deep convection a review of processes “everything we hear is an opinion, not a fact. everything we...
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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
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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.
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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
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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
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Buoyancy and shear processes
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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
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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
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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
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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
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• 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
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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
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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
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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
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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
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Cloud modelling results
3a. Linear terms
z = 6km, t = 40mins; p’ cont. intervals at 0.5 hPa;Updraft (heavy) contours at 10m/s intervals.
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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
+
-
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Positive vorticity Negative vorticity
Straight line hodograph
3a. Linear terms
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• 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
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• 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
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3b. Non-Linear terms
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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
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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
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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
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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.