Lecture 13: Map discussion + Cloud development and forms (Ch 6)

1. 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

2. Thurs 5 Oct. Fine, calm, sunny day… frost on rooves near university; patchy on lawns (we hit 20oC by late afternoon) • dry atmosphere • light winds • strong nocturnal inversion Nocturnal cooling

3. 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.

4. 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

5. warm & dry over Edmonton • weak flow • identify temperature advection Weak cold advection Stronger warm advection CMC 850 mb analysis,12Z Thurs 5 Oct

6. 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

8. anticlockwise winds • crossing the isobars • p falling on east side • p rising on west side

9. Fig. 6-5 Frontal lift Localized convection Weather system convergence (cross-isobar flow into a Low) 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 Orographic lift • slow lift • large area • fast lift • small area

10. 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

11. 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

12. 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”)

13. “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)

14. Fig. 6-7 “absolutely stable atmospheric layer” - air parcel rising becomes colder than surrounding (“environmental”) air • both DALR and SALR exceed the ELR

15. Sec 6-1 Fig. 1 SALR DALR absolutely stable ELR3 ELR1 absolutely unstable

16. “conditionallyunstable** 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

17. Sec 6-1 Fig. 1 SALR DALR absolutely stable ELR2 conditionally stable

18. (see Sec 6-1, 3rd edition) Nb! It is slope that matters… not the temperature itself DALR SALR ELR absolutely stable height conditionally unstable If ELR is parallel to DALR (MALR) the layer is said to be “neutral” with respect to dry (saturated) adiabatic motion absolutely unstable temperature

19. … 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