Hurricanes

Tropical Weather Tropics: the belt between the Tropic of Cancer (23.5N) and the Tropic of Capricorn (23.5S) The weather is very different from that of the mid-latitudes ♦ Temperature: small seasonal changes (the noon sun is always high) ♦ Precipitation: either wet all seasons (Af) or distinct dry and rainy seasons (Aw, Am) depending on the location of the ITCZ ♦ Winds: mostly easterlies The surface temperature of the ocean is relatively high all year around resulting is high evaporation rate and water content in the atmosphere. In the NH the ocean temperature is maximum August-September. This is the time when large amount of latent heat is released in the atmosphere.

Satellite Infrared Image

Tropical Waves Streamlines: depict actual wind flow at the surface (account for friction as well). Exhibit regions of surface convergence and divergence Tropical wave: a weak trough of low pressure disturbing the streamlines Tropical waves have wavelengths ~2500 km, move slowly to the west. In the zone of divergence: ♦ West side of the trough ♦ Downward air motion ♦ Fair weather In the zone of convergence: ♦ East side of the trough ♦ Upward air motion ♦ Precipitation and storms Occasionally the tropical wave will intensify and become a hurricane

Hurricanes A hurricane is a tropical cyclone with sustained winds over 74 mi/hr (64 knots), formed over the tropical region of the oceans ♦ Forms around centers of very low surface pressure ♦ Surface winds in the NH are counterclockwise and in the SH are clockwise. ♦ The winds are spiraling and intensifying towards the center ♦ Typical diameter ~ 500 km ♦ The center of the storm (the hurricane eye~40 km) is cloud free and with very weak winds. ♦ The eye is surrounded by a ring (the eye wall) of intense winds and storms. ♦ The clouds are aligned into spiral rain bands Aka typhoons (NE Asia), cyclones (India) and tropical cyclones (Australia). “Tropical cyclone” is the common international term for a storm which developed in the tropics.

Summary of the 2004 Hurricane Season

Summary of the 2005 Hurricane Season

Summary of the 2006 Hurricane Season

Hurricane Movement The direction of movement of hurricanes is determined by the prevailing winds. In the tropics they move to the west, northwest. The direction depends on the pressure and the wind system in the rest of the atmosphere. Over the oceans they follow closely the flow around the subtropical highs. The strength depends very much on the temperature of the ocean water. H H H

The Structure of a Hurricane The surface winds are cyclonic (counterclockwise) and toward the low pressure center. In the center the converging air is swirling and moving upwards. The rising air condenses, forms clouds, and releases latent heat that heats the core of the storm. The warm central core is a column of warmer air that is taller than the surrounding air and corresponds to a center of high pressure aloft. The resulting winds aloft a therefore anticyclonic and outward of the high pressure center.

The Structure of a Hurricane

Hurricane Katrina’s eye wall

The Structure of a Hurricane

Forming Conditions: Warm and moist air that is conditionally unstable. ♦ The peak of the hurricane season is late summer and early fall. ♦ A cold air mass aloft can create unstable conditions Surface air convergence. ♦ Converging winds along the ITCZ ♦ Preexisting atmospheric disturbance ♦ Tropical wave Upper air divergence - a divergence in the jet stream. Hurricanes typically form in the tropics degrees latitude. ♦ not at the equator (why?) ♦ away from the subtropical highs (why?)

Where is the energy coming from? The energy is ultimately derived from the warm ocean water. During evaporation latent heat is taken from the ocean surface into the atmosphere. The rising warm moist air further transports the energy upward. During condensation at higher levels, the heat is released in the form of sensible heat and the air aloft is heated This creates higher pressure aloft and drives diverging anticyclonic winds aloft. The diverging winds aloft intensify the low pressure center at the surface. This drives stronger convergence of the surface winds, stronger winds and bigger waves (stronger air-ocean friction) -> positive feedback. When the hurricane moves over land the supply of moisture in the “engine” is cut off and the storm dissipates.

Middle Latitude Cyclones/Tropical Cyclones/Tornadoes In common: the central pressure is LOW and the surface winds are cyclonic Difference: ♦ the size: MLC > TC >T ♦ the pressure drop: MLC < TC < T What does that tell us? ♦ The pressure gradient force is biggest in tornadoes, is smaller in hurricanes and is smallest in middle latitude cyclones ♦ PGF is balanced by the Coriolis force, which is proportional to the wind speed ♦ The winds are strongest in tornadoes, less strong in hurricanes and weakest in middle latitude cyclones.

Storm Surge and Flooding. The storm surge results from several different effects: ♦ Heavy rainfall ♦ Elevated sea level due to the low pressure center. ♦ Large ocean waves generated by the winds (high surf) ♦ Strong winds push the ocean water towards the beach (Eckman transport)

Storm surge due to Ekman transport: the net transport of water is to the right of the surface wind Notice the velocity difference of the rain bands on both sides of the eye.

Saffir-Simpson Scale

Florida (Keys) 1935 Camille 1969 Andrew 1992

Naming Hurricanes