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Montgomery, MT, Davis C, Dunkerton TJ, Wang Z, Velden C, Torn R, Majumdar S, Zhang F, Smith RK, Bosart L, Bell MM, Haase JS, Heymsfield A, Jensen J, Campos T, Boothe MA.  2012.  The Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) Experiment: Scientific basis, new analysis tools and some first results. Bulletin of the American Meteorological Society. 92:153-172.   10.1175/BAMS-D-11-00046.1  
Haase, JS, Maldonado-Vargas J, Rabier F, Cocquerez P, Minois M, Guidard V, Wyss P, Johnson AV.  2012.  A Proof-of-Concept Balloon-borne GPS Radio Occultation Profiling System for Polar Studies. Geophysical Research Letters. 39:doi:10.1029/2011GL049982.   doi:10.1029/2011GL049982   Abstract

Global warming has focused attention on the polar regions and recent changes in sea and land ice distribution. Accurate modeling of the future evolution of climate and weather in the Antarctic relies heavily on remote sensing observations. However, their reliable assimilation into numerical weather models and reanalyses is challenging because of the unique environment and sparsity of in-situ observations for validation. We developed a stratospheric balloon-borne GPS radio occultation system for the 2010 Concordiasi campaign to provide refractivity and derived temperature profiles for improving satellite data assimilation. The observed excess phase delay profiles agree with those simulated from model and dropsonde profiles. 711 occultations were recorded from two balloons, comparable to the number of profiles acquired by 13 driftsonde balloons. Of these profiles, 32% descended to 4 km above the surface, without open-loop receiver tracking technology, demonstrating it is possible to retrieve useful information with relatively simple low cost instruments.

Haase, JS, Nowack RL.  2011.  Earthquake scenario ground motions for the urban area of Evansville, Indiana. Seismological Research Letters. 82:176-185.
Garrison, JL, Voo J, Yueh SH, Grant MS, Fore AG, Haase JS.  2011.  The estimation of sea surface roughness effects in microwave radiometric measurements of salinity using reflected Global Navigation Satellite System signals. IEEE Geoscience and Remote Sensing Letters. PP:1-5.   10.1109/LGRS.2011.2159323   Abstract

In February–March 2009, an airborne field campaign was conducted using the Passive Active L- and S-band (PALS) microwave sensor and the Ku-band Polarimetric Scatterometer to collect measurements of brightness temperature and near-surface wind speeds. Flights were conducted over a region of expected high-speed winds in the Atlantic Ocean, for the purposes of algorithm development for sea surface salinity (SSS) retrievals. Wind speeds encountered during the March 2, 2009, flight ranged from 5 to 25 m/s. The Global Positioning System (GPS) delay mapping receiver from the National Aeronautics and Space Administration (NASA) Langley Research Center was also flown to collect GPS signals reflected from the ocean surface and generate postcorrelation power-versus-delay measurements. These data were used to estimate ocean surface roughness. These estimates were found to be strongly correlated with PALS-measured brightness temperature. Initial results suggest that reflected GPS measurements made using small low-power instruments can be used to correct the roughness effects in radiometer brightness temperature measurements to retrieve accurate SSS.

Haase, JS, Choi YS, Nowack RL.  2011.  Liquefaction hazard near the Ohio River from midwestern scenario earthquakes. Environmental & Engineering Geoscience. 17:165-181. Abstract

Evansville, Indiana, and Henderson, Kentucky, located on the banks of the Ohio River, are susceptible to liquefaction-induced damage in the event of significant earthquake shaking. A sequence of three earthquakes with magnitudes greater than 7 occurred near New Madrid, MO, in 1811-1812, producing ground motions with Modified Mercalli Intensity VII in the Evansville-Henderson area near the Ohio River, 180 km away from the source. In addition, liquefaction evidence has been documented less than 40 km from Evansville from two large earthquakes that occurred within the past 12,000 years in the Wabash Valley. As a complement to recent work on the probabilistic seismic hazard analysis and scenario earthquake ground motions, we have calculated the liquefaction hazard within the 33 X 42-km(2) area containing Evansville and Henderson, based on scenario earthquakes from each of these source regions. Cone penetrometer test data were used to estimate the factor of safety against liquefaction at 58 sites in the study region. Liquefaction potential index (LPI) maps were calculated using a probabilistic method to account for the uncertainty due to spatial variability of soil profiles. The site response and peak ground accelerations for the scenario earthquakes vary across the study area, resulting in significant variations in LPI. The LPI is highest in the outwash terraces at the edges of the Ohio River Valley for both scenario earthquakes. However, the probability of liquefaction severe enough to produce lateral spreading (LPI > 12) was less than 20 percent in most of the study area for both scenarios.

Haase, JS, Nowack RL, Choi YS, Bowling T.  2011.  Probabilistic seismic hazard assessment including site effects for Evansville, Indiana, and the surrounding region. Bulletin of the Seismological Society of America. 101:1039-1054.   10.1785/0120090322   Abstract

The central US has a low rate of seismicity, but because of the occurrence of past earthquakes such as the 1811-1812 New Madrid events, as well as prehistoric events in the Wabash Valley Fault Zone, there is a significant seismic hazard. Evansville, Indiana, is one of the closest large urban areas to both the New Madrid and Wabash Valley Fault Zones. For this reason, it has been targeted as a priority region for urban seismic hazard assessment. The probabilistic seismic hazard methodology that will be used in the Evansville, region incorporates information from new surficial geologic mapping efforts on the part of the USGS and state geological surveys, as well as information on the depth and properties of near surface soils and their associated uncertainties. The subsurface information has been compiled to determine a depth dependent shear wave velocity model proxy for the major lithologic units. The probabilistic seismic hazard calculation follows the method used for the USGS national probabilistic seismic hazard maps, with the modification of the attenuation curves and their uncertainties based on estimates of local site conditions. The maps show deamplification at periods of 0.2 seconds relative to the 2002 national seismic hazard maps in the Ohio River Valley soils underlying most parts of the cities of Evansville and Henderson, KY, presumably due to nonlinearity. The Ohio River Valley soils increase expected ground motion amplitudes at periods of 1 second. The peak ground acceleration values are not significantly affected, except for in localized lacustrine deposits.

Frankel, A, Harmsen S, Mueller C, Calais E, Haase J.  2011.  Seismic Hazard Maps for Haiti. Earthquake Spectra. 27:S23-S41.   10.1193/1.3631016   Abstract

We have produced probabilistic seismic hazard maps of Haiti for peak ground acceleration and response spectral accelerations that include the hazard from the major crustal faults, subduction zones, and background earthquakes. The hazard from the Enriquillo-Plantain Garden, Septentrional, and Matheux-Neiba fault zones was estimated using fault slip rates determined from GPS measurements. The hazard from the subduction zones along the northern and southeastern coasts of Hispaniola was calculated from slip rates derived from GPS data and the overall plate motion. Hazard maps were made for a firm-rock site condition and for a grid of shallow shear-wave velocities estimated from topographic slope. The maps show substantial hazard throughout Haiti, with the highest hazard in Haiti along the Enriquillo-Plantain Garden and Septentrional fault zones. The Matheux-Neiba Fault exhibits high hazard in the maps for 2% probability of exceedance in 50 years, although its slip rate is poorly constrained.

Rabier, F, Bouchard A, Brun E, Doerenbecher A, Guedj S, Guidard V, Karbou F, Peuch V-H, El Amraoui L, Puech D, Genthon C, Picard G, Town M, Hertzog A, Vial F, Cocquerez P, Cohn SA, Hock T, Fox J, Cole H, Parsons D, Powers J, Romberg K, VanAndel J, Deshler T, Mercer J, Haase J, Avallone L, Kalnajs L, Mechoso RC, Tangborn A, Pellegrini A, Frenot Y, Thépaut J-N, McNally A, Balsamo G, Steinle P.  2010.  The Concordiasi project in Antarctica. Bulletin of the American Meteorological Society. 91:69-86.   DOI: 10.1175/2009BAMS2764.1   Abstract

Within the framework of the International Polar Year, the Concordiasi project will makeinnovative observations of the atmosphere above Antarctica. The most important goals ofConcordiasi are:* To enhance the accuracy of weather prediction and climate records in Antarctica throughthe assimilation of in-situ and satellite data, with an emphasis on data provided byhyperspectral infra-red sounders. The focus will be on precipitation and the mass budget ofthe ice sheets. The improvements in dynamical model analyses and forecasts will be utilizedin chemical-transport models that describe the links between the polar vortex dynamics andozone depletion, and to advance the understanding of the Earth system by examining theinteractions between Antarctica and lower latitudes.* To improve our understanding of microphysical and dynamical processes controlling theozone content of the polar air masses, by providing the first quasi-Lagrangian observations ofozone and particle content of air masses, in addition to an improved characterization of the 3Dpolar vortex dynamics. Techniques for assimilating these Lagrangian observations will bedeveloped.A major Concordiasi component is a field experiment during the Austral springs of 2008 and2009. The field activities are based on a constellation of up to eighteen long durationstratospheric balloons deployed from the McMurdo station. Six of these balloons will carryGPS receivers and in-situ instruments measuring temperature, pressure, ozone, and particles.4All the balloons are capable of releasing dropsondes on demand for measuring atmosphericparameters. Finally, radiosounding measurements are collected at various sites, including theConcordia station.

Muradyan, P, Haase JS, Xie F, Garrison JL, Lulich T, Voo J.  2010.  GPS/INS navigation precision and its effect on airborne radio occultation retrieval accuracy. GPS Solutions. 10.1007/s10291-010-0183-7
Haase, JS, Nowack RL, Park CH, Hill J.  2010.  Probabilistic seismic hazard estimates incorporating site effects - an example from Indiana. Environmental & Engineering Geoscience . 16:369-388. Abstract

The United States Geological Survey has published probabilistic earthquake hazard maps for the United States based on the current knowledge of past earthquake activity and geological constraints on earthquake potential (Frankel et al. 1996, 2002). These maps assume standard site conditions with S-wave velocities of 760 m/sec in the top 30 meters. For urban and infrastructure planning and long term budgeting, the public is interested in similar probabilistic seismic hazard maps that take into account near surface geological materials, that give a more realistic assessment of the actual conditions. We have implemented a probabilistic method for incorporating site effects into the USGS seismic hazard assessment calculation (Cramer, 2003) that takes into account the first order effects of the geologic conditions. The thicknesses of unconsolidated sediments, which play a large role in amplification, were derived from a P-wave refraction database with over 13,000 profiles, and a preliminary geology-based velocity model was constructed from available information on S-wave velocities. The information is averaged over 7.5 minute quadrangles, so the associated uncertainties are an important input to the probabilistic calculation as well. An interesting feature of the preliminary hazard maps incorporating site effects is the approximate factor of two increase in the 1 Hz spectral acceleration with 2% probability of exceedence in 50 years for parts of the greater Indianapolis metropolitan region and surrounding parts of central Indiana. This effect is primarily due to the relatively thick sequence of unconsolidated sediments infilling ancient bedrock topography that has been deposited since the Pleistocene Epoch. As expected, the late Pleistocene and Holocene depositional systems of the Wabash and metropolitan region and surrounding parts of central Indiana. Ohio Rivers produce additional amplification in the southwestern part of Indiana. Ground motions decrease as would be expected towards the bedrock units in south central Indiana where motions are significantly lower than the values on the USGS maps. Given the resolution of the input data, the results are useful for understanding the effects of amplification on the probabilistic hazard assessment at a large scale and the effects of proposed new design guidelines however, site specific surveys are still required for engineering purposes. In particular, the results indicate that the number and location of individual counties that are assigned seismic hazard levels based on the probabilistic seismic hazard calculations can differ significantly depending on the methodology used for incorporating the site effects.

Haase, JS, Dautermann T, Taylor MJ, Chapagain N, Calais E, Pautet D.  2010.  Propagation of plasma bubbles observed in Brazil from GPS and airglow data. Advances in Space Research. in press Abstract

Equatorial Spread-F is a common occurrence in the equatorial ionosphere that isassociated with large variations in plasma density that often cause scintillation andinterference in communication signals. These events are known to result from Rayleigh-Taylor instability, but the day-to-day variability of their occurrence is not wellunderstood. The triggering mechanism of plasma depletions is still a matter of debate, butmay be linked to gravity waves that under favorable conditions propagate to the middleatmosphere. Understanding the triggering of ESF was the focus of the SpreadFEXcampaign near Brasilia, Brazil in 2005. The campaign provided co-located airglow andGPS observations to study the onset of plasma depletions and their evolution as theytraversed the region. Comparisons between the 630.0 nm airglow data and GPS datademonstrate the ability of the compact dual frequency GPS array to detect the plasmabubbles and retrieve reliable propagation characteristics of the depletions. In this casestudy, a plasma depletion was detected and moved over the array at velocities of 85-110m/s, slowing as it moved towards the east. Correlation of consecutive airglow imagesgives consistent estimates of the eastward drift over the same time period. Mapping theairglow data to the GPS line-of-sight geometry allows direct comparison and reveals aresolvable westward tilt of the plasma depletion that may be due to vertical shear.However, in this case it appears that horizontal gradients in drift velocity are moreimportant than vertical shear in the evolution of the depletion. The uniqueness of thisstudy is the ability to resolve locally the characteristics of the plasma depletion withoutrelying on assumptions about the mapping of the depletion along magnetic field lines tolarge latitudinal distances. It presents new information for understanding ESFdevelopment and the development of depletions strong enough to produce scintillation.

Fritts, DC, Abdu MA, Batista BR, Batista IS, Batista PP, Buriti R, Clemesha BR, Dautermann T, de Paula E, Fechine BJ, Fejer B, Gobbi D, Haase J, Kamalabadi F, Laughman B, Lima PP, Liu H-L, Medeiros A, Pautet D, Riggin DM, Sabbas SF, Sobral JHA, Stamus P, Takahashi H, Taylor MJ, Vadas SL, Wrasse C.  2009.  Overview and Summary of the Spread F Experiment (SpreadFEx). Annales Geophysicae. 27:1-15.
Fritts, DC, Abdu MA, Batista BR, Batista IS, Batista PP, Buriti R, Clemesha BR, Dautermann T, de Paula E, Fechine BJ, Fejer B, Gobbi D, Haase J, Kamalabadi F, Laughman B, Lima PP, Liu H-L, Medeiros A, Pautet D, Riggin DM, Sabbas SF, Sobral JHA, Stamus P, Takahashi H, Taylor MJ, Vadas SL, Wrasse C.  2009.  The Spread F Experiment (SpreadFEx): Program overview and first results. Earth Planets Space. 61:411-430.
Chen, SH, Zhao Z, Haase JS, Chen AD, Vandenberghe F.  2008.  A study of the characteristics and assimilation of retrieved MODIS total precipitable water data in severe weather simulations. Monthly Weather Review. 136:3608-3628.   10.1175/2008mwr2384.1   AbstractWebsite

This study determined the accuracy and biases associated with retrieved Moderate Resolution Imaging Spectroradiometer (MODIS) total precipitable water (TPW) data, and it investigated the impact of these data on severe weather simulations using the Weather Research and Forecast (WRF) model. Comparisons of MODIS TPW with the global positioning system (GPS) TPW and radiosonde-derived TPW were carried out. The comparison with GPS TPW over the United States showed that the root-mean-square (RMS) differences between these two datasets were about 5.2 and 3.3 mm for infrared (IR) and near-infrared (nIR) TPW, respectively. MODIS IR TPW data were overestimated in a dry atmosphere but underestimated in a moist atmosphere, whereas the nIR values were slightly underestimated in a dry atmosphere but overestimated in a moist atmosphere. Two cases, a severe thunderstorm system (2004) over land and Hurricane Isidore (2002) over ocean, as well as conventional observations and Special Sensor Microwave Imager (SSM/I) retrievals were used to assess the impact of MODIS nIR TPW data on severe weather simulations. The assimilation of MODIS data has a slightly positive impact on the simulated rainfall over Oklahoma for the thunderstorm case, and it was able to enhance Isidore's intensity when the storm track was reasonably simulated. The use of original and bias-corrected MODIS nIR TPW did not show significant differences from both case studies. In addition, SSM/I data were found to have a positive impact on both severe weather simulations, and the impact was comparable to or slightly better than that of MODIS data.

Xie, FQ, Haase JS, Syndergaard S.  2008.  Profiling the Atmosphere Using the Airborne GPS Radio Occultation Technique: A Sensitivity Study. IEEE Transactions on Geoscience and Remote Sensing. 46:3424-3435.   <u>10.1109/tgrs.2008.2004713</u>   AbstractWebsite

Global Positioning System (GPS) radio occultation (RO) sounding, with its high vertical resolution temperature and humidity profiling capability, is revolutionizing atmospheric science, particularly through assimilation in numerical weather prediction (NWP) models. Currently, the observations are derived from GPS receivers onboard low Earth orbiting satellites. However, with the current number of satellites, it is difficult to provide dense sounding measurements in a specific region within a limited time period. With a GPS receiver onboard an airplane, the GPS RO technique offers such an opportunity while retaining the high vertical resolution sounding capability. The GNSS Instrument System for Multistatic and Occultation Sensing is currently under development for the National Science Foundation's High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft. This paper presents a sensitivity analysis of the airborne occultation technique that will be used for the HIAPER system. The results demonstrate an anticipated overall accuracy of better than 0.5% for the retrieved refractivity from the surface to about 1 km below the airplane, where the expected airplane velocity errors of up to 5 mm/s limit the accuracy. The effects on the retrievals due to horizontal variations in atmospheric refractivity are significant, and retrieval errors may reach several percent inside frontal systems when the front is perpendicular to the ray paths and within 200 km of the tangent point. In general, the airborne GPS RO system provides a promising new data source for NWP and targeted observational studies.

Dautermann, T, Calais E, Haase J, Garrison J.  2007.  Investigation of ionospheric electron content variations before earthquakes in southern California, 2003-2004. Journal of Geophysical Research-Solid Earth. 112   B0210610.1029/2006jb004447   AbstractWebsite

[1] It has been proposed that earthquakes are preceded by electromagnetic signals detectable from ground- and space-based measurements. Ionospheric anomalies, such as variations in the electron density a few days before earthquakes, are one of the precursory signals proposed. Since Global Positioning System (GPS) data can be used to measure the ionospheric total electron content (TEC), the technique has received attention as a potential tool to detect ionospheric perturbations related to earthquakes. Here, we analyze 2 years ( 2003 - 2004) of data from the Southern California Integrated GPS Network (SCIGN), a dense network of 265 continuous GPS stations centered on the Los Angeles basin, for possible precursors. This time period encompasses the December 2003, M6.6, San Simeon and September 2004, M6.0, Parkfield earthquakes. We produce TEC time series at all SCIGN sites and apply three different statistical tests to detect anomalous TEC signals preceding earthquakes. We find anomalous TEC signals but no statistically significant correlation, in time or in space, between these TEC anomalies and the occurrence of earthquakes in southern California for the 2003 - 2004 period. This result does not disprove the possibility of precursory phenomena but show the signal-to-noise ratio of a hypothetical TEC precursor signature is too low to be detected by the analysis techniques employed here. Precursors may still be revealed for future large earthquakes in well instrumented areas such as California and Japan, if the tests can be developed into techniques that can better separate external influences from the actual TEC signal.

Garrison, JL, Lee SCG, Haase JS, Calais E.  2007.  A method for detecting ionospheric disturbances and estimating their propagation speed and direction using a large GPS network. Radio Science. 42   Rs601110.1029/2007rs003657   AbstractWebsite

A technique is developed for detecting short period (3-10 min) ionospheric disturbances and estimating their propagation speed and direction using data from a large GPS network (a hundred or more receivers). This method increases the signal-to-noise ratio of small signals and could be applied, autonomously, to process a large set of data for the study of the potential signal sources and statistical distributions of these disturbances. The integral electron content (IEC) for every satellite-station pair in the network is extracted from dual frequency phase data. These IEC time series are then band-pass-filtered and cross-correlated with each other. The resulting correlation power is an indication of the presence of a common disturbance recorded at the two stations, and the delay to the maximum correlation is a measurement of the propagation time between the ionospheric pierce points of the respective stations. A threshold on correlation power is used to select a subset of these delay measurements. The velocity of the detected perturbation is then estimated by fitting a two-dimensional plane wave model to this subset of measurements. A technique is developed to remove the effects of time-varying satellite motion and to reconstruct the waveform that would have been observed at a fixed point within the ionosphere. Consistency of the resulting velocity estimates is checked using a stacking-alignment method and a time-distance mapping that accounts for the motion of the GPS satellites. The sensitivity of the velocity estimate to both the assumed height of a thin-layer ionosphere and the detection threshold value is studied. A simulation is used to demonstrate the IEC waveform distortion due to satellite motion, and an example is shown in which this distortion is able to shift the dominant frequencies of an actual disturbance outside of the passband of the filter, thereby preventing detection. Four weeks of data, in different seasons, collected using the Southern California Integrated GPS Network (SCIGN), were processed. Over the total of 28 days, 127 significant disturbances were detected, most with horizontal propagation speeds between 50-1000 m/s and westward directions of propagation. A few cases with exceptionally high speed (>2000 m/s) were observed. It is hypothesized that these are manifestations of disturbances that occur simultaneously throughout the ionosphere, rather than traveling waves. The rate of occurrence of disturbances in the 3-10 min band was found to be larger than expected. Observational biases of this method are discussed.

Emore, GL, Haase JS, Choi K, Larson KA, Yamagiwa A.  2007.  Recovering seismic displacements through combined use of 1-Hz GPS and strong-motion accelerometers. Bulletin of the Seismological Society of America. 97:357-378.   10.1785/0120060153   AbstractWebsite

Retrieving displacement from seismic acceleration records is often difficult because unknown small baseline offsets in the acceleration time series will contaminate the doubly integrated record with large quadratic errors. One-hertz Global Positioning System (GPS) position estimates and collocated seismic data are available from the 2003 M-W 8 Tokachi-Oki (Hokkaido) earthquake. After a process of correcting for possible misorientation of the seismic sensors, an inversion method is used to simultaneously solve for ground displacement with both data sets as input constraints. This inversion method takes into account the presence of unknown offsets in the acceleration record, and the relatively large uncertainties in the estimated 1-Hz GPS positions. In this study, 117 channels of seismic data were analyzed. Only 5% of the time does the static displacement retrieved from traditional baseline correction processing without GPS information agree with the absolute displacement measured with 1-Hz GPS to within the errors of the GPS data. In solving simultaneously for constrained displacements that agree with both the seismic and GPS data sets, an optimal solution was found that included only one- or two-step functions in the acceleration records. Potential explanations for the offsets are analyzed in terms of tilt of the sensor or electronic noise. For nine stations, clear misorientations of the seismic sensors of more than 20 deg from the reported orientation were found. For this size event, the 30-sec sampled GPS solutions were also a sufficient constraint for establishing the offset errors and recovering reliable displacements. The results significantly extend the frequency band over which accelerometer data are reliable for source inversion studies.

Miyazaki, S, Larson KM, Choi KH, Hikima K, Koketsu K, Bodin P, Haase J, Emore G, Yamagiwa A.  2004.  Modeling the rupture process of the 2003 September 25 Tokachi-Oki (Hokkaido) earthquake using 1-Hz GPS data. Geophysical Research Letters. 31   L2160310.1029/2004gl021457   AbstractWebsite

[ 1] High-rate GPS has the potential to recover both dynamic and static displacements accurately. We analyze 1-Hz GPS data recorded during the 2003 Tokachi-Oki earthquake. The 1-Hz GPS displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records. The GPS waveforms were inverted to model the spatio-temporal evolution of the fault slip during the rupture. The slip is found to propagate downdip in the subduction zone with largest moment release -50 km northwest of the hypocenter. The region of largest slip agrees in general with traditional seismic studies, indicating that 1-Hz GPS can be used for finite fault studies. The 1-Hz GPS slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data, possibly because 1-Hz GPS is more sensitive to cumulative slip distribution.

Vedel, H, Huang XY, Haase JS, Ge M, Calais E.  2004.  Impact of GPS Zenith Tropospheric Delay data on precipitation forecasts in Mediterranean France and Spain. Geophysical Research Letters. 31   L0210210.1029/2003gl017715   AbstractWebsite

[1] Forecasting precipitation in the western Mediterranean is difficult because of the interactions among dynamical forcing, orographic lifting and moisture advection from the warm Mediterranean Sea. Torrential rainfall events are not uncommon, especially during the autumn. This type of event motivated an effort to improve precipitation forecasting by incorporating additional information on the initial state of the humidity field from Global Positioning System measurements of refractive delay. In this study we process data from a network of sites in Western Europe and assimilate the data over a two week period into the HIRLAM numerical weather prediction model. The overall impact for the two week period is neutral, however, for a severe rain event taking place during that period, the forecasts show improved skill when including GPS data. The work implies that the GPS data have good potential for influencing numerical models in rapidly developing, high moisture flux situations.

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.

Calais, E, Haase JS, Minster JB.  2003.  Detection of ionospheric perturbations using a dense GPS array in Southern California. Geophysical Research Letters. 30 AbstractWebsite

[1] We present observations of high-frequency ionospheric perturbations detected using Global Positioning System (GPS) data from the Southern California Integrated GPS Network (SCIGN), a densely spaced GPS array of about 250 stations centered on the Los Angeles area. We show examples of perturbations with 3 - 10 minute periods that may result from coupling between the ionosphere and neutral gravity waves generated in the lower atmosphere. Although the signal-to-noise ratio of the perturbations is relatively small, we show how multi-station array processing techniques can take advantage of the high spatial density of the measurements and the coherence of the signal over a region the size of the SCIGN to considerably improve the detection capability.

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.

Lesne, O, Haase J, Kirchengast G, Ramsauer J, Poetzi W.  2002.  Sensitivity Analysis of GNSS radio occultation for airborne sounding of the troposphere. Phys. and Chem. of the Earth. 27:291-299. AbstractWebsite

The usual geometry for radio occultation sounding using GNSS (Global Navigation Satellite System) signals has the receiver placed on a LEO (Low Earth Orbit) satellite. We investigate a new geometric approach, assuming an airborne rather than a spaceborne receiver. Information on the refractivity structure and hence the pressure, temperature, humidity can be retrieved from accurate airborne measurements of amplitude and phase delay of the signals occulted by the troposphere. We present some advantages and disadvantages for the concept of making measurements from commercial aircraft equipped with proper GNSS receivers and antennae compared to the spaceborne case. We simulated realistic airborne occultation observations and assessed the characteristics of their geometry and sampling. We also compared the dynamic range of the signal with the magnitude of error sources that affect the measurements. Findings include that an airborne system has the potential to provide many more profiles below 10 km height than a single LEO or constellations of up to 25 satellites over the North Atlantic (though with inferior global coverage), and that the SNR (signal to noise ratio) should be better below 5 km than in the LEO case. Though the receiver velocity error is larger than for the LEO system, it is still small enough relative to the signal level to retrieve useful information. Because of the large horizontal drift of the tangent point of up to 450 km, the assumption of spherical symmetry in the existence of significant 3D variations in structure is expected to be a major error source, in addition to the airplane velocity uncertainty.