Precipitation VFR Research R Hudson Precipitation Measurement Types

Precipitation VFR Research - R. Hudson

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

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 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 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 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 VFR Research - R. Hudson

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

Effect of rainfall angle on catch • Vertical rain sees a circular crosssection on the gauge – true “as calibrated” precipitation catch • Angular rain sees an elliptical crosssection 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 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 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 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 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 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 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 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 VFR Research - R. Hudson

Local orographic effects 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 VFR Research - R. Hudson

Seasonal distribution Total Monthly Ppt. (mm) 200 Upper Penticton Creek 100 0 Total Monthly Ppt. (mm) 400 Russell Creek 300 200 100 0 J F M A M J J A S VFR Research - R. Hudson O N D

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 • 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. . . • 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. . . • 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

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