General Theme: ….Consider the evolution of convection in the absence of significant larger-scale forcing influences…or even boundary layer features….

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Presentation transcript:

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:

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 of Wavelength 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)