j. l. brenguier, f. burnet, and l. chaumat 4th ims turbulence workshop, london, 23-25 march 2009...

J. L. Brenguier, F. Burnet, and L. Chaumat4th IMS Turbulence Workshop, London, 23-25 March

2009

Discussion

or, back to the real world!


2009

Droplet Inertia

Impact on Condensational Droplet Growth

Impact on Droplet Collection

Turbulence and Droplet Spectra


2009

Droplet Spatial Distribution and Clustering

Impact on Condensational Droplet Growth


2009

The droplet spatial distribution as observed from droplet counting

Chaumat & Brenguier, 2000


2009

Clustering, as derived from droplet counting is smaller than what was expected when the Fast-FSSP was built.

New instruments are developed (Holodec, Fugal) that will provide a different perspective, but that will not change significantly the fact that droplet counts are desesperatly Poissonian (even in highly turbulent cloud volumes)

The impact on droplet growth by vapour diffusion is negligible because condensational growth is an integral process and chances for a lucky droplet to remain lucky over a few hundreds of meters is infinitely small considering that the droplet spatial distribution is continuously rearranged on scales of seconds and cm.


2009

Inertial Effects

Impact on Droplet Collection(Collision + Coalescence)


2009

Droplet collection is a totally different story. Unlike condensational growth, once droplets have coalesced, the process is irreversible.

Inertial trajectories leading to enhanced collision do not necessarily generate a signature visible in the droplet spatial distribution and droplet counting because very short duration collision events are sufficient for droplets to coalesce.

Enhance collision and coalescence can have a significant impact on the onset of precipitation and the time needed for a cloud a produce precipitation.

? Is there a theory that supports enhanced collision with droplet spatial distributions consistent with observations in real clouds ??


2009

It is not feasible to analyze in situ data up to the point where they can be directly compared to theory. Some parameters are not measurable (velocities, third dimension) or hardly corrected (coincidence dead-time)

It is however much easier to analyse model simulations using a simulator of the instrument to compare with measurements in actual clouds. Instrument parameters (beam diameter, depth of field, time response, etc) are well known !


2009

Discussion

Do we need enhanced collection to explain rain formation ?

Are theories proposed here to explain enhanced collection, producing droplet spatial distributions consistent with the observed ones?

Are there any obstacles in trying to translate modelled droplet fields into observable properties by simulating the instrument principles


2009

Turbulence and Droplet Spectra


2009

If, beyond the common theory of droplet growth by vapour diffusion (dr/dt=AS/r), there are additional processes broadening droplet spectra in adiabatic cores (low turbulence), we should never observe narrow spectra high above cloud base.

~2 km above cloud base

Do we really need another (more complicated) theory to explain droplet growth ??


2009

Can we explain observed broad spectra


2009


310 cm-3

202 cm-3

103 cm-3

40 cm-3


2009


650 m280 cm-3

675 m280 cm-3

660 m , 190 cm-3

655 m , 125 cm- 3

650 m140 cm-3


2009

Large eddy simulations of cloud dynamics, with no subgrid (<25 m) processes, are sufficient to simulate the broadening of droplet spectra accounting for : Variability of droplet concentration due to variability of vertical velocity at cloud baseVariability of air parcels trajectories

BUT

How can we simulate inhomogeneous mixing feature ?


2009

The model has to simulate the dilution of droplet concentration with little or no droplet evaporation, but also the difference we observe between different cloud types

d /T = 6.6 d /T = 1.9 d /T = 0.05


2009

Discussion

Are there any observed features of droplet spectra in clouds that cannot be explained by the common theories of cloud dynamics-microphysics coupling at the LES scale ?

Are there models that explain the inhomogeneous mixing features (see W. Gabowski end S. Krueger presentation on Wednesday) ?


2009

Electrical charge impact on collection

j. l. brenguier, f. burnet, and l. chaumat 4th ims turbulence workshop, london, 23-25 march 2009...

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