星载微波散射计的研究现状及发展趋势

介绍了美国宇航局(NASA)和欧空局(ESA)星载微波散射计传感器的类别、特点以及发展历程,详细描述了星载散射计海面风矢量在大洋环流、海洋大气交互作用、中短期极端天气状况预报等领域的应用以及散射计分辨率增强数据在极地海冰制图、监测、分类、陆地植被参数反演等方面的应用,展望星载散射计研究在全天候、全天时、高时间分辨率、高空间分辨率等提高观测能力方向的发展趋势,最后简述国内星载散射计神州4号(SZ 4)散射计(CN/SCAT)和海洋卫星2号(HY 2)散射计的特点及研究现状。     

A new relationship between radar cross-section and ocean surface wind speed using ERS-t scatterometer and buoy measurements

The ERS–I spacecraft scatterometer, C-band VV polarization, acquired radar cross-section measurements over the global oceans during 1992 and 1993. We investigate the cross-section dependence on mean wind speed U using collocated buoys within ±25km of the scatterometer cells. These collocated measurements result in over 75000 matches in two diITerent oceanic regions. The buoys measure hourly mean wind speeds from 0·2–10 mS ¹ and 0·2–18ms ^-1 in the equatorial Pacific Ocean and at mid^-latitudes off the North American coasts, respectively. We present experimental evidence for a new and compact exponential model dependence on wind speed. The previously used power–law form inadequately characterizes the cross-section measurements based on a single index over a large wind speed range. The cross-sectional slope varies from about zero dB/ms^-1 at high wind speeds U=18ms ^-1 and small incidence angles 0=20° to about 1·3dB/ms ^-1 at low wind speeds U=3ms ^-1 and large incidence angles, 0=55°. The CMOD4 model significantly underestimates the radar cross section measurements for U≤3ms ^-1 whereas the exponential model exhibits less bias.     

Regional accuracy of QuikSCAT gridded winds

The quality of gridded 00 UTC and 12 UTC QuikSCAT wind speed fields provided by the Florida State University (FSU) and NASA Jet Propulsion Laboratory (JPL) are analysed over the Bay of Bengal during May–August 2001. Additionally, an examination of these fields is performed over the Gulf of Mexico for the May–August period from 2001 to 2003. Both 00 UTC and 12 UTC time almost coincide with QuikSCAT sampling times (twice a day) and correspond to either early morning or late evening local time over these regions. The primary restriction for generating accurate maps with a temporal resolution of 12 hours and less is a lack of adequate sampling. Due to non‐uniform spatial‐temporal sampling of the scatterometer, interpolation procedures cannot provide proper estimates in data gaps over the regions not covered by a swath. Wind speed autocorrelation coefficients for gridded datasets have been compared with that of original QuikSCAT data and buoy winds. It is shown that the spatial and temporal interpolation used to obtain these datasets results in smoothing of the QuikSCAT wind speeds, reducing their original variance. This smoothing is amplified where substantial diurnal wind variability occurs. A comparison with buoy data shows that FSU and JPL gridded fields are unable to reproduce correctly observed low correlations in wind speed between morning and evening breeze flows and diurnal variability of winds, leading to a reduced difference between 00 UTC and 12 UTC values in comparison with buoys and swath QuikSCAT data. Rather, the FSU and JPL maps describe daily mean fields. Another consequence of the spatial‐temporal interpolation is that the winds are distorted at a frequency matching the dominant sampling interval (3–4 days) of QuikSCAT measurements over the Bay of Bengal.     

Using sentinel-1A SAR wind retrievals for enhancing scatterometer and radiometer regional wind analyses

Scatterometer surface wind speed and direction observations in combination with radiometer wind speeds allow to generate surface wind analyses with high space and time resolutions over global as well as at regional scales. Regarding scatterometer sampling schemes and physics, the resulting surface wind analyses suffer from lack of accuracy in areas near coasts. The use of the synthetic aperture radar (SAR) onboard the Sentinel-1A satellite attempts to address the enhancement of surface wind analyses issues. In this study, SAR wind speeds and directions retrieved from backscatter coefficients acquired in interferometric wide (IW) swath mode are used. Their accuracy is determined through comprehensive comparisons with moored buoy wind measurements. SAR and buoy winds agree well at offshore and nearshore locations. The statistics characterizing the comparison of SAR and buoy wind speeds and directions are of the same order as those obtained from scatterometer (Advanced SCATterometer (ASCAT) and RapidScat) and buoy wind comparisons. The main discrepancy between SAR and buoy data are found for high wind speeds. SAR wind speeds exceeding 10 m s^–1 tend to be underestimated. A similar conclusion is drawn from SAR and scatterometer wind speed comparisons. It is based on the underestimation of SAR backscatter coefficient (σ°) with respect to σ° estimated from scatterometer winds and the geophysical model function (GMF) named CMOD-IFR2 (Ifremer C band MODel). New SAR wind speeds are retrieved using CMOD-IFR2. The corrected SAR retrievals allow better determination of the spatial characteristics of surface wind speeds and of the related wind components in near-coast areas. They are used for enhancing the determination of the spatial structure function required for the estimation of wind fields gridded in space and time at the regional scale. The resulting wind fields are only determined from scatterometer wind observations in combination with radiometer retrievals. Their qualities are determined through comparisons with SAR wind speeds and directions, and through their application for determination of wind power off Brittany coasts.     

Diurnal variability in sea surface temperature in the Arctic

The formation of diurnal warming events in sea surface temperature (SST) observations in the Arctic is investigated using multiple satellite derived SST products and in situ buoy temperature measurements. Significant diurnal warming events (of the order of several K) are shown to occur even in the Arctic during summer months, when the total daily insolation at high latitudes is, in fact, higher than that at low and mid latitudes. The observed Arctic diurnal warming events are shown to usually happen in persistent low wind conditions, and are more frequent in shallow waters than deep waters. During the studied period of June and July 2008, significant diurnal warming events were observed over most of the studied area, although with smaller spatial extent and reoccurring less often when compared to events reported at low and mid latitudes.     

Russian scatterometer: discussion of the concept and the numerical simulation of wind field retrieval

Orbital scatterometry is briefly overviewed and its trends are indicated. Two scatterometer concepts are currently considered for trade-offs: with fixed and rotating antenna systems. The concept with a rotating antenna system was selected and SeaWinds was chosen as the prototype for the first Russian scatterometer. The scatterometer concept was then further developed and instead of two pencil beams, a fan-beam antenna was proposed (about 1° × 6°). The fan-beam antenna allows successive measurements for horizontal and vertical polarization in each wind vector cell (WVC). This increases the number of observations of the WVC at different incidence and azimuth angles during flight. The scatterometer parameters required to implement the proposed measurement geometry for an orbit altitude of 650 km and a swath width of 1525 km are discussed. A numerical scatterometer model that accounts for both the specifications and the observation geometry is developed. The scatterometer performance, with subsequent formation of a swath and splitting into WVCs, is simulated. The procedure of wind vector retrieval includes two stages: 1) determining wind speed and wind direction in a single WVC; and 2) using the information from adjacent WVCs to correct wind direction. It is shown that the accuracy of wind direction retrieval by a WVC can be increased by simultaneous radar cross-section (RCS) measurements at vertical and horizontal polarization. The basic error in determining wind direction is due to a 180° wind direction ambiguity caused by the form of RCS azimuth dependence. Two-dimensional median filtering is commonly employed in scatterometry to increase the accuracy of wind direction retrieval. In this study, two-dimensional angular median filtering was employed and it is shown that the error in wind direction retrieval significantly decreased. The results of the research indicate that wind field can be retrieved by the new scatterometer with the level of precision required.     

Characteristics of atmospheric divergence and convergence in the Indian Ocean inferred from scatterometer winds

Large-scale surface atmospheric convergence and divergence patterns in the Indian Ocean are mapped using high-spatial resolution, merged scatterometer wind vectors during 1991-2000. The convergence zone evolves to north of 15°S as a result of convection promoted by warm (> 28 °C) equatorial sea surface temperature (SST), and it exhibits strong intensity during boreal summer and winter. A divergence zone evolves to the south of 15°S as a result of subdued convection caused by colder SST (< 24 °C) that reduces outgoing long-wave radiation; it exhibits enhanced intensity in the eastern Indian Ocean during boreal winter. The interannual variability shows that the divergence in the eastern Indian Ocean lags its western counterpart by 5-7 months. The convergence in the eastern Indian Ocean is stronger than its western counterpart during boreal summer. Relationship between Southern Oscillation Index and spatially averaged convergence time series indicate that the latter weakened during strong El Niño years 1994 and 1997. Spatially averaged divergence time series show a near-contemporaneous relationship with all-India rainfall, with a temporal lag of 1∼2 months.     

A laboratory study of friction-velocity estimates from scatterometry: low and high regimes

Measurements from scatterometers pointing at wind-waves in three large wave-tanks are examined to study fetch effects and the correlation with wind friction-velocity u_?,. Time-series measurements were made at 13, 35 and 95 m with a K_a-band scatterometer aimed upwind at 30° incidence angle and vertical polarization. Average normalized radar cross-section σ_o values from all fetches follow a common trend for σ_o as a function of u_? so the fetch dependence is negligible. An empirical power-law model yields a high correlation between σ_? and u_?but because systematic anomalies arise, we re-examine a turbulence approach that delineates low and high regimes with a transition at u_? of approximately 25cms^-1. Using this criteria, the data are well represented by a two-section power-law relationship between σ_o and u_?.     

Retrieval of bare soil and vegetation parameters from wind scatterometer measurements over three different climatic regions

Medium to low resolution (1-50 km) active microwave sensors such as spaceborne scatterometers and wide-swath mode synthetic aperture radars have great potential as tools for long term monitoring over land and ice. To optimise the use of this kind of data, the heterogeneity of the target and its effects on the radar measurements need to be investigated and modelled, particularly in the view of retrieving geophysical parameters. In this paper, wind scatterometer measurements over three different test sites, the NOPEX region in Sweden, the HAPEX-Sahel site in Niger and the Niger delta area in Nigeria, are analysed. For these regions, a forward model is developed by considering the backscatter contributions of the bare surface, the seasonal and evergreen vegetation and the open water areas. Colocated high spatial resolution SAR data and ground information are used to characterise the target scene. The model is then inverted to retrieve monthly soil roughness, dielectric properties and vegetation parameters. It is shown that the measurements contain enough information to characterise these three different regions and to monitor their temporal evolution. The retrieved values obtained for the bare surface and the vegetation parameters are consistent with ground measurements collected in these areas. Further improvements are achieved by incorporating the time scale variability of the variables investigated into the retrieval scheme.     

Calculation of the sensible heat flux of the global ocean using satellite data

The sensible heat flux of the global ocean is derived using satellite data of Special Sensor Microwave/Imager (SSM/I) precipitable water, Advanced Very High Resolution Radiometer (AVHRR) sea-surface temperature (SST) and scatterometer wind speed. Prior to heat flux derivation, the air temperature over the sea surface in the global ocean is obtained with an iterative solving technique applied to a simplified equation that specifies the relationship among boundary-layer parameters. It is found that a bias exists between the calculated air temperature and the climatology data, which is corrected by a linear model based on the climatology of air temperature. Using the corrected air temperature and the bulk formula, we calculate the sensible heat flux from January 1992 to October 1998. The heat flux calculation is consistent with previous results. An error analysis suggests that based on the bulk formula, the uncertainty caused by the air temperature is comparable with the error components from the SST and wind. Empirical orthogonal function (EOF) analysis is used to extract the temporal and spatial characteristics of the calculated sensible heat flux. The first EOF is characterized by a winter and summer oscillation with an annual cycle, the second shows another annual cycle in spring and autumn oscillation, the third EOF is related to the El Niño and La Niña cycle, reflecting a certain relationship between El Niño Southern Oscillation (ENSO) events and the global ocean sensible heat flux, and the fourth EOF indicates an increasing trend with a quasi-biennial oscillation and atmospheric influences.     

海洋二号有效载荷微波散射计数据处理系统的设计

海洋二号有效载荷微波散射计是由中科院空间中心研制的一种星载微波遥感器。它采用笔型波束圆锥扫描脉冲体制,主要用来测量海面风矢量,具有广泛的应用需求。着重介绍了散射计的数据处理系统的功能、结构及部分实验结果。     

Comparison of Oceansat-2 scatterometer- to buoy-recorded winds and spatial distribution over the Arabian Sea during the monsoon period

For this wind resource assessment (WRA) study, wind speed and direction are the fundamental inputs. Also, these studies are data driven and require large historical wind speed data sets available on the site. This work explores the application of space-based scatterometer winds for assimilation into WRA studies towards the development of offshore wind energy. This article focuses on estimating the performance of Oceansat-2 scatterometer (OSCAT)-derived wind vector using in situ data from buoys at different locations in the Arabian Sea. A comparative study between three methods for estimating the equivalent neutral winds (ENW) for buoys is carried out. OSCAT winds were closest to ENW estimated by the Liu–Katsaros–Businger (LKB) method. The spatial and temporal windows for comparison were 0.5° and ±60 minutes, respectively. The monsoon months (June–September) of 2011 were selected for study. The root mean square deviation for wind speed is less than 2.5 m s^?1 and wind direction is less than 20°, and a small positive bias is observed in the OSCAT wind values. From the analysis, the OSCAT wind values are consistent with in situ-observed values. Furthermore, wind atlas maps were developed with OSCAT winds, representing the spatial distribution of winds at a height of 10 m over the Arabian Sea.     

CFOSAT散射计几何定位验证与精度初步评估

采用STK软件对中法海洋卫星（CFOSAT）散射计几何定位算法进行了验证。针对CFOSAT散射计扇形波束扫描观测体制,分别就扇形波束扫描散射计几何定位原型算法、包含天线安装误差的完整几何定位算法利用STK进行了详细验证。结果表明了CFOSAT扇形波束散射计几何定位算法的准确性,与STK几何定位结果偏差在100 m以内。在此基础上,结合有源定标器实验数据,对CFOSAT实际观测数据几何定位参数进行了校正,给出了CFOSAT散射计几何定位精度的初步评估。     

An assessment of veering wind effects on scatterometry from the sea surface

To characterize scatterometer returns from the sea surface near meteorological fronts, we investigated microwave scattering from seas in which long waves are at oblique angles to short waves. We simulate the effects of veering winds on C- and K_u-band scatterometers by using models in which the short waves align with the wind friction velocity u_? , but the long waves are at oblique angles to the u_? direction. The analysis reveals two main effects due to the rotation of the long wave slope probability density distribution. Azimuthally averaged normalized radar cross-section a _o decreases as the oblique angle increases. Additionally, two regimes exist. In the small angle regime, azimuthal scans of normalized radar cross-section σ^o exhibit features similar to the classic double-maxima pattern for non-veering wind conditions, but the axis of σ^o maxima is rotated toward the long-wave axis. In the large angle regime, more than two maxima are apparent in azimuthal scans. Therefore it may be inappropriate to use standard three term Fourier cosine models for some veering wind conditions.     

Wind-fetch dependence of Seasat scatterometer measurements

Abstract

This work is focused on the effects of large-scale wind-generated gravity waves on scatterometer wind-measurement accuracy. We present theoretical and experimental evidence of the important role played by the degree of wind-wave coupling in surface geometry and therefore in microwave signatures. Seasat scatterometer wind-measurement errors are analysed, and an error bias is found to be related to the degree of wind-wave development. Attention is focused on a dynamic measure of wind-wave development that characterizes the rate of energy transfer from the mean wind to the energy-carrying portion of the wave spectrum. An explanation of the bias is suggested based on the consideration of an additional component of surface scattering caused by electromagnetic-wave diffraction at the crests of individual, sufficiently steep wavelets.     

Mapping of Sahelian vegetation parameters from ERS scatterometer data with an evolution strategies algorithm

The West African Sahel rainfall regime is known for its spatio-temporal variability at different scales which has a strong impact on vegetation development. This study presents results of the combined use of a simple water balance model, a radiative transfer model and ERS scatterometer data to produce map of vegetation biomass and thus vegetation cover at a spatial resolution of 25 km. The backscattering coefficient measured by spaceborne wind scatterometers over Sahel shows a marked seasonality linked to the drastic changes of both soil and vegetation dielectric properties associated to the alternating dry and wet seasons. For lack of a direct observation, METEOSAT rainfall estimates are used to calculate temporal series of soil moisture with the help of a water balance model. This a priori information is used as input of the radiative transfer model that simulates the interaction between the radar wave and the surface components (soil and vegetation). Then, an inversion algorithm is applied to retrieve vegetation aerial mass from the ERS scatterometer data. Because of the nonlinear feature of the inverse problem to be solved, the inversion is performed using a global stochastic nonlinear inversion method. A good agreement is obtained between the inverse solutions and independent field measurements with mean and standard deviation of −54 and 130 kg of dry matter by hectare (kg DM/ha), respectively. The algorithm is then applied to a 350,000 km² area including the Malian Gourma and Seno region and a Sahelian part of Burkina Faso during two contrasted seasons (1999 and 2000). At the considered resolution, the obtained herbaceous mass maps show a global qualitative consistency (r²=0.71) with NDVI images acquired by the VEGETATION instrument.     

A comparison of ship- and scatterometer-derived wind speed data in open ocean and coastal areas

Quality controlled wind speed observations from merchant ships have been compared with ERS-1 scatterometer data. The ship and satellite wind speed pairs were well correlated at 120 km separation in the open ocean, reducing to 40 km in the North Sea. The maximum allowed separation of ship and scatterometer wind speed pairs had to be further reduced to 20 km in the North Sea to avoid matching of coastal ship wind speed data with scatterometer data from more exposed regions. Spurious biases in the comparisons were caused by the error variability of the scatterometer data (0.5 m s-1) being significantly less than that for the ships (2.0 m s-1). The unbiased regression was: U10n(ship)=1.025 U10n(satellite)+0.255 No significant enhancement of the scatterometer wind speeds occurred in the coastal region.     

降雨影响散射计测风的初步研究

给出了降雨影响C、Ku波段微波散射计测量海面风速的初步结果。研究结果显示,海面风速为25～30 m/s时,雨速为15 mm/hr的降雨会使这两个波段的微波散射计测量的风速偏低10 m/s。
     

Gridded surface wind fields from Metop/ASCAT measurements

This article deals with the calculation and validation of daily surface wind vector fields from wind speed and direction observations derived from Advanced SCATterometer (ASCAT) scatterometer measurements over the global ocean. According to the ASCAT sampling scheme, the objective method allowing for the determination of regular in space and time wind speed and direction fields uses ASCAT observations as well as European Centre for Medium Weather Forecasts (ECMWF) analyses. The latter are considered as external drift for the kriging method and as the temporal interpolation basis for ASCAT retrievals. This study focuses on the investigation of the capability of the method to add valuable wind information to the operational atmospheric analyses and on the quality of the resulting wind fields. The accuracy of the former is determined through comprehensive comparisons with daily winds calculated from moored buoy data. At global and regional scales, comparisons are performed with surface wind patterns derived from the ECMWF analysis and from ECMWF Re-analysis project (ERA-Interim) re-analyses.     

Using sea surface temperature measurements from microwave and infrared satellite measurements

The Tropical Rainfall Mapping Mission Microwave Imager (TMI) instrument Sea Surface Temperature (SST) product (v1.0) is compared with in situ observations obtained in the Atlantic Ocean. The TMI SST has a mean warm bias of 0.25?K±0.7?K when compared to in situ SST at a depth of 7?m. When TMI SST are compared to in situ skin SST measurements, the bias is 0.6?K±0.5?K. A limited global comparison between TMI SST and co-incident ERS-2 Along-Track Scanning Radiometer (ATSR/2) skin SST demonstrates a bias of 0.6?K±0.6?K consistent with the result obtained using in situ observations. These results are consistent with the predicted accuracy of the TMI SST data products. Based on these results, a simple method to merge the TMI and ATSR data is proposed.