Re: Frequency of strongest T/S to rise?

On Sep 7, 3:42 pm, "Norman" <normanthis...@thisbitweather-
consultancy.com> wrote:
Martin Rowley wrote:
... this has been covered before, but is, perhaps, particularly
relevant at the moment.

http://news.bbc.co.uk/1/hi/sci/tech/7596643.stm

Still highly controversial, as there are some experts who believe
that hurricanes/cyclones/typhoons obey periodic fluctuation as well
as the problem with shearing interfering with the development.
However, if the seas are getting warmer, and water vapour is a 'fuel'
for these storms, then other factors being right (the key statement),
it would seem that such features could be more intense.

Very controversial indeed. I hardly think that there's a reliable
enough database of sufficient length to enable firm conclusions to be
drawn. In most parts of the world tropical cyclone intensity is
inferred from satellite imagery. There are no direct measurements. In
the North Atlantic it is not uncommon for the intensity measured by
'Hurricane Hunter' aircraft to differ substantially from what is
inferred from satellite imagery. This must call into question the
reliability of any intensity database constructed solely from satellite
imagery.

One tantalising event is the hurricane that made landfall in Brazil in
March 2004. That had no historical precedent and may be linked to
global warming. On the other hand, it may be just an isolated
exceptional event that won't be repeated for centuries. Only time will
tell.

I think the idea might be to introdiuce a method of recalibrating them
with new telemetry techniques. Scroll to the end:

Saffir-Simpson Scale

Category One Hurricane: 64-82 knots,
74-95 mph,or 119-153 km/hr.

Category Two Hurricane: 83-95 knots;
96-110 mph, or 154-177 km/hr.

Category Three Hurricane: 96-113 knots;
111-130 mph, or 178-209 km/hr.

Category Four Hurricane: 114-135 knots;
131-155 mph, or 210-249 km/hr.

Category Five Hurricane:
Winds greater than 135 knots;
155 mph, or 249 km/hr.

http://www.nhc.noaa.gov/aboutsshs.shtml

THE BEAUFORT WIND SCALE

One of the first scales to estimate wind speeds and the effects was
created by Britain's Admiral Sir Francis Beaufort (1774-1857). He
developed the scale in 1805 to help sailors estimate the winds via
visual observations. The scale starts with 0 and goes to a force of
12.
The Beaufort scale is still used today to estimate wind strengths.

The Beaufort scale:

For use on land
For use at sea

BEAUFORT SCALE: Specifications and equivalent speeds for use on land

FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE ON LAND
10 m above ground
miles/hour knots0 0-1 0-1 Calm Calm; smoke rises
vertically.

1 1-3 1-3 Light air Direction of wind shown by smoke drift, but not by
wind vanes.

2 4-7 4-6 Light Breeze Wind felt on face; leaves rustle; ordinary
vanes moved by wind.

3 8-12 7-10 Gentle Breeze Leaves and small twigs in constant motion;
wind extends light flag.

4 13-18 11-16 Moderate Breeze Raises dust and loose paper; small
branches are moved.

5 19-24 17-21 Fresh Breeze Small trees in leaf begin to sway; crested
wavelets form on inland waters.

6 25-31 22-27 Strong Breeze Large branches in motion; whistling heard
in telegraph wires; umbrellas used with difficulty.


Force miles/hour knots description

0 0-1 0-1 Calm Sea like a mirror

1 1-3 1-3 Light air Ripples with the appearance of scales are formed,
but without foam crests.

2 4-7 4-6 Light Breeze Small wavelets, still short, but more
pronounced. Crests have a glassy appearance and do not break.

3 8-12 7-10 Gentle Breeze Large wavelets. Crests begin to break. Foam
of glassy appearance. Perhaps scattered white horses.

4 13-18 11-16 Moderate Breeze Small waves, becoming larger; fairly
frequent white horses.

5 19-24 17-21 Fresh Breeze Moderate waves, taking a more pronounced
long form; many white horses are formed. Chance of some spray.

6 25-31 22-27 Strong Breeze Large waves begin to form; the white foam
crests are more extensive everywhere. Probably some spray.

7 32-38 28-33 Near Gale Sea heaps up and white foam from breaking
waves begins to be blown in streaks along the direction of the wind.

8 39-46 34-40 Gale Moderately high waves of greater length; edges of
crests begin to break into spindrift. The foam is blown in well-marked
streaks along the direction of the wind.

9 47-54 41-47 Severe Gale High waves. Dense streaks of foam along the
direction of the wind. Crests of waves begin to topple, tumble and
roll over. Spray may affect visibility.

10 55-63 48-55 Storm Very high waves with long over-hanging crests.
The resulting foam, in great patches, is blown in dense white streaks
along the direction of the wind. On the whole the surface of the sea
takes on a white appearance. The 'tumbling' of the sea becomes heavy
and shock-like. Visibility affected.

11 64-72 56-63 Violent Storm Exceptionally high waves (small and
medium-size ships might be for a time lost to view behind the waves).
The sea is completely covered with long white patches of foam lying
along the direction of the wind. Everywhere the edges of the wave
crests are blown into froth. Visibility affected.

12 73-83 64-71 Hurricane The air is filled with foam and spray. Sea
completely white with driving spray; visibility very seriously
affected.

The Beaufort scale was extended in 1944, when Forces 13 to 17 were
added. Hitherto, Force 12 (Hurricane) had been the highest point on
the scale, referring to a sustained wind speed of 64 knots (32.7 m/s)
or more - that is, the wind speed averaged over a period of 10
minutes.

The additional five points extended the scale to 118 knots (61.2 m/s),
with Force 12 referring only to speeds in the range 64 to 71 knots
(32.7-36.9 m/s). However, Forces 13 to 17 were intended to apply only
to special cases, such as tropical cyclones. They were not intended
for ordinary use at sea - indeed, it's impossible to judge Forces 13
to 17 by the appearance of the sea. For all normal purposes, the
Beaufort scale extends from Force 0 (Calm) to Force 12 (Hurricane),
with Force 12 defined as a sustained wind of 64 knots (32.7 m/s) or
more.

http://www.metoffice.gov.uk/education/secondary/students/beaufort.html

The Dvorak technique (developed in 1974 by Vernon Dvorak) is a widely
used system to subjectively estimate tropical cyclone intensity based
solely on visible and infrared satellite images. Several agencies
issue Dvorak intensity numbers for cyclones of sufficient intensity.
These include the National Hurricane Center's Tropical Analysis and
Forecast Branch (TAFB), the NOAA/NESDIS Satellite Analysis Branch
(SAB), the Joint Typhoon Warning Center at the Naval Pacific
Meteorology and Oceanography Center in Pearl Harbor, Hawaii, and the
Air Force Weather Agency (AFWA).

In a developing cyclone, the technique takes advantage of the fact
that cyclones of similar intensity tend to have certain characteristic
features, and as they strengthen, they tend to change in appearance in
a predictable manner. The structure and organization of the tropical
cyclone are tracked over 24 hours to determine if the storm has
weakened, maintained its intensity, or strengthened. Various central
cloud and banding features are compared with templates that show
typical storm patterns and their associated intensity.

If infrared satellite imagery is available for a cyclone with a
visible eye pattern, then the technique utilizes the difference
between the temperature of the warm eye and the surrounding cold cloud
tops to determine intensity (colder cloud tops generally indicate a
more intense storm). In each case a "T-number" and a Current Intensity
(CI) value are assigned to the storm. These measurements range between
1 (minimum intensity) and 8 (maximum intensity). The T-number and CI
value are the same except for weakening storms, in which case the CI
is higher.

Dvorak # Knots Millibars
Atlantic NW Pacific
1.0 25 ---- ----
2.0 30 1009 1000
2.5 35 1005 997
3.0 45 1000 991
3.5 55 994 984
4.0 65 987 976
4.5 77 979 966
5.0 90 970 954
5.5 102 960 941
6.0 115 948 927
6.5 127 935 914
7.0 140 921 898
7.5 155 906 879
8.0 170 890 858


The National Hurricane Center will often quote Dvorak T-numbers e.g.

DVORAK SATELLITE INTENSITY ESTIMATE OF T2.5/35 KT.

The Cooperative Institute for Meteorological Satellite Studies (CIMSS)
at the University of Wisconsin-Madison has developed the Objective
Dvorak Technique (ODT). This is a modified version of the Dvorak
technique which uses computer algorithms rather than subjective human
interpretation to arrive at a CI number. This is generally not
implemented for tropical depressions or weak tropical storms.

http://en.wikipedia.org/wiki/Dvorak_technique

.

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