Liling Liu

Characterization of soil dust aerosol in China and its transport and distribution during 2001 ACE-Asia: 1. Network observations

[1] Mass loading, 20 elemental concentrations, and time series of aerosol particles were investigated over the China Dust Storm Research (ChinaDSR) observational network stations from March to May 2001 during the intensive field campaign period of ACE-Asia. Four extensive and several minor dust storm (DS) events were observed. Mass balance calculations showed that 45 - 82% of the observed aerosol mass was attributable to Asian soil dust particles among the sites, in which Ca and Fe contents are enriched to 12% and 6%, respectively, in the Western High-Dust source regions compared with dust aerosols ejected from the Northern High- Dust source regions. For the latter areas, elemental contents exhibited high Si (30%) and low Fe (4%). For all major source areas and depositional regions, aluminium (Al) comprises 7% of Asian dust. Air mass back-trajectory analysis showed that five major transport pathways of Asian dust storms dominated dust transport in China during spring 2001, all of which passed over Beijing. Measurements also suggest that the sand land in northeastern China is a potential source for Asian dust. The size distribution for estimating vertical dust flux was derived from the observed surface dust size distributions in the desert regions. For particle diameters between 0.25 and 16 mum, a lognormal distribution was obtained from averaging observations at various deserts with a mass mean diameter of 4.5 mum and a standard deviation of 1.5. This range of soil dust constitutes about 69% of the total dust loading. The fractions for particles in the size ranges of 16 mum are around 1.7% and 30%, respectively.

An Improved Adaptive Regularization Method for Forward Looking Azimuth Super-Resolution of a Dual-Frequency Polarized Scatterometer

Dual-frequency polarized scatterometer (DFPSCAT) is a pencil-beam rotating scatterometer which is designed for snow water equivalent (SWE) measurement, and Doppler beam sharpening (DBS) technique is proposed for DFPSCAT to achieve the azimuth resolution. However, the DBS technique is inapplicable for the forward-looking and afterward-looking regions. Based on an approximate aperiodic model of scatterometer echo signal, an improved adaptive regularization deconvolution algorithm with gradient histogram preservation (GHP) constraint is implemented to settle the problem. To investigate its performance of resolution enhancement and resulted accuracy, both a synthetic backscattering coefficient (σ0) field reconstruction and SWE σ0 reconstruction are carried out. The results show that the proposed method can recover the truth signal and achieve azimuth resolution of 2 km with the designed scatterometer system, which is required by the SWE retrieval. Moreover, the relative errors of reconstructed σ0 are less than 0.5 dB that satisfy the accuracy requirement for SWE retrieval, and comparisons with observed results show that the error reduction is more than 0.03 dB. Meanwhile, a comparison between the proposed algorithm and some existing resolution enhancement methods is analyzed, which concludes that the proposed method can obtain a comparable resolution enhancement as L1 method and has less noise. The technique is also verified with advanced scatterometer (ASCAT) scatterometer data.

Adaptive regularization method for forward looking Azimuth super-resolution of a Dual-Frequency Polarized Scatterometer

Dual-Frequency Polarized Scatterometer (DFPSCAT) is a pencil-beam rotating scatterometer which is used to measure snow water equivalence (SWE). Respecting the low azimuth resolution of its forward-looking region, an adaptive regularization deconvolution super-resolution method, based on the scatterometer echo signal model, is proposed. Compared with the classical SIR and MAP algorithms, the proposed method can better reconstruct the original signal, and has less noise amplification. The algorithm processing accuracy with different Kpc is also studied, and the results show that when the value of Kpc is less than 0.1, nearly the entire restored data can satisfy the requirement of 0.5dB accuracy.

Spatial Resolution and Precision Properties of Scatterometer Reconstruction Algorithms

Various reconstruction methods have been used to enhance the spatial resolution of scatterometer data. Most of the image reconstructions are two-dimensional problems, which combine multiple passes of overlapping data over the temporally homogeneous surface, and thus are only suitable for land and ice applications. This paper attempts to address the one-dimensional reconstruction to enhance the azimuth resolution of scatterometer data using a single pass of observations. Since the range resolution determined by the on-board dechirping technique is generally up to several hundred meters, the one-dimensional reconstruction is adequate for certain near real-time ocean applications, such as the development of coastal scatterometer winds. Three well-known reconstruction algorithms, including additive algebraic reconstruction technique (AART), multiplicative algebraic reconstruction technique (MART), and scatterometer image reconstruction (SIR), are evaluated. The spatial resolution and the reconstruction precision resolved by each algorithm are separately analyzed using the local impulse response and Monte Carlo methods. The dependence of the spatial resolution and the reconstruction precision on a variety of parameters, such as the mean backscatter coefficient and its variance, the beamwidth of spatial response function (SRF), and the SRF function type, is evaluated using a simulation framework. In particular, the tradeoff between the spatial resolution and the reconstruction precision is examined for three algorithms. The results show that SIR offers the quickest convergence and lowest noise.

Analysis of Backus-Gilbert approach on resolution enhancement of dual-frequency polarized scatterometer

Dual-Frequency Polarized Scatterometer (DPS) is a new system utilizing Doppler Beam Sharpening (DBS) technology for resolution enhancement. Respecting the low azimuth resolution over nadir swath, the Backus-Gilbert method is investigated to improve the resolution. According to the precision definition of scatterometer measurement, a Backus-Gilbert method for scatterometer is derived. Using the method, the tradeoff of resolution and precision of resolution-enhanced backscatter (normalized radar cross section, σ0) is established in a quantitative manner. The results show that resolution enhancement is achieved by the price of greatly decreased precision of reconstructed backscatter.

Deconvolution method for resolution enhancement of Dual-Frequency Polarized Scatterometer

Dual-Frequency Polarized Scatterometer (DFPSCAT) is a new system utilizing Doppler beam sharpening (DBS) technology for azimuthal resolution enhancement. Considering the DBS technology is inapplicable for the middle areas of the swath, a theoretical framework of deconvolution signal processing is proposed to improve resolution. A deconvolution method of the nonlocally centralized sparse representation (NCSR) is adopted to verify its feasibility, and simulation results show that the deconvolution method have obviously better resolution enhancement and higher recovery accuracy than these of the classical scatterometer image reconstruction (SIR) method.

Dual Frequency Polarized Scatterometer for global snow observation

Dual Frequency Polarized Scatterometer (DFPSCAT) is one of the three payloads onboard the satellite of Water Cycle Observation Mission (WCOM). DFPSCAT is an X/Ku band rotating pencil beam scatterometer with 2-5 km resolution and 1000km swath for mapping of snow water equivalent (SWE) and freeze-thaw process [1]. DFPSCAT achieves fine resolution by linear frequency modulation pulse compression along the elevation direction, and by unfocused synthetic aperture processing (a technique where the Doppler effect is exploited to synthesize a longer aperture to achieve an improved resolution), as well as super-resolution reconstruction by oversampling in the direction of the azimuth.