Satellite Observations Gerald van der Grijn Meteorological Operations Section Thanks to: Anthony McNally (satellite section) Lars Isaksen (data assimilation section) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Coverage of conventional observations used in NWP 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Coverage of satellite observations used in NWP NOAA AMSUA/B HIRS, AQUA AIRS DMSP SSM/I SCATTEROMETERS GEOS TERRA / AQUA MODIS OZONE 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Current data count 28R1 (10/03/04 00Z) Screened Assimilated Synop: 193616 (0.27%) Aircraft: 254294 (0.36%) AMV’s: 2186744 (3.06%) Dribu: 10803 (0.02%) Temp: 116442 (0.16%) Pilot: 94889 (0.13%) UpperSat: 68105926 (95.38%) PAOB: 814 (0.00%) Scat: 247320 (0.35%) TOTAL: 71.210.848 Synop: 39142 (1.57%) Aircraft: 158219 (6.35%) AMV’s: 73574 (2.95%) Dribu: 3547 (0.14%) Temp: 66405 (2.67%) Pilot: 49818 (2.00%) UpperSat: 1985939 (79.72%) PAOB: 290 (0.01%) Scat: 114290 (4.59%) TOTAL: 2.491.224 99.06% of screened data come from satellites 87.26% of assimilated data come from satellites 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Evolution of Forecast Skill 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Met-OP Training Course Use and Interpretation of ECMWF Products Impact of withdrawing different types of observations on forecast quality Anomaly correlation of 500hPa height for Southern Hemisphere 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Met-OP Training Course Use and Interpretation of ECMWF Products Impact of withdrawing different types of observations on forecast quality Anomaly correlation of 500hPa height for Northern Hemisphere 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Met-OP Training Course Use and Interpretation of ECMWF Products 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Importance of Satellite Data Satellite data have progressively become an essential part of the observing system used at ECMWF Satellite data represent by far the largest volume of data (and associated computing cost) used in the ECMWF data assimilation system Satellite data have recently caught up radiosondes in terms of forecast skill impact over NH 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Two Types of Satellites Polar Orbiters Orbits are sun-synchronous circular orbits that almost pass over the poles. Altitudes are typically at 850 km. Each satellite will complete about 14 orbits in one day. Imagery from successive orbits overlay with each other to give global daily coverage. This data is used in NWP models. Geostationary Satellites Orbit at a height of 35,800 km At this height the satellite is stationary with respect to a point on the earth’s surface. High temporal resolution Ideal for making sequential observations of clouds Polar orbiter at 850 km. Geostationary satellite at 35,800 km. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Coverage of polar orbiter 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Instruments / Satellites currently used at ECMWF Polar Orbiters NOAA: High Resolution IR Sounder (HIRS) on NOAA-17 Advanced Microwave Sounding Unit (AMSU-A and AMSU-B) on NOAA-14, NOAA-15, NOAA-16, NOAA-17 Solar Backscatter UltraViolet radiometer (SBUV/2) DMSP Special Sensor Microwave Imager (SSM/I) on DMSP-13, DMSP-14, DMSP-15 Aqua Atmospheric InfraRed Sounder (AIRS) MODerate resolution Imaging Spectroradiometer (MODIS) TERRA Quikscat Scatterometer 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Instruments (or satellites) currently used at ECMWF Polar Orbiters cont’d ERS-2 Radar Altimeter Synthetic Aperture Radar (SAR) ENVISAT Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Geostationary Satellites ATMOSPHERIC MOTION VECTORS (satellite derived winds) MET-5, MET-7, GOES-10, GOES-12, (MET-8 under evaluation) RADIANCES MET-5, MET-7,GOES-9, GOES-10, GOES-12, (MET-8 under evaluation) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

What do satellites actually measure ? They DO NOT measure TEMPERATURE They DO NOT measure HUMIDITY They DO NOT measure WIND Satellite observations are obtained using remote sensing techniques based on measurements of electromagnetic radiation. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Electromagnetic Radiation Every object with a temperature larger than 0 K emits electromagnetic radiation. Electromagnetic radiation therefore extends over a wide range of energies and wavelengths. The distribution of all radiant energies can be plotted in a chart known as the electromagnetic spectrum. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Electromagnetic Radiation In the earth’s atmosphere, the radiation is partly to completely transmitted at some wavelengths; at others those photons are variably absorbed by interaction with air molecules. Blue zones mark minimal passage of incoming and/or outgoing radiation, whereas, white areas denote atmospheric windows in which the radiation doesn’t interact much with air molecules. Most remote sensing instruments operate in one of these windows by making their measurements tuned to specific frequencies that pass through the atmosphere. Some sensors, especially those on meteorological satellites, directly measure absorption phenomena. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

What do satellites actually measure ? In essence, satellite instruments measure the radiance L that reaches the top of the atmosphere at a certain frequency v. Radiance transfer equation: where is the black body emission at a given temperature at altitude , and the change in transmittance with height. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Different instruments and channels Depending on the frequency, the measured radiance will be sensitive to different geophysical variables. In general the channels used for NWP can be considered as one of three different types. Atmospheric sounding channels from passive instruments Surface sensing channels from passive instruments Surface sensing channels from active instruments In practice, satellite instruments have channels which are a combination of atmospheric sounding and surface sensing. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Passive atmospheric sounders These channels are located in parts of the infrared and microwave spectrum. Main contribution to the measured radiance is from the atmosphere and can be written as: They avoid frequencies for which surface radiation or cloud contribution are important. Channels are primarily used to obtain temperature and humidity profiles. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Passive atmospheric sounders 0° 30°N 60°N 150°W 120°W 90°W 60°W 30°W 30°E 60°E 90°E 120°E 150°E 200 210 220 230 240 250 260 270 280 290 300 HIRS Ch. 12 (6.7 micron) 60°S 30°S 0° 30°N 60°N 150°W 120°W 90°W 60°W 30°W 30°E 60°E 90°E 120°E 150°E 190 200 210 220 230 240 250 260 270 280 290 AMSU-A Ch. 5 (53 GHz) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Passive surface sensing channels These channels are located in atmospheric window regions at frequencies where there is very little interaction with the earth’s atmosphere. Also known as “imaging channels” Main contribution to the measured radiance is: where Tsurf is the surface temperature and ε the surface emissivity. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Passive surface sensing channels Primarily used to obtain information on surface characteristics such as Surface temperature Quantities that influence surface emissivity Wind (roughness over the sea) Vegetation Also used for Cloud top temperatures (infrared) Rain (microwave) Deriving wind information through sequence of geostationary satellite images. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Passive surface sensing channels HIRS Ch. 8 (6.7 micron) 60°S 30°S 0° 30°N 60°N 150°W 120°W 90°W 60°W 30°W 30°E 60°E 90°E 120°E 150°E 90 110 130 150 170 190 210 230 250 270 290 SSM/I Ch. 7 (85 GHz) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Active surface sensing channels These instruments illuminate the earth’s surface by emitting energy in atmospheric window regions and measure the radiance that is scattered back. Main contribution to the measured radiance is: Provide information on ocean winds (scatterometers) Similar class instruments such as altimeters and SARS (Synthetic Aperture Radars) provide information on wave height and spectra. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Active surface sensing channels Big gaps in data coverage of Quikscat data due to rain contamination. Hurricane Lili 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Atmospheric temperature sounding We saw that the radiation measured by passive atmospheric sounders can be written as: If the primary absorber is a well mixed gas (e.g. O2 or CO2) than it can be seen that the measured radiance is essentially a weighted average of the atmospheric temperature profile, The function K(z) that defines this weighted average is known as a WEIGHTING FUNCTION. It specifies the layer from which the radiation emitted to space originates, and hence it determines the region of the atmosphere which can be sensed from space at this frequency. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Ideal weighting functions If the weighting function was a delta-function, this would mean that the measured radiance is sensitive to the temperature at a single level in the atmosphere. z K(z) If the weighting function was a box-car function, this would mean that the measured radiance is sensitive to the mean temperature between two atmospheric levels. z K(z) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Real atmospheric weighting functions High in the atmosphere very little radiation is emitted, but most will reach the top of the atmosphere. z At some level there is an optimal balance between the amount of radiation emitted and the amount reaching the top of the atmosphere. K(z) A lot of radiation is emitted from the dense lower atmosphere, but very little survives to the top of the atmosphere due to absorption. K(z) 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Real weighting functions cont’d The altitude at which the peak of the weighting function occurs depends on the strength of absorption for a given channel. Channels in parts of the spectrum where the absorption is strong (e.g. near the centre of CO2 or O2 lines ) peak high in the atmosphere. Channels in parts of the spectrum where the absorption is weak (e.g. in the wings of of CO2 or O2 lines) peak low in the atmosphere. AMSUA By selecting a number of channels with varying absorption strengths we sample the atmospheric temperature at different altitudes 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Real weighting functions cont’d HIRS AMSUA AIRS Ch-14 Ch-1 Ch-13 Ch-12 Ch-11 Ch-2 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Characteristics of weighting functions The weighting functions are broad, i.e. several kilometres. the instrument can sense the mean properties of broad layers very well, but the width of the weighting functions limits the capability of satellite sounders to detect atmospheric structures which have relatively small scale in the vertical. For most instruments the weighting functions are highly overlapping. although the instrument may make measurements at N separate frequencies, we obtain fewer than N pieces of independent information. 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Extracting temperature profiles from satellite measurements If we know the entire atmospheric temperature profile T(z) then we can compute (uniquely) the radiances a sounding instrument would measure using the radiative transfer equation. This is sometimes known as the forward problem In order to extract or retrieve the atmospheric temperature profile from a set of measured radiances we must solve what is known as the inverse problem Unfortunately with a finite number of channels and weighting functions that are generally broad, the inverse problem is formally ill-posed (an infinite number of different temperature profiles could give the same measured radiances) Extracting temperature profiles from satellite measurements 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

for more information on how this data is actually used: We’ve learned what satellites measure, but … for more information on how this data is actually used: ECMWF MET-OP lecture: ‘Data assimilation’ by Lars Isaksen ECMWF MET-DA lecture: “Data assimilation and use of satellite data”. ECMWF newsletter articles ( http://www.ecmwf.int/publications/newsletters/ ) Spring 2003: “Assimilation of high-resolution satellite data.” Spring 1999: “The use of raw TOVS/ATOVS radiances in the ECMWF 4D-Var assimilation system” ECMWF Technical Memoranda ( http://www.ecmwf.int/publications/library/do/references/list/14 ) TM 345: “An improved general fast radiative transfer model for the assimilation of radiance observations” 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products

Satellite Data Monitoring on the Web Products publicly available on the public ECMWF server: Data coverage maps http://www.ecmwf.int/products/forecasts/d/charts/monitoring/coverage/dcover Data monitoring plots http://www.ecmwf.int/products/forecasts/d/charts/monitoring/satellite 7 June 2004 Met-OP Training Course Use and Interpretation of ECMWF Products