Hurricane Cloud Physics

Robert A. Black

NOAA/AOML/HRD

Main Research Topics

• 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, measures0 - 5 g m-3 LWC,Sensitive to drops 50 µm

Next, a few images from hurricanes

1.6 mm

PMS 2D-C images 50µm Res.

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.

Some very large drops. (9.6 mm between white lines)

9.6 mm

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

• 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 unknown2) 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?