Transcript Lecture 10 - University of Oklahoma

Lecture 7 (10/21)
Hurricanes and
Tropical Meteorology
Tropical Meteorology
• Anything between about 30N and 30S in
latitude
• Or between Tropic of Cancer (23.5N) and
Tropic of Capricorn(23.5S)
• Fuzzy definition
• Radiation budget > 0 in tropics
• Job of tropics = export heat poleward
Tropical Meteorology
• Very difficult compared to mid-latitudes
• Can’t just measure pressure and get an
approximate wind field
• Geostophic Balance - doesn’t happen in in
tropics
• Effects of Earth’s rotation not a big player
(Coriolis force is small)
• Try to measure convergence (winds coming
together and divergence- winds pulling
apart) to deduce areas of upward motion
Other Problems
• Lack of data in tropics
• Most of area is not covered with land so less
observations
• Countries with less advanced obs networks
• Best sources for winds are: aircraft reports,
wind profilers, reconnaissance flights,
satellite-derived wind reports (cloudtagging)
Tropical Cyclone Classification
• TC’s form from a wave or disturbance
• Trop disturbance = any area of “disturbed
weather” ex: stalled out front
• Trop wave = elongated area of low pressure
(just a trough). Go from East to West
(direction of winds at these latitudes).
• Trop depression = closed low (trough
pinches off)
• Trop storm = 39-73mph, hurricane=74+
Strength Versus Intensity
• Intensity = core region (center to 100 km)
• An increase in intensity = stronger max
winds or lower minimum surface pressure
• Intensity can change quickly
• Strength = outer part of core
• Associated with area weighted average
wind speed outside of core
Eyewall Cycle
• Happens in intense hurricanes
• Closed rainbands form a bit outside eyewall and
move towards eye
• Old eye deteriorates from subsidence (sinking air)
induced by rainband
• Closed rainband becomes new eyewall (most
intense when eyewall is well defined)
• Intensity changes during this but not strength
• Whole cycle repeats itself
Eyewalls & Rainbands
• Region of clouds/intense rain/strongest
winds that seperates sinking air in eye from
rest of storm
• Typically not vertical (45 deg slope =
common)
• Rainbands = stronger areas of rain with
higher clouds between them
• Sometimes, tornadic supercells will form in
rainbands as storm hits land (b/c of added
friction)
Inflow and Outflow
• Inflow in a TC happens near ocean’s surface
(under clouds) in planetary boundary layer
• Little known about hurricane inflow
• Outflow = usually anticyclonic aloft (upperlevel high)
• Anticyclonic outflow helps push out the
stable “exhaust” of a hurricane
Fun Facts
• Anticyclonic outflow (almost) never displaced
toward equatorward side
• TC’s need 80 degree Fahrenheit water
• TC’s need deep, warm water too (winds stir it up)
• recurvature = term for TC switching from
Northeastward to Northwestward movement
• Usually means the peak for the storm - starts to
decay after that
• Bill Gray at CSU = hurricane guru - uses
climatology and old navy radiosonde data to come
up with theory/predictions
Websites
FAQ’s from the National Hurricane Center
http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.ht
ml (G12 & D7 on FAQ’s)
Current Satellite Loops
http://www.cira.colostate.edu/ramm/rmsdsol/TRO
PICAL.html
Tropical model data output
http://www.essc.psu.edu/~rhart/tcgengifs/ (GFDL)
The coolest website ever (shows movie of enhanced
satellite imagery of Michelle) (need high speed
connection)
http://rsd.gsfc.nasa.gov/pub/goes/QTmovies/0111.
michelle.mov
For Next time:
• Mid-semester gift. No reading assignment.
We will be discussing Radar next week and
the book does not have anything useful to
read about it.
• However, still be sure to do Homework 7
(Hurricanes and Tropical Meteorology)
• We will still have the quiz next week