Tropical Low Pressure Systems

What is a Tropical Low Pressure System?

– An intense low pressure system with a closed circulation. Also known as a hurricane, typhoon, cyclone and willy willies.

– Violent storms between 200 and 650km in diameter which can last for 2-3 days

– The most significant atmospheric hazard in terms of damage to property and loss of life

Hurricane Mitch(visible)

What are they like?– The eye is very calm. It is the center of rotation and

contains cool sinking air, so clouds can’t form. The lowest pressure is found here.

– Deep convection is concentrated near the center in the eyewall. Warm, rising air with very violent thunderstorms are found here. The strongest winds are found here.

– Spiral rainbands branch out from the eyewall and also contain strong thunderstorms, heavy rain, and high winds.

The Birth of a Hurricane

– Steps to becoming a hurricane:

• Wave/Disturbance (disorganized low pressure)

• Tropical Depression (closed circulation)

• Tropical Storm (winds higher than 39mph)

• Cyclones (winds higher than 74mph)

So how does a tropical disturbance develop into a

full blown cyclone, hurricane or typhoon?

7 Conditions (after Musk 1988)• Ocean with surface temperature over 26°C with

warm water to a depth of 70m• A disturbance in lower atmospheric circulation• A large enough Coriolis Force to produce circular

pattern of wind (7-20° N and S of the equator)• Winds at all levels in the same direction (little or no

vertical wind shear)• Excess heat is transported away from the storm by

winds in the upper troposphere• Rapid outflow of air in the upper troposphere is

replaced by an inflow at ground level• Unstable air with high humidity

The Role of Latent Heat

Spatial Distribution (Where?)

Temporal Distribution (When?)

• In the northern hemisphere storms can occur between June and late November

• The vast majority of activity is concentrated in September and October. Why is this the case?

Saffir-Simpson Intensity Scale

– A scale used to label the strength of a tropical low pressure event by its pressure and winds.

– CAT 1 = minimal damage, CAT 3 extensive; CAT 5 catastrophic

Category Maximum Sustained Surface Winds[in kts (mph)]

Storm Surge[in feet]

Minimum Sea-level Pressure[in mb (" Hg)]

CAT 1 64-82 (74-95) 4-5 >980 (>28.94)

CAT 2 83-95 (96-110) 6-8 965-979 (28.50-28.91)

CAT 3 96-113 (111-130) 9-12 945-964 (27.91-28.47)

CAT 4 114-135 (131-155) 13-18 920-944 (27.17-27.88)

CAT 5 >135 (>155) 18+ <920 (<27.16)

Why do hurricanes, typhoons and cyclones decay and die? Under what conditions will this

take place?

Real Hurricanes

Hurricane Mitch(visible)

Hurricane Mitch(infrared)

Hurricane Georges(radar)

What hazards are associated with tropical low pressure

systems

How do humans respond to the hazard?How can the hazard be

managed?

Modifying Vulnerability vs.

Modifying the Event

Monitoring, Forecasting and Warnings• Mostly done by use of geotsationary satellites,

doppler radar and aircraft.

Satellite Image

Cyclone Sidr, Bay of Bengal. November 2007

Radar Image

Radar image of Hurricane Katrina, August 2005

Aircraft Reconnaissance

The End Product - Evacuation• In the US for example, The National Hurricane

Center (NHC Miami, FL) then inputs this evidence into complex computer models which generate storm track and strength predictions.

• These are used to issue watches and warnings for the areas that are likely to be affected.

• Accurate warnings can save lives because people have time to evacuate.

• Timing is however very important. Evacuating the Florida Keys along the only highway would take 31 hours. Accurate Hurricane warnings are only really possible 12-18 hours in advance

Other ways to modify vulnerability

• Community preparedness

• Land-use planning

Modifying the Event

• Environmental control• Hazard resistant design• Modifying the loss; recovery and reconstruction

Spatial and Temporal Variations in the Human Response – Case

Studies

Bangladesh Katrina