Precipitation I: Processes and

measurement

For precipitation to occur, you must have water vapor

Saturation vapor (es) pressure over water

The amount of water vapor that the

atmosphere can carry (the saturation

vapor pressure, es) is a function of

temperature. The saturation vapor

pressure does not increase linearly

with temperature but rather

exponentially through the Clausius-

Clapeyron equation.

Having water vapor in the atmosphere is a necessary but insufficient condition for

precipitation. There must also be a mechanism to promote uplift, cooling and condensation.

Magnus-Tetens formula – a good approximation:

es(T)= 6.1094 exp[(17.625.T)/(T+243.04)]

Precipitation requirements and processes

•Condensation occurs when the temperature is ≤ the dewpoint Td.

•Without small particles in the air (e.g., dust, soot, sea salt, clay, sulfate,

phytoplankton) to act as cloud condensation nuclei (CCN), the air

can become supersaturated. The number of cloud condensation

nuclei in the air ranges between around 100 to 1000 per cm3.

•Typical CCN = 0.2 μm

•Typical cloud droplets are 20-100 μm

•Rain drops are around 2000 μm - 2 mm

• Downward velocity must exceed uplift velocity for precipitation to fall.

Uplift velocities in convective storms can be large.

Relative sizes of CCN, cloud droplets and

rain droplets

http://www.flame.org/~cdoswell/wxmod/wxmod.html

Precipitation requirements and processes (cont). •Uplift and cooling to the dew point

•Condensation

•Growth of droplets (two basic processes)

1) Bergeron-Findeisen theory (top figure): If ice

and supercooled water droplets exist together in a

cloud, the ice crystals grow at the expense of water

droplets, the reason being that the saturation vapor

pressure over ice is lower than over water. Solid

precipitation that falls may (or may not) melt at a

lower, warmer atmospheric level to become rain.

2) Coalescence (bottom figure): Occurs in warm

(liquid) clouds. Falling droplets have terminal

velocities directly related to their diameter, such

that the larger falling drops overtake and absorb

smaller drops, the smaller drops can also be swept

into the wake of larger drops and be absorbed by

them.

Key processes that provide uplift, cooling and

condensation are convection, frontal uplift and

forced ascent over orographic barriers. http://schepping.punt.nl/?a=2007-10

http://schepping.punt.nl/?a=2007-10

Bergeron-Findeisen theory (cont.)

Arapahoe Basin – May 2005

http://www.geminibv.nl/tidbits/dauwpunt-druk-

en-temperatuur?set_language=en

http://www.gi.alaska.edu/alison/ALISON_Science_Snow.html

The process of vapor diffusion is most efficient at around -12 to -15oC,

corresponding to the temperature of largest difference between water and

ice saturation vapor pressures. Different combinations of temperature and

supersaturation determine the type of snowflake that forms.

Precipitation requirements and processes (cont.)

Import of water vapor is important to maintain precipitation

Consider a thunderstorm

•For a column within the thunderstorm of unit area (m2),

the volume = 10,000 m3

•Typical average water mass is 0.5 g m-3

= 5000 g water in column

= 5000 cm3/10,000 cm2

= 0.5 cm precipitation, which is not much

Thus to maintain precipitation there must be an influx (entrainment)

of vapor-rich air to replace the water that falls from precipitation.

Precipitation quality

Dingman 2002 Figure 4-53

Precipitation is not pure, but rather

contains a number of ions. The

straight lines indicate concentration

ranges for continental rain, while the

wavy lines indicate concentrations for

marine rain. Ranges found in high

pollution areas are indicated by

dashed lines.

http://www.grc.k12.nf.ca/climatecanada/precipfactors.htm

http://www.ux1.eiu.edu/~cfjps/1400/stability.html

Convective precipitation

Keys to convective storm development are low atmospheric stability and ample atmospheric

water vapor. The stronger the decrease in temperature with height (the more negative the

environmental lapse rate), and the more water vapor that is available, the more favorable the

conditions for convective storms.

Convective precipitation tends to be

localized. The typical convective

storm occurs when local surface

heating makes air parcels warmer

(less dense) than their environment,

such that they rise and condense.

Lapse rates and stability Environmental lapse rate: The vertical rate of change

of temperature with height that we would measure

with a thermometer

• Is usually negative (temperature decreases with

height), the reason being that the atmosphere is

heated primarily from the surface and rising air

cools. An increase in temperature with height is

called an inversion.

• Typical values are -6.5oC km-1

Dry adiabiatic lapse rate (DALR): The rate of cooling

of a rising parcel (a given mass) of air due to

expansion and doing work against the

environment.

• The DALR is -9.8oC km-1

Saturated (wet) adiabatic lapse rate: The lapse rate

after condensation occurs.

• Condensation releases latent heat, such that the

saturated adiabatic lapse rate is les than the

DALR.

• Value varies; a typical value is -5oC km-1

A rising air parcel cools and expands. If

the temperature of the parcel is greater

than (less than) the temperature of the

surrounding environment, it is less (more)

dense than the environmental air and will

continue to rise (it will fall). If condensation

occurs, then the latent heat release makes

the parcel warmer (hence less dense) than

it would otherwise be, making it more likely

that it will stay warmer than the surrounding

environment and continue to rise.

Condensation may result in precipitation.

Key: If the environmental lapse rate

exceeds the DALR, the situation is unstable

and the parcel will rise.

Tropical cyclones

Dingman 2002 Figure 4-4

NOAA composite satellite image of Hurricane Katrina

Tropical cyclones feed on evaporation and latent heat release. They form between

5-20o latitude in each hemisphere where the sea surface temperature is at least

27oC. The grow through a process called CISK (Conditional Instability of the Second

Kind) through which rising air, condensation and latent heat release results in a drop in

surface pressure, causing inflow (converging winds) and more evaporation, further latent

heat release, and a further drop in surface pressure. Rainfall totals can be impressive.

http://earthobservatory.nasa.gov/IOTD/view.php?id=7079

48 hour precipitation from hurricane (and

tropical storm) Irene

National Weather Service

Frontal precipitation is associated with

travelling extratropical cyclones (low

pressure systems). They tend to be

strongest in winter. Precipitation is

associated with uplift, cooling and

condensation in warm fronts, cold fronts

and occluded fronts. Extratropical

cyclones should not be confused with

tropical cyclones (hurricanes, typhoons). Dingman 2002, Figure 4-2

Frontal precipitation

Surface weather analysis from the Canadian Meteorological

Center for Nov. 11, 2010 showing locations of cyclones and their

associated fronts, as well as anticyclones

This starts an animation from a numerical weather prediction model (Canadian Climate

Center) that shows traveling eddies. You loop through the forecasts through 144 hours (six

days). Look especially at the upper right-hand panel, showing sea level pressure (SLP) and

the 1000-500 hPa thickness. Keep looking to get a feel for how the eddies behave.

Traveling cyclones and anticyclones

http://www.weatheroffice.gc.ca/model_forecast/global_e.html

From here (screen shot above):

Click on “anim” for either the 00z run of the 12z run

Vertical structure of cyclones and anticyclones

The low (cyclone) at the surface is located

to the east of the shortwave trough at higher

levels. The is necessary to place the

surface low under an area of mass

divergence aloft, evacuating air. In turn, this

is compensated by convergence near the

surface, with rising motion in between

(fostering cloud formation and precipitation).

If the upper level divergence exceeds the

lower level convergence, surface pressure

falls. Similarly, the anticyclone at the

surface is located to the east of the ridge in

the shortwave at higher levels, necessary to

place the surface high under an area of

mass convergence. There is compensating

divergence at low levels, with sinking motion

in between, fostering clear skies and fine

weather.

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter12/vert_struct_tilted2.html

Temperature (thickness) advection

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter12/cold_warm_air_advection.html

Temperature advection is the primary process that cyclones and anticyclones intensify.

The temperature advection amplifies the shortwave, making the upper-level patterns of

divergence and convergence stronger.

Initially (a), streamlines and isotherms parallel each other (the atmosphere is barotropic). In

(b), the shortwave has cause the streamlines to cross the isotherms west and east of the trough (the

atmosphere there is now baroclinic). In the baroclinic region west (east) of the trough, cold (warm)

advection is occurring. Along with amplifying the wave, cold-air advection west of the trough will

produce sinking motion as the cold air descends to the surface behind the cold front, while warm-air

advection east of the trough will produce rising motion near the center of the low. In (c) the

temperature advection is cut off and the cyclone occludes.

Patterns of extratropical cyclone activity

Extratratropical cyclone frequency (left) and tracks (right) based on a detection and tracking

algorithm applied to data from the NCEP/NCAR reanalysis for the winter of 1989/1990.

(http://data.giss.nasa.gov/stormtracks/).

Sea level pressure pattern for October 24, 1997, showing

a memorable upslope storm that affected Boulder CO

With a cyclone centered over the Oklahoma panhandle, winds over the eastern plans of

Colorado have a component from east to west, moving “upslope”, promoting cooling,

condensation and precipitation. The system draws in water vapor from the Gulf of Mexico.

Orographic precipitation and chinooks

Because of the latent heat release during ascent,

the air at the top of the mountain barrier is warmer

that it would have been without the latent heat

release. As it descends on the leeward side, it

warms at the adiabiatic lapse rate, and arrives at

a higher temperature than it had before the ascent

process. These warm, leeside conditions are

often associated with strong gusty winds called

chinooks.

Orographic precipitation results from

ascent of air over a mountain barrier,

resulting in adiabiatic cooling,

condensation and precipitation on the

upwind side of the barrier. The leeward

side experiences a rain shadow. This

is in large part why the west slope of the

Front Range of Colorado receives

more precipitation than the plains to the

east. However, the plains can receive

considerable precipitation from “upslope”

storms when a low pressure system lies to

the south. These are most common

in winter.

Some notable temperature changes

linked to chinooks:

Loma, MT: -56oF to 49oF in 24 hours

Spearfish SD: -4oF to 45oF in 2 minutes!

Precipitation recycling

What fraction of precipitation that falls within a watershed is due to

water that evapotranspires from that watershed and then falls back

within the watershed?

F+

F+ PL/P = 1/(1+ 2.F+/ET.A)

P = Total precipitation

PL = Precipitation of local origin

E T = Evapotranspiration

A = Area of watershed

F+ = Vertically integrated vapor flux directed into the

watershed (advective moisture term)

E

T

From the formulation of

Brubaker et al. (2003):

To get a high recycling ratio (P/PL)

you want a large evaporation rate

and a small advective moisture term.

The ratio is very scale dependant.

At the global scale, ALL precipitation

is recycled!

P

Dingman 2002 Figure 2-3

Precipitation recycling (cont.)

A few estimates for the annual

ratio from Brubaker et al (1993)

Amazon 25%

Eurasia 10%

Sahel 35%

Mississippi Basin 24%

Key points:

For most regions, the bulk of precipitation is

“imported” in that the water vapor associated

with the precipitation comes from outside of

the region. However, recycling is important

for some areas, such as the Amazon and the

Sahel of Africa. The concern is that

deforestation in the Amazon will significantly

affect the hydrologic cycle there.

www.treehugger.com http://www.experimentearth.com/amazonrainforest.html

Precipitation gauges

http://www.weatherworks.com.au/?p=3082

http://www.hubbardbrook.org/w6_tour/rain-

gauge-stop/precipitation.htm

http://www.novalynx.com/260-2501.html

There are many types of

gauges, which vary in their

catch efficiency. Shielded

gauges, like the one above,

tend to perform better than

unshielded gauges. The liquid

water equivalent of snowfall

is especially hard to measure.

Wyoming snow gauge

Wyoming snow gauges are designed to provide accurate measurements

of snowfall water equivalent. The one pictured above is located on the

drainage divide of Imnaviat Creek, near Toolik Lake, on the North

Slope of Alaska. Photo by M. Serreze.

Typical SNOTEL sites (summer)

SNOTEL (SNOwpack TELemetry) is an automated system to measure snowpack water

equivalent. There are over 600 SNOTEL sites across the western U.S. Snow pillows

measure the weight of the overlying snowpack. SNOTEL sites complement snowcourses,

where snow water equivalent is measured manually.

Snotel Sites

From Serreze et al., 2001

Areal or gridded estimates of precipitation

A problem often faced in hydrology is using point

measurements of precipitation to come up with a

regional average (such as for a watershed) or gridded

field of precipitation. This generally requires

interpolation. While there are many types of

interpolation, ranging from a simple average of all

stations within a given region to inverse distance

weighting to optimal interpolation, they pretty much all

boil down to the following:

Pinterp= Σ (Pi .wi)/Σ wi

Where Pinterp is the regional or grid point value we wish

to interpolate to, Pi are measurements at each

precipitation gauge (as in the watershed shown at

right) and wi are the interpolation weights. Simply

phrased, the interpolated value is the sum of the station

values times the weights divided by the sum of the

weights. The key is how one determines the weights.

A practical note: If there is a

dense station network, all

interpolation techniques work well.

If the network is sparse, none of

them work well.

Dingman Figure 4-20

Station Density: Contiguous U.S

The station density from the U.S. cooperative network looks rather

dense, but at the scale of medium of small watersheds is can be

quite sparse. The country with the densest station network is Israel,

followed by the UK.

The figure at left shows

monitoring stations north

of 40oN with at least ten

years of record for the

period 1960-1989. Note

the very sparse network

in high northern latitudes.

The situation over the

Antarctic continent is

pathetic. Coverage is

also very sparse over

large areas of the world’s

ocean.

The Arctic: A very sparse network 3

From Serreze and Barry, 2005

Getting the weights: Examples

Dingman 2002 Figure 4-27

Dingman 2002 Figure 4-28

One can develop a function based on the

distance decay of the correlation in precipitation.

(see the left hand figure). While attractive, the

problem is that the correlation length scale of

precipitation tends to be short, especially in

mountainous terrain. One can also make use of

relationships between precipitation and elevation

(hypsometric method). A problem is that in

mountainous terrain, relationships between

precipitation and elevation can be complex.

An alternative: The aerological approach

∂W/∂t = ET - P - •Q

∂W/∂t

ET P

•Q

∂W/∂t = time change in

precipitable water

(column water vapor)

ET = Evapotranspiration

P = Precipitation

•Q = Vertically-integrated vapor

flux divergence

Rearrange:

P- ET = - •Q - ∂W/∂t

Key: While P and ET may be hard to measure over large areas, we can get

the net precipitation (P-ET) using atmospheric winds and humidities.

Aerological estimates of mean annual precipitation minus evapotranspiration (P-ET) based on NCEP/NCAR data for the period 1970-1999 (mm) for the region north of 60oN. Contours are at every 100 mm up to 500 m (negative values dashed) and at every 200 mm for amounts of 600 mm and higher [from Serreze and Barry, 2005]. Annual P-ET is positive everywhere except locally in the Norwegian Sea and southern Barents Sea where evaporation rates are high from autumn through spring (because of cold, dry air blowing over a fairly warm open ocean).

Annual P-ET from the aerological approach