Transcript Document
ATOC 4720 class22 • Hurricanes and climate Hurricanes Hurricanes intense rotational storm that develop in regions of very warm SST (typhoons in western Pacific Hurricanes • Evolved from tropical depression or cyclones (winds near the center of the vortex is <32 m/s) • Horizontal radial scale: 500km • Vertical depth: 10-15km (deep convection) • Intense convection and strong winds region are typically only 100km in radius (this is why hurricanes are often classified as mesoscale systems) Hurricanes • Organized convective precipitation on two distinct scales: individual convective cells are grouped in mesoscale bands, and the bands in turn are arranged in characteristic configurations on the scale of the hurricane • Highly organized: efficient to produce rainfall Hurricanes: Eye Eye: 20-50km in diameter low pressure (880mb) eye-wall cloud and rainband: most strongly developed in the quadrant of the storm centered about 45degree to the right of the direction of the storm movement in NH Centralfugal force expells clouds outwards Hurricanes: region and condition • • • • Between 5-20N(S) in latitudes (not on EQ); SST>26.5C; Conditionally unstable atmosphere; Weak vertical shear (near barotropic: many barotropic hurricane models) Mechanism: CISK Atmospheric CISK Theory •Conditional instability of second kind (CISK) • (early 60-70’s):cooperation between convection •And large-scale convergen • Weak disturbances containing vorticity; • Ekman layer or boundary layer (BL) convergence through Ekman pumping; • Increased convection; • Latent heat release; • Temperature increase; CISK theory • Surface pressure falls; • Increased vorticity; • Increased convergence CISK Rapid rotation Warm core due to latent heat release: stretches the vortex upward---increases the rotational rate. Ignore mixing in a vortex: Potential vorticity is conserved. Relative vorticity; is Coriolis parameter; is the thickness of vortex For two pressure levels p1 (surface) and p2 ( high level pressure), If Increase, thickness increases and thus vortex Is stretched. Fast spine. [ f does not change at the same location] Gradient wind balance In hurricanes, centrifugal Force cannot be neglected compared To the Coriolis force, and the balance of forces is “gradient wind balance”, The balance between radial pressure gradient force, centrifugal force, and Coriolis force. [Different from Geostrophic Balance.] [Detail: dynamics section] CISK: Little evidence shows that such interaction leads to a growth rate maximum on the observed scale of hurricanes Air/sea interaction theory • 1980’s (Emanuel) • Based on the fact that the potential energy for hurricanes arises from the thermodynamic disequilibrium between the atmosphere and the underlying ocean. • Since the subcloud BL is unsaturated, the BL air has a potential for significant icnrease in specific humidity therefore for moist entropy: real energy source is believed from warm sea surface of the tropical oceans. Hurricane:Carnot heating engine Heat absorbed at the ocean surface at Expelled by radiative cooling at the top at K K Efficiency: =33.3% It is used to make destructions. Damagement Strong winds, convective gusts; Sea level rise (1-2m) due to low pressure; Storm surge (2-10m); Waves, tornadoes; Flooding from rains. Andrew 1992: $30 billion (17billion insured) Relation to climate Hurricanes transport heat northward, big amount: maybe Important in regulating climate. ENSO can modify SST, so coupled climate modes may affect Hurricane formation; Recent research indicates that global warming May increase the frequency and intensity of hurricanes because Of the increased SST. No real trends, however, are observed up to now in both Atlantic and Pacific.