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Discussion

Discussion. or, back to the real world!. Droplet Inertia Impact on Condensational Droplet Growth Impact on Droplet Collection. Turbulence and Droplet Spectra. Droplet Spatial Distribution and Clustering Impact on Condensational Droplet Growth.

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Discussion

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  1. Discussion or, back to the real world!

  2. Droplet Inertia Impact on Condensational Droplet Growth Impact on Droplet Collection Turbulence and Droplet Spectra

  3. Droplet Spatial Distribution and Clustering Impact on Condensational Droplet Growth

  4. The droplet spatial distribution as observed from droplet counting Chaumat & Brenguier, 2000

  5. 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.

  6. Inertial Effects Impact on Droplet Collection (Collision + Coalescence)

  7. 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 ??

  8. 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 !

  9. 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

  10. Turbulence and Droplet Spectra

  11. 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. Do we really need another (more complicated) theory to explain droplet growth ?? ~2 km above cloud base

  12. Can we explain observed broad spectra

  13. 310 cm-3 202 cm-3 103 cm-3 40 cm-3 Can we explain observed broad spectra

  14. 650 m 280 cm-3 675 m 280 cm-3 660 m , 190 cm-3 650 m 140 cm-3 655 m , 125 cm- 3 Can we explain observed broad spectra

  15. 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 ?

  16. d/T = 6.6 d/T = 1.9 d/T = 0.05 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

  17. 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) ?

  18. Electrical charge impact on collection

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