Hurricane Cloud
Physics
Robert A. Black
NOAA/AOML/HRD
Main Research Topics
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Rainfall and the warm rain process
Precipitation Chemistry
Aerosols
Ice microphysics
– Lightning
– Ice particle nucleation
– Bergeron Process precipitation growth
How to get rain from vapor
• Hygroscopic particles
(CCN)
• Absorb water from
vapor to provide
droplets
• Problem - This process
narrows the size
distribution
Köhler Curves
Collision-Coalescence
• Fortunate larger
droplets fall faster,
collide, & stick
together to form ever
larger drops
• Numerical simulations
show raindrop
formation in 25-30
min.
Bergeron Process
• Ice particles grow at
the expense of
(supercooled) cloud
• These get large
enough to fall out,
melt, & produce rain
Ice Crystal Morphology
• Ice crystal shape is
governed by the
temperature and
saturation ratio.
• Above the black line,
the air is
supersaturated with
respect to water
• Below it is ice
saturation only.
Diagram of snow crystal patterns as functions of temperature and
the excess vapor density. This diagram was revised from the
original Nakaya Diagram by Kobayashi[12]. The excess vapor
density is easily translated to the supersaturation; namely,
(supersaturation)=(excess vapor density)／(equilibrium vapor
density).
Where does all the ice originate?
• Ice nucleation is very
inefficient at T > -15ºC
• Hallett & Mossup,
(1974) Provided the clue
• -3ºC > T > -8ºC,
growing graupel ejects
numerous ice splinters,
the “ice multiplication”
mechanism.
Graupel
Instruments
• Aircraft-mounted
• Particle Imaging
Probes DMT CCP
shown.
• Doppler Radar
• Analog measurement
devices (T, P, RH,
LWC)
Instruments - Cont.
• Aircraft-mounted
• DMT PIP Probe (top)
• Measures precipitation
0.1 - 6.4 mm diameter
• DMT CAPS Probe
(Bottom) includes
aerosols and cloud
drops 0.1 - 50µm
LWC-100
A “Hot wire”
Cloud water
Meter, measures
0 - 5 g m-3 LWC,
Sensitive to
drops
50 µm
Next, a few
images from
hurricanes
PMS 2D-C images 50µm Res.
1.6 mm
PMS 2D-C images 50µm Res.
PMS 2D-C images 50µm Res.
PMS 2D-P images 200µm Res.
PMS 2DG-P Images 150 µm Res.
9.6 mm
Some very large drops. (9.6 mm between white lines)
PMS 2DG-C images 30µm Res.
1.92 mm
PMS 2DG-P images 150µm Res.
PMS 2DG-C images 30µm Res.
PMS 2DG-P images 150µm Res.
Platform Limitations
• WP-3D maximum altitude too low (-8ºC),
or about 25K ft. (+6 hrs into flight)
• Jets won’t fly below 35K ft (-40ºC).
• Microphysically important -10ºC to -20ºC is
unreachable.
• A/C safety precludes flying below 1.5 km in
eyewall when in precipitation.
• Aircraft and instruments subject to damage
from ice particle impacts and lightning
What do hurricanes offer?
• Strong horizontal wind, but not usually
damaging turbulence
• Long lifetime - often a week or more.
• Moderate updraft
• No hail or tornados
• Usually, little or no lightning
Hurricane Allen 5 Aug. 1980
• Strong Cat-4
• First mission dedicated to
ice microphysics
• First circumnavigation
above melting level
• First documented eyewall
replacement cycle
• Provided the evidence that
killed Project Stormfury
Hurricane Irene 26 Sept. 1981
• First time for a
circumnavigation in
convection
• Updraft maxima < 10
m/s
• Cloud LWC < 0.5 g m-3
• Precip. Conc. < 30 l-1
• Peak 2D-C > 100 l-1
What purpose for the ice?
• Willoughby et al, 1984 suggested the ice, by
spreading out radially around the storm and
inducing downdraft, aided the creation of
“convective rings” (the eyewall, in other words)
• Microphysically, the ice saturates and stabilizes
the storm environment, thereby suppressing other
convection outside the eyewall.
LIGHTNING
Lightning Origins
• Takahashi, 1978, Saunders et al, 1991, 1992, 1996
all showed that graupel, supercooled cloud water,
and ice crystals are all necessary for charge
separation.
• These researchers differ greatly on the relative
quantities of these particles that are necessary.
• These are very difficult measurements to make in
natural clouds. Hurricanes provide one good place
to try.
Hurricane Study
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Black & Hallett, 1999
Measured Ez and Ey
2-D particle imagery
No Cloud LWC
No cloud droplet
measurements of any
kind
Lightning in Hurricanes - Cont.
• Only convective areas
were charged
• Stratiform areas had
little or no charge
• Transition between
areas with much liquid
water and little ice to
all ice and little water
were abrupt.
Conceptual model
Conclusions
After 30 years, there are still outstanding questions.
1) The vertical distribution of cloud water is unknown
2) The partitioning of the condensate mass between
precipitating and non-precipitating particles, graupel, rain,
and snow remains hard to quantify.
3) The relationship between updraft speed and all these
particles at various altitudes is still uncertain
4) The influence of the SSA and other aerosols on the
convection remains to be measured.
5) Lightning is a sign of stronger updraft - what is the
relationship between flash rate, storm structure, and the
microphysics that separates the charge?