Nationwide Differential GPS (NDGPS) Water Vapor Observations

During Hurricane GeorgesNational 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

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.

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 isAnother Person's Signal"

GPS Signal Delays Caused by the Atmosphere

IONOSPHERICDELAY

TOTALATMOSPHERICDELAY

TROPOSPHERICDELAY

HYDROSTATICDELAY

WETDELAY

IPWV

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 ~ 95%:5%.

• 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.

-6

-4

-2

0

2

4

6

8

58 91 126 157 194 232 271 304

700mb Relative Humidity

Diff

ere

nce

(n

og

ps-

gp

s) %

Julian Day (2000)

Better with gps 173 (39%)Worse with gps 93 (21%)Same 174 (40%)

-6

-4

-2

0

2

4

6

8

58 91 126 158 195 233 272 305

850mb Relative Humidity

Diff

ere

nce

(n

og

ps-

gp

s) %

Julian Day (2000)

Better with gps 161 (37%)Worse with gps 100 (23%)Same 178 (40%)

77% Same

or better with GPS

79% Same

or better with GPS

850mb RH 700mb RH

200

300

400

500

600

700

800

90014.5 15 15.5 16 16.5 17 17.5 18 18.5

Relative Humidity 3-h Forecast Errors (Forecast-RAOB)February - November 2000 (421 verification times)

Control

with GPS

Pre

ssu

re (

hP

a)

Percent

GPS Improves NWP Forecast

Accuracy

Dual-Use of NDGPS Sites for GPS Meteorology

Integrity Monitor Antenna

Reference Station Mast

NOAA GSOS Sfc Met Sensors

Reference Antenna

Nuclear Power Plant

(not standard)

Equipment Shelter

NDGPS Site at Hartsville, TN

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.

• 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.

Hurricane Georges

NDBC

MOB1

Hurricane Georges Near Landfall on 9-27-98

GPS Sites Monitoring IPWV During Hurricane Georges

USCG Maritime DGPS Site at Mobile Pt., ALNWS GPS-IPWV Site at SSC, MS

268 269 270 271 272 273 274Day of Year (1998)

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6.0

7.0

Inte

g rat

ed P

reci

pita

ble

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e r V

apo r

(cm

)

NOAA-FSL @ Stennis, MS (NDBC)

USCG/NOAA-FSL @ Mobile, AL (MOB1)

9/26/98 10/1/98

(A)

(B)

(C)

(D) (E) (F)

(G)

(A)

(B)(C)

(D)

(E)

(F)

(G)

GEORGES_WV.PPT

Ground-Based GPS Water Vapor ObservationsDuring Hurricane Georges

9/25/98

Approximate time of landfall

Frontal Passage

Cloud Band

Georges stalled over Mobile, AL

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).

Comparison of GPS Observations & 3-h RH Forecasts by RUC-2 NWP Model

268 269 270 271 272 273 274

Day of Year (1998)

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Inte

grat

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reci

pita

ble

Wat

er V

apor

(cm

)

Water Vapor at NDBCDuring Hurricand Georges

GPS-IPW at NDBC3-h MAPS/RUC-2 Analysis

268 269 270 271 272 273 274

Day of Year (1 998)

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Wat

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apor

(cm

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Water Vapor at MOB1Dur ing Hurri cane George

GPS-IPW at MOB13-h MAPS/RUC-2 Analysis

NDBC MOB1

What Did We Learn?

• GPS is capable of making continuous upper-air water vapor measurements during (at least) a Category 2 hurricane. We believe it’s the only ground-based observing system that can do this.

• GPS provides upper-atmospheric moisture observations to forecasters and modelers when they’re 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?

• 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.

Next Steps

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.