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)