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

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

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 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

Coriolis Effect http://www.youtube.com/watch?v=_36MiCUS1ro&feature=related CEE 3430 – Spring 2011

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

Lifting Mechanisms From Bedient

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

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

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

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: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

Hurricane Katrina in the Gulf - 8/29/2005 From Bedient 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 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

Rainfall Averaging Methods 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