lecture 7 hk climate, its long term trend and variability
TRANSCRIPT
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Lecture 7 Hong Kong’s climate, it’s long term
trends and variability
LSGI1B02 Climate Change and Society
LEUNG Wing-mo
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“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.
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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.
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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)
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One form of climate change
Distribution of a weather parameter (say, temperature)
cold hot
Pro
bab
ility
of
occ
urr
ence
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Other modes of climate change
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An illustration of our changing climateDistribution of (a) daily minimum and (b) daily maximum
temperatures anomalies (relative to 1961-1990 climatology)
IPCC AR5
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HK Weather Rainfall in a certain day of a certain year
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HK Climate1981-2010 annual mean rainfall
2398.5mm(HKO)
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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
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Climate of Hong KongMonthly Normals (1981-2010)
Most cloudy in SpringThundery in Summer
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Climate of Hong KongMonthly Normals (1981-2010)
Strong Monsoon Signal
Tropical Cyclone Warning Signal
SW monsoon
NE monsoon
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Monthly mean no. of reported hail
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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
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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)
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http://www.hko.gov.hk/cis/normal/1971_2000/5daymean_e.htm
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Long Term Trends in HK
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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”
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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
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Decadal global temperature(horizontal line indicates the 1961-1990 long-term average)
Source: WMO, The Global Climate 2001-2010, A decade of climate extremes
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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
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Trends in global mean surface temperature
UK
USA
USA
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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 )
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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)
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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
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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.
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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.
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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
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Spatial variation of nighttime temperatures in HK(5 am, 2007)
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Diurnal variations of the average THKO and TTKL (1989 – 2007)
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Diurnal variations of the rate of change of THKO and TTKL (1989 – 2007)
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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)
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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
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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.
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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
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White background indicates increase, yellow background indicates decrease
Regional differences in rainfall trend (1900 – 2005, over land, IPCC AR4)
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Hong Kong rain days (daily rainfall>=1mm)(1885-2012)
Note: Total annual rainfall increases while rain days decrease
- 1.1 day/decade
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Days of heavy rain (hourly rainfall >30mm) (1885-2012)
+ 0.2 day/decade
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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
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Highest hourly rainfall record Hong Kong Observatory, Tsimshatsui (1885–2012)
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Hours of reduced visibility at Hong Kong Observatory 1968 - 2012
(Visibility < 8km, RH<95%, excluding rain, mist or fog)
+53hour/decade
+ 450 hours/decade
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Mean sea level of Victoria Harbour (1954-2012)
Data is probably contaminated by land subsidence in pre-GPS era
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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
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No. of tropical cyclones within 300 km of HK (1961-2012)
5-year moving average
Linear trend
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Hong Kong’s changing climate summary
Global warming + urbanization change in HK climate
Parameters Trend
temperature
rainfall
mean sea level
Time of reduced visibility
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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