L. Cucurull

Improving the vertical resolution of ionospheric tomography with GPS Occultations

We combine GPS/MET data from 29 occultations and IGS ground data collected from 160 stations around the world to perform stochastic tomography of the ionosphere with a 4×20×20 global grid of voxels extending from 200 to 650 km above the mean surface of the Earth. A correlation functional approach that limits the spatial high frequency content of the images is used, and a Kalman filter is applied in the time direction. The combination of ground and occultation data and the use of smoothing techniques is robust enough for vertical resolution in this four-layer model analysis. We discuss the role of noise on the choice of the correct range of eigenvalues in the inversion problem, and the impact of occultation data, showing that ground data alone is insufficient for vertical resolution even in a three-layer, noise-less simulation.

The Use of GPS to Validate NWP Systems: The HIRLAM Model

Abstract A meteorological synoptic situation using Global Positioning System (GPS) observations and a numerical weather prediction (NWP) model in the vicinity of the Madrid Sierra, Spain, between 2 and 15 December 1996 has been studied. The experiment was characterized by high precipitable water (PW) values associated to rainfall events. The PW was estimated at the level of 1 mm with five GPS receivers to study the passage of a winter frontal system. The GPS network had baselines ranging from 5 to 50 km. These observations have been used to study the spatial and temporal variations of PW. For this same location and time period, PW calculations were carried out by HIRLAM (High-Resolution Limited Area Modeling), the hydrostatic NWP system operational at the Spanish National Weather Service. HIRLAM has been run in two modes: analysis (HIRLAM/A) and forecast (HIRLAM/F). The comparison of PW values obtained using GPS and high-resolution HIRLAM/A shows a PW bias of −0.4 mm (GPS-derived PW higher), and a root-me...

Three-Dimensional Variational Data Assimilation of Ground-Based GPS ZTD and Meteorological Observations during the 14 December 2001 Storm Event over the Western Mediterranean Sea

Abstract The impact of GPS zenith total delay (ZTD) measurements on mesoscale weather forecasts is studied. GPS observations from a permanent European network are assimilated into the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) using its three-dimensional variational data assimilation (3DVAR) system. The case study focuses on a snow storm that occurred during the period of 14–15 December 2001 over the western Mediterranean Sea. The experiments show that the most significant improvement in forecast is obtained when GPS ZTD data are assimilated together with local surface meteorological observations into the model within a cycling assimilation framework. In this case, the root-mean-square (rms) differences between forecasted and observed values are reduced by 1.7% in the wind component, 4.1% in the temperature variable, and 17.8% in the specific humidity field. This suggests the deployment of GPS receivers at surface stations to better initialize numerical weather prediction mo...

Assessing the impact of simulated COSMIC GPS radio occultation data on weather analysis over the Antarctic: A case study

The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission was launched in April 2006. As part of its mission, COSMIC will provide approximately 2500–3000 global positioning system (GPS) radio occultation (RO) soundings per day distributed uniformly around the globe. In this study, a series of sensitivity experiments are conducted to assess the potential impact of COSMIC GPS RO data on the regional weather analysis over the Antarctic. Soundings of refractivity are assimilated into the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model using its three-dimensional variational data assimilation system. First, the sensitivity of the analysis to the background error statistics and balance constraints is analyzed. Then the effects of the data distribution and the observational error of the simulated refractivity observations are examined. In this study, the simulated soundings are based on a realistic set of orbit parameters of the COSMIC constellation. Analysis of the assimilation results indicates the significant potential impact of COSMIC data on regional analyses over the Antarctic. In the one case studied here, the root-mean-square differences between the background and observed values are reduced by 12% in the horizontal wind component, 17% in the temperature variable, 8% in the specific humidity, and 22% in the pressure field when COSMIC GPS RO data are assimilated into the system by using a 6-h assimilation time window. These preliminary results suggest that COSMIC GPS RO data can have a significant impact on operational numerical weather analysis in the Antarctic.

Estimation of tropospheric zenith delay and gradients over the Madrid area using GPS and WVR data

We have analyzed data from an experiment over the Madrid (Spain) area obtained from 5 GPS receivers and 3 Water Vapor Radiometers (WVR), in order to compare their retrievals of Tropospheric Slant Delays. For this purpose we have fitted a simple gradient model to both types of data, using a Kalman filter to account for the temporal variability of the zenith and gradient parameters. We show that the retrieved gradients with the two instruments are compatible, thus suggesting that the derived slant delays can be useful for tomographic analysis. We compare the estimates of ZWD obtained with the GIPSY and the Bernese software packages. Finally, we compare the estimated gradients with those obtained with HIRLAM, a Numerical Weather Prediction model.

The contributions of the MAGIC project to the COST 716 objectives of assessing the operational potential of ground-based GPS meteorology on an international scale

Abstract MAGIC (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean) is a 3 year project financed in part by the European Commission for research on deriving and validating robust GPS integrated water vapor (IWV) and zenith tropospheric delay (ZTD) data sets and developing methods to assimilate the data into numerical weather prediction models (NWP) and test their impact. It was conceived independently from the COST 716 action, which seeks to coordinate research in the domain at an international scale, but addresses some of the same objectives. This has led to a productive cooperation between the two initiatives and their participants, and motivated the decision of MAGIC participants to provide research results as part of the COST demonstration system. Currently a database of 1.5 years of ZTD data are available on the MAGIC web site which has been validated through comparisons with radiosondes which gives differences with a standard deviation of 10 mm ZTD or the equivalent error in IWV of 1.6 kg/m2. NWP assimilation tests will be carried out in the final year of the project.

Ionospheric calibration of radar altimeters using GPS tomography

We compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate different GPS data analysis strategies. We verify that global tomographic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day fit of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU. Tomographic inversion using simulated data from the Parametrized Ionospheric Model highlights the strong correlation between GPS bias constants, electronic densities at the highest layer, and unmodeled protonospheric TEC. This suggests that GPS TEC estimates at the TOPEX/POSEIDON altitude are more accurate if the bias constants are estimated and if a layer above TOPEX/POSEIDON is added to the grid.

Sensitivity of PAZ LEO Polarimetric GNSS Radio-Occultation Experiment to Precipitation Events

A Global Navigation Satellite System (GNSS) radio occultation (RO) experiment is being accommodated in the Spanish low Earth orbiter for Earth Observation PAZ. The RO payload will provide globally distributed vertical thermodynamic profiles of the atmosphere suitable to be assimilated into weather numerical prediction models. The Ground Segment services of the U.S. National Oceanographic and Atmospheric Administration and standard-RO processing services by University Corporation for Atmospheric Research (USA) will be available under best effort basis. Moreover, the mission will run, for the first time, a double-polarization GNSS RO experiment to assess the capabilities of polarimetric GNSS RO for sensing heavy rain events. This paper introduces the Radio-Occultation and Heavy Precipitation experiment aboard PAZ and performs a theoretical analysis of the concept. The L-band GNSS polarimetric observables to be used during the experiment are presented, and their sensitivity to moderate to heavy precipitation events is evaluated. This study shows that intense rain events will induce polarimetric features above the detectability level.

MM5 derived ZWDs compared to observational results from VLBI, GPS and WVR

Abstract Modelled values of zenith wet delay (ZWD) from the non-hydrostatic numerical weather prediction (NWP) model MM5 are compared to estimated values retrieved from observations by geodetic very long baseline interferometry (VLBI), global positioning system (GPS) receivers, and water vapour radiometers (WVRs). In addition, sparse radiosonde (RS) data are used to augment the available data sets. The comparison is done for three stations of the European geodetic VLBI network for six observing sessions during the year 1999. The stations (Madrid, Onsala, and Wettzell) were primarily chosen to have the maximum number of collocated measuring techniques. In general, the time series for the different techniques show a good agreement. The correlation values between the techniques amount to 75–95%. The RMS differences of MM5 with respect to the other techniques obtain values of ±1.3–1.6 cm. The bias between MM5 and VLBI lies at about 1.0 cm, the bias between MM5 and GPS varies in the range of 0.0–0.6 cm and appears to be station dependent.

Integrating NWP products into the analysis of GPS observables

Abstract The processing of the Global Positioning System (GPS) observables is regularly performed using some “a priori” standard meteorological values. These can be very unrealistic if only few observations are available since the actual atmospheric conditions over the geodetic network are neglected. This seems to be the case of randomly moving GPS receivers far from continuously observing reference stations, as geodetic coordinates and tropospheric parameters cannot be accurately estimated. More realistic initial values for the troposphere may be obtained from a Numerical Weather Prediction (NWP) model. Thus, the impact of the ingestion of NWP forecasts into the GPS data processing is analyzed for short time series in terms of the geodetic vertical coordinate. The non-hydrostatic MM5 model with boundary and initial conditions given by the ECMWF analysis is used to compute estimates of the zenith wet and dry delays. These modeling simulations are then ingested into the GPS data analysis of the GIPSY software package. First, this approach is applied to GPS data gathered at the UK permanent stations of the COST-716 first benchmark campaign where it is possible to estimate a stable 24 h reference solution. Here, an improvement of about 60% in the vertical coordinate bias is found. Still, the absolute accuracy cannot expected to be better than 1 dm due to the remaining mismodeling of the troposphere. Then, the modeling approach is applied to a kinematic analysis of GPS buoy data. In this case also, a qualitative improvement is noticeable as demonstrated by a comparison with independent satellite radar altimeter data.