CWR 4101 Precipitation

1
CWR 4101 Precipitation

• Dr. Marty Wanielista
• 407.823.4144
• wanielis_at_mail.ucf.edu
• www.stormwater.ucf.edu
• http//classes.cecs.ucf.edu/CWR4101/wanielista

2
Meteorology

• Weather specific short term conditions
• Climate long term conditions
• Measures
• Relative Humidity
• Wind and Temperature
• Precipitation
• Evaporation
• Transpiration

3

• Relative Humidity, f () or degree of saturation
• The ratio of actual vapor pressure to that at
  saturation for a given temperature, expressed as
  a percentage.

? e Actual vapor pressure
?vRvT ? es Saturated vapor pressure which
occurs when the rates of evaporation and
condensation are equal.
Note as f approaches 100 the chance of
precipitation increases
4
Wind And Temperature

• Wind caused by thermal and earth rotation.
• Measured as direction from and speed.
• Low pressure causes counter clockwise movement.
• High pressure causes clock wise movement.
• Temperature measures reported as average,
  maximums and minimums.
• Temperature changes with elevation. Inversions
  occur when temperature decreases with elevation
  increase.
• All have diurnal components. (daily fluctuations)

5
Nucleation Particles
Precipitation Processes

• Nuclei must be present in order to begin the
  raindrop formation process
• Dust particles
• Sulfur
• Any number of particulates
• The nucleation process results in the transport
  of pollutants

6
(No Transcript)
7
Precipitation Formation

• For precipitation formation to occur the
  followings conditions must be present
• Condensation onto nuclei
• Cooling of the atmosphere
• Growth of water droplets
• Mechanism to cause a sufficient density of the
  droplets

8
Droplets form by nucleation
9
Weather Systems

• Convective Storms are the result of warm, humid
  air rising into cooler overlying air. (e.g.
  summer thunderstorm)
• Heat island effects along beaches and in urban
  high impervious areas.

Figure 3.1 on page 77
10
Weather Systems, cont.

• Orographic Storms can be formed if warmer air
  rises over a high geographic feature and the
  rising air mass has a condensation level of
  moisture.

Figure 3.2 on page 77
11
Weather Systems, cont.

• Cyclonic Storms are caused by the rising of air
  as it converges on an area of low pressure. This
  process results in the formation of a front.

12
Weather Systems, cont.

• Types of fronts
• Cold front formed by cold air advancing under
  warmer air-intense and cover small area.
• Warm front formed by warm air advancing over
  colder air less intense and cover larger area.

Figure 3.3 on page 78
13
Weather Systems, cont.

• Tropical Cyclone It is an intense cyclone with
  its source in the tropics regions. Its wind speed
  is generally greater than or equal to 75 mph. It
  can cause a great volume of rainfall in a short
  period of time.

Figure 3.4 on page 79
14
Hurricane Charlie August 2004
15
(No Transcript)
16
(No Transcript)
17
Hurricane Hugo Track (September 11-25, 1989)
Source NHC / NOAA (ftp//ftp.nhc.noaa.gov/pub/sto
rm_archives/atlantic/prelimat/atl1989/hugo)
18
Hurricane Impact Before Hugo (1989)
Folly Beach, SC
Source NOAA Photo Library (http//www.photolib.no
aa.gov/historic/nws/wea00465.htm)
19
Hurricane Impact After Hugo (1989)
Folly Beach, SC
Source NOAA Photo Library (http//www.photolib.no
aa.gov/historic/nws/wea00466.htm)
20
Hurricane Hugo (September 10-22, 1989)

• Death Toll 49 (26 within the U.S.)
• Damages 9.7 billion
• People Evacuated 215,000
• Hurricane Category 5 (Saffir/Simpson Scale)
• Storm Tide 20 ft (Bulls Bay SC)
• Min. Pressure 918 mb
• Max. Wind Speeds 170 mi/hr
• Max. Precipitation 13.5 in (Puerto Rico)

21
Precipitation Measurements

• Point Measurement Rainfall Gages
• Tipping bucket, weighing, and float
• Network of Rainfall Gages
• The number of stations depend on precipitation
  and its variability
• Area Measurement Radar
• Missing Data at a Rainfall Gage
• Source of Data - NOAA, local gages

22
Source of Data

• http//www.cityoforlando.net/public_works/stormwat
  er/rain/rainfall.htm
• http//arcimspub.sjrwmd.com/website/dahds/design/i
  ndex.html
• http//www.crh.noaa.gov/ind/precip.php
• Many other publications, Universities, etc.

23

• Rainfall Depth to Watershed Volumes

If V cubic feet, P inches and A Acres, Then
V 3630PA

• Intensity to Watershed Discharges

If Q cubic feet per second, P inches/hour and
A Acres, Then Q 1.008CiA
24
Presentations (tabular and graphical)

• Point and area extrapolations
• Volume and Rate estimates
• Hyetograph
• Cumulative with time for a single duration
• Dimensionless Graphs
• IDF Curves

25
Interpretation and Quantification of
Precipitation Data

• Records of rainfall data at weather stations are
  used to construct

• Hyetograph (Fig 3.6 on p. 88) 15 minutes
  hyetograph

26
Fig 3.6 Rainfall Intensity

• 25-yr., 6-hr. Orange County Storm (6.0 in.)
• Between hours 2.0 and 2.5 (30 min.)
• (2.22) inches of rain fall that is volume!
• ? (2.22 inches/0.5 hr.) 4.44 in./hr.
• Between hours 0 and 6
• 6 inches of rain fall!
• ? (6 inches/6.0 hr.) 1.00 in./hr.

27

• Problem No. 2 on page 110 (Use spread sheet)

28

• Cumulative Rainfall Diagram (Figure 3.7 on page
  88)

• Duration time when the cumulative rainfall
  diagram (mass diagram) reaches plateau
• Intensity the slope of the cumulative diagram

29

• Dimensionless Cumulative Rainfall Diagram- Since
  maximum rainfall volumes vary for different
  regions, a dimensionless mass diagram is helpful
  in time distribution for any rainfall volume
• Figure 3.8 on page 90 gives rainfall distribution
  as function of locations over the nation. NRCS
  Type 1, Type 1A, Type II, and Type III
• Tables C.1 to C.3 on pages 453 to 455 give the
  data of dimensionless mass diagrams

30
Rainfall Distribution
NRCS Natural Resources Conservation Services
(NRCS)
Refer to Appendix C Table C.1
31
Rainfall Distribution
Specific for Orange County
Constructed based on Table C1 in Appendix C
32
Area Wide Data and Averages

• Point measures (see pages 91-95)
• Arithmetic Average
• Isohyethal (contours of equal precipitation)
• Thiessen
• Distance Square
• Radar is most accurate
• More accurate spread of intensities and volumes.
• Not always available.
• See http//www.wildwildweather.com/radar.htm
• OneRain, Inc http//www.onerain.com/index.htm

33
3. 6 Average Watershed Precipitation

• Arithmetic Average Method

• Isohyetal Method Wi Ai/A
• Pi precipitation of the i-th cell
• Ai area of the i-th cell

• Thiessen Method Wi Ap/A
• Pi gage precipitation for polygon i-
• Ap area of the polygon representing the i-th
  station

34
Intensity-Duration-Frequency Curves

• Duration of a storm is the time that a rainfall
  event lasts.
• Duration is used in concert with intensity, thus
  the intensity is the average for that time
  interval.
• Frequency is the return period to establish the
  average intensity given a particular duration
  storm event.

35

• Data Source NOAA 15 minutes or hourly data,
  e.g., Table 3.8 on page 100
• Graphical Representation e.g., Figures 3.12 and
  3.13 on pages 98 and 99
• Equation Fitting

• E.g., given data in Table 3.9 on page 103, fit
  the equation to obtain a and b in Table 3.0 on
  page 104 for different frequency

36
IDF Intensity, Duration, and Frequency

• Figures C.1 through C.5 on pages page 457 through
  461, respectively.
• REGRESS

• Using Equations

• i average intensity, D duration
• a and b fitting constants for each frequency

• Table 3.10 on Page 104.