[GTP] MMM seminar at NCAR-Rich Rotunno

[Silvia Gentile]

MMM SEMINAR NCAR
Mesoscale Modeling at High (but not Turbulence-Resolving) Resolution
Rich Rotunno
National Center for Atmospheric Research
Mesoscale and Microscale Meteorology


This talk discusses an issue associated with meteorological modeling 
with fine [O(1km)] grid meshes. The scales resolved by these meshes are 
small enough to capture features of the larger turbulent eddies that may 
exist in the planetary boundary layer (PBL), but too large for the 
simulation of the turbulent cascade that regulates their amplitude and 
structure, as would occur in a Large-Eddy Simulation (LES) where grid 
meshes are O(10m). Until recently larger [O(10km)] grid meshes were 
typically used for mesoscale modeling, but now increases in 
computational capability are making possible higher-resolution mesoscale 
modeling. The issue addressed in this talk is that PBL parameterizations 
developed for the larger-grid-mesh simulations may in fact set up 
situations in which the mesoscale model produces convectively unstable 
motions, the reason being that in an attempt to represent observed 
heated-PBL profiles, most parameterizations produce 
potential-temperature profiles that are superadiabatic over much of 
their depth. From the point of view of a mesoscale model, however, these 
profiles are absolutely unstable and are prone to producing circulations 
at the minimum resolved scale (typically 4-6 times the horizontal mesh 
size) at short lead times since the growth rate is inversely 
proportional to the resolved scale. Although the production of these 
circulations corresponds to a real physical process, the smaller-scale 
turbulence that determines the ultimate form taken by these motions 
(i.e. whether they be regular or randomly distributed in space) depends 
on scales not resolved by the mesoscale model. For mesoscale models with 
grid meshes O(1km), the motions immediately downscale of the minimum 
resolved scale are too large to fall within the inertial subrange of 
turbulence and no theory based on first principles exists for the 
parameterization of their effects. In the present study we turn to 
simulations with resolution high enough to simulate turbulent transfer 
(i.e. LES) as a guide to the physical regimes where structural features 
such as quasi-steady, quasi two-dimensional circulations may occur in 
the heated PBL.

This seminar will be recorded and available via webcast at:
http://www.fin.ucar.edu/it/mms/fl-live.htm
Thursday, 12 September 2013, 3:30 PM
Refreshments 3:15 PM
NCAR-Foothills Laboratory
3450 Mitchell Lane
Bldg 2 Main Auditorium, Room 1022

-- 
Silvia Gentile
NCAR IMAGe
1850 Table Mesa Drive
Boulder, CO 80305
303 497 2480
www2.image.ucar.edu