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Precipitation Manual gauges Tipping bucket gauges Weighing gauges Calibration of rain gauges PowerPoint Presentation
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Precipitation Manual gauges Tipping bucket gauges Weighing gauges Calibration of rain gauges Measuring snow. Features of a good rain gauge. Receiving funnel with small hole to reduce evaporation.

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Precipitation Manual gauges Tipping bucket gauges Weighing gauges Calibration of rain gauges


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Presentation Transcript
slide1

Precipitation

  • Manual gauges
  • Tipping bucket gauges
  • Weighing gauges
  • Calibration of rain gauges
  • Measuring snow
features of a good rain gauge
Features of a good rain gauge
  • Receiving funnel with small hole to reduce evaporation.
  • Receiving funnel has larger area than collection container, or collection container narrows toward the bottom. Gives amplification for ease of reading rain amounts.
  • Collection container is shaded to reduce evaporation.
  • What score would you give to the gauge in the picture… ? / 10
slide4

The “gold standard” (but impractical) rain gauge exposure. Pit gauge sits in a hole with its lip at the same level as a wind-baffling grid laid around it.

Some things that might cause errors in rainfall measurements.

  • Nearby obstacles. Minimum recommended ratio of height of nearby objects to distance from gauge is ~1:5.- wind flow around gauge may deflect smaller droplets (under-catch)- wetting and evaporation losses (under-catch)- splash-out from funnel in heavy rain (under-catch)- plugging, then evaporation from funnel (under-catch)

Fig. from Strangeways

tipping bucket rain gauge
Tipping bucket rain gauge

Each tip causes a switch to close and re-open (pulse data).

Rain amount and rate can be determined.

standard tipping bucket gauge
Standard Tipping Bucket Gauge

Percent error

Rain rate (mm/h)

tipping bucket gauge with siphon
Tipping bucket gauge with siphon

Rain rate (mm/h)

Percent error

slide9

Calibration of a Tipping Bucket Rain Gauge

  • You have a TBRG with a 20 cm diameter funnel. It tips and sends a pulse for every 0.2 mm of rainfall
  • You have a calibration device that holds 650 ml of water, and then slowly releases this water into the gauge. What total rainfall will have been simulated when the calibrator is empty?
  • How many tips would you expect if the gauge is working properly?
  • What rainfall rate does your test simulate if the calibrator emptied over a period of 12 minutes?
slide10

Nipher shielded snow gauge

  • central cylinder can be lifted out
  • measure depth of snow in cylinder
  • melt snow in cylinder to get water content
  • turbulence generated by unshielded gauge deflects falling snow
  • unshielded gauge catches only ~60% of snowfall

Wind

single double alter wind shields around weighing gauges
Single & double Alter wind shields around weighing gauges

Wind is deflected down by the blades, to create calmer air at the mouth of the gauge and reduce snow under-catch.

slide12

For electronic recording, a “load cell” positioned under the bucket sends a signal proportional to the weight of precipitation collected.

Older models recorded the weight on a rotating chart.

A weighing gauge for precipitation measurements

Funnel is removed in the version currently used by Met Service, so that both snow and rain are measured. Anti-freeze in bucket in winter melts snow, so gauge measures rain plus snow water content.

slide13

Ultra-sonic snow depth sensor

Time for sound pulse to bounce back to sensor is a measure of the snow depth.

Meteorological Service uses a standard gravel surface under the sensor.

GPS for sensing snow depth

Look at difference between signal received directly from GPS satellite and signal that bounces off snow.

Difference related to distance between GPS receiver and snow surface.

Averages over a larger area than the ultra-sonic sensor.