Basics of Satellite Imagery Analysis
VS 03UTC, 8 July 2015
29 June 2016Japan Meteorological Agency
Basics of satellite imagery Review of radiation physics Features of each channel VS IR, I2 WV I4 S1 (IR-I2), S2 (I4-IR)
Basics of Satellite Imagery Analysis
Meteorological Satellite
Observation of infrared and reflected solar radiation from the Earth and its atmosphere
Provision of data on atmospheric/cloud movement Useful especially for analysis of weather
phenomena at sea Very important for identification of TS areas
Global ObservingSystem in 2025 4
1978
GMS (JMA)
Satellite Orbits
Geostationary Satellite The geostationary satellite orbits in line with the
earth’s rotation, so remains over the same spot on the earth.
It is positioned at approximately 35,800 km above the equator.
It conducts ongoing observation over a large fixed area (1/3 of the Earth).
It provides data with high temporal resolution. Due to the satellite’s high orbit, the spatial
resolution of its data is limited.
wikipedia
Polar-Orbiting Satellite The polar-orbiting satellite circles the Earth over
the north and south poles at low altitude (for NOAA, around 850 km) within a short period.
As its orbit is lower than those of the geostationary satellites, it can conduct microwave observation.
It can observe the polar regions, which the geostationary satellite cannot.
It passes over the same point on the earth only twice a day.
It observes a swath only about 2,000 km wide centering on the nadir.
Satellite Orbits
Satellite pictures arenot snap-shots.
Scanning takes time.
Physical quantities are averaged over pixels.
Developing Cb smaller than a pixel cannot be visualized
2 km (IR)
- 70℃
- 30℃
- 50℃
- 40℃
- 20℃
0℃
- 40℃
- 80℃
0℃
- 40℃
- 80℃
Resolution
Just under the satellite Longitude
Latitude1 km
Basics of satellite imagery Review of radiation physics Features of each channel VS IR, I2 WV I4 S1 (IR-I2), S2 (I4-IR)
Basics of Satellite Imagery Analysis
Every physical body emits electromagnetic radiation.
There is a relationship between emitted radiation and temperature.
Wikipedia
LowTemperature
Radiation EnergyHigh
• Planck’s law
• Stefan-Boltzmann's Law
I = σ T4 [Wm-2]
• Wien's DisplacementLawλMAX = 2,987 / T [μm]
Relationship between radiation and temperature
wikipediawavelength
radi
atio
n
Source: http://www.nasa.gov/images/content/706436main_20121114-304-193Blend_M6-orig_full.jpg
Wien's Displacement Lawλ MAX = 2,987 / T [μm]
T ~ 6000 K λ MAX = 2,987 / 6000
~ 0.5 [μm] (Visible)
T ~ 300 K λ MAX = 2,987 / 300
~ 10 [μm] (Infrared)
Electromagnetic Spectrum
http://www.colonlove.com/Relax-Far-Infrared-Sauna_ep_41.html
1μm 103μm 1m10-3μm10-6μm10-9μm
VIS IR
Absorption by H2O, CO2, O3, aerosol
Land/ Sea
Absorption
Reflection
Reflection from Land/Sea
Absorption by H2O, CO2, O3
Radiation from H2O, CO2, O3
Radiation from Clouds
↓ Atmosphere
aerosol
IR channelsVisible channels
Absorbed by H2O, CO2, O3, aerosol
Land/ Sea
Absorption
Reflection
Reflection from Land/Sea
↓ Atmosphere
aerosol
Atmospheric Absorption (Solar Radiation)
VS
Land/ Sea
Absorption by H2O, CO2,O3
An Introduction to Atmospheric Radiation (K. N. Liou, 2002)
Atmospheric Absorption (Earth Radiation)
IRI2 WV
Meteorology (M. Morgan, 1997)
Absorption by Atmospheric Gases
Basics of satellite imagery Review of radiation physics Features of each channel VS IR, I2 WV I4 S1 (IR-I2), S2 (I4-IR)
Basics of Satellite Imagery Analysis
16 Bands of AHI(Advanced
Himawari Imager)
Characteristics of Wavebands
VS (0.64 μm) The waveband of the strongest solar radiation
IR (10.4 μm) / I2 (12.3 μm) The waveband of the strongest earth radiation with little intermediate absorption and re-emission (atmospheric window)
WV (6.2 μm)IR channel with significant absorption by water vapor (Water Vapor channel)
I4 (3.9 μm)Affected by both solar radiation and earth radiation. The radiation characteristic for water cloud is different from that of IR1, which enables visualization of low-level cloud (fog) at night.
• Shows intensity of reflected solar radiation (=sunshine).
• Appearance depends on solar elevation angle.
VS 06 UTC, 8 October 2014VS 00 UTC, 8 October 2014
Visible ChannelVS (0.64 μm)
In the morning and evening and in high-latitude areas, images appear darker because there is little incident light due to oblique sunlight and a low number of reflected rays.
Usage:• Distinction between thick and thin clouds
The reflectance of a cloud depends on the number and density of droplets and raindrops in it.
• Distinction between convective and stratiform typesAllows identification of cloud types from cloud top surface texture.
Thick Cloud
Thin Cloud
Smooth and uniform Rugged and uneven
Visible ChannelVS (0.64 μm)
Thin or Thick?
IR VS
Thin or Thick?
IR VS
Thin! Thin is the correct answer.
IR VS
Thin or Thick?
Thick!
IR VS Thick is the correct answer.
Thin or Thick?
• Show distribution of brightness temperature.• Each channel has different radiation
characteristics.
IR 00 UTC, 8 October 2014
• Colder = lighter (white); warmer = darker (black)
• A cloud appears white not because it is a cloud but because it is cold.
Infrared Channels (IR, I2, WV, I4)
IR 00 UTC, 8 October 2014 I2 00 UTC, 8 October 2014
WV 00 UTC, 8 October 2014 I4 00 UTC, 8 October 2014
10.3-11.3 μm
Example:Sensor (IR channel) receives energy at around 10.4 μm.
Converted into TBB(equivalent Black Body Temperature)
I = σT42. エ10-6 5. エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
300 +20℃+10℃0℃-10℃-20℃-30℃-40℃-50℃
2. エ10-6 5. エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
300 +20℃
-10℃
+10℃0℃
-20℃-30℃-40℃-50℃
white
dark
Infrared Channels (IR)
31
3120℃ 1000 hPa 2. エ10-6 5.エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
3002. エ10-6 5.エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
300
850 hPa10℃
0℃500 hPa
Observation of cloud top temperature produces data that can be used to determine cloud top height if the temperature profile is known.
0℃
10℃
20℃
Infrared Channels (IR, I2, WV, I4)
20℃ 1000 hPa 2. エ10-6 5.エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
300
2. エ10-6 5.エ10-6 0.00001 0.00002 0.00005
50
100
150
200
250
300
IR, I2 (Thin Cloud)
5℃600 hPa
Transmission
Total sensor input
0℃ 500 hPa
Which cloud is higher: A or B?
BA
Whiter: colder; darker: warmer
IR
BA
Low temperature: higher
IR
Which cloud is higher: A or B?Whiter: colder; darker: warmer
Infrared Difference Imagery (IR - I2) S1
S1 00UTC, 8 October 2014
• S1: Yellow Sand, Volcanic Eruption, Thin Ci, etc.
Thin Ci
IR - I2 (10.4 – 12.3 μm) is≃0 Positive =0 Negative Positive( Gray Black Light gray White Black )
12.3 μm (I2)10.4 μm (IR)
SiO2
Thick cloud
Different transmissivity characteristicsIR1: cleaner window
Radiation characteristics of
quartz (SiO2)
Volcanic ash and Yellow sandBlackbody
Dry Wet
Cloud free area
Infrared Difference Imagery (IR - I2) S1
Features of Cirrus in Satellite Imagery
37
Here, 11 mm is around 3 - 5 [K] higher in terms of brightness temperature.
I2 IR (10.4 mm) - I2 (12.3 mm)
Brightness temperatureIR > I2
Cirrus or Cb? Darker areas are cirrus.
Yellow sand
Yellow sand forecast
Volcanic Ash
Lower-level Air
Upper-level Air
Mid-level Air
Absorption
Absorption
Absorption
Re-emission
Re-emission
Re-emission
The Higher The Colder
Total sensor input
• Corresponds to the amount of water vapor in the upper and middle layers.
• Cannot detect low-level water vapor.
WV (Water Vapor Channel)
Which area is moister: A or B?
B
A WV
B
A WV
BIAK/FransKaseipo
Minamitorishima/Japan
B is moister.
I4 (3.9 μm) Edge of Solar and Earth Radiation
5. エ10-7 1. エ10-6 5. エ10-6 0.00001 0.00005
250
500
750
1000
1250
1500
1750
2000
Solar radiation
Earth radiation
0.5μm5800K 10μm
290K
Very small amount of energy
Night: earth radiation onlyDay: contaminated by sunlight
3.9 μm
3.9μm11μm
1 pixel
When 5% of a 300 K pixel is 500 K,• 320 K is detected in IR1 (11 μm) • 360 K is detected in IR4 (3.9 μm)
IR1 IR4
300K
320K 360K
Sub-pixel Effect
Fire Detection with I4 (3.9μm)
more affected
Less affected
Fire Fraction
T(11μm)
T(3.9μm)
T surface = 300K
T fire = 500K
Tem
pera
ture
0.05
Response functions to fire differ between IR1 and IR4
Rad
iatio
n
Wave length
I4 IR
More energy needed
Less energy needed
What is this? I4 (3.9 μm) IR (10.4 μm)
What is the black dot?
From Google Maps
Volcanic Eruption
Eruption of Mt. Raung
I4 (3.9 μm) IR (10.4 μm)
What are these? I4 (3.9 μm) VS (0.64 μm)
What are these black dots?
Quoted from Google map
Here
What is this gray area?
I4 (3.9 μm) IR (10.4 μm)
Nothing
What are these?
What are these black dots?
Forest FireI4 (3.9 μm) VS (0.64 μm)
Forest fireSmoke
3.9 μm Difference Imagery (I4 - IR) S2
S2 00UTC, 8 October 2014
• S2: Fog, Forest Fire, Volcanic Eruption, Convective cloud, etc.
Thin Ci
Water Cloud
Split image S2 (I4 - IR (3.9 μm – 10.4 μm))≃0 >0 =0 <0Gray White Gray Black
IR
10.4 μm
I4
3.9 μm
Difference between I4 and IR (Nighttime)
Surface
Ice Cloud
IR
10.4 μm
I4
3.9 μm
IR
10.4 μm
I4
3.9 μm
IR
10.4 μm
I4
3.9 μm
Fog (night time)
S2 (I4 - IR)
IR I4
white Gray Black
VS Large (Reflection) Small
IR, I2 Low (Temperature) High
WV Humid (WV amount) Dry
I4 (daytime)
Small (Reflection) Large
I4(night time)
Low (Temperature) High
S1: IR – I2 < 0 (negative) = 0 > 0 (positive)
S2: I4 – IR < 0 = 0 > 0
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