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Precipitation. Precipitation. Measurement Types of precipitation Precipitation distribution seasonal regional topographic controls Basin average precipitation. Total Precipitation. Total Precipitation = rain + snow Current standard: 16” diameter PVC standpipe gauge

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precipitation

Precipitation

VFR Research - R. Hudson

precipitation2
Precipitation
  • Measurement
  • Types of precipitation
  • Precipitation distribution
    • seasonal
    • regional
    • topographic controls
  • Basin average precipitation

VFR Research - R. Hudson

total precipitation
Total Precipitation

Total Precipitation = rain + snow

Current standard: 16” diameter PVC standpipe gauge

  • 1-2 metres gauge height
  • charged with antifreeze to melt snow falling into the gauge
  • circulating pump
  • pressure transducer measures gauge depth
  • depth recorded by a data logger

VFR Research - R. Hudson

rainfall
Rainfall

Current and past standard for measuring rain only is the tipping bucket rain gauge

  • rain is funnelled into a collector mechanism that tips back and forth when a specific volume of rain is received
  • each tip activates a reed switch, number of tips are counted by a recorder
  • usually calibrated to 1.0, 0.5 or 0.25 mm rain

VFR Research - R. Hudson

snowfall
Snowfall

Snow is measured as water equivalent in units of depth (mm or cm).

  • AES method at manned sites: limited capacity, manually operated shielded gauges
    • usually emptied daily
    • water equivalent of snow caught in gauge is measured either by weighing or by melting the snow and measuring in a graduated cylinder

VFR Research - R. Hudson

precipitation gauge siting
Precipitation gauge siting

All precipitation gauges experience undercatch

  • wind exacerbates gauge undercatch:
    • causes ppt to fall at an angle less than 90o
      • this affects rain and snow equally
    • the gauge will deform the wind field, creating updrafts and flow acceleration over the gauge orifice that inhibits ppt from falling in the gauge
      • this mostly affects snow, but can affect rain if gauge is poorly sited
  • if possible, gauge should be in sheltered site

VFR Research - R. Hudson

effect of rainfall angle on catch
Effect of rainfall angle on catch
  • Vertical rain sees a circular cross-section on the gauge
    • true “as calibrated” precipitation catch
  • Angular rain sees an elliptical cross-section with a larger area than true area of the orifice
    • a function of the angle at which the rain falls
    • catch ratio = orifice area/area of ellipse

VFR Research - R. Hudson

proper siting of a precip gauge
Proper siting of a precip. gauge

VFR Research - R. Hudson

effects of wind on snow catch
Effects of wind on snow catch
  • Usually, precipitation gauges cannot be sited in small forest clearings, and must be located at open windy sites
    • alteration of the wind field over the gauge orifice affects snow more than rain
    • gauge shields reduce undercatch by reducing wind field deformation over the orifice
    • types of shields:
      • Nipher shield for manually read AES snow gauge
      • Alter shield for remote total ppt gauge

VFR Research - R. Hudson

what causes precipitation
What causes precipitation?

Primary cause of precipitation: lifting of moisture laden air

  • As elevation increases, pressure decreases
    • Boyle’s law: at constant temperature, as pressure increases, the volume decreases
    • Charles’ law: at constant pressure, as temperature changes, volume changes

VFR Research - R. Hudson

types of precipitation
Types of precipitation

Ppt is typed according to the lifting mechanism that generates it

  • Cyclonic: lifting due to convergence into a low pressure cell
    • typical winter rains in coastal B.C.
    • wide spread, low to moderate intensity
  • Frontal: lifting of warm air over colder, denser air at frontal surface

VFR Research - R. Hudson

fronts
Fronts
  • warm front: warm air advances over cold air
    • wide spread - 300 to 500 km ahead of front due to low slope of front (1/100 to 1/300)
    • continuous, low to moderate intensity
  • cold front: cold air advances under warm
    • more localized and more rapid lifting due to steeper frontal surface (1/50 to 1/150)
    • much higher intensities than warm front and less uniform

VFR Research - R. Hudson

convective orographic
Convective / Orographic
  • Convective: lifting of unstable air that is warmer than surrounding air due to uneven surface heating
    • thunder storms
    • spotty and highly variable in intensity
  • Orographic: mechanical lifting over mountains
    • very important in B.C. - controls climate

VFR Research - R. Hudson

precipitation distribution
Precipitation distribution
  • Rain vs. snow
    • simply a function of temperature, governed by season and elevation
  • Spatial distribution
    • topographic control of precipitation
      • local vs. regional
  • Seasonal distribution
  • Rainfall intensity-duration

VFR Research - R. Hudson

precipitation distribution16
Precipitation distribution
  • In B.C., the most important effects are frontal and orographic
    • highest intensity storms on the coast are frontal, but the orographic effect works in conjunction with the front to produce very high volume and intensity of rainfall
    • orography generally defines B.C.’s climate and biogeoclimatic zonation

VFR Research - R. Hudson

local orographic effects
Local orographic effects

VFR Research - R. Hudson

rain shadow effect
Rain shadow effect
  • Moist air is forced over mountain barriers by westerly air flow
  • Ppt falls on windward (i.e., west) side of mountain range while the leeward (eastern) side is warmer and drier
    • strongest contrast in B.C. is the transition from the wet west side of the Coast mountains to the Fraser Valley

VFR Research - R. Hudson

seasonal distribution
Seasonal distribution

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VFR Research - R. Hudson

rainfall intensity duration
Rainfall intensity-duration
  • USA standard intensity criteria
    • Light: up to 2.5 mm/hour
    • Moderate: 2.6 to 7.6 mm/hour
    • Heavy: over 7.6 mm/hour
  • For a given storm, maximum insensity in mm/hour is inversely proportional to duration
    • temporal distribution of rain during a storm is not uniform

VFR Research - R. Hudson

types of precipitation networks
Types of Precipitation networks
  • Operational networks
    • Hydro
    • MOE
    • AES
    • Fire weather
  • Research networks
    • Density of network depends on purpose of research and resources

VFR Research - R. Hudson

there are multiple networks
There are multiple networks...
  • Hydro: monitoring for dam and reservoir management
  • Highways: monitoring for road conditions
    • e.g., Sea to Sky highway, there is a series of remote snow gauges at upper elevations to monitor snow and weather conditions to assess debris flow/avalanche hazard along the highway

VFR Research - R. Hudson

networks
Networks...
  • AES: mostly low elevation, volunteer manned on a daily basis
  • Fire weather: MOF and forest industry network to assess forest fire risk
    • seasonally monitored (April - October)
    • remote, a range of elevations

VFR Research - R. Hudson

networks24
Networks...
  • MOE: mostly remote, high elevation
    • in conjunction with snow courses
    • runoff forecasting
    • jointly operated with WSC
  • Research networks
    • tend to be denser than operational networks
    • localized, specific to research objectives
  • Actual network density unknown

VFR Research - R. Hudson