Deep Moist Convection (DMC) Part 2 – Modes of Isolated Organization AOS 453 – Spring 2014 4/3/2014

Deep Moist Convection (DMC)Part 2 – Modes of Isolated Organization

AOS 453 – Spring 20144/3/2014

Structure Of DMC Lectures This Week

• Tuesday– DMC (Convective/Convection) Initiation

• Chapter 7 in MR09

– Isolated DMC Organization (Part 1)• Chapter 8 in MR09

– SPC Mesoanalysis Introduction

• Thursday– Isolated DMC Organization (Part 2)


– Diagnostic Indices and Variables for Isolated DMC – SPC Mesoanalysis Continued

Tuesday Recap• What is DMC?

• Troporpheric Overturning• Parcels lifted to LFC and freely convect throughout significant portion of troposphere

• How does DMC get initiated?• Need to reach LFC by either:

– Mechanically Lifting Parcels» Air mass boundaries (including density current/outflow boundaries)» Orographic Lifting

– Decrease pre-existing CIN to make it easier to overcome (takes a long time)» Via large scale processes

• Synoptic UVM• Differential moisture advection• Surface heating

» Via manipulating lapse rate (lapse rate tendency)

• DMC initiation is a complex problem• Driven largely by mesoscale processes, but can be helped by synoptic scale• Need to account for entrainment.

– Drier mid-troposheric air will decrease buoyancy of plumes– Vertical wind shear will increase turbulent mixing across periphery of plumes increasing

entrainment and decreasing the buoyancy of plumes

Tuesday Recap

• Isolated DMC Organization (part 1)• The role of vertical wind shear

» Vertical wind shear is vertical gradient of horizontal wind component• Often represented as wind vector difference between

two layers• Can plot wind shear using a hodograph

» Wind shear alone prevents initiation, but wind shear + buoyancy (CAPE) promotes organization• Bulk Richardson Number

» Wind shears role in organization is twofold:1.) Displaces precipitation away from updraft2.) Induces a dynamic pressure gradient









Modes of Organization – And their relationship to wind shear






Ordinary/Single Cell Convection

Single-Cell Convection(Ordinary Thunderstorms)

• Think one updraft• Driven by buoyancy

• No organized, initiated convection from its outflow• Gust front lifting in associated (weak shear) environments is

weak and shallow (no dynamic PGF!)

• Usually initiated in concert with several other single cells (usually there is a field of deep convection on satellite imagery)

• COLLIDING outflow (not necessarily individual outflow) is often associated with new convection initiation– Not to be confused as “organized” or “multicell” convection. We’ll

see the difference soon…

Ordinary/Single Cell Convection- 3 Stages In Life Cycle



Ordinary/Single Cell Lifetime (τ)

• Can come up with approximate quantitative relationship for lifecycle time, τ

– Function of depth of convection: H

– Speed of updraft : wup

– Speed of precip (terminal velocity):vt

Ordinary/Single Cell Convection- Like The Mayfly

If H = 10km If H = 10km wup = 5 m s-1 wup = 10 m s-1

vt = 5 m s-1 vt = 10 m s-1

τ ≈ 1 hr τ ≈ 30 min

Ordinary/Single Cell Convection- Like The Mayfly

Multicell Thunderstorms


• ♬ Just a spoon-full of wind shear helps the thunderstorm live long…the thunderstorm live long…the thunderstorm live long ♬


• 0-6km wind shear ~ 10-20 m s-1

• So there is environmental HORIZONTAL vorticity (prove it to yourself)

• NOT just one individual updraft (so last slide’s little song number was a little deceiving)

• Multiple ordinary/single cells going through life cycle one after the other

• Driven by cold pool propagation

• Organized initiation of new cells along leading edge outflow boundary

• New cells initiate along the downshear outflow boundary or gust front

• Can think of collective low-level outflow (cold pool) as a thunderstorm “machine” or “conveyor belt”

Why Does Wind Shear Matter?

• How come the same organization doesn’t occur in ordinary/single cell convection?

• Multicell convection is composed of a bunch of ordinary/single cells that have same life cycle, so why in ordinary/single cell case do we not see the same organization?

First Need To Discuss Baroclinically Generated Vorticity and Vortex Rings

COLD / DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE


COLD / DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE


COLD / DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE

WARM /LESS DENSE

Once It Hits The Ground…

Once It Hits The Ground…

It Spreads Radially Away From Center Of Maximum Pressure (directly beneath the downdraft)

H

It Spreads Radially Away From Center Of Maximum Pressure (directly beneath the downdraft)

Who cares?

• Interaction with environmental wind shear promotes enhanced vertical lifting along downshear outflow boundary!

• Compared to ordinary/single cell case• Better/more-likely for lifting to lift air to LFC and initiate

new DMC

Who cares about the wind shear?

• Taller downshear outflow boundary• More mechanical lifting

• Cooperative horizontal vorticity between baroclinically generated vortex ring and environmental shear

WHAT DIRECTION IS THE SHEAR VECTOR?

Vertical Wind Shear Vector

Supercellular Convection


• ♬ Just the right amount of wind shear makes the thunderstorm spin ‘round…the thunderstorm spin ‘round…the thunderstorm spin ‘round ♬


• > 20 m s-1 0-6km wind shear• Long hodographs with/without curvature• Not the only important environmental diagnostic• Need very special environmental wind shear (one the has

helicity as we will see)

• Updrafts are actually rotating with vertical vorticity throughout most of its depth (typically AT LEAST lower half)…this is called the MESOCYCLONE

• Width/diameter ~ 5km• Vorticity magnitude on the order of .01 s-1


• When mesocyclone is rotating cyclonically– Propagation is right of the mean wind

• When mesocyclone is rotating antiyclonically– Propagation is left of the mean wind

Supercellular ConvectionBASE-ANGLE RADAR(MOORE, OK 2013)

Supercellular ConvectionBASE-ANGLE RADAR(MOORE, OK 2013)


Top-Down View

Supercellular ConvectionTop-Down View

Why Two Downdrafts?

• Forward Flanking Downdraft (FFD)• Precipitation advected ahead of storm by mean wind

• Rear Flanking Downdraft (RFD)• Mid-level flow forced down by collision with updraft• Precipitation that falls out of updraft core UPSTREAM and

evaporates, cooling the air and forcing air to subside around the periphery of the updraft

• Reason for the “kink” or “occlusion” in the outflow boundary near the low-level updraft’s inflow

Mean Wind Advected PrecipitationDown Wind

Mid-Level windsCollide with penetratingupdraft

Further precipitation Evaporation cools theAir and forces it to subside

How Does Environmental Vertical Wind Shear End Up As Storm Rotation?

• From TILTING the horizontal vorticity vertically via a local maximum in vertical velocity.

• Advection and Storm Relative Helicity (SRH) can play a role in determining the type of Supercells that develop as we will see

UPWARD TILTING OF ENVIRONMENTAL VERTICAL WIND SHEAR=

VERTICAL VORTICITY OF THE SUPERCELL’S MESOCYCLONE

How Does Environmental Vertical Wind Shear End Up As Storm Rotation?

• Storm-Relative Vertical Vorticity Tendency Equation

(after scaling out perturbation components)

Advection Tilting

Mean Vertical Wind Shear Vector

Storm Relative Wind Vector

If no original vertical vorticity,Tilting Must Come 1st

If no original vertical vorticity,Advection Comes 2nd

STRETCHING

NO STRETCHING

No Favored Rotationso

Left AND Right Movers

Cyclonic Rotation Is Favored

SoRight Movers FAVORED

The Difference is Storm Relative Helicity

• Storm Relative Helicity (SRH)

The “helical” nature of the flow

DNA DOUBLE Helix Helix


• How do you get helicity from rotation (vorticity)?


• How do you get helicity from rotation (vorticity)?

• Add a velocity through it! (So velocity DOT vorticity)


• How do you find SRH?• Use hodograph!• CURVED hodographs have some value of SRH to them

– Curved means the vertical shear is directional as well as speed

• STRAIGHT hodographs do NOT have SRH

So What Did We Learn?

Driven by buoyancy

Driven by outflow lifting

Driven by dynamic pressure gradient

FOR A FIXED VALUE OF CAPE

The Thunderstorm Project

• Until 1947, no one really had an idea what thunderstorms were or why they initiated.– Largely because they didn’t have a way to observe their composition

and lifecycle effectively– Three-mode model came from this project!

• Once RADAR came around, and the instruments became light enough to put onto airplanes…the government funded the first join meteorological research project into the origins of thunderstorms.

https://www.youtube.com/watch?v=JSKF6q2lIS4#t=1526









Documents

Deep Moist Convection (DMC) Part 2 – Modes of Isolated Organization AOS 453 – Spring 2014 4/3/2014