Lecture 14

• Tropical disturbances, tropical depressions, tropical cyclones

• Other tropical systems, remember the ITCZ

Tropical cyclones

• What are they?

• How are they structured?

• How and where do they form?

• What is the life cycle?

• Tropical cyclone tracks

• How do they cause destruction?

• Observing and forecasting them

Hurricane Mitch approaching Honduras on Oct. 26, 1998

5-6 Atlantic,

9 in E Pacific,

16 W Pacific

Hurricane Mitch in W Caribbean Oct 26

IR images

Structure

• The eye, clear of clouds (8 km or more)

• eye wall: narrow circular rotating region of intense thunderstorms

• Spiral rain bands

• Depend on warm water, warm air. Up moist, down dry.

Cross-section of a hurricane (vertical dimension exaggerated)

Large weather engines

• Driven by latent heat of water• Air of tropics dry because of subsidence in the

subtropics and trade winds blowing that air equatorw. Evaporation may take place and great amounts of energy are collected that may be released when the water vapor condenses into liquid water again.

• Air is driven into the center because there the pressure is lowest. Tight pressure gradient, wind speed is increasing toward the eye; even more energy in latent form

How strong a heat engine?

• “The energy released by condensation in a single day in an average hurricane is at least 200 times the entire world’s electrical energy production capacity.”

• Can calculate the minimum possible central sea-level pressure for a tropical cyclone based on SSTs

Minimum possible central pressure for a perfect tropical cyclone, avg Sept SST

Regions where most hurricanes form

What inhibits tropical cyclone development if SSTs are above 26.5C?

• Location too close to the equator because of the decreasing Coriolis effect and therefore less spin

• Vertical shear. It tends to tear apart the vertical structure in the storm

Hurricane Celia wind measurements

Winds are strongest on the NE side of the storm because of its movement

Wind damage from hurricane

What is the life cycle?

• Easterly waves, westward propagating disturbances (WPDs)

• Tropical depression, tropical cyclone• hurricane/typhoon/cyclone of category

– 1 33-42 m/s– 2 43-49 m/s– 3 50-58 m/s– 4 59-69 m/s– 5 above 70 m/sDecay

Vis satellite picture on 8/14/2001, easterly wave and a tropical depression are evident

Tropical depression later became tropical storm Chantal

Easterly wave (scale 2000-3000 km)

Georges at different stages in its life cycle (IR satellite images) 18-28 Sept 98

Hurricane Floyd’s track and SST Sept 99

Hurricane Allen ‘80

Category 5 storm that loses its powers when traveling over land.

After landfall

Tracks

• Determined by the vertically averaged background flow

• Under influence of subtropical highs

• It is also influenced by the perturbation the storm creates on the background vorticity gradient

• Additional ingradients still to be determined

The names, dates and stages of tropical storms in the Atlantic in 95

Two very damaging hurricane tracks

Weather Phenomena of the tropics

• Easterly waves• Tropical disturbances, tropical depressions, tropical

storms• Tropical upper tropospheric troughs (TUTTs)• Intertropical convergence zones (ITCZs)• Kelvin, Mixed-Rossby gravity (MRG), inertio-gravity

waves• Madden Julian Oscillation (MJO)

African Easterly Waves form due to the instability of the African Easterly jet. Period of 3-4 days, wavelength of 2000-2500 km

Easterly Waves

How do tropical cyclones form?

Note first baroclinic mode structure, warm ocean, low wind shear, moisture

How do tropical cyclones form ? (necessary but not sufficient conditions)

• Warm ocean (at least 50m deep)• Potentially unstable atmosphere to moist convection• Relatively moist layers near mid-troposphere (5 km)• At least 500 km from equator (~5 deg latitude)• Pre-existing near surface disturbance (vorticity)• Low vertical wind shear (<10m/s from low to upper)

– Sometimes the pre-existing disturbance is an MCC– Within envelope of the MJO– From breakdown of ITCZ

Sfc pressure vs wind speed for tropical systems. Hurricanes, p<980mb, V>118km/hr

Table 7u-1: Saffir-Simpson scale of hurricane intensity.

TypeCategory

Pressure (mb)

Winds (mph)

Winds (kmph)

Surge (meters)

Tropical Depression

TD ----- < 39 < 62 -----

Tropical Storm

TS ----- 39-73 63-118 -----

Hurricane 1 > 980 74-95 119-153 1.2-1.5

Hurricane 2 965-980 96-110 154-177 1.6-2.4

Hurricane 3 945-965 111-130 178-209 2.5-3.6

Hurricane 4 920-945 131-155 210-250 3.7-5.4

Hurricane 5 < 920 >155 >250 >5.4

 

A “typical” tropical cyclone measures about 600 to 800 km horizontally and extends 15 km vertically

Energy

• Latent energy, gained through evaporation, drives tropical cyclones. Within the storm, this energy is efficiently converted to mechanical energy reinforcing the storm's pressure gradient and winds. The process depends on unsaturated air slowly sinking from the upper atmosphere at the periphery of the storm and gaining moisture (latent heat) as it moves over the ocean surface and towards the eye wall.

Hurricane Hortense (1996) note the spiral structure

How much energy does a hurricane release?

• - Total energy released through cloud/rain formation: An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 n.mi) (Gray 1981). (More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands.) Converting this to a volume of rain gives 2.1 x 10^16 cm3/day. A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives

• 5.2 x 10^19 Joules/day or 6.0 x 10^14 Watts.

•This is equivalent to 200 times the world-wide electrical generating capacity - an incredible amount of energy produced!

Energy

• Notice the 1:400 ratio,

• KE of the hurricane : LH used

• Very inefficient heat engine

Tropical cyclone tracks and geography

• Tropical cyclones are steered by the background flow throughout the depth of the troposphere (eestward with slight poleward component)

• Depends on strength of subtropical ridge• May take a more northward component and then recurve

eastward if ridge is weak• However, a tropical cyclone will modify the background

flow and as in so many other cases, there is not clear cut way of teasing the two apart.

• Beta effect• A difficult forecast problem

Multiple storm tracks

Typical areas where hurricanes begin their path and typical early paths

Locations where hurricanes began their development (1950-2000)

Color coded tropical storm tracks