J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Discussion or, back to the real world!

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Droplet Inertia Impact on Condensational Droplet Growth Impact on Droplet Collection Turbulence and Droplet Spectra

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Droplet Spatial Distribution and Clustering Impact on Condensational Droplet Growth

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 The droplet spatial distribution as observed from droplet counting Chaumat & Brenguier, 2000

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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.

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Inertial Effects Impact on Droplet Collection (Collision + Coalescence)

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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 ??

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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 !

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Turbulence and Droplet Spectra

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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 ??

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Can we explain observed broad spectra

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Can we explain observed broad spectra 310 cm cm cm cm -3

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Can we explain observed broad spectra 650 m 280 cm m 280 cm m, 190 cm m, 125 cm m 140 cm -3

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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 base  Variability of air parcels trajectories BUT How can we simulate inhomogeneous mixing feature ?

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 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 ) ?

J. L. Brenguier, F. Burnet, and L. Chaumat 4th IMS Turbulence Workshop, London, March 2009 Electrical charge impact on collection