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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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