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VI. Precipitation in urban areas
URBAN CLIMATOLOGY
6.1 Urban precipitation
• precipitation is not continuous in time and space
• empirical studies sometimes show contradictory results
• it is hard to separate urban influence from others (position, relief, …)
Modification of precipitation regime in urban environment; a general model adopted
from http://www. ucar.edu/communications/staffnotes/0603/cities.shtml)
Urban precipitation
• It is not clear whether urban environments initialize new
precipitation events or whether they just intensify existing
precipitation
• Most studies proved that precipitation totals in cities and in their
leeward side are 5–15% higher compared to rural areas
• The summer is the time of maximum urban effect on precipitation
• Precipitation anomaly in La Porte,
USA (Changnon 1968) and project
METROMEX
Urban precipitation
Precipitation in urban areas is modified due to three different
effects:
• thermal effect (UHI and stronger convection in summer
-> more showers and thunderstorms)
• mechanical effect (higher roughness -> lower velocity of
atmospheric fronts -> more precipitation)
• pollution effect (more condensation nuclei)
• Due to UHI there is lower proportion of precipitation in
the form of snow
• Ice particles of anthropogenic origin -> condensation
nuclei for stratus clouds -> more frequent light snowfall
in city
6.2 Water runoff in urban environment
Before and after urbanisation hydrograph (adopted from
Christopherson 1997)
6.3 Precipitation regime in Brno
Network of stations in Brno area and its temporal development in the
period 1890–2012: a) period of measurements; b) number of stations
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Long term variability of precipitation in Brno
Fluctuation of mean annual and seasonal precipitation totals in Brno in the
period 1803–2010; annual values smoothed with Gaussian filter for 20 years
are in bold and horizontal line is the mean for 1961–2010 reference period
Comparison of mean annual
variation of precipitation at
selected stations in Brno in
the period 1961–2010
Precipitation regime in Brno
Spatial distribution of
mean annual precipitation
totals in Brno region in
the period 1961–1990
Number of rainy days
Variability in annual number of days with precipitation totals ≥ 0.1, 1.0, 10.0
and 20.0 mm including linear trends at Tuřany station, period 1961–2010
Variability in annual number of days with precipitation totals ≥ 0.1, 1.0, 10.0
and 20.0 mm including linear trends at Tuřany station in the period 1961–
2010
Number of rainy days
Maximum daily precipitation totals
Maximum daily precipitation totals and assessment of mean return periods
at selected stations in Brno in the period 1961–2010
Variability of maximum daily precipitation totals including linear trends at
two stations in Brno in the period 1961–2010
Rainy (dry) periods
Number of precipitation (above) and dry (below) periods at selected
stations in Brno in individual decades of the period 1961–2010
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6.4 Final remarks and questions
1. What are the main impacts of changed precipitation regime
on people living in cities?
2. How we can define extremity of precipitation regime?
3. What is the role of other factors such as relief, position,
land use etc.?
4. How can be negative effects mitigated in urban-planning
design?
• Higher probability of occurrence of short-term extreme
precipitation totals and flash floods
• Longer periods without any precipitation, higher probability of
drought occurrence
• Non-uniform precipitation distribution during the year
Urban precipitation and Global warming projections