Use of GPS to Adjust Radisondes: Validation using AIRS

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Use of GPS to Adjust Radisondes Validation using
AIRS

• Larry M. McMillin, Seth I. Gutman, Jiang Zhao,
• M. K. Rama Varma Raja, James G.Yoe

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Main Points

• Surface based GPS measurements can be valuable
  for
• Removing biases from radiosonde moisture reports
• Improving the initial state of numerical models
• Steps required to achieve the potential value
• Locate a GPS at each radiosonde site
• Make the measurements that are taken (sometimes
  for other purposes) available for moisture
  measurements in a timely fashion
• AIRS validation
• 3way total water vapor validations

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GPS radiosonde approach background

• Radiosondes are subject to calibration errors
• Vaisala packaging issue
• There can be significant radiosonde to radiosonde
  biases
• The United States has a policy of flying
  radiosondes from at least two different vendors
  at any one time
• Procedures have been developed at the ARM sites
• Use the upward looking microwave to adjust the
  individual radiosonde reports
• The microwave sensors are expensive and rare
• The GPS water vapor has many of the same
  features, but is cheaper and widely available

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GPS radiosonde approach background

• GPS sensors are installed at many locations for
  other purposes
• Many of these could make the water vapor
  measurements
• Some countries/agencies have a policy of not
  making the reports available
• There is no standard procedure reporting and
  distributing the information
• Needs national/international decisions and policy
  (WMO)
• In the United States, some GPS sites are at or
  close to existing radiosonde sites
• These have been used in this study

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GPS radiosonde approach

• Take the GPS IPW and use the radiosonde IPW to
  calculate an adjustment factor by taking the
  ratio
• Apply the ratio to all the radiosonde layer
  precipitable water values
• Compare the adjusted profile to the values
  retrieved by the AIRS instrument
• Note GPS takes continuous measurements
• Use the GPS at the radiosonde time for the
  radiosonde comparison
• Use the GPS at the satellite time for the
  satellite comparison
• Use the GPS to track changes in moisture with
  time
• Discard cases with large changes
• Use the GPS to adjust for the time changes

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GPS radiosonde approach

• Evaluate the 3 U.S. RAOB types
• RS80-37H
• VIZ B2
• RS MSS

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Figure 1. Locations of the GPS stations located
near radiosonde stations and used in this study.
Stations marked with xs are not used because
local terrain conditions made the results differ.
Stations marked without a radiosonde type had no
AIRS matches and could not be used.
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Figure 3 RMS and bias as a function of pressure
for the Meteorological Sounding System (MSS) made
by the Space Data Corporation. The lines labeled
as RGPS and AGPS show the match with radiosondes
and AIRS after adjustments. The AGPS has the
smaller time difference.
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Figure 4 RMS and bias as a function of pressure
for the VIZ-B2 radiosonde. The lines labeled as
RGPS and AGPS show the match with radiosondes and
AIRS after the GPS has been used to adjust the
radiosonde. The AGPS has a smaller time
difference.
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Figure 5 RMS and bias as a function of pressure
for the Vaisala RS80-57H radiosonde. The lines
labeled as RGPS and AGPS show the match with
radiosondes and AIRS after the GPS has been used
to adjust the radiosonde. The AGPS has the
smaller time difference.
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Costs for a GPS sensor

• Here are the approximate costs for a GPS water
  vapor sensor
• Receiver antenna 5,500
• PC for data logging and communications
  1,000
• antenna installation 100 gt 10,000 depending on
  the type of monument and desired degree of
  stability
• Communications depends on what's
  available.
• Collocated surface met sensors
• 5,500 and up depending on what's wanted.
• Average total cost
• 15,000 per system recurring costs for
  maintenance and communications.

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Recommendation 1

• Place a GPS sensor at all U.S. radiosonde
  stations
• Recommendation 1a
• North America
• Recommendation 1b
• North and South America
• Recommendation 1c
• The world

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GPS water vapor can be valuable input to
numerical models

• Note that satellite measurements based on emitted
  atmospheric radiation are subject to surface
  effects over land
• GPS sensor can offer a supplemental coverage over
  land areas
• Data are taken at frequent time intervals
• The next series of slides have been provided by
  Seth Gutman
• They illustrate the potential benefits of using
  the GPS IPW values in the model
• Note for this and the cases just discussed, the
  IPW has the greatest effect on the surface layers
  because they dominate the IPW values

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Overview

• The GPS-Met project started in 1993 as a
  collaboration between FSL, UCAR and NCSU to
  determine how well and under what circumstances
  GPS could be used to measure IPW.
• It has evolved into a collaboration between FSL,
  other NOAA organizations, other federal, state
  and local government agencies, universities, and
  the private sector.
• Major accomplishments include
  - specification of the GPS-IPW observation
  accuracy and error covariance
  - development of real-time data processing
  techniques for operational weather forecasting
  - verification of positive impact on Wx
  forecast accuracy - definition and exploration
  of new applications, including radiosonde
  moisture sounding QC and cal/val of satellite
  moisture soundings.

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Project Linkages
Ground-Based GPS-Met 11/94
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GPS-Met Network
318 sites operating with 28 in checkout or
waiting for antenna positions
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GPS NWP Impact Tests

• Multi-year study with the 60km RUC
  indicates that GPS makes a small but consistent
  positive impact on short-term weather forecast
  accuracy
• Primarily at the lower levels where most of the
  moisture resides - IPW more correlated w/
  low-level moisture
• Magnitude of impact consistently increases with
  the number of stations
• RH forecast improvement is greatest in the cool
  months when convection is less frequent and the
  moisture distribution is more synoptic scale.
• Impact on precipitation forecast accuracy
  generally increases with precipitation amount
  threshold

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Impact of GPS-IPW increases as the
number of GPS Observations Increase
With GPS
Without GPS
Improved PBL in RUC 20 increases 3-h CAPE
forecast by 500 J/kg, but with GPS, the CAPE
exceeds 1000 J/kg. Only model to forecast these
storms!
Positive impact to 500 hPa, but largest Impact at
700 and 850 hPa
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GPS Radiosonde Moisture QC for RRS
Table 1. GPS-Met sites within 10 km of an
Upper-Air site
Table 2. Other GPS-Met Sites evaluated for the

Radiosonde Replacement Program
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GPS Radiosonde Moisture QC for RRS
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One year, all sites sorted by mean difference
(sonde-GPS)
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Additional Links
GCOS/GUAN sites need GPS improved Comms
GPS for Space Wx forecasting
Global satellite
Cal/Val Global Climate Monitoring
International GPS Service sites have GPS, need
Sfc Met improved Comms
International DGPS sites have GPS, need Sfc Met
improved Comms
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3 way comparisons AIRS validation

• Radiosondes, AIRS and GPS can all provide a
  measurement of IPW
• comparing all 3 has significant advantages over a
  2 way comparison
• When an anomaly occurs, the suspect is easily
  identified
• Nose estimates are more robust

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3 way comparisons AIRS validation

• An important question for scatter plots
• Best prediction
• Best estimate of the underlying relationship
• Since all the values have errors, the best fit
  slope is always biased towards the independent
  variable
• For the best estimate, rotate both variables by
  45 degrees, do the least squares fit, and rotate
  the solution back.
• Errors then force the solution towards the 45
  degree line
• Effect of errors on the slope is reduced

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GPS Comparison Descriptions

• AIRS/GPS/radiosonde moisture comparisons
• GPS gives only Integrated Precipitable Water
  (IPW)
• GPS observations are continuous AIRS and
  radiosondes are not
• Use AIRS time for AIRS
• Use RABO time for radiosondes
• Discard cases with large changes in GPS TPW
  between the two times
• Note for the AIRS matches with GPS adjusted
  radiosondes, use the AIRS time for the GPS
• Note - This is equivalent adjusting the
  radiosonde at the radiosonde time, and then using
  the GPS to correct for the change in moisture
  with time

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Figure 2. Scatter plots of total precipitable
water for three instruments, AIRS, GPS, and
radiosondes. The values in the upper left hand
corner are least squares fit. The values in the
bottom right define the estimate of the true
slope. The line shown in the figures is the 45
degree line.
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IPW Conclusions

• All three instruments compare well
• The GPS versus radiosonde has the best fit, but
  only by a very small amount (.961 versus .943 for
  AIRS/GPS and .947 for AIRS versus radiosonde).
• AIRS compares well to both

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Summary and Conclusions

• When the GPS is used to validate AIRS in a three
  way AIRS, radiosonde, GPS comparison, the
  following can be observed
• The differences between any two are roughly the
  same meaning each has about the same error
• A GPS measurement at each radiosonde site can
  reduce calibration errors and produce a more
  uniform moisture effort for a combined network
• The equipment is relatively cheap
• GPS data by itself can be a valuable input to
  forecast models