Version 0.8, 03 February 2005 APPLICATIONS OF METEOSAT SECOND GENERATION (MSG) RGB IMAGES: PART 03 CHANNEL SELECTION AND ENHANCEMENTS Author:Jochen Kerkmann (EUMETSAT) Contributors:D. Rosenfeld (HUJ), HP. Roesli (MeteoSwiss) M. König (EUM)

Version 0.8, 03 February 2005 PART 3: CHANNEL SELECTION AND ENHANCEMENTS

Version 0.8, 03 February 2005 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: aReproduction of RGB schemes inherited from other imagers; bContrast 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: aConversion from radiances to brightness temperatures/reflectances; bSelection of display mode (inverted or not inverted); cStretching of the intensity ranges (linear stretching of active dynamic range); dGamma corection; eGamma2 correction; IV. Final enhancementIV. Final enhancement –e.g. Gamma correction of final RGB image, Channel Selection & Enhancements

Version 0.8, 03 February 2005 I. (Cloud) Physical Properties represented by the MSG Channels VIS0.6:optical thickness and amount of cloud water and ice VIS0.8:optical thickness and amount of cloud water and ice "greeness" of vegetation NIR1.6, IR3.9r:particle size and phase WV6.2, WV7.3:mid- and upper level moisture IR8.7, IR10.8, IR12.0:top temperature IR8.7 - IR10.8:phase and optical thickness IR IR10.8:optical thickness IR3.9 - IR10.8:optical thickness, phase, particle size IR IR10.8:top height WV6.2 - IR10.8:top height, overshooting tops

Version 0.8, 03 February 2005 II.a RGB Schemes inherited from NOAA AVHRR The most popular RGB schemes used for NOAA AVHRR images are: RGB (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

Version 0.8, 03 February 2005 II.a RGB Schemes inherited from MODIS For MODIS (27 channels, not considering channels 8-16) there are different ways (this is 27!/(27-3)!) to choose 3 channels from 27 channels. Some popular RGB schemes used for MODIS images are: (VIS0.6-VIS0.5-VIS0.4) (VIS0.6-VIS0.8-VIS0.4) (VIS0.6-NIR1.6-IR11.0) (NIR1.3-NIR1.6-IR11.0) MODIS 5 March 2000 ?? UTC RGB Composite

Version 0.8, 03 February 2005 II.b Personal Choice/View MSG-1, 16 March 2004, 16:00 UTC RGB RGB gives bluish surface colours gives more "natural colours"

Version 0.8, 03 February 2005 II.b Personal Choice/View MSG-1, 5 May 2003, 13:30 UTC RGB 04r RGB 02-04r-09 gives green Cb clouds gives better "warning colours"

Version 0.8, 03 February 2005 III.a Conversion from Radiances to Brightness Temperatures / Reflectances Counts Counts  Radiances Radiances  Brightness temp. (IR channels) Reflectances (VIS channels) The best RGBs are achieved using brightness temperatures for IR and reflectances for VIS channels !!!

Version 0.8, 03 February 2005 III.a Conversion from Rad to BT / Refl. Example: MSG-1, 5 June 2003, 14:45 UTC, Channel 09 (IR10.8) Radiance Brightness Temperature Range = 14 / 115 mW/m2/sr/cm-1,  =1.0 Range = 200 / 305 K,  =1.0 better contrast !

Version 0.8, 03 February 2005 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.

Version 0.8, 03 February 2005 Example: MSG-1, 5 June 2003, 14:45 UTC, RGB based on Radiances based on Reflectances better contrast, no sun correction needed ! no Gamma correction needed ! III.a Conversion from Rad to BT / Refl.

Version 0.8, 03 February 2005 The relation between the SEVIRI radiances and the equivalent brightness temperatures / reflectances is described in a separate Powerpoint file (see conversion.ppt) ! III.a Conversion from Rad to BT / Refl.

Version 0.8, 03 February 2005 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.

Version 0.8, 03 February 2005 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:

Version 0.8, 03 February 2005 III.c Stretching of Intensity Ranges - Examples Overview- FeatureChannelTotal RangeRange of Interest Cb Clouds EuropeIR K / 340 K203 K / 233 K Cb Clouds AfricaIR K / 340 K180 K / 220 K Dust StormIR12.0-IR K / +5 K-4 K / +2 K Dust StormIR8.7-IR K / +15 K-15 K / 0 K Fog NightIR3.9-IR K / +25 K-10 K / 0 K Thin CirrusIR8.7-IR K / 15 K0 K / +7 K

Version 0.8, 03 February 2005 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)

Version 0.8, 03 February 2005 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)

Version 0.8, 03 February 2005 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. IR IR10.8

Version 0.8, 03 February 2005 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

Version 0.8, 03 February 2005 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

Version 0.8, 03 February 2005 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

Version 0.8, 03 February 2005 III.d Gamma Correction 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 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.

Version 0.8, 03 February 2005 III.d Gamma Correction Mapping function for different Gamma corrections

Version 0.8, 03 February 2005 III.d Gamma Correction: Examples Reflectance [%] BRIT (  =1.0) BRIT (  =2.0) BRIT (  =0.5) Examples of mapping functions for different Gamma corrections for displaying the reflectance ( %) for the MSG visible channels.

Version 0.8, 03 February 2005 III.d Gamma Correction: Examples MSG-1, 3 March 2004, 12:00 UTC, Channel 01 (VIS0.6) Range = %,  =0.5 Range = %,  =1.0 Range = %,  =2.0

Version 0.8, 03 February 2005 III.d Gamma Correction: Examples Left:Channel 01 (VIS0.6), various Gamma corrections (  =1.0 …  =5.0) Right:resulting RGB Composite 03,02,01 Click on the icon to see the animation (AVI, 2805 KB) !

Version 0.8, 03 February 2005 III.e Gamma2 Correction The Gamma2 correction helps to enhance the middle part of the selected intensity range. One possibility is a Gamma function operated double sided from the middle of the intensity (physical) range. The larger it is the more stretching is on the middle value at the expense of the extremes.

Version 0.8, 03 February 2005 III.e Gamma2 Correction The formula to perform a Gamma2 correction on a MSG IR brightness temperature difference image, within a range of Tmin and Tmax (see stretching of intensity ranges) is: where BRIT is the brightness intensity (0-255) of the displayed image and Tmed is (Tmax + Tmin)/2. Gamma2 should be  1. For T  Tmed: For T  Tmed:

Version 0.8, 03 February 2005 III.d Gamma2 Correction: Examples Reflectance [%] BRIT (  2=1.0) BRIT (  2=2.0) BRIT (  2=3.0) Examples of mapping functions for different Gamma2 corrections for displaying the reflectance ( %) for the MSG visible channels.

Version 0.8, 03 February 2005 III.e Gamma2 Correction: Examples MSG-1, 6 March 2004, 12:00 UTC, Diff. IR IR10.8 Range = -2 / +2 K,  2 =1.0 Range = -2 / +2 K,  2 =3.0

Version 0.8, 03 February 2005 IV. Final Enhancement To be written