chapter 5: image interpretation Flashcards
Satellite imagery can be in one of the following two formats:
Analog and digital
Analog:
which data is displayed in a pictorial or photograph‐type format, independent of what type of sensor was used to collect the data and how the data were collected
interpretation and identification of targets in this imagery is performed manually or visually, ie by human interpreter
Digital
data is represented in a computer as arrays of pixels, with each pixel corresponding to a digital number, representing the brightness level of that pixel in the image
When remote sensing data are available in digital format,digital processing and analysismay be performed using a computer.
Both analogue and digital imagery can be displayed as
black and white (also called monochrome) images, or as
color images by combining different channels or bands representing different wavelengths.
Visible Imagery (VIS)
Images obtained using reflected sunlight at visible wavelengths, 0.4 to 0.7 um
Visible imagery is displayed in such a way that:
- high reflectance objects, e.g. dense cirrus from CB clusters, fresh snow, nimbostratus etc., are displayed as white, and
- low reflectance objects, e.g. much of the earth’s surface, is black
There are grey shades to indicate
different levels of albedo (or reflectivity)
Visible imagery is not available
at night
Infra Red (IR)These images are obtained by measuring
the intensity of the thermal emissions from the earth and the atmosphere, at IR wavelengths in the range 10‐12 um
This so‐called ‘window’ need to be
chosen to allow the satellite sensors to detect such radiation unhindered, and the 10‐12 um band is one such.
For example, the GOES (8‐11) sensors use
the atmospheric infrared window centered at 10.7 micrometers
For example, the GOES (8‐11) sensors use the atmospheric infrared window
centered at 10.7 micrometers
at this wavelenght
energy radiated by the earth’s surface and clouds is not significantly attenuated by atmospheric gases.
For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.
At this wavelength, energy radiated by the earth’s surface and clouds is not significantly attenuated by atmospheric gases
in this channel
most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.
For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.
In this channel, most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.
therefore
the brightness temperature sensed by the satellite is close to actual surface skin or cloud top temperature for other scenes.
IR imagery is so presented that warm/high intensity emissions are
dark grey or even black
IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are
white
IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are white. This convention was chosen so that the output would correspond with that from
the VIS channels
Color slicing is also frequently used whereby
different colors are assigned to various temperature ranges, thus rendering the cooling/warming of cloud tops (and thus the development/decay) easy to appreciate:
- warming/darkening of the imagery with time indicates descent and decay
- cooling/whitening images imply ascent and development
Water Vapor (WV)
This imagery is derived from
emissions in the atmosphere clustered around a wavelength of 6.7 um
The infrared water vapor channel on board GOES‐8to‐11 is located at 6.7 micrometers where the earth’s
emitted spectrum is highly attenuated by water molecules. Thus, this channel senses radiation from the mid‐and upper‐ levels of the atmosphere, from both water vapor and clouds.
IR channel, this wavelength undergoes
strong absorption by WV in the atmosphere (i.e. this isnota ‘window’), and so can also be used to infer vertical distribution and concentration of WV ‐ an important atmospheric constituent
WV imagery uses
the radiation absorbed and re‐emitted by water vapor in the troposphere.
If the upper troposphere is moist
WV emissions will be dominated by radiance from these higher levels is conventionally shown white.
If the upper troposphere is dry
then the sum of the radiation is biased towards lower altitude WV bands: and this is displayed as a shade of grey, or even black.
Because water vapor is transported by
atmospheric circulations
Because water vapor is transported by atmospheric circulations, it allows
the detection of features in the mesoscale flow as well as hemispheric patterns
WV imagery is also very important in the study of
cyclogenesis, often being displayed as a time‐sequence.
Near‐IR (Shortwave IR)
Imagery from a specific wavelength of 3.9um, lies in the overlap region of the electromagnetic (EM) spectrum betweensolar and terrestrial radiation.
Near‐IR (Shortwave IR)
Radiation in this wavelength region is
not significantly attenuated by the earth’s atmosphere.
Near‐IR (Shortwave IR)
these images uses
a mixture of reflected solar radiation plus radiation emitted by the earth and atmosphere.
Near‐IR (Shortwave IR)
it is used in
fog/very low cloud studies
Near‐IR (Shortwave IR)
It is used in fog/very low cloud studies. Interpretation is
sometimes complex, especially in the presence of other tropospheric clouds.
Enhancements are used to
improve the appearance of the imagery and assist in visual interpretation and analysis.
In raw imagery, the useful data often populates
only a small portion of the available range of digital values (commonly 8 bits or 256 levels).
Contrast enhancement involves
changing the original values so that more of the available range is used, thereby increasing the contrast between targets and their backgrounds.
Understanding contrast enhancements requires the concept of
an image histogram
A histogram is
a graphical representation of the brightness values that comprise an image. The brightness values (i.e. 0‐255) are displayed along the x‐axis of the graph. The frequency of occurrence of each of these values in the image is shown on the y‐axis
By manipulating the range of digital values in an image, graphically represented by its histogram, we can apply
various enhancements to the data
There are many techniques of enhancing contrast and detail in an image; the simplest type of enhancement is a
linear contrast stretch
Linear Contrast Stretch
This involves identifying lower and upper bounds from the histogram (usually the minimum and maximum brightness values in the image) and applying a transformation to stretch this range to fill the full range
In our example, the minimum value in the histogram is 84 and the maximum value is 153. These 70 levels occupy
less than one‐third of the full 256 levels available. A linear stretch uniformly expands this small range to cover the full range of values from 0 to 255.
In our example, the minimum value in the histogram is 84 and the maximum value is 153. These 70 levels occupy less than one‐third of the full 256 levels available. A linear stretch uniformly expands this small range to cover the full range of values from 0 to 255.
This enhances
the contrast in the image with light toned areas appearing lighter and dark areas appearing darker, making visual interpretation much easier
Histogram‐Equalized Stretch
A uniform distribution of the input range of values across the full range may not always be an appropriate enhancement, particularly if the input range is not uniformly distributed. In this case, ahistogram‐equalized stretchmay be better
(histogram equalized stretch)
This stretch assigns
more display values (range) to the frequently occurring portions of the histogram.
This stretch assigns more display values (range) to the frequently occurring portions of the histogram.
• In this way, the detail in these areas will be
better enhanced relative to those areas of the original histogram where values occur less frequently.
This stretch assigns more display values (range) to the frequently occurring portions of the histogram.
• In this way, the detail in these areas will be better enhanced relative to those areas of the original histogram where values occur less frequently.
• In other cases, it may be desirable to
enhance the contrast in only a specific portion of the histogram.
For example, suppose we have an image of the clouds, that occupy the digital values from 40 to 76 out of the entire image histogram. If we wish to enhance the detail in the clouds, we could
stretch only that small portion of the histogram represented by the clouds (40 to 76) to the full grey level range (0 to 255).
For example, suppose we have an image of the clouds, that occupy the digital values from 40 to 76 out of the entire image histogram. If we wish to enhance the detail in the clouds, we could stretch only that small portion of the histogram represented by the clouds (40 to 76) to the full grey level range (0 to 255). All pixels below or above these values would be assigned to
0 and 255
For example, suppose we have an image of the clouds, that occupy the digital
values from 40 to 76 out of the entire image histogram. If we wish to enhance the
detail in the clouds, we could stretch only that small portion of the histogram
represented by the clouds (40 to 76) to the full grey level range (0 to 255). All
pixels below or above these values would be assigned to 0 and 255, respectively,
and the detail in these areas would
be lost
For example, suppose we have an image of the clouds, that occupy the digital values from 40 to 76 out of the entire image histogram. If we wish to enhance the detail in the clouds, we could stretch only that small portion of the histogram represented by the clouds (40 to 76) to the full grey level range (0 to 255). All pixels below or above these values would be assigned to 0 and 255, the details in the clouds would be
greatly enhanced
the purpose of a meteorological satellite image interpretation is to
relate atmospheric features on an image to physical processes
the purpose of a meteorological satellite image interpretation is to relate atmospheric features on an image to physical processes. image interpretation can help
in identifying the mechanism
list some of the atmospheric features that are commonly identified on meteorological satellite imegery:
- inter tropical convergence zone
- severe thunderstorms and squall lines
- hurricane structure
- frontal systems
- jet streams
*
the ITCZ is
an easily recognizable, intermittent band of cloudiness that circles the Earth in the vicinity of the equator
The ITCZ is an easily recognizable, intermittent band of cloudiness that circles the Earth in the vicinity of the equator.
The cloudiness is associated with
numerous rain showers and thunderstorms formed by the convergence of the northeast and southeast trade winds.
Developing thunderstorms often look like
popcorn.
Severe thunderstorms quickly extend to
the top of the troposphere or beyond
Severe thunderstorms quickly extend to the top of the troposphere or beyond and are identifiable on
VIS (IR ) images by very bright (cold) cloud tops and by rapid areal expansion of the anvil region.
As storms grow into larger organized systems, they can
take on linear shapes (squall lines) or group into clusters (convective complexes).
A fully matured hurricane consists of:
- eye
- eyewall
- spiral bands
Eye:
The center of a hurricane is known as the eye. It is a 20‐65 km wide region of relatively clear and calm conditions brought about by descending air.
Eyewall
Immediately outside of the eye is the eyewall region, an area of vigorous tall/deep clouds, heavy rainfall, and the strongest observed winds.
Spiral Rain Bands:
Localized areas of tall/deep clouds, heavy rain, and high winds, known as spiral bands, may extend a few hundred kilometers outward from the center of a hurricane.
A more distinct eye usually indicates a
strong system
A more diffuse, less differentiated eye can indicate
either a weak system or one that is transitioning toward a stronger storm.
Eye in IR Imagery
In infrared satellite imagery, a region of relatively warm temperatures is associated with the eye. In this region, the IR sensor may perceive warm, low cloud tops or the surface temperature if the eye is clear.
Eyewall in VIS Imagery
The eyewall can be identified as a bright white ring of clouds associated with tall convective thunderstorms immediately outside the eye.
Eyewall in IR Imagery Eyewall in IR Imagery
The infrared imagery shows the eyewall region as a ring of the coldest cloud tops corresponding to the tops of deep convective cumulonimbus clouds
Spiral Rain bands in VIS Imagery
On the visible satellite image, spiral bands appear as brighter bands of convection spiraling inward toward the center (eye) of a hurricane.
Spiral Rain bands in IR Imagery
Spiral bands may appear as colder features in infrared imagery, but may not be as easilydiscernible as in thevisible imagery.
Satellite cloud photographs are a most valuable aid in locating
fronts and other atmospheric phenomena on a weather map.
………………………………….. indetify fronts on satellite imagery
Distinctive, long cloud bands
Distinctive, long cloud bands identify fronts on satellite imagery. Some extend
several thousand miles and exceed 300 miles in width
Frontal zones contain
both stable and unstable cloud forms
Distinctive, long cloud bands identify fronts on satellite imagery. Some extend several thousand miles and exceed 300 miles in width.
• Frontal zones contain both stable and unstable cloud forms, and through the use of ………………. we can determine …………
infrared imagery, we can determine frontal stability and whether all or part of a front is active or inactive.
Active fronts or ……………… produce ……..
or portions thereof produce appreciable cloudiness and usually precipitation
inactive fronts or …………………………
portions thereof have few clouds and no precipitation.
As a rule, frontal cloud bands are more continuous over
oceans than over land
As a rule, frontal cloud bands are more continuous over oceans than over land.
This rule is related to the
availability of moisture
As a rule, frontal cloud bands are more continuous over oceans than over land. This rule is related to the availability of moisture. Over land, frontal cloud bands can be …………………………………….. while the same front over water may appear ……………………
discontinuous and fragmented, while the same front over water may appear well‐developed and continuous.
Active Cold Fronts
These fronts appear as continuous, well‐developed cloud bands, especially over oceans. Over land, they are much more fragmented and discontinuous because of topography.
These fronts appear as continuous, well‐developed cloud bands, especially over oceans. Over land, they are much more fragmented and discontinuous because of topography.
the bands are made up of
stratiform, cimiliform and cirriform cloud layers
The bands are made up of stratiform, cumuliform, and cirriform cloud layers. In IR imagery, the frontal band appears
off‐white with lines of bright white (convective activity) within the band.
Inactive Cold Front
Over oceans, these fronts appear most often as narrow, fragmented, discontinuous cloud bands.
Inactive cold front
Over oceans, these fronts appear most often as narrow, fragmented, discontinuous cloud bands. They are similar in appearance to
active cold fronts overland.
Inactive Cold Front
Over oceans, these fronts appear most often as narrow, fragmented, discontinuous cloud bands. They are similar in appearance to active cold fronts overland. The clouds are mainly
low‐level cumuli form and stratiform, but some cirriform clouds may be present
Inactive Cold Front
Over oceans, these fronts appear most often as narrow, fragmented, discontinuous cloud bands. They are similar in appearance to active cold fronts overland. The clouds are mainly low‐level cumuli form and stratiform, but some cirriform clouds may be present. Overland, inactive cold fronts or portions thereof, have
few or no clouds
Warm Front are difficult to locate on
difficult to locate on satellite imagery
Anactive warm front may be associated with
a well organized cloud band, but the frontal zone is difficult to locate.
An active front maybe placed
somewhere under the bulge of clouds that are associated with the peak of the warm sector of a frontal system
An active front maybe placed somewhere under the bulge of clouds that are associated with the peak of the warm sector of a frontal system. The clouds are a combination of
stratiform and cumuliform beneath a cirriform covering.
Inactive warm fronts
cannot be located at all, because they are, for the most part, cloud free.
It is to be noted that each front presents a different situation with respect to the
air masses involved
It is to be noted that each front presents a different situation with respect to the air masses involved. Each front must be treated as a
separate case
It is to be noted that each front presents a different situation with respect to the air masses involved. Each front must be treated as a separate case, by using
- present indications
- geographical locations
- stability of the air masses
- moisture content
- intensity of the front
A cold front is always drawn on
the warm side of the frontal zone, so most of the frontal clouds appear just behind the front, as the cold air mass wedges under and lifts the warmer air ahead of it.
Cold fronts usually extend quite far …………….. from ……….. pressure systems
far south from the low pressure system
The warm front is also always drawn on the
warm side of the frontal zone, so a lot of the clouds appearaheadof it, as the warm air rises up slowly over the cooler air ahead of it.
The occluded front extends from
the junction of the warm and cold fronts back towards the low center
jet streams are driven by
the temperature contrast baused by the uneven heating of the earth’s surface
The streams are driven by the temperature contrast, caused by the uneven heating of the Earth’s surface. The greater the contrast in temperature, the
faster the wind speed in the jet.
In water vapor images the location of the jet stream can be identified as
a strong gradient from high to low humidity (from white to dark colors) on side of the frontal cloud bands that is facing towards the pole.
Water surfaces normally appear darker than land in
visible image
A sea surface with a high sun angle has an albedo of about
0.05-0.10
A sea surface with a high sun angle has an albedo of about 0.05‐0.10, where as land (soil) can have an albedo of
0.10 to a s high as 0.35 over sand
Forests typically have albedos of
0.03 to 0.10
The contrast between land and ocean is not so great over a
rough ocean surface where less of the solar radiation is reflected back toward the satellite.
In the infrared, the difference in the land/water signature is a function of their
temperatures
In the infrared, the difference in the land/water signature is a function of their temperatures. A cold water surface next to a warm land surface gives
sharp contrast
In the infrared, the difference in the land/water signature is a function of their temperatures. A cold water surface next to a warm land surface gives sharp contrast, whereas a cold water surface next to a cold land surface may be
indistinguishable.
In addition, the IR images can show
the warm currents such as the Gulf stream based on the temperature contrast
A sandstorm or duststorm is a meteorological phenomenon usually caused by
strong and turbulent winds blowing over loose soil or sand and sweeping up large quantities of sand or dust particles from the ground, clouding the air and reducing the visibility drastically.
In general, sandstorm/duststorm can be defined as an
ensemble of particles of sand and dust energetically lifted to great heights by a strong and turbulent wind
Dust, when carried aloft by strong winds, can be detected on
satellite imagery
Dust is most easily seen on the visible image when it is
advected over a surface of relatively low reflectivity, such as an ocean
Dust, or blowing sand, is not as easily distinguishable over
brighter regions, such as deserts, because it exhibits a reflectivity that is similar to the surface.
Sun glint is a
Sun glint is a
Sun glint is a bright spot on the visible image caused by the sunlight reflecting from a smooth surface (Figure).
The signature is most often seen over
the ocean
The signature is most often seen over the ocean, and the intensity of the spot
indicates
the character of the surface
when is the spot well defined
If surface winds are calm, the ocean is smooth, the reflection is strong and the spot is well defined
when is the spot less well defined
With stronger winds, the sea surface is rough, the reflection is more diffuse, and the spot is less well defined.
Note that the dust and sun glint are not discernible on the
infrared image.
snow and ice make ………………………….. difficult
cloud discrimination
In the visibleimage, snow and ice can have
a reflectivity that is close to that of a cloud
In the visibleimage, snow and ice can have a reflectivity that is close to that of a cloud, whereas in theinfrared
the surface is close to that of a low cloud
The primary difference between cloud and snow
snow does not move
This simple fact is the basis for distinguishing snow from cloud by using
successive images to detect clouds by their movement across a snow field
Snow and ice can sometimes be identified by
the fact that they conform to geographic features.
……………………….are often free of snow
valleys, rivers and lakes
Valleys, rivers, and lakes are often free of snow, whereas …………………………….. are covered
sounding land surfaces
Valleys, rivers, and lakes are often free of snow, whereas surrounding land surfaces are covered, making it easier to
distinguish between clouds and snow.
Sandy surfaces have a ………………………………than soil and vegetation in the ………………… channel
higher reflectiity
visible
Some sandy regions, such as ………………………………………….. have a reflectivity that is close to ………………………………..
the white sands in New Mexico
snow and clouds
In the infrared, the sandy surface appears
cooler at night and warmer during the day than a corresponding soil or vegetation surface.
Agricultural land exhibits
variable reflectivity and emittance during different seasons of the year.
When the fields are freshly plowed they normally appear
darker than a few days later when the ground is dry.
When the fields are freshly plowed they normally appear darker than a few days later when the ground is dry.
• The field is normally even darker after
crops have grown to cover the bare ground.
