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Atmospheric Instrumentation M. D. Eastin
Outline
Measurement of Precipitation
• Review of Precipitation
• Precipitation Gauges• Accumulation• Tipping Bucket• Optical Disdrometer• Snowfall
• Exposure / Measurement Errors
Atmospheric Instrumentation M. D. Eastin
Definitions and Concepts:
Precipitation Rate: Mass flow rate of liquid or solid water crossinga horizontal plane per unit time
Depth to which a flat horizontal surface wouldbe covered per unit time if no water were lost by run-off or evaporation
where: R = precipitation rate (mm hr -1 OR mm day-1) Mw = mass flow rate of water (kg m-2 s-1) ρw = density of water (kg m-3)
SI unit: millimeters per unit time (mm hr -1 OR mm day-1)[ millimeters and hour or day are used ][ to make the numbers “manageable” ]
Meteorology: 1 inch = 25.4 mm1 day = 24 hour1 day = 86400 seconds
Instrument: Precipitation Gauge or Disdrometer
Review of Precipitation
w
wMR
Atmospheric Instrumentation M. D. Eastin
Definitions and Concepts:
•Precipitation is observed 3-5 days per week at a typical location in the United States•Most precipitation rates are less than 10 mm hr -1 (~0.25 inches hr -1)•Extreme precipitation rates can reach 200 mm hr -1 (~5.0 inches hr -1)•Precipitation gauges should exhibit a dynamic range → 0 – 200 mm hr -1
Review of PrecipitationDefinitions and Concepts:
Atmospheric Instrumentation M. D. Eastin
Definitions and Concepts:
•Some tropical locations experience daily precipitation•Most precipitation rates are less than 10 mm hr -1 (~0.25 inches hr -1)•Extreme precipitation rates can reach 400 mm hr -1 (~10.0 inches hr -1)
Review of PrecipitationDefinitions and Concepts:
Atmospheric Instrumentation M. D. Eastin
Accumulation Gauges:
•Collects precipitation in a containerand holds its, usually in the form ofliquid water, until the gauge is emptiedeither manually or automatically
•Measurements are based on heightof the water (h) in the gauge over thetime interval between successivemeasurements
•Automated gauges will empty at regular time intervals (dt = constant)
•Manual gauges (most common) are often emptied at irregular time intervals (perhaps once a day -- more often during intense precipitation)
Precipitation Gauges
t
dttRth0
)()(
Atmospheric Instrumentation M. D. Eastin
Accumulation Gauges:
•The U.S. has two national networks of manual rain gauges:
(1)The NOAA Cooperative Observer Program (COOP) consists of over 8700 volunteers who are expected to record relevant weather and climate observations (including the 24-hr precipitation total) each day
Observers use a “standard” 8-inch diameter (wind-shielded) manual rain gauge (that can be read to within 0.01 inches)
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Accumulation Gauges:
•The U.S. has two national networks of manual rain gauges:
(2)The Community Collaborative Rain, Hail, and Snow (CoCoRaHS) network consists of over 15000 volunteers who report 24-hr precipitation totals when they can – there are no formal expectations
Observers use a high capacity 4-inch diameter manual rain gauge (with a wind shield) (that can be read to within 0.01 inches)
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Accumulation Gauges:
Accuracy ±0.50 mmResolution 0.25 mmResponse Time N/A
Advantages
• Easy to use • Inexpensive• No calibration required• No instrument drift
Disadvantages
•Manual reporting•Irregular observation times•Infrequent observation times•Susceptible to wind errors if not shielded•Susceptible to evaporation errors if not measured
and emptied frequently•Difficult to automate•Difficult to measure frozen precipitation
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Tipping-Bucket Gauge:
•Measures precipitation by collecting rainwaterin a funnel and then passing it to a pair of
smallidentical “buckets” balanced on a yoke
•When one bucket fills with rainwater, the yokepivots to one side, emptying the first bucket
•The second bucket is then able to fill, and whenit’s full, pivoting occurs in the opposite
direction•Each time pivoting occurs, an electric pulse is
generated by a magnetic or optical switchwhich represents a unit of rainfall related tothe bucket volume (B)
•The number of pulses (N) is proportional tothe total precipitation amount (P)
•If the times of each pivot are also recorded, thenthe precipitation rate (R) can be computed
via
Precipitation Gauges
NBP
12 tt
NBR
Atmospheric Instrumentation M. D. Eastin
Tipping-Bucket Gauge:
•It is important to remember thatprecipitation is only “measured”when the bucket fills and pivots
•Thus, precipitation “event” start andstop times are not well recorded
•Typical equivalent bucket depths are0.01 in (or 0.25 mm) – both ASOSand Davis stations – so timing isonly an issue in light precipitation
•Some versions are heated – snowand ice can be readily melted – someasurements of liquid equivalentwater depth can be automatedwithout significant error in eventtiming:
ASOS → HeatedDavis → Not Heated
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Tipping-Bucket Gauges:
•The U.S. has one national network of automated tipping- bucket rain gauges:
(1)The FAA / NWS automated surface weather observing stations (ASOS and AWS) consists of over 5000 stations that regularly report hourly precipitation observations
Stations use a heated tipping-bucket precipitation gauge with a wind shield[ resolution of 0.01 in (or 0.25 mm)]
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Tipping-Bucket Gauges:
Accuracy ±0.50 mmResolution 0.25 mmResponse Time Variable
Advantages
• Easily automated• Frequent observations• Any evaporation errors are minimal• Can measure frozen precipitation (with a heater)
Disadvantages
•Requires significant electric power•Under-reports rainfall in light precipitation•Difficult to measure frozen precipitation (w/o heater)
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Optical Disdrometers:
•Detects the passage of precipitation through a beam of light, causing arapid fluctuation in received light by an opposing detector
•The amplitude and frequency of thelight fluctuations are a function of
1. Drop diameter [ D ]2. Drop fall speed [ w(D) ]3. Drop concentration [ N(D) ]
•These three can be combined toestimate precipitation rate (R)
•A horizontal slot makes the gaugesensitive to only the drop’s verticalcomponent → gauge becomes
insensitive to wind
Precipitation Gauges
dDDwDND
R )()(60
3
LEDDetectorSlot
Atmospheric Instrumentation M. D. Eastin
Optical Disdrometers:
Accuracy ±0.05 mm hr-1
Resolution 0.01 mm hr-1
Response Time 1-5 min
Advantages
• Easily automated• Can measure frozen precipitation• No need for wind corrections
(can be mounted to aircraft)
Disadvantages
•Expensive•Large power consumption•Must manage large volumes of data•Requires more frequent calibration•Requires several minutes to collect
a sufficient number of samplesfor a “stable” mean rain rate
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Snowfall:
•Measured and reported as two metrics
1. Snow Depth [ inches ]2. Liquid Equivalent [ mm ]
•Snow depth is report at least onceper day (new snowfall or not)
•Liquid equivalent can reported daily orat regular intervals if the precipitationgauge is heated (snow / ice will melt)
Accumulation gauges → requires observer to melt snow→ cannot measure snowfall rate
Tipping-bucket gauges→ most are heated (automated melting)→ can measure snowfall rate
Optical disdrometers → no meting required → can measure snowfall rate
Precipitation Gauges
Atmospheric Instrumentation M. D. Eastin
Winds and Turbulent Flow:
•Affects both accumulation and tipping-bucket gauges•Winds around the gauge can cause small drops to be deflected out → underestimate•Experiments suggest the reduction may be 20% for winds ranging from 5 to 10 m/s•Gauges should be sited free from obstructions and shielded
1. Alter wind shield → rain2. Nipher wind shield → snow3. Turf Wall wind shield → rain and snow
Exposure / Measurement Errors
Turf Wall
Alter
Nipher
Atmospheric Instrumentation M. D. Eastin
Evaporation:
•Affects accumulation gauges•Long periods between manual measurements may allow a portion of the collected water to
evaporate (especially in drier climates) → under-estimate of measured precipitation
Splash-Out:
•Affects both accumulation and tipping-bucket gauges•Large water drops hitting the top portion of the funnel may splash and part of the drop may
be ejected → under-estimate of measured precipitation
Plugging:
•Affects both accumulation and tipping-bucket gauges•Funnel becomes clogged by debris (grass, leaves, snow/ice, bird droppings) and collected water is prevented from entering the bucket or water storage area → under-estimate•Can be minimized by placing a “debris screen” on top of the funnel
Dew Accumulation:
•Affects both accumulation and tipping-bucket gauges•Heavy dew formation can accumulate to a measurable extent and be recorded by the human
observer or tipping bucket data logger (as a “trace” of precipitation) → an over-estimate
Exposure / Measurement Errors
Atmospheric Instrumentation M. D. Eastin
Summary
Measurement of Precipitation
• Review of Precipitation
• Precipitation Gauges• Accumulation• Tipping Bucket• Optical Disdrometer• Snowfall
• Exposure / Measurement Errors
Atmospheric Instrumentation M. D. Eastin
References
Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp.
Costello, T. A., and H. J. Williams, 1991: Short duration rainfall intensity measured using calibrated time-of-tip data from a tipping bucket rain gauge. Agriculture and Forest Meteorology, 57, 147-155.
Delany, A. C., and S. R. Semmer, 1998: An integrated surface radiation measurement system. Journal of Atmospheric and Oceanic Technology, 15, 46-53
Golubev, V. S., P. Y. Groisman, and R. G. Quayle, 1992: An evaluation of the United Sates standard 8-in non-recording rain gauge at the Valdai Polygon in Russia. Journal of Atmospheric and Oceanic Technology, 9, 624-629.
Groisman, P.Y., and D. R. legates, 1994: The accuracy of the United States precipitation data. Bulletin of the American Meteorological Society, 75, 215-227.
Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp.
Lindroth, A., 1991: Reduced loss in precipitation measurements using a new wind shield for rain gauges. Journal of Atmospheric and Oceanic Technology, 8, 444-451.
Perrin, T., M. Cabane, A. Rigaud, and C. Pontikis, 1989: An optical device for the measurement of droplet size spectra in clam or low wind conditions. Journal of Atmospheric and Oceanic Technology, 6, 850-860.
Snow, J.T., and S. B. Harley, 1988: Basic Meteorological observations for schools: rainfall. Bulletin of the American Meteorological Society, 69, 497-507.
Wang, T, K.B. Earnshaw, and R. S. Lawrence, 1979: Path-averaged measurements of rain rate and raindrop size distribution using a fast-response optical sensor. Journal of Applied Meteorology, 18, 654-660.