# Presentation: 047141

## Nationwide Differential GPS (NDGPS) Water Vapor Observations            During Hurricane Georges

*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**                                                                     **

## 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 is***

***Another Person's Signal"***

## GPS Signal Delays Caused by the Atmosphere

## 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.

## GPS Improves NWP Forecast Accuracy

- 77% Same or better with GPS

- 79% Same or better with GPS

- 850mb RH

- 700mb RH

## 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.

## 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.

## Hurricane Georges                       Near Landfall on 9-27-98

## GPS Sites Monitoring IPWV           During Hurricane Georges

## Slide 12

## 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

## 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?*

## Next Steps

- 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

- 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.