Download Lecture 8: Condensation and Cloud Formation

METR125 –Physical Meteorology
Lecture 8: Mixing and Convection
Modified from
Steve Platnick
Notes
Cold Cloud Processes
Warm Cloud
Processes
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Water Cloud Formation
Water clouds form when RH slightly greater than 100% (e.g., 0.3%
supersaturation). This is a result of a subset of the atmospheric aerosol
serving as nucleation sites (to be discussed later). Common ways for
exceed saturation:
1.
Mixing of air masses (warm moist with cool air)
2.
Cooling via parcel expansion (adiabatic)
3.
Radiative cooling (e.g. ground fog, can lead to process 2)
Platnick
Clouds Formation
Clouds are formed when air containing water vapor
is cooled below a critical temperature called
the dew point and the resulting moisture condenses
into droplets on microscopic dust particles
(condensation nuclei) in the atmosphere.
CLOUDS: A visible mass of liquid water droplets suspended
in the atmosphere above Earth's surface.
http://earthobservatory.nasa.gov/Library/glossary.php3
Clouds can form along warm and cold fronts, where air flows up
the side of the mountain and cools
as it rises higher into the atmosphere,
and when warm air blows over a colder surface,
such as a cool body of water.
A good repository of cloud photos in various
categories can be found
at www.cloudappreciationsociety.org/gallery
Video: cloud formation in Tucson
• http://www.youtube.com/watch?v=NiCSk1z
xMEs
Timelapse of Tucson cloud formations
11
MET 112 Global Climate Change
Video
• http://www.met.sjsu.edu/metr112videos/MET%20112%20Video%20Librarywmv/clouds/DTS-5.wmv
DTS-5.mp4
Clouds-1.mp4 –clouds and aerosols
12
MET 112 Global Climate Change
Clouds Roles - Importance of
Clouds
• Clouds is part of hydrological cycle to move water
• Clouds is key in energy
– Clouds absorb/reflect short wave radiation
(clouds alebedo effect)
– Clouds emit longwave radiation back to space
– clouds absorb surface longwave radiation and keep the
heat in the atmosphere to warm the surface
(clouds greenhouse effect)
13
MET 112 Global Climate Change
100% of the incoming energy from the sun is balanced by
100% percent total energy outgoing from the earth.
incoming energy from the Sun = outgoing energy from the Earth.
since the Earth is much cooler than the Sun, its radiating energy is much weaker
(long wavelength) infrared energy. energy radiation into the atmosphere as heat,
rising from a hot road, creating shimmers on hot sunny days.
The earth-atmosphere energy balance is achieved as the energy received from the Sun
balances the energy lost by the Earth back into space.
So, the Earth maintains a stable average temperature and therefore a stable climate.
15
http://www.srh.noaa.gov/jetstream//atmos/energy.htm
MET 112 Global Climate Change
Earth System Water Cycle
Water freely
evaporating and
condensing
Since more water
molecules are
evaporating than
condensing, then net
evaporation is
occurring.
19
MET 112 Global Climate Change
Q: How and why do clouds form on some
days and not on others?
Q: Why does the atmosphere sometimes produce
stratus clouds (thin layered) while other times we get cumulus,
or cumulonimbus clouds to form?
The answer is largely related to the concept of
atmospheric stability.....
Assessing Atmospheric Stability
to assess stability, what two pieces
of information do we need ?
We need to know
• the vertical temperature profile
•The temperature of parcel of the air
Stability of the environment
• To determine the environmental stability, one must
calculate the lapse rate for a sounding
• lapse rate = DT/DZ = T2-T1/Z2-Z1
• Since the environment is often composed of layers with
different stabilities, it is useful to first identify these layers
and then calculate their respective lapse rates
• recall the stability criteria:
• Γe < Γm - Absolutely stable
• Γm < Γe < Γd - Conditional Instability
• Γm < Γd < Γe - Absolutely unstable
HW5
Stability of the environment
• Characterize the stability of the layers in the
sounding to the right -->
• layer 1
• layer 2
• layer 3
• layer 4
• layer 5
• layer 6
Atmospheric Instability and Cloud
Development
What determines the base (bottom) of a cloud??
Q: What determines the height to which the cloud will grow??
let's use the previous example of a rising air parcel -->>
Q: On this diagram, where is cloud base?
Q: On this diagram, where is cloud top?
Atmospheric Instability and Cloud
Development
•
•
•
•
•
Q: On this diagram, where is cloud base?
A: Where the parcel reaches saturation - 2 km
Q: On this diagram, where is cloud top?
A: Where the parcel will no longer be able to rise - 9 km
Here, Tp = Te - this is often referred to as the
equilibrium level
Concepts of Mixing
es(T)
(T1,e1)
e
saturated
Radiative
Cooling
Mixing
(T2,e2)
unsaturated
T
Hygrometric Chart - Isobaric mixing of two air samples
Platnick
q, w, e, T of the mixed air
• See textbook and notes
M2
• q= M1
q +
M1+M2
W=
e=
T=
1
M1+M2
q2
Saturation Vapor Pressure (Clausius-Clapeyron equation)
At equilibrium, evaporation and condensation have the
same rate, and the air above the liquid is saturated
with water vapor; the partial pressure of water vapor, or
the Saturation Vapor Pressure (es) is:
es (T)  es Ttr  e

Air and
water vapor
T
T Water
L 1 1
( 
)
R v T Ttr
Where Ts=triple point temperature (273.16K), L is the latent heat of
vaporization (2.5106 J/kg), es(Ttr) = 611Pa (or 6.11 mb). Rv is the
gas constant for water vapor (461.5 J-kg1-K1).
specific
Platnick
Saturation Vapor Pressure
An approximation for the saturation vapor pressure
(Rogers & Yau):
e s (T )  Ae
Over liquid water:
L = latent heat of vaporization/condensation,
A=2.53 x 108 kPa, B = 5.42 x 103 K.
Over ice:
L = latent heat of sublimation,
A=3.41 x 109 kPa, B = 6.13 x
103 K.
Platnick

B
T
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
dry adiabat
saturation
adiabat
saturation
mixing ratio
PHYS 622 - Clouds, spring ‘04, lect.2, Platnick
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
ex. Parcel at T=30C, 1000mb, w=5 g/kg
dry adiabat
saturation
adiabat
saturation
mixing ratio
PHYS 622 - Clouds, spring ‘04, lect.2, Platnick
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
ex. Parcel at T=30C, 1000mb, w=5 g/kg
dry adiabat
LCL ≈ 670mb
saturation
adiabat
saturation
mixing ratio
PHYS 622 - Clouds, spring ‘04, lect.2, Platnick
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
ex. Parcel at T=30C, 1000mb, w=5 g/kg
2 g/kg
dry adiabat
saturation
adiabat
saturation
mixing ratio
Platnick
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
ex. Parcel at T=30C, 1000mb, w=5 g/kg
2 g/kg => parcel water content at 500mb = 3 g/kg
dry adiabat
saturation
adiabat
saturation
mixing ratio
Platnick
p R/Cp
Pseudoadiabatic Chart (Stuve diagram)
ex. Parcel at T=30C, 1000mb, w=5 g/kg
LWC = 3 g/kg * rd(T,p) = 3 g/kg * (p/RdT)
= 3 g/kg * 0.68 kg/m3 ≈ 2 g/m3
dry adiabat
saturation
adiabat
saturation
mixing ratio
Platnick
Convective development
(mesoscale, local)
Synoptic development
Cold front - steep frontal slopes
Warm front - shallow frontal slopes
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Clouds are difficult, in part, by the nature
of the relevant spatial scales and interdisciplinary fields
Scale
Relevant Physics
synoptic
~1000s km
(large scale dynamics/thermodynamics, vapor fields)
mesoscale
~100s km
local (cloud scale)
<1-10 km
(dynamics/thermodynamics, turbulence, mixing)
particle
µm - mm
(nucleation, surface effects, coagulation,
turbulence, stat-mech)
molecular
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Convective condensation level
(CCL)
• Text book P47
• The convective condensation level (CCL)
represents the height where an air parcel
becomes saturated when lifted adiabatically
to achieve buoyant ascent. It marks where
cloud base begins when air is heated from
below to the convective temperature,
without mechanical lift.
CCL vs LCL
• The convective temperature (CT or Tc) is the
approximate temperature that air near the surface
must reach for cloud formation without
mechanical lift. In such case, cloud base begins at
the convective condensation level (CCL), whilst
with mechanical lifting, condensation begins at the
lifted condensation level (LCL). Convective
temperature is important to forecasting
thunderstorm development.
• LCL and CCL often agree closely with one
another (R&Y, p48)
Convection: elementary parcel
theory
• See Text book p48-50
• Convection can be induced by buoyant or
mechanical forces
• Buoyant convection represents a conversion
of potential energy to kinetic energy.
• Velocity can be determined from eq. of
motion (4.14)
METR125 Clouds
Emphasis on cloud microphysics: cloud particle nucleation, growth
•
Water Clouds
– Formation concepts
– Water path for adiabatic cloud parcel
– Nucleation theory for water droplets (In October)
•
•
•
Ice Clouds
Aerosol-cloud interaction
Precipitation mechanisms
•
Cloud Modeling (Guest lecture)
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick