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Introduction to Radio Occultations

Introduction to Radio Occultations. Georg Bergeton Larsen GRAS SAF Project Manager Atmosphere Ionosphere Research Division (AIR) Danish Meteorological Institute (DMI) Copenhagen. Outline of presentation. The satellite system Derivation of atmosphere parameters Bending angle

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Introduction to Radio Occultations

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  1. Introduction to Radio Occultations Georg Bergeton Larsen GRAS SAF Project Manager Atmosphere Ionosphere Research Division (AIR) Danish Meteorological Institute (DMI) Copenhagen

  2. Outline of presentation • The satellite system • Derivation of atmosphere parameters • Bending angle • Refractivity • Temperature • Humidity • Distribution of measurements • Advantages and limitations • Satellite missions • Summary

  3. The satellite system The occultation measurement The GPS constellation

  4. GRAS Atmosphere profiling Metop t1 t2 t3 GPS

  5. Neutral Atmosphere Bending Angle • The bending angle is computed from the atmospheric phase delay • Ionosphere correction on bending angle

  6. Refractivity profile • The refractivity is determined using the Abel transform • Uncertainty: 0.3 % measurement range: 4 - 450 N-units

  7. Atmosphere Parameters • The density of the dry atmosphere is computed by assuming an ideal gas • where Rd = 0.287 J/(gK) and k1=77.6 K/hPa, k2 = 37,39104 K2/hPa, k3 = 70,40 K/hPa. The pressure profile is now obtained by using hydrostatic equilibrium • Uncertainty: 0.3 % measurement range: 10-1100 mb

  8. Temperature profile • The temperature profile is derived by using the ideal gas law and the estimated profiles of refractivity and pressure. • Uncertainty: 1K measurement range: 180K - 335 K

  9. Water vapor profile • The water vapor pressure is derived by an iterative process using T(z) from NWP model (Offline Products) and using the 1DVAR method (NRT Products). • 1) Total pressure • 2) Water vapor pressure • 3) Total density • Equations 1) to 3) are solved with the dry pressure as initial input in eq. 1) and Rw=0.461 J/(gK). • Uncertainty: < 20 % Measurement range: 1 - 45 mb • Assumed uncertainty on T(z) less than 1K

  10. GRAS SAF Prototype TemperatureProfile from CHAMP Data

  11. Retrieved Water Vapour Profile GPS/MET occultation Feb 9, 1997 at UT 16:15 lat: –14 lon: 141 Tdry eocc TNWP esaturated

  12. Distribution of GRAS measurements GRAS occultations during 24 hrs. Approximately 600 atmosphere profiles distributed globally Distribution of NWP Radio sondes

  13. Advantages and limitations of GPS Atmosphere Profiling • Absolute measurement • The basics of the observations are a measurement of time. Calibration of clocks can be achieved using assisting ground observations. • Global coverage • The geometry of the observation leads for one satellite to evenly distributed data on a 24-hour interval. Observations over seas and oceans (covering 70% of the Earth) minimize the major error source concerning accuracy of weather forecast and climate models. • High vertical resolution • The vertical resolution limited by the Fresnel zone of the observation leads to information of atmosphere phenomena with scale sizes less than 1 km. • Insensitive to clouds and precipitation • The wavelengths applied makes the measurement transparent to clouds and rain hampering other space techniques.

  14. Satellite missions • Research / demonstration • GPS-Met (1995-97) • Ørsted (1999-) • CHAMP (2000-) • SAC-C (2000-) • GRACE • FedSat • Operational • METOP/GRAS (2005-) • NPOESS/GPSOS (2010-) • Constellations of micro-satellites • COSMIC (2006-) • ACE+ (2008-)

  15. Summary • Key parameters and method introduced • Bending angle • Refractivity • Temperature • Humidity • Examples and distribution of RO measurements • Advantages and limitations • Global coverage (not synoptic) • High vertical resolution (averaged horizontally) • Insensitive to clouds • Satellite missions • GRAS on Metop - first operational RO mission

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