CLOUDS AND SATELLITE IMAGERY

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CLOUDS AND SATELLITE IMAGERY

• MSC 243 Lecture 3
• 9/10/09

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Road Map September

• Today Homework 1 given
• Tuesday 15th Lab on Satellite imagery
• Tuesday 15th Homework 1 due
• Thursday 17th Hurricane Forecasting (Chris
  Landsea, NHC)
• Tuesday 22nd Hurricane Lab (if there is a
  hurricane!)
• Thursday 24th Homework 2 given
• Monday 28th We start forecasting!

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Surface analysis based on Station Models
How is wind related to pressure?
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Winds No Friction

• Winds, in the absence of any frictional forces,
  are the result of a balance of forces acting on a
  parcel of air
• Pressure Gradient Force
• -(1/r)(dP/dy)
• Coriolis Force
• 2Wsin(latitude)(velocity)
• Net wind blows parallel to lines of constant
  height or pressure, with lower values on to the
  left of the wind direction

PGF
WIND
CORIOLIS
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Upper tropospheric Winds
Winds blow parallel to lines of constant height
or pressure The tighter the gradient of
pressure, the stronger the wind The balance of
the pressure gradient force and Coriolis force is
called GEOSTROPHIC BALANCE
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Surface Winds
LOW
PGF

• Winds are the net result of forces acting on a
  parcel of air
• Pressure Gradient Force
• Coriolis Force
• Friction Caused by the surface roughness

996 mb
1000 mb
WIND
1004 mb
FRICTION
1008 mb
CORIOLIS
1012 mb
HIGH
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Surface Winds
Surface winds spiral into and counter clockwise
around a low pressure center Surface winds
spiral out from and clockwise around a high
pressure center
http//www.rap.ucar.edu/weather/model/
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Temperature Advection
If Surface winds carry (advect) warmer/colder
air, the local temperature will
increase/decrease Blue circle area of cold
advection Red circle area of warm advection
http//www.rap.ucar.edu/weather/model/
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This Lecture

• Types of Clouds
• Satellites
• Geostationary and Polar
• Radiative Transfer
• Visible, Infrared and Water Vapor
• Identifying clouds from satellites
• Other types of satellite imagery

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Types of Clouds

• Sorted by height
• High (gt 7km) Cirrus (ci), Cirrostratus (cs),
  Cirrocumulus (cc)
• Middle (2-7km) Altostratus (as), Altocumulus (ac)
• Low (lt 2km) stratus (st), cumulus (cu),
  strato-cumulus (sc)

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Types of Clouds

• Sorted by structure
• Cirrus Composed of Ice, wispy
• Cumulus Detached elements puffy
• Stratus Uniform Layer, sheetlike
• Nimbo Precipitation producing
• Nimbostratus
• Cumulonimbus

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Cirrus Clouds
Ice crystals (frozen super-cooled water
droplets). Fair weather, point in direction of
air movement at their elevation. Often associated
with changing weather.
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Cumulus Clouds
As Earth is warmed by the sun, bubbles of warmer
air (thermals) rise upward from the surface.
Water vapor in a thermal cools and condenses as
it rises and mixes with the surrounding air.
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Stratus Clouds
Uniform gray cloud that covers the entire sky.
Sometimes accompanied by a light mist or drizzle.

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Cumulonimbus
Thunderstorm clouds, much larger and more
vertically developed than fair weather cumulus.
Formed by convective updrafts. Water droplets at
lower levels, ice crystals at higher levels.
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Radiation in the Atmosphere
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Effect of Clouds on Temperature

• When forecasting maximum temperatures, remember
  that thick clouds block the sun, reducing heating
  at the surface.
• When forecasting minimum temperatures, remember
  that thick clouds inhibit surface cooling.
• Note that thin cirrus clouds have less of an
  effect than thick low clouds.

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Satellite Imagery

• 1st weather satellite TIROS 1 (1960)
• Satellites observe clouds as well as non-visible
  radiation (aerosols, ozone)
• Rest of lecture
• Orbits
• Radiative Transfer
• Interpretation of images
• Other types of satellite imagery

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Geostationary Orbiting Satellites

• Satellite observes same area at all times.
  Continuous sampling.
• 36000km from Earths surface!

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Polar Orbiting Satellites

• Satellite observes globe in swaths
• 700-1700km from Earths surface

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Polar Orbiting Satellites
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Radiative Transfer

• Electromagnetic radiation alternating electric
  and magnetic fields.

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Radiative Transfer

• Radiation is specified by wavelength or
  frequency. Speed freq x length

microwave
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Radiative Transfer

• Electromagnetic radiation can transport energy.
• Fundamental radiation unit radiance
• Radiance energy per unit time per unit
  wavelength per unit solid angle crossing a unit
  area 90o to beam.
• RADIOMETER measures energy per unit time striking
  it, at select frequencies (or wavelengths).

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NASA Satellite Radiometer
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Radiative Transfer

• Each object (including us!) emits radiation with
  a particular frequency spectrum.
• Satellite instruments are designed to measure
  radiances in specific frequency channels,
  corresponding to the desired frequency that
  objects / gases emit at a given temperature.

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Radiative Transfer

• Radiative energy depends on wavelength l and also
  temperature T.
• One can estimate temperature from brightness
  brightness temperature.
• Total emitted radiation E s T4
• Radiation reaching a body is then ABSORBED,
  REFLECTED or SCATTERED OUT by the body.

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Visible Image
VIS imagery indicates the amount of solar
radiation reflected from the Earth, in the 0.4 to
0.7 micron wavelength range. A VIS image is an
approximation of the Earth's albedo ( of
sunlight reflected by a surface). Light tones
show high reflectivity, darker tones show low
reflectivity. Features on the surface of the
Earth or in the atmosphere vary in their
reflectivity and can be discerned on a VIS image.
No data at night.
The large, thick clouds appear white since they
have a high albedo. Thinner clouds appear light
to medium gray. The ocean, with a very low
albedo, appears nearly black. The land,
characterized by albedos that depend on the
nature of the surface, appears as various shades
of gray.
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Infrared Image
The IR sensors on board the polar orbiting and
geostationary satellites measure the amount of
infrared energy emitted by the Earth and the
atmosphere. Because the amount of energy emitted
depends on the temperature of the surface, IR
imagery is essentially a picture of the surface
and cloud top temperatures portrayed in black,
white, or gray shades. This information can be
used to observe thermal properties of the Earth
and the atmosphere. In conventional IR imagery,
colder areas appear as white or light gray tones
and warm areas appear black or dark gray.
The highest (and coldest) cloud tops appear
white. Lower clouds appear as lighter shades of
gray, and warmer land and water surfaces appear
as darker shades of gray.
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Water Vapor Image
In addition to the visible and thermal infrared
images, it is possible to record energy absorbed
and re-radiated by water vapor. Infrared energy
radiated by water vapor at wavelengths of 6.7 and
7.3 microns is measured by the satellite
radiometer and converted to brightness
temperature. The grayness of each pixel in a WV
image corresponds to a specific brightness
temperature. The darker regions are areas where
very little water vapor exists in the middle and
upper troposphere, and the lighter regions are
moist.
Water vapor imagery shows the altitude of the
highest moist layer in the atmosphere, not just
cloud patterns. Meteorologists are able to
observe large-scale circulation patterns even
when clouds are not present.
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What is this?
IR
VIS
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What this is
Snow
Snow
Cirrus
Cirrus
Stratus
Stratus
Snow is reflective, but not too cold. Stratus in
Mississippi is the same, and the wispy but cold
values in Ontario are cirrus
http//www.rap.ucar.edu/weather/satellite/
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What is this?
IR
VIS
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What this is
Cumulus
Cumulus
Cumulo Nimbus
Cumulo Nimbus
Cirrus
Cirrus
The low (warm) and scattered clouds in Georgia
are cumulus. The scattered by higher clouds in
central Florida are cumulonuimbus. The wispy
clouds southwest of Florida are cirrus clouds
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What this is
Cumulus
Cumulus
MCN
MCN
Cumulo Nimbus
Cumulo Nimbus
TPA
TPA
NQX
NQX
Cirrus
Cirrus
KMCN 272053Z 28008KT 10SM FEW055 33/19 A2999 RMK
AO2 SLP152 T03280194 58023 KTPA 272053Z 01015KT
6SM TS HZ FEW020 BKN040CB BKN080 OVC250 28/23
A3005 RMK AO2 TSB45 SLP175 FRQ LTGICCG TS OHD-N
MOV S TCU E VCSH VIS LWR N T02780228 56015 KNQX
271955Z 11008KT 6SM HZ SCT020 BKN080 BKN250 30/24
A3006 RMK SLP181 T03000244
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Other Satellite Products

• Satellites observe at many frequencies. The most
  widely used frequencies are not able to see
  through clouds.
• Here is collection of satellite products useful
  for hurricane analysis and prediction.

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Winds from Satellite Upper Level
Water Vapor and Infrared. http//cimss.ssec.wisc.
edu/tropic2/
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Winds from Satellite Low Level
Tracking the velocity of low-level clouds
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Wind Shear
200 hPa wind 850 hPa wind
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Upper Level Divergence
Spreading out of wind at upper levels.
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Low Level Vorticity
Local spin of atmospheric elements
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• Seawinds on QuikSCAT
• Pencil-beam scatterometer

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Sea Surface Winds (QuikScat)
Scatterometer is an ACTIVE satellite sensor. It
sends a high-frequency microwave beam to the
ocean surface. The back-scatter is measured by
the satellite sensor.
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Saharan Air Layer
This imagery is created by differencing the 12.0
and 10.7µm infrared channels. This difference is
sensitive to the presence of dry and/or dusty air
in the lower atmosphere (600-850 hPa or
4,500-1,500 m) and is denoted by the yellow to
red shading. This imagery is useful for
monitoring the position and movement of dry air
masses such as the Saharan Air Layer (SAL).
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Advanced Microwave Sounding Unit
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Tropical Rainfall Measuring Mission (TRMM)
Microwave Imager
Before eyewall replacement
After eyewall replacement
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Sea Surface Temperature
Infrared Satellite data measures the temperature
of whatever is radiating into space. By
accounting for effects of the atmosphere, an
estimate of the surface temperature can be made.