Todays lecture objectives:

1
ATMS 455 Physical Meteorology

• Todays lecture objectives
• Cloud Morphology (WH 5.1)
• How do microscopic processes impact weather
  phenomena on the mesoscale?

http//www.artcyclopedia.com/feature-2001-08.html
2
ATMS 455 Physical Meteorology

• Todays lecture topics
• Cloud Morphology (WH 5.1)
• Mechanisms of formation
• Types of clouds
• Convective clouds
• Layer clouds
• Orographic clouds

3
Introduction

• At any one time, about one half of the earths
  surface is covered by clouds
• Clouds can occur at altitudes from the surface of
  the earth up to about 20 km
• Nacreous (mother-of-pearl) clouds occur at
  heights up to 30 km and noctilucent clouds occur
  at about 80 km exact compositions of these
  clouds are unknown

http//www.carlwozniak.com/clouds/Graphics/New20P
ix/clouds14.jpg
4
Mechanisms of formation

• Clouds form in air which has become
  supersaturated with respect to liquid water or
  ice happens most commonly through ascent
  accompanied by adiabatic expansion and cooling

http//www.carlwozniak.com/clouds/Graphics/New20P
ix/clouds03.jpg
5
Mechanisms of formation

• A brief review of environmental stability
• Absolute stability (G
• Conditional instability (Gd
• Absolute instability (Gd

Gd g/cp 9.8 oC/km
http//weather.uwyo.edu/upperair/sounding.html
6
Mechanisms of formation

• Principal types of ascent
• Local ascent of warm buoyant air parcels in a
  conditionally unstable environment produces
  convective clouds
• Vertical velocities 3 m s-1
• Cloud lifetimes range from minutes to hours

http//www.carlwozniak.com/clouds/Graphics/New20P
ix/clouds36.jpg
7
Mechanisms of formation

• Principal types of ascent
• Forced lifting of stable air which produces layer
  clouds
• Vertical velocities range from 3-10 cm s-1
• Cloud lifetimes are in the range of tens of hours

http//www.carlwozniak.com/clouds/Graphics/New20P
ix/clouds39.jpg
8
Mechanisms of formation

• Principal types of ascent
• Forced lifting of air as it passes over hills or
  mountains produces orographic clouds
• Vertical velocities several m s-1
• Cloud lifetimes depend on wind direction steady
  winds means long lifetimes

http//www.cartage.org.lb/en/themes/Sciences/Earth
science/Hydrology/Meteorology/Clouds/CloudDescript
ions/Lenticular2/Lenticular2.htm
9
Mechanisms of formation

• Clouds may also form by the cooling of air below
  its dew point when it comes into contact with a
  cold surface (e.g. radiation fog, advection fog)

http//asd-www.larc.nasa.gov/SCOOL/New_Clouds/Fog/
fog6.jpg
10
Mechanisms of formation

• The mixing of two parcels of air with different
  temperatures (e.g. steam fog)

http//www.pbase.com/clickaway/image/34671223
11
Mechanisms of formation

• Adiabatic expansion and cooling due to a rapid
  local reduction in pressure (e.g. funnel clouds)

http//hms.pnl.gov/funnels.htm
12
Types of clouds

• Currently used cloud classification scheme was
  first proposed back in 1803

13
Types of clouds

• Currently used cloud classification scheme was
  first proposed back in 1803 by Howard (not Moe,
  Curly, Larry, or Shemp) - Luke
• Cumulus (heap or pile)
• Stratus (a layer)
• Cirrus (filament of hair)
• Nimbus (rain clouds)

14
Types of clouds

• Other names for the classification scheme
• alto (mid-level)
• lenticularis (lens-shaped clouds)
• castellanus (turrets)
• Main characteristics for classifying clouds are
  depth and altitude

15
Types of clouds
16
Convective clouds
55 minute period
17
Convective clouds

• Physical mechanisms
• Well-defined cloud bases indicates
• that the air at lower levels is well mixed
  due convective stirring (a)
• Boundaries of young clouds are sharp, giving them
  the cauliflower appearance due to the clouds
  consisting mainly of liquid water (a)
• Cumulus congestus in (a) likely due to growth
  above a particularly hot spot on the ground

18
Convective clouds

• Physical mechanisms (cont.)
• As clouds age, their outlines become
• more ragged due to decrease buoyancy and due
  to the increasing presence of ice particles
  (b)-(e)
• Anvil begins to form as upper regions of the
  cloud are starting to spread out horizontally by
  the wind at this level (b). Largest cloud in (b)
  has transformed into a cumulonimbus.
• due to (1) growth of ice at expense of liquid
  droplets and (2) ice particles evaporate slower
  than water droplets at the edges of the cloud
  see WH Fig. 2.7

19
Convective clouds

• Physical mechanisms (cont.)
• Regions on the right of the largest
• cloud in (c) and (d) have become increasingly
  diffuse as the concentrations of ice particles
  increase and the anvil becomes larger
• The upper regions of the cumulonimbus have
  glaciated (become dominated by ice particles) in
  (e)
• Both anvil cirrus and stratocumulus clouds can
  restrict the heating of the ground and thereby
  inhibit the formation of new convective clouds

20
Convective clouds

• Cloud towers are produced by elements of rising
  buoyant air called thermals

21
Convective clouds

• As a thermal rises, it pushes environmental air
  away from its upper boundary
• At the same time, environmental air is entrained
  into the turbulent wake beneath the thermal (a)
  and (b)
• Some environmental air is entrained through the
  sides and top of the thermal due to turbulent
  mixing
• As a result of entrainment, the diameter of a
  thermal initially increases as it rises (a)

22
Convective clouds

• Thermal widening ceases above the LCL due to the
  entrainment of cool, dry air
• Some cloud water evaporates and the thermal
  buoyancy is reduced
• Buoyancy is completely destroyed once the thermal
  has been thoroughly turned inside out
• Evaporation at the cloud boundary causes cooling
  and sinking motions which help to keep the
  boundaries well defined

23
Convective clouds

• Net upward movement of air in convective clouds
  is compensated by slower subsidence of air over
  the much larger area between the clouds
• This subsidence produces warming and drying and
  hinders the growth of thermals in the regions
  between clouds
• There is a tendency for thermals to feed
  previously formed clouds regions that have been
  moistened by earlier thermals have new thermals
  form that have reduced evaporation

Parameterize a thermal
24
Convective clouds

• In situ aircraft measurements of a convective
  cloud

25
Convective clouds

• Within the cloud the air is generally moving
  upward
• Larger liquid water contents are generally
  associated with the higher updraft velocities
• Three droplet spectra measured 100 m apart in the
  cloud
• Bimodal droplet distribution with peak
  concentrations at droplet radii of 6 and 11 mm
• Bimodal distribution is often observed in clouds
  growing in an unstable environment- may be
  produced by the mixing of cloudy air and drier
  environmental air at the growing cloud top

26
Convective clouds

• The weight of falling precipitation can influence
  convective overturning through downward motions
  influenced by evaporation downdrafts are
  visible as large protuberances called mamma

Often forms under the anvil associated with
severe convective storms
http//www.invectis.co.uk/cloud/cloud.htmlpic
27
Layer clouds

• Widespread ascent of air
• associated with the
• development of cyclones
• Cirrus often the first sign of
• an approaching warm front.

http//www.stormeyes.org/tornado/SkyPix/dngeness.h
tm
28
Layer clouds

• Cirrus (cont.)
• High level ( 9 km)
• Composed of ice particles
• Large in size
• Low concentrations

http//www.stormeyes.org/tornado/SkyPix/dngeness.h
tm
29
Layer clouds

• Cirrus - fallstreaks
• Due to the large sizes of ice particles in cirrus
  and the low saturation vapor pressure of ice, ice
  particles often fall through distances of a
  kilometer or more before evaporating

http//ww2010.atmos.uiuc.edu/guides/mtr/cld/cldtyp
/hgh/gifs/crs1.gif
30
Layer clouds

• Cirrostratus
• As a warm front moves closer to the observer, the
  cirrus clouds give way to cirrostratus. Thin
  forms of cirrostratus often give rise to a bright
  halo (22o and 46o halos)

http//www.invectis.co.uk/cloud/cloud.html
31
Layer clouds

• 22o and 46o halo
• Produced by refraction of sunlight in hexagonal
  prisms of ice

32
Layer clouds

• 22o and 46o halo
• Inner edge of 22o halo is sharp and colored red
• Outer edge of 22o halo is colored blue
• Sky immediately inside the 22o halo is always
  darker than the outside
• Halos with an angular radius 46o of can also be
  produced by refraction of ice crystals but they
  are less common and less bright than the 22o halo

33
Layer clouds

• As warm front continues to approach, clouds
  thicken and lower in the form of altostratus
  sometimes the sun and/or moon are visible through
  altostratus

http//www.stormeyes.org/tornado/SkyPix/asundula.h
tm
34
Layer clouds

• altostratus corona
• Produced by the diffraction of light in small
  water droplets
• If the droplets are uniform in size, rings may be
  seen (inside ring is blue
• or violet, outside ring
• is red)

http//www.du.edu/jcalvert/astro/corona.htm
http//www.planetterragen.com/clouds/
35
Layer clouds

• Multiple cloud layers
• Ice particles from an upper cloud layer may fall
  into lower cloud layers where they grow into snow
  particles, melt as they pass through the 0oC
  level, and reach the ground as rain
• Melting level can often be seen looking toward
  the horizon in the direction of sun
• Snow scatters more light than rain cloud is much
  darker just above the melting layer than below it
• As the air is moistened by rain, fragments of
  low-level cloud (pannus or scud) often form

36
Layer clouds

• Distribution of liquid water in warm layer clouds
  (fog)
• Radius of droplets ranges from a few micrometers
  up to 30 40 mm
• LWC ranges from 0.05 to 0.1 g m-3
• Average droplet radius is about 5 mm and
  increases with height into the cloud

37
Layer clouds

• Other types cirrocumulus (shown below),
  altocumulus, and stratocumulus
• Two types of motions that break them up into
  small cumulus-type elements
• Rayleigh convection
• Shear instability

http//www.stormeyes.org/tornado/SkyPix/ccvirga.ht
m
38
Layer clouds

• Rayleigh convection vertical shear of
  horizontal flow

39
Layer clouds

• Rayleigh convection
• Small convective cells warmed by radiation from
  the ground (aided at times by convective heat
  transfer) while cloud tops are cooled as they
  radiate to space a related phenomena was first
  observed in a pub!
• Critical rate of differential heating above which
  cellular motion occurs (Benard cells)

40
Layer clouds

• Rayleigh convection vertical shear of
  horizontal wind
• Benard cells give way to cloud streets or rolls
• Winds stronger than 6 m s-1
• Lapse rate is neutrally stable
• Align along mean direction of the wind
• Horizontal spacing on order of 10 km
• Ratio of horizontal wavelength to depth of the
  convective layer is 101

41
Layer clouds

• Shear instability
• Vertical wind shear exceeds some critical value
  within a stably stratified atmosphere

http//amath.colorado.edu/student/petersem/header.
html
42
Orographic clouds

• Formation due to the lifting of moist air above
  LCL as streamlines are perturbed by orography
  (mountains or hills)
• Mountain wave first wave in streamlines that
  forms over the barrier can produce mountain
  wave clouds

flow
43
Orographic clouds
flow

• Mountain wave cloud particle distribution
• LWC a peak value over the windward slopes (a)
• For cold cloud, ice content peak is on downwind
  of crest (b)
• Cloud base differences

44
Orographic clouds

• Mountain wave clouds
• Moisture layers ? layered (stacked) clouds
• Can be extensive and important
• Augment clouds in cyclonic storm systems
  producing heavy precipitation on windward slopes
• Rain shadow areas on leeward (downwind) slopes
• A train of lee waves (lee-wave clouds) downwind
  of a mountain (Fig. 5.15)

45
Orographic clouds

• Moisture layers

The first modern sighting of a flying saucer
(1947) was made over Mt. Rainier, WA, where
disk-shaped wave clouds are very common.
http//www.invectis.co.uk/cloud/cloud.htmlci
Photo credit Jenny Roye
46
Orographic clouds

• Moisture layers

http//www.invectis.co.uk/cloud/cloud.htmlci Phot
o credit Dave Appel
47
Orographic clouds

• Mountain wave clouds
• Within 25o of the sun are often tinted with
  brilliant (iridescent green, purple-red, blue)
  colors
• Common in newly formed wave clouds when the cloud
  droplets tend to be uniform in size (WH 4.4.1)
• Billow clouds are sometimes confused with wave
  clouds and rarely exhibit iridescence because
  their droplet size distributions are broader

48
Orographic clouds

• Lee wave clouds
• Steepest slopes of streamlines (vertical air
  velocity maxima) are a few kilometers downwind of
  the barrier
• Sailplane records, waves can extend to 30 km,
  nacreous clouds

49
Orographic clouds

• Leeward (downwind) winds
• Fohn winds in Alps, Chinooks in North America
• Rotor and rotor cloud winds may reverse
  direction near the ground due to the formation of
  a vortex. Very turbulent!

50
Orographic clouds

• A train of lee waves (lee-wave clouds) downwind
  of the Appalachian Mountains (Fig. 5.18)

51
Orographic clouds

• Orographic wave phenomena
• Vertical profiles of wind and temperature play an
  important role
• Increase in wind speed and decrease in static
  stability from the lower to the upper troposphere
• If the width of the mountain range is comparable
  to the natural wavelength of the waves, the
  amplitude of the waves is increased by the
  resonance effect.