Transcript Lecture 21

ATOC 4720 class21
• Clouds and storms
1. The air-mass thunderstorm
2. Severe storms
Cirrus clouds
Cirrostratus: halos
A question from Tom: atmospheric stability
IMPORTANT:
: conditionally unstable
Dry lapse rate and T profile
[Derive on the blackboard]
Saturation lapse rate and T
profile
Height (z)
Archimedes’ principle:
Td
Ts
Tobs
FB
FG
Acceleration of air parcel
Temperature (T)
1. The air mass
thunderstorm:isolated
Air mass thunderstorm character
Isolated thunderstorm;
Occurs in warm and humid regions: tropics and
warm marine air-masses drift to middle
latitude (say,air from the Gulf Stream);
Life cycle: Self destructing mechanism-downdraft counteracts updraft;
Weak or no vertical shear in winds;
2. Severe thunderstorms
Occur only in convectively unstable situation;
Considerable vertical shear;
Downdraft reinforce updraft;
Squall lines; multicell storms, supercell storms
Squall lines
Squall lines
• Individual cumulonimbus towers (close to
each other) are often arranged in long lines;
gusty surface winds--squall lines;
• Often occur in summer: West Africa, south
of the Sahara, and mid-latitude lands,
including central and eastern US;
Squall lines
Conditions, structure, and development:
Vertical wind shear;
Convectively unstable;
Moves with a speed of winds in middle troposphere: so it takes
Over the air in the prestorm environment at low levels,
while it acts as a massive, slow-moving obstacle to
The winds in the upper troposphere;
Structure: see figure: downdraft formation, Anvil formation,
Mamma;
Updraft and downdraft reinforce each other;
Surface wind gust 25m/s;
Right-moving multicell storms
Supercell thunderstorm
Hails
Tornado