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Lecture 14: Cloud development and forms (Ch 6)

Lecture 14: Cloud development and forms (Ch 6). Quiz 2 next Friday 13 Oct covers to end of Ch. 5 (not Ch. 6 as earlier announced) Factors influencing the ELR Limitations on the lifting of unstable air Cloud classification/types Slide show – cloud identification.

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Lecture 14: Cloud development and forms (Ch 6)

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  1. Lecture 14: Cloud development and forms (Ch 6) • Quiz 2 next Friday 13 Oct covers to end of Ch. 5 (not Ch. 6 as earlier announced) • Factors influencing the ELR • Limitations on the lifting of unstable air • Cloud classification/types • Slide show – cloud identification

  2. CMC 500 mb analysis,12Z Wed 11 Oct

  3. Wed Oct 11 09:10:21 2006 GMT… SNOW AND MXD PCPN ALNG THE QSTNRY FNT FROM AB FOOTHILLS TO MONT ON D1 WHILE COLD ADVECTION TAKES PLACE OVR PRAIRIES. THERE CUD BE UP TO 10 CM OF SNOW LOCALLY OVR AB. • storm has wrapped the freezing contour far south and westeward towards rockies CMC 850 mb analysis,12Z Wed 11 Oct

  4. absolutely stable below 550 mb (compare ELR with DALR & SALR) Cold, saturated, stable airmass

  5. … 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

  6. Height Fig. 6-9 conveys same general idea but gives an unrealistic T(z) Neutral layer (ELR=DALR) is destabilized due to heating from below 1400 0600 Nocturnal ground-based inversion is eroded by the addition of daytime heat Temperature Factors influencing the ELR 1. Surface heating & cooling

  7. Factors influencing the ELR 2. “Differential advection” Fig. 6-10

  8. What limits the ascent of parcels? This figure assumes parcel does not saturate – if it did, equil. lvl. is higher • entrainment of cooler, unsaturated environmental air into the rising parcel – may result in evaporation of water droplets, further cooling the parcel • and/or, more importantly: • ascent of the parcel into a stable layer Here the capping stable layer is shown as an inversion – but any lapse rate weaker than the SALR will do the job… Fig. 6-12

  9. Fig. 6-13 • frontal inversion Fig. 6-14 Photo :Keith Cooley • subsidence inversion • (common in lee of Rockies) Types of inversions… • radiation inversion

  10. CLOUD “concentration of suspended droplets and/or ice crystals in air well above the surface” (p146, 4th edition) Luke Howard’s four basic cloud forms: stratus sheet-like (Latin - “layer”) cumulus puffy (“heap”) cirrus wispy (“curl of hair”) … high, cold; thin, ice nimbus raining (“violent rain”) … others described by combining basic types, and considering their height (low/mid/high), depth (vertical development) and causal origin

  11. Ice crystals Mostly liquid droplets Weak lift

  12. Fig. 6-15

  13. Lenticular Clouds Moist air rises on the upwind side… cools and condenses…on the downwind side, air sinks and warms – the cloud evaporates… clouds appear motionless as the air rushes through

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