Observations of Equatorial Atmosphere and Ionosphere by GPS Occultation Technique Toshitaka Tsuda1,

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Observations of Equatorial Atmosphere and
Ionosphere by GPS Occultation TechniqueToshita
ka Tsuda(1), Klemens Hocke(2), Yuichi Aoyama(1)
and Hisao Takahashi(3) (1) Radio Science Center
for Space and Atmosphere (RASC), Kyoto
University(2) Communications Research Laboratory
(CRL)(3) Instituto Nacional de Pesquisas
Espaciais (INPE), Brasil
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Excitation of Atmospheric Waves by Convection in
Equatorial Atmosphere
Because of intense solar radiation in the
tropics, convections are actively generated,
which further excite various atmospheric waves,
such as equatorial waves (Kelvin waves, mixed
Rossby gravity waves), atmospheric tides and
gravity waves. Energy and momentum are
transported upward by the vertically propagating
atmospheric waves. Wave-wave and/or wave-mean
flow interactions drive QBO and SAO in the middle
atmosphere.
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Time-Height Section of Zonal Wind Perturbations
Observed with Ground-based Techniques
Meteor Wind Radar

• Fluctuating components of zonal wind velocity
  observed with the Jakarta meteor wind radar (MWR)
  and radiosondes from Nov 1992 to Apr 1993 (150
  days).
• 30-50 day oscillations existed in the troposphere
  (below about 15 km).
• Kelvin waves (10-20 day) propagated in the lower
  stratosphere (15-30 km).
• Semiannual oscillation (SAO) and various waves
  (3-6 day) are seen in the mesosphere (above about
  70 km).

100 km
70
Radiosonde (every 6 hours)
30
15
0
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Comparison of a temperature profile between
GPS/MET and a nearby radiosonde (Indonesia)
RMS Difference Upper Troposphere 1 K
Stratosphere 2 K (Fluctuations due to
Atmospheric Waves)
(6.9S,107.6E)
?
? Temperature perturbations in the stratosphere
seems to be caused by atmospheric
gravity waves
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Determination of Potential Energy (Ep) due to
Atmospheric Gravity Waves using GPS/MET
Temperature Profiles

• Meso-scale temperature fluctuations (T) are
  extracted from a single GPS/MET profile by using
  a high-pass filter with cutoff at 10 km.
• GPS/MET profiles are already low-pass filtered to
  reduce noise, with cut-off of 1-2 km in the
  stratosphere and 200m in the troposphere.
• Accordingly, T in the stratosphere consists of
  temperature fluctuations having a vertical scale
  between 1-2 km and 10 km.
• We then evaluate a potential energy per unit
  mass, Ep½(g/N)2(T/T)2 (J/kg), where N2 is
  also estimated from the GPS/MET profiles.

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Altitude (km)
40
20
0
-10
10
Observed T H-P filtered T T Variance
N2
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Global Distribution of Gravity Wave Energy (Ep)
at 20-30 km in Northern winter (Nov-Feb, 1995-7)
The Ep value is averaged in an area extending 10
and 20 degrees in latitude and longitude, and the
center coordinates are shifted every 1 and 2
degrees, respectively Tsuda et al., 2000.
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Correlation between Gravity Wave Energy (Ep) and
Cloud amount in Equatorial Region inferred from
OLR
Top Latitude-longitude distribution of the wave
potential energy Ep½(g/N)2(T/T)2 in Nov-Feb at
20-30 km from GPS/MET. Bottom Black body
temperature (OLR) in Feb, 1997. Blue area in
tropics indicates tall clouds. Large Ep values
are detected at low latitudes (25o N - 25o S),
and they are particularly enhanced over the
regions of active convection, i.e, Indonesia,
Indian ocean, Africa and South America.
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Estimation of Horizontal TEC (total electron
content) from GPS occultation measurements
? s ?
GPS Satellite
LEO satellite carrying a GPS receiver
Ionosphere
?r

• Assuming that TEC is mostly attributed to the
  tangent point (h) of a straight GPS ray path,
  horizontal TEC (hTEC) is determined as function
  of h.
• Fluctuating components of hTEC(h) is calculated
  by a sliding 7 km-window.
• Thin layers of Ne disturbances are assumed to
  have a constant vertical scale (?r 600 m in
  this study). Then, ?hTEC is scaled to ?Ne by
  dividing with a constant factor, s2 (2r ?r)1/2
  (r6370 100 km).
• We do not investigate the absolute values
  of ?Ne, but,
• we are mainly interested in the global
  distribution of ?Ne.

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Longitude Distribution of temperature variance,
water vapor and surface topography at 5-25S on
February 2-16, 1997
? Maximum of Ne perturbations in the MLT region
(80-120 km) ? Temperature variance at 22-28
km (solid) and 32-38 km (dot) ? Water vapor
pressure at 6 km from GPS/MET (solid) and
ECMWF (dot) ? Mean height of surface topography
at 5-25S ? Number of occultation events from
GPS/MET
South America Africa Indonesia
Good correlation between ?Ne, Ep and humidity is
recognized.
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Convergence of Ne (sporadic E) by the combined
neutral wind shear and geomagnetic effects
(Hines, 1974 Kelley, 1989)
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Dynamical Coupling Processes in the Equatorial
Atmosphere
Mesosphere
Thermosphere - Ionosphere
Stratosphere
Troposphere
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• GPS occultation technique provides profiles of
  water vapor, temperature and electron density
  fluctuations.
• Using GPS/MET data, longitude variation of the
  stratospheric gravity wave activity in the
  tropics was determined, which was particularly
  enhanced over regions of active convections,
  i.e., Africa, South America and Indonesia.
• Large ?Ne in the equatorial ionosphere are
  frequently detected in the same longitude region
  of the enhanced gravity wave activity and high
  humidity, which suggests a dynamical coupling of
  different layers by vertically propagating
  atmospheric waves.

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EQUARS (INPE, Brazil) 2005 - 6
Dense data rate in equatorial region by a low
inclination angle Altitude 700
km (90 min.) TBD Inclination
angle 20 deg TBD
Distribution of OccultationEvents with EQUARS
in 24 hours
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• Because inclination angle of the EQUARS orbit
  will be less than 20o, the occultation data is
  denser at low latitudes, which is very useful to
  study dynamical coupling processes in the
  equatorial atmosphere.
• As EQUARS can sample nearly the same area every
  about 90 minutes, it will be able to determine
  time evolution of atmospheric phenomena with a
  time resolution comparable to other ground-based
  measurements, e.g., balloon soundings.
• EQUARS data, denser at low latitudes, are
  valuable to complement a global data-set with
  COSMIC.

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Data Analysis System for GPS Occultation Measureme
nts with EQUARS/COSMIC (Proposed)
Meteorological Agencies USA, Taiwan,Brazil,
Japan