Download Lecture_2a.severeprimer.2016

Lecture 2a
Severe Thunderstorm Primer
Synoptic Laboratory II – Mesoscale
Professor Tripoli
Severe Weather Definitions
I) World Meteorological Organization (WMO)
A) General
–
–
–
–
–
–
–
Heavy rain
Strong wind/ wind gusts
Hail
Lightning
tornadoes
flash floods
extreme temperature
B) Specific Events
–
–
–
–
Snow storms
dust/sand storms
sea swell/ tsunamis/ storm surge
extended area of fog for transport (aviation especially)
Severe Weather
(National Weather Service)
• Weather that “poses a threat to life and/or property”
• May include:
–
–
–
–
–
–
–
–
–
Heavy snow
Freezing rain
High winds
Flash flooding
River flooding
Thunderstorms
Tornadoes
Tropical storms
Hurricanes
Severe Thunderstorm
• National Weather Service Definition
– Thunderstorm having at least one of:
• Large hail (1 inch in or 2.5 cm in diameter or larger)
• Damaging winds (at least 58 mph or 55 kts))
• Tornado (visible funnel cloud reaching the surface)
– Not included:
• Lightning
• flooding
Basic conditions for Thunderstorms
• Potential Conditional Instability
– Environmental lapse rate is greater than the moist
adiabatic rate and less than the dry adiabatic lapse
rate
– Sufficient lifting of air parcels from low levels will
result in free convection (LFC), i.e. positive CAPE
– Some Convective inhibition <50 J/kg
– Existence of a Theta_e minimum usually ~3 km
above the surface, or ~700 hPa
Basic Conditions for Severe
Thunderstorms
1. Enhance basic conditions for thunderstorms
– significant CAPE (strength of updraft )
– Significant Theta_e minimum in vertical (strength
of downdraft)
– Moderate Cap (distribute to CAPE to a limited
number of isolated storms)
Basic Conditions for Severe
Thunderstorms
2. Vertical Wind Shear
–
–
–
–
Significant environmentally-supplied vorticity/PV on scale of thunderstorms
Inertially stabilizes storm structure
Organizes updraft/downdraft structure of storm
Strength of shear required is proportional to CAPE, i.e. matching important
• Turbulence tempered by a relationship between static stability and wind shear such as a
Richardson number (static stability/wind shear or thermal forcing/ inertial forcing)
• Goldilocks Ri:
– `Too much shear blows storm apart!
– Too little shear insignificant compared to thermodynamic forcing
– Certain shears “just right”
– Straight line versus curved hodograph
• Curved hodograph implies more organization of specific updraft/downdraft structures
– Thermal wind: low level veering (backing) shear in NH (SH) suggestive of warm
air advection
• Low level WAA preferable to CAPE forcing
– Tornadogenesis very dependent on curved wind shear profiles
– Bottom Line: Severe weather season is during the spring and Fall when there
exists strong jet streams and CAPE simultaneously
Basic Conditions for Severe
Thunderstorms
3. Helicity
– Critical for supercells and tornadoes
– Helicity density:
h = V iV ; z =Ñ ´ V
– Helicity:
3km
H=
ò h dz
o
– Helicity layers :
• 0-1 km
• 0-3 km
Basic Conditions for Severe
Thunderstorms
4. Large Scale dynamic forcing (QG PVA, low PV at outflow level)
– Thunderstorm has to form full 3D vertical circulation
– Downdraft helpful at low levels providing an “energy releasing”
downdraft, Kind of like a biker pushing a pedal down on one side
(updraft CAPE release) and lifting up on the other (downdraft
evaporationally enhanced cooling)
– Large scale QG forcing or low inertial stability, ie reduced “outflow
resistance” helpful also
• Required vertical motion for QG adjustment can be manifested as cumulus
clouds, ie the sum total of cumulus updrafts ARE the QG lifting
• Upward cumulus mass flux will build outflow along isentropic surface of
cumulus updraft theta_e
• This will force air down in compensation if not part of a large circulation, and
that will require energy-stealing WORK
• Energy consumed related to outflow resistance, which is proportional to inertial
stability, ie vorticity or PV along outflow surface
• Low PV lessens work, and so increases potential strength of convection
• Lowest resistance toward anticyclonic side of jet stream, ie poleward if jet core
is poleward
Basic Conditions for Severe
Thunderstorms
5. Low Freezing Level
– Particular condition formation of hail
– Almost all thunderstorms have hail since riming of
ice is what causes lightning
– For hail to reach surface it cannot melt
• Hail must be sufficiently large not to melt (CAPE wind
shear induced organization)
• Freezing level sufficiently low
Basic Conditions for Severe Weather
• Composite of predicted or observed
(nowcasting) environmental dynamical and
thermodynamical state
• Assessment of dynamical and
thermodynamical forcing, ie QG forcing, local
mesoscale circulations
• Composite diagrams
• Composite indices