The Hong Kong Polytechnic UniversityDepartment of Applied Physics

GE Subject : Climate and Our Environment

Weather Monitoring & Forecasting in Hong Kong

29 September 2008

What is WEATHER FORECASTING ?

a complex and difficult job

a risky business too !

atx

y

z

3 D

12

3

6

9

1D

in = 4D problem

Predict the future weather

Forecasting range Lead time

Nowcasting ~ several hours

Very short range ~ 12 hours

Short range ~ a few days

Medium range ~ one week

Long range over 10 days

Seasonal a month to a year

Forecasting Range

Time

Length

2 km

2 000 km

5 000 km

10 000 km

s to min min to hrs hrs to days days to weeks, months …

200 km

20 km

200 m

2 m

Sub-tropical ridge,

Monsoon,

ITCZHigh / Low son weather

map,

Cold / Warm fronts

Tropical cyclones

Land / Sea Breeze

Squall linesThunderstorms

TornadoesWaterspouts

Turbulenteddies

planetary, seasonal scale

synopticscale

mesoscale

microscale

Scale of Weather Systems & Atmospheric Motion

Weather Forecasting Procedures

Meteorological Information

Forecasting Methods/Analysis

Forecast Products/Warnings

Procedures of Weather Forecasting

Meteorological observations

Data analysis

Forecasting methods and techniques

Forecast and warning dissemination

Observation and MonitoringSurface Observations :-

Automatic Weather Stations (AWS)Telemetering Rain GaugesWeather BuoysVisibility Meters

Upper Air Observations :-

Radiosonde and Wind Profiler

Remote Sensing :-

Doppler Weather RadarsWeather SatellitesLightning Location Networks

Meteorological Observations

Surface observations over land

Stevenson Screen:Temperature,Dew pointPressure

Wind mask:wind speed and direction

Telemetering rain gauge

Human observation

Surface observations over the ocean Voluntary Observing Ship (VOS) Program

Temperature, Dew point, Sea surface temperature, wind direction/speed, pressure, sea wave, swell, weather observations, etc.

Weather Buoy

Automatic Weather Station and Sea surface temperature

Traditionally, visibility monitoring of Victoria Harbour has been carried out by trained observers at the Observatory Headquarters.

With the increasing number of high rise buildings obscuring the visibility observations, there is a need to devise a new method to maintain the long-term visibility monitoring in Victoria Harbour.

To this end, an automatic visibility meter has been installed at Central to provide visibility readings.

Visibility Meter

Data Coverage (Asia) - surface

Station circle

Wind barbs (1knot ~ 2km/hr)

Each station is represented by a “station circle” and the observational data are plotted around this circle in standard positions so that the meteorologist can see at a glance all the parameters at every station.

From : Dr P. M. Inness

Weather Chart

• To measure upper-air temperature, dew point, pressure, wind speed and direction

• 2 times per day (00, 12 UTC), where 00 UTC = 0800 HKT

Radiosonde

Upper Air Observations

Wind Profiler

Uses radar to detect the wind speed and direction at various elevations above the ground.

Upper air observations – Tephigram

The Tephigram is a thermodynamic diagram designed to aid in the interpretation of the temperature and humidity structure of the atmosphere and used widely throughout the worldmeteorological community.

Allows a radiosonde profile to be analysed for stability

Upper air observations – Upper air charts

• 大老山多普勒天氣雷達 (Tates' Cairn Doppler Weather Radar System) since 1994

• 大帽山多普勒天氣雷達 (Tai Mo Shan Doppler Weather Radar System) since 1999

• 機場多普勒天氣雷達 (Terminal Doppler Weather Radar System) since 1996

Remote Sensing Observations

Weather Radar Systems

Tai Mo Shan Radar Station(TMS Radar)

Tates' Cairn Radar Station(TCR Radar)

Radar Images

35,800 km

S

N

Geostationary satellites

Polar-orbiting satellites

Meteorological SatellitesTwo major types of meteorological satellites characterized by their orbits:

Satellite Antenna

圖片顯示 Image Display

1966 1999

可見光 Visible 紅外線 Infrared

水氣圖 Water vapour

地球靜止衛星 - Geostationary satellites

極軌衛星 – Polar Orbiting Satellite

Special Senor Microwave / Imager (SSM/I)

QuikSCAT

Lightning sensor stations

The network comprises six stations jointly established by the Hong Kong Observatory, the Guangdong Meteorological Bureau and the Macao Meteorological and Geophysical Bureau. The stations are located at Chung Hom Kok, Tsim Bei Tsui, Sha Tau Kok, Taipa in Macao, Sanshui and Huidong in Guangdong.

http://www.weather.gov.hk/wxinfo/llis/index.htm

The lightning location information service

Internet lightning alert service to provide lightning information according to individual needs in an interactive way.

People can choose their location of interest, either from the popular places listed on the webpage or entering the name of a building, estate or street. The user can also click directly on the lightning map to specify the place of concern.

An alert will be provided automatically when there is lightning within a 15km range of the specified location.

Weather Forecasting Procedures

• Subjective:– human experience

• Statistical:– Analogue, persistence, trend analysis

• Dynamical:– Chart analysis

• Numerical weather forecast

Weather Forecasting MethodsWeather Forecasting Methods

Met. Information -> forecasting methods Met. Information -> forecasting methods forecast forecast productsproducts

In the old days -

Forecasting methods :

mainly statistical or climatological + human experience

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 (Tropical cyclone) to synoptic scale (cloud front)

• Valid time:– 1 - 48 hr (short-range)– 24 - 196 hr (medium range)– > 10days (long-range)

Numerical Weather Predictions

Equations

+

Conceptual Models

+

High Performance Computers

Major Weather Systems affecting Hong Kong

Air masses affecting us

Summer Monsoon Rain

Tropical Cyclones

Winter Monsoon

Air masses affecting us

Summer Monsoon Rain

What is a trough ?

A trough of low pressure is an extension of an area of low pressure. It is called a trough because the atmospheric pressure over the region is lower than its two adjacent sides.

Tropical Cyclones

Classification of Tropical Cyclones

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 km/h or more

Locations of tropical cyclone formation

Winter Monsoon

HH

What is an anticyclone or high pressure area ?

When the atmospheric pressure over a region is higher than its surrounding, it is called an anticyclone or an area of high pressure. The air mass surrounding an anticyclone will move clockwise in the northern hemisphere. The weather under an area of high pressure is generally stable.

Wind (air mass theory):Wind (air mass theory): Northerly winds- dry and cold (in winter)Northerly winds- dry and cold (in winter)

NW – hazy and dryNW – hazy and dry

Southerly winds - warm and humidSoutherly winds - warm and humidSW – unstableSW – unstable

SE – misty in springSE – misty in spring

Easterly winds – tricky !Easterly winds – tricky !

Rule of Thumb for Forecasting Weather in Hong Kong

High pressure area : Stable weather

Low pressure area : Unstable weather

Some examples of severe weather

•Rainstorms more frequent in summertime

•Can cause widespread flooding and landslips

•Can also accompanied with hail and tornado (not frequent in Hong Kong)

A Black rainstorm (2000/04/23)

Rainstorms

A hail storm (1998/02/18)

Hailstorms © Hong Kong Observatory

a hail storm with"hook" signature

© Hong Kong Observatory

Squall lines and “Shi Hu Feng (石湖風 ) ”

A squall line is a cluster of several severe thunderstorm cells along a line. Squall lines travel quickly and are destructive.

• heavy downpour and active lightning, • sudden change in surface wind direction and abrupt increase in wind speed• severe gust associated with squall lines can exceed 100 kilometres per hour • some of them are even embedded with hails and tornadoes

Time series of 1-minute maximum gust recorded at Kawi Chungfrom 11:00 a.m. to 2:00 p.m. on 9 May 2005

0

50

100

150

11:00 12:00 13:00

Hong Kong Time

1-m

inu

te M

axi

mu

m G

ust

(km

/hr)

135 km/h

Radar Reflectivity of Typhoon Yutu

Tropical Cyclones

Typhooneye

Rainbands

© Hong Kong Observatory

© Hong Kong Observatory

Typhoon Zeb (1998) The image was originally captured by the Geostationary Meteorological Satellite (GMS-5) of Japan Meteorological Agency (JMA). Anyone wishing to further disseminate the satellite image should seek permission from JMA, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (Fax: +81-3-3211-2032).

Typhoon Lupit (2003)The satellite imagery was originally captured with GOES-9, which backs up GMS-5 and is operated over the western Pacific by the joint effort of JMA and US NOAA NESDIS. Anyone wishing to further disseminate these satellite images should seek permission from JMA, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (Fax: +81-3-3211-2032).

El Niño and La Niña

El Niño refers to the above-normal sea surface temperatures from the coasts of Peru and Ecuador to the equatorial central Pacific. El Niño usually peaks around Christmas, hence the name of the phenomenon (Spanish for "the little boy" or "the Christ Child"). It occurs on average every three to five years, lasting 12 to 18 months.

La Niña, "the little girl" in Spanish, is characterized by below-normal sea surface temperatures in the eastern equatorial Pacific. La Niña occurs less frequently compared with El Niño. La Niña conditions typically last approximately 9 to 12 months.

El Niño and La Niña

El Niño and La Niña can cause a disruption of the ocean-atmosphere system in the tropical Pacific and result in climate impacts in different regions

http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

El Niño Situation

More rain in South America, while tropical Australia, Papua New-Guinea and Indonesia experience drought conditions.

http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

La Niña Situation

Increased convection or cloudiness over tropical Australia, Papua New-Guinea, and Indonesia

Sea Surface Temperature Status from 2005 to 2008

Effect of El Niño and La Niña on Hong Kong Climate

Rainfall

Wet springs in Hong Kong seem to be associated with El Niño activity in the previous winter, such as the springs of 1983, 1992 and 1998.

Tropical cyclones

El Niño years tend to have fewer tropical cyclones affecting Hong Kong, relating to the favoured location for the formation of tropical cyclones shifting towards the eastern and central Pacific.

Hong Kong tends to have more tropical cyclones in years with strong La Niña activity

Seasonal Forecasting

Seasonal Forecasting

• Monthly, Three-Monthly, or Annual Forecast

• Usually forecast anomaly (above normal, normal, or below normal)

• Methods :

(i) Conceptual/Statistical :- based on climatology and conceptual model on the effects of different climate system on local climate (e.g. El Nino, La Nina, Winter Monsoon Strength, Snow Cover, etc.)

(ii) Dynamical Approaches :- based on outputs of computer model simulations and post-processing methods

Seasonal Forecasting in Hong Kong

Annual outlook :- total rainfall and tropical cyclone activities of the yearhttp://www.weather.gov.hk/wxinfo/season/anlf.htmA conceptual-statistical approach is used to formulate the annual outlook

Detailed methods : http://www.weather.gov.hk/wxinfo/season/met.htm

Seasonal forecast :- total rainfall and mean temperature of the seasonhttp://www.weather.gov.hk/wxinfo/season/season.htmThe Hong Kong Observatory adapted a Global-Regional Climate Model (G-RCM) suite consists of a Global Spectral Model and the latest version of the regional climate model to provide seasonal forecasts for Hong Kong on experimental basis

Notes :Blue: +ve anomaly (above normal rainfall)Orange: -ve anomaly (below normal rainfall)Unit expressed in standard deviation (σ)±0.5σ being classified as near normal

Regional Climate Model Rainfall Forecast ChartSummer 2008 (June to August 2008)

Notes :Red: +ve anomaly (above normal temperature)Blue: -ve anomaly (below normal temperature)Unit expressed in standard deviation (σ)±0.5σ being classified as near normal

Regional Climate Model Temperature Forecast ChartSummer 2008 (June to August 2008)

Thank you

Please visit HKO home page @

www.weather.gov.hk