Lecture 5: Precipitation

Key Questions

1. What physical concepts control the formation of precipitation (air pressure, expanding gases and cooling, and vapor pressure)?

2. What are some air mass lifting mechansims?

3. What cause precipitation spatial variability in WA?

4. What is a tipping bucket rain gauge?

5. Where can precipitation data for WA be found?

PNW radar image January 16, 2011

Precipitation

Precipitation is the primary input into a basin and has a dominant effect on streamflow and hillslope processes.

• Type• Duration• Intensity• Distribution (spatial variability)

Q

rain/snow

Assignment: open up the link below and read the section about Precipitation

Precipitation formation involves multiple physical concepts:

2. Rising clouds expand due to a decrease in atmospheric pressure with altitude.

3. Clouds (gases) get cooler as they expand.

4. Water vapor condenses when clouds cool.

1. Processes lift air masses up higher in the atmosphere.

ground surface

air has mass

air has density

Density = ρa =mass

volume

atmosphere

ground surface

air pressure is equal to the density of the air times the distance below the upper atmosphere times the acceleration of gravity

upper atmosphere

Pa = air pressure = ρa x g x h

ρa = air density

g = acceleration of gravity

h = depth below the upper atmosphereh

Actually, it is a little more complicated because air density changes with altitude, but we will assume a uniform density.

http://www.altitude.org/air_pressure.php

Mt. Rainier

upper atmosphere

Pa = 101.1 KPa

h

Pa = 60 KPa

h

sea level

balloonground surface

upper atmosphere

Pa = ρa x g x h high pressure

low pressure

ground surface

upper atmosphere

high pressure

low pressure

Concept 1: the balloon expands with altitude because the pressure drops

ground surface

upper atmosphere

high pressure

low pressure

Concept 2: the balloon temperature decreases with altitude because expanding gases cool off

coolest

cooler

cool

For an adiabatic process

V x T = constant

V = volumeT = temperature

Vapor Pressure

According to Dalton’s Law, each gas in a parcel (volume) of air exerts a pressure independent of the other gases (oxygen, nitrogen, carbon dioxide, water vapor).

Actual vapor pressure is a measure of the amount of water vapor molecules present in a given parcel of air.

low vapor pressure high vapor pressure

Saturation vapor pressure is the maximum amount of water vapor a parcel of air could hold at a specific temperature (which decreases with temperature).

cool air has a low saturation vapor pressure

warm air has a higher saturation vapor pressure(more water molecules)

Saturation vapor pressure is the maximum amount of water vapor a parcel of air could hold at a specific temperature (which decreases with temperature).

Relative humidity is the ratio of the amount of water vapor in a parcel of air (absolute vapor pressure) to how much water vapor the parcel could hold at a given temperature (saturation vapor pressure).

relative humidity =

Dew point temperature is the temperature at which a parcel of air reaches saturation.

actual vapor pressure

saturation vapor pressureX 100

Air Mass Lifting Mechanisms

cool air

Warm Front

Warm air slowly rises over cool air – produces light rain over large areas.

Warm Front

cold air

Cold Front

Steep cold air mass collides with warm air and forces warm air up quickly – produces moderate to heavy rain over smaller areas.

warm air

Cold Front

Convective Storm (thunderstorm)Warm, humid air rises fast– produces heavy rain over isolated areas.

warm, humid air

ground heat warms humid air

warm, humid air is less dense and rises fast

cool down drafts cool down drafts

Convective Storm (thunderstorm)

Orographic Effect

Humid air is forced to rise over mountains

http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0712/07120318_jetstream_pac_anal.gif

Our Maritime climate brings moisture from the Pacific Ocean

Orographic Effect

Orographic Effect

high pressure

low pressure

clouds cool when they expand

Orographic Effect

clouds cool when they expand

Orographic Effect

water vapor condenses when air cools, which produces rain

Orographic Effect

water vapor will condense because the saturation vapor pressure decreases

If warm, humid air is cooled off

Condensation nuclei are required as nucleation points for water vapor condensation (dust, pollutants, salt, ash) to form droplets. Droplets collide and aggregate to form drops (0.4 to 4 mm in diameter).

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter5/ccn.html

http://news.discovery.com/earth/heavy-hurricanes-and-their-climate-effects.html?print=true

http://www.bentler.us/washington‐state/maps/img/washington‐state‐map.jpg

http://www.prism.oregonstate.edu/

http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0901/09010900_jetstream_pac_anal.gif

Pineapple Express: January 8, 2009

Janauary 6‐9 storm event, 2009

http://wa.water.usgs.gov/data/realtime/adr/interactive/maps/NooksackSC_basin.pdf

high topographic relief

increase in rainfall due to the orographic effect 

Point Measurement of Rainfall

Accurate measurements are necessary for quantitative hydrologic analyses. Two questions arise:

1. How accurate are point measurements?

2. How accurately can point measurements be extrapolatedover an area?

City of Bellingham Rain Gauge Locations

Tipping Bucket Rain Gauge

8 inch diameter

Collects 1 mm (0.01 inch) of rain and tips, empties and send an electronic digital signal that is recorded. 

Brannian Creek Rain Gauge

Variables that affect accuracy

• Wind (keep about 1 m above the ground)

• Obstacles (place in open areas away from trees and structures)

• Splashing

• Evaporation

• Annual measurement accuracy is 5-15% up to 75% for a single storm

Lake Whatcom Watershed Rain Gauge Locations

North Shore Meteorological (MET) Station

Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation

Brannian Creek Rain Gauge

Geneva Rain Gauge

Bloedel Donovan Rain Gauge

North Shore Hyetograph: 2010 Water Year

Oct 1 Apr 1 Sep 30

North Shore Monthly Totals: 2010 Water Year

North Shore Meteorological (MET) Station

Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation

There are 8760 hours in one year.

In 2010, the North Shore gauge recorded rainfall 1221 hours out of 8760.

Meaning that it rained14% of the year.

Hourly rainfall frequency for the 1221 hours of recorded rainfall at the North Shore gauge in 2010.

92 % of the 1221 hours of recorded rainfall in 2010, were ≤ 0.1 inches.

What does this say about rainfall intensity in the watershed?

Cumulative Rainfall: 2010 Water Year

Oct 1 Apr 1 Sep 30

Why does it rain more in the southern part of the Lake Whatcom Watershed?

47.7

50.0 50.0

67.4

http://weather.wsu.edu/

http://www.wrcc.dri.edu/summary/Climsmwa.html