Hurricanes

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.

The Structure of a Hurricane

The Structure of a Hurricane

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.♦ Right side:100+25=125 knots♦ Left side:100-25=75 knots

Saffir-Simpson Scale

1900-1999

Florida (Keys) 1935Camille 1969Andrew 1992

(at landfall)

Naming Hurricanes

Air Masses and Fronts

Air mass source regions and their paths

Types of Fronts

• Cold front: cold, dry stable air is replacing warm, moist unstable air. • Warm front: warm, moist unstable

air is replacing cold dry stable air.

• Stationary front: boundary between the two air masses is not moving.

• The symbols on a map are in the direction of the air mass motion.

• Occluded front: when a cold front catches up with a warm front

•Steep leading edge•Fast movers•Survive the longest

overrunning

Occluded fronts.

• Cold fronts move faster than warm fronts. They can catch up and overtake their related warm front. When they do, an occluded front is formed.  

• Cold occlusion: very cold air behind, not so cold air ahead of, the warm front

• The upper warm front follows the surface occluded front

Cold occlusion

Warm Occlusion

• Very cold air ahead of, not so cold air behind, the warm front

• The cooler air from the cold front cannot lift the very cold air ahead, rides “piggyback”

• The warm front aloft precedes the surface occluded front

Middle-Latitude Cyclones

Polar front theory

•Air convergence at the surface is aligned with air divergence aloft.

•Air divergence at the surface is aligned with area of convergence aloft

•Surface L intensifies (weakens) if the divergence aloft is stronger (weaker) than the surface convergence.

•Surface H intensifies (weakens) if the convergence aloft is stronger (weaker) than the surface divergence.

Favorable conditions for a strong midlatitude storm

Convergence

Divergence

L

surface

aloftConvergence

Divergence

H

Another way of looking at it.

Weather Forecasting

Forecasting Methods (1)

• Persistent forecast: (short term forecast) There is no reason for the weather conditions to change.

•Steady state forecast (trend method). If we know the speed and the direction of the weather system, we can extrapolate its future location.

•Analogue forecast (pattern recognition): “I have seen this atmospheric conditions before and based on what happened back then, I can predict the weather today or tomorrow. ”

•Statistical forecast: the forecast is made based on past computer simulations that are weighted for the actual humidity, cloud cover, wind direction.

Forecasting Methods (2)

• Probability forecast: based on historical data. “What is the chance to snow in Gainesville on Christmas?”

•Weather type. Uses general criteria such as the position of the subtropical highs, upper-level flow, prevailing storm track…

•Climatological forecast: Based on information for the typical weather conditions at a given location for a given season.

Type of Forecasts

•Very short-range forecasts (up to 6 h): based on satellite images and Doppler radar. Not likely to include numerical models.

•Short-range forecast: (2-3 days) variety of techniques - satellite images, Doppler radar, wind and pressure maps of the surface and aloft … also numerical models – usually beyond 12 hrs.

•Medium range forecast: (3-8 days) Mostly based on computer generated weather predictions.

• Long-range forecast (up to 16 days). It is not very accurate. Gives mostly the trends of the weather and the global expectations.

•Extended forecast – one that goes beyond 3 days

•Outlook – how the average weather conditions for a particular month or a season compare to normal ones.

Thunderstorms and Tornadoes

Stages of Development - I

•Cumulus (growth) stage: ♦ Updraft: warm and moist air is

uplifted, expands, condenses and forms towering clouds.

♦ Latent heat is released -the temperature in the cloud is higher than outside the cloud, air keeps rising.

♦ The updrafts are strong so that the cloud droplets remain suspended in the cloud.

♦ Usually no precipitation and lightning during this stage.

Stages of Development - II

•Mature stage:♦ Precipitation starts♦ Entrainment: dry air from

around the cloud is drawn into the cloud. Some of the cloud droplets evaporate and chill the air.

♦ Downdraft: the cold and heavy air is sinking

♦ The updraft and the downdraft form a storm cell.

♦ Gust front: the boundary between the cold and the warm air at the surface

♦ Lightning and thunder

Stages of Development - III

•Dissipating stage: ♦ the updraft weakens♦ the gust fronts move away♦ the precipitation is light♦ the downdrafts dominate:

cut off the fuel supply of the storm

Stages of Cloud-to-Ground Lightning.• The lightning is initiated with a flow of electrons from the base of the

cloud towards the ground (stepped leader)

• When the leader gets close to the ground a flow of positive charges surges upwards.

• When they meet a strong current (return stroke) transfers a charge between the cloud and the ground.

• The process can be repeated several times in the same channel (dart leader, return stroke…)

1 2 3

Severe Weather and Doppler Radar

•A Doppler Radar uses radio waves

•To measure precipitation it measures the intensity of the reflected radio waves.

•To measure wind speed it measures the frequency of the reflected radio waves

•Doppler effect: the change of the wave frequency as the object moves – the siren of an ambulance has a higher pitch when it is approaching.

•Cannot measure the speed of parallel winds: need 2 radars

•Doppler radar can reveal a Tornado Vortex Signature (TVS).