ATOC 185 Lecture Notes - Lecture 10: Mesocyclone, Tornado Watch, Alarm Device
8 Jun 2018
School
Department
Course
Professor
November
7th
and
9th–
Tornadoes
Cont.
Where
and
When?
-Over 85% of tornadoes occur in North America (won’t ask this on final)
- Boundary between warm and cold air in the middle of North America
- Moves depending on time of year
- ‘Tornado Alley’
- Changes month to month
-Where the largest numbers of hurricanes occur
-US tornado averages per month
- Most common in May and June (spring and summer generally)
-Time occurrence
- Most common at 6-7pm
- Increases throughout afternoon, peaks at 6pm, decreases through the night
Tornado
Detection
- Storm spotters
-Doppler radar
-Fixed site, or mobile
- Radar can be placed on trucks and moved and placed in viewing range of tornado
- Measure speed of wind because the beam that is received by the radar will have
different frequency depending on the motion of the water drops
- Velocity signal in Doppler Radar
- That there is a likely tornado occurring
- Isn’t observed at the ground – a few meters above the ground
- Reflectivity in doppler radar
-Where precipitation is occurring
- Detects where strongest intensity of precipitation is occurring
-All tornadoes form within thunderstorms
Thunderstorm
formation
- Require 3 ingredients
1. Moisture
a. Moist air flow comes in from warm/moist oceanic air from Gulf of Mexico or gulf
stream current in Atlantic
2. Conditionally unstable atmosphere
3. A trigger
a. A mechanism to trigger thunderstorm updraft, either through forced lifting or
heating
b. Air going in at low levels that forces air to rise and go out
c. Convergence of air
d. Triggers that produce rising motion and therefore precipitation are similar to
hurricane triggers
-Go out on a LIM – lift, instability, moisture (three ingredients that make up a thunderstorm)
Air
masses
Dry Line – important
feature
Cold air migrating towards
East
Area between cold and
warm front characterized by
warm dry air
Left to right arrow is
jetstream
Existence of cold and warm
front focuses areas of
forced lifting - trigger
Instability
- Stable: anything you do to ball (or parcel of air) will accelerate it back to equilibrium position
- Unstable: ball is precarious, any kind of tweak will cause it to fall
- Parcel of air is very unstable, any type of tweak to air parcel forces it to accelerate
Hurricanes
vs
Tornadoes
–
Instability
- Both need instability (warmer air near the surface, considerably colder air aloft) in order to
form thunderstorms – we measure this by estimating the amount of convective available
potential energy (CAPE) in the atmosphere
-Two ways to get instability – warming at low levels, cooling at high levels
-On right side of images, air is too
stable
and lift will not occur
Lapse
Rate
Temperature decrease with
increased elevation
0 C to -6
Document Summary
Over 85% of tornadoes occur in north america (won"t ask this on final) Boundary between warm and cold air in the middle of north america. Where the largest numbers of hurricanes occur. Most common in may and june (spring and summer generally) Increases throughout afternoon, peaks at 6pm, decreases through the night. Measure speed of wind because the beam that is received by the radar will have different frequency depending on the motion of the water drops. Isn"t observed at the ground a few meters above the ground. Detects where strongest intensity of precipitation is occurring. Stable: anything you do to ball (or parcel of air) will accelerate it back to equilibrium position. Unstable: ball is precarious, any kind of tweak will cause it to fall. Two ways to get instability warming at low levels, cooling at high levels. On right side of images, air is .
