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Ad Stoffelen - One of the best experts on this subject based on the ideXlab platform.
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Assessing the High and Extreme-force Sea Surface Wind Capabilities for the Next Generation of Spaceborne Scatterometers
'European Space Agency', 2020Co-Authors: Portabella Marcos, Polverari Federica, Lin Wenming, Sapp Joe, Ad Stoffelen, Mouche, Alexis Aurélien, Verhoef Anton, Jelenak Zorana, Chang PaulAbstract:2019 Living Planet Symposium, 13-17 May 2019, Milan, ItalyGlobal information on the motion near the ocean surface is generally lacking, limiting the physical modelling capabilities of the forcing of the world’s water surfaces by the atmosphere. This also limits our knowledge of the exchange of momentum across the ocean-atmosphere interface, affecting meteorological and ocean applications. A particularly pressing requirement in the Ocean Surface Vector Wind (OSVW) community is to obtain reliable extreme winds in hurricanes (> 30 m/s) from wind Scatterometers, since extreme weather classification, surge and wave forecasts for societal warning are a high priority in nowcasting and in Numerical Weather Prediction (NWP). The main goal of this study is therefore to consolidate a wind reference for assessing Scatterometer high and extreme-force wind capabilities. Scatterometers have proved to have very good performances when retrieving low to moderate winds. Over the years, improvements have been done on the development of the empirical Geophysical Model Functions (GMF) in order to obtain more accurate wind estimates with respect to in situ measurements. However, measuring high and extreme winds is still challenging as vicarious calibration is needed and calibrated in situ reference winds are scarce. Theoretical statistical descriptions of the high-wind ocean surface, where patchy foam, droplets, spume and wave breaking occurs are much simplified, while the microwave interaction on cm scales is rather complex too. Moored buoy data are usually used as absolute reference to calibrate the GMFs, however, for very high and extreme winds above 25 m/s, moored buoys may not be reliable. Moreover, controversy exists in the OSVW satellite community on the quality of moored buoys above 15 m/s rather than 25 m/s. Therefore, collaboration has been sought with the National Oceanic and Atmospheric Administration (NOAA) Ocean Science Team hurricane hunters to have additional high and extreme winds reference data sets. The NOAA hurricane hunters fly into hurricanes to drop sondes, and thus obtain wind profiles in the lowest few kilometers of hurricanes, and operate dedicated microwave instrumentation on aircraft to obtain detailed wind patterns in hurricanes, such as the Stepped-Frequency Microwave Radiometer (SFMR). Ideally, local dropsonde winds may be statistically used to calibrate SFMR as they have similar spatial representation (“footprint”), which in turn, after spatial aggregation to Scatterometer footprints, may be used to calibrate satellite Scatterometers and radiometers in overflights. Although this approach is credible in principle and physically more consistent than any other global method to obtain maximum winds in hurricanes, research is ongoing to understand the exact physical interpretation of dropsonde winds and SFMR in the inherently extremely variable conditions in tropical hurricanes for which more hurricane flight data is required. Since processing artifacts are known to exist for both dropsondes and SFMR, NOAA has recently reprocessed both data sets to provide a more accurate reference. In the framework of the EUMETSAT-funded “C-band High and Extreme-Force Speeds (CHEFS)” project, the main goal is to assess the extreme wind capabilities of the next generation of C-band wind Scatterometers onboard Metop Second Generation. One important goal within CHEFS is to provide an appropriate high and extreme-wind reference at Scatterometer scales. To this end, five wind data sets have been collected so far: (i) different types of moored buoy data; (ii) NOAA-reprocessed SFMR 10m surface winds from 2008 to 2017; (iii) NOAA dropsonde wind profiles from 1996 to 2017; (iv) reprocessed ASCAT-A winds at 12.5 km resolution; and (v) the latest European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset ERA5 from 2007 to 2017. The quality of buoy winds between 15 m/s and 25 m/s is thoroughly evaluated. Analysis of the wind characteristics of different types of moored buoys in terms of height and mooring is performed. The buoy wind performance, estimated with triple collocation analyses of buoy, ASCAT and ERA5 winds, shows that although buoy wind uncertainty increases with wind speed, the quality of buoy wind vectors up to 25 m/s is within 2 m/s, indicating that buoy winds can indeed be used for wind Scatterometer GMF calibration in the mentioned wind range. An accurate calibration procedure for surface high winds retrieved from SFMR will be also developed using dropsonde data as reference. To this end, a SFMR/dropsonde collocation criterion is defined. In addition, the so-called WL150 algorithm used to estimate 10m surface winds from dropsonde wind profiles is also revised and its impact on the SFMR calibration process is tested. SFMR wind data are also analyzed at different temporal/spatial scales in order to assess the spatial representativeness error when compared against 12.5 km resolution ASCAT winds for different wind conditions. Moreover, the SFMR rain data are also used to evaluate the impact of rain on both SMFR and ASCAT winds, notably for winds higher than 25 m/s. The findings of this analysis will be then used to investigate extreme winds cases such as those close to the hurricane eyewall. In such high wind gradient conditions, new SMFR-dropsonde collocation approaches will also be explored in order to reduce the collocation error and improve the wind consistency in the analysis, which should lead to a more suitable SFMR calibration at extreme wind conditions. Finally, SFMR calibrated winds can be aggregated along the flight track and used as wind reference for re-tuning the C-band vertically-polarized (e.g., ASCAT) and cross-polarized (e.g., Sentinel-1) GMFs in preparation for the Metop Second Generation Scatterometer (SCA). The latest results of this ongoing work will be presented at the conferenc
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Development of an ASCAT coastal wind product
'European Space Agency', 2020Co-Authors: Portabella Marcos, Ad Stoffelen, Verhoef Anton, Vogelzang J.Abstract:Second EPS/MetOp RAO Workshop, 20-22 May 2009 Barcelona, Spain.-- 6 pages, 4 figures, 3 tablesThe Advanced Scatterometer, ASCAT, on MetOp-A was launched on 19 October 2006 as the third wind Scatterometer currently in space joining up with the ERS-2 and the SeaWinds Scatterometers. Scatterometers measure the radar backscatter from wind-generated cm-size gravity-capillary waves and provide high-resolution wind vector fields over the sea with high quality. In this paper we show progress in high resolution processing and its verification, and in processing closer to the coast
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Improvement of the ERA* high-resolution forcing product: benefits from the Scatterometer constellation
'European Space Agency', 2020Co-Authors: Portabella Marcos, Ad Stoffelen, Verhoef Anton, Trindade Ana, Vall-llossera MercèAbstract:2019 Living Planet Symposium, 13-17 May 2019, Milan, ItalyHigh-resolution satellite derived sea surface wind data, such as those from Scatterometers, are increasingly required for operational monitoring and forecasting of the ocean. We present an improved version of ERA*, an ocean forcing product which keeps the time and space coverage of atmospheric model fields, but adds the accurately observed local mean and variability of wind Scatterometers, to make these datasets suitable for, among others, high-resolution ocean model forcing. Recent attempts of combining Scatterometer data and numerical weather prediction (NWP) outputs, i.e., blended ocean forcing products, allows for an increased temporal resolution (e.g., daily) but generally only resolves NWP spatial scales of about 100-150 km. Therefore, information on the wind-current interaction, the diurnal wind cycle and the wind variability in moist convection areas is lost in such products. Moreover, known systematic NWP model (parameterization) errors are in fact propagated at times and locations where no Scatterometer winds are available. The alternative, direct forcing from NWP results in even more extensive physical drawbacks. We propose to maintain the increased temporal coverage in a gridded wind and stress product (ERA*), but also to maintain the most beneficial physical qualities of the Scatterometer winds, i.e., 25-km spatial resolution, wind-current interaction, variability due to moist convection, etc., and, at the same time correct the large-scale NWP parameterization and dynamical errors. Additionally, we correct these winds for the effects of atmospheric stability and mass density, using stress equivalent 10 m winds, U10S. A Scatterometer-based correction, using accurate, unbiased, high spatial resolution ocean vector winds from several Scatterometers, i.e., the Advanced Scatterometers (ASCATs) on board Metop satellites and the OSCAT Scatterometer onboard Oceansat-2, is proposed as a new angle to tackle this problem, i.e., to reduce NWP local wind biases accounting for satellite sampling characteristics (note that the Scatterometer temporal sampling is latitudinal dependent and rather poor in the tropics). The correction consists of geolocated (i.e., at every ocean grid point), temporally - averaged differences between the Scatterometer and the collocated NWP reanalysis (ERA-interim) U10S. Since ERA local biases are relatively persistent over time but such persistence is regionally dependent (e.g., such persistence is longer in the tropics than in the extratropics), we test different configurations of ERA*, i.e., with different temporal windows (from 1 to 5 days) and varying number of Scatterometers (i.e., different combinations of the above mentioned Scatterometer systems) in order to find the best quality forcing product. The new ERA* gridded ocean forcing product is validated against independent Scatterometer data, i.e., the 25-km HSCAT (onboard HY-2A) wind product. HSCAT is a good wind reference since the orbit pass (6am/6pm) is very different from that of ASCAT-A/B (9:30am/9:30pm) and OSCAT (12:00am/12:00pm). Globally, there is a reduction of the vector root-mean-square error in ERA* w.r.t. ERA interim of more than 10%. Overall, the 1-day temporal window and the multiple Scatterometer correction lead to the best quality ERA* global wind product, although such wind quality depends on the region. In particular, this ERA* product outperforms ERA in the tropics, where moist convection and wind-current interaction are not well resolved by the latter. Scores of the bias and standard deviation error support the above statements. Moreover, in contrast with ERA, ERA* is able to resolve eddy scales similar to those resolved by Scatterometers, as shown by wind spectral analysi
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Advancements in Scatterometer Wind Processing
EUMETSAT, 2020Co-Authors: Ad Stoffelen, Portabella Marcos, Verhoef Anton, Verspeek Jeroen, Vogelzang J.Abstract:Ninth International Winds Workshop, Annapolis, Maryland, USA, 14-18 April 2008.-- 8 pages, 7 figures, 1 tableThe EUMETSAT Advanced Scatterometer ASCAT on MetOp-A was launched on 19 October 2006 as the third wind Scatterometer currently in space joining up with the ESA ERS-2 and the NASA SeaWinds Scatterometers. Scatterometers measure the radar backscatter from wind generated cm-size gravity-capillary waves and provide high-resolution wind vector fields over the sea. Wind speed and wind direction are provided with high quality and uniquely define the mesoscale wind vector field at the sea surface. The all-weather ERS Scatterometer observations have proven important for the forecasting of dynamical and severe weather. Oceanographic applications have been initiated using winds from SeaWinds on QuikScat, since Scatterometers provide unique forcing information on the ocean eddy scale. Together, ERS-2, ASCAT and SeaWinds provide good coverage over the oceans and are now used routinely in marine and weather forecasting. In this paper we show progress in high resolution processing and its verification, in providing gridded winds with mesoscale detail, and in processing closer to the coast with improved geophysical interpretatio
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ERA*: Towards an eddy resolving ocean forcing
European Geosciences Union, 2020Co-Authors: Trindade Ana, Portabella Marcos, Ad Stoffelen, Verhoef Anton, Vall-llossera MercèAbstract:European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 pageHigh-resolution satellite derived sea surface wind data, such as those from Scatterometers, are increasing lyrequired for operational monitoring and forecasting of the ocean. We present an improved version of ERA*, an ocean forcing product which keeps the time and space coverage of atmospheric model fields, but adds the accurately observed local mean and variability of wind Scatterometers, to make these datasets suitable for, among others, high-resolution ocean forcing. Recent attempts of combining Scatterometer data and numerical weather prediction (NWP) outputs, i.e. blended ocean forcing products, allows for an increased temporal resolution (e.g., daily) but generally only resolves NWP spatial scales of about 100-150 km. Therefore, information on the wind-current interaction, the diurnal wind cycle and the wind variability in moist convection areas is lost in such products. Moreover, known systematic NWP model (parameterization) errors are in fact propagated at times and locations where no Scatterometer winds are available. The alternative, direct forcing from NWP results in even more extensive physical drawbacks. We propose to maintain the increased temporal coverage in a gridded wind and stress product (ERA*), but also to maintain the most beneficial physical qualities of the Scatterometer winds, i.e. 25-km spatial resolution, wind-current interaction, variability due to moist convection, etc., and, at the same time correct the large-scale NWP parameterization and dynamical errors. Additionally, we correct these winds for the effects of atmospheric stability and mass density, using stress equivalent 10 m winds, U10S. A Scatterometer-based correction, using accurate, unbiased, high spatial resolution ocean vector winds from several Scatterometers, i.e. the Advanced Scatterometers (ASCATs) on board Metop satellites and the OSCAT Scatterometer onboard Oceansat-2, is proposed as a new angle to tackle this problem, i.e. to reduce NWP local wind biases accounting for satellite sampling characteristics. Since ERA local biases are relatively persistent over time but such persistence is regionally dependent (e.g., persistency is longer in the tropics than in the extratropics), we test different configurations of ERA*, i.e. with different temporal windows (from 1 to 5 days) and varying number of Scatterometers (i.e. different combinations of the above mentioned Scatterometer systems) to find the best quality forcing product. The new ERA* gridded ocean forcing product is validated against independent Scatterometer data, i.e. the 25-km HSCAT (onboard HY-2A) wind product. HSCAT is a good wind reference since the orbit pass (6am/6pm) is very different from that of ASCAT-A/B (9:30am/9:30pm) and OSCAT (12:00am/12:00pm). Globally, there is a reduction of the vector root-mean-square error in ERA* w.r.t. ERA interim of more than 10%. Overall, the 1-day temporal window and the multiple Scatterometer correction lead to the best quality ERA* global wind product, although such wind quality depends on the region. In particular, this ERA* product outperforms ERA in the tropics, where moist convection and wind-current interaction are not well resolved by the latter. Scores of the bias and standard deviation error support the above statements. Moreover, in contrast with ERA, ERA* is able to resolve eddy scales similar to those resolved by Scatterometers, as shown by wind spectral analysi
D G Long - One of the best experts on this subject based on the ideXlab platform.
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calibration and validation of the rapidscat Scatterometer using tropical rainforests
IEEE Transactions on Geoscience and Remote Sensing, 2016Co-Authors: Nathan M Madsen, D G LongAbstract:Launched in September2014, RapidScat is currently operating on the International Space Station (ISS). RapidScat estimates ocean vector winds via the measurement of the normalized radar coefficient (σ 0 ) of the ocean's surface. Measurements are also collected over land. The ISS orbit permits, for the first time, the observation of the diurnal variation in Ku-band σ 0 at midto high-incidence angles. To complement calibration efforts over the ocean, in this paper the calibration and validation of the σ 0 measurements are performed using natural land targets, namely the Amazon and Congo rainforests. The diurnal σ 0 cycle of the targets is estimated with respect to incidence angle, azimuth angle, and season using measurements from previous sensors. Understanding this diurnal backscatter response enables the comparison of RapidScat measurements with measurements from the QuikSCAT, NASA Scatterometer, SeaWinds, and Oceansat-II Scatterometers. RapidScat σ 0 measurements are found to be consistent but biased low compared to those of QuikSCAT by up to 0.3 dB. The effectiveness of slice balancing is evaluated and found to be dependent on the pitch of the ISS. Extreme pitches of the ISS are also found to introduce azimuth dependencies in egg measurements. By accounting for seasonal and diurnal cycles, we find that the rainforests are well suited for Scatterometer sensor cross-calibration, even for disjoint years.
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high resolution measurements with a spaceborne pencil beam Scatterometer using combined range doppler discrimination techniques
IEEE Transactions on Geoscience and Remote Sensing, 2003Co-Authors: M W Spencer, Wuyang Tsai, D G LongAbstract:Conically scanning pencil-beam Scatterometer systems, such as the SeaWinds radar, constitute an important class of instruments for spaceborne climate observation. In addition to ocean winds, Scatterometer data are being applied to a wide range of land and cryospheric applications. A key issue for future Scatterometer missions is improved spatial resolution. Pencil-beam Scatterometers to date have been real-aperture systems where only range discrimination is used, resulting in a relatively coarse resolution of approximately 25 km. In this paper, the addition of Doppler discrimination techniques is proposed to meet the need for higher resolution. The unique issues associated with the simultaneous application of range and Doppler processing to a conically scanning radar are addressed, and expressions for the theoretical measurement performance of such a system are derived. Important differences with side-looking imaging radars, which also may employ Doppler techniques, are highlighted. Conceptual design examples based on Scatterometer missions of current interest are provided to illustrate this new high-resolution Scatterometer approach. It is shown that spatial resolution of pencil-beam Scatterometer systems can be improved by an order of magnitude by utilizing combined range/Doppler discrimination techniques, while maintaining the wide-swath and constant incidence angle needed for many geophysical measurements.
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sea ice extent mapping using ku band Scatterometer data
Journal of Geophysical Research, 1999Co-Authors: Q P Remund, D G LongAbstract:Although spaceborne Scatterometers such as the NASA Scatterometer have inherently low spatial resolution, resolution enhancement techniques can be used to increase the utility of Scatterometer data in monitoring sea-ice extent in the polar regions, a key parameter in the global climate. The resolution enhancement algorithm produces images of A and B, where A is the normalized radar backscatter coefficient σO at 40° incidence and B is the incidence angle dependence of σO. Dual-polarization A and B parameters are used to identify sea ice and ocean pixels in composite images. The A copolarization ratio and vertically polarized B are used as primary classification parameters to discriminate between sea ice and open ocean. Estimates of the sea-ice extent are obtained using linear and quadratic (Mahalanobis distance) discriminant boundaries. The distribution parameters needed for the quadratic estimate are taken from the linear estimate. The σO error variance is used to reduce errors in the linear and Mahalanobis ice/ocean classifications. Noise reduction is performed through binary image region growing and erosion/dilation techniques. The resulting edge closely matches the NASA Team algorithm special sensor microwave imager derived 30% ice concentration edge. A 9-month data set of global sea-ice extent maps is produced with one 6-day average map every 3 days.
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radar backscatter measurement accuracy for a spaceborne pencil beam wind Scatterometer with transmit modulation
IEEE Transactions on Geoscience and Remote Sensing, 1997Co-Authors: D G Long, M SpencerAbstract:Scatterometers are remote sensing radars designed to measure near-surface winds over the ocean. The difficulties of accommodating traditional fan-beam Scatterometers on spacecraft has lead to the development of a scanning pencil-beam instrument known as SeaWinds. SeaWinds will be part of the Japanese Advanced Earth Observing Satellite II (ADEOS-II) to be launched in 1999. To analyze the performance of the SeaWinds design, a new expression for the measurement accuracy of a pencil-beam system is required. In this paper the authors derive a general expression for the backscatter measurement accuracy for a pencil-beam Scatterometer which includes the effects of transmit signal modulation with simple power detection. Both separate and simultaneous signal+noise and noise-only measurements are considered. The utility of the new expression for Scatterometer design tradeoffs is demonstrated using a simplified geometry. A separate paper, ibid., 1997, describes detailed tradeoffs made to develop the SeaWinds design.
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spaceborne radar measurement of wind velocity over the ocean an overview of the nscat Scatterometer system
Proceedings of the IEEE, 1991Co-Authors: F M Naderi, Michael H Freilich, D G LongAbstract:Scatterometry and Scatterometer design issues are reviewed. The design of the NASA Scatterometer (NSCAT) to be flown on the Japanese ADEOS mission is presented. Building on Seasat experience, the NSCAT system includes several enhancements, such as three antenna azimuths in each of two swaths, and an onboard digital Doppler processor to allow backscatter measurements to be colocated everywhere within the orbit. These enhancements will greatly increase the quality of the NSCAT wind data. The ground processing of data is discussed, and Scatterometers of the next decade are briefly described. >
David G. Long - One of the best experts on this subject based on the ideXlab platform.
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Ultrahigh Resolution Scatterometer Winds near Hawaii
Remote Sensing, 2020Co-Authors: Nolan Hutchings, Thomas Kilpatrick, David G. LongAbstract:Hawaii regional climate model (HRCM), QuikSCAT, and ASCAT wind estimates are compared in the lee of Hawaii’s Big Island with the goal of understanding ultrahigh resolution (UHR) Scatterometer wind retrieval capabilities in this area, which includes a reverse-flow toward the island in the lee of the predominate flow. A comparison of Scatterometer measured σ 0 and model predicted σ 0 suggests that Scatterometers can detect the reverse flow in the lee of the island; however, neither QuikSCAT- nor ASCAT-estimated winds consistently report this flow. Furthermore, the Scatterometer UHR winds do not resolve the wind direction features predicted by the HRCM. Differences between Scatterometer measured σ 0 and HRCM predicted σ 0 indicate possible error in the placement of key reverse flow features predicted by the HRCM. We find that coarse initialization fields and a large size median filter windows used in ambiguity selection can impede the accuracy of the UHR wind direction retrieval in this area, suggesting the need for further development of improved near-coastal ambiguity selection algorithms.
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Architectures for Earth-observing CubeSat Scatterometers
CubeSats and NanoSats for Remote Sensing II, 2018Co-Authors: M. Patrick Walton, David G. LongAbstract:Earth-observing satellite Scatterometers are important instruments capable of measuring a variety of geophysical properties. Historically, the Scatterometer design space has revolved around two main architectures: the fan beam and the scanning pencil beam. Since the implementation of these architectures, developments in satellite- relevant technology, spacecraft standards, and engineering practice have expanded the potential design space for Earth-observing Scatterometer systems. This expanded design space is investigated and example designs are presented that utilize the expanded design space to improve performance and reduce cost.
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Polar Applications of Spaceborne Scatterometers
IEEE journal of selected topics in applied earth observations and remote sensing, 2016Co-Authors: David G. LongAbstract:Wind Scatterometers were originally developed for observation of near-surface winds over the ocean. They retrieve wind indirectly by measuring the normalized radar cross section ( $\sigma ^o$ ) of the surface, and estimating the wind via a geophysical model function relating $\sigma ^o$ to the vector wind. The $\sigma ^o$ measurements have proven to be remarkably capable in studies of the polar regions where they can map snow cover; detect the freeze/thaw state of forest, tundra, and ice; map and classify sea ice; and track icebergs. Further, a long time series of Scatterometer $\sigma ^o$ observations is available to support climate studies. In addition to fundamental scientific research, Scatterometer data are operationally used for sea-ice mapping to support navigation. Scatterometers are, thus, invaluable tools for monitoring the polar regions. In this paper, a brief review of some of the polar applications of spaceborne wind Scatterometer data is provided. The paper considers both C-band and Ku-band Scatterometers, and the relative merits of fan-beam and pencil-beam Scatterometers in polar remote sensing are discussed.
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Global ice and land climate studies using Scatterometer image data
Eos Transactions American Geophysical Union, 2001Co-Authors: David G. Long, Mark R. Drinkwater, Benjamin Holt, Sasan S. Saatchi, Cheryl BertoiaAbstract:Scatterometers have provided continuous synoptic microwave radar coverage of the Earth from space for nearly a decade. NASA launched three Scatterometers: the current SeaWinds Scatterometer onboard QuikSCAT (QSCAT, 13.4 GHz) launched in 1999; the NASA Scatterometer (NSCAT, 14.0 GHz), which flew on the Japanese Space Agency's ADEOS-1 platform during 1996–1997; and the Seasat-A Scatterometer system (SASS, 14.6 GHz), which flew in 1978. The European Space Agency's (ESA) 5.3-GHz Scatterometer (ESCAT) has been carried onboard both the ERS-1 and ERS-2 satellites since 1991. properties, including the phase state, of a particular surface type. Varying response from the surface also results from different polarizations, viewing angles and orientations, and radar frequencies. The wide swath of Scatterometers provides near daily global coverage at intrinsic sensor resolutions that are generally between 25–50 km.
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Status of the SeaWinds Scatterometer on QuikScat
Earth Observing Systems IV, 1999Co-Authors: David G. LongAbstract:The QuikScat satellite carrying the SeaWinds Scatterometer was developed as a replacement mission for the aborted Japanese Advanced Earth Observation System-I (ADEOS-I) mission carrying the NASA Scatterometer. Like NSCAT, SeaWinds is an active microwave remote sensor designed to measure winds over the ocean from space. SeaWinds can measure vector winds over 95 percent of the Earth's ice-free oceans every day, a significant improvement over previous Scatterometers. Such data is expected to have a significant impact on weather forecasting and will support air-sea interaction studies. SeaWinds will also fly aboard ADEOS-I sensor scheduled for launch in Nov. 2000. QuikScat was successfully launched on June 19, 1999, though as of this writing the instrument has not ben turned on. This paper provides a brief overview of the SeaWinds instrument and discussed new applications of Scatterometer data for the study of land and ice.
Klaus Scipal - One of the best experts on this subject based on the ideXlab platform.
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assimilation of a ers Scatterometer derived soil moisture index in the ecmwf numerical weather prediction system
Advances in Water Resources, 2008Co-Authors: Klaus Scipal, M Drusch, Wolfgang WagnerAbstract:Abstract The European Centre for Medium-Range Weather Forecasts (ECMWF) currently prepares the assimilation of soil moisture data derived from advanced Scatterometer (ASCAT) measurements. ASCAT is part of the MetOp satellite payload launched in November 2006 and will ensure the operational provision of soil moisture information until at least 2020. Several studies showed that soil moisture derived from Scatterometer data contain skillful information. Based on data from its predecessor instruments, the ERS-1/2 Scatterometers we examine the potential of future ASCAT soil moisture data for numerical weather prediction (NWP). In a first step, we compare nine years of the ERS Scatterometer derived surface soil moisture index (ΘS) against soil moisture from the ECMWF re-analysis (ERA40) data set (ΘE) to (i) identify systematic differences and (ii) derive a transfer function which minimises these differences and transforms ΘS into model equivalent volumetric soil moisture Θ S ∗ . We then use a nudging scheme to assimilate Θ S ∗ in the soil moisture analysis of the ECMWF numerical weather prediction model. In this scheme the difference between Θ S ∗ and the model first guess ΘFG, calculated at 1200 UTC, is added in 1/4 fractions throughout a 24 h window to the model resulting in analysed soil moisture ΘNDG. We compare results from this experiment against those from a control experiment where soil moisture evolved freely and against those from the operational ECMWF forecast system, which uses an optimum interpolation scheme to analyse soil moisture. Validation against field observations from the Oklahoma Mesonet, shows that the assimilation of Θ S ∗ increases the correlation from 0.39 to 0.66 and decreases the RMSE from 0.055 m3 m−3 to 0.041 m3 m−3 compared against the control experiment. The corresponding forecasts for low level temperature and humidity improve only marginally compared to the control experiment and deteriorate compared to the operational system. In addition, the results suggest that an advanced data assimilation system, like the Extended Kalman Filter, could use the satellite observations more effectively.
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initial soil moisture retrievals from the metop a advanced Scatterometer ascat
Geophysical Research Letters, 2007Co-Authors: Zoltan Bartalis, Hans Bonekamp, Vahid Naeimi, Stefan Hasenauer, Josep Figa, Klaus Scipal, Wolfgang Wagner, Craig AndersonAbstract:[1] This article presents first results of deriving relative surface soil moisture from the METOP-A Advanced Scatterometer. Retrieval is based on a change detection approach which has originally been developed for the Active MicrowaveInstrument flownonboardtheEuropeansatellites ERS-1 and ERS-2. Using model parameters derived from eight years of ERS Scatterometer data, first global soil moisture maps have been produced from ASCAT data. The ASCAT data were distributed by EUMETSAT for validation purposes during the ASCAT product commissioning activities. Several recent cases of drought and excessive rainfall are clearly visible in the soil moisture data. The results confirm that seamless soil moisture time series can be expected from the series of two ERS and three METOP Scatterometers, providing global coverage on decadal time scales (from 1991 to about 2021). Thereby, operational, nearreal-time ASCAT soil moisture products will become available for weather prediction and hydrometeorological applications. Citation: Bartalis, Z., W. Wagner, V. Naeimi, S. Hasenauer, K. Scipal, H. Bonekamp, J. Figa, and C. Anderson (2007), Initial soil moisture retrievals from the METOP-A Advanced Scatterometer (ASCAT), Geophys. Res. Lett., 34, L20401, doi:10.1029/2007GL031088.
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Azimuthal anisotropy of Scatterometer measurements over land
IEEE Transactions on Geoscience and Remote Sensing, 2006Co-Authors: Zoltan Bartalis, Klaus Scipal, Wolfgang WagnerAbstract:Studies of the Earth's land surface involving Scatterometers are becoming an increasingly important application field of microwave remote sensing. Similarly to Scatterometer observations of ocean waves, the backscattering coefficient (sigma0) response of land surfaces depends on both the incidence and azimuth angle under which the observations are made. In order to retrieve geophysical parameters from Scatterometer data, it is necessary to account for azimuthal-modulation effects of the backscattered signal. In the present study, this paper localizes the regions affected by a strong azimuthal signal dependence when observed with the European Remote Sensing Satellite Scatterometer and the SeaWinds Scatterometer on QuikSCAT (QSCAT). The possible physical reasons for the azimuthal effects, relating the very detailed QSCAT azimuthal response to the spatial orientation of special topographic features and land cover within the sensor footprint, were then discussed. Different methods for normalizing the backscattering coefficient with respect of observation azimuth angle were also proposed and evaluated. First, the mean local incidence angle of the sensor footprint using the shuttle radar topography mission digital elevation model (DEM) were modeled and concluded that the resolution of the DEM is too coarse to characterize most of the observed azimuthal effects. A more effective way of normalizing the backscatter with respect to azimuth is then found to be by using historical backscatter observations to statistically determine the expected backscatter at each observation azimuth and incidence angle as well as time of the year. The efficiency of this method is limited to the availability of past measurements for each location on the Earth
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Soil moisture-runoff relation at the catchment scale as observed with coarse resolution microwave remote sensing
Hydrology and Earth System Sciences Discussions, 2005Co-Authors: Klaus Scipal, C. Scheffler, W. WagnerAbstract:Microwave remote sensing offers emerging capabilities to monitor global hydrological processes. Instruments like the two dedicated soil moisture missions SMOS and HYDROS or the Advanced Scatterometer onboard METOP will provide a flow of coarse resolution microwave data, suited for macro-scale applications. Only recently, the Scatterometer onboard of the European Remote Sensing Satellite, which is the precursor instrument of the Advanced Scatterometer, has been used successfully to derive soil moisture information at global scale with a spatial resolution of 50 km. Concepts of how to integrate macro-scale soil moisture data in hydrologic models are however still vague. In fact, the coarse resolution of the data provided by microwave radiometers and Scatterometers is often considered to impede hydrological applications. Nevertheless, even if most hydrologic models are run at much finer scales, radiometers and Scatterometers allow monitoring of atmosphere-induced changes in regional soil moisture patterns. This may prove to be valuable information for modelling hydrological processes in large river basins (>10 000 km2. In this paper, ERS Scatterometer derived soil moisture products are compared to measured runoff of the Zambezi River in south-eastern Africa for several years (1992?2000). This comparison serves as one of the first demonstrations that there is hydrologic relevant information in coarse resolution satellite data. The observed high correlations between basin-averaged soil moisture and runoff time series (R2>0.85) demonstrate that the seasonal change from low runoff during the dry season to high runoff during the wet season is well captured by the ERS Scatterometer. It can be expected that the high correlations are to a certain degree predetermined by the pronounced inter-annual cycle observed in the discharge behaviour of the Zambezi. To quantify this effect, time series of anomalies have been compared. This analysis showed that differences in runoff from year to year could, to some extent, be explained by soil moisture anomalies.
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Soil moisture-runoff relation at the catchment scale as observed with coarse resolution microwave remote sensing
Hydrology and Earth System Sciences Discussions, 2005Co-Authors: Klaus Scipal, C. Scheffler, W. WagnerAbstract:Microwave remote sensing offers emerging capabilities to monitor global hydrological processes. Instruments like the two dedicated soil moisture missions SMOS and HYDROS or the Advanced Scatterometer (ASCAT) onboard METOP will provide a flow of coarse resolution microwave data, suited for macro-scale applications. Only recently, the ERS Scatterometer, which is the precursor instrument of ASCAT, has been used successfully to derive soil moisture information at global scale with a spatial resolution of 50 km. Concepts of how to integrate macro-scale soil moisture data in hydrologic models are however still vague. In fact, the coarse resolution of the data provided by microwave radiometers and Scatterometers is often considered to impede hydrological applications. Nevertheless, even if most hydrologic models are run at much finer scales, radiometers and Scatterometer allow monitoring of atmosphere-induced changes in regional soil moisture patterns. This may prove to be valuable information for modelling hydrological processes in large river basins (0.85) clearly demonstrate that the seasonal change from low runoff during the dry season to high runoff during the wet season is well captured by the ERS Scatterometer. Additionally, differences in runoff from year to year could be to some extend, explained by soil moisture anomalies.
W. Wagner - One of the best experts on this subject based on the ideXlab platform.
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Soil moisture-runoff relation at the catchment scale as observed with coarse resolution microwave remote sensing
Hydrology and Earth System Sciences Discussions, 2005Co-Authors: Klaus Scipal, C. Scheffler, W. WagnerAbstract:Microwave remote sensing offers emerging capabilities to monitor global hydrological processes. Instruments like the two dedicated soil moisture missions SMOS and HYDROS or the Advanced Scatterometer onboard METOP will provide a flow of coarse resolution microwave data, suited for macro-scale applications. Only recently, the Scatterometer onboard of the European Remote Sensing Satellite, which is the precursor instrument of the Advanced Scatterometer, has been used successfully to derive soil moisture information at global scale with a spatial resolution of 50 km. Concepts of how to integrate macro-scale soil moisture data in hydrologic models are however still vague. In fact, the coarse resolution of the data provided by microwave radiometers and Scatterometers is often considered to impede hydrological applications. Nevertheless, even if most hydrologic models are run at much finer scales, radiometers and Scatterometers allow monitoring of atmosphere-induced changes in regional soil moisture patterns. This may prove to be valuable information for modelling hydrological processes in large river basins (>10 000 km2. In this paper, ERS Scatterometer derived soil moisture products are compared to measured runoff of the Zambezi River in south-eastern Africa for several years (1992?2000). This comparison serves as one of the first demonstrations that there is hydrologic relevant information in coarse resolution satellite data. The observed high correlations between basin-averaged soil moisture and runoff time series (R2>0.85) demonstrate that the seasonal change from low runoff during the dry season to high runoff during the wet season is well captured by the ERS Scatterometer. It can be expected that the high correlations are to a certain degree predetermined by the pronounced inter-annual cycle observed in the discharge behaviour of the Zambezi. To quantify this effect, time series of anomalies have been compared. This analysis showed that differences in runoff from year to year could, to some extent, be explained by soil moisture anomalies.
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Soil moisture-runoff relation at the catchment scale as observed with coarse resolution microwave remote sensing
Hydrology and Earth System Sciences Discussions, 2005Co-Authors: Klaus Scipal, C. Scheffler, W. WagnerAbstract:Microwave remote sensing offers emerging capabilities to monitor global hydrological processes. Instruments like the two dedicated soil moisture missions SMOS and HYDROS or the Advanced Scatterometer (ASCAT) onboard METOP will provide a flow of coarse resolution microwave data, suited for macro-scale applications. Only recently, the ERS Scatterometer, which is the precursor instrument of ASCAT, has been used successfully to derive soil moisture information at global scale with a spatial resolution of 50 km. Concepts of how to integrate macro-scale soil moisture data in hydrologic models are however still vague. In fact, the coarse resolution of the data provided by microwave radiometers and Scatterometers is often considered to impede hydrological applications. Nevertheless, even if most hydrologic models are run at much finer scales, radiometers and Scatterometer allow monitoring of atmosphere-induced changes in regional soil moisture patterns. This may prove to be valuable information for modelling hydrological processes in large river basins (0.85) clearly demonstrate that the seasonal change from low runoff during the dry season to high runoff during the wet season is well captured by the ERS Scatterometer. Additionally, differences in runoff from year to year could be to some extend, explained by soil moisture anomalies.
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Monitoring soil moisture over the Canadian Prairies with the ERS Scatterometer
IEEE Transactions on Geoscience and Remote Sensing, 1999Co-Authors: W. Wagner, J. Noll, M. Borgeaud, H. RottAbstract:The capability of the Scatterometers onboard the European Remote Sensing Satellites (ERS-1 and ERS-2) for soil moisture retrieval is investigated. The ERS Scatterometer consists of three antennas that illuminate the Earth's surface from three different viewing directions. This allows the authors to study the dependence of the backscattering coefficient /spl sigma//sup 0/ on the azimuth and the incidence angle. An analysis of ERS Scatterometer data over the Canadian Prairie region shows that land surfaces are slightly anisotropic with respect to the azimuth angle. It is proposed to consider the azimuthal anisotropy as an additional error source to /spl sigma//sup 0/. The variation of /spl sigma//sup 0/ with the incidence angle was found to be linked to vegetation, but independent of soil moisture. Based on these observations, a method for the normalization of the backscattering coefficient with respect to the incidence angle is proposed. The normalized backscattering coefficient at an incidence angle of 40/spl deg/, /spl sigma//sup 0/(40), is sensitive to vegetation and, in the case of moderate vegetation (grassland to sparsely forested areas), to the soil moisture content. Soil moisture maps derived from ERS-1 Scatterometer measurements are compared to maps representing conditions on annually cropped land showing agreement. Results suggest that, over the Canadian Prairies, estimates of the total water content in the soil profile might be possible with an accuracy of about 10% of field capacity if little or no rainfall has occurred for three days before radar image acquisition.
