Title: Application of GPS Slant Water Vapor Tomography to an IHOP Storm Case with Simple Constraints
1Application of GPS Slant Water Vapor Tomography
to an IHOP Storm Casewith Simple Constraints
- Y. F. Xie, J. Braun,
- A. E. MacDonald and R. H. Ware
Xie/FSL
2Outline
- GPS application in meteorology.
- Development of a GPS tomography
- A variational approach based on GPS
slants - Incompleteness of GPS slant observation
- Schemes to resolve the incompleteness.
- Numerical experiments
- Real time GPS observation experiments over
IHOP area. - Conclusions.
Xie/FSL
3References
- MacDonald, A., Y. Xie, and R. Ware, 2002
Diagnosis of Three-Dimensional Water Vapor Using
Slant Observations from a GPS Network, Monthly
Weather Review, 130,386-397. - Xie, Y., J. Braun, A., MacDonald, and R. Ware,
2004 IHOP GPS Water Vapor Retrieval. In
preparation.
Xie/FSL
41. The GPS application
- The most interesting application Integrated
Slant Water Vapor - Water vapor variability is largely responsible
for signal errors (delays) in GPS positioning - Verification of the accuracy of the delays led to
a new atmospheric remote sensing tool - ground-based GPS observation network.
- For example FSL/GPS-Met.
Xie/FSL
5GPS slant delay
These delays are integrations of refractivity
along GPS ray paths between GPS receivers and
satellites
D ?r N ds
Where N Cdry Pd/T C1Pw/T C2Pw/T2
GPS software Bernese can convert these delays
into integrated water vapor along these ray
paths, Dq ? q dl, where q water vapor.
6GPS tomography
- GPS tomography is a direct analysis of GPS
- observations independent of other obs or
- NWP models.
- For example, study of GPS slant water
- vapor through double-differencing under the
- zero mean assumption.
72. GPS tomography
- Minimizing the differences between observed
integrated water vapor (Dobs) and integrated
water vapor (D) along the same ray paths
estimated using the control grid function (q). - The cost function is
-
- minimize ?(D Dobs)2
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82. GPS tomography (continue)
- GPS slants cannot uniquely determine a
solution to the GPS tomography problem. We call
this incompleteness of these integrated water
vapor. - A more complex example
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92. GPS tomography (continue)
- Schemes for resolving the incompleteness
- A. Other water vapor observations.
- In our OSSE experiments, water vapor
observations from microwave profilers are used. - B. Simple physical constraints.
- In our real data experiments, some simple
physical constraints are used, for examples,
non-negativeness and exponential decay with
height.
Xie/FSL
10GPS tomography with simple constraints
- minimize ?(D Dobs)2
- subject to qi,j,k 0
- qi j k 10e-z, k large
- (g/m3)
11Recursive filter
- Due to limited information provided by
GPS slant observations, an analysis is restricted
over certain scales. That is, a grid analysis is
correlated between its gridpoints. - A. Multigrid technique (used in OSSE)
- B. Recursive filter. (used in here)
-
123. Numerical Experiments.
- OSSE experiments.
- Using mesoscale model water vapor fields as
nature, e.g., QNH from FSL and MM5 from NCAR - High accuracy line integrations of these water
vapor fields along GPS slant paths from ideal
ground GPS network to existing GPS satellites are
computed. They are used as GPS observations. - Adding 16 microwave profiler observation sites
and solving the GPS tomography problem to
generate three-dimensional water vapor
distributions.
Xie/FSL
133. Numerical Experiments (cont.)
Tomography analysis
Nature from QNH
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143. Numerical Experiments (Cont.)
Tomography analysis
Nature from QNH
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153. Numerical Experiments (Cont.)
Vertical profile (Moist)
Vertical profile (dry)
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163. Numerical Experiments (Cont.)
- Real time GPS data over IHOP area
- Bernese 4.2 is used to process the GPS slant
delays from 25 ground-based GPS sites over IHOP
(International H2O Project) during June 12-13,
2002. - The domain is over the right center of IHOP.
(see the right figure)
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17GPS slant observations
- Over this domain, there are 25 GPS sites
- Elevation angle down to 7 degree
- Collection of GPS slant during a half hour time
interval.
183. Numerical Experiments (Cont.)
- Collect data over a half hour time interval and
treat them as one time observations (assuming
large-scale water vapor does not change much). - Add some simple physical constraints, requiring
water vapor field is not negative and restricting
water vapor close to an exponential decay
function with height at the top 5 levels. The
water vapor control grid is 21 21 21 over a
500km by 500km by 10km domain. - Solve the GPS tomography problem with the simple
constraints and a recursive filter.
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193. Numerical Experiments (Cont.)
Jun. 13, 2002, 0000UTC
Goes water vapor image
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203. Numerical Experiments (Cont.)
Jun. 12, 2004, 2200UTC
21Comparison with Sounding
- At the central facility
- of IHOP, we plot the
- sounding data (red)
- and our tomography
- analysis (black).
Jun. 13, 2002, 0000UTC
224. Conclusions
- In a separate study, we found a GPS receiver
network with 100km density could provide useful
information for mesocale water vapor analysis - The GPS tomography provides a direct and
independent analysis of GPS slants comparing to
other data assimilation techniques, such as,
3DVAR or 4DVAR. It helped to discover and solve
the incompleteness of GPS slant observation - GPS integrated water vapor observations provide a
practical mesoscale water vapor observations and
analysis for improving weather research and
forecasts - The study here shows the GPS slant retrieval
contains useful information under the
double-differencing and zero mean assumption. - Future work would focus on improvement of
mesoscale water vapor forecasts from GPS
tomography and 3DVAR/4DVAR.
Xie/FSL