- Background on GPS radio occultations
- Inversion Technique
- GPS Radio Occultation Features
- Isolating Atmospheric Delay
- Relevence of GPS Occultation to Atmospheric Science
1. Background on GPS radio occultations
When a signal transmitted
by the global positioning system (GPS) and received by a low-Earth orbiter
(LEO) passes through the Earth's atmosphere its phase and amplitude
are affected in ways that are characteristic of the index of refraction
of the propagating medium. By applying certain assumptions on the variability
of the index of refraction of the propagating media (e.g. spherical
symmetry in the locality of the occultation), phase change measurements
between the transmitter and the receiver yield refractivity profiles
in the ionosphere (~60-1000 km) and neutral atmosphere (0-50 km). The
refractivity, in turn, yields electron density in the ionosphere, and
temperature and pressure in the neutral atmosphere. In the lower troposphere,
where water vapor contribution to refractivity is appreciable, independent
knowledge of the temperature can be used to solve for water vapor abundance.
The radio occultation
technique has a 30 year tradition in NASA's planetary program and has
been a part of the planetary exploration programs to Venus, Mars and
the outer planets [
see references on planetary occultation experiments]. However, the application
of the technique to sense the Earth's atmosphere using GPS, first suggested
by Yunck et al. [1988],
was tested for the first time with the launch of the GPS/MET
mission on April 3, 1995. GPS/MET is an experiment managed by the University
Corporation of Atmospheric Research (UCAR)
[Ware et al., 1995] and
it consists of a 2 kg GPS receiver piggybacked on the
MicroLab
I satellite which has a circular orbit of 730 km altitude and 60
deg. inclination. The GPS receiver is a space qualified TurboRogue
[Meehan et al., 1992] capable of tracking up to 8 GPS satellites simultaneously
at both frequencies transmitted by GPS. Under an optimal mode of operation,
the GPS receiving antenna boresight is pointed in the negative velocity
direction of the LEO and provides 100-120 globally distributed setting
occultations per day. Tens of thousands of occultations have been collected
over the past two years and can be used to assess the accuracy and potential
benefit of the GPS radio occultations.
2. Inversion Technique
The basic observable
for each occultation is the phase change between the transmitter and
the receiver as the signal descends through the ionosphere and the neutral
atmosphere. After removal of geometrical effects due to the motion of
the satellites and proper calibration of the transmitter and receiver
clocks, the extra phase change induced by the atmosphere can be isolated.
Excess atmospheric Doppler shift is then derived. This extra Doppler
shift can be used to derive the atmospheric induced bending as a function
of the asymptote miss distance, a. [see
figure above defining these terms]. Assuming a spherically symmetric
atmosphere, the relation between the bending and extra Doppler shift
induced by the atmosphere is given by
 (1)
where f is the operating frequency, c is the speed of light,
 and
 are the transmitter and receiver's velocity respectively, and
 and
are the unit vectors in the
direction of the transmitted and received signal respectively,
 is the unit vector
in the direction of the straight line connecting the transmitter to the receiver.
The spherical symmetry assumption can also be used to relate the signal's bending to the medium's
index of refraction, n,
[Born and Wolf, 1980]
via the relation
 (2)
where a= nr and r
is the radius at the tangent point [see figure above ]. This integral equation
can then be inverted by using an Abel integral transform given by
 (3)
The refractivity, N, is related to atmospheric quantities via
 (4)
 (5)
 (6)
where Pis total pressure (mbar), T is temperature (K), Pw is water
vapor partial pressure (mbar), Ne is electron density (m-3), f is operating frequency (Hz),
 is density, R is the gas constant, m is the gas effective molecular weight,
h is height, g is gravitational acceleration.
When the signal is passing through the ionosphere (tangent point height > 60 km),
use of a single GPS frequency is sufficient to estimate the bending
to be used in Eq. (3). Moreover, the first two terms on the right hand
side of Eq. (4) are negligible, therefore, knowledge of the index of
refraction leads directly to electron density.
When the signal is going through both the neutral atmosphere and the ionosphere (tangent
point height < 60 km), a linear combination of the two bending angles,
associated with the two GPS frequencies, is used to isolate the neutral
atmospheric bending and its refractivity profile is derived by use of
Eq. (3) [
Vorob'ev and Krasil'nikova,
1994]. In the stratosphere and the region of the troposphere
where temperature is colder than ~250K, the water vapor term in Eq.
(4) is negligible. Therefore, knowledge of refractivity yields the density
of the medium by use of the ideal gas law (Eq. 5). The density in turn
yields the pressure by assuming hydrostatic equilibrium (Eq. 6) and
a boundary condition at some height. Applying the gas law once more,
knowledge of density and pressure yields the temperature. In the troposphere,
at height where the temperature is larger than 250K, the water vapor
term in Eq. (4) becomes significant and it is more efficient to solve
for water vapor given some independent knowledge of temperature
[
Kursinski et al., 1995].
(back
to begining of page)
3. GPS Radio Occultation Features
Details about vertical and horizontal resolution of the technique, and refractivity, temperature,
pressure, water vapor or electron density accuracies as a function of height, are discussed by Kursinski et al., 1997. Here we highlight
some of these GPS occultation features.
Due to the nature
of the measurement, which is a pencil-like beam of the electromagnetic
signal probing the atmosphere, the technique has a much higher vertical
and across-beam resolution than horizontal (i.e. along the beam). The
vertical resolution of the technique is essentially set by the physical
width of the beam where geometrical optics is applicable. This scale
is set by the Fresnel diameter which, in vacuum, is given by
where
is the signal's wavelength,
RGPS and RLEO
are the distances of the tangent point to the GPS and LEO respectively.
For a LEO, Dvacuum is ~1.5 km. In the presence of a medium, due
to bending induced on the signal, the Fresnel diameter is ~0.5 near
the surface and approaches 1.5 km above 20 km altitude where bending
becomes small. When the signal encounters sharp gradients in refractivity
due to either water vapor layers near the surface or sharp electron
density changes at the bottom of the ionosphere, the Fresnel diameter
shrinks to ~200 meters.
A horizontal resolution
scale is set by the length of the beam inside a layer with a Fresnel
diameter thickness. This length is 160-280 km for a Fresnel diameter
of 0.5-1.5 km.
In the ionosphere, the vertical scale is still set by the Fresnel diameter; however, the
horizontal scale can extend several thousands of kilometers due to the
large vertical extent and scale height of the ionosphere. These features
of the ionosphere allow one to use tomographic approaches in order to
combine information from neighboring occultations to solve for horizontal
and vertical structure [
Hajj et al., 1994].
Under ideal conditions,
when a LEO tracking GPS has a 360o field of view of the Earth's horizon,
about 750 occultations per LEO per day can be obtained. However, side-looking
occultations (GPS-LEO link > 45o from velocity or anti-velocity of
LEO) sweep across a large horizontal region, and the spherical symmetry
assumption becomes inaccurate. Discarding side-looking occultations,
one LEO provides up to 500 occultations per day.
A high inclination LEO provides a set of occultations that covers the globe fairly uniformly.
This feature is particularly advantageous when comparing LEO-GPS occultation
coverage to that obtained from balloon launched radiosondes. A total
of about 800 radiosondes are launched each 12 hours from sites around
the world. The vast majority of these sites are over the northern hemisphere
continents, particularly Europe and North America. This creates the
need for high resolution temperature/pressure/water vapor profiles in
the southern hemisphere and over the oceans. The contribution of radio
occultation retrievals to climate and weather modeling should be particularly
important in these regions. (Global data provided by spaceborne nadir
sounders average over large-3-7 km-vertical distances.)
When compared to infrared spaceborne sounders, the radio occultation technique has the
advantage of being an "all-weather" system. Namely, it is
insensitive to aerosols, cloud or rain due to the relatively large GPS
wavelengths.
Unlike other techniques such as radiosonde or microwave sounders, where instruments need constant
calibration, the GPS radio occultation provides a self calibrating system,
as will be discussed in more detail below. The long term stability inherent
in radio occultation make this an excellent system to keep an accurate
record of climate changes.
4. Isolating Atmospheric Delay
Click on image to view in full size
The main observable
used in an occultation geometry is the phase change between the transmitter
and the receiver as the occulting signal descends through the atmosphere.
This phase change is due to (1) the relative motion of the LEO with
respect to the GPS, (2) clock drifts of the GPS and LEO and (3) delay
induced by the atmosphere. In order to derive the excess atmospheric
Doppler shift, one must remove the contribution of the first two effects.
Accurate knowledge
of the GPS orbits comes from an overall solution involving all 24 GPS
satellites and a global network of ground receivers. The LEO orbit is
determined by use of other links tracking the non-occulting GPS satellites.
When the occultation is mostly radial (i.e. GPS-LEO link has no horizontal motion out of
the occultation plane), the occultation link descends through the ionosphere
and stratosphere at a rate of about 3 km/sec; thus, crossing a Fresnel
diameter in about 0.5 seconds. However, in order to investigate sub-Fresnel
structure (by examining the diffraction pattern of the received signal's
phase and amplitude) and for other purposes (such as eliminating different
signals caused by atmospheric multipath in the lower troposphere) the
occulting data is taken at a rate of 50 Hz. In order to calibrate the
LEO clock, one more GPS transmitter is tracked by the LEO at the same
high rate (labeled as link 2 on the figure). In addition, in order to
calibrate the GPS clocks, a ground receiver tracks both GPS satellites
at 1 Hz (labeled as links 3 and 4). One can interpolate the lower rate
GPS clock solutions to 50 Hz, due to the greater clock stability (of
order 10^12 sec/sec, as opposed to 10^9 sec/sec for the LEO clock),
and the smoothness of the DoD Selective Availability dithering.
Knowing the position
of all four participants (i.e. two GPS satellites, one LEO and one ground
receiver), and modeling various physical effects such as light travel
time, the three spaceborne clocks can be solved for w.r.t. to the ground
clock. The net result of the calibration is the excess phase due to
the atmosphere as a function of time.
(back
to begining of page)
5. Relevence of GPS Occultation to Atmospheric Science
The radio occultation technique holds the promise of providing atmospheric sounding which
combines the high-resolution vertical profiling characteristic of radiosondes
and the global coverage usually provided by passive remote sounders.
The technique depends on the proven ability to measure the time delay
of transmitted radio signals with high precision and stability as they
traverse the atmosphere. This ability to determine time differentials
very precisely and to provide an observational data base with long-term
stability is enhanced by the use of GPS. Also, the use of radio frequencies
means that cloud and aerosol particles have little effect on retrieved
quantities. Although measurements are sensitive to free electrons in
the ionosphere, the use of dual frequencies can remove their effects.
Sensitivity to very high concentrations of water vapor complicates the
retrievals near the Earth's surface, but also holds forth the promise
that key boundary layer structure may be retrieved.
The ability of radio occultation to provide high-vertical-resolution profiles has been
demonstrated repeatedly in planetary missions. GPS occultations provide key
measurements in four areas of concern to the scientific community:
dynamics in the vicinity of the tropopause, measurements of
climate change, atmospheric structure in the troposphere,
and the calibration of IR sensors.
Tropopause dynamics
The tropopause is the transition region between the dynamically controlled thermal structure
in the troposphere below and radiative control in the middle atmosphere
above. It is a region of active scientific interest and a difficult
region to characterize via traditional remote sensing. Early results
of GPS/MET (
Kursinski et al., 1995) suggest that it can measure temperature as a function
of pressure to an accuracy of 1K, or less, with a vertical resolution
on the order of 1 km at and near the tropopause. This suggests that
the technique may have a special role to play in this altitude regime.
Here we discuss some of the scientific issues related to dynamics associated
with the tropopause, explain why GPS occultations are important in addressing
these issues in the context of the present network of operational remote
and in situ sensors.
Understanding the exchange processes between the stratosphere and the troposphere is essential
in understanding chemical and dynamic processes in both regions. Such
an understanding is difficult to obtain because vertical transport across
the tropopause depends on irreversible mixing associated with smaller
scale processes. This interaction tends to be difficult to monitor and
depends strongly on the local temperature and water vapor.
Observing System
Simulation Studies (OSSEs) have shown that GCM assimilation systems
are extremely sensitive to wind and geopotential inputs at upper tropospheric
jet levels. The GPS measurements promise to provide accurate measurements
of the 200 mb geopotential on a global basis and, if sufficient measurement
density is achieved, should have a positive impact on global assimilation
systems.
Accurate knowledge of the vertical and horizontal temperature structure near the tropopause
may also greatly aid in planning air flight routes. Air traffic usually
takes advantage of the strong jet streams associated with the tropopause,
which can be estimated by examining horizontal temperature gradients
in this region. High accuracy temperature measurements with high vertical
resolution are needed on a global scale to determine and predict the
location and strength of these jet streams. Furthermore, areas of intense
clear air turbulence can be sensed by high vertical resolution temperature
data because clear air turbulence leaves a distinctive structure in
the temperature field.
Analysis of GPS/MET data suggests that occultation data may play an important role in assimilation
models where the available data is sparse. For instance, we have noticed
significant disagreements between the ECMWF global model and GPS/MET
temperatures, especially near the tropopause, in the southeastern Pacific.
There are typically ~60 radiosonde flights daily in the southern hemisphere,
and consequently the ECMWF model relies heavily upon TOVS observations
to infer tropopause temperatures in such regions. Accurate temperatures
from GPS, at high southern latitudes, should improve the determination
of the structure and evolution of the southern polar vortex during its
annual cycle, and the characterization of the ozone hole formation and
the subsequent dispersal of ozone-poor air over the southern hemisphere.
Dynamically, the tropopause represents the boundary between high potential vorticity
(PV) air in the stratosphere and relatively low PV air in the troposphere.
As such, the structure and topography of the tropopause is dynamically
coupled to the troposphere below and therefore, as implied by the invertibility
principle for potential vorticity, a detailed knowledge of tropopause
topography can be utilized to infer the dynamical structure of the troposphere
below (Hoskins et al., 1985). GPS observations provide the static portion
of the PV and, with sufficient sampling density, should provide useful
information on horizontal gradients and therefore the wind field and
the relative vorticity. This implies that GPS observations near the
tropopause may prove important in this regard for model initialization
and assimilation. As cyclogenesis is often associated with high PV stratospheric
air pushing downward into the troposphere depressing the tropopause,
interest has been expressed by a number of researchers in exploring
how GPS data may be used in the study of cyclogenesis particularly in
regions such as the Northern Atlantic storm track where high vertical
resolution soundings are sparse.
The GPS occultation technique is particularly powerful in retrieving temperatures with good
accuracy, vertical resolution, and adequate spatial coverage near the
tropopause. The ~200 km "line-of-sight" spatial averaging
is comparable to or smaller than the horizontal scale of interest. Together
with temperature retrievals by nadir viewing microwave radiometers,
occultation temperature retrievals in this region should provide a powerful
combination for climate studies of the Earth's lower atmosphere
(
Spencer & Christy, 1990). Such a combination would dramatically increase
our ability to study dynamics in the vicinity of the tropopause beyond
the current network of passive infrared instruments and microwave radiometers.
Although the current infrared and microwave instruments on operational weather satellites
offer better horizontal resolution than GPS occultations, they have
difficulty measuring temperature in the vicinity of the tropopause with
sufficient vertical resolution. First, any temperature profile retrieved
from a radiometer measurement must be deconvolved from a set of radiances
by an associated set of weighting functions typically having half widths
of one half to a full scaleheight. Second, the comparatively sharp change
in the lapse rate and the cold temperatures near the tropopause pose
difficulties in the retrieval process. The results of comparisons between
radiosondes and retrievals from satellite radiometric data consistently
show increased errors in the retrieved temperature near the tropopause.
For instance, temperature retrievals from the High-resolution Infrared
Sounder (HIRS/2) and the Microwave Sounding Unit (MSU) show errors of
~2.5 K near the tropopause while only ~1.5 K in the mid-troposphere
(
Reale et al., 1988). In fact, for extreme
cases in the tropics, simulations show that the coldest retrieved temperature
can be more than 10 K warmer than the actual temperature
(
Yates et al., 1989).
Temperature measurements by radiosondes are themselves subject to systematic errors at tropopause
heights. Because of the low atmospheric density at these altitudes,
the physical temperature of the radiosonde thermistor is actually determined
by a balance between conductive heat transfer with the air temperature,
absorbed long-wave and solar radiation and the long-wave radiation emission
of the sensor. This makes the measured temperature sensitive to a variety
of factors such as cloud cover, cloud top temperatures, solar zenith
angle, surface temperature, vertical temperature structure and vertical
distribution of aerosols, ozone, water vapor and carbon dioxide
(
Finger and Schmidlin 1991;
McMillin et al. 1988). Present estimates indicate that the magnitude of these
errors may be of order 1 to 3 K
(Ahnert 1991;
Schmidlin 1991;
McMillin et al. 1988).
Climate change
The occultation technique is well suited to tracking changes in regional and global
climate. First, no long time scale drifts in calibration are associated
with the occultation technique. For example, no degradation in accuracy
was experienced when Voyager acquired radio occultation observations
of Neptune and of Jupiter although ten years elapsed between them. In
the GPS case, because the occulted satellite can be simultaneously viewed
by other GPS receivers, the instabilities of the signal source during
the observation can be measured and removed by the "double
differencing" technique. By contrast, calibration drifts present
a significant obstacle in detecting climate change with operational
remote sensors. Second, the relatively long horizontal averaging interval
of the limb sounding geometry is advantageous in this context because
it inherently produces a spatial average of the quantity being measured.
For instance, occultations effectively smooth over small scale internal
gravity waves. Finally, with the increased accuracy of occultation temperature
retrievals, the limiting factor in detecting long term climate trends
and in determining climate averages becomes the natural variability
of the climate instead of errors in the measurements themselves. Consequently,
fewer measurements are required in determining averages and secular
trends. Below we discuss some of the climate related issues which might
be addressed by future GPS occultation measurements.
Fundamental toward understanding the response of the atmosphere to changes in its constituents
is the balance between incoming short-wave/warming and outgoing long-wave/cooling
radiation. In particular, temperatures in the upper troposphere/tropopause
region are of tremendous interest because they determine the ability
of the atmosphere to cool. Changes in concentrations of atmospheric
radiative forcing constituents, such as aerosol and cloud particles
and greenhouse gases, including water vapor, force the atmosphere to
alter its temperature structure in this region in order to maintain
an energy balance with incoming solar radiation. Major volcanic eruptions
inject a large amount of aerosol material into the lower stratosphere,
significantly altering the radiative forcing of the atmosphere in this
region. Detailed monitoring of the evolution of the atmospheric thermal
structure following these events can only be done on a global scale
with remote sensing and can only be done remotely with wavelengths sufficiently
long to be unaffected by the enhanced aerosols concentrations. As discussed
earlier, initial results suggest that occultation temperature retrievals
are most accurate in this cooling layer.
The reaction of high cirrus cloud formation to anthropogenic increases in greenhouse
gas concentrations has been proposed as a climate feedback mechanism.
If global warming displaces a given cloud layer to a higher and colder
region of the atmosphere, then the colder cloud will emit less radiation
forcing the troposphere to warm to compensate for this decrease
(Houghton
et al., 1990; referred to as IPCC hereafter). Accurate high vertical
resolution temperature reconstructions in the tropopause region where
cirrus clouds exist will assist in our understanding of the conditions
under which they form and evolve. Currently, the changes in this altitude
region associated with anthropogenic increases in greenhouse gases are
poorly understood as indicated by the variability of GCM predictions
( IPCC). This present
uncertainty in understanding combined with the importance of this altitude
regime in maintaining radiative energy balance illustrates the importance
of accurate, high resolution monitoring in this regime.
Tracking the geopotential height of constant pressure surfaces over time spans of decades allows
one to detect an integrated expansion of the lower atmosphere. Looking
for a systematic elevation of constant pressure surfaces over times
spans of years would be a useful technique in searching for secular
warming of the Earth's atmosphere. Thus far we have focused on temperature
and water vapor as the only parameters of interest produced by the occultation
technique; however, very accurate reconstructions of pressure as a function
of geopotential height will also be provided wherever accurate temperature
retrievals are produced. Since pressure is a vertical integral of the
atmospheric mass density, an elevation of a surface of constant pressure
would indicate that the mass of gas below that particular level expanded,
which can only arise when the integrated temperature of that mass of
air increases (Gary, 1992).
Therefore, if the troposphere warms in response to the anthropogenically
increased greenhouse gas forcing as is generally predicted, the average
pressure scale height across the troposphere will increase, causing
the height of a given pressure level to rise. The largest inflation
should occur in the vicinity of the tropopause because the heights of
pressure levels there represent the integrated effect of the warmer
temperatures below. A simple order of magnitude estimate indicates that
a 2 K increase (a magnitude often discussed in climate change simulations)
will produce about a 70 m increase in height. This is equivalent to
a change in pressure at fixed altitude of approximately 1%. We have
not yet assessed the accuracy of pressure retrievals from GPS/MET, but
our theoretical predictions indicate that pressure errors should be
~0.1%. In fact, the theorized sensitivity implies changes may be detected
more easily than with other data, and that the sensitivity may be used
to distinguish between and evaluate the climatic change predictions
of different models, depending on the ability to separate the climate
change signal from synoptic, diurnal, seasonal, and other climatic variability.
The change in surface temperatures due to an increase in greenhouse gas densities is generally
predicted to be largest and may therefore become apparent first at high
latitudes (IPCC). Occultation temperature retrievals at high latitude
winter conditions are an obvious example of how the radio occultation
observations can complement data from passive sounders. Accurate temperature
measurements by passive remote sensing techniques under these conditions
are difficult because of very cold temperatures, common near-surface
thermal inversions, and the presence of ice clouds which limit IR soundings
to the cloud tops. In contrast, the radio occultation technique has
an advantage with colder temperatures because the air is dry, relatively
dense, and scale heights are relatively small, all of which result in
accurate temperature retrieval.
Despite the fundamental role played by water in weather and climate, an adequate climatology
of atmospheric water vapor does not exist (Starr
and Melfi, 1991). Radiosondes presently provide most of the high
vertical resolution profiles of humidity with a highly in-homogeneous,
land-biased spatial distribution. In the lower troposphere, GPS observations
should deliver hundred meter to one km vertical resolution with global
coverage and 100-200 km horizontal averaging. This vertical resolution
lies between that of radiosondes and current satellite remote sounding
instruments and should yield a significant improvement in the vertical
scales observable globally. In addition, horizontal averaging produces
profiles that are more representative climatologically than point measurements.
Finally, insensitivity to particulates allows GPS occultations to measure
humidity structure representative of the entire range of climatological
variation by retrieving water vapor profiles within and below clouds.
The GPS water vapor retrieval accuracies estimated by
Kursinski et al.
(1995a) at low latitudes
compare favorably with goals of 5 and 10% established by Starr and Melfi
for the boundary layer and overlying troposphere respectively, and are
generally conservative. Climatological boundary layer humidities may
in fact be retrieved at the 1.5% level. Changes in water vapor abundance
at low latitudes in the lower and mid-troposphere have been identified
as reliable indicators of modeled climate change
(Santer
et al., 1990; Schlesinger
et al., 1990). These are precisely the regions of greatest accuracy
for GPS occultations.
Tropospheric retrievals
GPS occultations present the only method of probing the troposphere in a limb sounding
geometry routinely. Typically, tropospheric temperature retrievals from
limb sounders are complicated by the presence of aerosols and clouds
in the troposphere. The GPS L-band dual frequencies, however, are well
away from atmospheric absorption lines and the effect of particle extinction
will be negligible with an extreme upper bound of ~1% change in the
total water refractivity (Kursinski
et al.,1995). On the other hand, the permanent dipole moment of
water vapor makes it a large contributor to atmospheric refractivity
in the lower troposphere at microwave wavelengths. This is particularly
true at low latitudes where the air is warm and absolute humidity levels
are high. By using a priori knowledge of the temperatures in an individual
profile, the water vapor profile can be retrieved. Our studies suggest
that, in warm conditions, it should be possible to produce high vertical
resolution profiles of water vapor accurate to better than 5 % in the
boundary layer and 10% in the lower half of the troposphere
(Kursinski
et al., 1995). Again, a strength of the GPS occultation technique
is its high vertical resolution in the troposphere. Vertical resolution
in the troposphere can range from ~1 km down to 100 meters where the
refractivity gradients are large. Under conditions where horizontal
gradients are small, the vertical resolution can be pushed beyond the
Fresnel diffraction limit by sampling and subsequently inverting the
diffraction pattern.
Although occultations involve long horizontal path lengths through the atmosphere, this horizontal
scale is consistent with the expected vertical resolution. Lindzen and
Fox-Rabinovitz (1989) have pointed out that horizontal and vertical
resolution are strongly coupled and must therefore be self-consistent
in both atmospheric models and observational systems. Their work indicates
that vertical resolution is inadequate in virtually all large scale
models and observing systems. With GPS/MET, the ratio of vertical to
horizontal scale resolution is typically of ~1:160, which is in good
agreement with the constraints set by Lindzen
and Fox-Rabinovitz (1989).
Retrievals of water vapor in tropical and subtropical latitudes are of interest because
water vapor is responsible for most of the transfer of heat and energy
in the tropics, where most of the Earth's insolation lies. Much of the
insolation is directly transformed into the latent heat of evaporation
of water from the ocean surface. The moist air is then advected in a
Hadley circulation, in which it eventually rises and rains out. The
details of the evaporation/ precipitation cycle in the tropics are not
well understood, and consequently tracking the quantity and movements
of water vapor is essential to characterizing heat transfer at low latitudes.
Unfortunately, because of the limited number of radiosondes flights
conducted in the tropics, only sparse and unevenly sampled information
on water vapor concentrations exists. A GPS occultation data set would
greatly augment the capacity for studying water vapor transport and
the hydrological cycle. The impact of low level water vapor measurements
in the low latitudes should have a dramatic impact on important assimilated
parameters such as water vapor transport calculations.
As we extend the GPS retrievals to or near the surface, the height of the marine boundary
layer should be determined with great accuracy as have been found in
simulations ( Kursinski et
al., 1997). The reason is simply the very sharp and large contrast
in refractivity between the moist air below and relatively dry air above
which results in dramatic changes in the amplitude and phase of the
signal as the signal path crosses this region. This is of interest for
weather and climate characterization and is extremely difficult to recover
accurately with nadir-viewing sounders. Knowledge of this height can
also potentially greatly improve the accuracy of nadir sounding retrievals
of water vapor distribution of the same region.
Under conditions when it is difficult to separate the wet and dry contributions to the
refractivity of the lower troposphere, the refractivity profiles themselves
should provide a very accurate representation of the sampled volume.
Although refractivity is not currently considered a variable of climatological
interest, the accuracy available with GPS occultation retrievals could
cause it to become so.
Calibration of IR sensors
The anticipated accuracy of the radio occultation temperature retrievals, their long
term stability, and the physical simplicity of the observable are key
factors which suggest these observations may be useful as a calibration
tool to aid other remote sensing observations. First of all, frequency
and time differences can be determined much more accurately than intensity.
Secondly, the use of well-known frequencies generated by highly precise
clocks makes the physics of probing the atmosphere at all levels far
simpler. In contrast, passive IR observations observe intensity as a
function of frequency, a physically more complex relationship. For passive
IR observations, at each frequency the observed intensity is the result
of emission/absorption and subsequent radiative transfer through the
intervening media. Both the emission and radiative transfer are functions
of pressure and temperature dependent line-shapes and the densities
of the emitting, absorbing and scattering constituents in the atmosphere.
(back to beginning of page)
|
){kind=link}