1. Hurricane Cloud Physics
Robert A. Black
NOAA/AOML/HRD

2. Main Research Topics Rainfall and the warm rain process
Precipitation Chemistry
Aerosols
Ice microphysics
Lightning
Ice particle nucleation
Bergeron Process precipitation growth

3. 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

4. Collision-Coalescence
Fortunate larger droplets fall faster, collide, & stick together to form ever larger drops
Numerical simulations show raindrop formation in min.

5. Bergeron Process
Ice particles grow at the expense of (supercooled) cloud
These get large enough to fall out, melt, & produce rain

6. 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).

7. Where does all the ice originate?
Graupel
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.

8. Instruments Aircraft-mounted Particle Imaging Probes DMT CCP shown.
Doppler Radar
Analog measurement devices (T, P, RH, LWC)

9. Instruments - Cont. Aircraft-mounted DMT PIP Probe (top)
Measures precipitation mm diameter
DMT CAPS Probe (Bottom) includes aerosols and cloud drops µm

10. 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

11. PMS 2D-C images 50µm Res.
1.6 mm

12. PMS 2D-C images 50µm Res.

13. PMS 2D-C images 50µm Res.

14. PMS 2D-P images 200µm Res.

15. Some very large drops. (9.6 mm between white lines)
PMS 2DG-P Images 150 µm Res.
9.6 mm
Some very large drops. (9.6 mm between white lines)

16. PMS 2DG-C images 30µm Res.
1.92 mm

17. PMS 2DG-P images 150µm Res.

18. PMS 2DG-C images 30µm Res.

19. PMS 2DG-P images 150µm Res.

20. 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

21. 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

22. 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

23. 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

24. 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.

25. LIGHTNING

26. 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.

27. Hurricane Study Black & Hallett, 1999 Measured Ez and Ey
2-D particle imagery
No Cloud LWC
No cloud droplet measurements of any kind

28. 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.

29. Conceptual model

30. Conclusions After 30 years, there are still outstanding questions.
The vertical distribution of cloud water is unknown
The partitioning of the condensate mass between precipitating and non-precipitating particles, graupel, rain, and snow remains hard to quantify.
The relationship between updraft speed and all these particles at various altitudes is still uncertain
The influence of the SSA and other aerosols on the convection remains to be measured.
Lightning is a sign of stronger updraft - what is the relationship between flash rate, storm structure, and the microphysics that separates the charge?