Introduction
The new generation satellite instruments provide more and more data. This poses a challenge: figuring out how to extract and package the data into products that are easy for forecasters to interpret and use. The aim is to provide fast, easily understandable visual information.
The so-called RGB composition techniques offer the possibility of compressing multispectral information content for optimum visualization while preserving the pattern and texture of cloud and surface features as well as temporal continuity. RGB is the abbreviation of 'Red, Green, Blue' - these are the primary colors of light. RGB images are created by combining a number of spectral channels or channel differences and visualizing them in the three primary colors. Numerous different kinds of RGB images could be created with this technique. However, which ones are the most useful or effective?
Some years ago satellite experts developed optimally tuned RGB types for METEOSAT SEVIRI (Spinning Enhanced Visible and Infrared Imager) data to highlight specific atmospheric and surface features. These are the so-called standard RGBs recommended by EUMETSAT. The MSG Interpretation Guide (Kerkmann et al, 2006) contains training material on EUMETSAT's recommended RGB schemes for the SEVIRI instrument, along with instructions on how to create them. The recipes in the MSG Interpretation Guide describe not only which channels (or channel differences) are visualized in red, green and blue, but also how to enhance them properly.
Over the course of two WMO workshops (2007, 2012) a list was made of recommended RGB schemes for other satellite instruments as well - which channels or channel differences were to be used for which purpose. The way they are tuned or enhanced depends on the spectral characteristics of the channels.
This training module describes METOP AVHRR (Advanced Very High Resolution Radiometer) RGB schemes that are based on EUMETSAT recommendations (Putsay et al., 2014). The 'recipes' were tuned to create good quality METOP/AVHRR RGB images as similar to the EUMETSAT recommended SEVIRI RGB schemes as possible.
The AVHRR instruments on the polar orbiting MetOp satellites have fewer channels than the METEOSAT SEVIRI instrument. It uses 6 channels, two of which are alternatives: channel 3a only works in daytime, while channel 3b only works at night. Table 1 lists the central wavelengths of the MetOp AVHRR instrument and the central wavelengths of the closest SEVIRI channels together with the names of the channels.
| MetOp/AVHRR | 'Closest' SEVIRI channel | Central wavelength of the closest SEVIRI channel [μm] | ||
|---|---|---|---|---|
| Channel name | Central wavelength[μm] | |||
| 1 | VIS0.63 | 0.630 | VIS0.6 | 0.635 |
| 2 | NIR0.87 | 0.865 | VIS0.8 | 0.81 |
| 3a (daytime) | NIR1.61 | 1.610 | NIR1.6 | 1.64 |
| 3b (nighttime) | NIR3.74 | 3.740 | NIR3.9 | 3.90 |
| 4 | IR10.8 | 10.800 | IR10.8 | 10.80 |
| 5 | IR12.0 | 12.000 | IR12.0 | 12.00 |
Table 1: The names and central wavelengths of the MetOp AVHRR and the closest SEVIRI channels
Table 1 shows the AVHRR channel names in two columns. The numbered channels in the first column are the channel names used in the AVHRR user's guide. As the SEVIRI channels are named by their central wavelength (see the fourth column), similar channel names were defined and will be used in this training module for the AVHRR channels as well (see the second column). However, to make differentiation between AVHRR and SEVIRI channels easier, two decimal digits are used for channels 1-3b.
Four 'recommended' RGB types can be created from the AVHRR channels: Night Microphysics, Day Microphysics, Natural Colour and Cloud RGBs. They are discussed following the structure below:
- The aim of the RGB type
- Physical background
- How to create the given RGB type
- Typical colors
- Examples of interpretation
- Benefits and limitations
- Comparisons with the corresponding standard SEVIRI RGB type
- Exercises
At the end of the training module the three daytime AVHRR RGBs are compared and the effects of the scanning geometry are discussed.
The main aim of the training module is to help the users (weather forecasters and other experts) to understand and use these RGB types by giving them background information, examples and exercises.
Note that the RGB Colour Interpretation Guide on the EumeTrain webpage provides examples of the typical colors of the AVHRR color composites.
Short description of the images presented in this training module
The AVHRR and SEVIRI images were processed at the Hungarian Meteorological Service. The images were visualized by the Hungarian Advanced Weather Workstation (HAWK) visualization system in stereographic map projection.
With SEVIRI images 10 minutes were added to the satellite nominal time to get a 'European time', a time when SEVIRI starts to scan the northern part of the full disk containing Europe.
Acknowledgements
The authors wish to thank EUMETSAT for the beneficial training workshops on MSG applications and RGB images. This training module has benefited from advice and suggestions provided by Jochen Kerkmann (EUMETSAT). The authors are grateful to Márta Diószeghy (Hungarian Meteorological Service) for checking the text.