Report of GPS-99 Session 2
GPS meteorology: atmospheric sensing with ground and space-based GPS receivers

Session 2-1: Occultation Method

The session was very well attended. The attendence was steady throughout the session at about 70 people. Each of the four presentations generated a lively question and answer period. The session accumulated about 10 minutes of delay overall.

I neglected to remind the audience in time about the 3 poster presentations, but that session has also generated a sizeable crowd.

In terms of content, the papers in this session were presented by the clear leaders in the field, and all discussed the technology state-of-the-art in a highly authoritative manner. This is the primary reason, in my mind, that this session was very successful.

The presentation aids worked flawlessly.

It was a great pleasure presiding over this interestinng session.

Yoaz Bar-Sever

Session 2-2: Project Plan and Estimation of Total Precipitable Water

The session started with a presentation by R. WARE on Real Time GPS Networks: Opportunities for Atmospheric Sciences. Co-authors include D. Fulker, S. Stein, D. Anderson, S. Avery, R. Clark, K. Droegemeier, J. Kuettner, J. Minster, and S. Sorooshian. Ware described "SuomiNet, an NSF-funded real-time national GPS network for atmospheric research". The project will establish and operate 100 university-based GPS receivers with real-time data flow and coordination via the Internet. Ware encouraged coordination of SuomiNet with real-time GPS networks in other countries.

GIULIO RUFFINI followed with his paper, co-authored by A. Rius, E. Cardellach, A. Escudero, A. Flores, M. Garcia, J. Vila, and J. M. Aparicio: "The GNSS-OPPSCAT Project and related GPSR work at the IEEC". Ruffini described their work on GPS ocean reflection experiments, including results from a balloon-borne GPS receiver. The initial results showed that the method is viable. The goal of the work is to evaluate and prepare for a spaceborne experiment.

NIGEL PENNA, with co-authors A. H. Dodson, H. C. Baker, B. Burki, G. Elgered, A. Rius, M. Rothacher, and D. Offiler, presented the invited paper: "The WAVEFRONT Project on GPS Water Vapour Estimation". This is a collaborative project between research institutions in Spain, Sweden, Switzerland and the U.K. The goal of the project is to develop the potential for estimation of atmospheric moisture fields using GPS network data. Results demonstrate agreement with radiometer and radiosonde measurements at the 1.5 kg per square meter level.

ERIC CALAIS, with co-authors J. Haase, Maorong Ge, Julia Talaya, Antonio Rius, Francesco Vespe, Renato Santangelo, Xiang-Yu Huang, and Jose Davila, presented "The MAGIC Project: GPS Meteorology in the Western Mediterranean". The goal of the experiment is the routine retrieval of zenith total delay (ZTD) from a 50-station GPS network, validation of the ZTD products, development of methods to assimilate ZTD data into weather models, and evaluation of the impact of ZTD on the models. Calais discussed comparisons with radiosonde measurements and model output.

NIGEL PENNA returned to the podium to present "The UK GPSIPW Meteorological Forecasting Impact Experiment". Co-authors include A. H. Dodson, N. C. Baker, D. Offiler, and M. Higgins. Penna described results from evaluation of precipitable water (PW) data from eight GPS stations in the U.K. Four of the sites are co-located with radiosonde launch sites to validate the GPS estimates. The other four are placed at non-meteorological sites to access the impact of PW data on forecast models in locations where moisture measurements are not available. Penna showed that the method is working and that it shows promise for improvement of weather forecasting.

ASHRAF EL-KUTB MOUSA presented two posters: "Effect of Using GPS as a Meteorological Sensor on the Baseline Precision", and "GPS Meteorology in Egypt: Test Results". Mousa described results from a three station GPS network in the Cairo area that he is using to evaluate the use of GPS sensed PW data to improve GPS survey accuracy and for meteorology. He found that baseline accuracy improved significantly, particularly in the vertical, when GPS PW data are applied. He also found good agreement between GPS and radiosonde sensed PW data, with improved agreement when the GPS cutoff angle is lower than 10 degrees elevation.

ZHIZHAO LIU, with co-author Jingnan Liu, presented the poster: "Experimental Comparison of Zenith Tropospheric Delays Derived from GPS Observations and Meteorological Measurements". Liu described comparisons of ZTD derived from GPS data and from a model based on meteorological data. The GPS data were observed by the IGS stations at Wuhan and Shanghai. He found a bias between the GPS and model results. He also evaluated a device that automatically records meteorological data by comparison with manually recorded data and concluded that the automatic device compares favorably.

Randolph Ware

Session 2-3: Accuracy and Error Source in Observation

This session clearly established the mature nature of technical/geodetic side of GPS meteorology. We have reached the point of " dotting the i's and crossing the t's ", as we say in England. We are concerned largely with refinments rather than breakthroughs. The technical emphasis is on such matters as particular choices for the mapping functions, the need to model ocean loading effects in the GPS software (in order that they do not appear as artifacts in the PW time series), etc. Several themes, such as that pursued by Chida et al., have beeen developed in previous talks, but are now being treated with better datasets and with greater authority. One exception to this rule is water vapor tomography. This topic was addressed by several research groups in the IUGG meeting and at GPS'99, but it is still in its early stages of technical development. With respect to PW mapping the major frontier now lies in the assimilation of GPS PW estimates, not in the mere production of the basic measurement.

The only major suprise in this session was a rather sustained attack on the Niell Mapping Function by Dr. Ifadis in his talk 02-22.

Mike Bevis

Session 2-4: Observations of Meteorological Phenomena

The main topics of this session were time and spacial variations of precipitable water vapor (PWV) observed with a ground-based GPS receiver in various regions. A total of 13 papers were presented at this session, where 3 were given orally, and 10 were displayed as posters. Many of them were accompanied by lively and productive discussions.

Large time variations of PWV were accompanied with meteorological disturbances such as a passage of a typhoon or a heavy snowfall event. Campaign observations employing a dense GPS receiver network detected a horizontal gradient of PWV affected by a local topography, or an orographically excited mountain lee wave.

Toshitaka Tsuda

Session 2-5: Detection of Atmospheric Moisture with SAR

Although this session was compact, including only two oral papers, lively discussions were generated on the basis of excellent presentations. The west zenith delay was estimated by a GPS network, which was utilized to correct interferometric SAR images. The phase pattern obtained with the SAR interferometry showed an evidence of a stationary mountain wave.

Toshitaka Tsuda

Session 2-6: Numerical Weather Prediction

Two papers were presented in this session. The first paper, given by Tracy Smith, discussed the use of integrated water vapor (IPW) data from the NOAA ground-based GPS receiver network in the Mesoscale Analysis and Prediction System (MAPS) at the Forecast System Laboratory of NOAA. The FSL/MAPS system has been assimilating the IPW data since November 1997. Comparison of forecasts made with and without the use of GPS IPW data indicated that the GPS runs only provided very modest improvement in skill over the non-GPS runs. However, for individual active weather events the GPS runs showed increased skill in the forecasts of relative humidity and precipitation.

Nobutaka Mannoji reported on the use of precipitable water (PW) data derived from the 949 GPS stations maintained by the Geographical Survey Institute (GSI) of Japan in the Japan Meteorological AgencyĠs numerical weather prediction (NWP) models. The PW data retrieved from the GPS array were incorporated into the initial conditions of water vapor of JMA models, and the result of forecasts with and without the GPS PW data were compared. Mannoji concluded that the effect of initial PW distribution altered by the GSI GPS network was strongly affected by the direction of prevailing wind, due to the fact that the GPS stations were located only over land of Japan.

Bill Kuo

Session 2-7: Estimating the Structure of the Atmosphere

Sigurjon Jonsson used the data from the 14 permanent GPS stations in operation on Kilauea volcano to estimate the horizontal gradient of the tropospheric path delay, and to use that information to improve the accuracy of the estimated GPS station positions. He showed that by using lower elevation cut-off angles and by additionally estimating the tropospheric gradient the average improvement in station repeatabilities is at least 10% and 15% for the horizontal and vertical component, respectively. Moreover, the zenith delay for all the stations in the network showed a similar pattern in time, except for a small time lag between the stations. Such time lag could be used to provide estimates of wind speed and direction with reasonable accuracy.

T. Ragne Emardson discussed the use of time series of atmospheric delay from networks of GPS receiver to estimate the motion of atmospheric water vapor above GPS networks. The motion above each site was determined by comparing the time series from different sites and estimating relative time offsets in these time series. The velocity field of the atmospheric delays, as estimated by this method, could be used to improve the understanding of the energy transport in the atmosphere, the spatial interpolation of water vapor, and for calibrating interferometric SAR (imSAR) observations for delays caused by water vapor.

Hiromu Seko presented an innovative approach to derive the three dimensional moisture structure associated with a mesoscale convective system. He first estimated integrated water vapor along paths from GPS receivers to GPS satellites using IPW data from RINEX. Analysis region was divided into grid boxes, which moved with the mean speed of the rainfall system. Water vapor distribution around the precipitating system was assumed to be stationary during the analysis period. The observed IPW along a path corresponded to the sum of path length multiplied by water vapor amount per unit volume in each grid box which the path crossed. Observation equations were then obtained using this relation and the most probable three dimensional distribution of water vapor were estimated by least square method. He successfully applied this method to a cloud cluster near a medium-scale cyclone in the Baiu front and revealed the dry air intrusion from the rear side of the cloud cluster.

Kazuro Hirahara discussed a strategy for GPS atmospheric tomography which could be used to reveal the four dimensional structure of refractivity in the neutral atmosphere. By dividing the space above the GPS stations into 3-D cells, and by assuming that the refractivity is constant within each cell in an time interval, observations equations for the refractivity in each cell can be derived with coefficient of length for the ray segment within each cell. By using the tomographic techniques and the slant delays, four-dimensional structure of atmospheric moisture can be derived. Hirahara then proposed two GPS tomography strategies, one for the local scale (with a station separation of 1 km) and the other for the Japan national scale (with a station spacing of 20 km).

A. Flores reported results from a tropospheric tomography campaign at the Onsala Space Observatory in Sweden. Seven GPS receivers, as well as two water vapor radiometers, distributed within a radius of 3 km, were operated for 21 days during the summer of 1998. By using the tomographic techniques and the GPS data from this network, they were able to observe a strong horizontal water-vapor gradient associated with a small-scale warm front that was not detected with conventional methods.

Bill Kuo

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