Learning Objectives - Precipitation. Understand precipitation processes and forms Understand the spatial and temporal distribution of precipitation Learn where to find precipitation data: will get experience with homework Understand the difference between normal rainfall and average rainfall
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Learning Objectives - Precipitation • Understand precipitation processes and forms • Understand the spatial and temporal distribution of precipitation • Learn where to find precipitation data: will get experience with homework • Understand the difference between normal rainfall and average rainfall • Learn how to measure precipitation • Understand the sources of error in measuring precipitation: web page handout • Learn how to change point precipitation data into areal information • Learn how to read depth/duration/frequency/ graphs • Introduce concepts of probability and return periods for events
Precipitation • Single strongest variable driving hydrologic processes • Formed from water vapor in the atmosphere • As air cools its ability to ‘hold’ water decreases and some turns to liquid or ice-e.g., condensation on glasses with cool liquids
40% of the terrestrial precipitation originates from land evaporation 57% of all terrestrial evaporation returns as precipitation over land. Moisture evaporating from the Eurasian continent is responsible for 80% of China’s water resources. In South America, the Río de la Plata basin depends on evaporation from the Amazon forest for 70% of its water resources. The main source of rainfall in the Congo basin is moisture evaporated over East Africa, particularly the Great Lakes region. The Congo basin in its turn is a major source of moisture for rainfall in the Sahel. Ent el. 2010 http://www.agu.org/journals/wr/wr1009/2010WR009127/2010WR009127.pdf
What are hydrologists most concerned with respect to precipitation? How much? (depth) over what time period? (intensity)Where? (space)When? (time)What type?
Solar-driven air circulation and latitude of the major circulation cells 02_04.jpg Figure 2.4 bama.ua.edu/~rfindlay/Lecture%204.ppt
Precipitation • Precipitation requires air mass lifting. This can occur in 3 ways: • Convective cells (thunderstorms) • Fronts • Orographic (mountains) • Or by a combination of the above
> 400 41-60 81-120 21-40 81-120 201-400 January precip in mm for British Columbia
Rising Air Cools • As air rises, it cools “adiabatically” that is it does not exchange heat with its surroundings (they are cooling similarly) • Lapse rate ranges: • ~6°C/km Wet • ~10°C/km Dry http://ess.geology.ufl.edu/ess/Notes/AtmosphericCirculation/lapserate.jpeg
Where do nuclei come from? • Ocean salt • Pollution • Wind born dust • Cloud seeding • Microbes
Cloud Seeding • silver iodide • dry ice (solid carbon dioxide). • Liquid propane, which expands into a gas, has also been used. This can produce ice crystals at higher temperatures than silver iodide. • The use of hygroscopic materials, such as salt, is becoming more popular Microbes Ice nucleating strains of P. syringae possess a gene that encodes a protein in their outer membrane that binds water molecules in an ordered arrangement, providing a very efficient nucleating template that enhances ice crystal formation
www.atmo.arizona.edu/students/courselinks/fall05/nats101s5/lecture13.pptwww.atmo.arizona.edu/students/courselinks/fall05/nats101s5/lecture13.ppt Cloud Droplets to Raindrops A raindrop is 106 bigger than a cloud droplet Several days are needed for condensation alone to grow raindrops Yet, raindrops can form from cloud droplets in a less than one hour What processes account for such rapid growth? 106 bigger 106 bigger Ahrens, Fig. 5.15
Big water drops fall faster than small drops As big drops fall, they collide with smaller drops Some of the smaller drops stick to the big drops Collision-Coalescence Drops can grow by this process in warm clouds with no ice Occurs in warm tropical clouds www.atmo.arizona.edu/students/courselinks/fall05/nats101s5/lecture13.ppt Collision-Coalescence Area swept is smaller than area of drop small raindrop Collection Efficiency 10-50%
Forms of Precipitation Fog drip Liquid (Rainfall) Precipitation Snow (vapor condensed to ice) Solid (Ice) Hail (water condensed to ice) We are going to concentrate on rainfall and not worry about snowfall, snowmelt, etc (which are very important subjects in the North and West of the US)
Fog Drip (Occult Precipitation) • Moisture/vapor in clouds and fog condenses on needles and leaves • When drops get large enough, they fall to the ground beneath the tree • No precipitation is measured in the open Data: Bull Run watershed Oregon -30% more precip in woods Other studies suggest 30-50% increased precipitation
Fog nets in Lomas de Lachay Peru Studies in Peru show up to 5-10 l/day/m2 A 8x4 m2 net could capture ~ 300 l/day
Weather Patterns • Weather (day to day) vs. climate (years-decades and patterns) • Climate is what you expect, weather is what you get • Climate and geography result in biome classification
Biomes and Rainfall Figure 2.2
Continental precipitation recycling rate, dependence on upstream continental evaporation to sustain precipitation Ent el. 2010 http://www.agu.org/journals/wr/wr1009/2010WR009127/2010WR009127.pdf
Climate terms of importance • Period of record- all information for a gage since data collection began • Normal value- average of last 30 years beginning with a decadal year- e.g.the most current normal average rainfall is for the 30 year period 1971-2000. • Average – average of time period of interest • Why use instead of period of record?
Climate terms of importance • Average Temperature- how compute?
Climate terms of importance • Average Temperature- how compute? • Historically daily maximum T plus daily minimum T divided by 2 • Need to know what 24 hour period • Could be midnight to midnight local time, could be midnight UTC (GMT), could be 9 am to 9 am • NWS is midnight to midnight local time
Climate terms of importance • Average Temperature- how compute? • Modern data loggers can record temperature at any specified time • If use hourly – leads to lots of data!
Climate terms of importance • Stream temperature for fish in Washington • Based on 7-day average of the daily maximum temperatures • Salmon and Trout Spawning 13C (55.4F) • Core Summer Salmonid Habitat 16C (60.8F) • Salmonid Spawning, Rearing, and Migration 17.5C (63.5F) • Indigenous Warm Water Species 20C (68F)
Measurement of Precipitation • Terminology (2.3) • Types of devices (2.4.2) • Snowfall conversions (2.4.1) • Location of devices (2.4) • Interpretation of data (2.3.3, 2.6)
Rainfall Terminology • Type-hail, rain, snow, sleet • Depth • Storm Duration • Average rate of precipitation-Intensity • Return Period or Recurrence Interval • Average vs normal
Snow Measurement • Determine the water equivalent • 5%-60% of snow depth may be water equivalent-- “density” • Snow pillows use antifreeze solution and pressure measurement to measure water equivalent
Location of Gages • Gauges measure point rainfall • True precipitation unaffected by surroundings-winds, trees, buildings • Clearance distance 2 times height of object • For large areas multiple gauges are needed for more accurate estimates
Interpretation of Data • Time distributions • Area distributions • Using point data to find average rainfall • See figure 2.8 • Thiessen method
Storm Patterns (Histograms) Figure 2.14
How to get area? • GIS, CADD • Planimeter • Graphical • Count cross hairs, not areas • Weight • Field survey
Graphic method of measuring areaCount the vertices within the area • Each vertix represents the center of the area around it 51 vertices Scale
Prediction-Frequency Distributions • To plan and design projects must be able to predict probability of rainfall events • Duration, Intensity, Return Period • How long? • How much? • How often?