1. VFR Research - R. Hudson Precipitation

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

3. VFR Research - R. Hudson 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

4. VFR Research - R. Hudson 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

5. VFR Research - R. Hudson 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

6. VFR Research - R. Hudson Precipitation gauge siting All precipitation gauges experience undercatch –wind exacerbates gauge undercatch: causes ppt to fall at an angle less than 90 o –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

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

8. VFR Research - R. Hudson 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

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

10. VFR Research - R. Hudson 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

11. VFR Research - R. Hudson 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

12. VFR Research - R. Hudson 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

13. VFR Research - R. Hudson 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

14. VFR Research - R. Hudson 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

15. VFR Research - R. Hudson 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

16. VFR Research - R. Hudson 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

17. VFR Research - R. Hudson Local orographic effects

18. VFR Research - R. Hudson 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

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

20. VFR Research - R. Hudson 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

21. VFR Research - R. Hudson Types of Precipitation networks Operational networks –Hydro –MOE –AES –Fire weather Research networks –Density of network depends on purpose of research and resources

22. VFR Research - R. Hudson 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

23. VFR Research - R. Hudson 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

24. VFR Research - R. Hudson 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