PHYS0629 / PHYS1056

Weather and Climate

Tropical Cyclones

29 November 2013

Dr T C Lee

Hong Kong Observatory

Source: NASA image courtesy the MODIS Rapid Response Team at NASA GSFC

Content

1. Introduction

2. Interactions and Associated phenomena

3. Monitoring and Forecasting Tropical Cyclones

4. Tropical Cyclone Warning History

5. Devastating Storms

6. Long term variation of Tropical Cyclones

1. Introduction

Formation conditions Structure Intensity metric & classification Movement Tropical cyclone basins Naming of tropical cyclones Climatology of tropical cyclones Impacts of tropical cyclones

Favourable Conditions

Important environmental conditions for a tropical cyclone to

form :

Warm ocean waters (usually about 26 oC or above) An unstable atmosphere for convection (cools fast enough with height) Sufficient moisture in the atmosphere, especially in the mid-level troposphere

Generally a minimum distance of at least 300 miles (480 km) from the equator (sufficient Coriolis Force)

A pre-existing near-surface disturbance or low pressure area (to start the spin)

Low values of vertical wind shear between the surface and the upper troposphere (small change in wind speed / direction with height).

warm sea surface

water vapor rises and cools

water vapor condenses into a liquid (clouds) and releases heat

The heat warms the atmosphere and make the air lighter, warmed air

continues to rise into the atmosphere.

With more air moves in and rise, the positive feedback creates strong

winds in the storm.

The Energy Source of Tropical Cyclones

(Source : NOAA, NWS http://www.srh.noaa.gov/jetstream/tropics/tc.htm)

Structure of Tropical Cyclones

(Source : NOAA, NWS http://www.srh.noaa.gov/jetstream/tropics/tc.htm)

Air spirals in toward the center in a counter-clockwise pattern (in the northern hemisphere), and out the top in the opposite direction.

In the very center of the storm, air sinks, forming an "eye" that is mostly cloud-free with relatively low wind speed.

Super Typhoon Zeb (1998) The image was originally captured by the Geostationary Meteorological Satellite (GMS-5) of Japan Meteorological Agency (JMA).

Outflow Cirrus

Spiral Rain Band

Eye

Eye wall

Wind speed record of Waglan Island during the direct hit of Typhoon York

over Hong Kong on 16 September 1999. Notice the dramatic fall and rise in

wind strength during the passage of the eye of York.

Radius Height Cross Section Tangential Wind Field

Positive values are cyclonic (or counter-clockwise)

Negative values are anticyclonic (or clockwise)

(Source : Lecture notes of M. D. Eastin

http://clas-pages.uncc.edu/matt-eastin/files/2013/08/METR4320-TC-

structure.ppt)

Maximum is near the surface in the eyewall Significantly lower wind speed in the eye

Structure of Tropical Cyclones

Uncorrected record of barometric pressure (mean sea level) at the Hong Kong

Observatory during the passage of Typhoon Wanda on 1st September 1962.

Structure of Tropical Cyclones

Minimum pressure in the center of the storm

Tropical Cyclones Intensity and classification

(based on Hong Kong Observatory classification)

Max 10-min mean wind near the centre Tropical Depression up to 62 km/h Tropical Storm 63 to 87 km/h Severe Tropical Storm 88 to 117 km/h Typhoon 118 to 149 km/h Severe Typhoon 150 to 184 km/h Super Typhoon >=185 km/h

Classification of Tropical Cyclones of Different Warning Centers

Maximum Sustained Wind

Speed at the centre of the

tropical cyclones

Hong Kong

(10-minute average)

Mainland China

(2-minute average)

Japan

(10-minute average)

US Pacific

(1-minute average)

US Atlantic

(1-minute average)

kts km/h

< 34 < 63 Tropical Depression (TD) Tropical Depression Tropical Depression Tropical Depression Tropical Depression

34 47 63 87 Tropical Storm (TS) Tropical Storm Tropical Storm

Tropical Storm Tropical Storm

48 63 88 117 Severe Tropical Storm (STS) Severe Tropical Storm Severe Tropical Storm

64 80 118 149

Typhoon (T) Typhoon Typhoon :

64 84 kts

--------------------------------------

Very Strong Typhoon

85 104 kts

--------------------------------------

Violent Typhoon

>=105 kts

Typhoon

64-129kt

--------------------------------------

Super Typhoon: >= 130 kt

Hurricane categories

1: 64 82 kts

--------------------------------------

2: 83 95 kts

--------------------------------------

3: 96 113 kts

--------------------------------------

4: 114 135 kts -------------------------------------

5: >135 kts

81 99 150 184

Severe Typhoon (ST) Severe Typhoon

>=100 >=185 Super Typhoon (SuperT) Super Typhoon

Tropical cyclone Movement

Left on its own, a tropical cyclone in the Northern Hemisphere has an inertial tendency to drift towards the northwest.

Environmental current steers the tropical cyclone similar to the movement of a cork within the main flow of a stream or river.

Over the western North Pacific, tropical cyclones use to travel along the southern or southwestern flank of the subtropical ridge to the northwest.

In gist, a three body problem between the tropical cyclone, the subtropical ridge,

and the westerly trough (upper air).

Some common tracks of Tropical Cyclones in

western North Pacific and South China Sea

Tropical cyclone basins

(Source: Atlantic Oceanographic and Meteorological laboratory, NOAA)

1. North Atlantic 2. Eastern North Pacific 3. Western North Pacific 4. Northern Indian Ocean 5. Southwestern Indian Ocean 6. Southeastern Indian Ocean 7. Southwestern Pacific

Major Tropical Cyclone Warning Centers

in the western North Pacific

China Meteorological Administration, Beijing

Hong Kong Observatory, Hong Kong

Joint Typhoon Warning Center, Hawaii

Tokyo Typhoon Centre of the Japanese Meteorological Agency (WMO Regional Specialized Meteorological Center )

Naming of Tropical Cyclones in the western North Pacific

Since WWII, the US military weather forecasters have named tropical cyclones forming in the western North Pacific. The names used were

almost exclusively English feminine names through 1978 (Girl friend,

spouse, etc.).

But beginning in 1979 men's names were used with women's names in an alternating manner.

Starting from 1 January 2000, tropical cyclones in the western North Pacific are named from a new list of names contributed by 14 members of

the World Meteorological Organization's Typhoon Committee (10 x14 =140

names).

The new names will be allotted to tropical cyclones reaching tropical storm strength by the Tokyo Typhoon Centre of the Japanese

Meteorological Agency (WMO RSMC).

Tropical Cyclone Names Contributed by the Hong Kong

Kai-tak Shanshan Man-yi Lingling Fung-wong Dolphin Choi-wan Lionrock Ma-on Banyan http://www.weather.gov.hk/informtc/sound/tcname2013e.htm

Retired Tropical Cyclone Names in the western North Pacific (up to 2010)

A total of 31 typhoons (in the western North Pacific Ocean) have been removed from the list

from 1947 to 2010, most of which retired after 2000.

(Source : Lei et al., 2012 : Summary of Retired Typhoon within the Western

North Pacific Ocean[J]. Tropical Cyclone Research and Review1(1): 23-32.)

Tropical cyclone frequency in western North Pacific The most active basin with about 30 tropical cyclones each year

0

1

2

3

4

5

6

7

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Ave

rag

e N

um

be

r o

f T

rop

ica

l C

yclo

ne

s

(1971-2000 average, based on HKO data)

Average No. of Tropical Cyclone Affecting Hong Kong (Signal Issued)

from 1971 to 2000

0

0.5

1

1.5

2

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

No

. o

f T

rop

ica

l C

yclo

ne

Tropical cyclones affecting Hong Kong About 5 tropical cyclones affecting Hong Kong each year

Impacts of tropical cyclones on Hong Kong

High winds (strong, gales, storm force, hurricane force winds)

Heavy rain

Swell and rough Seas

Storm surge

Very hot weather with afternoon or evening thunderstorms (due to continental flow)

Haze and high API situation (subsidence ahead of the storm, low level inversion)

Storm Surge

strong winds piling up the sea water near the coast

low atmospheric pressure of the tropical cyclone uplifts the sea surface on its path

Storm Surge of Hurricane

Sandy in Oct 2012

Ocean surface winds for Hurricane Sandy

observed at 9:00 p.m. PDT Oct. 28 (12:00

a.m. EDT Oct. 29) by the OSCAT radar

scatterometer on the Indian Space Research

Organization's (ISRO) OceanSat-2 satellite.

Colors indicate wind speed and arrows

indicate direction. The image shows the large

extent of high winds associated with this

system. (Source : ISRO/NASA/JPL-Caltech)

(Source : NRL, http://www.nrlmry.navy.mil/tc-bin/tc_home2.cgi)

Storm Surge Prediction and Observations

(Source : NOAA/NOS

http://tidesandcurrents.noaa.gov/data_menu.shtml?bdate=20121028&edat

e=20121030&datum=6&unit=1&shift=d&stn=8518750+The+Battery%2C+

NY&type=Tide+Data&format=View+Plot)

(Source : http://www.nhc.noaa.gov/refresh/graphics_at3+shtml/211343.shtml?gm_psurge)

Tropical Cyclone Storm Surge Probabilities

Sandy-battered New York & New Jersey

(Source : Daily Mail, http://www.dailymail.co.uk/news/article-2226762/Hurricane-Sandy-Terrifying-picture-taken-space-shows-devastation-wreaked-New-Jerseys-shores.html )

Sea water floods the Ground Zero construction site, Oct. 29,

2012, in New York. (Source : AP Photo/ John Minchillo)

Damage to the South Ferry station of the No. 1 subway

line, in lower Manhattan (Patrick Cashin, MTA / AP Photo )

Annual mean sea level rise in Battery Park, New York

from Permanent Service for Mean Sea Level (PSMSL) tide gauge

data.

On 24 September 2008, storm surge due

to Typhoon Hagupit caused a sea level

rise of about 1.4 m above normal.

Some houses near the shore in Cheung Chau were damaged

Storm Surge of Typhoon Hagupit in 2008

(courtesy of TVB)

Flooding in Tai O after Typhoon Hagupit (September 2008)

Haze and Reduced Visibility

Due to accumulation of local and regional suspended particulates and photochemical effects

Common when tropical cyclone approaching -

Conceptual model of the occurrence of reduced visibility in Hong Kong due to a tropical cyclone near Taiwan.

High temperatures on 21 September 2013

Affected by the subsiding continental airstream

associated with the outer circulation of super

typhoon Usagi, the weather in HK became dry

and very hot on 20 and 21 September.

The temperatures at the Hong Kong

Observatory rose to a maximum of 34.7 oC on

the afternoon of 21 September, the highest

record for September since 1969.

http://www.youtube.com/watch?v=-Mst13yIy38

High winds, squally heavy rain and storm surge

Typhoon Danas hitting Okinawa Japan

http://www.youtube.com/watch?v=XTvkrLESrwU

Storm Surge

(Just watch YouTube, dont risk your life !)

http://www.youtube.com/watch?v=06AJkSD0HiU

http://www.youtube.com/watch?v=-Kou0HBpX4A

Hurricane Katrina Historic Storm Surge

2. Interactions and Associated phenomena

Fujirawa effect Tropical cyclone and monsoon Extra-tropical transition Double Eye Walls (Eyewall replacement cycles)

Fujiwhara effect

When tropical cyclones come near each other (usually within 1000 km), their circulations can affect each other, causing them to rotate against each

other in a counter-clockwise direction.

The phenomenon is known as the Fujiwhara effect, named after Dr. Sakuhei Fujiwhara (1884-1950) who described it in a 1921 paper about the

motion of vortices in water. He later became the director of the Japan

Meteorological Agency in 1941.

The interaction of two tropical cyclones

Two tropical cyclones situation

October 2009

The track of Parma was affected by Super Typhoon Melor over the western North

Pacific. It lingered near northern Luzon for four days before moving west-

northwestwards across the northern part of the South China Sea.

Parma

Melor

(Source : NASA image courtesy the MODIS Rapid Response

Team at NASA GSFC.)

6 October 2009

[The satellite imagery was originally captured by Multi-functional

Transport Satellite-2 (MTSAT-2) of Japan Meteorological Agency (JMA).]

Three tropical cyclone situation

August 2010

Three tropical cyclones, Lionrock, Namtheun and Kompasu interacting with each

other

8 p.m. on 31 August 2010

2012 Tembin & Bolaven Satellite Images - Fujiwhara Effect

( Source : http://www.youtube.com/watch?v=l82K0bVmibQ )

Tropical Cyclone Northeast Monsoon

Tropical Cyclone + Northeast Monsoon

Tropical Cyclone and Northeast Monsoon

Both Tropical Storm Parma and the northeast monsoon affecting Hong Kong on 12

October 2009

Under the combined effect of the northeast monsoon and Parma, Hong Kong was

windy from 11 to 14 October.

Dry air intrusion Northeast monsoon - Dry and cold air mass

Tropical Storm Parma and the northeast monsoon in October 2009

(Source : NRL Monterey Marine Meteorology Division http://www.nrlmry.navy.mil/tc_pages/tc_home.html)

Interact with the upper air westerlies associated with the surface northeast monsoon

Abrupt change in track or Weaken quickly due to vertical wind shear increase

upper air westerlies

(Source : Central Weather Bureau, Taiwan)

An extra-tropical cyclone is a low pressure system that primarily gets its

energy from the temperature

difference in the horizontal direction

across the cyclone (known as

temperature gradient in meteorology) .

Extra-tropical cyclones have frontal features, i.e. they are associated with

cold fronts, warm fronts, and occluded

fronts. Structurally, extra-tropical

cyclones are "cold-core (i.e. the center is colder than the surroundings

at the same height).

Tropical cyclone & Extra-tropical cyclone

Tropical cyclones typically have little temperature differences across the

storm. Their energy are derived

from the release of heat due to

cloud/rain formation from the warm

moist air of the tropics.

Tropical cyclones are "warm-core".

A tropical cyclone will transform into an

extra-tropical cyclone as it recurves

poleward.

While most typhoons have a single eye wall, many mature typhoons develop the

double-eye-walled structure.

When typhoons show a double eye-walled structure, they are often in the process of

undergoing a "eye wall replacement cycle"

where a new eye wall develops and

replaces an existing one.

The double-eye-walled may last for a day or two.

Double-eye-walled

Double-eye-walled structure is an intermediate phase of intensification, it can

happen in typhoon to super typhoon stages. As such, it does not imply that a

double eye walled typhoon is the strongest typhoon !

3. Monitoring and Forecasting Tropical

Cyclones

Surface and Upper Air Observations Aircraft Reconnaissance Satellite images (Dvorak Analysis) Radar images Kidney and Beach Ball Numerical Weather Predictions

Surface and Upper Air Observations

Land stations Ship reports Upper air soundings

Locating tropical cyclone using the inflow angle method

Aircraft Reconnaissance

Looking straight up from inside the eye of Super typhoon Forrest (1983) from a

hurricane hunter reconnaissance aircraft.

(Source Photo courtesy of Scott A. Dommin. www.ral.ucar.edu/guidance/)

A NOAA WP-3 Orion turboprop Hurricane Hunter aircraft. (Photo credit: NOAA)

(Source : NASA

http://www.gsfc.nasa.gov/gsfc/earth/pictures/c

amex4/dropsonde.gif)

(Source : Randy Redman of the US Air Force)

Aircraft reconnaissance in the western

North Pacific discontinued since in 1987

Dropsonde Observation for Typhoon Surveillance near the TAiwan Region"

(DOTSTAR)

a project of Department of Atmospheric Sciences, National Taiwan University

perform GPS dropwindsonde airborne observation of typhoons near Taiwan

The meteorological measuring system installed on the fixed-wing

aircraft, with the inset showing the air data probe

Using aircraft to collect data of tropical cyclones in Hong Kong

The Observatory collaborates with the Government Flying Service (GFS) to use its fixed-wing

aircraft for collecting meteorological data in the vicinity of Hong Kong. On 22 June when

Tropical Storm Haima affected the northern part of the South China Sea, for the first time the

fixed-wing aircraft flew near the centre of the storm, collecting unique data including wind and

pressure up to 20 readings per second. Such data are helpful in determining the strength of the

storm and supporting decision-making in the provision of public weather service.

Meteorological Satellites

35,800 km

S

N

Geostationary satellites

Polar-orbiting satellites

Two major types of meteorological satellites characterized by their orbits:

Satellite Images

Infra-red images show the temperatures of the observed objects (e.g. clouds). Such "night vision" capability makes them useful round the clock.

In general, the higher the top of the clouds, the lower its temperature and

the brighter it will appear in the image.

Visible images liken the black-and-white photos captured from space. They are available in day time only. The resolution of visible images is higher

than those of infra-red images. This enables visible images to show more

detailed structure of clouds.

Visible Infrared Water vapour

- Geostationary satellites

Dvorak enhancement used for Tropical Classifications with the Dvorak Technique

Visible image IR image

IR with rainbow color enhancement image

(Source : NOAA, http://www.ssd.noaa.gov/PS/TROP/ )

Super Typhoon HAIYAN (2013)

This visible image of Super Typhoon Sanba was captured by the MODIS instrument aboard NASA's Aqua satellite

on Sept. 13, 2012 at 0450 UTC (12:50 a.m. EDT). (Credit: NASA Goddard MODIS Rapid Response Team)

Polar Orbiting Satellite Images

(Source : NASA, http://www.nasa.gov/mission_pages/hurricanes/archives/2012/h2012_Sanba.html)

Passive microwave imagery

enable the user to "see through" non-raining clouds and view rainbands, eyewalls and eyes even when obscured by upper-level clouds that hinder

the user of visible and IR imagery.

data is currently only available on polar orbiter platforms that fly over tropical cyclones at most several times/day/satellite.

- Special Sensor Microwave/Imager (SSM/I)

- Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI)

- Advanced Microwave Sounding Unit (AMSU-B)

- Advanced Microwave Scanning Radiometer (AMSR-E)

- The WindSat Passive Microwave Radiometer

(Reference : NOAA, http://mirs.nesdis.noaa.gov/index.php)

Typhoon Nanmadol, 26 Aug 2011

Infrared Visible Microwave

(TRMM)

(source : NRL NRL Monterey Marine Meteorology Division (Code 7500) TC_PAGES Page

Super Cyclone Phailin (2013)

http://mirs.nesdis.noaa.gov/amsub.php

AMSU-B (Microwave image) Visible Image

NASA's TRMM satelilte data was used to create this 3-D view of Typhoon Sanba on September

14, 2012, at 0541 UTC. The inner eye wall and older eye are both shown to extend to heights

above 13km (~8.8 miles). (Credit: SSAI/NASA, Hal Pierce)

(Source : NASA, http://www.nasa.gov/mission_pages/hurricanes/archives/2012/h2012_Sanba.html)

Dvorak Analysis

A technique developed by Vernon Dvorak in 1970s to estimate tropical cyclone intensity based on infrared satellite images

Commonly used by warning centers in assessing tropical cyclone intensity Utilizes a special satellite image look-up table to assist the assessment Main assessment parameters include, pattern, shear off distance, cloud top temperature,

eye temperature, etc.

(source : NRL NRL Monterey Marine Meteorology

Division (Code 7500) TC_PAGES Page

(Source :http://en.wikipedia.org/wiki/Dvorak_technique#cite_note-2)

(Reference : Dvorak Tropical Cyclone Intensity Estimation Technique http://www.nhc.noaa.gov/pdf/06velden.pdf)

Note : conversion factor of 1 min mean wind to 10-min mean wind speed is about 0.93

Doppler weather radar

Doppler weather radar is capable of measuring the approach (or departing) speed

of raindrops

Faster the raindrops move towards the radar,

the higher will be the frequency of the

microwave reflected from raindrops.

The raindrops' approach speed is a good estimation of the winds which carry the

raindrops.

Semicircle Effect

In the Northern Hemisphere, winds surrounding a tropical cyclone blow in an anticlockwise direction.

winds on the right semicircle will be in the same direction of the storm's translation motion while winds over the left semicircle will be in the opposite direction.

winds on the right semicircle are usually stronger than those on the left semicircle, which are thus termed the dangerous semicircle and navigable semicircle

respectively.

subtropical ridge usually stays on the northeastern side of a tropical cyclone, resulting in a tighter pressure gradient and thus stronger winds in between them

The 30%, 50% and 70% probability for strong winds at Waglan brought by typhoons.

Diagram showing the prevailing wind direction at Waglan when a tropical cyclone is located in a

particular sector

Kidney and beach ball

Kidney - an area on the map where it is probable that windy conditions will affect a location in Hong Kong once a tropical cyclone comes within that area.

Beach Ball - to determine the prevailing wind direction in Hong Kong based on the tropical cyclone location relative to Hong Kong

A narrow escape from Super Typhoon Usagi 2013

A small change in the forecast track of Usagi (yellow and blue arrows) may result in vastly

different effects on Hong Kong's weather.

Yellow arrow Usagi makes landfall to the east of Hong Kong, Usagi will weaken on her way to Hong Kong. Northerly winds blocked by terrain to the north effect, the wind strength

over Hong Kong will be relatively lower.

Blue arrow Usagi maintains strength on her way to Hong Kong. HK will be more prone to the effect of southeasterly winds and storm surge (together with high tide near the Mid-

Autumn Festival).

Actual - Usagi passed about 80 kilometres to the north of Hong Kong with a storm surge of

about 0.5 to 1 metre.

What would happen then if Usagi took on a track of about 100 kilometres southward when it

was near Hong Kong (red line) with the time of occurrence of storm surge matching with that of

the astronomical high tide?

Computer storm surge simulated results showed that storm surge of about 1.7 metres would

occur at Quarry Bay and added up with the astronomical high tide (2.2 metres), resulting in a

sea level of nearly 4 metres, rather close to that caused by Wanda in 1962.

Modern days Numerical Weather Prediction

NWP = Using physical equations to simulate the evolution of the atmospheric motions

Methods: integration of the evolution equations, based on the initial and

boundary conditions

Applicable Scale: mesoscale (thunderstorm) to synoptic scale (frontal system)

Valid time: 1 - 48 hr (short-range)

24 - 196 hr (medium range)

> 10days (long-range)

Numerical Weather Predictions

Equations

+

Conceptual Models

+

High Performance Computers

Numerical Weather Predictions

Different Models give Different Forecast Tracks !

Ensemble Forecasts Strike Probability

4. Tropical Cyclone Warning History

The changes of tropical cyclone warning system of Hong Kong over the last

century

Starting from 1884, a system of drum, ball and cone was employed. Typhoon gun was used to warn imminent tropical cyclone gale force winds

In 1907, explosive bombs replaced the typhoon gun. The last typhoon boom was exploded in 1937.

In 1917, the first numbered signal system from 1 to 7. Numbers 2 to 5 signifying gale force winds expected from the four quadrants, namely N, S, E and W.

In 1931, the signals from 1 to 10 with signals 2 and 3 signifying strong winds from SW and SE respectively, signal 4 being a non-local signal, signals 5 to 8 signifying gales

from the four quadrants, namely NW, SW, NE ad SE, signal 9 signifying increasing

gales and signal 10 indicating the threat of hurricane force winds. Signals 2, 3 and 4

were used intermittently afterwards and were discontinued in the late 1930s.

In 1956, the No. 3 Strong Wind Signal was introduced between the No. 1 Stand-by Signal and the gale signals.

Starting from 1 January 1973, signals 5 to 8 were replaced by 8 NW, 8 SW, 8 NE and 8 SE respectively.

11/21/2013

Observatory in action Storm warning signals 1890 Red and Black signals

11/21/2013

1897 from North or East from South or West 1904

Typhoon Gun on the Peak

Time ball tower and typhoon mast 1885 to 1907

Tsimshatsui Marine Police Base

Time ball tower on Signal Hill 1908 to 1933

Signal stations in Hong Kong

The number of signal stations in Hong Kong peaked at 42 in the 1960s (Figure 3).

As the electronic media became popular, these stations were progressively closed, beginning

in the late 1970s. The last signal station in

Hong Kong, on the island of Cheung Chau, was

decommissioned on 1 January 2002 marking

the end of the era of the hoisting of tropical

cyclone warning signals.

5. Devastating Storms

Tracks of the tropical cyclones that necessitated the issuance of the Hurricane

Signal No. 10 in Hong Kong since 1946

1962

Wanda:

Formed in western North Pacific. First became a severe typhoon over

South China Sea about 390 km ESE of HK (20.6N 117.5E), then a super typhoon for a short while about 160 km ESE of HK (21.7N 115.6E). Weakened to a severe typhoon about 30 km SSE of HK (22.1N 114.3E).

This table shows the average wind speed in knots as recorded (uncorrected) at the Observatory during each of the three hours before and the four hours after the maximum wind

Date AUG. 17

1936

SEPT. 2

1937

GLORIA

SEPT. 22

1957

MARY

JUNE 9

1960

ALICE

MAY 19

1961

WANDA

SEPT. 1

1962

Mean hourly wind speed

3 hrs. before

Max.

wind

37 20 44 42 34 36

2 hrs. before 46 30 49 44 38 45

1 hr. before 54 58 51 44 42 54

Maximum hourly mean

wind 62 59 59 50 43 68

1 hr. after

Max. wind

56 58 56 44 12 48

2 hrs. after 52 43 46 36 14 40

3 hrs. after 29 34 37 37 39 36

4 hrs. after 24 29 36 45 42 33

Maximum gust (knots) 115 130* 101 103 89 140

Instantaneous Minimum

Pressure (mb.) 979.3 958.3 984.3 973.8 981.1 953.2

Intensity of the previous tropical cyclones which necessitated the issuance of

the No. 10 Signal

Maximum gusts and minimum mean sea level pressure recorded in Hong Kong

during the passage of Vicente and the previous tropical cyclones that necessitated

the issuance of the No. 10 Signal in Hong Kong

Casualties caused by Typhoons with No. 10 or equivalent (1884-2002)

(SOURCE : Weathering the Storm by P.Y. Ho, 2003)

In Hong Kong, the Tolo Harbour over the northeast is more susceptible to high storm surges than the Victoria Harbour For the Typhoon of 1937, the villages along the coast of Tolo Harbour were severely flooded by the storm surge of the typhoon. Thousands of lives were lost, mostly fishermen who were living in their boats. The high water mark in the area left by the typhoon was estimated to be about 6 metres above CD, which meant a surge of about 3.8 metres.

TROPICAL CYCLONE TRACKS FOR THE TOP 20 STORM SURGE RECORDS AT QUARRY BAY/NORTH POINT TIDE GAUGE STATION

(1947-2006)

18 Sept 1906

1-2 Sept 1937

Typhoon Wanda, Sept 1962

7-10 September 1983, Typhoon Ellen caused great destructions to fishing boats and sea vessels

(Courtesy : Hong Kong Museum of History)

Typhoon MORAKOT ( ) August 2009

Typhoon MORAKOT

Windward slope

Heavy rain of Morakot

Intense rainband over the southern semi-circle

Slow movement

Terrain effect (2000-3000m height)

Radar image of Typhoon Morakot at 09 UTC on 8 August 2009

Source : Central Weather Bureau, Taiwan

Typhoon MORAKOT ( ) August 2009

Over 1000 mm in many areas Some places with near 3000mm rainfall

The "Geng-Zi" typhoon disaster in 1900 () - The Deadly Autumnal Storm

Re-analysis of mean sea level pressure and 10-min Wind on 9 November 1900.

(Source : NOAA 20th Century "Reanalysis")

As per the reports from the Governor and newspapers, the Observatory had given

due notice of this impending typhoon to the public and the inclement weather also

fully validated the Observatorys prediction. Regrettably, the public gave less attention to the warnings and little precautions were taken as many of them

disbelieved that such a violent storm would affect the territory in this season.

The Geng-Zi Typhoon caused extensive damages and heavy casualties to the territory. Numerous sampans and boats were sunk or even smashed to matchwood by the raging waves.

Ten steam-launches and over 110 junks were also sunk and the harbour was full of wreckage.

Many trees were damaged or uprooted. Lamp posts and telephone posts were bent at all angles

by the furious winds. Over 200 lives were lost in these few fatal hours.

Damages and casualties

(Source : the Illustrated London News on 22 December 1900)

Estimate the possible hourly positions as well as the pressure pattern of the

typhoon during its passage over Hong Kong

(Prepared by Mr K.Y. Kong, Weather Prediction Center NOAA)

A closer look of the Geng-Zi Typhoon

Super Typhoon Haiyan (2013)

Dvorak Analysis with T-number reaching 7.5-8.0 One of the most intense super typhoons since satellite era May be the most intense super typhoon during landfall (subject to further analysis)

(Source : ABC News http://mobile.abc.net.au/news/specials/typhoon-haiyan-photos-before-after/)

Before After Tacloban City, the Philippines

Inter-annual and inter-decadal variations Climate change and TC activities

Long term variation of Tropical Cyclones

in the western North Pacific and the

South China Sea

(a) storms of tropical storm

intensity and above

Annual storm counts based on the categories assigned according to reported

maximum sustained winds converted into 10-min mean

(b) storms of typhoon intensity

Source : Lee et al., Impacts of Climate Change on Tropical Cyclones in the Western North Pacific Basin. Part I: Past Observations. Tropical Cyclone

Research and Review, 2012, 1(2): 213-230

In gist, these oscillations affect the SST and/or atmospheric circulation over the WNP, subsequently affect the steering flow, favorable TC

genesis frequency, TC formation locations, etc.

There are many factors modulating the inter-annual and inter-decadal

TC activities in the WNP and the SCS

For examples :

ENSO Pacific Decadal Oscillation (PDO) Quasi-Biennial Oscillation (QBO) East Indian Ocean SST

.

Spectral Analysis of Tropical Cyclone Activities over the western North Pacific

MTM spectrum of the annual number of tropical cyclones

in the western North Pacific.

2.4 year peak Quasi-Biennial Oscillation (QBO)

3-4 year peak ENSO

18 year peak Pacific Decadal Oscillation (PDO)

(Source : Yeung, K.H., M.C. Wu, W.L. Chang and Y.K. Leung, 2005, Long-term Change in Tropical Cyclone Activity in the Western North Pacific. Presented in the Scientific Assembly of International Association of Meteorology and Atmospheric Science (IAMAS) 2005, Beijing, China, 2-11 August. Hong Kong Observatory Reprint No. 601.)

Taking the ENSO as an example :

Genesis position shift to

the east

Sub-tropical ridge split into

two

Genesis position shift to the

west

Sub-tropical ridge continuous

Composite circulation in the late season at 850 and 500 hPa for (top) El Nino, (middle) neutral, and (bottom) La Nina years.

Mean tropical cyclone genesis location map for El Nino and La Nina years

STY activity could be related to the ENSO events. Generally speaking, there were more

STYs in El Nio years than in La Nia years. Possible causes :

In El Nio years, affected by SST pattern, monsoon trough and weak vertical wind shear, the

formation locations of the TCs drifted eastward, so they could be in a weak vertical shear

environment during their movements. As a result, there were more STYs in El Nio years,

and less in La Nia years.

ENSO and Super Typhoon Activities

Weak vertical wind shear, positive

low-level vortex and longer developing

time are all advantageous to TC

intensity in El Nio years.

(Source : Huang and Xu, 2010 : Super Typhoon Activity over the Western North Pacific and Its Relationship with ENSO, J. Ocean Univ. China (Oceanic

and Coastal Sea Research) 9 (2): 123-128.)

Frequencies and positions of TC formation during the typhoon season for (a) El Nino

and (b) La Nina years. The numbers in the top-right corner indicate the TS to TY and

STY genesis frequencies in the west and east WNP.

(Source : Zhan et al., 2011: Contributions of ENSO and East Indian Ocean SSTA to the Interannual variability of Northwest Pacific Tropical Cyclone

Frequency. J. Climate, 24, 509-521. doi: 10.1175/2010JCLI3808.1.)

ENSO and Super Typhoon Activities

Climate Change and Tropical Cyclone Activities in the WNP

Analysis of available TC data from different databases shows that most of the datasets depict a decreasing trend, and some statistically significant, in

the annual number of TCs and typhoons in the WNP.

For TC intensity, differences in TC datasets for the WNP do not permit a convincing detection of a long term trend in this basin.

Climate model projections of future TC activity in the WNP suggest a noticeable decrease in the frequency is likely in the 21st century.

Most of the the model simulations also report an increase in the number of intense TCs and the TC rainfall rate in the WNP in a warmer climate.

The possible influence of climate change on the shift of TC track and formation location over the WNP is also noted in some studies utilizing

observational analysis and model simulations.

Linear trends in the June-October mean frequency of TC occurrence and in the TC

motion vectors. The areas with confidence level exceeding 95% for the TC occurrence

changes are shaded. The contour interval is 0.3 year-1 and the unit of the vectors is

ms-1. the thick solid lines with arrows denote the prevailing typhoon tracks.

(Source : Wu, L., B. Wang, and S. Geng, 2005: Growing typhoon influence on east Asia, Geophys. Res. Lett., 32, L18703, doi:10.1029/2005GL022937.)

Anomaly in annual mean TC occurrence frequency for 2001-2010. Superimposed is the 500 hPa

steering flow anomaly averaged over May-November (unit of ms-1). Anomalies are with reference

to the 1961-90 mean. The TC occurrence frequency is calculated based on the HKO TC dataset

and the 500 hPa anomalous flow is drawn from NCEP-NCAR re-analysis 1 data.

Number of TCs (including TDs, blue) and typhoons (red) landfalling in China (1949-2010). The

solid, thick and dashed lines represent the annual number, 5-year running mean and linear

trend respectively. trends are estimated in the period of 1949-2010.

Source : Lee et al., Impacts of Climate Change on Tropical Cyclones in the Western North Pacific Basin. Part I: Past Observations. Tropical Cyclone

Research and Review, 2012, 1(2): 213-230

Possible impacts of climate change on TC tracks

JJASO typhoon frequency climatology averaged over the period of 19702006. The contour interval (CI) is 0.5 per season (JJASO) per grid box (2.58 3 2.58). The bold arrows represent the majority of

typhoon paths in the western North PacificEast Asian region. The box represents the area in the vicinity of Taiwan.

(Source : Tu, Jien-Yi, Chia Chou, Pao-Shin Chu, 2009: The Abrupt Shift of Typhoon Activity in the Vicinity of Taiwan and Its Association with Western North

PacificEast Asian Climate Change. J. Climate, 22, 36173628.)

Time series of seasonal (JJASO) typhoon numbers passing the vicinity of Taiwan from 1970 to 2006. (b) The conditional posterior

probability mass function of change points is plotted as a function of time. (c) Posterior density function of seasonal typhoon rate

before (dashed line) and after (solid) the shift, with the changepoint year being set in 2000. (d) JuneOctober typhoon frequency differences for the period of 200006 minus the period of 197099

(Source : Tu, Jien-Yi, Chia Chou, Pao-Shin Chu, 2009: The Abrupt Shift of Typhoon Activity in the Vicinity of Taiwan and Its Association with Western North

PacificEast Asian Climate Change. J. Climate, 22, 36173628.)

Time series of (a) the number of landfall TCs over Korea and Japan, (b) the number of TC

genesis events over the WNP, and (c) the ratio of number of landfall TCs over Korea and

Japan to the number of TC genesis events over the WNP. The gray and black lines indicate

the raw and lowpass filtered values, respectively

(Source : Park, D.S.R., C.H. Ho, J.H. Kim and H.S. Kim, 2011: Strong landfall typhoons in Korea and Japan in a recent decade, J.

Geophys. Res., 116, D07105, doi:10.1029/2010JD014801.)

Tropical cyclones affecting Hong Kong

Modulated by El Nio-Southern Oscillation (ENSO) and Pacific Decadal

Oscillation PDO, there are strong inter-annual and inter-decadal

variations in the tropical cyclone activity in the South China Sea.

Analysis of the Hong Kong Observatory tropical cyclone records since

1961 shows that there is a long term decreasing trend in the number of

tropical cyclones entering the 500 km range of Hong Kong, but the trend

is not statistically significant at 5% level.

There is no significant trend in the number of tropical cyclones landing

over the south China coast within 300 km of Hong Kong.

Inter-annual and inter-decadal variations of TC activity in SCS

El Nio Southern Oscillation (ENSO)

El Nio - there are fewer TCs developed in April and May and the genesis positions are usually located further east over the western North Pacific compared to normal condition.

Hence, TCs are unlikely to affect the territory before June.

La Nia - TCs in August-October are likely driven by an anomalous steering flow into the South China Sea and hence more tropical cyclones are likely to affect Hong Kong

compared to normal condition.

Pacific Decadal Oscillation (PDO)

PDO switches between warm and cold phases could affect the average position and

strength of the subtropical ridge over the WNP and therefore influence the TC frequency

and genesis position.

PDO+ENSO

Overall speaking, TC activity in the SCS during El Nio and warm phase of PDO tend to be

below normal, while above-normal TC activity could be found during La Nia and cold

phase of PDO

East Indian Ocean (EIO) Sea Surface Temperature anomaly (SSTA)

Warm (cold) EIO SSTA suppresses (promotes) TC genesis over the WNP. There is a

significant negative correlation between the TC genesis frequency to the east of 120oE and

EIO SSTA.

Annual number of tropical cyclones come within 500 km range of Hong Kong

0

2

4

6

8

10

12

1961 1966 1971 1976 1981 1986 1991 1996 2001 2006

Year

Nu

mb

er

of

tro

pic

al

cyc

lon

es

5-year Running Mean

Linear Trend

01

2

3

4

5

6

7

8

1961 1966 1971 1976 1981 1986 1991 1996 2001 2006

Year

Nu

mb

er

of

tro

pic

al

cy

clo

ne

s

5-year Running Mean

Linear Trend

Annual number of tropical cyclone (including tropical depressions) landing over

the south China coast within 300 km of Hong Kong from 1961-2010

1960 1970 1980 1990 2000 2010

02

46

810

Annual number of TS affecting within 500km range of HK

Year

Num

ber of TS

Formed_TS

Entered_TS

Trend (per decade) P-value

Total TS -0.2118 0.2066

TS Formed in SCS 0.02545 0.7369

TS Entered from WNP -0.2372 0.1682

010

20

30

40

50

60

1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

Win

d s

pee

d (

m/s

)

10 minute mean wind

5 year running mean (10 min wind)

Gust

5 year running mean (Gust)

10

20

30

40

50

60

70

1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

Win

d s

pee

d (

m/s

)

10 minute mean wind 5 year running mean (10 min wind) Gust 5 year running mean (Gust)

There is no significant trend in the TC-induced high winds in the remote station at Waglan

Island during 1961-2010 but the decreasing trend in the urban station at Kai Tak is statistically

significant. The significant decreasing trend of the high winds in Kai Tak is very likely due to the

dense urban development in Hong Kong over the last few decades

Annual maximum 10 minute mean wind speed and gust brought by

TCs entering 500km range of HK from 1961 to 2010.

at Kai Tak at Waglan Island

020

40

60

80

100

120

140

160

180

1961 1966 1971 1976 1981 1986 1991 1996 2001 2006

Year

An

nu

al

Ra

infa

ll p

er T

C (

mm

)

Annual Rainfall per TC

5 year running mean

0

20

40

60

80

100

1961 1966 1971 1976 1981 1986 1991 1996 2001 2006

Year

An

nu

al

Ma

xim

um

Ho

url

y R

ain

fall

(m

m)

Maximum Hourly Rainfall

5 year running mean

Annual rainfall per TC brought by TCs entering

500km range of Hong Kong from 1961 to 2010.

Annual maximum hourly rainfall for TCs entering

500km range of HK from 1961 to 2010.

Thank You