Nationwide Differential GPS NDGPS Water Vapor Observations During Hurricane Georges

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Nationwide Differential GPS (NDGPS) Water Vapor
Observations During Hurricane
Georges National Hurricane Conference (NHC)
Workshop Road Weather Management Portion
April
9, 2001 Presented by

Seth I. Gutman

Chief, GPS-Met Observing Systems Branch

NOAA Forecast Systems Laboratory
Boulder,
Colorado

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The Role of NDGPS

• NDGPS was originally developed as a public safety
  navigation system, but it has evolved into much
  more.
• NDGPS was designed to support multi-modal surface
  transportation applications including high
  accuracy vessel navigation in navigable
  waterways positive train control integrated
  vehicle highway safety systems situational
  awareness and emergency (911) response.
• Other significant applications have emerged,
  including precision farming and high accuracy
  surveying using Continually Operating Reference
  Stations (CORS).
• Due to a fortuitous synergy between the
  positioning and navigational applications of GPS
  and meteorological remote sensing, NDGPS sites
  can be also be used for weather forecasting and
  climate monitoring.

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Ground-Based GPS Meteorology

• GPS-Met is a collection of techniques that
  measure signal delays caused by the neutral
  atmosphere. The most mature is a ground-based
  technique called GPS-IPWV.
• GPS-IPWV uses stationary dual frequency GPS
  receivers, improved orbits, and data processing
  techniques derived from geodesy to estimate
  signal "delays" caused primarily by the
  troposphere. Ancillary Psfc Tsfc data are
  needed to retrieve precipitable water vapor from
  the GPS signal delays.
• Because of its variability in time and space,
  water vapor introduces delays in the GPS signal
  that cannot be accounted for analytically.
• For positioning and navigation, these delays are
  considered nuisance parameters, and techniques
  have been developed to estimate and remove them
  for high accuracy positioning.

"One Person's Noise is
Another Person's Signal"
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GPS Signal Delays Caused by the Atmosphere
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GPS Water Vapor Measurements

• The tropospheric signal delay has dry and wet
  components that are in the same ratio as the dry
  and wet constituents of the atmosphere 955.
• Comparisons with independent IPWV observations
  indicates that the wet signal delay can be mapped
  into total column (integrated) precipitable water
  vapor with an error of only about 3.5.
• Integrated precipitable water vapor (IPWV) is
  defined as the height of a vertical column of
  liquid water that would form if all of the
  moisture in the atmosphere condensed.
• IPWV is an important parameter in weather
  forecasting since it is generally correlated with
  the amount of precipitation in a given storm.

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GPS Improves NWP Forecast Accuracy
700mb RH
850mb RH
77 Same or better with GPS
79 Same or better with GPS
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Dual-Use of NDGPS Sites for GPS Meteorology
NDGPS Site at Hartsville, TN
Integrity Monitor Antenna
Reference Antenna
Reference Station Mast
NOAA GSOS Sfc Met Sensors
Nuclear Power Plant (not
standard)
Equipment Shelter
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Hurricane Georges

• Hurricane Georges came ashore between Biloxi and
  Gulfport, Mississippi as a strong Category 2
  hurricane early in the morning of Sept. 28,
  1998. -
• Two GPS sites monitored the event ?
  NOAA/NWS GPS-IPWV site at Stennis Space Center,
  MS (NDBC) about 60 km (37 miles) west of the eye
  at landfall ? USCG DGPS site at Mobile
  Point, AL (MOB1), located about 93 km (58
  miles) east of the eye at landfall. -
• These are believed to be the first-ever
  ground-based measurements of total column
  precipitable water vapor during a land-falling
  hurricane.

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Hurricane Georges

• Flooding was a major concern because there was so
  much moisture associated with this Hurricane.
  -
• Storm precipitation totals between 10 and 20
  were measured throughout southern Alabama,
  Mississippi and parts of Florida. 25 inches fell
  at Munson, FL. Precipitation estimates from
  Doppler radar showed that a small area in Alabama
  may have received as much as 35 inches of rain.
  -
• Extreme precipitation totals from this hurricane
  resulted from a combination of high rainfall
  rates and a stationary and/or very-slowly-moving
  storm. -
• Billions of dollars in damage were reported in
  Puerto Rico and along the Gulf Coast.

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Hurricane Georges Near
Landfall on 9-27-98
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GPS Sites Monitoring IPWV During
Hurricane Georges
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Comparison of GPS Observations 3-h
RH Forecasts by RUC-2 NWP Model
Differences between observations and forecasts
are caused by several factors, including
? Poorly defined initial conditions
? Temporal spatial aliasing ?
Simple GPS data assimilation method
? Lack of other observations (e.g. no satellite
data because of cloud cover, no commercial
aircraft data because of diversions).
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What Did We Learn?

• GPS is capable of making continuous upper-air
  water vapor measurements during (at least) a
  Category 2 hurricane. We believe its the only
  ground-based observing system that can do this.
• GPS provides upper-atmospheric moisture
  observations to forecasters and modelers when
  theyre needed most when other observing systems
  cannot operate reliably or at all.
• GPS-IPWV observations improve the description of
  the moisture (RH) field and predictions of its
  short-term future state.
• But will this improve precipitation forecasts?

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Next Steps

• Our ultimate goal is to improve quantitative
  precipitation forecast (QPF) accuracy, including
  precipitation type, location, quantity, time of
  onset and cessation.
• Aside from the transportation safety issues, cost
  effective maintenance decisions require accurate
  forecasts and long lead-times to optimize
  responses in rapidly changing and sometimes
  dangerous conditions.
• Precipitation forecast skill is generally poorer
  than for other parameters because, among other
  things, moisture tends to be under-observed in
  time and space, and most assimilation schemes are
  not handling it correctly.
• NDGPS data, more conventional observations, and
  improvements in high resolution analysis systems
  like LAPS should help mitigate this problem.

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Next Steps

• NOAA/FSL, in collaboration with DOT and other
  agencies, will continue to expand the network of
  GPS water vapor observing systems.
• Enhancements to high resolution data assimilation
  systems are needed to permit moisture to be
  handled more effectively than is currently
  possible.
• This should result in improved QPF forecasts, but
  we have to prove it.
• A key component of this is forecast verification
  and the development of cases to objectively
  demonstrate impact on QPF accuracy.
• The various FSL divisions working with FHWA have
  all of the expertise and tools needed to do this
  in a cost effective manner.