Deep Convection
A 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.
Large Hail
(> 2cm in
diameter)
Damaging
Wind
> 90kt gust at
surface
Physical Process
Parameters used Radar products
to diagnose
Radar signatures
Strong updraft;
Hail embryos reside in
regions of high supercooled liquid water in the
hail growth region;
- CAPE
- Lifted Index
- Freezing level
heights (0 C, -20C)
Minimal melting of
hailstone
Weak shear
- Mid-level flow vector
(transfer of momentum)
& evaporatively driven
downdraft
Strong shear
- Rear inflow jet
- WBFZL (1.5 3.6km)
- Shear / SREH
- Mid-level wind
strength
- DMAPE
- CAPPI
- WER / BWER
- low-level
reflectivity gradient
- storm-top
displacement
- TBSS
- anomolous
propagation
- mid-level rotation
- splitting /
-Storm top div
- Radar algorithms
-Mid-level
convergence
- low-level
divergence
- low to mid-level
velocity maximum
- bow echo
- Strong precipitation
Heavy
precipitation
- Long lasting
(based on 1 in
10 year R I)
Tornado
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
- Shear (0-1km)
- Strong lowlevel rotation
- TVS
[10ms-1]
- SREH (0-1km)
- EHI
- Low LCL heights
- 0-1km CAPE
Buoyancy
and shear processes
Measuring buoyancy - CAPE

1
T  T dz
integrating:
 d( w )  g 
EL
LFC
2
2
EL
LFC
V
V
TV
1
w  CAPE
2
2
TV cloud
parcel
curve
EL
Where w is updraft strength in m/s
Maximum 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 environment
curve
DMAPE
The method used in
Australia to determine the
downdraft psuedo-adiabat
Mid-way moist
adiabat
Predicted saturated
adiabat for updraft
parcel
Mean moist adiabat
650-450hPa
1 2
w  DMAPE
2
w  downdraft speed
LFS=Level of Free sinking
DMAPE area
Estimating the convective gust strength
Mean Layer Flow (MLF) Vector (650-450hPa)
The resultant Surface
Convective Gust is the
Vector addition of the MLF
vector and the downdraft
vector (calculated via
DMAPE).
Vertically
orientated
downdraft vector
due to negative
buoyancy –
magnitude
calculated from
DMAPE area on
sounding
2.7 Summary - Special buoyancy factors
associated with Severe Convection
Strong Instability
immediately above LFC
(strong Lapse Rate)
Height of environmental
WBFZL
Dry slots in mid and lowlevels or deep moist layer
to 500hPa
Stable layer capping moist
low-levels (CIN)
Low-level moisture
• Plot the following hodograph.
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
Activity
Equations of Motion
F  ma
a  F

1
m
2. Building tools
Du
1 p

(1)
Dt
 x
Dv
1 p

( 2)
Dt
 y
Dw
1 p

 g (3)
Dt
 z
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.
2. Building tools
The environment in which we will grow
a storm - the basic state
z
Activity
u  u ( z )  z
vv0
ww0
p  p z 
p
   g (4)
z
   z 
x
The over-bar denotes basic –
state values.
2. Building tools
Vorticity in our (basic-state)
environment
Right – hand rule: The
(environmental) vorticity
vector points to the left of the
shear vector
u  w
 
z x
2. Building tools
An updraft grows in our
environment
z
small

In and around the
storm values of
'
  ( z )   ( x, y, z, t )(5) pressure, density
and wind are
p  p( z )  p ' ( x, y, z, t )
perturbed away
from their
environmental
values
x
3a. Linear terms
Cloud modelling results
z = 6km, t = 40mins; p’ cont. intervals at 0.5 hPa;
Updraft (heavy) contours at 10m/s intervals.
2. Building tools
Pressure and vorticity that arise when
updraft interacts with a shear layer
• Consider a slice of atmosphere (say at
z = 3km).
'
'  u w
p ~
z x
z
y
w0
-
z  3km
p 0
w’
p 0
'
+
u
'
x
3a. Linear terms
Straight line hodograph
Positive vorticity
Negative vorticity
Curved hodograph – 3 layer model
2
3
1
1
3
Activity
2
• 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 ?
3a. Linear terms
Anti-clockwise turning
hodograph
• 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.
3b. Non-Linear terms
The storm inflow layer and3c.
itsStorm-Relitive
rotationalHelecity
potential
• Board exercise. We will develop the accompanying conceptual
model “hodograph picture” in the lectures.
Storm motion vector
SREH ( z ) 
z

(U  c). dz
z0
Storm relative wind vector
Vorticity vector
* Board derivation
The mathematical definition of SREH
(U – c) is the storm relative flow vector
 is the vorticty vector
SREH – Straight line
hodograph
3c. Storm-Relitive Helecity
S
E
W
“Steering” flow vector
sfc
800 hPa
500 hPa
N
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.
3c. Storm-Relitive Helecity
SREH – Backing shear vector
hodograph
profile
S
E
sfc
W
500 hPa
800 hPa
N
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.
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.