HARMONISING SEVIRI RGB COMPOSITES FOR OPERATIONAL FORECASTING

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1 HARMONISING SEVIRI RGB COMPOSITES FOR OPERATIONAL FORECASTING HansPeter Roesli (1), Jochen Kerkmann (1), Daniel Rosenfeld (2) (1) EUMETSAT, Darmstadt DE, (2) The Hebrew University of Jerusalem, Jerusalem IL Abstract It is proposed to use the AIRMASS and DUST RGB composites for general weather monitoring throughout 24-hours/ 7-days, complemented by the DAY/NIGHT cloud microphysics couple for convection monitoring in particular. This would benefit the duty forecaster and training efforts. A PARADIGM CHANGE MSG SEVIRI imagery for the first time provides operationally multi-spectral imagery at high repetition rate from the geostationary orbit. Display of individual channels or images of channel differences fall short of imparting in a concise way to the duty forecaster the wealth of pertinent weather and weather related information buried in the image data content. Based on experience with AVHRR and MODIS products, the application on SEVIRI image data of the RGB compositing technique offers the possibility of compression of the multi-spectral information content for optimum visualisation, while at the same time preserving pattern and texture of cloud and surface features as well as continuity in the time domain. Also, by a careful choice of the inputs to an RGB composite scheme a multitude of phenomena may be captured in one go. Adequately reprojected RGB composites may be used right out to regions on the limb of the SEVIRI coverage, though image processing artefacts are cluttering the images somewhat (volcan eruption in figure 9 and fog/fire in figure 13). OPERATIONAL AND TRAINING CONSIDERATIONS The RGB compositing technique is an efficient way for a continuous monitoring weather and weatherrelated phenomena. It preserves the natural-look-and-feel of mono-channel imagery the duty forecaster is used to, in particular when when viewing image sequences. RGB compositing also has its drawbacks. The number of channels invites to a confusing number of possible combinations. There are individual (subtle or more serious) colour perception problems and difficulties in memorising the resulting colour schemes, i.e. in relating colour shades to particular meteorological or non-meteorological features. For an operational forecaster and for training it is important to reduce the set of RGB schemes to a minimum while maximising the number of identifiable phenomena relevant to nowcasting and shortrange forecasting. At the same time the selected RGB schemes should cover 24-hours/7-days including twilight conditions with at most small changes in colouring. Even then there remain many possible schemes that may render communication across services difficult and training inefficient. Therefore, any effort to harmonise RGB compositing schemes for operational use and as a training tool should have a positive impact on the weather services. This proposal tries to steer in this direction.

2 Most importantly, the proposed three RGB composting schemes are based on physical considerations as regards the selection of the channel combinations as well as the attribution of them to the individual colour planes of the RGB display device. This assists in understanding and memorising the image content and its relation to the resulting colour landscape. A SET OF HARMONISED RGB COMPOSITING SCHEMES COVERING 24/7 IR-BASED ONLY Strict application of the 24/7 consideration limits the set of applicable SEVIRI channels to those sensitive to IR emission from the Earth surface and its atmosphere, i.e. channels 4-11 at night and 5-11 in daylight. Restricting even more to channels 5-10 two RGB schemes emerge: the AIRMASS and the DUST product. They complement each other (figure 12). In extreme cases both schemes might need slight regional/seasonal re-tuning. AIRMASS (devised by Jochen Kerkmann) includes information from the split water vapour channels and ozone channel (Figure 1). It identifies airmass and gives indications of atmospheric dynamics (jet streams, stratospheric intrusions (proxy for potential vorticity)) in the middle-upper troposphere. At the same time it gives an overview on the accompanying cloud system (figures 4-7). DUST (devised by Daniel Rosenfeld) is a slight misnomer. Though it excels in monitoring desert dust, it identifies many other features indirectly through the dust behaviour (gust fronts) or directly based on differing transmission/emission characteristics of channels 7 (8.7μm) and 10 (12.0μm) with respect to channel 9 (10.8μm) over cloud tops and land surface (Figure 2). The list also includes the identification of many cloud types (including fog, ship tracks and contrails), dry lines as well as of ash and SO 2 plumes from active volcanoes (figures 8-11). Figure 1: AIRMASS scheme, physical interpretation

3 Figure 2: DUST scheme, physical interpretation ADDING CONVECTION MONITORING INCLUDING SOLAR CHANNELS Though AIRMASS and DUST allow for very limited convection monitoring, only the inclusion of the solar SEVIRI channels gives clues into the intensity of the convective processes below the cold cloud tops. The obvious problems of 24/7 coverage can be alleviated to a certain extent when using day and night alternately, the schemes "DAY Cloud Microphysics and NIGHT Cloud Microphysics respectively, both devised by Daniel Rosenfeld. The couple conveys indications on thickness, particle size/phase and temperature at the cloud tops (figure 3 and 13). Using conceptual models these features give indications on the severity of the convection. DAY relies heavily on the solar part of channel 4 (3.9μm) and NIGHT on the difference between channel 4 and 9 (10.8μm). NIGHT, being on one hand less capable as regards convection, on the other hand identifies fog and fires. The resulting colour schemes for identical cloud features are quite similar in DAY and NIGHT. The couple has its difficulties at twilight, in particular with convection signatures. This capability is really limited to the well-lit hours (particle size!). Figure 3: DAY/NIGHT cloud microphysics scheme scheme

4 AIRMASS RGB COMPOSITE EXAMPLES Figure 4: Cloud analysis Figure 5: Jet stream positions

5 Figure 6: Airmass analysis Figure 7: Proxy for potential vorticity

6 DUST RGB COMPOSITE EXAMPLES Figure 8: Dust outbreak due to earlier thunderstorm outflow Figure 9: Contrails (upper left), fog patches (lower left) and volcano activity (black: ice cloud, bright blue: SO 2 plume)

7 Figure 10: Cloud analysis Figure11: Dry/moist boundary layer and ECMWF dew-point analysis

8 COMPLEMENTARITY OF AIRMASS AND DUST RGB COMPOSITES Figure 4: DUST RGB left and AIRMASS RGB right giving excellent view of general meteorological situation DAY/NIGHT CLOUD MICROPHYSICS COMPOSITE EXAMPLE Figure 13 CONCLUSIONS RGB composites are excellent monitoring and teaching tools when addressing operational forecasting in particular. They retain the natural look-and-feel of traditional mono-channel imagery while enhancing a multitude of weather and weather-related phenomena. It appears that limiting the great number of possible compositing schemes and harmonising them would benefit the duty forecaster and training efforts at the same time. It is proposed to use the AIRMASS and DUST RGB composites for general weather monitoring throughout 24-hours / 7-days, complemented by the DAY/NIGHT cloud microphysics couple for convection monitoring in particular. The EUMETSAT website offers more information on the SEVIRI channels and their application as well as on case studies and real-time imagery using the proposed RGB schemes. The following links may be consulted: Home > What We Do > Training > Resources Home > Image Gallery > Meteosat Images of the Month > Gallery Home > Image Gallery > Derived Product Imagery Home > What We Do > Training > Distance Learning