Lecture 7 (10/21)

1. Lecture 7 (10/21) Hurricanes and Tropical Meteorology

2. 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

3. 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

4. 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 (cloud-tagging)

5. 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+

6. 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

7. 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

8. 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)

9. 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 (upper-level high) • Anticyclonic outflow helps push out the stable “exhaust” of a hurricane

10. 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

11. Websites FAQ’s from the National Hurricane Center http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html (G12 & D7 on FAQ’s) Current Satellite Loops http://www.cira.colostate.edu/ramm/rmsdsol/TROPICAL.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

12. 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