Michael Miceli Webpage

Hurricanes and Thermodynamics



Background Information on Hurricanes


Hurricanes are rotating weather systems with low pressure that generally occur over warm tropical oceans.
The term "Hurricane" is only used in the Eastern Pacific, Gulf of Mexico, Carribean Sea, and Atlantic.
  -The rest of the world uses typhoons or tropical cyclones
Wind speeds are 33m/s at 10m above the ocean surface on average
Certain conditions that need to be met:
  -Ocean water warmer than 26.5 degrees celsius at depth of 50m
  -pre-existing low-pressure disturbance
  -air must be humid
  -wind speeds constant with height
  -Generally form at lattitude greater than 5 degrees from equator due to Coriolis Effect

Coriolis Effect-An effect whereby a mass moving in a rotating system experiences a force (the Coriolis force) acting perpendicular to the direction of motion and to the axis of rotation. On the earth, the effect tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern and is important in the formation of cyclonic weather systems. (google.com)



Physics Behind Hurricanes


Two Components of Motion of Air
Primary Circulation- the tangential motion of air about the central rotation axis
  -pressure gradient forces air parcels to the low-pressure center of the storm
  -Coriolis Force drives air perpendicularly to its motion
  -Centrifugal Force makes them move radially outward
Secondary Circulation-the radial motion of air, which is outward at large altitudes and inwards closer to the surface of the ocean
  -the Centrifugal and Coriolis Forces are not in perfect equilibrium with the pressure gradient
  -Centrifugal and Coriolis Forces are dependent on wind speed, but pressure gradient is not
  -air is forced to center and then rises
  -Conservation of Angular Momentum states that at small radius, speed is faster, which increases the rotational speed of the hurricane


Storm Surge Air pressure at average sea-level presses down against the water at 1013 millibars
At low-pressure center, the pressure presses down at 974 millibars
At the eye of the storm, water will rise above average sea level and the hurricane will carry this "storm surge" on to land


Thermodynamics
-Inflowing air gets increasingly moist as it flows inward due to evaporation from the warm ocean surface
-Temperature of the rising air increases with moisture content
-As wind speeds increase and surface pressure falls, the evaporation rate and moisture gradient increase
-The hurricane is created by the increase in negative radial temperature gradient due to the increase in moisture gradient



The Carnot Heat Engine and Mature Hurricane


Kerry Emanuel (1988, Massachussetts Institute of Technology) linked a mature hurricane to a Carnot Heat Engine
  -Assumptions-axisymmetric vortex in hydrostatic and rotational balance
  -Hurricane acquires heat energy in the form of latent heat
  -At the surface of the sea, the temperature ranges from 26-30 degrees celcius, and the rising air enters the upper troposphere (15 km above sea) where the temperature ranges from -60 to -70 degrees celsius


Problems With This Model
  -assumes axisymmetric, but at high altitudes, the hurricane becomes asymmetric
  -Storms are sometimes more intense, but the vast majority are less intense
  -What may cause these problems are asymmetries, such as asymmetric waves
Vortex Rossby Waves
  -propogates in direction opposite of tangential wind
  -excited by changes in large-scale vertical wind shear and moist convection
  -these waves are able to transport angular momentum radially within a hurricane, which plays an imortant role in the intensity


Kleinschmidt (1951) was the first to explain that the energy source of hurricanes resides in the thermodynamic disequilibrium between tropical atmosphere and the ocean
  -can be seen in under-saturation of near-surface air
  -evaporation of water transfers heat from the ocean, and the heat capacity of the ocean is extremly large, when compared to that of the overlying atmosphere
  -in order to bring the troposphere into thermodynamic equilibrium with the ocean, there would need to be a transfer from the ocean of 108 J/m2



References


http://www.meteo.physik.uni-muenchen.de/~roger/Publications/PWJUNE06smith.pdf
http://www.physicscentral.com/buzz/blog/index.cfm?postid=1236450869816874403
http://www.math.nyu.edu/caos_teaching/hurricanes/emanuel91.pdf
http://aerosols.ucsd.edu/classes/sio217a/EmanuelK2005ch10.pdf
http://astro.berkeley.edu/~echiang/oom/oomsol08.pdf
exerpt from textbook you gave me