General Theme:
….Consider the evolution of convection in the absence of significant larger-scale forcing influences…or even boundary layer features….
The spectrum of convective storms and convective systems can largely be explained based on just two environmental parameters:
…..Buoyancy
….Vertical Wind Shear
Ordinary Cell:
Multicell:
Supercell:
Archetypes: Building blocks of the
observed spectrum
Ordinary Cells: short lived (30-60 min), propagate with the mean wind
Multicells: long-lived group of ordinary cells
Supercells: quasi-steady, rotating, propagate right or left of the vertical wind shear vector
•Buoyancy processes: basic updraft/downdraft, (ordinary cells)
•Gust front processes: triggering of new cells, upscale growth, (multicells)
•Dynamic processes: rotating updraft, dynamic vertical pressure gradient forcing, (supercells)
Physical processes controlling cell types:
http://www.meted.ucar.edu/convectn/csmatrix/
What Goes Up……
Must Come Down
Ordinary Cell Evolution:
What Goes Up……
Basic Equations:
(Buoyancy)--
+ ice….
Buoyancy Force:
Archimedes Principal: Buoyancy is simply the difference between the weight of a body and the fluid it displaces.
Wmax = (2 CAPE)1/2
Parcel Theory:
.…ignores pressure effects
Buoyancy is Scale-Dependent!!!
…real bubble in 3D simulation
Dynamic Pressure:
Buoyancy Pressure:
Diagnostic Pressure:
**For wavelike disturbances:
Vertical Momentum Eq. (rewritten)
(dynamic) + (buoyancy)
Basic 2D Equations:
--
Or, more simply, consider the 2D horizontal vorticity equation:
where
⁄
Buoyant Processes:
Buoyancy is Scale-Dependent!!!
Basic Equations:
(Buoyancy)--
+ ice….
Cold Pools: Density Currents
Droegemeier and Wilhelmson, JAS, 1987
…2D …30 – 40 km …100 – 200 m
You’ve all heard of “Kelvin” Helmholtz instability…????
Shallow (Trapped) Wave-Like Disturbances
Density Current Internal Bore ofWavelength
• Gravity-wave related phenomena can be excited by antecedent convection
• Statically stable nocturnal PBL provides an environment where such disturbances can maintain coherence
From Simpson (1997), An Introduction to Atmospheric Density Currents
Density Current:
Theoretical speed of propagation:
“Optimal” condition for cold pool lifting
C/∆u > 1
C/∆u = 1
C/∆u < 1
RKW Theory
Rotunno et al. (JAS, 1988)