The formation of mesoscale fluctuations by boundary layer convection

Harm Jonker Multi-Scale Physics Faculty of Applied Sciences

A spectral gap?

Multi-Scale Physics Faculty of Applied Sciences

(Stull)

Cold Air Outbreak

Multi-Scale Physics Faculty of Applied Sciences

LES of Sc (ASTEX)

Dx = Dy = 100m

L = 6.4km

(8hr) L = 12.8km

(12hr)

Liquid water path

L = 25.6km (16hr)

“Large Eddy Simulations: How large is large enough?”, de Roode, Duynkerke, Jonker, JAS 2004 “How long is long enough when measuring fluxes and other turbulence statistics?”, Lenschow, et al. J. Atmos. Oceanic Technol., 1994

qt lwp u w

Intermediate Conclusions

1) the formation of dominating mesoscale fluctuations is an integral part of PBL dynamics!

- no mesoscale forcings - what is the origin (mechanism) ? - latent heat release - radiative cooling - entrainment - inverse cascade Atkinson and Zhang Fiedler, van Delden, Muller and Chlond, Randall and Shao, Dornbrack, ……

Convective Atmospheric Boundary Layer

penetrative convection

entrainment entrainment

z i

heat flux flux

 Multi-Scale Physics Faculty of Applied Sciences

Saline convection tank Laser Induced Fluorescence (LIF)

digital camera Han van Dop, IMAU Mark Hibberd, CSIRO Jos Verdoold, Thijs Heus, Esther Hagen

fresh water

Laser r

(z) buoyancy flux & tracer flux salt water (2%) fresh water + fluorescent dye

D

p

Laser Induced Fluorescence

Laser Induced Fluorescence (LIF) “bottom-up” tracer boundary layer depth structure

(see also van Dop, et al. BLM 2005) (Verdoold, Delft, 2001)

Conclusions

1) the formation of dominating mesoscale fluctuations is an integral part of PBL convective dynamics!

2) latent heat and radiation are not essential (but speed up the process considerably) 3) budgets: no inverse cascade on average.

significant backscatter (on all scales) 4) production: ineffective (slow), but spectral transfer is just as ineffective 5) the spectral behaviour of w at large scales is crucial Multi-Scale Physics Faculty of Applied Sciences