make summary for each lucture

Wave

Atop the storm surge ride enormous waves driven by hurricane winds. The waves act as battering rams and greatly increase the destruction caused by the storm surge flooding.

Sea surface usually undulates with a spectrum of waves having different heights, wavelengths, and speed and direction of travel.

Waves in deep water move at a speed given by: cdeep = (gL/2 )0.5 L is the distance between wave crests (i.e. wavelength)

Waves in shallow water move at a speed given by: cshallow = (gh)0.5 h is the ocean depth

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Wave

Deep-water ocean waves, whose propagation speed depends on their wavelength, are called dispersive.

Longest waves move out in front and shortest waves fall behind. The energy of waves moves at the packet speed (group velocity). In case of deep-water waves, the packets move at half the speed of individual wave crests.

[Ref: Divine Wind, 2005]

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Wave

Waves are produced by wind: the energy input to waves increases with the cube of the wind speed.

The longer the wind acts on wave packets, the larger the waves. When waves interact with each other, energy is usually transferred “upscale,” from shorter to longer waves. When waves break, their energy is turned into heat.

The biggest waves in hurricanes are those moving at about 1/3rd the wind speed. In deep water, the group velocity is thus about 1/6th the wind speed.

In a storm moving at about 1/6th of its maximum wind speed (e.g., a storm with 100 mph winds moving at 17 mph), the wave packets containing the largest waves are moving right with the storm – in which case the waves have plenty of time to grow.

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Wave

In the Northern Hemisphere the biggest waves are in the right-front quadrant (left-front in the Southern Hemisphere)

Scanning Radar Alimeter Measurements of Waves Heights and Directions [Ref: Divine Wind, 2005]

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Wave

As wave packets approach the coastline, their front side slows down first and rear part of the packet catches up.

The hurricane waves pounding ashore, often riding atop a storm surge, release enormous quantities of energy accumulated at the ocean.

The pounding is sometimes so strong that it can be detected by seismographs thousands of miles away.

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Rain

Hurricanes are often accompanied with torrential rain which can cause devastating floods (including flash flooding) and mudflows.

Since 1944, radar reflectivity measures roughly how much rain, snow, and hail is in the air. In 1997, NASA launched the Tropical Rainfall Measuring Mission (TRMM), which for the first time put a meteorological radar into orbit for measuring precipitation at sea.

Floyd displayed a fairly common feature of intense hurricanes: a double eyewall structure, with inner and outer eyewalls and reduced rainfall in between.

Dramatic double eyewall was also observed in Hurricane Gilbert in 1988. There was no radar return whatsoever between the two eyewalls.

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Rain

Radar Reflectivity Measured from Aircraft Flying in Hurricane Floyd [Ref: Divine Wind, 2005]

Radar reflectively measured from a hurricane reconnaissance aircraft flying in the eye of Hurricane Floyd of 1999 showed the heavy rainfall regions.

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Rain

Radar Reflectivity Measured from Aircraft Flying in Hurricane Gilbert [Ref: Divine Wind, 2005]

Rainfall rates in spiral bands are sometimes as large as those of the eyewall

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Rain

Four factors determine how much rain will fall in a given place: the amount of water vapor in the air, how fast the air ascends, and the vertical extent and duration of the updraft.

As air ascends, its pressure drops, since pressure decreases with altitude. Based on 1st law of thermodynamics, falling pressure causes falling temp. (1°C for each 100 m the air ascends). Amount of water vapor air can hold declines with falling temp. Eventually, when air’s capacity for water vapor drops below its actual water vapor content, the vapor begins to condensate into tiny droplets, forming cloud. Condensation heats the air (release of latent heat), so the rising air doesn’t cool as fast as before.

Precipitation efficiency is defined as the fraction of water vapor ascending through the cloud base that ultimately falls on the surface as precipitation. [see Fig. 24.5, Divine Wind 2005]

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Rain

The precipitation efficiency in the core of a strong hurricane can be almost 100%.

Eyewalls are always tall and their precipitation efficiency is usually large. The main factor controlling the rainfall rate in eyewalls is the updraft speed which is controlled by two factors – (1) Uplflow associated with strong intensification, (2) Frictional inflow caused by friction between the storm’s wind and the underlying surface. [see ‘spiraling inward’ in Fig. 24.6b, Divine Wind, 2005]

These two factors combined together produce the ring of intense rainfall surrounding the eye [Figs. 24.1 and 24.2 in Divine Wind, 2005].

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Rain

Average hurricane at sea precipitates about one trillion gallons of rainwater per day. Big slow moving storms produce the most rainfall at any given point.

Rainfall diminishes quickly after landfall; but if the hurricane passes over swamp instead of dry land, the rainfall decays much more slowly [see Fig. 24.7, Divine Wind, 2005]

If the storm encounters mountains, the air (with wind speed at 1 to 2 km above ground remaining high) is forced to ascend along the windward slopes and very heavy rainfall can occur. Some of worst flooding disasters, including flooding in Hurricane Mitch, happened when hurricanes or their remnants passed over mountains.

Even when the terrain is flat and dry, TC can produce enormous rainfall when they interact with other weather systems [e.g. Hurricane Floyd].

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Recap

The Science of Hurricanes

https://www.youtube.com/watch?v=qgont5jQjcQ

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