Deep ConvectionA review of processes

“Everything we hear is an opinion, not a fact. Everything we see is a perspective, not truth”

Marcus Aurelius: AD121-180

Learning Outcomes

• Review severe weather processes and associated parameters.

• Review hodograph concepts• Understand the role of shear-related processes

and parameters in determining propagation and updraft rotation.– Bulk shear; SREH

• Review radar signatures associated with severe convective weather.

Physical Process Parameters used to diagnose

Radar productsRadar signatures

Large Hail

(> 2cm in diameter)

Strong updraft;Hail embryos reside in regions of high super-cooled liquid water in the hail growth region;

Minimal melting of hailstone

- CAPE- Lifted Index- Freezing level heights (0 C, -20C) - WBFZL (1.5 -3.6km)

- Shear / SREHShear / SREH

- CAPPI- WER / BWER- low-level reflectivity gradient

- storm-top displacement- TBSS- anomolous propagation - mid-level rotation- splitting /

-Storm top div

- Radar algorithms

Damaging Wind

> 90kt gust at surface

Weak shear

- Mid-level flow vector (transfer of momentum) & evaporatively driven downdraftStrong shear

- Rear inflow jet

- Mid-level wind strength- DMAPE

-Mid-level convergence

- low-level divergence- low to mid-level velocity maximum- bow echo

Heavy precipitation

(based on 1 in 10 year R I)

- Strong precipitation

- Long lasting convection (slow moving cells / Large cells / Stationary trigger)

- Precipitable Water > monthly (ave +1 SD)

- Warm Cloud Depth > 3.0km

-Adiabatic Liquid Water Content > 12g/kg

- weak steering flow

- Large accumulations

Tornado - Shear (0-1km) [10ms-1]- SREH (0-1km)

- EHI - Low LCL heights- 0-1km CAPE

- Strong low-level rotation- TVS

Buoyancy and shear processes

Measuring buoyancy - CAPEintegrating:

EL

LFCV

VV

EL

LFC TdzTT

gwd )21

( 2

CAPEwEL

2

21

Where w is updraft strength in m/sMaximum possible value

(excluding super cells)

CAPE is the positive area between the parcel and environment virtual temperature curves between the LFC and the EL on the skew T –log P diagram

TV cloud

parcel curve

TV environment curve

Predicted saturated adiabat for updraft parcel

Mean moist adiabat 650-450hPa

Mid-way moist adiabat

DMAPE area

speeddowndraftw

DMAPEw

2

21

The method used in Australia to determine the downdraft psuedo-adiabat

LFS=Level of Free sinking

DMAPE

Mean Layer Flow (MLF) Vector (650-450hPa)

Vertically orientated downdraft vector due to negative buoyancy – magnitude calculated from DMAPE area on sounding

The resultant Surface Convective Gust is the Vector addition of the MLF vector and the downdraft vector (calculated via DMAPE).

Estimating the convective gust strength

Strong Instability immediately above LFC (strong Lapse Rate)

Height of environmental WBFZL

Dry slots in mid and low-levels or deep moist layer

to 500hPa

Stable layer capping moist

low-levels (CIN)

Low-level moisture

2.7 Summary - Special buoyancy factors associated with Severe Convection

• Plot the following hodograph.

Activity

Level (hPa)

height WindDir Speed (kts)

1010 Surf 050 16

943 500m 360 14

910 1000m 344 11

850 1500m 330 13

785 2000m 309 15

700 3000m 290 27

650 3600m 273 31

600 4200m 270 30

500 6000m 275 39

Equations of Motion

)3(1

)2(1

)1(1

gzp

DtDw

yp

DtDv

xp

DtDu

p = total pressure; = density

These equations contain density because they are in height (z) co-ordinates. They appear simpler than those for synoptic-scale motions because the effect of the Coriolis force and Friction is not included.

Fa

maF

m1

2. Building tools

The environment in which we will grow a storm - the basic state

0

0

)(

ww

vv

zzuu

z

x

zpp z

)4(gzp

The over-bar denotes basic – state values.

2. Building tools

Activity

Vorticity in our (basic-state) environment

xw

zu

Right – hand rule: The (environmental) vorticity vector points to the left of the shear vector

2. Building tools

An updraft grows in our environment

z

x

),,,()( ' tzyxpzpp )5)(,,,()( ' tzyxz

In and around the storm values of pressure, density and wind are perturbed away from their environmental values

small

2. Building tools

Cloud modelling results

3a. Linear terms

z = 6km, t = 40mins; p’ cont. intervals at 0.5 hPa;Updraft (heavy) contours at 10m/s intervals.

Pressure and vorticity that arise when updraft interacts with a shear layer

• Consider a slice of atmosphere (say at z = 3km).

xw

zu

p

'

' ~

z

u

w’

x

y0w kmz 3

0' p 0' p

2. Building tools

+

-

Positive vorticity Negative vorticity

Straight line hodograph

3a. Linear terms

• Consider these anti-clockwise and clockwise turning hodograph to be composed of 3 shear layers (1. a low-level layer , 2. a mid-level layer and 3. a high-level layer) stacked on top of one another.

• Determine the pressure perturbations relative to the shear vector for each layer.

• Where does high perturbation pressure near the ground underlie low perturbation pressure aloft ?

Activity

Curved hodograph – 3 layer model

1

2

3

1

2

3

• When the shear vector turns with height the orientation of the H and L pressure areas turn with height so that an upwardly directed pressure gradient force drives new updraft on the left flank of the storm. This forcing makes the storm propagate to the left of the steering flow. The new updraft is correlated with mid-level cyclonic vorticity.

Anti-clockwise turning hodograph

3a. Linear terms

3b. Non-Linear terms

dzcUzSREHz

z 0

).()(

The storm inflow layer and its rotational

potential

• Board exercise. We will develop the accompanying conceptual

model “hodograph picture” in the lectures.

The mathematical definition of SREH

(U – c) is the storm relative flow vector

is the vorticty vector

Storm motion vector

Storm relative wind vector Vorticity vector* Board derivation

3c. Storm-Relitive Helecity

E

sfc

500 hPa

W

S

N

800 hPa

SREH – Straight line hodograph

Area proportional to the SREH calculated between (0-2km) AGL for left moving versus right moving storms – straight hodograph.

In the straight line hodograph case both the left and right moving members of the original split have equal magnitudes of SREH.

“Steering” flow vector

3c. Storm-Relitive Helecity

Esfc

500 hPa

W

S

N

800 hPa

Area proportional to the SREH calculated between (0-2km) AGL for left moving versus right moving storms – curved hodograph. In this case the hodograph curvature produces a larger magnitude of SREH for the right - moving storm.

SREH – Backing shear vector hodograph profile

3c. Storm-Relitive Helecity

Learning Outcomes

• Review severe weather processes, associated parameters.

• Review hodograph concepts• Understand the role of shear-related processes

and parameters in determining propagation and updraft rotation.– Bulk shear; SREH

• Review radar signatures associated with severe convective weather.