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Precipitation

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

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  1. Precipitation VFR Research - R. Hudson

  2. Precipitation • Measurement • Types of precipitation • Precipitation distribution • seasonal • regional • topographic controls • Basin average precipitation VFR Research - R. Hudson

  3. 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

  4. 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

  5. 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

  6. 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

  7. Rain falls on different cross-sectional areas depending on angle of fall VFR Research - R. Hudson

  8. 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

  9. Proper siting of a precip. gauge VFR Research - R. Hudson

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. Local orographic effects VFR Research - R. Hudson

  18. 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

  19. Seasonal distribution 200 ) m m Upper Penticton Creek ( . t p P 100 y l h t n o M l a t o T 0 400 • Russell Creek ) m 300 m ( . t p P 200 y l h t n o 100 M l a t o T 0 J M M J J S N D F A A O VFR Research - R. Hudson

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

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