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Moore, AW, Small IJ, Gutman SI, Bock Y, Dumas JL, Fang P, Haase JS, Jackson ME, Laber JL.  2015.  National Weather Service forecasters use GPS precipitable water vapor for enhanced situational awareness during the Southern California summer monsoon. Bulletin of the American Meteorological Society. 96:1867-1877.   10.1175/bams-d-14-00095.1   AbstractWebsite

During the North American Monsoon, low-to-midlevel moisture is transported in surges from the Gulf of California and Eastern Pacific Ocean into Mexico and the American Southwest. As rising levels of precipitable water interact with the mountainous terrain, severe thunderstorms can develop, resulting in flash floods that threaten life and property. The rapid evolution of these storms, coupled with the relative lack of upper-air and surface weather observations in the region, make them difficult to predict and monitor, and guidance from numerical weather prediction models can vary greatly under these conditions. Precipitable water vapor (PW) estimates derived from continuously operating ground-based GPS receivers have been available for some time from NOAA's GPS-Met program, but these observations have been of limited utility to operational forecasters in part due to poor spatial resolution. Under a NASA Advanced Information Systems Technology project, 37 real-time stations were added to NOAA's GPS-Met analysis providing 30-min PW estimates, reducing station spacing from approximately 150 km to 30 km in Southern California. An 18-22 July 2013 North American Monsoon event provided an opportunity to evaluate the utility of the additional upper-air moisture observations to enhance National Weather Service (NWS) forecaster situational awareness during the rapidly developing event. NWS forecasters used these additional data to detect rapid moisture increases at intervals between the available 1-6-h model updates and approximately twice-daily radiosonde observations, and these contributed tangibly to the issuance of timely flood watches and warnings in advance of flash floods, debris flows, and related road closures.

Villamil-Otero, G, Meiszberg R, Haase JS, Min KH, Jury MR, Braun JJ.  2015.  Topographic-thermal circulations and GPS-measured moisture variability around Mayaguez, Puerto Rico. Earth Interactions. 19   10.1175/ei-d-14-0022.1   AbstractWebsite

To investigate topographic-thermal circulations and the associated moisture variability over western Puerto Rico, field data were collected from 15 to 31 March 2011. Surface meteorological instruments and ground-based GPS receivers measured the circulation and precipitable water with high spatial and temporal resolution, and the Weather Research and Forecasting (WRF) Model was used to simulate the mesoscale flow at 1-km resolution. A westerly onshore flow of similar to 4ms(-1) over Mayaguez Bay was observed on many days, due to an interaction between thermally driven [3 degrees C (10 km)(-1)] sea-breeze circulation and an island wake comprised of twin gyres. The thermally driven sea breeze occurred only when easterly synoptic winds favorably oriented the gyres with respect to the coast. Moisture associated with onshore flow was characterized by GPS measured precipitable water (PW). There is diurnal cycling of PW > 3 cm over the west coast during periods of onshore flow. The WRF Model tends to overestimate PWon the west side of the island, suggesting evapotranspiration as a process needing further attention. Fluctuations of PW affect local rainfall in times of convective instability.

Haase, JS, Ge MR, Vedel H, Calais E.  2003.  Accuracy and variability of GPS tropospheric delay measurements of water vapor in the western Mediterranean. Journal of Applied Meteorology. 42:1547-1568. AbstractWebsite

As a preliminary step for assessing the impact of global positioning system (GPS) refractive delay data in numerical weather prediction (NWP) models, the GPS zenith tropospheric delays (ZTDs) are analyzed from 51 permanent GPS sites in the western Mediterranean. The objectives are to estimate the error statistics necessary for future assimilation of GPS ZTD data in numerical models and to investigate the variability of the data in this area. The time series, which were derived continuously from November 1998 to June 2001, are compared with independent equivalent values derived from radiosonde profiles and the High-Resolution Limited-Area Model (HIRLAM) NWP model. Based on over two years of data, the difference between radiosonde and GPS ZTD has a standard deviation of 12 mm of delay and a bias of 7 mm of delay. Some sites have biases as high as 14 mm of delay. The bimodal distribution of residuals, with a higher bias for daytime launches, indicates these biases may be due to radiosonde day-night measurement biases. The biases between the GPS ZTD and HIRLAM estimates are smaller, but the 18-mm ZTD standard deviation is significantly greater. The standard deviation of the residuals depends strongly on the amount of humidity, which produces an annual signal because of the much higher variability of water vapor in the summer months. The better agreement with radiosonde data than HIRLAM estimates indicates that the NWP models will benefit from the additional information provided by GPS. The long-term differences between the observational data sources require further study before GPS-derived data become useful for climate studies.

Vey, S, Calais E, Llubes M, Florsch N, Woppelmann G, Hinderer J, Amalviet M, Lalancette MF, Simon B, Duquenne F, Haase JS.  2002.  GPS measurements of ocean loading and its impact on zenith tropospheric delay estimates: a case study in Brittany, France. Journal of Geodesy. 76:419-427.   10.1007/s00190-002-0272-7   AbstractWebsite

The results from a global positioning system (GPS) experiment carried out in Brittany, France, in October 1999, aimed at measuring crustal displacements caused by ocean loading and quantifying their effects on GPS-derived tropospheric delay estimates, are presented. The loading effect in the vertical and horizontal position time series is identified, however with significant disagreement in amplitude compared to ocean loading model predictions. It is shown that these amplitude misfits result from spatial tropospheric heterogeneities not accounted for in the data processing. The effect of ocean loading on GPS-derived zenith total delay (ZTD) estimates is investigated and a scaling factor of 4.4 between ZTD and station height for a 10degrees elevation cutoff angle is found (i.e. a 4.4-cm station height error would map into a 1-cm ZTD error). Consequently. unmodeled ocean loading effects map into significant errors in ZTD estimates and ocean loading modeling must be properly implemented when estimating ZTD parameters from GRS data for meteorological applications. Ocean loading effects Must be known with an accuracy of better than 3 cm in order to meet the accuracy requirements of meteorological and climatological applications of GPS-derived precipitable water vapor.

Walpersdorf, A, Calais E, Haase J, Eymard L, Desbois M, Vedel H.  2001.  Atmospheric gradients estimated by GPS compared to a high resolution numerical weather prediction (NWP) model. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:147-152. AbstractWebsite

The estimation of horizontal atmospheric gradients, in addition to zenith delays, is a strategy now commonly used in geodetic Global Positioning System (GPS) positioning. This strategy compensates for inhomogeneities in the atmospheric water vapor distribution above GPS sites, and has shown to increase the positioning precision, e.g. in geodynamic networks. While the zenith delay has been successfully related to the pressure at the GPS site and the water vapor above the site, the relation of the GPS estimated horizontal gradients to atmospheric quantities remains unclear. To get a better understanding of the nature of these gradients inferred by GPS, this study compares GPS tropospheric observations from the MAGIC permanent network on the NW side of the Mediterranean Sea with simulations based on the high resolution NWP model ALADIN (Meteo France). To verify the model performance, we use meteorological measurements from the FETCH ship campaign in the Gulf of Lyon in March-April 1998. For this study, five stations of the MAGIC network close to the Golf of Lyon have been selected. Results from two periods, representing two different weather situations occuring within the FETCH observation campaign, are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.

Ge, M, Calais E, Haase J.  2001.  Automatic orbit quality control for near real-time GPS zenith tropospheric delay estimation. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:177-181. AbstractWebsite

We implement an automatic orbit quality control procedure in order to reduce the effect of satellite orbit error for near real-time estimating of zenith total delay (ZTD). We estimate the three Keplerian parameters of the GPS orbits that represent the main error sources (semimajor axis, inclination, and argument of perigee). We start with an a priori constraint corresponding to the typical orbit accuracy for each parameter and adjust it iteratively according to its estimated value and variance. This data analysis procedure shows an improvement of 20% in ZTD rms compared to a strategy based only on the quality index provided with the IGS predicted orbits. A real-time test with IGS ultra-rapid orbits also shows significant improvement compared to fixing IGS ultra-rapid orbits. This strategy allows for a totally automated estimation of orbital parameters directly from the real-time GPS data, without altering the accuracy of the ZTD estimates. (C) 2001 Elsevier Science Ltd. All rights reserved.