Lecture 7 hk climate, its long term trend and variability
1. Lecture 7 Hong Kong’s climate, it’s long term
trends and variability
LSGI1B02 Climate Change and Society
LEUNG Wing-mo
2. “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.
3. 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.
4. Climate Variability and Climate Change
Normals
Climate Change
Climate Oscillations
Climate Variability
Normals
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)
5. One form of climate change
Distribution of a weather parameter (say, temperature)
cold hot
Probabilityofoccurrence
7. An illustration of our changing climate
Distribution of (a) daily minimum and (b) daily maximum
temperatures anomalies (relative to 1961-1990 climatology)
IPCC AR5
14. 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
15. 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)
20. Warming trend in Hong Kong (1885-2012)
Rise of 1.2 oC/100 years
(0.74 oC in 20th century
globally)
Annualmeantemperature,HKO,TST
Note the increasing rates of “change” and “variability”
21. 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
22. Decadal global temperature
(horizontal line indicates the 1961-1990 long-term average)
Source: WMO, The Global Climate 2001-2010, A decade of climate extremes
26. Increase in hot nights and very hot days
Decrease in cold days
No. of hot
nights > 28 C
No. of very hot
days > 33 C
No. of cold
days < 12 C)
27. Return period analysis
Extreme weather
events
Return period
in 1900
Return period
in 2000
Daily min. temp.
≤4o
C
6 years 163 years
Daily max. temp
≥35o
C
32 years 4.5 years
28. 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.
29. 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.
30. 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
34. 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)
35. 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
37. 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
38. White background indicates increase, yellow background indicates decrease
Regional differences in rainfall trend
(1900 – 2005, over land, IPCC AR4)
39.
40. Hong Kong rain days (daily rainfall>=1mm)
(1885-2012)
Note: Total annual rainfall increases while rain days decrease
- 1.1 day/decade
41. Days of heavy rain (hourly rainfall >30mm)
(1885-2012)
+ 0.2 day/decade
42. Extreme event
Return period in
1900 (year)
Return period in
2000 (year)
Hourly rainfall
≥ 100mm
37 18
2-hourly rainfall
≥ 150mm
32 14
3-hourly rainfall
≥ 200mm
41 21
More frequent extreme rainfall events
42
44. Hours of reduced visibility at Hong Kong Observatory
1968 - 2012
(Visibility < 8km, RH<95%, excluding rain, mist or fog)
+53hour
/decade
+ 450 hours/decade
45. Mean sea level of Victoria Harbour (1954-2012)
Data is probably contaminated by land subsidence in pre-GPS era
46. 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 average
Linear trend
47. No. of tropical cyclones within 300 km of HK
(1961-2012)
5-year moving average
Linear trend
48. Hong Kong’s changing climate summary
Global warming + urbanization change in HK climate
Parameters Trend
temperature
rainfall
mean sea level
Time of reduced visibility
49. 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
