabatic lapse rate (DALR) is an idealised "benchmark"
process - we vizualize the vertical motion (upward or downward) of an
intact "parcel" of unsaturated air, contained in an imaginary flexible
skin that freely permits the parcel to expand or contract so as to remain
at the local pressure, but, which prevents heat exchange with the
outside atmosphere. The thermodynamic process undergone by such
an idealised parcel as it ascends (or descends) is said to be "adiabatic
expansion" (adiabatic compression, if descending).
by comparing the ELR with the “benchmarks” (DALR, SALR) we
can assign a stability class to any layer
Will an “air parcel” that has been dislodged on the vertical axis
experience:
- a restoring buoyancy force (statically stable)
- no buoyancy force (“statically neutral”)
- a buoyancy force exacerbating (reinforcing) the original
disturbance (“statically unstable”)
“absolutely** unstable atmospheric layer” - air parcel rising
becomes warmer than surrounding (“environmental”) air
Fig. 6-6
• ELR exceeds both DALR and SALR
** absolute stability (instability) is also named “unconditional” stability (instability)
“absolutely stable atmospheric layer” - air parcel rising becomes
colder than surrounding (“environmental”) air
Fig. 6-7
• both DALR and SALR exceed the ELR
Sec 6-1 Fig. 1
SALR
DALR
absolutely stable
ELR3
ELR1
absolutely unstable
“conditionally unstable** atmospheric layer”
Fig. 6-8
“If the atmosphere is conditionally unstable, an air parcel becomes buoyant if
lifted above some critical altitude… the level of free convection” (p163)
** same as conditionally stable
Sec 6-1 Fig. 1
SALR
DALR
absolutely stable
ELR2
conditionally
stable
Nb! It is slope
that matters…
not the
temperature
itself
(see Sec 6-1, 3rd edition)
DALR
SALR
ELR
absolutely stable
height
If ELR is parallel to
DALR (MALR) the layer
is said to be “neutral”
with respect to dry
(saturated) adiabatic
motion
conditionally
unstable
absolutely
unstable
temperature
… deep layers of the atmosphere are seldom absolutely
unstable … Why? Because any transient instability
will result in vertical motion that mixes heat and so
reduces or removes the instability (the layer in question
returning spontaneously towards neutral stratification).
… since a stably-stratified layer strongly resists the
upward motion of parcels, those parcels that, due to
being forced to do so, do rise, will tend to spread
horizontally in thin layers… flat tops and bases…
“stratiform” clouds