Download Lecture 23

Lecture 23: Map discussion + Thunder & Tornadoes (Ch 11)
• brief map discussion and/or storm video
• cloud buoyancy & order-of-magnitude estimate for vertical velocity
• potential instability
• airmass thunderstorm
• severe thunderstorm & thunderstorm complexes
• warming aloft since Wed
• saturated, stable lower troposphere
• low-level SE, W aloft
12Z Fri 3 Nov., 2006
• the more zonal current and associated increase in thickness, anticipated by
Wednesday’s GEM 48 hr prog, has appeared
• warm thickness adv. at 500 mb
• warm adv. from SE at 850 (lifting over cold dome of surface air?
12Z Fri 3 Nov., 2006
• noise problem with sfc isobars on
preliminary CMC sfc chart (result
on newly refined resolution of
global forecast runs, used in the
analysis which blends obs + fcst)
• easterlies in C. Ab… upslope
• weak sfc low in SW Ab.
Buoyant acceleration of an air parcel
Let W be the vertical velocity of an air parcel, and let W be the change in W over
time interval t. Then W/t is the parcel’s acceleration.
Let T0(z) be the height-varying temperature of the environment, and let T’ be the
difference between the parcel’s temperature and the environment at the same level
(z).
Newton’s law can be written:
(again, this is an approximation)
W
T'
 g
t
T0
The pressure gradient force and gravity almost balance each other (they do so
exactly in a well-mixed, hydrostatic atmosphere); but the parcel’s temperature
deviation T’ gives rise to the “reduced gravity” force gT’/ T0 which may have
either sign.
Back-of-envelope calculation for
vertical velocity in a deep convective
cloud
W
depth
h
• note: TC(z) folllows moist adiabat
Cloud
TC
T’ = Tc – T0
• accel’n: g (TC – T0) / T0 ~ 10 * 5 / 300
• rise time: h / W
• accel’n * time: W = g (TC – T0 ) / T0 * h / W
• W 2 = g (TC – T0 ) / T0 * h
Envir.
T0
 Thunderstorm: organised (coherent) 3-d atmospheric motion
 takes place in an atmosphere that is in a “conducive” state, and
locally modifies that state
 “organised” implies some sort of a co-ordinated, self-perpetuating
pattern of winds (U,V,W), pressure (P), temperature (T) and
humidity (Q) that can persist for many minutes and perhaps even for
hours
 energy for the motion derives from a pre-existing store of energy
(gravitational potential energy & latent heat)
• Airmass thunderstorm: short-lived, isolated, scattered within
warm humid airmass, self-extinguishing
• Severe thunderstorm: winds exceed nearly 100 kph or
hailstones exceed nearly 2 cm or storm spawns tornado
For this to occur updrafts and downdrafts must remain
separated and reinforce each other to intensify and prolong the
storm. Require very warm, humid surface air, conditional
instability, wind shear + trigger
Thunderstorm - occurs in conditionally unstable atmosphere - why?
• To get energetic cloud, must release stored potential energy (warm, moist
near-ground air) in a small region - “concentration” or “focusing” of energy
release
• In conditionally-unstable atmosphere most unsaturated parcels rising will
experience a restoring force… but those few that rise high enough to saturate,
will result in deep, energetic clouds whose updraft causes surface
convergence - sucking in the energy (warm, moist air) to this “focal point”
• “Trigger” selects the storms which “succeed” - unequal surface heating, or
terrain slopes or irregularities, or (in case of “severe thunderstorm”) frontal
lifting
•
An elevated temperature inversion may suppress deep convection for a time,
but the “potential instability” (Sec. 11-3) is such that an eventual storm that
does develop is likely to be more explosive
Potential Instability
• dewpoint lapse rate 2oC per kilometer
• imagine both parcels lifted 500 m… lower parcel (T,Td)=21,21 but upper (22,18)
• lift a further 500 m… lower (18.5,18,5) but upper (17,17)
• column has been destabilized
• elevated inversion common
in lee of Rockies due to
subsidence
(“capping inversion”)
Warm & dry
Warm & moist
Sec. 11-3, Fig 1
Airmass thunderstorm
• successive surges of
warm moist air form light
Cu whose evaporation
humidifies the column;
progressively deeper Cu
• cloud grows above
freezing level – Bergeron
process initiated
• weight of rain and/or
graupel initiates
downdraft; if precip
falls into unsaturated
air (eg. mixed in by
entrainment), its
evaporation chills the
downdraft
• downdraft kills off the
updraft
• most of the precip
particles (water & ice)
evaporate again
• storm consists of
several such cells
(updraft + downdraft)
of differing ages
T0(z)
Ice crystals
Fig 11-7
Severe thunderstorm
• favourable mesoscale pattern permits
prolonged separation of updraft & downdraft
• wind shear is the key (low level SE or S +
mid level SW or W)
• favourable conditions over large
area causes clusters of storms that
interact (mesoscale convective
system) – organized linearly (“squall
line” along front) or not (“MCC”)
Fig 11-14
The shear is better
seen in the
animation/video
Mesoscale Convective Complex (MCC)
• downdraft from individual cell moving to ENE drives under warm moist SE stream
(“outflow boundary”) initiating a new cell on the south flank of the complex. Thus
cluster as a whole moves in a direction deviating from individual storms
older cells on N flank cut off from the low-level warm, humid SE
SW or W or NW
flow in midtroposphere
Warm humid
low-level SE
Fig 11-11
Downdraft produces gust front, associated with which may see shelf
cloud (“arcus”) and occasionally roll cloud
Fig 11-16
Shelf cloud…
Photo by Tom Eklund
Downburst
Particularly intense downdraft over small area (diameter < 5 km) spreads out at
ground… many larger airports now protected by warning system based on
“Doppler acoustic radar” (dopplerized “sodar”… sound detection and ranging)
Fig 11-20
Supercell Storm
• isolated, single powerful cell with diameter 20-50 km, ie. smaller than MCC, life
2-4 hours
• setting for most large tornados
• large scale rotation in the cloud, not seen in other types of severe storm
• often radar reveals a
“hook echo”
• doppler radar may
reveal the rotation
Fig 11-18
Fig 11-19
Small droplets in the warm
updraft do not reflect
probing radar waves