ALBERT

Description: High-rate GPS and seismic sensors are mutually contributing to seismological applications for capturing earthquake-induced coseismic displacements. In this study, we propose an approach for tightly integrating GPS and strong motion data on raw observation level to increase the quality of the derived displacements. The performance of the proposed approach is demonstrated using 5 Hz high-rate GPS and 200 Hz strong motion data collected during the El Mayor–Cucapah earthquake ( M w 7.2, 2010 April 4) in Baja California, Mexico. The new approach not only takes advantages of both GPS and strong motion sensors, but also improves the reliability of the displacement by enhancing GPS integer-cycle phase ambiguity resolution, which is very critical for deriving displacements with highest quality.

Description: This paper presents an altimetric method for ocean monitoring by remote sensing. It uses carrier observations of reflected GNSS signals. The method is illustrated in a simulation study and applied to a long term experiment yielding an ocean tide spectrum. The altimetric concept is based on residual observations of Doppler frequency. A linear relation between Doppler residuals f0 and height departures ΔH from the surface level is derived. In contrast to existing phase-based methods which are constrained by smooth ocean conditions, the frequency-based retrieval here described holds good for rougher ocean conditions. Two retrievals of Doppler residuals are distinguished: Tracking Retrieval and Spectral Retrieval. A simulation study investigates the performance of Spectral Retrieval for a rough ocean surface with a noise-like sea level deviation ξ(t). Simulation settings were adjusted to reflection events in coastal experiments with an elevation range of [5…15] deg. In this range Tracking Retrieval tolerates a surface standard deviation σξ 〈 5 cm, whereas Spectral Retrieval tolerates σξ ≤ 30 cm. These limits correspond to significant wave heights of 20 cm for Tracking Retrieval and 1.2 m for Spectral Retrieval. The simulation results are confirmed by applying the altimetric method to the experimental data. The recovery of continuous phase tracks in experimental data is onerous and Tracking Retrieval only works for a period of smooth ocean conditions (162 events). By contrast, Spectral Retrieval yields altimetric estimates throughout the whole experiment (2607 events). The altimetric time series extends over more than 60 days and results in a tide spectrum that resolves diurnal (K1) and semidiurnal (M2, S2) constituents. The formal precision for these estimates lies in the decimeter range.

Description: This paper evaluates the usage of reflected GPS signals for Earth observations to study changes of sea level and sea-ice in remote sensing. In a coastal setup, ∼670 m above Disko Bay (Greenland), signals with different carriers L1 and L2 were recorded. A method is presented that analyses the interferometric phase between the reflected and the direct signals and derives the height of the reflecting surface. The analysis includes a ray tracing and an estimation of signal coherence. It is shown that coherent reflections are related to sea-ice coverage. Absolute heights are derived with a time interval of ∼30 min. The altimetric results show semidiurnal tides that are validated using the AODTM-5 tide model. The residual height has a mean of 9.7 cm for L1 and 22.9 cm for L2. The dispersion is not significant but a significant tropospheric bias is detected with an error of up to 20 cm.

Description: The ‘Convective and Orographically-induced Precipitation Study’ (COPS) analyses the processes driving deep convection over complex terrain. Convection initiation (CI) is mainly not only expressed by a single process, but by a variety of them, which interact on different scales in time and space and finally can lead to deep convection. A study of such a case over inhomogeneous terrain is presented in this article. Data from the COPS network of stations are taken to identify the time and location of CI. In many cases this is not the same location as the first convective clouds, showers or even thunderstorms. It is shown that the interaction of the CI processes on multiple scales locally leads to either deep convection and severe storms or calm weather. The boundary conditions between the different outcomes are narrow. During IOP 9c (20 July 2007), a mesoscale convective system (MCS) embedded in a surface low over eastern France propagated north-eastward and a gust front of the MCS reached the COPS area. During the passage of the gust front through the Rhine valley, convective activity was significantly reduced. The gust front reached the slope of the Black Forest, and the warm and humid air above the mountain range prior to the gust front was lifted up to 2000 m within a short time period. Within the air prior to the gust front aloft from the surface, CI started along a north–south oriented line above the crest. Due to insolation ahead of the gust front, a convergence line developed. The interaction of local-scale orographic winds, the regional-scale gust front, the mesoscale convergence line, and the synoptic-scale cold front led to a squall line. Finally, individual severe convective cells formed along the outer section of the synoptic-scale cold front. Copyright © 2011 Royal Meteorological Society

Description: [1]  Sea surface topography observations are deduced from an airborne reflectometry experiment. The GORS (GNSS Occultation Reflectometry Scatterometry) receiver was set up aboard the German HALO (High Altitude LOng range) research aircraft. Flights were conducted over the Mediterranean Sea about 3500 m above sea level. A signal path model divided into large and small scale contributions is used for phase altimetry. The results depict geoid undulations and resolve anomalies of the sea surface topography.For the whole experiment 65 tracks over the Mediterranean sea are retrieved and compared with a topography model. Tracks distinguish between RHCP/LHCP. The differnce, however, is not significant for this study. Precision and spatial resolution decrease disproposionately at low elevations. Eight tracks with centimeter precision are obtained between 11 ∘ and 33 ∘ of elevation. At higher elevation angles the number of tracks is significantly reduced due to surface roughness. In future such retrievals could contribute to ocean eddy detection.

Description: Above the southern Andes range and its prolongation in the Antarctic Peninsula, large-amplitude mountain and shear gravity waves observed with Weather Research and Forecasting (WRF) mesoscale model simulations during winter 2009 are analyzed. Two specific reasons motivated this study: (1) a decade of satellite observations of temperature fluctuations in the stratosphere, allowing us to infer that this region may be launching the largest-amplitude gravity waves into the upper atmosphere, and (2) the recent design of a research program to investigate these features in detail, the Southern Andes Antarctic Gravity wave Initiative (SAANGRIA). The simulations are forced with ERA-Interim data from the European Centre for Medium-Range Weather Forecasts. The approach selected for the regional downscaling is based on consecutive integrations with weekly reinitialization with 24 h of spin-up, and the outputs during this period are excluded from the analysis. From 1 June to 31 August 2009, five case studies were selected on the basis of their outstanding characteristics and large wave amplitudes. In general, one or two prevailing modes of oscillation are identified after applying continuous wavelet transforms at constant pressure levels and perpendicularly to the nominal orientation of the dominant wave crests. In all cases, the dominant modes are characterized by horizontal wavelengths around 50 km. Their vertical wavelengths, depending on a usually strong background wind shear, are estimated to be between 2 and 11 km. The corresponding intrinsic periods range between 10 and 140 min. In general, the estimated vertical wavelength (intrinsic period) maximizes (minimizes) around 250–300 hPa. The synoptic circulation for each case is described. Zonal and meridional components of the vertical flux of horizontal momentum are shown in detail for each case, including possible horizontal wavelengths between 12 and 400 km. Large values of this flux are observed at higher pressure levels, decreasing with increasing height after a progressive deposition of momentum by different mechanisms. As expected, in the wintertime upper troposphere and lower stratosphere in this region, a prevailing zonal component is negative almost everywhere, with the exception of one case above the northern tip of the Antarctic Peninsula. A comparison with previous experimental results reported in the region from in situ and remote sensing measurements suggests a good agreement with the momentum flux profiles computed from the simulations. Partial wave reflection near the tropopause was found, as considerable departures from equipartition between potential and kinetic wave energy are obtained in all cases and at all pressure levels. This ratio was always less than 1 below the lower stratosphere.

Description: [1]  This paper describes an altimetric study based on a Zeppelin flight experiment over Lake Constance. The receiver uses a Master–slave configuration to obtain interferometric observations. These observations contain the relative phasing of direct and reflected signals and are used for altimetry. Separated antennas were attached to the receiver to record direct and reflected signals at slant elevation angles. Due to slant geometry in some cases filtering is required to remove direct contributions. Filtered observations are corrected with an altimetric model and Doppler residuals are retrieved. Correction reduces the width of the spectral reflection peak from 3Hz to less than 10mHz. Doppler residuals are sensitive to the surface height. Lake level is estimated inversely for the residuals at different trial heights. In a case study reflection events are examined and lake level is estimated using right-handed and left-handed circularly polarised observations. Reference level is determined from tide gauge stations around the lake. Mean deviation of estimates from reference level is 50 cm. Doppler shifts of different model corrections are compared. The altimetric correction is the most important one with mean Doppler shifts between 316 and 560mHz. Mean Doppler shifts are much smaller for baseline correction (less than 0.2mHz) and water vapour correction (0.1-1.0mHz). In addition the geoid undulation effect (up to 25 cm amplitude) is predicted with mean Doppler shifts between 0.1 and 0.9mHz. Precision of Doppler residuals (0.5-0.6mHz) is not suitable to resolve the geoid undulation effect. Phase residuals prove to have a better resolution instead. The effect of geoid undulation, however, is not dominating in phase residuals.

Description: The impact of higher-order ionospheric effects on the estimated station coordinates and clocks in Global Navigation Satellite System (GNSS) Precise Point Positioning (PPP) is well documented in literature. Simulation studies reveal that higher-order ionospheric effects have a significant impact on the estimated tropospheric parameters as well. In particular, the tropospheric north-gradient component is most affected for low- and mid-latitude stations around noon. In a practical example we select a few hundred stations randomly distributed over the globe, in March 2012 (medium solar activity) and apply/do not apply ionospheric corrections in PPP. We compare the two sets of tropospheric parameters (ionospheric corrections applied/not applied) and find an overall good agreement with the prediction from the simulation study. The comparison of the tropospheric parameters with the tropospheric parameters derived from the ERA-Interim global atmospheric reanalysis shows that ionospheric corrections must be consistently applied in PPP and the orbit and clock generation. The inconsistent application results in an artificial station dis-placement which is accompanied by an artificial 'tilting' of the troposphere. This finding is relevant in particular for those who consider advanced GNSS tropospheric products for meteorological studies.

Description: A reflectometry station has been setup in 2013 near Ny-Ålesund, Svalbard at 78.9082 ∘ N, 11.9031 ∘ E. The main goal of the setup is to resolve the spatial and temporal variations in snow and ice cover, based on reflection power observations at grazing elevations. In this study, we develop a method to map the recorded signal power to the main reflection contributions while also discussing the spatial characteristics of the observations. A spectral analysis resolving differential Doppler between direct and reflected signals is presented to identify reflections contributions for a complete year (2014). Strong water reflections are identified with power ratios higher than 70 dB/Hz and constant Doppler shifts of 0.5 − 0.6 Hz for all elevations. Contributions with ratios higher than 40 dB/Hz can be related to specular land or glacier reflections, for which Doppler shift usually increases with the elevation angle and the distance between reflection point and receiver. Reflections nearby, around 3 − 5 km, show differential Doppler of 0.4 − 0.5 Hz while for reflections further than 16 km away, Doppler shift is usually larger than 0.8 Hz. Azimuth variations cause cross-track drift of up to 4 ∘ during the observation year. Topography-induced shadowing of very low-lying satellites limits the extent of the monitoring area. However, the amount of satellites tracked daily, up to 30, allows the GNSS-R station to constantly record reflections over areas with thick snow cover and glaciers. This offers the possibility to compare the derived reflected power with local meteorological data to resolve snow and ice variations on the area.

Description: [1]  This paper presents an altimetric method for ocean monitoring by remote sensing. It uses carrier observations of reflected GNSS signals. The method is illustrated in a simulation study and applied to a long term experiment yielding an ocean tide spectrum. The altimetric concept is based on residual observations of Doppler frequency. A linear relation between Doppler residuals f 0 and height departures Δ H from the surface level is derived. In contrast to existing phase-based methods which are constrained by smooth ocean conditions, the frequency-based retrieval here described holds good for rougher ocean conditions. Two retrievals of Doppler residuals are distinguished: Tracking Retrieval and Spectral Retrieval. A simulation study investigates the performance of Spectral Retrieval for a rough ocean surface with a noise-like sea level deviation ξ ( t ). Simulation settings were adjusted to reflection events in coastal experiments with an elevation range of [5…15] deg. In this range Tracking Retrieval tolerates a surface standard deviation σ ξ 〈 5 cm, whereas Spectral Retrieval tolerates σ ξ ≤ 30 cm. These limits correspond to significant wave heights of 20 cm for Tracking Retrieval and 1.2 m for Spectral Retrieval. The simulation results are confirmed by applying the altimetric method to the experimental data. The recovery of continuous phase tracks in experimental data is onerous and Tracking Retrieval only works for a period of smooth ocean conditions (162 events). By contrast, Spectral Retrieval yields altimetric estimates throughout the whole experiment (2607 events). The altimetric time series extends over more than 60 days and results in a tide spectrum that resolves diurnal (K1) and semidiurnal (M2, S2) constituents. The formal precision for these estimates lies in the decimeter range.