EO 306.1 Explain the role of satellite imagery in PBS
and forecasting.
Satellite imagery is an indispensable tool. In order to properly interpret
satellite imagery, the briefer must have:
- A strong knowledge of satellite systems and imagery
- A strong knowledge of interpretation techniques
- Skill in interpreting satellite
Role of satellite during the briefing
Satellite imagery:
- Is an excellent visual aid
- Helps to fill in information in data-sparse areas
- Is a source of real-time information
- Is a good tool to update charts with changing weather conditions
Role of satellite in forecasting
Satellite imagery:
Role of satellite in forecasting
Satellite imagery:
- Is used as an analytic tool, especially in data-sparse areas
- Is a direct aid in short-term forecasting
- Provides input into numerical weather prediction (NWP) models
- Is used to monitor performance in the early stages of a forecast
EO 306.2 Describe the two types of operational weather satellite systems.
LIST THE TWO TYPES OF SATELLITES SYSTEMS:
- Geostationary (GOES)
- Polar-orbiting (POES)
Where does the satellite point
The satellite always points in the same direction and is maintained at the
same position over the surface of the equator. This position is known as the
satellite sub-point
EO 306.3 Describe the basic principle of satellite
imagery obtained from a satellite radiometer.
LIST THE TWO TYPES OF RADIATION MEASURED BY SATELLITE
RADIOMETERS:
- Solar (visible) radiation reflected from the Earth’s surface and the top of
clouds (shortwave radiation).
- Terrestrial (infrared) radiation emitted by the Earth’s surface and the
atmosphere (longwave radiation)
What us the basic principle of satellite
Basic principle
Satellite imagery is obtained from sensors aboard the satellite called
radiometers that measure electromagnetic radiation emanating from the
Earth’s surface and the atmosphere.
Substances that absorb all types of radiation at every wavelength are
called
black bodies (BOLD). A black body emits the maximum amount of radiation
corresponding to its temperature
There are other regions
where the absorption by atmospheric gases is so weak that the atmosphere
is almost transparent. These regions of minimal absorption are known as
windows and are used for most imagery production as follows:
- Window between 0.3 to 0.8 µm is used for VIS (visible) imagery
- Window between 10 to 13 µm is used for IR (infrared) imagery
- Window between 6 to 7 µm is used for WV (water vapour) imagery
LIST THE ADVANTAGES AND DISADVANTAGES OF THE TWO SATELLITE
SYSTEMS:
- Distance from Earth
- Availability of images
- Resolution at sub-point
- Resolution at high latitude
- Cost
- Life expectancy
- Solar eclipses
EO 306.4 Describe the four spectral bands currently
used by NAV CANADA for the provision of PBS.
LIST THE FOUR SPECTRAL BANDS USED BY NAV CANADA:
- Visible (VIS)
- Near infrared (NIR)
- Water vapour (WV)
- Infrared (IR)
VISIBLE IMAGERY:
1-4 km resolution
› Daytime only
› Similar to a black and white
photograph
› Brightness depends on
albedo and sun angle
› Shadows add dimensions
Characteristics affecting a cloud’s albedo
Clouds have a higher albedo than land (other than snow cover) and appear
white or light gray in a normal display. Their brightness, or albedo, depends
on their physical properties.
Clouds with a high albedo have:
Clouds with a low albedo have:
- Large vertical height or thickness
- High cloud-water content
- Small average cloud-droplet size
Clouds with a low albedo have:
- Shallow height or thickness
- Low cloud-water content
- Large average cloud-droplet size
Problem areas in interpreting VIS imagery
a. Distinguishing clouds from snow-covered ground
b. Small clouds
The radiation reaching a satellite from a field of small, broken
cumulus clouds is emitted or scattered partly from the clouds and
partly from the Earth’s surface. If the clouds are smaller than the
resolution of the satellite radiometer, the imagery will have a uniform
character, unlike a normal view of convective clouds.
Thin clouds
Thin clouds may allow scattered light from lower clouds (or from the
Earth’s surface) to reach the satellite. The corresponding image will
be a mixture of the brightness originating at more than one level.
Thin cloud over a high-albedo desert surface may look misleadingly
bright and thick, whereas the extent of thin cloud over dark surfaces
such as the ocean may be underestimated.
What are the advantages and disadvantages of visible imagery
Advantages of visible imagery:
- Representation is unambiguous (what you see is what you have)
- Image is similar to a black and white photo
- Important for daytime cloud and surface mapping
- Brightness is proportional to the degree of solar reflection
- The atmosphere is transparent to this type of radiation
- Image can be enhanced to bring out fringe areas
Disadvantages of visible imagery:
- Only available during daylight hours
- Brightness of surfaces, orientation and sharpness of shadows vary
diurnally and seasonally
- Surfaces with similar albedo difficult to distinguish
What are the advantages and disadvantages of Near-infrared
Advantages of near-infrared imagery:
- Similar advantages to visible images
- As good for high albedo/warm versus low albedo/cold contrast
Disadvantages of near-infrared imagery:
- Similar advantages to visible images
- Not as good as VIS for high albedo/cold versus low albedo/warm contrast
Near Infrared Characteristics:
NEAR-INFRARED IMAGERY
1 km resolution
› Similar to visible imagery
› Mostly albedo, very small
contribution from temperature
› Good for land / water
differentiation
› Available with POES
› Daytime only
WATER VAPOUR IMAGERY Characteristics
8 km resolution
› Displays high and mid level
moisture
› No clouds required
› Black = dry = high flux
› Animation displays circulation
patterns
› Only available on GOES
Advantages and disadvantages of water vapour imagery
Advantages of water vapour imagery:
- A good representation of moisture distribution in mid and high-levels
- A good representation of mid and high-level circulation
- Cold upper lows, jet streams, dry air intrusions, deformation zones
Disadvantages of water vapour imagery:
- Low water vapour concentrations semi-transparent
- Surfaces with similar gray tones difficult to interpret
- Difficult to interpret moisture height and low-level moisture
- Measures water vapour concentrations through a layer, not at a specific
level
- Coarse resolution, only 8 km
- Only available on GOES
Advantages and disadvantages of infrared imagery
Advantages of infrared imagery:
- Provides a temperature representation of Earth-atmosphere system
- Is available continuously (i.e., not dependent on daylight)
- Is important for day and night cloud and surface mapping
- Shows atmosphere transparent to its wavelength
Disadvantages of infrared imagery:
- Surfaces with same temperature are impossible to distinguish
- Misinterpretations are common with thin or scattered clouds due to
contamination from below
- Non-enhanced images are difficult to interpret
- Resolution (4 km) is not as good as VIS (1 km) on GOES images
Overview of image sources
What are the frequency of images provided for each type of satelitte
Frequency of images:
Geostationary: Four per hour for each
satellite (over North America)
Polar:Two per day per satellite for
any particular location
Types of image available:
Geostationary: VIS, Water vapour, IR
Polar: VIS, NIR, IR
Received by NAV CANADA
wavelength in microns >:
Geostationary
VIS 0.6
WV 6.7
IR 10.7
Polar: VIS 0.6
NIR 1.0
IR 10.7
O 306.5 Contrast diurnal and seasonal variations in visible and infrared satellite images.
DIURNAL VARIATIONS in satellite
Seasonal Variations:
Strong sunlight / no sunlight
› Usually warmer during daytime
› Sun angle changes
› Sun glint
› Cumulus type clouds
› Nocturnal inversions
Seasonal:
Snow cover
› Convective clouds
› Pressure systems
› Northern latitude
› Vegetation
› Water bodies
› Water temperature
EO 306.6 Explain the greyscale code on visible and
infrared satellite images.
STATE THE PURPOSE OF ENHANCING THE GREYSCALE
Image enhancement is the process of assigning new values to a range of
pixels in order to identify phenomena of interest.
All satellite images received by NAV CANADA contain up to 256 shades of
grey. However, the human eye can only distinguish 15 to 40 tones.
EO 306.7 Decode the label on any meteorological
satellite image.
There are two types of image labels:
- The label that is displayed in the list of products in an image manager
text window - The label that is part of the image
