Clouds in Low Latitudes
Cumulonimbus
Cumulus
Mesoscale
Convective
System
Stratocumulus
Stratus
Tropical Cyclone
R. A. Houze
Lecture, Indian Institute of Tropical Meteorology, Pune, 2 August 2010
Clouds in Low Latitudes
Lecture Sequence
1.
2.
3.
4.
5.
6.
7.
Basic tropical cloud types
Severe convection & mesoscale systems
Tropical cloud population
Convective feedbacks to large-scales
Monsoon convection
Diurnal variability
Clouds in tropical cyclones
Basic Tropical
Cloud Types
Cumulonimbus
Cumulus
Mesoscale
Convective
System
Stratocumulus
Stratus
Tropical Cyclone
Stratus and stratocumulus
Stratus
Low base, shallow cloud layer, overcast
Stratocumulus
Sun shining through breaks in cloud layer
Viewed from aircraft
10 km
~100 km
1 km
Photograph: Gabor Vali
Satellite-Visible
Marine stratocumulus
Satellite--Infrared
Marine stratocumulus
Stratus & Stratocumulus Climatology
IPCC, 2007
Ocean currents
SST Climatology (January)
January Surface Pressure and Wind
Stratocumulus Regime
Moderate SST
Idealized mixed layer
qvs
z
St, Sc, Cu in a Boundary Layer Heated from
Below
well
mixed
sea
surface
heat flux
heat flux
heat flux
Note:
Lilly’s model only
Based
on Lilly
explains
the1968
formation of the
stratus
Heating & Cooling Associated With Drizzle
Courtesy Robert Wood
Cellularity of Stratocumulus layer
Closed
cells
Open
cells
Open and Closed Cells
Open
Open
cells
Low albedo
Closed
Closed
cells
Satellite
High albedo
100 km
Cloud albedo
strongly
dependent
upon
open/closed
cells
More robust
drizzle
associated
with open
cell structure
Strong drizzle
Ship Radar
Weak drizzle
Conceptual model of POC formation
Closed
cells
Open
cells
Courtesy Robert Wood
Breakup of stratocumulus layer
and Decoupling
Bretherton & Wyant 1997 extended Lilly’s model to explain the breakup of the St/Sc layer
Cumulus and cumulonimbus
All Cumulus and Cumulonimbus
Buoyancy phenomena

B  g
0
o
Condensation
level
Cumulus
Condensation level
Buoyant
Updraft
Fair weather type
Lifecycle of Small Cumulus
Often due to surface heating—LFC is at the ground
Development of Cumulus
Congestus
Cumulus congestus
Cumulus congestus
Cumulonimbus
Rain
Glaciation and Anvil Formation
Cumulonimbus with an older anvil
The cumulonimbus produces a layer of cirrostratus
Cumulonimbus with Anvil
Hail
Rain
Cumulonimbus anvils viewed from space
100 km
Cumulonimbus and Thunderstorms
“Cumulonimbus (Cb)”
Overshooting
top
Near the tropopause
0°C
“Thunderstorm”
“Cumulonimbus”
“Thunderstorms”
• ALL precipitate
• NOT all Cb contain ice
• NOT all Cb have lightning
& thunder
• NOT all precipitate
• ALL contain ice particles
• ALL have lightning
& thunder
Global Lightning Occurrence
Many deep Cb over tropical oceans have extensive
glaciated anvils but do NOT exhibit lightning & thunder.
Why?
Christian et al. 2002
Compare soundings over tropical land and
ocean regions where deep cumulonimbus
occur
Indo/Pacific Warm Pool
Can’t
generate
large
buoyancy
West African Squall Line
Can
generate
huge
buoyancy
Cumulonimbus may be rotational
Tornado
Why do some cumulonimbus exhibit extreme
rotation?
Think of vertical equation of motion:
Ask: What determines p’ ?
Note that pressure
perturbation is governed
by:
 2 p  FB  FD

FB 
o B
z x
v v
FD    o v  v
 

p ~  2 p

p
B>0
p’ due to buoyancy
shear
min p’
PG force
p’ due to wind field
Most cumulonimbus clouds have pressure perturbation
fields dominated by the buoyancy forcing. The most
common is the
“Single-cell Thunderstorm”

p 
o
H
L
Byers and Braham (1949)
“Multicell
Thunderstorm” is
made up of single
cells
Byers and Braham (1949)
Two Types of Thunderstorm
Multicell
Thunderstorm
Why?
Supercell
Thunderstorm
To be continued
Conclusion of Lecture 1 :
We have looked at the most basic forms
Cumulonimbus
Cumulus
Stratocumulus
Next lecture
Mesoscale
Convective
System
Last lecture
✔
Stratus
Tropical Cyclone
Clouds in Low Latitudes
Lecture Sequence
1.
2.
3.
4.
5.
6.
7.
Basic tropical cloud types
Severe convection & mesoscale systems
Tropical cloud population
Convective feedbacks to large-scales
Monsoon convection
Diurnal variability
Clouds in tropical cyclones
Next
End
This research was supported by
NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G,
NSF grants, ATM-0743180, ATM-0820586,
DOE grant DE-SC0001164 / ER-6