Transcript Slide 1

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 2-3km.
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 ~500m
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 $300K 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 age 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, 19,100km, 11
Hr 15mts)
NAVSTAR (NAVigation Satellite
Timing and Ranging), 24+4 GPS on 6
orbits (55o, 20,200km, 12 Hr)
European GALILEO (European Satellite Navigation
System), 27+3 GPS on 3 orbits (56o, 23,616km, 14
Hr, 22mts) 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 (L1=1575.42 MHz and
L2=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 3x108 m/s, with an accuracy of 10 cm, we need to use a
clock with a stability of 3x10-9 .
This can easily achieved with GPS, as it employs a clock with a
stability of 1x10-12, corresponding to a distance resolution of 0.3 mm.
(1 hour stability of Cesium atomic clock is 1x10-12, and that for 1
week is as good as 1x10-13.)
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
DoD, 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
DoD, 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
1
nx  exp

COSMIC


x
 a

da
2
2

a x

N= (n-1) x 10-6
N=77.6*P/T + 3.73*105(e/T2) + ionospheric term
Dry
Wet
In Dry Atmosphere, second term is assumed to be zero – valid above ~10km
Using Hydrostatic equation, profile of temperature can be estimated from N
Prior information on temperature will help in estimating profile of water vapor
2TN – 77.6p
e~
=
ΔT
3.73 x 105
For typical values of N = 300, P = 1000mb 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
(6.9S,107.6E)
(2) Detailed
temperature
structure near
the tropopause
(1) Humidity profile by combining
a temperature model (1D-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
40km
Kelvin waves
Gravity waves (UTLS region)
Gravity waves
Radiosonde
Trapped by
vertical shear
- Eastward propagation
- Downward phase
Radiosonde
GPS RO
Spatial and temporal
variations using GPS
RO
Tropopause
Vertical structure of fast
and ultra-fast waves
Generation mechanism
Regional convections
10km
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
Identification of Tropopause using Bending Angle profile from GPS Radio
Occultation (RO): A Radio Tropopause
Rao, Ratnam et al., 2007, GRL (this issue)
(a)
16.7km
(c)
(b)
16.8km
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
STP 
TTP  z  TTP TTP  TTP z

z
z
Detection/prediction of onset of Indian Summer Monsoon
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 (850mb – 200mb)
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
p
e
N  77.6( )  3.73  10 5 ( 2 )
T
T
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 (20o) 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 (~20o) to provide adequate number of occultations in the tropical latitudes.
Development of dual frequency GPS receiver
Thanks for your kind attention!
[email protected]
List of Publications using GPS RO data in last 3 years
1.
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, L16101, doi:10.1029/2004GL019789.
3. T. Tsuda, M. Venkat Ratnam, P. T. May, M. J. Alexander, R. A. Vincent, and A. MacKinnon (2004), Characteristics of gravity waves
with short vertical wavelengths observed with radiosonde and GPS occultation during DAWEX (Darwin Area Wave Experiment), JGR,
Vol. 109, D20S03, doi:10.1029/2004JD004946.
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. 84A, 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 BashaV. V. M. Jagannadha Rao, B. V. Krishna
Murthy, T. Tsuda, and Kenji Nakamura, (2007), Validation of the COSMIC Radio Occultation data over Gadanki (13.48oN, 79.2oE):
A tropical region, Terrestrial Atmospheric and Oceanic journal (Submitted)