Precipitation Rate objectives:

1
ATMS 320 Meteorological Instrumentation

• Precipitation Rate objectives
• Learn how precipitation rate is measured
• Know the limitations of these measurements
• Appreciate the unique calibration, exposure, and
  error sources of the various measurement
  techniques

http//www.msc-smc.ec.gc.ca/education/imres/42_ins
truments_e.cfm
2
ATMS 320 Precipitation Rate

• Precipitation varies significantly by region
• Traditionally measure precip by covering a large
  area using many distributed point measurements
• Remotely sense precip having large areal coverage
  (radar or satellite)

http//www.art.com/
3
ATMS 320 Precipitation Rate

• Precipitation rate
• The depth to which a flat horizontal surface
  would have been covered per unit time if no water
  were lost by run-off, evaporation, or percolation

http//www.gov.mb.ca/conservation/watres/water_gui
de/cycgif.html
4
ATMS 320 Precipitation Rate

• Point precipitation measurement accumulation
  gauges
• Collect precip and hold it, usually as water,
  until it is emptied either manually or
  automatically
• Recording or non-recording
• Measurand is precip rate (R) and output is depth
  of water in the gauge, h1 in meters.

evaporation causes h1(t) to decrease slightly
with time when R 0
5
ATMS 320 Precipitation Rate

• Point precipitation measurement accumulation
  gauges (cont.)
• High resolution measurement of h1 can be quite
  difficult with a simple bucket gauge because it
  is difficult to determine h1with sufficient
  accuracy

http//amsr-gv.gsfc.nasa.gov/Gauge/DAILY/ERK_003B_
200302.DAILY.gif
6
ATMS 320 Precipitation Rate

• Accumulation gauges fence post gauge
• A small amount of rain falling into this gauge
  makes h2 larger than the equivalent h1, thus
  easier to read
• Variable amplification since the amplification
  ratio decreases as the gauge fills

http//www.weatherequipment.com/rainfall.htm
7
ATMS 320 Precipitation Rate

• Point precipitation measurement accumulation
  gauges (cont.)
• Non-recording gauge, so a person must read it
  periodically

Ac
Am
8
ATMS 320 Precipitation Rate

• Point precipitation measurement weighing gauge
• Springs and levers convert weight to displacement
  which is converted to a voltage

9
ATMS 320 Precipitation Rate

• Point precipitation measurement accumulation
  gauges (cont.)
• Automatic reset gauges
• Pressure gauge (Fig. 9-1a)
• Siphon gauge (Fig. 9-3)
• Tipping bucket gauge (Fig. 9-5)

10
ATMS 320 Precipitation Rate

• Point precipitation measurement Automatic reset
  gauges
• Pressure gauge (Fig. 9-1a)
• Signal output will exhibit a small diurnal cycle
  when there is no rain

Aneroid sensor connected to gauge via tubing at
the bottom. Transducers are used to produce
voltage output. Some provision must be made to
empty the gauge when it fills up in order to make
this gauge fully automatic
11
ATMS 320 Precipitation Rate

• Point precipitation measurement Automatic reset
  gauges
• Siphon gauge (Fig. 9-3)
• Rain that collects in the funnel drains into a
  measuring chamber. When the chamber is full, the
  addition of more water starts a siphon that
  drains the water out of the chamber. Measuring
  chamber is equipped with a capacitive transducer
  that senses the depth of water in the gauge.

12
ATMS 320 Precipitation Rate

• Point precipitation measurement Automatic reset
  gauges
• Siphon gauge

13
ATMS 320 Precipitation Rate

• Point precipitation measurement Automatic reset
  gauges
• Siphon gauge issues
• Loses any rain that falls during the period when
  the siphon is emptying (30 s)
• Must be kept very clean to maintain the siphon
  process
• Must be heated to protect it from freezing

14
ATMS 320 Precipitation Rate

• Point precipitation measurement Automatic reset
  gauges
• Tipping bucket gauge (Fig. 9-5)
• Incoming water falls into one of the buckets.
  When it is full, its center of gravity is outside
  the point of support and it tips, dumping the
  water collected and bringing the other bucket
  into position to collect water. When bucket tips,
  it momentarily closes a reed or mercury switch.

15
ATMS 320 Precipitation Rate

• Point precipitation measurement Tipping bucket
  gauge
• Time required to fill a bucket, TB, is given by

16
ATMS 320 Precipitation Rate

• Point precipitation measurement Tipping bucket
  gauge
• Temporal granularity due to the time period over
  which pulses are counted
• Amplitude granularity due to the bucket size and
  time period used

a.g.
t.g.
17
ATMS 320 Precipitation Rate

• Tipping bucket gauge issues
• Under-reporting of rainfall when the rain rate is
  too light (evaporation) or too heavy (splashing)
• Adding a heater causes under-reporting

http//www.usatoday.com/weather/wtipgage.htm
18
ATMS 320 Precipitation Rate

• Point precipitation measurement snow gauges
• Snow can plug tubes
• Susceptible to be blown into or out of the gauge
  by strong winds

http//www.coscosci.com/products/rain_snow_gauge.h
tm
19
ATMS 320 Precipitation Rate

• Snow gauges liquid-water content measurement
  techniques
• Weight of snow (snow pillow, see to the right)
• Collect in open gauge and melt it
• Detect water attenuation of gamma radiation

http//wsoweb.ladwp.com/Aqueduct/snow/pillow.htm
20
ATMS 320 Precipitation Rate

• Point precipitation measurement optical rain
  gauge
• Precipitation particles passing through the beam
  cause scintillation (rapid fluctuation) of the
  light received at the detector

21
ATMS 320 Precipitation Rate

• Point precipitation measurement optical rain
  gauge
• Amplitude and frequency of scintillation is a
  function of
• Drop size
• Fall speed of the drop
• The number of drops in the beam at an instant

http//www.opticalscientific.com/Org.htm
22
ATMS 320 Precipitation Rate

• Point precipitation measurement optical rain
  gauge
• scintillation spectral variance measured in two
  wave bands
• 1 to 4 kHz proportional to the rain rate
• 25 to 250 Hz along with signal strength in first
  band is used to discriminate between rain and snow

http//www.opticalscientific.com/Org.htm
23
ATMS 320 Precipitation Rate

• Point precipitation measurement optical rain
  gauge
• Introduction of a horizontal slot between the
  lens and the photodetector makes the gauge
  sensitive only to the vertical component of the
  precipitation particle velocity (eliminates
  blowing snow)
• Capable of detecting rain rates of 0.01 to 3000
  mm/h and snow water equivalent rates of 0.005 to
  300 mm/h

http//www.opticalscientific.com/Org.htm
Note skip optical rain gauge simulation on
p.175- 177
24
ATMS 320 Precipitation Rate

• Point precipitation measurement calibration
• Tipping bucket pour a measured amount of water
  slowly into the gauge and count the number of
  bucket tips produced
• Or the high tech approach

http//images.amazon.com/images/P/B00000IJVE.01.LZ
ZZZZZZ.jpg
25
ATMS 320 Precipitation Rate

• Point precipitation measurement calibration
• Siphon water out of the flask at a rate
  controlled by the valve
• Data logger counts the bucket tipping rate
• Computer determines the mass loss of water from
  the flask per unit time, using the digital
  balance
• Equivalent rain rate can be determined
• Gauge can be tested over a wide range of rain
  rates by using various valve settings

An advantage of the high tech calibration
technique erratic or noisy behavior of the
gauge can be readily detected
26
ATMS 320 Precipitation Rate

• Point precipitation measurement exposure
• Ideal area free from obstructions that would
  create large eddies that deflect the flow and
  where horizontal winds are light
• Set the gauge orifice a few feet above ground
  well away from obstructions
• In turbulent flow, small rain drops and snow may
  be deflected out of the gauge and the gauge catch
  reduced
• 20 for wind in the range of 5 to 10 m/s
• Over 80 for winds above 10 m/s

http//www.deltaforce.net/jnu/pg/
27
ATMS 320 Precipitation Rate

• Point precipitation measurement exposure
• Ideal a turf wall around the rain gauge
• Alter windshield
• Nipher gauge is ideal where snow is the
  predominant precip type

snow
28
ATMS 320 Precipitation Rate

• Point precipitation measurement sources of
  error
• Representativeness
• Wind
• Wetting and evaporation
• Splash out
• Plugging
• Dew accumulation

http//www.columbuslibrary.org/cmlpcbk/browseresul
ts.cfm?searchterm98
29
ATMS 320 Precipitation Rate

• Sources of error (cont.) unique to gauge type
• Tipping bucket loss at high rain rates, bucket
  jams
• Pressure, siphon, and weighing gauges
  temperature sensitivity proportional to the
  diurnal cycle, sensitive to wind flow over the
  gauge that can register a false signal, fail to
  record precip while emptying

http//www.city-interactive.com/sud/sud_shot09.jpg
30
ATMS 320 Precipitation Rate

• Error impacts
• Representativeness either over- or
  under-estimation
• Dew formation, temperature sensitivity, and wind
  pumping slight overestimation
• All other error underestimation, mostly due to
  exposure problems

http//www.suddenimpact.com/
31
ATMS 320 Precipitation Rate

• Radar rain measurement
• Complements ground or point measurement
  techniques that are severely compromised by the
  under-sampling problem
• Provides complete areal coverage and timely
  reporting

http//www.erh.noaa.gov/radar/latest/DS.p19r0/si.k
gsp.shtml
32
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• Used to estimate raindrop concentration in the
  atmosphere from the strength of the returned
  signal
• Requires some assumptions about the raindrop size
  distribution and fall rate

http//www.roc.noaa.gov/
33
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• Single radar can measure rainfall over an area of
  at least 70,000 square kilometers
• NWS WSR-88D radar network can estimate area
  rainfall over most of the US

34
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• Weather radar equation

Radar beam would propagate in a straight line but
for the variation of atmospheric density with
height that causes a gradient in the index of
refraction which, in turn, causes the beam to
bend down slightly even in normal
atmospheric conditions. Earths curvature causes
beam to appear to bend up.
35
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• radar turns on transmitter for a short period of
  time (pulse width) peak power during this period
  is Pt
• Electromagnetic pulse is transmitted at 767 to
  3067 ms
• Between pulses, radar listens for any signal
  reflected back from particles in the atmosphere
  that were illuminated by the transmitted beam

36
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• Return echo at T1 is detected after 600 ms
• If pulse repetition interval is 1 ms, max
  unambiguous range is 150 km
• Echo from T2 is masked by next transmission
• Echo from T3 at about 170 km returns after the
  next pulse has been transmitted
• Radar cannot tell whether the target is T3 or T3

37
ATMS 320 Precipitation Rate
http//www.airs-icing.org/publications/Korolev-QJR
MS2000-126-IceParticleHabitsinStratiformClouds.pdf

• Radar rain measurement (cont.)

(empirical Marshall-Palmer formula)
38
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.)
• Pr ? Z ? R
• Z ? R
• Relationship depends on type of rain and varies
  with storm type and location within the storm

http//www.srh.noaa.gov/radar/latest/DS.p19r0/si.k
lzk.shtml
Previous equations were derived under the
assumptions (1) the beam at the target range is
uniformly filled with water or ice
particles with a certain size distribution
implied by the Z-R relationship (2) there
is nothing to absorb or scatter the beam between
the radar and the target
39
ATMS 320 Precipitation Rate

• Radar rain measurement (cont.) difficulties
• Calibration
• Unknown drop size distribution
• Horizontal and vertical winds
• Attenuation by atmospheric gases, rain, and a
  wetted radome (covering)
• Reflectivity enhancement (bright band associated
  with melting hydrometeors)
• Incomplete beam filling
• Evaporation and rain rate gradients

http//www.noaanews.noaa.gov/stories/images/pacjet
-021301.gif