The water vapour channels of SEVIRI (Meteosat). An introduction

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1 The water vapour channels of SEVIRI (Meteosat). An introduction Cachoeira P. July 2006 Formats 1.5 1

2 Objectives 2 Describe the characteristics of WV channels on board of SEVIRI Describe the relation between signal and the temperature in the atmospheric water vapour Learn on options for channel information display Learn of some synoptic clues contained in the WV channels Apply the previous knowledge to the interpretation of the airmass RGB

3 Contents 3 Characteristics of WV channels Radiative transfer considerations Synoptic features in WV Use in RGBs of WV channels

4 WV channel characteristics 4 SEVIRI channels at: 6.2 µm and 7.3 µm Strong absorption by water vapour, stronger for 6.2 Contribution from Hpa ( K) Contribution from lower levels in a dry atmosphere Strongest humidity impact on signal: 100HPa above contribution peak No depiction of low atmosphere or ground Nile river due to humid convection. Mountain ranges in desert areas. High cloud (Cb, anvils) appears as in other IR Lower cloud masked by water vapour absorption above

5 Spatial and spectral resolution 5 Horizontal: 3.5 km at SSP 7.3µm has a smaller bandwidth

6 Deeper into the atmosphere:

7 Vertical resolution 7 Transfer: absorption + weaker emission at colder T Strong absorption by cloud, as in other IR channels Dependent on humidity and temperature profiles Warm-or-dry ambiguity in WV channels Similar signal in the Arctic as in the tropics More humidity: decreased, upper levels signal Reversed channel difference for thermal inversions

8 Top of the atmosphere R Hpa Radiation Transfer ( WV ) R= B(T) (p--top) 900 Hpa Amount of absorber Earth s surface

9 Noisy channels? 9 MSG noise: 0.12K@250 (7.3µm) Contribution 6.2 [ 0.11K@300 (10.8µm) ] Very accurate information Contribution 7.3 However, WV provides a hazy weather depiction: spread vertical contribution mixes humidity with temperature in one count humidity fields are smoother than temperature

10 Humidity layers 10 Humid at mid-level Dry Humid

11 Applications of WV imagery 11 Synoptic and meso-scale diagnosis Jet stream Ascent regions Dry intrusions Vorticity centres Wedges and troughs Relative movement Deformation zones

12 Synoptic diagnosis 12

13 Wind and shear maxima 13 Wind maxima along dark slots in the WV image Maximum shear occurs on the left hand side, creating shear and circulation vortices

14 Applications of WV imagery 14 Numerical model validation

15 Applications of WV imagery 15 Atmospheric motion and humidity retrieval

16 Potential vorticity (PV) 16 = Stability * Absolute vorticity Growing from ground to stratosphere Conserved along the flow (except for turbulence and heating) Vorticity anomalies generate ascent ahead and subsidence behind

17 Potential vorticity (PV) Storm Emma, Met-9 Channel 5 1-Mar UTC, and height for PV=1 unit 17

maxima are close to dry slots Dry intrusions from stratosphere supply

18 Synoptic recipes 18 Dry zones, subsidence and tropopause intrusions are the darkest areas in the image Gray filling of dark areas means ascent Wind maxima are close to dry slots Dry intrusions from stratosphere supply potential vorticity and cyclogenesis 1.5 PVU height and 6.2µm Browning, Georgiev C. Georgiev

19 Synoptic recipes 19 Dry slots = Subsidence = [PV anomalies >1.5 PVU] = = Tropopause folding = downstream Cyclogenesis WV count is not a measure of PV (in particular for cut-off lows) C. Georgiev

20 Count daily cycle and monthly evolution 20 Ch.6 Ch.5 Ch.1 Ch.9 Ch.8 WV channel-6 daily cycle: 1K diurnal amplitude, 0.2K inter-day change Causes: sun absorption, dry air over hot ground, convective cooling Over the Sahara, afternoon: Ch.6 cooling but Ch.5 warming (convective balancing)

21 Overshooting 21 Water vapour or ice crystals overshoot the tropopause and reach warmer environments (reversed sign in 5-6 difference, or 5-9) Channel 9 image with overshooting tops in colour

Ozone and air masses 22 Airflow product, 25-Feb-2008 1845 UTC Ozone

stratospheric folding in the airflow product Low correlation of ozone

22 Ozone and air masses 22 Airflow product, 25-Feb UTC Ozone temperature (dots) and Dobson/10 (lines) Poor indication of ozone or stratospheric folding in the airflow product Low correlation of ozone with cloud systems High correlation between water vapour channels and vertical movement

Ozone and air masses 23 Airflow product, 11-Mar-2008 19 UTC Ozone

areas and O3 estimates Difficult tracing of tropopause foldings in O3

23 Ozone and air masses 23 Airflow product, 11-Mar UTC Ozone temperature (dots) and Dobson/10 (lines) Poor correlations between reddish areas and O3 estimates Difficult tracing of tropopause foldings in O3 estimates High independence between ozone temperature (dynamics) and ozone thickness (chemistry)

24 Ozone estimate MSG-2 12:00 12-Sept µm radiation (ch.8) is absorbed by stratospheric O3 B8(T8)=(1- α)*b8(t9) + α*b8(t) B8 is the Planck function for channel 8, at 9.7µm Minimisation on 21x21 pairs (T8,T9) i allows an estimate of (T, α) for the super pixel Independence between T and αretrievals

25 Airmass colour composite MSG-2 12:00 12-Sept

26 Airmass colour composite Kelvin 5-6 Channels

27 Airmass colour composite MSG-2 12:00 12-Sept Meaning of components for clear areas: RED: humid at mid-level or stable GREEN: cold surface (~ stable) BLUE: humid at high level or unstable BROWN: stable area

28 Conclusion 28 Profile in the atmosphere (instability) Not as intuitive as the window channels: dirt in the window Wind tracer Time for questions

INTERPRETATION GUIDE TO MSG WATER VAPOUR CHANNELS

INTERPRETATION GUIDE TO MSG WATER VAPOUR CHANNELS C.G. Georgiev1 and P. Santurette2 1 National Institute of Meteorology and Hydrology, Tsarigradsko chaussee 66, 1784 Sofia, Bulgaria 2 Météo-France, 42,