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Chapter VI: Channels and channel combinations

With the increasing amount of available spectral channels from imager instruments that provide information from the atmosphere and the Earth's surface, composite images have turned out useful to help forecasters extract the meteorological meaningful information.

This section contains webcasts and training modules related to individual satellite channels and to channel combinations. The material comprises training resources on short- (visible) and long-wave radiation (infra-red and water vapour channels) as well as on their combination to RGB images.

Single Channels

This section contains training resources on the spectral characteristics of imager channels (mainly MSG-SEVIRI and MTG-FCI).

Evaluation of the two-moment-microphysical scheme using observations from radar and SEVIRI visible channels (Webcast, 25 minutes), 2019

The goal of the SINFONY project of Deutscher Wetterdienst is to complement, refine and improve the methods of nowcasting (NWC) and numerical weather prediction (NWP) so that a continuous representation of atmospheric conditions and weather phenomena from the current time to the short-term forecast, i.e. in the period 0 to 12 hours, is possible. The pilot project focuses on summertime convective heavy-precipitation events, with the aim of providing more concise and accurate information for our warning service.

We employ different observational systems to evaluate how different models and microphysical parameterizations represent clouds and precipitation over Germany. We compare simulations with ICON and COSMO using different microphysical schemes (operational one-moment vs. Seifert and Beheng two-moment). All simulations are driven by the same boundary conditions from the ICON-EU analysis fields. The simulated period is spring/summer 2016, which is characterized by heavy convection over Germany. In the presentation we mainly focus on the observations from the radar network and on the observations in the visible channels from SEVIRI.

New capabilities with high resolution cloud micro-structure facilitated by MTG 2.3 μm channel (Webcast, 60 minutes), 2016

Major advancements of the MTG with respect to MSG are the superior resolution and the added 2.3 um channel, which in combination allow retrieving cloud particle effective radius and discerning cloud phase at the very high resolution of 500 m. These cloud properties determine whether a cloud would be precipitating, and if so whether it would be rain or snow. This information is particularly advantageous for observing small boundary layer clouds, which often produce rain showers in very clean air masses, such as at the Atlantic coast, but stop raining when polluted inland. This is also the case for post frontal clouds that may produce snow showers. It will be possible to detect it with the MTG.

The ability of retrieving high resolution cloud drop effective radius in convective clouds makes it possible to retrieve cloud drop concentrations, which are dominated by aerosol particle concentrations. The retrieved cloud drop concentrations can be reversed to infer the concentrations of particulate air pollution that occur under cloud decks that are rooted in the boundary layer. A proxy to the retrievals is an RGB color combination that can show the same qualitatively.

Furthermore, the inferred cloud drop concentrations has a predictive value for the nature of forecasted convective storms in that air mass, because the aerosols can invigorate the storms and make them more prone to producing lightning and hail.

CO2 and O3 Absorption Channels (Webcast, 60 minutes), 2014

CO2 and Ozone channels are difficult to interpret directly because they are not "clean" channels that behave in a simple way. To understand them better, the basic concepts of atmospheric radiative transfer are introduced. In this way, it will be easier to understand how to interpret the meaning of these channels, and by extension, of all the other channels.

WV Channels (Webcast, 60 minutes), 2014

The use of WV imagery to analyze and diagnose the 3-D structure and evolution of the atmosphere and meteorological processes. Real time WV data can provide a high-resolution understanding of the atmosphere in time and space that is independent of NWP.

IR Channels (Webcast, 60 minutes), 2014

This presentation deals with the origins of infrared (IR) radiance, with description of different IR spectral regions and selection of spectral bands for meteorological satellites.

In next part practical aspects of utilization of IR satellite data will be presented, from processing fundamentals through image mapping, visualization, practical usage up to recognition and interpretation of various meteorological features. Presentation will explain also how can measured IR radiation provide physical information from different atmospheric layers and how this information can be associated with model outputs. Finally, some notes on anisotropy of Earth-Atmosphere system in IR window will be demonstrated on real comparison of simultaneous measurements of the same scene but from different satellite positions in space.

Solar Channels (Webcast, 60 minutes), 2014

Meteosat solar channels are fantastic, and after 11 years of operation still largely untamed. Additional four solar channels are recruited for the next generation, from 2019, for an eight-a-side match against the infrared team. Have you ever seen midnight sun glints, fire traffic lights or fog dips? Google will not help you. This presentation might cast solar light on those concepts.

Water Vapour Imagery Pallet (Training Module, 180 minutes), 2014

This training module gives an overview on applications of MSG water vapour channels for operational weather forecasting. It handles the concept of potential vorticity which is a key feature to understand the dynamic processes in the higher Troposphere such as cyclogenesis. The CAL module also shows practical applications of the WV images from the geostationary satellites for locating tropopause foldings, clear air turbulence and deformation zones. It handles the effects of WV boundaries on the initiation of convective processes and finally presents some meteorological products heavily based on WV imagery.

WWW-Briefing - Canada (Webcast, 50 minutes), 2013

Trained at Queen's University as a nuclear physicist, "Phil the Forecaster" has been a professional meteorologist since 1976. Officially retired in 2011 Phil still continues to advance his research efforts with COMET and the Meteorological Service of Canada (MSC). Satellite and radar meteorology are his forte. Much of this research has been published by COMET under the Satellite Palette banner. Phil has also completed original and extensive research on performance measurement. This large body of work resides in the Case Studies section of Northern Latitude Meteorology (NorLatMet/COMET). Aside from the above Phil has been painting en plein air forever - - art and science are indeed similar endeavours. A sense of humour bundles all of this together and makes learning and teaching fun.

Meteosat WV Images (Webcast, 60 minutes), 2012

WV images are like a footprint of absorption of radiation by a water vapor, which is the main absorber in the atmosphere. Therefore, it is clear that understanding of these images can tell us a lot about footprint of atmosphere itself and about processes in it. For knowing which processes are present in the atmosphere good interpretation of WV satellite images is needed.

For gathering information about water vapor, SEVIRI instrument on board MSG satellite is scanning atmosphere in two channels in water vapor absorption band; 6.2 and 7.3 μm. Among these two WV channels, the radiation in channel 6.2 μm is more easily absorbed by water vapor and has a larger information content. Thus is called primary WV channel and is broadly used in image format for weather analysis and forecasting based on synoptic scale interpretation. 7.3 μm channel can also be used in operational forecasting environment for detecting mid-level moisture features associated with low-level thermodynamic conditions. Presenter for this one-hour lesson is Christo Georgiev, from Bulgarian National Institute of Meteorology and Hydrology.

Meteosat IR images (Webcast, 30 minutes), 2012

Overview and application of infrared channels presented by Jan Kaňák. The scope of this 30-minute lecture is to define origins of IR radiation and how it is measured by meteorological satellites. Thermal radiation wavelengths are spread in wide interval from basically 1 μm up to 1000 μm, but in this case interest is only in middle IR (3 - 8 μm) and long IR (8 - 15 μm) spectrum.

Physical background and meteorological interpretation of satellite images is similar to all meteorological satellites, but in this lecture emphasis will be on those derived from SEVIRI instrument, on board MSG satellite. Some of the practical aspects of utilization of MSG SEVIRI data, examples and useful remarks on IR images visualization you can see here. IR imagery is historically very basic observation method of atmosphere and clouds, together with visible imagery.

Meteosat Solar Channels (Webcast, 60 minutes), 2012

This one-hour lecture, held by José Prieto, is covering topic of solar satellite channels, mostly from SEVIRI instrument on board MSG. Of all twelve channels form SEVIRI these are the first four, with central wavelengths of 0.3-1.1; 0.6; 0.8 and 1.6 μm, respectively. First one is a high-resolution broad band, second two are narrow band and the fourth one is a microphysics channel. There is significant correlation between channels 0.6 and 0.8 μm, where channel 1.6 μm is somewhat different from them. Also small correlation between solar and other thermal channels is discussed.

Some of the topics that are covered in this lecture are; Solar channels characteristics, Vegetation monitoring, Cloud phase and particle size, Sun glint occurrence and Aerosols. Questions like How to discern clouds from forest, how to find ice on planet Earth, how to avoid squinting on satellite images or escape the smoke of a fire are discussed and answered within all these topics.

Spectral bands and their applications (Webcast, 36 minutes), 2011

Satellite instruments measure at various wavelengths. Since atmospheric gases reveal different properties when being measured at different wavelengths, this information can be essential to filter information. This lecture guides you the basic characteristics of the various SEVIRI channels to their applications.

Water Vapour Channels (Training Module, 120 minutes), 2007

Water vapour images have traditionally been used as additional information which tells about the air streams and water vapour content in the upper and middle levels of the troposphere.

Water vapour is a trace gas, that tells about the origin and kinematics of an airmass. In this respect water vapour images can reveal features that Infrared or Visible channel images are not capable of showing.

Since the introduction of RGB techniques in satellite products, water vapour information is been used more and more in these RGB combination images. Water vapour information is particularly important in Airmass and Severe Storms RGB products.

Channel combinations

The combination of single spectral channels to composite images (a.k.a. RGB images) unfolds the full potential of modern imager instruments. In contrast to former times, when only a few spectral channels were available, the wide range of today's channels offer a multitude of meaningful channel combinations. The increased number of available spectral channels allow for detecting many different physical properties of clouds and ocean/land surface. Roughly a dozen combinations have become generally accepted and they are used in many forecasting offices.

This chapter gives an overview of the different channel combinations and demonstrates some of their applications.

Product Tutorial on Sandwich Products (Product Tutorial, 120 minutes), 2019

This training module describes the Sandwich Products. These products help to detect and analyse various cloud top features of storms (storm systems) in their mature phase. It eases the detection of specific cloud-top features related to storm dynamics and microphysics, structure, and possible storm severity - such as overshooting tops, cold-U/V (enhanced-V) or cold-ring features, embedded warm spots/areas, gravity waves, above-anvil ice plumes, areas composed of very small ice particles, etc. These products directly support monitoring and nowcasting of convective storms. In areas with no, or poor, weather radar and surface observation coverage, this product is essential for proper storm detection.

Training Module on MetOp AVHRR RGB Images (Training Module, 120 minutes), 2017

This training module describes MetOp AVHRR (Advanced Very High Resolution Radiometer) RGB schemes that are based on EUMETSAT recommendations. The 'recipes' were tuned to create high quality MetOp/AVHRR RGB images as similar as possible to the SEVIRI RGB schemes recommended by EUMETSAT.

The main aim of the training module is to help the users (weather forecasters and/or other experts) understand and use these RGB types by giving them background information, examples and exercises.

The module takes the following structure:

  • 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 other RGB types and/or single channel images
  • Exercise

Himawari-8 RGB product use and development amongst RAV and RAII stakeholders: The Australian VLab Centre of Excellence perspective (Webcast, 30 minutes), 2016

This presentation summarises the adaptation of WMO / EUMETSAT endorsed RGB products to Himawari-8 data over the Australasian-Pacific region by WMO RAV and RAII stakeholders from the perspective of the Australian VLab Centre of Excellence. The content includes the tuning of the RGB products to the Himawari-8 data, the training conducted to permit effective use of the RGB products. Evidence is provided of the effective stakeholder use and development of the RGB products, including the development of new RGB products.

Tuning of RGB products and new RGB products from FCI (Webcast, 30 minutes), 2016

The amount of data from the world's weather satellites is overwhelming. While each type of data is valuable, it's almost impossible to use them all operationally. It's like trying to drink from a fire hose; there's simply too much data to absorb, and much of it ends up not being used.

"Red, Blue, Green" or RGB processing is a simple but powerful technique that consolidates different channels of satellite imagery into single products that are easy for forecasters to use. RGB processing used to be a visualization technique used mainly in research. But due to its popularity, it is increasingly available to operational forecasters. A pre-requisite for this, however, is the standardization of RGB products, i.e. the selection of the most useful RGB products for operational forecasting, generated at each Meteorological Service with the same identical standard method/recipe.

The combination of individual images into RGB colour composites is modernizing the interpretation of satellite imagery. While black and white imagery still has its uses, it often cannot match the effectiveness of RGB products. In fact, RGB images are often more useful than traditional colour image enhancements.

The Flexible Combined Imager (FCI) of MTG will open new possibilities of RGB products, with higher temporal and spatial resolution and better accuracy (less noise). Of particular interest will be the new NIR2.25 band that will improve cloud phase detection and detection of hot/large fires. Two new RGBs related to this channel will be presented. Furthermore, some standard RGBs will need to be tuned to account for the slight changes in central wavelength and band width. Some Himawari examples for this tuning will be given. Finally, the issue of local versions of RGB products will be addressed using the examples of “tropical” Night Microphysics and Airmass RGBs.

Introduction to RGBs (Webcast, 60 minutes), 2014

What is the benefit to work with RGBs not only with single channel images? How to create an RGB image from raw data? How to create a good RGB image? How to enhance features and which features to enhance? Why use standard RGBs? How to extract, distill, and package the data into products that are easy to interpret and use for forecasters?

Airmass RGB (Webcast, 30 minutes), 2014

The Airmass RGB is for sure one of the more complex RGB composites as it uses brightness temperature differences and not single channels on each colour beam. But - as this presentation will show - it is nevertheless a very powerful tool for discriminating between different air masses and highlighting dynamic properties of the atmosphere.

Dust RGB (Webcast, 30 minutes), 2014

Dust and smoke detection with SEVIRI RGB products: the lecture focuses on dust and smoke clouds and their identification in RGB products. The first part looks at the solar channels and the natural colours RGB product; the second part presents the triple window IR channels and the resulting dust RGB product. Some challenges of detecting dust clouds (like low level dust at night over water) will be discussed.

Detection of Dust with MSG (Training Module, 180 minutes), 2012

Atmospheric dust storms are common in many of the world's semi-arid and arid regions and can impact local, regional, and even global weather, agriculture, public health, transportation, industry, and ocean health. This globally-relevant three-hour module takes a multifaceted approach to studying atmospheric dust storms. The first chapter examines the impacts of dust storms, the physical processes involved in their life cycle, their source regions, and their climatology. The second chapter explores satellite products (notably dust RGBs) and dust models that are used for dust detection and monitoring. It also presents a process for forecasting dust storms. The third and final chapter of the module examines the major types of dust storms: those that are synoptically forced, such as pre- and post-frontal dust storms and those induced by large-scale trade winds; and those caused by mesoscale systems, such as downslope winds, gap flow, convection, and inversion downburst storms.

Meteosat Microphysics RGBs (Webcast, 60 minutes), 2012

Convective clouds can be characterized by three cloud top properties that can be detected by satellites and represented by respective tree RGB components;

  • Visible brightness, reflecting more solar radiation for thicker clouds with more water and ice (associated with RED color on RGB composites)
  • Cloud particle size and phase (water or ice), having larger drops with greater depth (associated with GREEN color on RGB composites)
  • Temperature, lover for higher tops (associated with BLUE color in RGB composites)

Microphysical processes within the clouds are specially discussed in this lecture, because they present great importance to the net reflectance of the clouds. Various combinations can be made of all the satellite channels to produce desired RGB combination (e.g. Airmass RGB and Dust RGB product) in order to track properties of interest like formation of fog, drizzle, rain clouds, intensive convective storms, etc. This presentation is given by Daniel Rosenfeld from Hebrew University of Jerusalem, Institute of Earth Sciences.

Operational use of RGBs: Part 1 (Training Module, 240 minutes), 2009

Technological advances and the increasing sophistication of weather forecasting have created a demand for more frequent and more accurate and higher resolution observations from space. To meet this demand on 28th August 2002 the first of four satellites known as Meteosat Second Generation (MSG) was launched.

MSG transmits more than 20 times the information of its predecessor. The improved resolution of frequency of data significantly contributes to the accuracy of both short-term and medium range weather forecasts. Since 2004, the MSG satellites have been providing full Earth disc images every 15 minutes, in 12 spectral bands.

Twenty times more information is also a challenge for the user to cope with. To present all of this extra data in an understandable way to the user, so-called RGB (red, green and blue) images were developed that allow you to easily make a qualitative analysis. In RGB images the different properties of the twelve spectral bands of MSG are combined in one powerful coloured image.

Fog, snow, atmospheric dust, SO2 clouds from erupting volcanoes, severe updrafts in convective systems, Potential Vorticity (PV) anomalies are just a few keywords and applications that we will teach you to recognise in satellite imagery. On several occasions questions and exercises will help you to test your gained knowledge.

Operational Use of RGBs: Part 2 (Training Module, 240 minutes), 2010

The new generation satellite data contains more and more information offering increased insight into cloud and air mass characteristics. This poses a challenge: figuring out how to extract, distill and package the data into products that are easy for forecasters to interpret and use.

One might create numerous different kinds of RGB images. Satellite experts developed some optimally tuned RGB types for highlighting specific features. These are the so called standard RGBs recommended by EUMETSAT. The advantage of using standard RGBs is their easy comparability.

The aim of creating RGBs is to provide fast, easily understandable VISUAL information. A 'good' RGB should convey information that would be difficult or time consuming to assess visually from one or more individual single channel images. RGB image should be unambiguous and use intuitive colours to help highlighting important meteorological and surface features. RGBs provide useful information to forecasters, in particular when looking at animated image sequences. They preserve the "natural" look-and-feel of "traditional" satellite images, e.g. they preserve texture, and the patterns are continuous in time.

In this module you will learn more about the EUMETSAT standard RGBs: HRV Fog RGB, Snow RGB, Night Microphysics RGB and the Ash RGB.