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

• dry atmosphere• light winds• strong nocturnal inversion

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

(we hit 20oC by late afternoon)

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.

CMC 500 mb analysis,12Z Thurs 5 Oct

• warm & dry over Edmonton• Edmntn on edge of main stream, which is carrying cloud to our N • ridge over Saskatchewan

CMC 850 mb analysis,12Z Thurs 5 Oct

• warm & dry over Edmonton• weak flow• identify temperature advection

Weak cold advection

Stronger warm advection

• 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) – 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:the four mechanisms that cause lift:

Orographic lift

Fig. 6-2

Frontal lift

Fig. 6-5

Weather system convergence (cross-isobar flow into a Low)

Localized convection

• slow lift• large area

• fast lift• small area

“… 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.”)

Static Stability and the ELR in relation to cloudsStatic Stability and the ELR in relation to clouds

These “whyswhys” can be explored by considering the causal origin of the lift (spontaneous?spontaneous?, eg. Cumulus due to thermals in an unstable boundary layer; or forced?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

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).

Assessing Static Stability of the AtmosphereAssessing 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.

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** absolutely** unstableunstable atmospheric layer” - air parcel rising atmospheric layer” - air parcel rising becomes warmer than surrounding (“environmental”) airbecomes warmer than surrounding (“environmental”) air

** absolute stability (instability) is also named “unconditional” stability (instability)

Fig. 6-6Fig. 6-6

• ELR exceeds both DALR and SALRELR exceeds both DALR and SALR

““absolutely absolutely stablestable atmospheric layer” - air parcel rising becomes atmospheric layer” - air parcel rising becomes colder than surrounding (“environmental”) aircolder than surrounding (“environmental”) air

Fig. 6-7

• both DALR and SALR exceed the ELRboth DALR and SALR exceed the ELR

Sec 6-1 Fig. 1

DALRDALR

SALRSALR

ELRELR11

ELRELR33

absolutely stableabsolutely stable

absolutely unstableabsolutely unstable

““conditionallyconditionally unstableunstable** atmospheric layer”** atmospheric layer”

** same as conditionally stable

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)

Sec 6-1 Fig. 1

DALRDALR

SALRSALR

ELRELR22

absolutely stableabsolutely stable

conditionallyconditionallystablestable

height

temperature

DALR SALRELR

absolutely stable

absolutelyunstable

conditionallyunstableIf ELR is parallel to

DALR (MALR) the layer is said to be “neutral” with respect to dry (saturated) adiabatic motion

(see Sec 6-1, 3rd edition)Nb! It is slope Nb! It is slope that matters… that matters… not the not the temperature temperature itselfitself

… since a stablystably-stratified layer strongly resists the upward motion of parcels, those parcels that, due to being forced to do so, dodo rise, will tend to spread horizontally in thin layers… flat tops and bases… “stratiform” clouds

… deep layers of the atmosphere are seldom absolutely unstableunstable … 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 neutralneutral stratification).