Precipitation Flashcards
What is precipitation
The water reaching a catchment as rainfall, snow, hail, dew or occult forms
What is average annual rainfall for the world and for Lancaster
1000 mm
Where was the highest recorded annual rainfall
At cherrapunji, India
How much rainfall did Cherrapunji receive
26,461 mm/yr in 1861
What was the highest rainfall in the Uk
Sprinkling Tarn (Scarfell Pike, Cumbria)
How much annual rainfall was at scarfell pike
6,528 mm/yr in 1954
What is rainfall measurement about
The areal integration (average across an area) over a network of point measurements (individual raingauges catches)
How many gauges are the UK Meteorological Office network
6,800 raingauges
What are most of the gauges at the UK meteorological office
Daily storage gauges though they also have a number of monthly storage gauges and rainfall recorders
What is a monthly storage gauge
Volumetric measurements taken after a month of collection
What do rainfall recorders do
Rainfall intensities recorded automatically on charts or electronic data loggers
How far above ground does the MET office mark II storage gauge stick
30cm
Why is the top of the Met price mark II storage gauge sharp
So when raindrops hit it they are chopped. Knife edge
What does the drop above funnel in the Met office mark II storage gauge prevent
Out splash - water jumping out
Why is the above funnel of the Met office mark II storage gauge narrow
To minimise evaporation that’s coming out of the bottle between daily measurements
Why does the Met office mark II storage gauge use a glass bottle inside
To prevent water from being attracted to side of bottle when you upend it so all water comes out
Why is the Met office mark II storage gauge 1ft above ground
Reduces insplash
Why is the base of the Met office mark II storage gauge splayed
So when it’s in the ground it’s less likely to move and water is less likely to evaporate
Why is there an outercan in the Met office mark II storage gauge
To catch overflow
Example of monthly storage gauge
Clougha Pike in Lancaster
Why are rainfall recorders useful
We might want to know rainfall every 5 mins or 15 mins so we need some form of automated rain gauge
What are the different rain gauges
Siphoning tank rain gauge.
Tipping bucket rain gauge.
What does the siphoning tank rain gauge do
Tells rainfall every 15 mins.
Gubbins - rainfall in big funnel getting directly into tank which fills with water. Inside is float connected to arm of gubbin. Arm goes up which is connected to pen that records volume.when at top it pivots and gets emptied and counter weight pulls it back up.
What is the chart created in a siphoning rain gauge
Hyetogram - need someone in lab digesting data. Not great
How does a tipping bucket rain gauge work
Inside is a double bucket and rain comes in funnel and first of all fills the inside bucket which tips and fills other bucket. Magnet behind bucket is reed switch so it you bring magnet towards it you can record time on data logger.
Why do you need a check gauge in tipping bucket
Due to greater error and freezing problems - if it’s frozen and locked nothing happens.
What is the effect of sheltering in raingauge sitting
Where you put the raingauge affects the total. Should always have an angle of 30* from rain gauge to highest nearest objected. Should be twice the height away if not degree measurement.
What is the effect of turbulence on rain gauges
If rain isn’t coming down vertically turbulence blows rainfall away from rain gauge on windy days so to counteract you build a Huddleston turf wall. 1.5 m away. Or institute of hydrology ground level gauge, need to generate rough surface so water can’t run straight in.
How do you produce rough ground
Anti splash grid around gauge in pit
What are the problems of snowfall measurement
Burial - not much bother in UK bc not for a lot of snow.
How do you counter act burial
Lift rain gauge above ground so it doesn’t get covered by drift. Need to get rid of turbulence on rain gauge.
After elevating the rain gauge to deter drift, how do you combat turbulence
Nipher shield
Alter shield - flap around in wind to remove turbulence
What are he problems of rainfall measurements in tropical rainforests like Damon valley field centre, Borneo
Has to be 40-60m high bc of high trees.
Have to build out of Borneo iron that doesn’t rust but shiny at night so insects dive into it.
Why do they have to put walls around rain gauges in Borneo
So elephants don’t trample it but only covers one rain gauge
What are the 2 things needed for areal estimation of rainfall
Catchment average and spatial patterns
What are the 6 methods of areal estimation of rainfall
Arithmetic mean Thiessen polygon Hypsometric curve Isohyetal Radar Mathematic surface
How to work out arithmetic mean of distribution
All rainfall and divide by number of gauges.
What does arithmetic mean require
Even spacing of rain gauges across catchment - uniform distribution. Can’t have relief
What is good for the arithmetic mean
Simple
Uniform distribution
Flat terrain
How does thiessen polygons work
Weight total rainfall according to local area that each gauge represents.
Draw line between rain gauge pairs, middle point of that line, draw another line at tangent to first line and join all them up to generate hexagons across catchment. Copes with uneven totals
What does the thiessen polygon deal with that arithmetic meal doesn’t
Uneven spacing
What is bad about the thiessen polygon method
Flat terrain required
How much more rainfall do you get if you go up a meter
1mm more due to orogrqphic enhancement of rainfall
What is a better version of the thiessen polygon
Altitude corrected thiessen polygon. Bisection point is median altitude
How does the hypsometric curve work
Curve of precipitation on the top right hand graph and area under the curve gives total rainfall weighted according to altitude
What is the isohyetal method
At a particular time you have a total amount of rainfall so you can draw lines between equal rainfall. Tot up rainfall for each area and divide by total area. Gives catchment average rainfall but also patterns of rainfall in catchment.
What can isohyetal method be subjectively adjusted for
Underlying topography
What is radar
Radar reflectively calibrates against point gauges means. Firing signals into atmosphere and receiving a return of things in the atmosphere
What is the mathematical surface method
If you got a rain gauge and their totals you can fit mathematical surfaces into it. Kriging is a method for fitting a mathematical surface through data.
