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| | Cyclone Hudah
deals late season
blow to Madagascar
Meteorologists in awe
of its unusual intensity
April, 2000
One of the most intense cyclones to cross the South West Indian Ocean in recent years dealt Madagascar a cruel late-season blow, killing at least 20 people, affecting 300,000, and making an already out-of control cholera epidemic even worse.
After battering the east coast with 8 metre waves and wind gust to 170 knots, it lost much of its punch as it climbed over the 9000 foot mountains separating the island's east and west coasts. Hudah re-formed over the warm waters of the Mozambique Channel to hassle northern Mozambique, but spared that country its fury, with the exception of one child who died from a blow to the head - the result of a coconut thrown down in the wind.
What makes Hudah's journey remarkable is not the devastation it brought (there was enough of that the previous month with cyclones Eline and Gloria) , but the very intense and localised power it displayed on its path towards the coast of Madagascar.
Cyclone Hudah, began life on March 21, about 6000km to the east, to the south of Australia's Christmas Island. It followed a remarkably straight line along the 17th parallel before the eye smashed into the eastern Madagascar city of Antalaha on the night of April 2.
Philippe Caroff, Chief forecaster with Meteo-France based at Reunion Island, east of Madagascar, described Hudah's unusual intensity to his colleagues by way of an email on April 6:
"In the early morning of the same day (that Hudah struck Madagascar), the cyclone reached its maximum intensity. At that time, satellite intensity analysis was almost 7.0 on the Dvorak scale, so that we classified Hudah as a very intense Tropical Cyclone (according to our Terminology; or minimal category 5 hurricane for Saffir Simpson scale addicts). This rare intensity (probably very close to the Maximum Potential Intensity) ranks this system as one of the most intense TC observed in recent history in the South West Indian Ocean basin, roughly at the same level of TC 'Geralda' in January 1994, with estimated central pressure of 905 hPa and associated max winds of 120 knots over a 10 minute average (135 knots sustained winds) gusting to 170 knots."
What made this storm different was its concentrated strength. Philippe Caroff continues:
"Besides the extreme intensity reached by Hudah, an interesting feature was the structure of the system, particularly its size, as partially
unveiled by the scarce observations available. 24 hours earlier before its passed near to Tromelin, Hudah's centre passed around 55 km to the south of Saint-Brandon (another small flat island). The synoptic station located there (belonging to Mauritius
Met Service) recorded minimum sea level pressure of 995.9 hPa and maximum 10 minute average winds of 39 knots (72 km/h) with maximum gusts of 55 knots (102 km/h). Though situated in the good semi-circle (speed of movement of Hudah was 12 knots at that time), those wind values were surprisingly low considering that the northern eyewall passed only 15 nautical miles to the south of the island! Extension of 50 knot winds was therefore extremely limited on this side of the eye. Strong dissymetry of the system was confirmed by scatterometer data."
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Cyclone Hudah on April 2, just before it collided
with the north-east coast of Madagasacar. Pictured here you can see the greatest intensity (white)
to the south-west of the eye. This is known as the
dangerous quadrant in sailing parlance. To the north of the eye there is much less violence,
and winds recorded in that vicinity 24 hours earlier
averaged 39 knots just 15 nm from the north wall of the eye.
Picture courtesy of Philippe Caroff, Chief forecaster
with Meteo-France at Reunion Island.
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Questions & Answers
Where did Hudah begin?
Philippe Caroff, chief meteorologist for the Indian Ocean island of Reunion:
"The incipient disturbance that later became Hudah was issued from a cluster (active cloud mass) seen first south of Java. Here in La Reunion, our area of responsibility starts west of
90°E, so we began to monitor more closely this system on the 23rd. It was still a very weak system when it crossed longitude 95°E, and was
only classified as a 'tropical disturbance' (according to the terminology used in the SouthWest Indian ocean), which roughly
corresponds to the first level of our Tropical Cyclone scale (winds less than 50 km/h), at midday the 24th. Second level of intensity -'tropical
depression'- was reached later that day (max winds in the 50-60 km/h range). As the system was about to cross longitude 90°E and leave
Perth's area of responsibility, third level was reached and the system christened Moderate Tropical storm 'Hudah' when west of 90°E. This stage of moderate tropical storm (max winds in the 63-88 km/h range)
corresponds to what is called a 'Tropical Cyclone' in Australian terminology (terminology is not the same in the different cyclone basins)."
Is there such a thing as a safe place
to be in such a storm?
Philippe: The 'safe' sectors depend of the direction of the storm's movement. In the case of a westward motion, the speed of movement adds to the clockwise winds (southern hemisphere) to the south of the track, so that the northern semi-circle is safer (there is slight difference between the northeastern and northwestern quadrant, but the latter
generally exhibits the weakest winds).
Was it unusual for Hudah to travel
so long east to west?
Philippe: "It is not unusual to see systems traveling east to west for so long. Most of the systems of last year did that, or remember the more spectacular case of Leon-Eline (responsible for the massive flooding in Mozambique) this year which traveled the entire basin for more than 11000 km, ending its life in Namibia!"
What drove the cyclone along?
Philippe: "The general circulation of the atmosphere is characterised by the existence of subtropical high pressure belts in the lower atmosphere (between 20 and 40° latitude in summer season). Those
highs drive the trade winds on their northern side (southern side for northern hemisphere). Trade winds are easterlies or southeasterlies blowing more or less continuously and more or less strongly depending on the extension and strength of the subtropical high. A tropical cyclone is steered by the average flow existing across the whole atmospheric layer, with maximum weight given to the lower levels (due to the greater density of the air). In that case the subtropical high was a dominant feature, responsible for the steady
easterly steering current that drove the motion of Hudah due west. The fact that the winds were light in the upper atmosphere (helped create) the severe intensity reached by Hudah as this was an extremely favourable factor for its intensification."
Tropical Cyclones can travel considerable distances over the open water when environmental conditions are favorable, says Dr Jeff Hawkins, meteorologist with the Naval Research laboratory in Monterey California. These conditions range from warm sea surface temperatures, inflow of warm moist air into the central region and weak winds aloft that will not shear the storm apart from its lower circulation.
Some factors which kept Hudah stoked
(these apply to all cyclones, hurricanes and typhoons)
When it comes to wind strength, each storm has its own size and wind distribution. Hudah was relatively small in overall size, with the radius of gale winds on the order of 70-100 nautical miles, especially to the south, There was a much smaller radius to the north side, as proved by Philippe Caroff's observations from Saint Brandon, where the eye passed just 15 miles to the south yet winds averaged only 39 knots, gusting to 55 knots. Larger storms can reach a size 2-4 times this in terms of the radius of gale winds.
What happens in the core of a hurricane is quite complex, according to Philippe Caroff.
Eyewall cycles are often observed, which means that a second eyewall can build and
replace the former disappearing one.
Tropical cyclones diminish rapidly when over land, especially when mountainous, because warm air-flow into the base of the storm is. This was the case when Hudah crossed Madagascar where the tallest peak in its interior (Massive du Tsaratanana) reaches 2876 m (9159 ft).
Once back over water, the environmental conditions were again favorable for intensification and this storm spun up again, but not to its previous intensity.
©
weather-wise.com 2000
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March 22: The
day before, this
mass of swirling,
moist cloud
grabbed the
attention of
forecasters.
March 25:
Whoops! Hudah almost decomposed before it really got going.
March 27:
Gaining strength
all the time,
Hudah crawls to
the west across
the Indian Ocean.
March 28
March 29
March 30: The
yellow-coloured
centre indicates
very cold cloud, a
sign of an
intensifying
storm.
March 31: for the
first time, an eye
begins to form.
April 1: Hudah
gains further
strength.
April 2: Hudah
closes on the
Madagascar coast
at maximum
intensity.
April 4: Hudah
out of its element
on top of the
Island mountain
chain separating
the east and west
coasts.
The
highest peak in
the Massif du
Tsaratanana rises
to 2876m.
April 4: After
crossing the
Island of
Madagascar,
Hudah reforms
over the warm,
southward-flowing
waters of the
Mozambique
Channel.
April 5: Gaining
strength all the
time ...
April 6: Hudah
stalls in the
Mozambique
Channel.
April 7: Hudah,
after circling
aimlessly in the
Mozambique
Channel begins to
collapse. Search
and rescue
co-ordinators on
both side of the
channel breathe a
collective sigh of
relief.
Above images courtesy of
NRL
Monterey Tropical Cyclone Homepage
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