[GTP] Seminar this afternoon: James Brasseur-3pm

[Silvia Gentile]

FYI,
This seminar will also be webcast: 
http://www.fin.ucar.edu/it/mms/fl-live.htm

RAL SEMINAR SERIES

Speaker: James G. Brasseur

               Department of Mechanical Engineering
                   Pennsylvania State University

Date:      August 5, 2013

Time:     3:00PM

Place:     FL 2 – Room 1022

Web-Cast Link for this Seminar:

NcastFL2_1022 (Large Auditorium) Webcast link
http://www.fin.ucar.edu/it/mms/fl-live.htm

  

Title:   The Surprising Transition in Atmospheric Boundary Layer 
Turbulence Structure from Neutral to Convective Stability States

  

Abstract:

The structure of the energy-dominant turbulent eddies in the 
atmospheric boundary layer (ABL) is known to depend strongly on the 
global stability state of the inversion-capped lower troposphere. In 
the purely shear-driven neutral limit, the ABL is characterized by 
(Coriolis-oriented) streamwise-elongated coherent eddies of negative 
fluctuating horizontal velocity, weakly correlated with vertical 
fluctuations. As the surface is heated and surface heat flux 
increases, buoyancy drives vertical fluctuations that are strongly 
correlated with the low-speed shear-driven motions. With sufficient 
surface heating, the interaction between buoyancy-driven vertical and 
shear-driven horizontal fluctuations leads to the creation of strong 
streamwise-coherent rolls with helical turbulent motions that couple 
the upper with lower boundary layer. We have studied this transitional 
process with a series of carefully designed spectral large-eddy 
simulations of the ABL with fixed geostrophic wind above the capping 
inversion and systematically increased surface heating. A goal was to 
identify the transition between "near neutral" and "moderately 
convective" by quantifying correlations and integral scales as a 
function of the global stability parameter -zi/L; we expected to find 
a rapid but systematic change in boundary layer structure below a 
well-defined –zi/L value of order 1. We were surprised to find a 
complex and rich transitional process that includes a critical 
transition in horizontal turbulence structure, a special stability 
state that is characterized by exceptionally coherent streamwise 
rolls, and an unexpected sensitivity in horizontal fluctuations to 
surface heating.

  

*This work is in close collaboration with Dr. Balaji Jayaraman, 
Research Associate in the "Cyber Wind Facility" program at Penn State, 
supported by the Department of Energy.

  


Silvia Gentile
NCAR IMAGe