Atmospheric InstrumentationM. D. Eastin Measurement of Precipitation

Atmospheric InstrumentationM. D. Eastin Outline Measurement of Precipitation Review of Precipitation Precipitation Gauges Accumulation Tipping Bucket Optical Disdrometer Snowfall Exposure / Measurement Errors

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Precipitation Rate:Mass flow rate of liquid or solid water crossing a horizontal plane per unit time Depth to which a flat horizontal surface would be 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 ) M w =mass flow rate of water (kg m -2 s -1 ) ρ w =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 mm 1 day= 24 hour 1 day = seconds Instrument: Precipitation Gauge or Disdrometer Review of Precipitation

Atmospheric InstrumentationM. 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 Precipitation Definitions and Concepts:

Atmospheric InstrumentationM. 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 Precipitation Definitions and Concepts:

Atmospheric InstrumentationM. D. Eastin Accumulation Gauges: Collects precipitation in a container and holds its, usually in the form of liquid water, until the gauge is emptied either manually or automatically Measurements are based on height of the water (h) in the gauge over the time interval between successive measurements 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

Atmospheric InstrumentationM. 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 InstrumentationM. 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 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 InstrumentationM. D. Eastin Accumulation Gauges: Accuracy ±0.50 mm Resolution 0.25 mm Response TimeN/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 InstrumentationM. D. Eastin Tipping-Bucket Gauge: Measures precipitation by collecting rainwater in a funnel and then passing it to a pair of small identical “buckets” balanced on a yoke When one bucket fills with rainwater, the yoke pivots to one side, emptying the first bucket The second bucket is then able to fill, and when it’s full, pivoting occurs in the opposite direction Each time pivoting occurs, an electric pulse is generated by a magnetic or optical switch which represents a unit of rainfall related to the bucket volume (B) The number of pulses (N) is proportional to the total precipitation amount (P) If the times of each pivot are also recorded, then the precipitation rate (R) can be computed via Precipitation Gauges

Atmospheric InstrumentationM. D. Eastin Tipping-Bucket Gauge: It is important to remember that precipitation is only “measured” when the bucket fills and pivots Thus, precipitation “event” start and stop times are not well recorded Typical equivalent bucket depths are 0.01 in (or 0.25 mm) – both ASOS and Davis stations – so timing is only an issue in light precipitation Some versions are heated – snow and ice can be readily melted – so measurements of liquid equivalent water depth can be automated without significant error in event timing: ASOS →Heated Davis →Not Heated Precipitation Gauges

Atmospheric InstrumentationM. 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 InstrumentationM. D. Eastin Tipping-Bucket Gauges: Accuracy ±0.50 mm Resolution 0.25 mm Response 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 InstrumentationM. D. Eastin Optical Disdrometers: Detects the passage of precipitation through a beam of light, causing a rapid fluctuation in received light by an opposing detector The amplitude and frequency of the light 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 to estimate precipitation rate (R) A horizontal slot makes the gauge sensitive to only the drop’s vertical component →gauge becomes insensitive to wind Precipitation Gauges LED Detector Slot

Atmospheric InstrumentationM. 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 samples for a “stable” mean rain rate Precipitation Gauges

Atmospheric InstrumentationM. D. Eastin Snowfall: Measured and reported as two metrics 1. Snow Depth [ inches ] 2. Liquid Equivalent [ mm ] Snow depth is report at least once per day (new snowfall or not) Liquid equivalent can reported daily or at regular intervals if the precipitation gauge 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 InstrumentationM. 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 → rain 2. Nipher wind shield → snow 3. Turf Wall wind shield → rain and snow Exposure / Measurement Errors Turf Wall Alter Nipher

Atmospheric InstrumentationM. 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 InstrumentationM. D. Eastin Summary Measurement of Precipitation Review of Precipitation Precipitation Gauges Accumulation Tipping Bucket Optical Disdrometer Snowfall Exposure / Measurement Errors

Atmospheric InstrumentationM. 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, Delany, A. C., and S. R. Semmer, 1998: An integrated surface radiation measurement system. Journal of Atmospheric and Oceanic Technology, 15, 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, Groisman, P.Y., and D. R. legates, 1994: The accuracy of the United States precipitation data. Bulletin of the American Meteorological Society, 75, 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, 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, Snow, J.T., and S. B. Harley, 1988: Basic Meteorological observations for schools: rainfall. Bulletin of the American Meteorological Society, 69, 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,