Download Weather and Precipitation

Subject: dramatic video footage of urban flash flooding from 10 Jan 2011
Date:
Wed, 12 Jan 2011 10:17:57 +1100
Two days ago there was a dramatic flash flood in Toowoomba (pop 130,000) in the
mountains west of Brisbane, Australia. The water came up in about an hour and was
in the middle of the day so there is amazing amateur video footage. One of the best is
of the rising limb downtown
http://www.youtube.com/watch?v=kYUpkPTcqPY&feature=player_embedded
A good reason not to have car parking next to storm water drains. Not an uncommon
problem in urban areas as we had similar problems in Newcastle two years ago with
storm drains, bridges and culverts clogged with cars making the flooding much worse.
Prof Garry Willgoose,
Director, Centre for Climate Impact Management (C2IM),
The University of Newcastle, Australia.
Weather and Precipitation
Learning Objectives
• Be able to describe weather systems responsible for
precipitation, explain the basic processes and
concepts involved and calculate atmospheric
properties involving water in the atmosphere
– Humidity, vapor pressure, dew point, latent heat, stability
• Be able to interpret point precipitation measurements
by plotting cumulative and intensity hyetographs
• Be able to construct and interpret intensity-durationfrequency graphs
• Be able to calculate area average precipitation
CEE 3430 – Spring 2011
David Tarboton 2
An Example (problems 1.13
and 1.14 actually)
Time P
min in
0
15
30
45
60
75
90
105
120
135
0
0.1
0.4
1
1.5
1.8
2
2.2
2.3
2.4
Time Q
hr
cfs
0
0
1 100
2 200
3 400
4 800
5 700
6 550
7 350
8 250
9 150
10 100
11
50
12
0
CEE 3430 – Spring 2011
Calculate
• Cumulative Mass Curve
• Rainfall Hyetograph
• Infiltration Volume
• Runoff Ratio
David Tarboton 3
Why Study
Weather/Climate?
Hydrology as we know it is driven by the climate,
primarily precipitation, but also temperature and
radiation. To understand the variability in hydrology we
need to understand something about the weather and
climate.
From Dingman, 1994
General Circulation of the Atmosphere,
Bedient Fig 1-3a
Three-Cell Model
From Simon Wang
Three-Cell Model: Scientific evolution
Halley
Thermally direct
circulation forcing
air towards equator
Hadley
Earth’s rotation and
the conservation of
linear momentum
cause the Trade Winds
Ferrel
Coriolis force deflects
winds toward the east
and pulls air from south
+ Conservation of
angular momentum
170 years!
Slide from Simon Wang
Coriolis Effect
http://www.youtube.com/watch?v=_36MiCUS1ro&feature=related
CEE 3430 – Spring 2011
David Tarboton 9
Formation of
Precipitation
1. Source of moisture
2. Lifting mechanism (orographic or heating)
3. Phase change from vapor to water - Energy
4. Small nuclei or dust for droplet formation
5. Droplets must grow as they fall to earth
From Bedient
Lifting Mechanisms
From Bedient
ATMOSPHERIC STABILITY
Dry Adiabatic Lapse
Rate (10 0C/km)
Environmental Lapse Rate
Moist Adiabatic Lapse Rate (6 0C/km).
Lower due to release of latent heat of
condensation from moist air
Temp
Modified from Bedient
Atmospheric Stability
Stable
Ambient lapse rate <
adiabatic lapse rate of
lifted parcel
From Brutsaert, 2005
Unstable
Ambient lapse rate >
adiabatic lapse rate of
lifted parcel
How much water can the air hold?
Saturation vapor pressure es(t). The maximum vapor pressure that is
thermodynamically stable.
4278.6 

es (T )  2.7489 108 exp  

T

242
.
79


mb, for T in oC
See Goff-Gratch (1946) for more precise
equation or Lowe (1977) for polynomials for
efficient evaluation
es(Ta)
ea
Td
Ta
Relative humidity. Vapor pressure relative to saturation vapor
pressure. (usually expressed as %)
RH 
e
es (T )
Dew point. Td. The temperature to which a parcel of air has to be cooled at
constant (vapor) pressure to reach saturation
Ralph, F. M., P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, and A. B. White (2006), Flooding on
California's Russian River: Role of atmospheric rivers, Geophys. Res. Lett., 33, L13801, doi:10.1029/2006GL026689.
Major Thunderstorm
From Bedient
Fronts and Low Pressure
• Cold/Warm Front
• Lifting/Condensation
• High and Low Pres
• Rainfall Zone
• Circulation Issues
• Main weather makers
From Bedient
From Bedient
Hurricane Katrina in the
Gulf - 8/29/2005
Katrina 08-28-2005 at 17:00 UTC
Measuring Rainfall - Tipping
Bucket
• Recording gage
• Collector and Funnel
• Bucket and Recorder
• Accurate to .01 ft
• Telemetry- computer
From Bedient
The Hyetograph
 Graph of Rainfall Rate (in/hr) vs Time (hr) at a
single gage location
 Usually plotted as a bar chart of gross RF
 Net Rainfall is found by subtracting infiltration
 Integration of Net Rainfall over time =
Direct RO Vol (DRO) in inches over a
Watershed
From Bedient
Mass Curves &
Rainfall Hyetographs
From Bedient
Intensity-Duration-Frequency
• IDF design curves
• All major cities
• Based on NWS data
• Various return
periods & durations
• Used for
drainage design of pipes
& roads
• Used for
floodplain designs watersheds
From Bedient
Design Rainfalls
 Design Storm from
 5, 10, 25, 50, 100
Year Events
 Various Durations
100 Year Storm
8.4 inches
3.50
25 Year Storm
6.6 inches
3.00
Rainfall (in)
HCFCD and NWS
 Based on
Statistical Analysis
of Data
4.00
10 Year Storm
5.6 inches
2.50
5 Year Storm
4.8 inches
2.00
1.50
1.00
0.50
0.00
0:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
Time
of 6 to 24 hours
Six Hour Rainfall
From Bedient
5:30
6:00
From
Bedient
Rainfall Averaging Methods
Thiessen Polygons - Areal
Average Rainfall from Gages
• Connect gages with lines
• Form triangles as shown
• Create perpendicular
bisectors of the triangles
• Each polygon is formed by
lines and WS boundary
P = S (Ai*Pi) / AT
From Bedient
RADAR Rainfall Estimates
 NEXRAD provides real-time data
on a ~16 km2 (6 mi2) grid
 Each estimate represents an
average rainfall amount over the
entire 4 x 4 km2 area
 NEXRAD rainfall estimates
compare well with point rain gage
measurements (r2 ~ 0.9)
From Bedient
Summary
• The atmosphere is the major link between oceans
and continents
• The hydrologic cycle is shaped by conditions in the
atmosphere with precipitation as the major input
• Precipitation due to atmospheric moisture, lifting,
cooling, condensation, release of latent heat,
instability
• Precipitation variability is summarized statistically in
intensity-duration-frequency curves used for design
• Spatial averaging methods are used to calculate
precipitation over a watershed
CEE 3430 – Spring 2011
David Tarboton 28