12th ams conf. on cloud physics/atmospheric radiation july, 2006

12th AMS conf. on cloud physics/atmospheric radiationJuly, 2006

14 sessions: 2 - aerosol + 2 - indirect effects (aerosol)2 - stratiform+ 2 RICO + 1 Cumulus2 deep convection + 2 precipitation2 cirrus

2. Recent field experiments:RICO, TWP-ICE, ACE-2, Crystal-Face

3. 2007 Radiation & Climate Gordon conf. theme:“Integrating multiscale measurements and models for key climate questions”

Where is current activity ??

1.

Presentations on:• Quantifying (measurements) aerosol, cloud nuclei & properties(smoke, pollution)• cloud dynamic/microphysics interfaces(cloud/dry air; drizzle dynamics; mixing; aerosol/precipitation)• subvisual cirrus• improvements on measuring the Earth’s RB from space (inc. clouds)

RT presentations de-emphasize new RT techniques, moreemphasis on improved calibrations, fine-tuning coefficientsand algorithms => greater accuracy, same concepts

Work shifting towards data-integration, model/data synthesis,more complex scenarios (e.g. amazonian burning or mineral dust/cloud &radiation impact)

“Parcel theory suggests that the microstructure of cumulus clouds is mainly a function of height”

What does that mean ??

• during moist processes (i.e. allows for phase changes),

the total water mixing ratio remains conserved:

1. Total water mixing ratio rt = rwater vapor + rliquid + rice

r = water vapor/dryair ; = density

As air is lifted, rwater vapor is determined by the saturation mixingratio, which depends on the temperature. As the temperaturedrops, water is released from the parcel.

Because everything depends on the release of water froma parcel with height, the cloud properties also are primarilya function of height.

MICROSTRUCTURE OF BOUNDARY LAYER CLOUD

d(rl)/dz - d(rv)/dz

r = water vapor/dryair and = density varies linearly as p/T

since

d(rl)/dz = is roughly constant

~ 2.0-2.2 in Chileanstratus

Then LWC = fadz where 0 < fad < 1

Given fad and it is possible to derive:

LWP, cloud radius as f(z), cloud-top effective radiuscloud vol. ext. coeff. ext(z), optical depth.

=> Adiabaticity is a powerful concept

• instrumental constraint• application to satellite remote sensing• departure from adiabaticity indicates physical processes• microphysical processes theoretically understood (e.g. Kohler)

Remarkable that it works…

Comparison of Observations and Adiabatic Model Predictions is poor

Modeled distributions are too narrow.

OBSERVEDN= 481 cm-3

<d>, = 17.7, 7.3 m

MODELEDN= 467 cm-3

<d>, = 17.9, 0.24 m

Comparison of Observations and Adiabatic Model Predictions is poor

Observed drop size distribution BROADENS w/ height; modeledDistributions NARROW

RY, fig. 5.7

Arctic example

• lidar-determined liquid cloud base parcel

• interpolated sounding temperature structure

• constrained w/ microwave radiometer-derived liquid water path

King LWC

adiabatic LWC

CB

excellent correspondencebetween adiabatic calc. andKing probe LWC

May 4

Z(km)

Liquid water content g/m^3

0 0.5

0.6

1.0

FSSP

An aside on the FSSP (Forward-Scattering Spectrometer Probe)

An optical sensor that sizes and counts drops, from whichLWC is derived. The optics rely on Mie scattering:

X=2pi*r/wavelength. The bigger x is, the more light is Forward-scattered.

South-eastern Pacific stratus

(also Californian stratus, but not so much in north Atlantic)

Fair-weather Cumulus: how often/much they are adiabatichas been debated

Kollias et al. 2001

Miami Cu had narrow (400m) adiabatic updraft region flankedby downdrafts

(Hess, 1959: Holt, Rinehart, and Winston, NY)

CORE

Pruppacher and Klett, 1978: Reidel ( Pub.)

Large resources have been devoted towards addressinghow clouds mix/entrain, with adiabaticity serving to constrainor measure how much mixing is occurring

2 field experiments (at least) devoted to this question:

• Small Cumulus Microphysics Study (SCMS), FL, summer 1995

• Rain in Cumulus over Ocean (RICO), Antigua, Jan 2005

SCMS; 28 July; 1434

1hz (100m) vs 1000hz (10 cm) LWC

PVM

(5. Gerber, 2000: 13th ICCP)

(XGLWC)

New tech. from SCMS: a fast FSSP

605 SCMS Cloud Passes 605 SCMS Cloud Passes

18

1. Indentification of LWCa requires a fast and accurate LWC sensor2. LWCa exists in SCMS Cu3. “Classical Adiabatic Cores” in SCMS Cu: none4. Entrainment/mixing already starts near cloudbase (one turn-over distance)5. Large LWCa parcels found only near cloud base6. Above cloud base mean LWC approaches 20% of LWCa

1. How does entrainment/mixing affect the evolution of LWC in Cu?2. What is the proper description of adiabatic cores?3. What size of LWCa parcel must be considered for modeling drop spectra evolution?4. Does the size and vigor of Cu affect the presence of LWCa?5. Does LWCa in RICO Cu differ from other small Cu, e.g., CCOPE or SCMS?

CONCLUSIONS

QUESTIONS

Parcel theory, with cloud formation described by Kohler etc.Can be usefully applied to all boundary layer clouds

ENTRAINMENT and MICROPHYSICS in RICO CuHermann Gerber

NASA/GISS WorkshopSept. 2006

CONDITIONAL SAMPLING FOR ACTIVE TURRETS

VERTICAL VELOCITY IS POSITIVE (~80%) IN AREA WITHVERTICAL VELOCITY IS POSITIVE (~80%) IN AREA WITH LWCLWC

TOP OF CLOUD IS VISIBLE IN FORWARD-LOOKING VIDEO TOP OF CLOUD IS VISIBLE IN FORWARD-LOOKING VIDEO

A SINGLE TURRET IS TRAVERSEDA SINGLE TURRET IS TRAVERSED

CLOUD IS TRAVERSED NEAR CLOUD TOPCLOUD IS TRAVERSED NEAR CLOUD TOP

(Raga, G.B., et al, 1990: J. Atmos. Sci., 47, 338-355.)

PVM

FSSP

Fast FSSP

10-cm RESOLUTION (1000 Hz) LWC DATA

PVM

TURRET SPECTRATURRET SPECTRA

Aircraft data across a Florida Cumulus CloudMoving on to other observations…..

• Higher LWC correlated w/ stronger updrafts• Downdrafts occurring at the edge• drop conc. doesn’t vary much

Moving on to other observations…..

Cloud interior humidity almost always between 98% and 102%

Supersaturation values typically ~ 0.1%, rarely > 0.2%

Soluble aerosoldeliquesce

• The supersaturation relative to a droplet(S’) is increased by two factors:

– The size of a droplet (Kelvin’s Law):• For a given “bulk” supersaturation, a droplet (having a curved

surface) has a lower relative supersaturation

– A solution droplet (Raoult’s Law):• For a given “bulk” supersaturation, the larger amount of solute

dissolved in the droplet, the higher the supersaturation relative to the droplet

Activation of CCN

• Consider a rising air parcel in which the RH just increased above 100%

• As the parcel continues to rise, the RH (or S) continues to increase, and solution droplets containing the largest nuclei would grow larger than r* and activate, growing into cloud droplets

• The supersaturation S continues to increase and more and more of the smaller droplets are activated

• As the droplets are growing, they are decreasing the amount of water vapor in the parcel, offsetting the increase in S from the rising (cooling) air parcel

• At some point the cloud droplets are taking up so much vapor that S starts to decrease in the air parcel-- the max S has occurred

According to parcel theory, the conditions at cloud basedetermine much of the microstructure of the cloud above

Experimentalists search for relationships betweenthe cloud base or sub-cloud layer and the cloud itself(or the lack thereof)

Droplet concentration near cloud base in updrafts in marine cumuli is controlled primarily by two processes:

1. Concentration of cloud condensation nuclei (CCN) entering cloud base

2. Peak supersaturation occurring in updrafts [Twomey, 1959]

CCN Concentrations• Cleaner, more maritime air masses contain

fewer aerosol particles and CCN than more polluted, continental air masses

• Fewer CCN result in fewer, but larger cloud droplets, accelerating rain production

Criteria was met in clouds sampled on 12 of the 15 flights

Areas Targeted

Updrafts and Downdrafts;Intensity = length of arrow

Criteria chosen to obtain droplet concentration 10Hz: 600-900m above the ocean surface

(nominal cloud base = 600m)LWC > 0.25gm-3

Updraft velocity > 0.5ms–1

No droplets > 65 μm 260X (avoids drop shattering) At least three consecutive data points

FSSP

Vertical velocities > 0.5ms-1 between 600-900m above the ocean surface show a relationship (R = 0.66) with average droplet concentration

Vertical velocities & Droplet concentration

Vertical velocities > 0.5 ms-1 between 600-900 m above the ocean surface show a relationship (R = 0.79) with 100-m wind speeds

Vertical Velocity – a proxy for peak supersaturation

Results – Droplet Concentration

Droplet concentrations between 600-900 m above ocean surface increase (R = 0.71) with 100-m wind speeds (5-14 ms-1).

CCN or peak supersaturations?

The total concentration of smaller CCN (PCASP and CN measurements) did not show a clear dependence on wind. This suggests that variability in the cloud base updraft was the most important control on the growth of drops.

The effect of more intense updrafts would be to increase the peak supersaturation, leading to activation of more cloud droplets and smaller cloud droplets near cloud base.

These conclusions were based on data from the most cleanlyAdiabatic cloud portions.

parcel (‘adiabatic’) theory appears able to explain someaspects of cumulus behavior, but note again that observedLWCs are often well below adiabatic values

• Factors that may be important:– Details of aerosol and CCN number

concentrations, composition, sizes, including giant/ultragiant aerosol particles

– Entrainment and mixing– Turbulence– Successive thermals– Preconditioning of cloud environment– “Time zero?”

Parcel theory also can’t explain why precipitation onset occursso quickly. Thus observations search for clues into othermechanisms;

Giant/Ultragiant Aerosol Particles

• Giant: aerosol particles with diameters between 2 and 20 micrometers

• Ultragiant: aerosol particles with diameters > 20 micrometers

• Soluble, giant aerosol particles (like sea salt!) do not have to grow long by vapor diffusion to be large enough to collect smaller droplets

• Ultragiant particles, if > ~45 micrometers, don’t even have to be soluble!

Work based on RICO data appears to discount this mechanism(but I don’t understand the argument)

Turbulence

Turbulence observed to increase with height, so that strongestUp/downdrafts are in top third of cloud

RY fig 5.5

Turbulent energy dissipation rate

This will increase the collision/collection rate of the drops &Can help explain a broader spectrum

Successive Thermals

• Some investigators have suggested that the drops from previous thermals within the same cloud may not completely evaporate, leaving some drops behind that may then be ingested by new thermals, giving them a “head start”

Preconditioning of Cloud Environment

• Numerical models of precipitation formation often start from pristine conditions in an undisturbed environment, but it is likely that earlier clouds change the local environment for the later clouds

600 m

High degree of structurein cloud field is compellingevidence (I think) of preconditioning, clouds coming and going as part ofa larger convective lifecycle

Good correlation betweenvertical and horizontalvelocities also seemsconsistent.

Entrainment and Mixing• The mixing in of dry air from outside the cloud via the

cloud’s own motions is called entrainment

• It is widely acknowledged that entrainment can lead to the production of smaller particles in the droplet size distribution

• It has been hypothesized that entrainment can actually lead to the production of larger drops, by significantly reducing the number of droplets in regions of the cloud that then experience less “competition” for the vapor

Total water

Equivalent potential temperature

Shows evidence of air from 380 mb mixing down

Most compelling in env. with dry air aloft (e.g., CO, NM)

Mixing w/ air from aloft

Homogeneous Mixing:

LWC decreases; drop conc. stays the same;effective radius decreases

Inhomogeneous Mixing:

LWC decreases; effective radius stays the same;Drop conc. decreases

Mixing time scale << evaporation time scale

Parcels mix& evaporate, then more mixing

All drops evaporate evenly

Some drops evaporate completely, resaturating air parcel& allowing some drops to stay the same size

(Lasher-Trapp, S., W. Cooper, and A. Blyth, 2005: QJRMS, 195-220)

SUPERADIABATIC

ADIABATIC PEAK

HOMOGENOUS MIXING INHOMOGENEOUS MIXING

Homogeneous mixing: all droplet size evaporate evenly.LWC decreases but N unchanged. Can’t increase drop size

Inhomogeneous mixing: some drops evaporate first, resaturatingMixed-in air & allowing other drops to grow (LWC decreases)

COMPOSITE OF 35 CuCOMPOSITE OF 35 Cu

IN

CLOUD EDGE

(Brenguier, J.-L, 1993: J. Appl. Meteor., 32, 783-793)

-60 -50 -40 -30 -20 -10 0 10 20

1000 Hz

COMPOSITE OF ENTRAINED PARCEL LENGTH

(Brenguier, J-L, and W.W. Grabowski, 1993: J. Atmos. Sci., 50, 120-136)

(Kreuger, S.K., et al, 1997: J. Atmos. Sci., 54, 2697-2712)

COMPOSITE OF ENTRAINED PARCEL PENETRATION

ENTRAINMENT SHEATH

NO HOLES SMALLPARCELS

DILUTIONDILUTIONDOMINATESDOMINATES

RH HALO?

NEW CCNACTIVATION

VORTEX RINGS?

SUPER-ADIABATIC DROPS?

ENTRAINMENT CONCEPTENTRAINMENT CONCEPT

X

REFERENCES

Much of what I’ve shown today is observations thatdon’t fit into an easy theory.

We need the theory however, as it conceptualizes forus what we are capable of modeling

In the long run the observations can tell us how toimprove the models and thereby improve weatherforecasts and climate predictions

Important to become comfortable with both