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Zhang G.,China University of Geosciences | Zhang G.,University of Texas at San Antonio | Zhang G.,East China Institute of Technology | Xie H.,University of Texas at San Antonio | And 4 more authors.
Remote Sensing of Environment | Year: 2011

In this study, ICESat altimetry data are used to provide precise lake elevations of the Tibetan Plateau (TP) during the period of 2003-2009. Among the 261 lakes examined ICESat data are available on 111 lakes: 74 lakes with ICESat footprints for 4-7. years and 37 lakes with footprints for 1-3. years. This is the first time that precise lake elevation data are provided for the 111 lakes. Those ICESat elevation data can be used as baselines for future changes in lake levels as well as for changes during the 2003-2009 period. It is found that in the 74 lakes (56 salt lakes) examined, 62 (i.e. 84%) of all lakes and 50 (i.e. 89%) of the salt lakes show tendency of lake level increase. The mean lake water level increase rate is 0.23. m/year for the 56 salt lakes and 0.27. m/year for the 50 salt lakes of water level increase. The largest lake level increase rate (0.80. m/year) found in this study is the lake Cedo Caka. The 74 lakes are grouped into four subareas based on geographical locations and change tendencies in lake levels. Three of the four subareas show increased lake levels. The mean lake level change rates for subareas I, II, III, IV, and the entire TP are 0.12, 0.26, 0.19, -0.11, and 0.2. m/year, respectively. These recent increases in lake level, particularly for a high percentage of salt lakes, supports accelerated glacier melting due to global warming as the most likely cause. © 2011 Elsevier Inc. Source

Gurnett D.A.,University of Iowa | Morgan D.D.,University of Iowa | Granroth L.J.,University of Iowa | Cantor B.A.,Space Sciences Inc | And 2 more authors.
Geophysical Research Letters | Year: 2010

Here we report the results of a nearly five-year search for impulsive radio signals from lightning discharges in Martian dust storms using the radar receiver on the Mars Express spacecraft. The search covered altitudes from 275 km to 1400 km and frequencies from 4.0 to 5.5 MHz with a time resolution of 91.4 s and a detection threshold of 2.8 × 10-18 Watts m -2 Hz-1. At comparable altitudes the intensity of terrestrial lightning is several orders of magnitude above this threshold. Although two major dust storms and many small storms occurred during the search period, no credible detections of radio signals from lightning were observed. © 2010 by the American Geophysical Union. Source

Padula F.P.,Integrity Applications Incorporated | Schott J.R.,Rochester Institute of Technology | Barsi J.A.,Space Sciences Inc | Raqueno N.G.,Rochester Institute of Technology | Hook S.J.,Jet Propulsion Laboratory
Canadian Journal of Remote Sensing | Year: 2010

The Landsat 5 thermal band lifetime calibration is being updated based on an improved calibration method that uses water temperatures observed by buoys at deep water sites and thermal and radiative transfer models. An uncertainty propagation analysis was constructed to determine the expected uncertainty in temperature (one standard deviation) at the sensor for this historic vicarious calibration process. The historical calibration effort fused environmental data sources that feed a forward modeling vicarious calibration process. The process consists of three major modeling efforts: subsurface temperature to water skin temperature, atmospheric radiative transfer, and sensor noise modeling. Each modeling effort was investigated uniquely, and the results were combined to derive the total process error. The uncertainty propagation results indicate that the historic vicarious calibration process has an expected uncertainty of ±0.5 K. This conclusion is consistent with the observed root mean square error (RMSE) between observed and predicted values using this method. After the instrument calibration was updated, the difference between instrument-derived radiance (observed data spanning a 23 year period) and radiance estimated using the subsurface buoy temperatures was 0.6 K RMSE. This demonstrates that the residual error in the observed calibration results and the expected process uncertainty are essentially comparable. Using the error analysis results, the data from the historical buoy temperature study were combined with data from traditional surface temperature and radiance studies (1999-2000) to generate a lifetime calibration update for the Landsat 5 instrument. The final calibration uses data from the long established thermal calibration sites on the Great Lakes (Erie and Ontario), the Salton Sea, and Lake Tahoe, as well as a number of additional deep water sites where National Oceanic and Atmospheric Administration (NOAA) bouys and atmospheric sounding data provide adequate ground truth for the historical calibration approach. This updated calibration has been implemented in the U.S. Geological Survey (USGS) - National Aeronautics and Space Administration (NASA) processing system. These results indicate that the image data is calibrated to better than 0.67 K (one sigma) over its 25+ year record. While this work rigorously investigated the historic thermal vicarious calibration process for Landsat 5 Thematic Mapper (TM), the approach and the new study sites can be easily extended to the investigation of similar systems. © 2010 CASI. Source

Xie H.,University of Texas at San Antonio | Tekeli A.E.,University of Texas at San Antonio | Tekeli A.E.,King Saud University | Ackley S.F.,University of Texas at San Antonio | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2013

Sea ice thicknesses derived from NASA's Ice, Cloud, and Land Elevation Satellite (ICESat) altimetry data are examined using two different approaches, buoyancy and empirical equations, and at two spatial scales - ICESat footprint size (70 m diameter spot) and Advanced Microwave Scanning Radiometer (AMSR-E) pixel size (12.5 km by 12.5 km) for the Bellingshausen and Amundsen Seas of west Antarctica. Ice thickness from the empirical equation shows reasonable spatial and temporal distribution of ice thickness from 2003 to 2009. Ice thickness from the buoyancy equation, however, additionally needing snow depth information derived from the AMSR-E, shows an overestimation in terms of maximum, mean (+63% to 75%), and standard deviation while underestimation in modal thickness (-20%) as compared with those from the empirical equation approach. When ICESat snow freeboard is used as the snow depth in the buoyancy equation, i.e., the zero ice freeboard assumption, the derived ice thicknesses match well with those from the empirical equation approach, within 5% overall. The AMSR-E, therefore, may underestimate snow depth and accounts for ~95% of the ice thickness overestimation as compared with the buoyancy approach. The empirical equation derived ice thickness shows a consistent asymmetrical distribution with a long tail to high values, and seasonal median values ranging from 0.8 to 1.4 m over the 2003-2009 period that are always larger than the corresponding modal values (0.6-1.1 m) and lower than the mean values (1.0-1.6 m), with standard deviation of 0.6-1.0 m. An overall increasing trend of 0.03 m/year of mean ice thickness is found from 2003 to 2009, although statistically insignificant (p = 0.11) at the 95% confidence level. Starting from autumn, a general picture of seasonal mean, modal, and median ice thickness increases progressively from autumn to spring and decreases from spring to the following autumn, when new thin ice dominates the ice thickness distribution. The asymmetric shape of the thickness distribution reflects the key role of ice deformation processes in the evolution of the thickness distribution. The statistical properties of the thickness distribution interannually (high range of mean thickness and standard deviation) indicate the variability of deformation processes. However, spring ice volume, the product of ice mean thickness and areal extent computed for the spring maximum, shows variability year to year but is primarily dominated by ice extent variability, with no increasing or decreasing trend over this record length. The dependence of the volume on the ice extent primarily suggests that ice thickness changes have also not covaried with the ice extent losses seen over the satellite record in this region, unlike the Arctic. These properties reflect the interactive processes of ice advection, thermodynamic growth and ice deformation that all substantially influence ice mass balance in the Bellingshausen-Amundsen Seas region. ©2013. American Geophysical Union. All Rights Reserved. Source

Huang X.,Lanzhou University | Huang X.,University of Texas at San Antonio | Xie H.,University of Texas at San Antonio | Liang T.,Lanzhou University | Yi D.,Space Sciences Inc
International Journal of Remote Sensing | Year: 2011

The Geoscience Laser Altimeter System (GLAS) instrument onboard the Ice, Cloud and land Elevation Satellite (ICESat) provides elevation data with very high accuracy which can be used as ground data to evaluate the vertical accuracy of an existing Digital Elevation Model (DEM). In this article, we examine the differences between ICESat elevation data (from the 1064 nm channel) and Shuttle Radar Topography Mission (SRTM) DEM of 3 arcsec resolution (90 m) and map-based DEMs in the Qinghai-Tibet (or Tibetan) Plateau, China. Both DEMs are linearly correlated with ICESat elevation for different land covers and the SRTM DEM shows a stronger correlation with ICESat elevations than the map-based DEM on all land-cover types. The statistics indicate that land cover, surface slope and roughness influence the vertical accuracy of the two DEMs. The standard deviation of the elevation differences between the two DEMs and the ICESat elevation gradually increases as the vegetation stands, terrain slope or surface roughness increase. The SRTM DEM consistently shows a smaller vertical error than the map-based DEM. The overall means and standard deviations of the elevation differences between ICESat and SRTM DEM and between ICESat and the map-based DEM over the study area are 1.03 ± 15.20 and 4.58 ± 26.01 m, respectively. Our results suggest that the SRTM DEM has a higher accuracy than the map-based DEM of the region. It is found that ICESat elevation increases when snow is falling and decreases during snow or glacier melting, while the SRTM DEM gives a relative stable elevation of the snow/land interface or a glacier elevation where the C-band can penetrate through or reach it. Therefore, this makes the SRTM DEM a promising dataset (baseline) for monitoring glacier volume change since 2000. © 2011 Taylor & Francis. Source

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