[GTP] IMAGe GTP Seminar--Julian Hunt, ASU, UCL, Univ Cambridge, TUDelft

[Silvia Gentile]

Developments in mesoscale  and turbulence modeling in critical 
geophysical flows.

Julian Hunt
ASU, UCL, Univ Cambridge, TUDelft.

April 24
Time: 11:00am
Location: FL Large Conf. room (FL-1022)

Abstract

        Recent developments  of  idealized theory of perturbed mesoscale 
flows over changes in surface roughness, temperature, and elevation , 
together with mesoscale numerical simulations and field and laboratory 
data, have been applied to atmospheric flows over polar orography  (and 
ice edges), Himalayas, and urban areas because of their  very  large 
sensitivities  to climate and environmental changes. (eg Hunt et al 
Qjrms 2004,Orr et al j atm sci 2008 ; jcrh AMS mtg 2009)

      Recent experimental , numerical and theoretical studies with 
colleagues at ASU , Europe and Nagoya have shown up some new critical 
features of turbulence that are important for modeling geophysical flows;

      a) there is a sudden relative drop in surface friction 
(tau/Usquared) when convective velocity w* exceeds the mean velocity U ; 
one explanation is  because of the interaction between entrainment into 
the isolated moving plume structures and the surface layer.This is the 
point when MO scaling  breaks down ? ( Owinoh et al Blmet 2005)

       b)  turbulent  shear layers above or below stably stratified 
layers induce wave motions with shear stresses (typically about 1/5 of 
those in the shear layer)  (Mahalov et al . Th Comp Fluid Dyn 
2007).These stresses  which are often overlooked in models, induce 
significant mean flows (eg double layers at night ) and contribute to 
intermittency near critical Richardson number. Ocean flows below the 
thermocline may also be driven by this mechanism .

   c)  models of thin  shear layers within turbulent flows  (Hunt et al 
J Fluid Mech 2006)can now be linked to the very high Reynolds number 
simulations by Prof Kaneda and colleagues (Ann Rev 2009) , which show no 
evidence of a Richardson like cascade , but an intermittent structure of 
shear layers where   intense microscale vortices tend to form. A new 
deterministic, non-cascade ,  scaling and statistical analysis is 
proposed for  intermittent processes at the smallest  scales   and in 
the inertial -range , where fast up and down scale transfer processes 
are in approximate equilibrium. This  approach could probably be 
developed to  model  small scale mixing and particle processes  in 
non-equilibrium turbulence .