BREAK-OUT SESSION ON FUNDAMENTAL
QUESTIONS IN CLOUD PHYSICS
1. What are the outstanding questions and key hypothesis in
cloud physics?
2. What facilities/observational capabilities/instruments are
needed and would make a difference in addressing these
key hypothesis/questions?
3. Can the scientific community make the observations now,
and what is needed?
4. How should we prioritize these new instrumental needs?
1. Science question: (Pellet) ICE NUCLEATION AND
SMALL ICE PARTICLES IN THE ATMOSPHERE;
MEASUREMENT DIFFICULTIES. What is the true ice
particle concentration? (Rogers) (Stith)
Hypothesis: (1) Aerosol particles are the dominant control of
primary ice formation in clouds.
Instrument needs: Ice nuclei, thermodynamics, and small
ice particles. Determine the extent of artifacts, develop new
instruments without ambiguity.
2. Science question: (Ruth) Cloud supersaturation,
measurement thereof.
Hypothesis: (1) Ice supersaturation can occur at extreme
relative humidity through processes that we do not
understand. (2) Warm cloud?
Instrument needs: Measure water vapor accurately. No
good, fast temperature measurements in clouds. Aerosol
measurements.
3. Science question: (Albrecht) How do aerosols and
dynamics of clouds affect precipitation efficiency?
Hypothesis: (1) There is an optimal amount of ultra-giant
nuclei that can increase the precipitation efficiency in clouds
with high background aerosol concentrations. (2)
Instrument needs: Quantifying ultra-giant nuclei, more
effective uce of cloud radar in precipitation. Also use active
techniques, - adding nuclei. Use remote sensing techniques
more efficiently, e.g. Zdr techniques. Dynamics
measurements is also important. New generation of CCN
instruments are needed: better, faster and replicable – in the
EOL suite of instruments. Determine soluble fraction in CCN
and relating it to sources of CCN.
4. Science question: (Cooper) What are cloud droplet size
distributions and what factors are most important?
Hypothesis: Cloud droplet size distribution in small cumuli
can be predicted from the aerosol distribution below and
around the cloud, and knowledge of entrainment. These
factors are more important than turbulence.
Instrument needs: Have a believable cloud droplet
spectrometer; high resolution. Instruments/tools to determine
entrainment, e.g. chaff. Needed for model improvements.
Accurate temperature and humidity measurements in clouds.
Apply circular polarization.
5. Science question: (French) Cloud microstructure near
cloud boundaries, supersaturation, temperature.
Hypothesis: Cl,oud microstructure features, driven by
entrainment and turbulence, have a dramatic impact on the
evolution of the droplet spectrum.
Instrument needs: Accurate high-frequency measurements
of temperature, humidity and cloud droplet spectrum (meter
scales, possibly finer). For cumulus, the scale is smaller than
can be measured with current radiation temperature
sensors, and water vapor also needs to be at small scales.
Supersaturation can be determined by deduction from the
drop spectrum (updraft + cloud droplet conc at a local scale)
6. Science question: (Lee ) Snow microphysics, snow
density from in-situ measurements. How do cloud
microphysics connect to dynamics?
Hypothesis: We can determine the effective density of snow
from shape and falling behavior of snow particles.
Instrument needs: In-situ: exists. Fall velocity. Difficult from
remote sensing. Density can be inferred from 2-D images
with a lot of assumptions; are there better ways? Easier in
the lab, on aircraft it is very difficult. Understanding snow
density in relation to remote sensing measurements (e.g.
new polarization). Do-able if we have good in-situ
measurements? Big difference between dry and wet snow.
7. Science question: (Ellis) Radar development of algorithms
for the cloud physics community needs.
Hypothesis:
Instrument needs:
8. Science question: (Loew) Radar instrumentation to help
cloud physics community
Hypothesis:
Instrument needs:
9. Science question:
Hypothesis:
Instrument needs:
10. Science question: (Kliche ) Cloud droplet size
distributions, theoretical, fitting methods. Compare with radar
meteorology. Gather in-situ measurements of in-situ particle
distributions.
Hypothesis: Equilibrium drop size distributions are a good
representation in clouds.
Instrument needs: Accurate drop-size spectrometers.
Accurate disdrometers for ice? 2D-video distrometers can
be replaced with images. Remove turbulence effects by
cameras structure by using remote cameras. Do we need to
measure all moments of the drop-size distribution? EOL
does not support disdrometer measurements?
11. Science question: (Stith) What new insights on
measurements do we need to predict the size distribution of
hydrometeors in models.
Hypothesis: We can predict the future size distribution
without a complete knowledge of the factors that impacted
this size distribution.
Instrument needs: Giant nuclei, CCN, entrainment, dynamics
are all important. Equilibrium drop-size distributions are
attained by nature regardless of the dynamics, aerosols …?
How accurate do measurements need to be?
12. Science question: (Tralli) What measurement are
needed for the community, what can we develop.
Hypothesis:
Instrument needs:
14. Science question: (Geerts) How does mechanical
turbulence or convective turbulence affect the cloud droplet
spectrum? Use radar data.
Hypothesis: Mechanical turbulence, close to the surface in
orographic clouds, is important for the development of the
drop-size distribution.
Instrument needs: Use radar instrumentation to measure
close to terrain. We need to be able to measure the vertical
air velocity field at low levels.
15. Science question: (Rodi) How well do we understand
the entrainment process to be able to understand the bulk
water content in a point of time and location, and how well
do we need this to be done for large-scale models?
Hypothesis:
Instrument needs:
18. Science question: (Salazar) Entrainment and aerosol
properties?
Hypothesis:
Instrument needs:
19. Science question: (Vivek) Discrepance between cloud
droplet formation from models and observations; what is the
impact of water vapor? What is the impact of time scales?
Hypothesis: Vapor fluctuations are important for the
development of the drop-size distribution.
Instrument needs: Water vapor radiometer or tomography
for spatial water vapor distribution. Cloud radars for droplet
formation.
20. Science question: (Smull) Mechanisms for generation
of supercooled liquid water, in particular for convection over
orography? Aircraft can’t sample at low levels.
Hypothesis: Modulation of flow by orography is capable of
generating supercooled liquid water and subsequent riming
growth of prcipitation.
Instrument needs: Depth is very necessary, e.g. if cloud
reaches to -30, then there is no liquid water. Input of ice from
above is a modulator. Dual-wavelength radar is needed to
measure the presence of liquid water. Microwave
radiometer, preferably with ranging ability. Updraft speed
determines liquid water, degree of ice determines the
removal of liquid water. Combination of radar and liquid
water radiumeter is useful.
21. Science question: (Haimov) From science questions,
what should the next radar development be? For instance, if
we can measure the full Doppler spectrum in two directions,
can we determine the drop-size spectrum?
Hypothesis:
Instrument needs:
22. Science question: (Oolman) How do supercooled drizzle
form near cloud top?
Hypothesis: Wind shear near cloud top generates
supercooled drizzle drops.
Instrument needs: Standard suite of clouds sensors, lidar
and radar. Microwave radiometer for drops and entrainment
structure. FMCW radar may give 5-m resolution.
23. Science question: (Coffey ) Determine requirements for
airborne platforms, current and future?
Hypothesis:
Instrument needs:
24. Science question: (Chandra) lack of hydrometeor
density? Raindrop oscillation relationship modes? The fall
dynamics of large ice particles? Spatial scales of variability
in particle size distributions?
Hypothesis:
Instrument needs: Spatial already dealt with.
25. Science question: (Smith) How does hail form and
evolve?
Hypothesis: The characteristics of the updraft and the
distribution of supercooled water in the updraft region
determine the growth of large hail.
Instrument needs: Trajectories in the periphery of the
updraft region are important. Narrow zones? Multiple
polarimetric Doppler radar and in-situ aircraft observations of
microphysics and updraft.
26. Science question: (Nesbitt) Determine factors controlling
the vertical structure of mixed-phase region, and what
controls it? What are the dynamical implications?
Hypothesis: The precipitation ice-mass produced in deep
mixed-phase clouds is a function of both thermodynamic
instability and the aerosol (CCN and IN) concentration.
(2) There are regions of the atmosphere that are sufficiently
clean that the concentration of IN is low enough that mixedphase clouds can exist for long periods of time.
Instrument needs: Arctic fall-season clouds occur in clean
air (low IN concentrations), and how do ice particles form.
What controls a balance between warm and cold cloud?
Measure updraft.
27. Science question: (Beasley) Electrification – how do
storms become electrified? What is the role of cloud and
precipitation particles in lightning initiation?
Hypothesis: Lightning can be in initiated when ice particles
become aligned in strong electric fields over lengths of
hundreds of meters.
Instrument needs: Electric field, Fly horizontally (not
balloons), ice particle concentration and habits. Polarimetric
radar.
28. Science question: (Blyth) Ice initiation, entrainment and
the evolution of cloud spectrum in warm cloud, development
of precipitation?
Hypothesis: (1) High concentration of ice particles can be
explained by the Hallett-Mossop process within a given
temparature range. (2) The development of warm rain can
be explained based on observed sub-cloud aerosol size
distributions. (3) Entrainment occurs at the top and bottom of
the thermal in cumulus clouds.
Instrument needs:
29. Science question: (Deirling) Cloud electrification and
cloud microphysics, total lightning and drop mass. How well
do we need to measure graupel size distribution?
Hypothesis: Total lightning is proportional to total ice and
droplet mass.
Instrument needs: Lightning mapping array and NALDN, insitu measurement of mass distribution, polarimetric radar.
30. Science question: (Zondlo) What mechanisms are
driving the large supersaturations in cirrus clouds, very large
supersaturations?
Hypothesis: see otherwise.
Instrument needs:
31. Science question: (Rogers) Role of aerosol particles in
ice formation in clouds?
Hypothesis:
Under given conditions characteristics of the aerosol
determine the number of ice particles forming by nucleation
mechanisms in selected clouds.
Instrument needs: Ice nuclei chamber, accurate particle
probes from 1 micron to mm sizes, polarimetric lidar, radar,
trace gas for tracers, aerosol mass spectrometers, etc.
If instruments exist, is measurement of various ice
nucleation processes necessary?
32. Science question: (Beaton) What should be new
instrumentation, adequacy or improvement of instruments?
Hypothesis:
Instrument needs:
33. Science question: (Friedrich) Polarimetric radar, can it
improve snow measurements, especially in orographic
clouds? Will the next generation polarimetric radars improve
our measurements of snow? Can we make measurements
that will ensure that the next generation polarimetric radars
improve our measurements of snow?
Hypothesis:
Instrument needs: Instrumentation question.
34. Science question: ( Medina) Vertical velocity may have
a great effect on collection efficiency and thus the impact on
the precipitation process in orographic storms?
Hypothesis: In orographic clouds, the precipitation and
collection fields are determined by vertical motion fields.
Look are other orographic section.
Instrument needs:
35. Science question: (Rauber) Role of aerosols on indirect
effect on clouds, albedo, and aerosol effects on climate.
Hypothesis: See aerosol section.
Instrument needs:
36. Science question: (Jensen) Giant aerosols and
precipitation formation.
Hypothesis: Bruce A.
Instrument needs:
38. Science question: Platforms - performance to climb with
clouds and ability to work near strong convection.
A second Doppler radar to go along with current one.
Ability to fly at low altitude in hurricane environments (salt,
strong wind).
Hypothesis:
Instrument needs: Next generation: Microphysics with bigger
sample volumes.
Many current measurements rely on 20-30 year old
technology; there has been some evolution but not dramatic.
How to move state-of-the-art instrument from private
companies to more general use, into a facility pool.
Lack of decent laboratory cloud physics facilities in the US.
Effort is needed to characterize the performance of the full
suite of instruments, this is a community task. In particular,
do a full uncertainty analysis. We must devote an
appropriate effort on the characterization. This should be
done by an independent laboratory to ensure a hands-off
evaluation. Many instruments generate larger data volumes;
in the past instruments went to many institutions. Now they
go to fewer institutions due to larger cost of instruments and
analysis. More collaborations are needed.
Model intercomparisons are common, we do not do that
sufficiently in the observational sciences.
Needed instruments: IN instruments for all mechanisms.
Well characterized air sample inlets. In-cloud temperature.
Water vapor inside and outside cloud. Automatic GNI
impactor coming online. Full characterization of particle size
distributions. Particle charge instruments. Cloud radars and
lidars needed. Disdrometers.
37. Science question: (Rauber) How do we collect and use
microphysical data for model usage? What about scale
differences?
Hypothesis:
Instrument needs: How do we conduct field programs and
use instrumentation in a manner that will provide data and
analysis that the modeling community can use to evaluate
models and develop parameterizations? Select relatively
tractable problems. E.g. look for 2-D barriers instead of 3-D
topography. If you use more complex topography, then you need
more platforms. Comparisons between model output and
observations need a metric, in order to compare apples-to-apples.
Take statistical approach or individual clouds (process), as an
example. Statistical approach may be better for larger scales,
following an individual cloud may be better for smaller scales.
Satellite community also needs in-situ observations taken at
‘scales’.
Fly aircraft through model space and direct actual flight path based
on model forecasts.
Observational scientists must also think about and do
parameterization developments.
We must work harder in early phases of field campaigns on
bringing modelers in.