APPLICATIONS OF METEOSAT SECOND GENERATION (MSG)

Version 0.6, 30 June 2004

APPLICATIONS OFMETEOSAT SECOND GENERATION (MSG)RGB IMAGES: PART 03

CHANNEL SELECTION AND ENHANCEMENTS

Author:Author: Jochen Kerkmann (EUMETSAT)Jochen Kerkmann (EUMETSAT)[email protected]@eumetsat.de

Contributors:Contributors: D. Rosenfeld (HUJ), HP. Roesli (MeteoSwiss)D. Rosenfeld (HUJ), HP. Roesli (MeteoSwiss)M. König (EUM)M. König (EUM)


PART 3:

CHANNEL SELECTIONAND ENHANCEMENTS


Optimum colouring of RGB image composites depends on:

• I. Selection of the channelsI. Selection of the channels– Physical properties represented by the channels (e.g. IR10.8 provides cloud top

temperature, VIS0.6 provides cloud optical thickness, VIS0.8 provides "greeness" of vegetation etc…)

• II. Attribution of images to individual colour beams depends on:II. Attribution of images to individual colour beams depends on:a Reproduction of RGB schemes inherited from other imagers;b Contrast and colours of the resulting RGB composite, which can be more or less pleasant

(depending on personal view);• III. Proper enhancement of individual colour channels requires:III. Proper enhancement of individual colour channels requires:

a Conversion from radiances to brightness temperatures/reflectances;b Selection of display mode (inverted or not inverted);c Stretching of the intensity ranges (linear stretching of active dynamic range);d Gamma corection;e Gamma2 correction;

• IV. Final enhancementIV. Final enhancement– e.g. Gamma correction of final RGB image,

Channel Selection & Enhancements


I. (Cloud) Physical Properties representedby the MSG Channels

VIS0.6: optical thickness and amount of cloud water and iceVIS0.8: optical thickness and amount of cloud water and ice

"greeness" of vegetationNIR1.6, IR3.9r: particle size and phaseWV6.2, WV7.3: mid- and upper level moistureIR8.7, IR10.8, IR12.0: top temperature

IR8.7 - IR10.8: phase and optical thicknessIR12.0 - IR10.8: optical thicknessIR3.9 - IR10.8: optical thickness, phase, particle sizeIR13.4 - IR10.8: top heightWV6.2 - IR10.8: top height, overshooting tops


II.a RGB Schemes inhereted fromNOAA AVHRR

The most popular RGB schemes used for NOAA AVHRR images are:

RGB 1-2-4 (Day) RGB 1-3A-4 (Day) RGB 3B-4-5 (Day/Night)VIS0.6-VIS0.8-IR10.8 VIS0.6-NIR1.6-IR10.8 IR3.7-IR10.8-IR12.0

O. Hyvärinen, FMI, 2003


II.a RGB Schemes inhereted from MODISFor MODIS (27 channels, not considering channels 8-16) there are 17550 different ways (this is 27!/(27-3)!) to choose 3 channels from 27 channels. Some popular RGB schemes used for MODIS images are:

01-04-03 (VIS0.6-VIS0.5-VIS0.4)01-02-03 (VIS0.6-VIS0.8-VIS0.4)01-06-31 (VIS0.6-NIR1.6-IR11.0)26-06-31 (NIR1.3-NIR1.6-IR11.0)

MODIS5 March 2000

?? UTCRGB Composite

01-04-03


II.b Personal Choice/View

MSG-1, 16 March 2004, 16:00 UTC

RGB 01-02-03 RGB 03-02-01 gives bluish surface colours gives more "natural colours"


II.b Personal Choice/View

MSG-1, 5 May 2003, 13:30 UTC

RGB 04r-02-09 RGB 02-04r-09 gives green Cb clouds gives better "warning colours"


III.a Conversion from Radiances to Brightness Temperatures / Reflectances

• CountsCounts • RadiancesRadiances • Brightness temp. (IR channels) • Reflectances (VIS channels)

The best RGBs are achieved usingbrightness temperatures for IR

and reflectances for VIS channels !!!


III.a Conversion from Rad to BT / Refl.Example: MSG-1, 5 June 2003, 14:45 UTC, Channel 09 (IR10.8)

Radiance Brightness TemperatureRange = 14 / 115 mW/m2/sr/cm-1, =1.0 Range = 200 / 305 K, =1.0 better contrast !


Example: MSG-1, 5 June 2003, 14:45 UTC, Channel 02 (VIS0.8)

Radiance Reflectance Range = 0 / 17 mW/m2/sr/cm-1, =1.0 Range = 0 / 100 %, =1.0 better contrast, no sun correction needed !

III.a Conversion from Rad to BT / Refl.


Example: MSG-1, 5 June 2003, 14:45 UTC, RGB 03-02-01

based on Radiances based on Reflectances better contrast, no sun correction needed ! no Gamma correction needed !



The relation between the SEVIRI radiances and the equivalent brightness temperatures / reflectances

is described in a separate Powerpoint file (see conversion.ppt) !



III.b Selection of Display Mode(Inverted or not Inverted)

There are no general rules as regards the display mode (in particular for the IR channels). As described under II.a and II.b, it is a matter of traditions and personal view to select the display mode. Traditionally, for AVHRR RGB composites the IR channels are inverted.

In this MSG Interpretation Guide, if not stated differently, all RGB composites are created from non-inverted (black = low energy, white = high energy) images.


III.c Stretching of Intensity Ranges

• the phenomenon of interest (high clouds, low clouds, surface features, dust, smoke … )

• the season (winter, summer)• the time of the day (day / night / twilight)

The range of interest for the MSG SEVIRI channels varies, depending on:


III.c Stretching of Intensity Ranges- Examples Overview-

FeatureFeature ChannelChannel Total RangeTotal Range Range of InterestRange of Interest

Cb Clouds Europe IR10.8 180 K / 340 K 203 K / 233 KCb Clouds Africa IR10.8 180 K / 340 K 180 K / 220 K

Dust Storm IR12.0-IR10.8 -15 K / +5 K -4 K / +2 KDust Storm IR8.7-IR10.8 -15 K / +15 K -15 K / 0 K

Fog Night IR3.9-IR10.8 -15 K / +25 K -10 K / 0 K

Thin Cirrus IR8.7-IR10.8 -15 K / 15 K 0 K / +7 K


III.c Stretching of Intensity Ranges- Example Cb Clouds Africa -

Range = 180 K / 340 K, =1.0 Range = 180 K / 233 K, =1.0

MSG-1, 20 May 2003, 14:00 UTC, Channel 09 (IR10.8)


III.c Stretching of Intensity Ranges- Example Cb Clouds Europe -

Range = 180 K / 340 K, =1.0 Range = 203 K / 233 K, =1.0

MSG-1, 5 June 2003, 14:45 UTC, Channel 09 (IR10.8)


III.c Stretching of Intensity Ranges- Example Dust Storm -

Range = -15 K / +5 K, =1.0 Range = -4 K / +2 K, =1.0

MSG-1, 3 March 2004, 12:00 UTC, Diff. IR12.0 - IR10.8


III.c Stretching of Intensity Ranges- Example Dust Storm -

Range = -15 K / +15 K, =1.0 Range = -15 K / 0 K, =1.0

MSG-1, 3 March 2004, 12:00 UTC, Diff. IR8.7 - IR10.8


III.c Stretching of Intensity Ranges- Example Fog at Night -

Range = -15 K / +25 K, =1.0 Range = -10 K / 0 K, =1.0

MSG-1, 9 November 2003, 03:15 UTC, Diff. IR3.9 - IR10.8


III.c Stretching of Intensity Ranges- Example Thin Cirrus -

Range = -15 K / +15 K, =1.0 Range = 0 K / +7 K, =1.0

MSG-1, 25 June 2003, 10:00 UTC, Diff. IR8.7 - IR10.8


III.d Gamma Correction

1

255minmax

min

BTBT

BTBTBRIT

In a nutshell, gamma correction changes the overall brightness (and color saturation) of an image as it is displayed on a monitor. The formula to perform a Gamma correction on a MSG IR (brightness temperature) image, within a range of BTmin and BTmax (see stretching of intensity ranges) is:

where BRIT is the brightness intensity (0-255) of the displayed image. For Gamma < 1.0, the image is darkened, with the biggest effect happening for the dark (low input) pixel values. If Gamma > 1.0, the image is brightened overall, with the largest changes happening again for the dark shadows. The formula for the Gamma correction on a MSG VIS (reflectance) image is analog.


III.d Gamma Correction

Mapping function for different Gamma corrections


III.d Gamma Correction: Examples

Reflectance 0 20 40 60 80 100

BRIT (=1.0) 0 51 102 153 204 255BRIT (=2.0) 0 114 161 197 228 255BRIT (=0.5) 0 10 41 92 163 255

Examples of mapping functions for different Gamma corrections for displaying the reflectance (0 - 100%) for the MSG visible

channels.



MSG-1, 3 March 2004, 12:00 UTC, Channel 01 (VIS0.6)

Range = 0 - 100%, =0.5 Range = 0 - 100%, =1.0 Range = 0 - 100%, =2.0



Left: Channel 01 (VIS0.6), various Gamma corrections (=1.0 … =5.0)Right: resulting RGB Composite 03,02,01

Click on the icon tosee the animation(AVI, 2805 KB) !


III.e Gamma2 Correction

The Gamma2 correction helps to enhance the middle part of the selected intensity range. This can be done e.g. with a tanh function.

To be written


IV. Final Enhancement

To be written

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