The water vapour channels of SEVIRI (Meteosat). An introduction jose.prieto@eumetsat.int Cachoeira P. July 2006 Formats 1.5 1
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
Contents 3 Characteristics of WV channels Radiative transfer considerations Synoptic features in WV Use in RGBs of WV channels
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 300-600 Hpa (240-260 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
Spatial and spectral resolution 5 Horizontal: 3.5 km at SSP 7.3µm has a smaller bandwidth
Deeper into the atmosphere: 5..6..7..9 6
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
Top of the atmosphere R 8 100 Hpa Radiation Transfer ( WV ) R= B(T) (p--top) 900 Hpa Amount of absorber Earth s surface
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
Humidity layers 10 Humid at mid-level Dry Humid
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
Synoptic diagnosis 12
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
Applications of WV imagery 14 Numerical model validation
Applications of WV imagery 15 Atmospheric motion and humidity retrieval
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
Potential vorticity (PV) Storm Emma, Met-9 Channel 5 1-Mar-2008 06 UTC, and height for PV=1 unit 17
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
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
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)
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 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 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)
Ozone estimate MSG-2 12:00 12-Sept-2006 24 9.7µm 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
Airmass colour composite MSG-2 12:00 12-Sept-2006 25
Airmass colour composite 26 9 6 5 0 5 208 Kelvin 5-6 Channels 8-9 -5-25 -40 243 9 8 5
Airmass colour composite MSG-2 12:00 12-Sept-2006 27 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
Conclusion 28 Profile in the atmosphere (instability) Not as intuitive as the window channels: dirt in the window Wind tracer Time for questions