Download Lecture 5: Precipitation

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
rain/snow
• Duration
• Intensity
• Distribution (spatial variability)
Q
Assignment: open up the link below and read the
section about Precipitation
Precipitation formation involves multiple physical concepts:
1. Processes lift air masses up higher in the atmosphere.
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.
air has mass
air has density
mass
Density = ρa =
volume
ground surface
atmosphere
upper atmosphere
air pressure is equal to the density of the air times the distance
below the upper atmosphere times the acceleration of gravity
Pa = air pressure = ρa x g x h
ρa = air density
h
g = acceleration of gravity
h = depth below the upper atmosphere
Actually, it is a little more complicated because air density changes with
altitude, but we will assume a uniform density.
ground surface
upper atmosphere
h
Pa = 60 KPa
h
Mt. Rainier
Pa = 101.1 KPa
sea level
upper atmosphere
low pressure
balloon
ground surface
Pa = ρa x g x h
high pressure
upper atmosphere
low pressure
Concept 1: the balloon
expands with altitude
because the pressure drops
ground surface
high pressure
upper atmosphere
low pressure
coolest
Concept 2: the balloon temperature
decreases with altitude because
expanding gases cool off
cooler
For an adiabatic process
V x T = constant
cool
V = volume
T = temperature
ground surface
high pressure
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).
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)
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 =
actual vapor pressure
saturation vapor pressure
X
100
Dew point temperature is the temperature at which a parcel of air
reaches saturation.
Air Mass Lifting Mechanisms
Warm Front
Warm air slowly rises over cool air – produces light rain over large areas.
cool air
Warm Front
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 air
Cold Front
Convective Storm (thunderstorm)
Warm, humid air rises fast– produces heavy rain over isolated areas.
cool down drafts
cool down drafts
warm, humid air
warm, humid air is less
dense and rises fast
ground heat warms humid air
Convective Storm (thunderstorm)
Orographic Effect
Humid air is forced to rise over mountains
Our Maritime climate brings moisture from the Pacific Ocean
http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0712/07120318_jetstream_pac_anal.gif
Orographic Effect
Orographic Effect
low pressure
high pressure
Orographic Effect
clouds cool when
they expand
Orographic Effect
water vapor condenses
when air cools, which
produces rain
clouds cool when
they expand
Orographic Effect
If warm, humid air is cooled off
water vapor will condense
because the saturation
vapor pressure decreases
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://www.bentler.us/washington‐state/maps/img/washington‐state‐map.jpg
http://www.prism.oregonstate.edu/
Pineapple Express: January 8, 2009
http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0901/09010900_jetstream_pac_anal.gif
Janauary 6‐9 storm event, 2009
high topographic relief
http://wa.water.usgs.gov/data/realtime/adr/interactive/maps/NooksackSC_basin.pdf
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 extrapolated
over 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. 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
Brannian Creek Rain Gauge
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
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.
Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation
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