Hydrology EESC BC 3025
Atmospheric
moisture,
condensation,
and
precipitation
Moisture in the atmosphere
- water undergoes huge expansion during evaporation: 1 g of
water
equals
1 ml volume in liquid form and 42 l as vapor (at 25oC)
- gravity concentrates the atmospheric gases near the surface,
the
pressure
drops to 1/e (= 37%) at about 8 km elevation
- 90% of water vapor content is confined to the lower 6 km
- water vapor pressure as a function of temperature (svp
=
saturation
water vapor pressure) (Fig),
can
explain
many phenomena in the atmosphere.
- absolute humidity (or water vapor mixing ratio):
mass
of
vapor per unit volume of air, in g m-3
- at 30oC, air has a svp of 42.43 hPa (hPa = mbar)
and
can
contain
up to 30 g m-3, at 0oC svp is only 4.5 g
m-3
- relative humidity: actual water vapor pressure / svp
in
%;
or: actual
water vapor content / water vapor content at saturation
- formation of fog, clouds,
mixing
clouds, can be understood in the
framework
of the vapor pressure diagram
Condensation and Precipitation
condensation:
- transition from vapor phase to liquid phase
precipitation:
- deposition of liquid water droplets and ice particles that are
formed
in
the atmosphere and grow to a sufficient size so that they are
returned
to the Earth's surface by gravitational settling. Solid and
liquid. Dew
and fog do not count as precipitation (can add 5-10% to
precipitation
in
the Pacific Northwest)
clouds and precipitation
explanantion of processes through the vapor pressure
diagram (Fig):
air rising => expansion => adiabatical (= no
heat
exchange with
environment) cooling => condensation
example: humidity and temperature for Black Rock Forest (Fig)
at T>0oC: warm
cloud
process: condensation,
gradual growth of water droplets by condensation,
collision and
coalescence
at T<0oC: cold
cloud
process:
involves also the formation and growth of ice crystals
two extra factors are needed to form
precipitation:
- sufficient moisture supply
- sufficient vertical motion
warm cloud process
- a moisture laiden air parcel rises,
cools
at dry
adiabatic lapse rate (~1oC/100m) until it
reaches the dewpoint, at
which point condensation occurs. After that, any further
rise causes
cooling
at the moist adiabatic lapse rate (0.5 - 0.9oC/100m),
because
of
the
released latent heat. (Fig)
- super saturation: relative
humidity
> 100%
- condensation nuclei are
needed
to
increase
condensation
- most efficient particles: Aitken
nuclei
(0.01-0.1
micro m)
- typical source: dust from land,
sea
spray
(hygroscopic!)
- 5 million/l air over land, 1
million/l
air over the
ocean
- experiment:
salt crystals as condensation
nuclei (Fig)
- experiment: when a beer
bottle
is
opened,
a cloud forms in the neck. If temp. of the bottle is 5oC,
temperature
drops
to ~-36oC when bottle is opened (Fig)
- experiment: when beer is
pored
into a glass,
bubbles form on scratches and dust particles, adding salt
can increase
the bubble formation: clouds in a glass of beer
- excercise: condensation
on
a
mirror
in the
bathroom (Fig);
condensation on
windshields
- condensation only creates droplets
<
100
micro
m radius, while raindrops are of the order of 1mm
- clouds are continuously forming and dissipating, some live
only
5 to 15
minutes
- excercise: how
many
cloud droplets form one
rain drop?
- droplets merge due to direct impact
and
collision
in the wake of falling drops
cold cloud process
- saturation vapor pressure is lower
over
ice
than
water => ice crystals grow in favor of liquid droplets
- ice crystals are very efficient
condensation nuclei
- most efficient in mid latitudes
(temperatures low
enough, but enough instability in the atmosphere)
Precipitation patterns
- kinds of precipitation: drizzle, rain,
ice
pellets,
snow, hail
- terminal velocity (v) is
achieved
when
gravitational
acceleration is counterbalanced by the friction of the air,
for 1mm
diameter
drop: v = 4 m/s = 9 miles/hour
- raindrops break up at 5 mm diameter,
snow
flakes can
reach
40mm, and hailstones over 50mm
- moisture in atmosphere: 25% condenses,
75%
forms
ice and snow; only 5% of that falls as snow and ice crystals,
the rest
melts; a lot of the precipitation re-evaporates before it
reaches the
ground
- most precipitation comes from bordering oceans, but up to 40%
can
come
from local ET.
- extremes in US: Kauai: 12,000 mm/y, Death Valley: 40mm/y
- dryest place on Earth: Calama in Atacama desert, Chile, rain
has
never
been recorded
- average annual precipitation (global (Fig)
and US (Fig)) onto
the
continents
is a function of:
- (a) latitude (precipitation highest in latitudes of rising
air-0°
and
60° north and south-and lowest in latitudes of descending
air-
30°
and 90° north and south);
- global circulation patterns in the
atmosphere (Fig)
- (b) elevation (due to orographic cooling, precipitation
usually
increases
with elevation (Fig)
;
- (c) distance from moisture sources (precipitation is usually
lower at
greater
distances from the ocean);
- (d) position within the continental land mass;
- (e) prevailing wind direction;
- global circulation patterns in the
atmosphere (Fig)
- (f) relation to mountain ranges (windward sides typically
cloudy and
rainy,
with leeward sides typically dry and sunny)
- (g) relative temperatures of land and bordering oceans
- global circulation patterns in the oceans (Fig)
- excercise:
spatial and temporal variability of precipitation
Point measurements of precipitation
- Obviously precipitation data are extremely important in
hydrology
- need to measure at many points and need to extrapolate
- point measurements performed by recording and non-recording
gages
- snow depth measurements by telemetry, 500 remote sites in US
- precipitation typically measured as depth
- many stations all over the world (Fig)
- the record of hourly precipitation over time is called a hyetograph
and shows that precipitation is organized into discrete storms (Figure
2.3, a station in North Carolina)
Spatial characteristics of precipitation and radar estimation
- averaging over an area using point measurements at stations (Fig2.4)
- measurement by radar, radar is reflected from raindrops
- storm track and total rainfall accumulation during a storm
on
June 27,
1995 based on radar measurements (Fig2.5)
- current
radar
image
- radar image of previous fieldtrip (Fig)
Temporal characteristics of precipitation
- our ability to forecast this temporal variation even a few
hours
in
advance
is
limited and our ability to forecast several days in advance is
almost
zero
- if you examined all of the rainfall data for a given region,
you
would
find an upper limit to
the depth of rainfall per time (precipitation intensity) for a
given
duration. (Envelope Curve
for
Precipitation)
- hydrologists apply a technique called frequency
analysis
to describe
the temporal characteristics of precipitation
- we assume that precipitation data are samples of a random
variable
characterized by a probablility density function
- only mean annual precipitation appears to be normally
(or Gaussian)
distributed (Fig2.7)
- if normally distributed, precipitation can be described by a mean
and and a standard deviation (Fig2.6)
- this information is useful to determine the exceedence
probability
(the probability that a certain annual precipitation value is
exceeded
in a given year) or the return period (the inverse of the
exceedence
probability).
- If data are normal distributed, EXCEL's NORMINV
function can be used to determine the value of a particular
parameter
(e.g.
annual precipitation) for a particular exceedence probability
and
recurrence
interval
- (Excercise:
Explore2,
global variability
of
precipitation),
determination
of
exceedence probablity using standard deviation, mean and the
normal
distribution. Denver, Seattle and LGA/NY as an example
(Fig2.6).
(Fig)
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