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

CMC 500 mb analysis,12Z Wed 11 Oct

CMC 850 mb analysis,12Z Wed 11 Oct

• storm has wrapped the freezing contour far south and westeward towards rockies

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.

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

… 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).

Factors influencing the ELRFactors influencing the ELR

Neutral layer (ELR=DALR) is destabilized due to heating from below

Height

Temperature

0600 1400Nocturnal ground-based inversion is eroded by the addition of daytime heat

Fig. 6-9 conveys same general idea but gives an unrealistic T(z)

1. Surface heating & cooling1. Surface heating & cooling

Factors influencing the ELRFactors influencing the ELR

2. “Differential advection”2. “Differential advection”

Fig. 6-10

What limits the ascent of parcels?What limits the ascent of parcels?

• 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

Fig. 6-12

Here the capping stable layer is shown as an inversion – but any lapse rate weaker than the SALR will do the job…

This figure assumes parcel does not saturate – if it did, equil. lvl. is higher

Types of inversions…Types of inversions…

Photo :Keith Cooley

• radiation inversion

• frontal inversion

Fig. 6-13

Fig. 6-14

• subsidence inversion(common in lee of Rockies)

CLOUDCLOUD

““concentration of suspended droplets concentration of suspended droplets and/or ice crystals in air well above and/or ice crystals in air well above the surface” (p146, 4the surface” (p146, 4thth edition) edition)

Luke Howard’s four basic cloud forms:Luke Howard’s four basic cloud forms:

stratusstratus 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

Mostly liquid droplets

Ice crystals

Weak lift

Fig. 6-15

Lenticular CloudsLenticular 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