Atmospheric InstrumentationM. D. Eastin Measurement of Precipitation

Atmospheric Instrumentation M. D. Eastin

Measurement of Precipitation


Outline


• Review of Precipitation

• Precipitation Gauges• Accumulation• Tipping Bucket• Optical Disdrometer• Snowfall

• Exposure / Measurement Errors


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



•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:



•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:


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

)()(



•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)




•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)




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



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


NBP

12 tt

NBR


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



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)]



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)



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


dDDwDND

R )()(60

3

LEDDetectorSlot


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



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



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


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


Summary


• Review of Precipitation

• Precipitation Gauges• Accumulation• Tipping Bucket• Optical Disdrometer• Snowfall

• Exposure / Measurement Errors


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.

Documents

Atmospheric InstrumentationM. D. Eastin Measurement of Precipitation