1. Joseph Kagenyi IMTR-NAIROBI Adopted from MSG Interpretation guide

2. MSG Channels Ch 5 & Ch 6 The source of Signal Radiations effects from different moisture profiles Interpretation Overlay with Other parameters (Z, PV, Wind, Jet streams, Relative humidity) Determination of Dynamic processes that change Synoptic features Courtesy: Christo Georgiev (Bulgaria) and Patrick Santurette (Météo-France) 2

3. Severe Weather Causes It will occur only when Thermodynamic Conditions are Satisfied: That is 1. Great Energy to sustain Strong Convection (Lifted Moisture). Temp Information 2. Reliable Source of Moisture ( local and Synoptic).>> WV distribution 3. Presence of Lifting Mechanism (convergence from either Local Wind interacting with Synoptic Wind field (such as Inter Tropical Convergence Zone)) WV represents presence of Moisture and Dynamics driving. ESAC TRAINING3

4. MSG WATER VAPOUR CHANNELS RADIATION EFFECTS FROM DIFFERENT MOISTURE PROFILES

5. Two of the 12 channels of SEVIRI radiometer of MSG are water vapour channels, centred at 6.2 and 7.3  m in WV absorption band. The 8.7  m exhibits properties of a WV and a window channel and may be considered as an IR channel in the WV absorption band. WV 6.2  m 7.3  m IR / WV 8.7  m

6. Absorption The radiation in 6.2  m band is more easily absorbed by water vapour in comparison with radiation in 7.3  m and 8.7  m channels. The 8.7  m channel is slightly absorbed by water vapour.

7. 8.7  m channel: slight absorption warmer sea – darker than the colder land low clouds, snow over Alps – white 7.3  m channel: moderate absorption warmer sea – not visible low clouds, snow over Alps – lighter high land – visible 6.2  m channel: strong absorption dry troposphere – dark moist troposphere – light low clouds, high land – not visible A l p s Low clouds Land S e a 8.7 m8.7 m 7.3 m7.3 m 6.2  m D r y M o i s t

8. Since the 6.2  m radiation exhibits a larger absorption, the 6.2  m radiation in image format better represents moisture distribution in mid- to upper troposphere over the cloud free areas. Absorption 7.3 m7.3 m 6.2  m

9.  Within the channels of the infrared band, the radiation will be emitted by objects such as cloud elements, precipitation and the earth surface.  For IR window channels this radiation will reach the satellite after no or very little absorption.  For the other channels, some portion of the upcoming radiation will be absorbed and then reradiated, this process being dependant on the pressure (density of molecules).  The radiation in a specific channel exhibits a specific ability to pass through the atmosphere, depending on the density of the absorbing atmospheric substances. Origin of the radiation

10. Weighting function  In order to analyse the transmittance ability of the atmosphere as regards to the radiance at any infrared channel, the weighting function is used.  The weighting function illustrates the relative vertical contribution of radiation produced by the atmospheric substances, which absorb / emits.  For the WV channels, the absorption substance is the water in its gaseous, liquid or solid states.

11. WV channel weighting functions The WV channel weighting functions are peaking at different altitudes and the levels of maximum contribution to the total radiation emitted by moisture are different for the two channels. The WV channels of MSG may serve as tools for observing moisture regime in different layers of the troposphere. MEAN WEIGHTING FUNCTIONS OF SEVIRI CHANNELS AT MIDDLE LATITUDES 400 hPa 600 hPa

12. 6.2  m 7.3 m7.3 m 10.8  m S e a 10.8  m window channel: Radiation reaching the satellite is not absorbed by any atmospheric substances. Various objects of the earth surface and cloudiness may be distinguished due to differences in their temperature. 7.3  m WV channel: Radiation is absorbed by mid- to low-level atmospheric moisture, higher humidity being displayed in lighter grey shades. High mountain surface may be seen as a dark feature if little moisture is present above. 6.2  m WV channel: Radiation is absorbed by upper- to mid-level atmospheric moisture, lower humidity being displayed in darker grey shades. Due to strong absorption, low-level features (earth surface, clouds) are not visible. A t l a s Land D r y M o i s t Low clouds

13. High altitude earth’s surface terrain is visible in the 7.3  m channel images in cases of dry mid- to upper troposphere. 7.3 m7.3 m A t l a s A t l a s Relative humidity (%) Light grey shades in the 7.3  m image indicate humid air at low levels, the mountain terrain appears darker (warmer) than the surrounding areas of low- level moist air.

14. The radiation in 6.2  m originates from above the earth surface, and it is much more useful for observing upper troposphere moisture content. 6.2  m Dark grey shades in the 6.2  m image indicate low humidity at mid/upper levels, but the mountain terrain is not visible. A t l a s Relative humidity (%)

15. Water Vapour channels Operational satellites carry out measurements in several ranges over which radiation is absorbed and reradiated by water vapour in the atmosphere. Channels of the main water vapour absorption band between 5.2  m and 8.5  m are referred to as WV channels.

16. WV channels The most useful WV channels are centered around 6.3  m at which the maximum absorption is observed. The 6.7  m band of GOES satellite of NOAA, USA is located in the 6.2  m band of MSG. As regards to qualitative application, the 6.2 and 6.7  m channels show similar radiation effects.

17. BRIGHTNESS TEMPERATURE FROM RADIATION MEASUREMENTS  The Brightness temperature is a measure of the intensity of radiation thermally emitted by an object, given in units of temperature since the intensity of this radiation correlates with the physical temperature of the radiating body that is given by the Stefan-Boltzmann equation.  For split window IR channels (10.8  m and 12  m), the brightness temperature equals the surface blackbody temperature of the object, which radiates.  Because of the absorption the brightness temperature derived in WV channels (especially 6.2  m and 6.7  m) may be totally different from the physical temperature of the object depending on the vertical distribution of humidity.

18.  Within the WV channels, as they are parts of the infrared band, the radiation will be emitted by objects of the earth surface and the atmosphere.  For IR window channels this radiation will reach the satellite after no or very little absorption, so that the instrument can detect the temperatures of cloud tops, land and sea surface.  For WV channels, the water vapour located anywhere in the troposphere will absorb some portion of the upcoming radiation and reradiate. Being commonly cooler than the earth’s surface or the cloud tops radiating from bellow, the water vapour usually radiates at a lower energy level. Origin of radiances in WV channels

19. ESAC TRAINING19 Low-level moist layers (650  800 hPa) Low-level clouds are not well distinguished in 6.2  m channel images. The Dry, cloud-free Moist and Cloudy areas appear slightly different, but not distinct in the image grey shades of the 6.2  m channel. 6.2  m Dry / Moist / Cloudy

20. Low-level moist layers (650  800 hPa) Both, low-level clouds and cloud-free moisture are seen by Ch. WV 7.3. The Dry, cloud-free Moist and Cloudy areas appear in black, grey and white shades of the 7.3  m channel image. Dry / Moist / Cloudy 7.3  m

21. Low-level moist layers (650  800 hPa) Low-level clouds are not well distinguished in 6.2  m channel images. The Dry, cloud-free Moist and Cloudy areas appear slightly different, but not distinct in the image grey shades of the 6.2  m channel. 6.2  m Dry / Moist / Cloudy

22. Low-level moist layers (650  800 hPa) 6.2 m6.2 m Therefore, Low level moist layers are detectable by the 7.3  m channel and may be visible in the 6.2  m images. The 7.3  m radiation is more sensible to detect differences in the water vapour content in the lower troposphere and may be useful for inspection of moisture commonly found there. 7.3 m7.3 m Dry / Moist / Cloudy

23. ESAC TRAINING23 Low-level moist layers (650  800 hPa) Both, low-level clouds and cloud-free moisture are seen by Ch. WV 7.3. The Dry, cloud-free Moist and Cloudy areas appear in black, grey and white shades of the 7.3  m channel image. Dry / Moist / Cloudy 7.3  m

24. NARROW 6.2 m6.2 m6.2 m6.2 m Ability to detect Mid-level moisture Sensitivity range 7.3 m7.3 m7.3 m7.3 m MODERATE to LARGE LOWHIGH Low-level moist layers (650  800 hPa) Low level moist layers are detectable by the 7.3  m channel, while such layers slightly affect the 6.2  m radiation. The 7.3  m channel is more sensible to the moisture content in a low-level layer, it’s sensitivity range being moderate to large depending on upper-level humidity content.

25. Operational applications of radiation measurements in MSG WV channels  In the air mass analysis as well as for assessing atmospheric stability and stability tendency in cloud free-areas to help solving the problem of convection Nowcasting.  For estimating water vapour content of two deep layers in clear atmosphere to predict pre- convective situations.  For analysis of changes in vertical distribution of humidity to help solving the problems of early warning and detection of convection.

26. MSG WATER VAPOUR CHANNELS DIFFERENCES IN MOISTURE REGIME RELATED TO DYNAMIC PROCESSES AS SEEN BY WV IMAGERY

27. INTERPRETATION OF WV CHANNELS RADIANCES IN IMAGE FORMAT Many pixels are considered as patterns and features of grey shades. The interpretation is aimed to relate these patterns of different grey shades and boundaries between them as well as their evolution to specific dynamical structures and processes.

28. SOURCE OF INFORMATION  Water vapour is accumulated in the troposphere from the earth’s surface by vertical motions and wind regime of all directions.  Therefore, the radiation measurements in water vapour absorption band may be a source of information for wind regime and atmospheric dynamics associated with vertical motion.

29. Different moisture regimes produced by dynamical features and processes DYNAMICAL APPROACH IN THE INTERPRETATION OF WV CHANNEL RADIANCE

30. Mid-/upper-level moisture regime 6.2  m Light image grey shades indicate moist cloud free or cloudy air at mid- to upper level. Dark image grey shades indicate dry air at mid- to upper level. Radiance in a WV channel may serve as a tool for observing moisture regime at altitudes close to the level of maximum weighting function for that channel.

31. Moisture boundaries 6.2 m6.2 m The boundaries between dark and light image grey shades indicate transition zones between different moisture regime or cloud regime.

32. Upper-level wind field 6.2 m6.2 m The moisture boundaries between dark and light image grey shades are associated with transition zones between different upper-level wind regime. 300 hPa wind > 40 kt

33. Mid-level geopotential field 6.2 m6.2 m 500 hPa heights This process develops in a cut-off regime seen by the 500 hPa heights field. At the deformation zones of blocking regime, the geostrophic approxima-tion is not valid, and the slope of the 500 hPa isobaric surface is not likely along the moisture boundaries. The 500 hPa height contours tend to follow the shape of moisture boundaries (dark/light shades) in the leading part of the trough.

34. The tropopause height 6.2 m6.2 m 1.5 PVU surface heights The tropopause height field reflects the moisture regime. The boundaries between dark (dry) and light (moist) features tend to follow the shape of the tropopause. The moisture boundaries represent strong gradient areas of height of the surface of constant Potential Vorticity (PV) equal to 1.5 PVU, this surface being correlated with the tropopause. = Tropopause surface heights

35. The tropopause height The changes in the field of tropopause height, at its strong gradient zones, are related to changes in moisture regime and upper-level wind. Tropopause surface height (solid) The moisture boundaries represent sloping areas of the tropopause. 6.2 m6.2 m Vertical cross-section of PV

36. DYNAMICAL APPROACH IN THE INTERPRETATION OF MSG WV CHANNELS RADIANCE Ability of 6.2  m and 7.3  m channels to detect synoptic scale differences in moisture regime produced by dynamical processes

37. Upper-level dynamical structures 6.2  m Tropopause surface heights Usually, the synoptic scale perturbations in atmosphere dynamics produce strong gradient zones of the tropopause height, associated with large differences in humidity at mid- upper levels.

38. Differences in humidity at mid-upper levels 6.2  m Tropopause surface height (solid) A synoptic scale dry air regime in dynamically active regions is produced by an intrusion of stratospheric air down to mid- tropospheric levels. The vertical distribution of potential vorticity (in brown) well depicts the folding of the tropopause (solid contour). S T R A T O S P H E R E T R O P O S P H E R E

39. 6.2  m7.3 m7.3 m Differences in humidity at mid-upper levels Significant moisture differences at mid-upper levels are very well depicted by 6.2  m channel imagery and they may not appear in the 7.3  m images.

40. Moisture regime at upper levels seen by MSG WV channels 6.2 m6.2 m7.3 m7.3 m The moisture regime produced by upper-level dynamics is usually much more distinctly seen by 6.2  m radiation measurements.

41. Upper-level wind field 7.3 m7.3 m6.2 m6.2 m Accordingly, the moisture regime as seen by 6.2  m channel imagery is more representative for upper-level wind field. Significant moisture boundaries associated with synoptic scale perturbations in the atmospheric motion may be obscured in 7.3  m channel images.

42. 6.2 m6.2 m Upper-level wind The synoptic scale moisture boundaries seen by radiation measurements in 6.2  m channel are associated with areas of folding of the tropopause and strong perturbations in upper-level wind field. Vertical cross-section of wind

43. 6.2 m6.2 m Upper-level wind Synoptic scale moisture boundaries seen by radiance in 6.2  m channel are associated with folding of the tropopause and strong perturbations in upper-level wind field. Vertical cross-section of wind

44. Upper-level wind 7.3 m7.3 m Such moisture boundaries may be not seen by the 7.3  m channel radiances, because the 7.3  m radiation is able to penetrate the water vapour to a great extent and its sensitivity range is overall less than that of 6.2  m channel. Vertical cross-section of wind

45. Tropopause folding 6.2 m6.2 m Synoptic scale moisture boundaries, detectable by radiance in 6.2  m channel are associated with areas of tropopause folding. The vertical distribution of potential vorticity (in blue) depicts the folding of the tropopause (solid contour). Height of the surface of PV = 1.5 PVU

46. Tropopause folding The tropopause folding is a very important feature, the evolution being closely related with the synoptic development. Interpreting WV imagery in the view of Potential Vorticity (PV) Concept enables to follow the tropopause folding, associated with PV anomalies, and provides with a set of powerful methods for synoptic scale analysis. 6.2 m6.2 m

47. Jet-stream related patterns Wind at 300 hPa (only > 100 kt) Typically, on a WV image, there are many well defined boundary features, and only some of them are associated with jet stream axes. In addition to the tropopause folding and PV anomalies, the jet stream related patterns are the other features seen on a WV imagery that are important for the operational forecasting.

48. Heights of the constant surface of PV = 1.5 PVU The jet stream axes are present along the boundaries of different moisture regimes produced by significant tropopause foldings, indicated by strong gradient in geopotential of the 1.5 PVU surface. Jet-stream related patterns The constant surface of potential vorticity (PV) equal to 1.5 PV Units represents the tropopause from a dynamical point of view (see Santurette & Georgiev, 2005).

49. Jet-stream related patterns Contours of wind speed > 100 kt / wind > 80 kt at 300 hPa Generally, there is well defined jet stream axes nearly coincident with the moisture boundaries oriented southwest- northeast. The jet axis of the maximum wind speed is likely along the most contrast part of moisture boundary in the WV image.

50. Synoptic evolution Animation of temporary changes in grey shade appearance of WV channel images provide insight into the evolution of moisture regime related to dynamical processes. 6.2 m6.2 m

51. Synoptic evolution 6.2 m6.2 m7.3 m7.3 m The animation of 6.2  m channel imagery more distinctly shows the evolution of upper-level wind field than sequences of 7.3  m images, because the 6.2  m radiance is more sensitive to differences in water vapour content at upper levels. Therefore, different moisture regimes related to significant dynamical processes and their evolution are clearly depicted by animation of 6.2  m channel images. UPPER Level Developments initiates surface developments.

52. 7.3 m7.3 m 6.2 m6.2 m The grey shade appearance of a 6.2  m channel radiances provides insight into the upper level dynamic pattern in which the dry areas represent low tropopause heights. The radiances in 7.3  m channel are useful for observing cloudy and low-level moisture features, associated with warm anomalies seen in the wet-bulb potential temperature (  w ) field. Tropopause heights / Wet-bulb potential temperature () at 850 hPacontours > 4 °C (  w ) at 850 hPacontours > 4 °C

53. The propagation of the dry feature to the south that is seen in 6.2  m radiances, reveals a process of interaction between the upper- level tropopause anomaly and low- level warm anomaly. The radiances in 7.3  m channel are not able to clearly depict approaching of low-level warm anomaly by the dry upper-level dynamic feature. 7.3 m7.3 m 6.2 m6.2 m Tropopause heights / Wet-bulb potential temperature () at 850 hPa Tropopause heights / Wet-bulb potential temperature (  w ) at 850 hPa contours > 4 °C

54. The 7.3  m radiance is not able to clearly catch the drying process, associated with a tropopause folding, which is going to interact with the low-level warm anomaly. 7.3 m7.3 m 6.2 m6.2 m WV image grey shades in 6.2  m show strengthening of upper-level dry regime (produced by a folding of the tropopause) that approaches the developing cloud pattern. Tropopause heights / Wet-bulb potential temperature () at 850 hPa Tropopause heights / Wet-bulb potential temperature (  w ) at 850 hPa contours > 4 °C

55. At the onset of surface cyclogenesis the upper-level dry features reaches the north-western edge of the warm anomaly cloud system, and this is seen by the radiances of the two WV channels. High level Ci clouds appear in a better contrast in the 7.3  m radiation, that is not sensitive to high-level moisture, which obscures these clouds in 6.2  m channel imagery. Tropopause heights / at 850 hPa Tropopause heights /  w at 850 hPa contours 4 °C 6.2 m6.2 m 7.3 m7.3 m Mean Sea Level Pressure

56. During the developing phase of the cyclone, the polar intrusion of stratospheric dry air, seen in 6.2  m radiances, plays an important role in strengthening the cyclonic circulation. The radiances in 7.3  m channel are not sensitive enough to depict this polar strip of dry air coming down from the stratosphere to the rear of the cyclone. 7.3 m7.3 m 6.2 m6.2 m Tropopause heights / Wet-bulb potential temperature () at 850 hPa Tropopause heights / Wet-bulb potential temperature (  w ) at 850 hPa contours > 4 °C

57. Operational applications of 6.2 and 7.3 MSG WV images  The radiation in 6.2  m band is highly absorbed by water vapour and it is more useful to be displayed and applied in image format for operational purposes.  Since the 6.2  m radiation is more sensitive to the water vapour content in mid and upper troposphere, the 6.2  m channel imagery is useful to be applied at synoptic scale for upper-level diagnosis.  The 7.3  m channel is able to detect low-level moisture, thus it is sensible to the moisture content at these altitudes.  Therefore, images in 7.3  m channel may be interpreted for studying low level humidity features.

58. References Santurette, P., Georgiev C. G., 2005. Weather Analysis and Forecasting: Applying Satellite Water Vapor Imagery and Potential Vorticity Analysis. ISBN: 0-12-619262-6. Academic Press, Burlington, MA, San Diego, London. Copyright ©, Elsevier Inc. 179 pp. Weldon, R. B., Holmes, S. J., 1991. Water vapor imagery: interpretation and applications to weather analysis and forecasting, NOAA Technical. Report. NESDIS 57, NOAA, US Department of Commerce, Washington D.C., 213 pp.