Lecture 7 Hong Kong’s climate, it’s long term

trends and variability

LSGI1B02 Climate Change and Society

LEUNG Wing-mo

“Climate is what you expect; weather is what you get.”-Robert A. Heinlein, science fiction writer

- John Wallace, atmospheric scientist

“Climate lasts all the time and weather only a few days.”-Mark Twain

“By climate we mean the average weather as ascertained by

many years’ observations. Climate also takes into account the

extreme weather experienced during that period. Climate is what

on an average we may expect, weather is what we actually get.”-Andrew John Herbertson, Geography Reader, Oxford University

Weather is the minute-to-minute, day-to-day state of the local atmosphere.

Climate is determined by the long-term (time) pattern of the averages of

temperature, precipitation etc and their extremes at a location (space).

Climate describes the long-term character (normally a 30-year period,

updated every 10 years) of all weather variations + extremes, and their

frequencies of occurrence.

Climate Variability and Climate Change

• Climate Variability: The range of fluctuations around long-term average climate conditions. It refers to variations in the state of climate beyond that of individual weather events.

Drivers of climate variability include El Nino Southern Oscillation (ENSO) and other phenomena. They produce warm, cool, wet or dry periods across different regions and are a natural part of climate variability.

• Climate Change: A significant and persistent change in the mean state of the climate or its variability. Climate change occurs in response to changes in some aspect of Earth’s environment.

It can include changes in temperature, precipitation, increase in occurrence of extreme weather events, as well as increased/decreased variability.

Climate Variability and Climate Change

Normals

Climate Change

Climate Oscillations

Climate VariabilityNormals

Short term: (years to decadal) rises and falls about the trend line (El Nino Southern Oscillation, ENSO)

Long Term Trends or major shifts in climate: (centuries)

Multi-decadal oscillations in regional climate: (e.g. Pacific Decadal Oscillation, PDO, North Atlantic Oscillation, NAO)

One form of climate change

Distribution of a weather parameter (say, temperature)

cold hot

Pro

bab

ility

of

occ

urr

ence

Other modes of climate change

An illustration of our changing climateDistribution of (a) daily minimum and (b) daily maximum

temperatures anomalies (relative to 1961-1990 climatology)

IPCC AR5

HK Weather Rainfall in a certain day of a certain year

HK Climate1981-2010 annual mean rainfall

2398.5mm(HKO)

Climate of Hong KongMonthly Normals (1981-2010)

Hong Kong has a sub-tropical climate (亞熱帶氣候) :HOT & WET in SummerCOOL & DRY in Winter

Climate of Hong KongMonthly Normals (1981-2010)

Most cloudy in SpringThundery in Summer

Climate of Hong KongMonthly Normals (1981-2010)

Strong Monsoon Signal

Tropical Cyclone Warning Signal

SW monsoon

NE monsoon

Monthly mean no. of reported hail

Climatological normals (1981-2010, 1971-2000…)

• Daily/monthly/annual mean rainfall;

• Daily/monthly/annual mean maximum, mean and minimum temperature;

• Daily/monthly/annual mean cloud amount and global solar radiation;

• Daily/monthly/annual mean no. of tropical cyclone warning signals and strong monsoon signals;

• Daily/monthly/annual mean of days with lightning, thunderstorms, fog, hail, tornadoes, frost,…

HKO climatological information webpage: http://www.weather.gov.hk/cis/climat_e.htm

Weather extremes

• Temperature:– 0 deg C (Jan 1893)– 36.1 (Aug 1900, Aug 1990)

• Rainfall:– 145.5 mm/h (Jun 2008) – Typhoon Fengshen (風神)– 534.1 mm/day (Jul 1926)– 1346.1 mm/month (Jun 2008)– 3343.0 mm/year (1997)– 901.1 mm (1963)

• Wind:– Gust at HKO: 259 km/h (Wanda, Sep 1962)– 60-min mean wind at HKO: 133 km/h (Wanda)– Gust at Waglan: 234 km/h (York, Sep 1999)– 60-min mean wind at Waglan (Ellen, Sep 1983)

http://www.hko.gov.hk/cis/normal/1971_2000/5daymean_e.htm

Long Term Trends in HK

Warming trend in Hong Kong (1885-2012)

Rise of 1.2 oC/100 years(0.74 oC in 20th century globally)

An

nu

al m

ean

tem

per

atu

re, H

KO, T

ST

Note the increasing rates of “change” and “variability”

Global temperature anomaly based on “1961 – 1990 normal” (decadal – red, annual – dark blue, linear trend – grey).

Source: WMO, The Global Climate 2001-2010, A decade of climate extremes

C/decade

Decadal global temperature(horizontal line indicates the 1961-1990 long-term average)

Source: WMO, The Global Climate 2001-2010, A decade of climate extremes

201020001980 199019701960195019401930192019101890 1900

JAN

FEB

MAR

APR

NOV

DEC

MAY

OCT

Less and less winter days (Oct - Apr)Lam Chiu-ying sound bite: there will be no winter by 2050

Blue –winter days :mean daily temperature ≤ 18.8 oC

(1971-2000 lowest 25% of daily mean temperature distribution)23

Trends in global mean surface temperature

UK

USA

USA

201020001980 199019701960195019401930192019101890 1900

AUG

APR

SEP

OCT

JUN

JUL

MAY

More and more summer days (Apr - Oct)

Orange – summer days : daily mean temp ≥ 27.7 oC(1971-2000 top 25% of daily mean temperature )

Increase in hot nights and very hot daysDecrease in cold days

No. of hot nights > 28 C

No. of very hot days > 33 C

No. of cold days < 12 C)

Return period analysis

Extreme weather

events

Return period

in 1900

Return period

in 2000

Daily min. temp.

≤4oC6 years 163 years

Daily max. temp

≥35oC32 years 4.5 years

One reason for HK’s larger temperature rising

trend - Urbanization

Heat is effectively released in rural areas at night.

Cooling at night is ineffective in urban areas due to high rises, more heat sources and less trees.

Other effects of urbanization

Wind flow

Higher wind speed conducive to faster heat dissipation and evaporation.

Lower wind speeds results in visibility reduction and less evaporation.

A snap shot of highly urbanized Hong Kong

Limited sky view

Air-conditioners

(heat source)

Street canyon, lack of

open space

Lack of greening

Busy traffic

(heat source)

High density

development

Spatial variation of nighttime temperatures in HK(5 am, 2007)

Diurnal variations of the average THKO and TTKL (1989 – 2007)

Diurnal variations of the rate of change of THKO and TTKL (1989 – 2007)

Diurnal variations of average Tu-r (urban – rural, in units of 0.10C, red denotes +ve, blue denotes –ve, the thick black line denotes Tu-r = 0) of

Hong Kong in different months of the year (1989-2007)

10-minute wind speed at King’s Park and Waglan(1968-2012)

橫瀾島

京士柏

Waglan: no significant trend

King’s Park: decreasing trendRelocation of anemometer

evaporation

rain

evapotranspiration

Sun

condensation

evaporation

Rainfall runoff

Ocean

cloud

Rising temperature enhances the WATER CYCLE

Rising temperatures increases the quantity of water vapour in air

There are now 4% more water vapour in the atmosphere than 30 years ago. A warmer world leads to a more vigorous water cycle, and hence changes in rainfall rate, rainfall amount,

frequency, intensity, time and type. The change is not local, and the changes will not be uniform.

Annual rainfall at Hong Kong Observatory,

Tsimshatsui (1885-2012)

1997 (3343mm)

A rise of 1% per decade

1963 (901mm)

Again, note not just the rising trend, but variability as well

White background indicates increase, yellow background indicates decrease

Regional differences in rainfall trend (1900 – 2005, over land, IPCC AR4)

Hong Kong rain days (daily rainfall>=1mm)(1885-2012)

Note: Total annual rainfall increases while rain days decrease

- 1.1 day/decade

Days of heavy rain (hourly rainfall >30mm) (1885-2012)

+ 0.2 day/decade

Extreme eventReturn period in

1900 (year)Return period in

2000 (year)

Hourly rainfall

≥ 100mm37 18

2-hourly rainfall

≥ 150mm32 14

3-hourly rainfall

≥ 200mm41 21

More frequent extreme rainfall events

42

Highest hourly rainfall record Hong Kong Observatory, Tsimshatsui (1885–2012)

Hours of reduced visibility at Hong Kong Observatory 1968 - 2012

(Visibility < 8km, RH<95%, excluding rain, mist or fog)

+53hour/decade

+ 450 hours/decade

Mean sea level of Victoria Harbour (1954-2012)

Data is probably contaminated by land subsidence in pre-GPS era

No. of tropical cyclones within 500 km of Hong Kong(1961-2012)

A decreasing trend, but not statistically significant at 5% level

5-year moving averageLinear trend

No. of tropical cyclones within 300 km of HK (1961-2012)

5-year moving average

Linear trend

Hong Kong’s changing climate summary

Global warming + urbanization change in HK climate

Parameters Trend

temperature

rainfall

mean sea level

Time of reduced visibility

References

• “Climate change in Hong Kong” Hong Kong Observatory Technical Note 107: http://www.hko.gov.hk/publica/tn/tn107.pdf

• “On climate change brought about by urban living” : http://www.hko.gov.hk/publica/reprint/r700.pdf