Transcript Chapter 6

Chapter 6
Remote Sensing
Satellite Thermal Sounders
Atmospheric gases
absorb radiation at
specific wavelengths.
They emit at those same
wavelengths.
The intensity of the radiation emitted is
proportional to the temperature of the gas.
4
I  T
Stephan Boltzman Equation.
As the radiation moves upward toward satellites,
that radiation emitted by specific molecules can be
absorbed by the same type molecules higher in
the atmosphere.

These molecules will then emit radiation, again at
particular wavelengths.
From the amount of energy received by a satellite
at a particular wavelength, the temperature of the
emitting gas can be determined.
Filters on the photomultiplier tubes that
detect radiation are set to allow only
specific wavelengths through.
These wavelengths (channels) are chosen
by the wavelengths of particular gases that
are fairly evenly distributed throughout the
atmosphere - primarily the troposphere;
e.g., CO2.
Some wavelengths it strongly absorbs and
emits, others it absorbs/emits less strongly.
And, some wavelengths it hardly absorbs
at all.
By choosing the
proper channels,
temperatures can be
determined for levels
high in the
troposphere,
(opaque), near the
middle of the
troposphere, (semitransparent), or near
the earth’s surface,
(nearly transparent).
•Clouds are opaque at all three
wavelengths.
•This image shows temperature variations
in upper troposphere.
•Warm tems over N. California - Ridge.
Cool over central US - Trough.
Shows surface temps. If no clouds
are present.
Southwest is warmest.
Cool water temps off west coast.
Low clouds show up over central
US.
Blurry view of ground.
Sierra Nevada shows as line of
cooler temps.
Warm temps off Baja California middle and lower troposphere is
warm.
By choosing channels sensitive to water vapor, a
profile of water vapor can be obtained.
Clouds (liquid water) is opaque and so water vapor
profiles can be made in these areas. (Shown as gray
or white on image).
Total column water vapor (precipitable water).
Shows depth of
water if all water
vapor in column
condensed and
precipitated.
(2) Images and gradients?
Most meteorological satellite images are presented
at about 4 km resolution, (pixel resolution) although
capabilities exist for much smaller resolution. The
latest GOES-13 can give images to 1km resolution,
and, of course, spy satellite at even much smaller
scale.
Since with infrared satellite images, a temperature
for each pixel can be determined, then a gradient
can be determined, (the derivative in the horizontal).
One must be careful near the edges of the image
because of the change of scale that occurs. It is
often difficult to determine distance there.
(3) Water Vapor Images
There are certain frequencies that water
vapor molecules absorb and emit radiation.
The intensity of emitted radiation is
dependent on the temperature of the
molecule.
Meteorological satellites detects radiation
from a variety of channels. Various
instruments detect radiation from those
channels in which water vapor emits
radiation. (AVHRR, AMSU A/B, HIRS, etc.)
Water Vapor Images
Channels are usually chosen to detect radiation from
water vapor that is in the levels about 500 mb up to
300 mb.
If a lot of water vapor in this region, radiation
reaching the satellite is primarily from the upper
portion (Temperatures are cold - Intensity is low).
Shown as white on image.
If the region is relatively dry, radiation is coming from
lower in the layer (Temperatures are warm - Intensity
is high). Shown as black on image.
Cannot detect low level water vapor.
Upper troposphere
off California is dry descending air.
This was region of
low clouds on
“transparent” image.
Very white areas are
high clouds - cirrus
and cb’s.
Circulation northeast
of Hawaii.
Cold front entering
northwest Oregon.
(4) Infrared Images
Receives radiation (11-12 m) which is in the
Atmospheric Window (8-12 m).
Atmosphere is transparent.
Satellite also detects emitted radiation for
earth’s surface.
Can determine surface temperatures - if no
clouds are present.
Can determine cloud top temperatures:
Colder - higher.
Warmer - lower.
Often colorized by temperature.
High clouds cold - are white
in this image.
Low clouds warm - are gray.
(5) Visible Images
Satellite detects radiation emitted from
clouds and the earth’s surface.
Can more easily distinguish high clouds
from low clouds.
Low clouds and snow covered surfaces
may be hard to detect.
Only available when Sun is shining.
(6) Wind Profilers and Radar
General weather radars emit radiation at 10-cm
which is a good wavelength for scattering of
raindrops and ice crystals, hail.
Other weather radars may use 3-cm or 5-cm
wavelengths.
The signal is emitted. Some reflects off the object of
interest and returns to the radar antenna.
The time it takes for the signal to go out and return
determines how far away the object is. The direction
the antenna is point when the signal is emitted and
returns along with distance determines the location
of the object.
For raindrops, ice
crystals, etc., what
is being detected
is a group, not
each individual
particle.
•Doppler Radar
The difference between Doppler
radar and non-Doppler radar is that
with Doppler, the phase change that
occurs due to the object moving can
be determined.
That difference is dependent on the
speed with which the object (or at
least a component of the speed) is
moving toward or away from the
radar antenna.
If the object is a volume of raindrops and it
moves horizontally because the wind blows
it horizontally, then the wind speed and
direction can be determined by how the
volume of raindrops move.
Because the weather radar usually does not
point horizontal but rather at some elevation
angle above horizontal, the objects reflecting
the signal are higher above ground the
farther the objects are from the radar.
Since Doppler radar will give the wind speed
along the radar beam and whether the wind
is moving toward the radar or way, the
horizontal wind can be determined.
The vertical wind component is usually
much less then the horizontal wind
components if the radar is not directed
upward at a high angle, the vertical wind
component can usually be neglected.
Notice the black
region roughly to the
east and west of the
center. And black
indicates very weak
winds, nearly calm.
Does this mean the
wind is not blowing in
this region?
Wind Profilers
Wind profilers are relatively low-power, highly
sensitive, clear-air radars, operating with
wavelengths from 33-cm to 6 meters. The
radars detect minute fluctuations in
atmospheric density caused by the turbulent
mixing of volumes of air with slightly different
temperature and moisture content. The
resulting fluctuations of the index of refraction
are used as a tracer of the mean wind in the
clear air.
The antenna is really a group of antennas in
a grid. The radar sends a signal vertically (all
antennas emitting at the same time).
Then it emits a signal with antennas emitting
at slightly later times so the wave front of the
signal moves upward at an angle 15o away
from vertical and toward the north.
Then another signal is emitted 15o away from
vertical toward the east.
The signal emitted vertically will give the
component of the wind moving directly up or
down.
The signal emitted 15o from vertical toward
the north will contain the components of the
wind blowing both vertically and toward the
north.
The signal emitted 15o from vertical toward
the east will contain the components of the
wind blowing both vertically and toward the
east.
The components:
u, v, w of the wind
can then be
determined from
these equations:
w  vel(A)
vel (C) - vel (A) cos15

u=
sin15
vel (B) - vel (A) cos15

v=
sin15
Knowing the u- and v-components, the horizontal
wind can be determined.

Wind information from wind profilers
are customarily plotted either with time
increasing to the right, or with time
increasing toward the left. With time
running backward, weather features
that pass a station must have started
out farther east than others.
This VAD (velocity azimuth display) shows time
increasing toward the right.
Profiler data
from
Bonneville,
Utah.
Time
increases
toward the
left.
(8) Other Time-Height Sections
Time-Height analyses may be made for
any parameter that is measured or
forecasted.
The next two examples show analyses
of temperature and potential
temperature.
Time-Height of Temperature
Warm
afternoon
temperatures
confined to
roughly the
lowest 100 150 mb.
(isotherms
begin to flatten
horizontally)
Note wind shift
about 18/06Z
Time-Height of Potential Temperature
Note: A wellmixed
atmosphere
(lot of vertical
motion)
exhibits a dryadiabatic lapse
rate and
constant
potential
temperature
with height.
A stratified atmosphere exhibits an increase of
potential temperature with height.
(9) Vertical Sections
Often used to depict the characteristics off
some particular feature; such as, a jet
stream, front, etc., by orienting the section
along a line perpendicular to the feature.
Data plotted on the section is taken from
stations within a designated distance of the
line.
Then the parameters of interest are
analyzed.
Questions
Do: 3, 4, 5