1DVAR analysis of temperature and humidity using GPS radio occultation refractivity data

[1] The constellation of Global Positioning System (GPS) satellites provides a source of continuous, phase-stable electromagnetic signals available for radio occultation observations of our planet. The atmospheric-induced bending of the transmitted rays observed during each occultation can be converted into a refractivity profile using an Abel transform. Since refractivity is related to temperature and humidity, it may potentially be used in global data assimilation for numerical weather prediction (NWP) and for creating climate data sets. We first compare GPS/Meteorology (GPS/MET) 1995 refractivity with various backgrounds and verify that the best expected background presents generally the best fit with the observed refractivity. We implement here an efficient one-dimensional variational (1DVAR) analysis of GPS refractivity that enables retrieving temperature, humidity, and sea-level pressure using the finite volume data assimilation system background. 1DVAR analyses with GPS/MET 1995 data are compared with collocated radiosondes. They show an excellent capacity of the GPS measurements to resolve the tropopause. In the Northern Hemisphere, we demonstrate a net reduction of temperature bias and standard deviation, as compared with the background. The 1DVAR humidity presents reduced standard deviation as compared to the background between 550 and 400 hPa. However, a refractivity bias between the observations and the background in the lower troposphere systematically shifts the 1DVAR humidity downward. A refractivity bias over the whole profile is transformed into a 1DVAR sea-level pressure bias. This study represents a step toward using the GPS radio occultation data in data assimilation systems to improve NWP forecasts and representation of Earth's climate in models.

[1]  S. Cohn,et al.  Assessing the Effects of Data Selection with the DAO Physical-Space Statistical Analysis System* , 1998 .

[2]  A. Jazwinski Stochastic Processes and Filtering Theory , 1970 .

[3]  George Antoine Hajj,et al.  A comparison of water vapor derived from GPS occultations and global weather analyses , 2001 .

[4]  Shian-Jiann Lin,et al.  A finite‐volume integration method for computing pressure gradient force in general vertical coordinates , 1997 .

[5]  Gottfried Kirchengast,et al.  Inversion, error analysis, and validation of GPS/MET occultation data , 1999 .

[6]  Michael E. Gorbunov,et al.  Remote sensing of refractivity from space for global observations of atmospheric parameters , 1993 .

[7]  J. R. Eyre,et al.  Retrieving temperature, water vapour and surface pressure information from refractive‐index profiles derived by radio occultation: A simulation study , 2000 .

[8]  P. Palmer Analysis of atmospheric temperature and humidity from radio occultation measurements , 1998 .

[9]  E. Kursinski Initial results of radio occultation observations of Earth's atmophere: using the Global Positioning , 1996 .

[10]  Ying-Hwa Kuo,et al.  Assimilation of Atmospheric Radio Refractivity Using a Nonhydrostatic Adjoint Model , 1995 .

[11]  X. Zou,et al.  Use of GPS/MET refraction angles in three‐dimensional variational analysis , 2000 .

[12]  J. Jenkins,et al.  Radio Occultation Studies of the Venus Atmosphere with the Magellan Spacecraft: 2. Results from the October 1991 Experiments , 1994 .

[13]  John R. Lanzante,et al.  An Assessment of Satellite and Radiosonde Climatologies of Upper-Tropospheric Water Vapor. , 1996 .

[14]  J. Schofield,et al.  Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System , 1997 .

[15]  J. R. Eyre,et al.  Assimilation of TOVS radiance information through one-dimensional variational analysis , 1993 .

[16]  W. H. Michael,et al.  Viking radio occultation measurements of the atmosphere and topography of Mars: Data acquired during 1 Martian year of tracking , 1979 .

[17]  Steven Businger,et al.  GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results , 1996 .

[18]  L. Rokke,et al.  Variational cloud‐clearing with TOVS data , 2000 .

[19]  X. Zou,et al.  Analysis and validation of GPS/MET data in the neutral atmosphere , 1997 .

[20]  R. Todling,et al.  Data Assimilation in the Presence of Forecast Bias: The GEOS Moisture Analysis , 2000 .

[21]  Henry B. Hotz,et al.  The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements , 1981 .

[22]  T. P. Yunck,et al.  The role of GPS in precise Earth observation , 1988, IEEE PLANS '88.,Position Location and Navigation Symposium, Record. 'Navigation into the 21st Century'..

[23]  Arlindo da Silva,et al.  Data assimilation in the presence of forecast bias , 1998 .

[24]  K. Hocke,et al.  Inversion of GPS meteorology data , 1997 .

[25]  G. F. Lindal,et al.  The atmosphere of Neptune : an analysis of radio occultation data acquired with Voyager 2 , 1992 .

[26]  A. Kliore,et al.  The neutral atmosphere of Venus as studied with the Mariner V radio occultation experiments , 1971 .

[27]  J. R. Eyre,et al.  A nonlinear optimal estimation inverse method for radio occultation measurements of temperature, humidity, and surface pressure , 2000, physics/0003010.

[28]  R. Wagoner,et al.  TOVS operational sounding upgrades: 1990-1992 , 1994 .

[29]  V. Eshleman,et al.  The Atmosphere of Jupiter: An Analysis of Voyager Radio Occultation Measurements. , 1981 .

[30]  Xiaolei Zou,et al.  The inclusion of GPS limb sounding data into NCEP's global data assimilation system , 1999 .

[31]  Michael S. Fox-Rabinovitz,et al.  Consistent vertical and horizontal resolution , 1989 .

[32]  Ernest K. Smith,et al.  The constants in the equation for atmospheric refractive index at radio frequencies , 1953 .

[33]  Stephen S. Leroy,et al.  Measurement of geopotential heights by GPS radio occultation , 1997 .

[34]  Larry J. Romans,et al.  Observing tropospheric water vapor by radio occultation using the Global Positioning System , 1995 .

[35]  W. G. Melbourne,et al.  Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System , 1996, Science.

[36]  Benjamin M. Herman,et al.  The GPS radio occulation technique , 2000 .

[37]  V. V. Vorob’ev,et al.  Estimation of the accuracy of the atmospheric refractive index recovery from Doppler shift measurements at frequencies used in the NAVSTAR system , 1994 .

[38]  Ying-Hwa Kuo,et al.  Assimilation of GPS radio occultation data for numerical weather prediction , 2000 .

[39]  C. Rodgers,et al.  Retrieval of atmospheric temperature and composition from remote measurements of thermal radiation , 1976 .

[40]  G. E. Wood,et al.  Radio Science with Voyager 2 at Saturn: Atmosphere and Ionosphere and the Masses of Mimas, Tethys, and Iapetus , 1982, Science.

[41]  Michael Seablom,et al.  Technical report series on global modeling and data assimilation. Volume 4: Documentation of the Goddard Earth Observing System (GEOS) data assimilation system, version 1 , 1995 .

[42]  G. Leonard Tyler,et al.  Systematic errors in atmospheric profiles obtained from abelian inversion of radio occultation data : Effects of large-scale horizontal gradients , 1999 .

[43]  John J. Barnett,et al.  Application of an optimal estimation inverse method to GPS/MET bending angle observations , 2001 .

[44]  W. Bertiger,et al.  A technical description of atmospheric sounding by GPS occultation , 2002 .

[45]  E. Robert Kursinski,et al.  Initial results of combining GPS occultations with ECMWF global analyses within a 1DVar framework , 2000 .

[46]  Ernest K. Smith,et al.  The Constants in the Equation for Atmospheric Refractive Index at Radio Frequencies , 1953, Proceedings of the IRE.

[47]  Lawrence L. Takacs,et al.  Data Assimilation Using Incremental Analysis Updates , 1996 .