Application of Global Positioning System GPS Radio Occultation

Application of Global Positioning System (GPS) Radio Occultation (RO) data for Climate change studies M. Venkat Ratnam Scientist National Atmospheric Research Laboratory (NARL) Department of Space (DOS), Gadanki, Tirupati – 517 502, India UN / Russian Federation / European Space Agency Workshop, 3 -7 Sep. 2007, Tarusa

Outline • Brief Introduction – Importance of Climate studies • Limitation in present techniques for climate studies • Advantages of GPS RO over convectional techniques • Historical development of Navigation (positioning) techniques • Basic Concept of GPS Occultation Measurement • Study of Climate change parameter - Tropopause • Application for prediction of onset of Indian Summer Monsoon • Near future Indian plans

Scientists find 'fingerprint' of human activities in recent tropopause height changes LIVERMORE, Calif. -- Scientists from the Lawrence Livermore National Laboratory have determined that human-induced changes in ozone and well-mixed greenhouse gases are the primary drivers of recent changes in the height of the tropopause.

‘‘Evidence for decadal tropical change includes an observed increase in the tropical mean temperature lapse rate which is not reproduced by the climate models’’ Source: D. J. Gaffen et al. , Science 287, 1242 (2000) ‘‘IPCC climate change report indicates an increase in lapse rate beginning about 1991’’ Source: IPCC climate 2001, The Scientific basis, J. T. Houhton et al. , eds, pp. 87 -91 ‘‘There is no serious threat to the climate. There is no need to dramatize the anthropogenic impact because the climate has always been subject to change under the Nature’s influence, even when humanity did not even exist’’ Source: Panic over the Global Warming is totally unjustified – Russian Academy of Sciences

Observations available for climate change studies Ground based measurements: Automatic weather station: provide surface measurements but have poor spatial resolution. Radiosonde provide measurements with good vertical resolution but network is too small – a large gap in oceans and also poor temporal resolutions. Ground based LIDAR: provide high-quality data with good vertical and temporal resolution but again poor spatial resolution. Space borne Instruments: IR sounders are crucial in measuring outgoing radiation to space, main limitation : Earth is covered with at least two-third of clouds creating a fundamental sampling problem for IR Microwave sensors can penetrate clouds to get Water Vapor retrievals but are generally limited to marine environments again main limitation : vertical resolution is poor i. e 2 -3 km. Need of high resolution measurements: The basic principle characterizing any atmosphere constituent is that its vertical structure should be resolved at least 3 times per scale height. GPS Radio Occultation (GPSRO): GPSRO has ability to probe the Troposphere and lower stratosphere in both clear and cloudy condition with a precision and vertical resolution of ~500 m

Advantages of GPS RO Scientists began to examine GPS as a tool for atmospheric sensing in the late 1980’s they found it offered so many attractions not found in the established space techniques. • Compact, low-power, low-data-rate sensors, costing of order $300 K rather than millions or tens of millions, easily embedded in spacecraft large and small. • An ability to sound the atmosphere from the stratopause to the earth’s surface. • A vertical resolution of a few hundred meters in the troposphere, compared with several kilometers or worse with other space instruments. • Self-calibrating profiles that never drift, can be compared between all occultation sensors over all time, and provide a calibration standard for other sensor types. • Virtually unbiased measurements that can be averaged over days or weeks to yield normal points with an equivalent temperature accuracy of order 0. 1 K. • Fully independent measurement of pressure and height, permitting recovery of absolute geo-potential heights with no external reference. • The prospect of concurrently sampling the full global atmosphere at low cost • An extraordinary diversity of applications outside of atmospheric science.

Historical Development of Navigation (Positioning) Techniques GPS (Global Positioning System): Triangulation by using artificial radio stars 1. In 15 -th century a safe voyage was realized by a celestial navigation by using an accurate clock and sextant (The great voyage of discovery) 2. Measurement s of radio stars with an interferometer (VLBI) in 1960’s; Tectonic plate motion, and Earth rotation 3. Active radio measurement s of an artificial star (satellite) with triangulation; accurate navigation and timing with GPS 4. Application of precise satellite positioning to monitoring of the Earth’s environment: GPS meteorology Satellite gravity mission

GPS (Global Positioning System) Russian GLONASS (GLObal NAvigation Satellite System), 21+3 GPS on 3 orbits (64. 80, 19, 100 km, 11 Hr 15 mts) NAVSTAR (NAVigation Satellite Timing and Ranging), 24+4 GPS on 6 orbits (55 o, 200 km, 12 Hr) European GALILEO (European Satellite Navigation System), 27+3 GPS on 3 orbits (56 o, 23, 616 km, 14 Hr, 22 mts) by 2010 Precise Satellite Positioning GALILEO The ubiquitous signals from GPS, together with Russia’s GLONASS, Europe’s Galileo, and a host of planned commercial and military craft in high orbits will become illuminating beacons enveloping the earth. Microwaves on two frequencies (L 1=1575. 42 MHz and L 2=1227. 60 MHz) are emitted from GPS satellites. Distance between the GPS satellites and a receiver is determined by measuring the propagation time of radio signals. Signals from individual GPS satellites are identified by a code. By receiving signals from at least 4 GPS satellites, location of the receiver can precisely be determined. Accuracy of the measurements is greatly improved to about a few millimeters by analyzing the carrier phase of the radio signals.

Watch our environment with a watch Time in one of the physical parameters that can be determined very accurately. The most accurate time standard can achieve 10 -15 sec If we measure a 100 m foot race (speed is about 10 m/s) with a stopwatch with an accuracy of 1/100 sec, we can determine the difference in distance to 10 cm. When we measure the traveling distance of radio wave, whose speed is 3 x 108 m/s, with an accuracy of 10 cm, we need to use a clock with a stability of 3 x 10 -9. This can easily achieved with GPS, as it employs a clock with a stability of 1 x 10 -12, corresponding to a distance resolution of 0. 3 mm. (1 hour stability of Cesium atomic clock is 1 x 10 -12, and that for 1 week is as good as 1 x 10 -13. ) Stable clock + Radio Wave ⇒ 　Accurate measurement of position and velocity

How GPS RO data contribute for climate studies ? • Non-linear characteristics of the climate system requires long-term observations • Understanding both its natural variability and its response to anthropogencally driven changes in radiative forcing • For studies of long term climate trends of a parameter (e. g. temperature, water vapor, tropopause height or geo-potential height of specific pressure levels) at high precision with sufficient accuracy, resolution, and spatial and temporal coverage of the parameter is required • Since only small variations are expected over the life time of an instrument. This is partly overcome by GPS RO method which requires no external calibration, but only relies on stable oscillators and hence most useful for climate research and weather prediction. • This data set has been successfully used for weather forecasting in which many studies have shown its uniqueness for better forecasting by incorporating the global data sets from GPS RO

　GPS RO occultation missions – Then and Existing 1995 - 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 GPS/MET SUNSAT ORSTED CHAMP UCAR, Apr 95 -Feb 97 S. Africa, Feb. 99 Denmark, Feb. 99 GFZ, July 2000 SAC-C Argentine + JPL, Nov 2000 IOX Do. D, JPL, Sep. 2000 GRACE DLR, NASA, Mar. 2002 COSMIC UCAR + Taiwan-NSPO, 6 LEO satellites, Apr. 2006 METOP EUMSAT, Germany, Oct. 2006 Data rate 2500/day

　GPS RO occultation missions – forthcoming　 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Terrasar X, Jul. 2007 C/NOFS Germany Do. D, USA EQUARS (? ) Oceansat-2 Brazil-INPE + RISH, 2007 (20 deg) India-ISRO+ Italy-ASI, 2008 (polar orbit) Megha Tropiques SWIFT-ARGO Data rate India-ISRO+France-CNES, 2009 (20 deg) Canada-CSA, 2009 (polar orbit) 2, 500/day > 5, 000/day

Basic Concept of GPS Occultation Measurement GPS observations GPS Signals received on a low earth orbiting (LEO) satellite are used for an active limb sounding of the atmosphere and ionosphere. Double Differencing During a rising or setting of a GPS satellite (occultation), the radio rays between the GPS and LEO satellites successively scan the atmosphere (and the ionosphere) from the receiver height down to the surface. A refractive index profile can be retrieved from the time variations of the ray bending angles. Precise orbit determination Atmospheric excess path Derivation of Bending angle Inospheric corrections Bending Angle Profile Abel Inversion Refractivity Profiling Dry Air Profiling Wet Air Dry Temperature Water Vapour

GPS RO Technique – Basic Principle Abel inversion COSMIC N= (n-1) x 10 -6 N=77. 6*P/T + 3. 73*105(e/T 2) + ionospheric term Dry Wet In Dry Atmosphere, second term is assumed to be zero – valid above ~10 km Using Hydrostatic equation, profile of temperature can be estimated from N Prior information on temperature will help in estimating profile of water vapor e~ = 2 TN – 77. 6 p ΔT 3. 73 x 105 For typical values of N = 300, P = 1000 mb and T = 273 K Δe≈0. 23ΔT, or ΔT = 4. 35Δe

Characteristics of temperature profiles with GPS occultation -- precise atmospheric profiles (temperature and humidity) -- excellent height resolution (about 1 km near the tropopause) -- wide height coverage (1 – 40 km) -- high data rate (150 -200/day/satellite) (3) Temperature fluctuations caused by atmospheric waves (6. 9 S, 107. 6 E) (2) Detailed temperature structure near the tropopause (1) Humidity profile by combining a temperature model (1 D-var) Comparison of a temperature profile between GPS occultation (GPS/MET) and a nearby radiosonde in Indonesia

Tropospheric and Lower Stratospheric (TLS Region) dynamics–local and Global Mesosphere 40 km Kelvin waves Gravity waves (UTLS region) Gravity waves Radiosonde - Eastward propagation Trapped by - Downward phase vertical shear Radiosonde GPS RO Spatial and temporal variations using GPS RO Tropopause Vertical structure of fast and ultra-fast waves Generation mechanism Regional convections 10 km Tropospheric disturbances - Eastward-propagating cloud system - Low-level convergence over Sumatera (OLR, GPS RO WV) ISM Provide a favorable condition for development Radiosonde, GPS RO 0 km

Identification of Tropopause using Bending Angle profile from GPS Radio Occultation (RO): A Radio Tropopause Rao, Ratnam et al. , 2007, GRL (this issue) (a) 16. 7 km (c) (b) 16. 8 km Radiosonde DRY WET Radio Tropopause: Definition Above the altitude range of moist atmosphere, the altitude at which the gradient of the Bending Angle is maximum (taking sign into account) and above which it decreases for at least 1 km is defined as the Radio Tropopause.

Temperature Lapse Rate from 13 km-16 km during May 2001–Dec. 2006

New Charecteristics observed in tropopause height TROPOPAUSE The tropopause heights defined by both lapse rate and cold point generally show large-scale, off-equatorial maxima, and even a ‘U-shaped’ feature along a particular meridian, in contrast to our previous knowledge. Although this feature has already been reported partially during the summer monsoon season, the present study shows the seasonal and geographical distributions of the tropical tropopause comprehensively using a new promising observational technique. In addition, the vertical shape of the tropopause is found to be sharp in the equatorial region and broad in the subtropics especially in northern winter. Possible mechanisms are discussed in light of dynamical and radiative processes. Ratnam et al. , 2005, Scientific Online Letters ?

Latitude Vs Longitude distribution Horizontal distributions of the cold point tropopause height (top), temperature (center), and the outgoing long-wave radiation (bottom). Ratnam et al. , 2005, SOLA

Anomalies from Climatological Mean (5 years) R=0. 56

Detection/prediction of onset of Indian Summer Monsoon : Periodical reversal of wind regimes due to differential heating between warmer continental area and adjoining tropical oceans Arabian Sea The onset of the monsoon is normally around the beginning of June over the Southern tip of India. Although there is no precise definition for the onset, it is conventionally identified by a sharp increase and persistency in the rainfall (Ananthakrishanan et al. , 1968) Because of the socio-economic-agricultural consequences, an attempt is made to study the onset of monsoon using GPS Radio Occultation Technique

Earlier Studies on onset of ISM Many investigations have been carried out on the onset of ISM over Arabian sea using various parameters such as 1. Ananthakrishanan & Soman (1988) Rainfall (over Kerala coast) 2. Fasullo & Webster( 2003) Vertical integrated moisture transport (VIMT) 3. Prasad & Hayashi (2005) Zonal asymmetric temperature anomaly (850 mb – 200 mb) 4. Taniguchi & Koike (2006) 850 mb Low level wind speed 5. Pearce & Mohanty (1984) 6. Soman & Kumar (1993) 7. P. L. S. Rao et al. , (2005)

Identification/prediction of Indian Summer Monsoon (ISM) Onset day is 146 according to Indian Meteorological Department (IMD) – based on integrated rainfall

How much before GPS RO can predict ISM? (Statistical analysis) Rao and Ratnam. , 2007 (Communicated to Geophysical Research Letters)

CONCLUSIONS Few interesting features are observed in Refractivity and Temperature parameters Over SE Arabian Sea 1. The Refractivity around 600 mb increases sharply by ~ 15 N units few days before the date of onset. This is due to moisture build up before onset probably due to evaporation 2. The mean Upper Tropospheric temperature increases by 1. 5 – 2 K at the time of onset. This is due to development of convective activity leading to release of latent heat causing increase of tropospheric temperature 3. At the time of onset, a dip in refractivity of nearly 5 units is observed which is due to fall in moisture content at the time of onset (PLS Rao et al. , 2005) 4. At the time of onset, the CPT increases by ~2 K and CPH decreases appreciably which supports earlier results (Ramanatham et al. , 1972)

Near Future Indian Plans Radio Occultation for Sounding Atmosphere (ROSA) (April. 2008) – in collaboration with Italy IGOR on Megha Tropiques (July 2009) - in colloboration with France Megha Tropquies will be a unique low inclination (20 o) LEO having a dual frequency GPS receiver for radio occultation observations GEMSS: Realizing the potential of GPS RO Technique for operational weather forecast, Atmospheric modeling, Communications, climate studies and also to carry out frontline research in Atmospheric sciences, ISRO has formulated a project, GPS RO based experiments for Meteorology and Space Sciences (GEMSS), with NARL Director as Project director. GEMSS perhaps will have a Constellation of LEO satellites at Low inclination angle (~20 o) to provide adequate number of occultations in the tropical latitudes. Development of dual frequency GPS receiver

Thanks for your kind attention! vratnam@narl. gov. in

1. List of Publications using GPS RO data in last 3 years M. Venkat Ratnam, G. Tetzlaff and Chirstoph Jacobi (2004), Study on stratospheric gravity wave activity: Global and seasonal variations deduced from the CHAllenging Minisatellite Payload (CHAMP)-GPS Satellite, JAS, Vol. 61, 1610 -1620. 2. M. Venkat Ratnam, Y. Aoyama, T. Tsuda and Ch. Jacobi (2004), Enhancement of Gravity wave activity observed during a major Southern Hemisphere stratospheric warming by CHAMP/GPS measurements, GRL, Vol. 31, L 16101, doi: 10. 1029/2004 GL 019789. 3. T. Tsuda, M. Venkat Ratnam, P. T. May, M. J. Alexander, R. A. Vincent, and A. Mac. Kinnon (2004), Characteristics of gravity waves with short vertical wavelengths observed with radiosonde and GPS occultation during DAWEX (Darwin Area Wave Experiment), JGR, Vol. 109, D 20 S 03, doi: 10. 1029/2004 JD 004946. 4. Chirstoph Jacobi, M. Venkat Ratnam, G. Tetzlaff (2005), Global analysis of stratospheric gravity wave activity using CHAMP radio occultation temperatures, Springer, Berlin Heidelber New York, 555 -560. 5. M. Venkat Ratnam, G. Tetzlaff and Chirstoph Jacobi (2005), Structure and variability of global tropopause, Springer, Berlin Heidelber New York, 561 -566. 6. M. Venkat Ratnam, Tsuda, M. Shiotoani, and M. Fujiwara (2005), Peculiar behavior of tropopause observed in tropical and extra tropical latitudes with CHAMP/GPS Radio Occultation measurements, Scientific Online Letters of Atmosphere, Vol. 1, 185‒ 188, doi: 10. 2151/sola. 7. M. Venkat Ratnam, T. Tsuda, T. Kozu, and S. Mori (2006), Long-term behavior of the Kelvin waves revealed by CHAMP/GPS RO measurements and their effects on the tropopause structure, Annales Geophysicae, Vol. 24, 1355– 1366. 8. T. Tsuda, M. Venkat Ratnam, T. Kozu, and S. Mori (2006), Characteristics of 10 -day Kelvin Wave Observed with Radiosondes and CHAMP/GPS Occultation during the CPEA Campaign (April - May, 2004), JMSJ, Vol. 84 A, 277 -293. 9. M. Venkat Ratnam, T. Tsuda, T. Kozu, and S. Mori (2006), Modulation of tropopause structure due to local and global-scale temperature variations: A case study using simultaneous radiosonde and CHAMP/GPS measurements, JMSJ, Vol. 84, 989 -1003. 10. D. Narayana Rao, M. Venkat Ratnam, B. V. Krishna Murthy, V. V. M. Jagannadha Rao, Sanjay Mehta, Debashis Nath and Ghouse Basha (2007), Identification of Tropopause using Bending Angle profile from GPS Radio Occultation (RO): A Radio Tropopause, Geophysical Research letters, (In Press) 11. D. Narayana Rao, M. Venkat Ratnam, Sanjay Mehta, Debashis Nath and Ghouse Basha. V. V. M. Jagannadha Rao, B. V. Krishna Murthy, T. Tsuda, and Kenji Nakamura, (2007), Validation of the COSMIC Radio Occultation data over Gadanki (13. 48 o. N, 79. 2 o. E): A tropical region, Terrestrial Atmospheric and Oceanic journal (Submitted)