Tropical Revolving Storms

Tropical Cyclogenesis Warm ocean waters - at least 26.5°C throughout a depth of 50m Unstable/moist atmosphere - rapid cooling with height Moist layers near the mid-troposhere At least 500km from the Equator with sufficient Coriolis force to provide geostrophic rotation

Tropical Cyclogenesis Pre-existing near-surface disturbance with sufficient vorticity & convergence Low levels of vertical windshear between the surface and upper troposphere

Tropical Cyclones Low pressure systems deriving energy primarily from evaporation from a warm sea Associated with high winds, lowered pressure and convective clouds concentrated around the centre A tropical cyclone is the generic term for a non- frontal synoptic scale low-pressure system over tropical or sub- tropical waters with organised convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation

The storms tend to affect certain areas: They occur mainly on the western edge of oceans after the winds have had a long sea track and the air has acquired a large amount of moisture. They occur in the summer hemisphere in areas where the ITCZ has reached a latitude of 10°N or S or greater and where there is sufficient geostrophic force to produce cyclonic rotation. They do not originate over land.

The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt) are called "tropical depressions".

Typical tropical cyclones have an “eye” in the centre. - The "eye" = roughly circular area of comparatively light winds and fair weather, approx. 30-60km dia. - Calm winds at the axis of rotation, but strong winds may extend well into the eye. - Little or no precipitation and sometimes blue sky or stars can be seen. - The eye is composed of air that is slowly sinking.

Once the tropical cyclone reaches sustained winds of at least 17 m/s (34 kt) they are typically called a "tropical storm" and assigned a name.

(the Northwest Pacific Ocean west of the dateline); If winds reach 33 m/s (64 kt), then they are called: "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); "typhoon" (the Northwest Pacific Ocean west of the dateline);

"severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); "severe cyclonic storm" (the North Indian Ocean); a "tropical cyclone" (the Southwest Indian Ocean near Madagascar)

In West Africa the squally trough is known locally as a tornado but is not to be confused with a tornado as described in Chapter 16.

TRS Names From 1979 individual storms were allocated alternate male and female first names alphabetically starting with A at the beginning of each storm season (e.g. Alfred, Barbara, Charles, etc.)

TRS Names In the Northwest Pacific basin, Asian names are selected by members of the WMO Typhoon Committee. N. Indian Ocean storms are allocated numbers

Weather associated with a TRS Very strong to extreme wind conditions All hazards associated with TS activity Severe pressure disturbances making pressure instruments unreliable Torrential rain Calm eye with subsidence

Weather associated with a TRS Anticylonic outflow above the storm with subsidence and fine weather around the edges giving a deceptive calm before the storm Increasing sea swell as the storm approaches

TRS Tracks and Seasons HURRICANES (July - Oct) CYCLONES (May - Oct) TYPHOONS (May - Nov) CYCLONES (Dec - April) CYCLONES (Nov - May)

Frequency of Tropical Cyclones Tropical Storm/Cyclone averages for the various areas are: N.W.Pacific (South China Sea) about 25 times per season, Atlantic about 9 -10 times per season N.E. Pacific (Coast of Central America) about 16 -17 times per season

Frequency of Tropical Cyclones N. Indian (Gulf of Arabia and Bay of Bengal); 5 - 6 times per season. S.W. Indian Ocean (Madagascar) about 10 - 11 times per season. S.E. Indian Ocean (W. Australia) about 7 times per season. S.W.Pacific (E. Australia) 9 times per season. Global Average 84 - 85 per annum.

Easterly wave

Easterly Waves The equatorial trough is a permanent series of thermal lows in a more or less continuous belt around the earth. On both sides of the equatorial low pressure belt, troughs sometimes extend out towards the sub-tropical high pressure belts. These troughs, known as easterly waves, move westwards under the influence of the easterly upper winds.

Easterly Waves The Easterly waves are extremely important phenomena since they are related to the formation of hurricanes in the north Atlantic. But not all easterly waves results in a violent tropical revolving storm.

Easterly Waves Easterly waves are most frequent during late summer when surface water temperature is highest. They occur mainly in the western parts of large ocean basins between 5° and 20° latitude.

Easterly Waves Easterly waves are usually very apparent over West Africa where they can be seen from their formation zone over Sudan/Ethiopia and westwards.

Easterly Waves

Easterly Waves Clusters are often formed east of the Philippines and north of New Guinea where the water is the warmest, at about 30°C. This is also the area where most tropical cyclones are generated in the world. The clusters formed in this area move westward towards Borneo but they seldom show the typical characteristics of an Atlantic Easterly wave.

Storm Tracks What determines the movement of tropical cyclones? Generally tropical cyclones tend to follow the circulation around the sub- tropical highs existing over the oceans.