Thunderstorm -1 - Lecture 33

A useful resource : Severe Weather (DAS/UIUC)


Thunderstorm (T-storm) :
* a storm with thunder and lightning,
* strong wind , heavy rain or hail,
* possibly accompanied by tornado

"severe" T-storm :
* produces 3/4 inch hail and/or gust of 50 knots :
* long lasting
* form along frontal boundaries and dry lines

Impacts
* flash-flood, hail, wind damages
* lightning : fire damage, electrocution
* strong updraft/downdraft/turbulence: dangerous for flying
* benefit : spring & summer rain

Overall structural characteristics
* horizontal size : 10's km or larger
* vertical extent, up to ~ 13 km
* contains one or more convective cells


duration : from 30 minutes to several hours


frequency :
* over 40,000 every day over the world, most in tropics in all seasons
* strongest ones are in the mid-latitude and in hurricanes
* most common in spring , summer afternoon, sometimes at night
* Florida has highest frequency in US

precise forecast of individual T-storm is impossible,
but forecasting the general location and timing of T-storms is possible;
* basis of forecast :
extrapolation of movement of existing storm in real time ,
and anticipating the change of environmental structure that would be favorable for T-storm development

Types of thunderstorm
(1) air-mass thunderstorm , single cell storm
(2) multicell cluster storm
(3) multicell line storm
(4) supercell thunderstorm (Fig.10.11)
* (1), (2), (3) may or may not be severe
* (4) is always severe ,
We will elaborate on each of them shortly.

Key essential processes :
(A) thermal instability
* necessary conditions :
a deep layer of air that is conditionally unstable;
meaning Gd > G > Gm, ; &
large moisture content ; &
sufficiently strong lifting.
Then the release of latent heat would give rise to strong buoyancy ;
would sustain the moist convection
* can generate strong Updraft ;
heavy precipitation, even hail

* strong Downdraft :
could also develop due to the frictional drag on the air by the falling water drops &
also possibly due to sinking due to evaporative cooling

* Four Updraft/Downdraft combinations :
weak, weak (ordinary T-storm)
weak, strong (downburst)
strong, weak (single & multicell T-storm)
strong, strong (severe T-storm)

(B) Impact of vertical shear in the wind ;
Strong shear , strong T-storm
(e.g. 100 mph from ahead aloft & 20 mph surface wind from behind )
* wind shear "organizes" the storm by producing :
. vertical tilting in storm
. downdraft in the hind sector
. continual influx of moist warm air from the front sector
then precipitation falls in the downdraft region rather than into the updraft region
. a self-sustaining strong Updraft/Downdraft Couplet (>30 m/s)
producing a long lasting T-storm

For details seewind shear effect on convection

Outflow phenomeona
(1) gust front
* rapid spreading of cool air in downdraft at surface (> 25 m/s)
* temperature drops sharply
* wind shift
* surface pressure rise
* shelf-cloud ahead
* an elongated roll cloud behind
* can induce a new generation of T-storm

(2) Downburst : strongly localized downdraft
(2a) Microburst : small downburst (< 4 km)
gust up to ~ 75 m/s , no rotation
. considerable wind damage
* small size ; hard to detect
* large wind shear
.change from head-wind to tail-wind in a short time
.airplane would suddenly lose altitude & control
Example: July 1982, New Orlans, 145 person died

* Low-Level Wind Shear Alert System in some major airports
measure wind speed and direction, shear
only partially effective
* Doppler radar, being developed, may be more effective
(2b) Macroburst : a large downburst (r > 4 km)
* non-severe gust front
covers the entire cold air outflow field

(3) A special category
Mesoscale Convective System (MCS) (Fig.10.17)
* a large cluster of storms ; occupying a very large area
* an elliptical structure , long-lasting (>12 hours) and slowly moving
* too large to be seen on a radar
MCS sat.view

Return to ATMOS100/C page