1. Subject: dramatic video footage of urban flash flooding from 10 Jan 2011
Date: Wed, 12 Jan :17:
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
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.

2. 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-duration-frequency graphs
Be able to calculate area average precipitation
CEE 3430 – Spring 2011

3. An Example (problems 1.13 and 1.14 actually)
Time P
min in 15
0.1 30
0.4 45 1 60
1.5 75
1.8 90 2
105
2.2
120
2.3
135
2.4
Time Q hr
cfs 1
100 2
200 3
400 4
800 5
700 6
550 7
350 8
250 9
150 10 11 50 12
Calculate
Cumulative Mass Curve
Rainfall Hyetograph
Infiltration Volume
Runoff Ratio
CEE 3430 – Spring 2011

4. 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.

5. From Dingman, 1994

6. General Circulation of the Atmosphere, Bedient Fig 1-3a

7. Three-Cell Model
From Simon Wang

8. Three-Cell Model: Scientific evolution
Halley
Earth’s rotation and
the conservation of
linear momentum
cause the Trade Winds
Hadley
Coriolis force deflects
winds toward the east
and pulls air from south
+ Conservation of
angular momentum
Ferrel
Thermally direct
circulation forcing
air towards equator
170 years!
Slide from Simon Wang

9. Coriolis Effect
CEE 3430 – Spring 2011

10. Formation of Precipitation
Source of moisture
Lifting mechanism (orographic or heating)
Phase change from vapor to water - Energy
Small nuclei or dust for droplet formation
Droplets must grow as they fall to earth
From Bedient

11. Lifting Mechanisms
From Bedient

12. ATMOSPHERIC STABILITY
Dry Adiabatic Lapse Rate (10 0C/km)
Environmental Lapse Rate
Altitude
Moist Adiabatic Lapse Rate (6 0C/km). Lower due to release of latent heat of condensation from moist air
Temp
Modified from Bedient

13. Atmospheric Stability
Stable
Unstable
Ambient lapse rate < adiabatic lapse rate of lifted parcel
Ambient lapse rate > adiabatic lapse rate of lifted parcel
From Brutsaert, 2005

14. How much water can the air hold?:
                                                                                                                                                                         
How much water can the air hold?
Saturation vapor pressure es(t). The maximum vapor pressure that is thermodynamically stable. Td ea Ta
es(Ta)
mb, for T in oC
See Goff-Gratch (1946) for more precise equation or Lowe (1977) for polynomials for efficient evaluation
Relative humidity. Vapor pressure relative to saturation vapor pressure. (usually expressed as %)
Dew point. Td. The temperature to which a parcel of air has to be cooled at constant (vapor) pressure to reach saturation

15. Ralph, F. M. , P. J. Neiman, G. A. Wick, S. I. Gutman, M. D
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: /2006GL

16. Major Thunderstorm
From Bedient

17. Fronts and Low Pressure
Cold/Warm Front
Lifting/Condensation
High and Low Pres
Rainfall Zone
Circulation Issues
Main weather makers
From Bedient

18. Hurricane Katrina in the Gulf - 8/29/2005
From Bedient
Katrina at 17:00 UTC

19. Measuring Rainfall - Tipping
Bucket
Recording gage
Collector and Funnel
Bucket and Recorder
Accurate to .01 ft
Telemetry- computer
From Bedient

20. 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

21. Mass Curves & Rainfall Hyetographs
From Bedient

22. 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

23. Design Rainfalls Design Storm from HCFCD and NWS
Based on Statistical Analysis of Data
5, 10, 25, 50, 100 Year Events
Various Durations of 6 to 24 hours
Six Hour Rainfall
From Bedient

24. Rainfall Averaging Methods
From Bedient
Rainfall Averaging Methods

25. 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

26. 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

28. 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