entrainment and mixing in cumulus clouds€¦ · justin peter alan blyth entrainment and mixing in...

IntroductionEntrainment

Edges of cloudsSummary

Entrainment and Mixing in Cumulus Clouds

Alan Blyth

NCAS, University of Leeds

25 March 2009

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

Outline

Alan Blyth Entrainment and Mixing in Cumulus Clouds

Cumulus clouds

Start Movie

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

Liquid water content

From: Warner (1955) Raga et al (1990)

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

From: Blyth and Latham 1990

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

Liquid water content

Trade-wind cumulus clouds 2005

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

LWC - COPS

COPS cumulus clouds, 15 July 2007. Justin Peter

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

OutlineCloud microphysics

Concentration (N) and mean diameter (d)

Trade-wind cumulus clouds 2005

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Entrainment

Two scales:

Cloud scale - where does entrainment occur and whathappens to entrained air?Small scale at boundaries - what is the mechanism for airexchange across boundaries?

Alan Blyth Entrainment and Mixing in Cumulus Clouds

Plumes, Thermals and Starting Plumes

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Entrainment in Plume

Expect entrained air from below

Alan Blyth Entrainment and Mixing in Cumulus Clouds

Thermals

From: Woodward 1959Expect entrained air from above and below

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Thermodynamic arguments

Use thermodynamic tracers in cloud - conserved variables

Use θq and Q (Paluch 1979). Invariant in adiabatic altitudechanges and mix linearly.

θq = T(

1000pd

)(Rd /Cp)/[1+Cw /Cpd Q]

× exp[

qLCpdT

/

(1 +

CwCpd

Q)]

and

Q = qv + ql

Alan Blyth Entrainment and Mixing in Cumulus Clouds

From Paluch 1979

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

From: Blyth et al. 1988

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Taylor and Baker

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Taylor and Baker

Distribution of points on Paluch diagram does notnecessarily mean ascent of adiabatic parcel followed bymixing – same distribution if mixing occurs continuously asparcel ascends as long as +vely buoyant.Different from Raymond and Blyth

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Wang and Geerts: cooling of reverse flow temperature probe

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Wang and Geerts

“The EC [evaporative cooling] correction proposed hereinshould be applied to all those studies. Since most of themeasurements in the papers listed above occurred attemperatures above -12C, their analyses are affected andsome of their conclusions may be flawed.”

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Downdrafts

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Observations of Thermals

From: Blyth et al, 1988

Alan Blyth Entrainment and Mixing in Cumulus Clouds

19 July 1981, CCOPE, Montana: 1625 - 1633

July, 1995, SCMS, Florida

Damiani et al

Damiani et al

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Damiani et al

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Damiani et al

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Schematic picture of thermals and entrainment

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Erosion of the core

Alan Blyth Entrainment and Mixing in Cumulus Clouds

Reduced LWC in middle of updraft

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Drop size distribution in the hole

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Cloud models: Richard Carpenter

From Carpenter et al (1990)

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Model of Cu cloud showing thermal

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Model of Cu cloud showing thermal

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Model of Cu cloud showing thermal

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Model of Cu cloud showing thermal

Alan Blyth Entrainment and Mixing in Cumulus Clouds

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

How clouds entrain

Courtesy of Dr. Sonia Lasher-Trapp

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Heus et al: Modelling results

“The obtained Paluch diagrams are found to be similar to manyresults in the literature, but the source of entrained air found byparticle tracking deviates from the source inferred from the Paluchanalysis.Whereas the classical Paluch analysis seems to provide someevidence for cloud-top mixing, particle tracking shows that virtually allmixing occurs laterally.Particle trajectories averaged over the entire cloud ensemble alsoclearly indicate the absence of significant cloud-top mixing in shallowcumulus clouds.”

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Plumes and thermalsObservations of source of entrained airThermalsModelling of Clouds

Heus et al

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

What happens at the boundaries?

From Baker et al. (1984)

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

From Jensen and Baker (1989)

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Cloud edges

The sharpness of cloud edges must tell us somethingabout the relative magnitudes of the processesstrengthening and weakening the gradients thereGradients are sharpened due to buoyancy and smoothedout due to turbulent eddies

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Gradients in LWC and vertical wind; convergence

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Does this happen at the edges of Clouds?

From Grabowski and Clark (1993)

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Heus and Jonkers: cloud-edge downdrafts

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Heus and Jonkers, 2003

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Cloud Boundaries

Schematic of cloud with descending shell

Rodts et al., 2003

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Summary

Does entrainment occur at ascending cloud top?

Not the edges of thermalAt rear of thermal

Some model results suggest lateral entrainment?Thermals important for circulating the entrained air downthe edges and into centreDilution of thermal about 3-5 mins?How long for molecular mixing vs bulk transportNeed improved in-cloud temperature probes to makeprogress

Alan Blyth Entrainment and Mixing in Cumulus Clouds

IntroductionEntrainment

Edges of cloudsSummary

Acknowledgements

Many thanks to:William Cooper, Sonia Lasher-Trapp and JorgenJensen,

Alan Blyth Entrainment and Mixing in Cumulus Clouds