1,720,992 research outputs found
Passive microwave algorithms for refractive index of Arctic sea ice: a comparison of two approaches and interpretations
No full textThe refractive index of sea ice can be estimated using the Debye relaxation model. However, the practical use of this model is difficult because it requires a large amount of information as inputs. To resolve this issue, two different approaches have been developed to retrieve the Arctic basin-scale refractive index from satellite passive microwave measurements using the bulk radiative transfer concept. One is the simplified three-layer sea ice radiative transfer (3SRT) approach and the other is the Brewster's angle approximation (BAA) approach. Although these methods are different, both find polarized emissivities for the microwave emitting layer of sea ice to estimate the refractive index using a relationship between polarized emissivities and the refractive index. A typographical erroneous version of the equation for this relationship has been used in numerous studies without noting the errors. This study provides the correct equations for the relationship. The refractive index obtained using the 3SRT method is within a reasonable range of 1.2-1.8. However, the refractive index obtained using the BAA approach shows too high values ranging from 1.2 to 3.2. The sensitivity of the approaches to variation in the emissivity is analysed. The BAA method is sensitive to the surface emissivity because of the assumptions considered. The refractive index of sea ice obtained from the 3SRT approach is compared with that obtained from the Debye relaxation model. The results show that a plausible refractive index over Arctic sea ice can be estimated from satellite passive microwave measurements using the 3SRT method.N
A Physically Based Two-Scale Ocean Surface Emissivity Model Tuned to WindSat and SSM/I Polarimetric Brightness Temperatures
No full textA two-scale ocean surface emissivity model tuned to WindSat and Special Sensor Microwave/Imager (SSM/I) polarimetric brightness temperatures for general passive microwave applications is detailed. The model provides a full Stokes vector emissivity calculation at arbitrary microwave frequencies and observation angles for wind speeds of up to 15 m/s. During model development, it was found that the untuned two-scale model generally produced plausible azimuthal behavior in the ocean surface emissivity vector; however, large discrepancies between the untuned model and WindSat and SSM/I observations were observed, in particular for the zeroth-azimuthal-harmonic coefficients. These discrepancies can be ascribed to inaccuracies in contemporary ocean foam coverage and emissivity models. Accordingly, foam influences were treated using machine-tunable correction parameters incorporated as a means of improving and extending the physically based two-scale model. In addition, a hydrodynamic modulation function and the lower cutoff wavenumber for small-scale perturbation integration were treated as empirically tunable. Model tuning was performed by minimizing the metric over all available wind bins, channel frequencies, polarizations, and azimuthal harmonics. The result is an approximately eightfold reduction infrom its initial untuned model value, indicating that machine tuning can considerably reduce model errors inherent in the two-scale model to levels acceptable for oceanic passive microwave remote sensing applications. The tuned model is independently validated against NASA Global Precipitation Measurement Microwave Instrument (GMI) measurements. The tuned model and GMI observed emissivities, when scaled to the surface temperature, are in agreement to within & x00B1;0.3 K root-mean-square (rms) error, thus suggesting good applicability of the model over a wide range of microwave frequencies and wind speeds.Y
Behavior of magnetic flux in granular high superconducting thin film
No full text학위논문(박사) - 한국과학기술원 : 물리학과, 2010.2, [ x, 112 p. ]Granular (SmBCO) 박막에서의 자기선속의 거동에 대해 연구하였다. 이러한 박막의 자기선속 밀도 분포와 전류 밀도 분포의 몇가지 특성을 발견하였고 이 특성을 초전도 박막의 히스테리시스 에너지 손실 예측에 적용하였다.
Granular SmBCO 박막은 /MgO///Hastelloy tape으로 구성된 Ion Beam Assisted Deposition (IBAD) 기판위에 Evaporation using a Drum in Dual Chambers (EDDC) 방식으로 성장시켰고, 박막의 여러 특성들을 측정하였다. 전이온도 와 임계전류 는 4-point probe 방법을 이용해 각각 측정과 측정으로부터 측정하였는데, 는 1 \muV/cm 기준에 의해 결정되었다. 결정의 정렬도(in-plane과 out-of-plane)는 X-ray diffraction을 통해 측정하였고, SmBCO의 성분비는 Energy Dispersive Spectroscopy (EDS)로 확인하였다. Scanning Electron Microscope (SEM) 이미지를 통해 박막의 granular structure를 확인할 수 있었고 grain의 크기를 측정하였다. SQUID magnetometer를 이용해 hysteresis loop을 구하였다. 임계전류의 자기장 세기 및 각도에 대한 의존성도 측정하였다.
SmBCO 박막에 수송전류 ()이나 자기장 (), 또는 그 둘을 동시에 가하며 scanning Hall probe 방법을 이용해 박막 표면 근처의 자기장 profile 를 측정하였다. 이렇게 측정된 profile로부터 수치적인 inversion 계산을 통해 sheet current density 분포 를 구하고, 이 전류밀도 분포를 갖고 Biot-Savart 법칙으로 박막의 자기선속밀도 분포 를 계산하였다. Analytic한 이론적 계산 결과가 존재하는, 자기장만 가해준 경우와 전류만 가해준 경우에 대해서 profile들은 이론 결과와 다른 몇가지 특성들을 나타내었는데, 이는 박막의 granular structure에 기인한다고 생각된다. 세가지 종류의 임계전류 : , , . 를 정의하였다. 는 I-V 측정을 통해 결정되는 값이다. 과 는 critical state model의 개념에 기반해 정의하였는데, 각각 current-like한 분포와 field-like한 분포에 대한 로부터 결정된다. 구해진 모든 값들이 의 함수로 plot하였을 때 한 라인을 이룬다는 것과 값보다 훨씬 작다는 것이 발견되었다. 값과 같은 값을 갖는 은 와 사이의 값을 갖는다.
Scanning Hall probe 측정으로부터 구한 profile들을 이용해 박막의 히스테리시스 에너지 손실 Q를 구하고 이론값과 비교하였는데, 이론값과 차이를 보이는 영역이 나타났다. 과 를 이용해 일반적인 경우의 에 model 전류밀도 분포를 계산하여 고자기장에서의 Q를 예측하는 데에 적용하였다. 이렇게 예측된 값들은 측정된 scanning Hall probe 데이터로부터 구한 Q값들과 잘 일치하였다.한국과학기술원 : 물리학과
Analytical relationship between polarized reflectivities on the specular surface
No full textAn analytical expression of the relationship between polarized reflectivities over the planar surface is obtained. A comparison with the reflectivities calculated using the Fresnel equations for a given refractive index and viewing angle demonstrated that the Hong approximation', which causes erroneous results, is no longer necessary and should be replaced by an analytical expression.N
A New Description of Small-Scale and Large-Scale Roughness in the Fast Ocean Surface Emissivity Model
No full textThe widely used Fast Microwave Ocean Surface Emissivity Model (FASTEM) does not include the interaction between small-scale and large-scale roughness, which seems to induce errors in the ocean surface emissivity estimation. In this study, we attempt to develop a new model that might be included in the FASTEM-like model. In the developed model, the large-scale roughness is expressed as a function of the local incidence angle (LIA) within the context of Fresnel reflection theory, incorporating the interactions between the small-scale and large-scale roughness into the fast ocean surface emissivity model, as done in the two-scale approach. With the new expression of the large-scale roughness, we also provide a more physically based form of the equation for the fast ocean surface emissivity calculation that includes the small-scale scattering over a geometrically rough surface. In addition, an algorithm for estimating two-scale roughness from the measured or modeled polarized emissivities in conjunction with the proposed fast ocean surface emissivity equation is provided. The results demonstrate that the interactions between two-scale roughness should be considered in order to estimate accurate two-scale roughness influences on the ocean surface emissivity.Y
A New Bias Correction Approach for Better Assimilation of Microwave Sounding Data Over Winter Sea Ice in the Korean Integrated Model
No full textMicrowave sounder observations are essential for numerical weather prediction (NWP) systems, but utilizing channels sensitive to surface over sea ice has been challenging due to difficulties in estimating the sea ice surface radiance. This study presents a preprocessing method to assimilate near-surface microwave-sounding observations over winter sea ice, including an estimation of a real-time surface emissivity from satellite radiance and a bias correction scheme to minimize the radiance discrepancy between observation and model simulation. Our results show that the radiance simulated using dynamic emissivity exhibits a much better agreement with the measured one, although a significant negative bias of about 0.61-1.18 K remains over the winter sea ice. Thus, a new bias correction procedure, based on the regression relationships between the residual bias and potential bias sources such as the surface temperature and surface emissivity, is added. When it is applied, the remained bias is successfully estimated. Moreover, the sea ice observations from all temperature-sounding channels have been better utilized in the Korean Integrated Model (KIM). The additional information on the polar regions has increased the analysis increment and reduced the ensemble spread. In addition, a neutral to slightly positive impact on temperature analysis errors in layers sensitive to surface radiance encourages further utilization of microwave sounder data over sea ice.Y
Impact of ice surface and volume scatterings on the microwave sea ice apparent emissivity
No full textEmissivity retrieval for sea ice from passive microwave measurements has been an important problem in climate/environmental research because of its link to various snow/ice variables. However, so far, it has been a difficult task because of the influences of surface and snow/ice induced volume scatterings. Here we examine the influences of scatterings on the ice emissivity from 10.65, 18.7, 23.8, and 36.5GHz Advanced Microwave Scanning Radiometer (AMSR)-E brightness temperatures over the Arctic Ocean. In doing so, we use a two-dimensional roughness parameterization, modified with surface facet orientations with an assumption that the facet emission follows the Fresnel relationship. Emitting layer temperature and refractive index retrieved from AMSR-E 6.9GHz brightness temperature measurements were used in this study and applied to other channels of interest. We demonstrated that the obtained roughness index has a strong linear relationship with the root-mean-square height measured by Atmospheric Terrain Mapper on the National Aeronautics and Space Administration (NASA) P-3 aircraft. The obtained roughness index showed that surface scattering on the emissivity is generally insignificant except for some first-year ice regions in particular at higher frequencies. This fact implies that Fresnel relations can be applicable for most of sea ice at the low-frequency microwave spectrum. By contrast, volume scattering is found to be significant in emissivity retrieval in case of multiyear ice. Nonetheless, volume scattering influence over first-year ice appears to be minor. We suggest that Fresnel-type emissivity can be estimated once a correction factor is used for removing surface scattering and volume scattering contributions from the apparent emissivity.OAIID:RECH_ACHV_DSTSH_NO:T201906496RECH_ACHV_FG:RR00200001ADJUST_YN:EMP_ID:A002329CITE_RATE:3.38DEPT_NM:지구환경과학부EMAIL:[email protected]_YN:NY
Influences of Two-Scale Roughness Parameters on the Ocean Surface Emissivity From Satellite Passive Microwave Measurements
No full textIn this study, a method for estimating two-scale roughness influences on the ocean surface emissivity is developed by solving a simplified two-scale ocean emissivity model equation. In this model, scatterings by small-scale roughness are described by the Kirchhoff approximation. For large-scale roughness, the mean local incidence angle (LIA) is introduced to describe slanted surface slope deviation from flat surface. This study focuses on the ocean state under low/moderate wind conditions in order to preclude foam and anisotropic influences within the model. Consequently, a unique pair of two-scale roughness parameters are estimated from the equation using observed ocean emissivities from AMSR2-measured radiances. The results show that the estimated small-scale roughness at 6.925 and 10.65 GHz is linearly correlated with the 10-m height wind speed U-10. As the frequency reaches 36.5 GHz, however, the scatters between small-scale roughness and U-10 are increased, which suggests that the Kirchhoff bistatic scattering function is not fully suitable to describe the small-scale roughness at this frequency. The linear relationships between mean LIA and U-10 are found with high correlation coefficients. In addition, the estimated mean LIA corresponds well with associated roughness calculated from both observed and modeled ocean wave height spectra. This evidence demonstrates that the proposed large-scale roughness parameterization is physically meaningful and, therefore, the mean LIA has a physical basis in large-scale roughness. In addition, the strong correlations between the roughness parameters and U-10 demonstrate the possibility to estimate U-10 from the AMSR2 data using intermediate parameters that are physically based on ocean surface characteristics.Y
Differentiating between first-year and multiyear sea ice in the Arctic using microwave-retrieved ice emissivities
No full textPolarized emissivities of the sea ice over the Arctic were retrieved at Advanced Microwave Scanning Radiometer-EOS 10.65, 18.7, 23.8, and 36.5 GHz channel frequencies. Results indicate that retrieved emissivities are consistent with other emissivity estimates. However, errors in the retrieved emissivity for multiyear sea ice at 23.8 and 36.5 GHz can be large up to 8% and 20%, respectively, because of ignoring the freeboard ice scattering and the use of the same emission layer as in 6.925 GHz. It is shown that the emissivity slope for first-year ice between 10.65 and 18.7 GHz is opposite to that for multiyear sea ice, enabling a distinction between first-year ice and multiyear ice. Using these differences in spectral features with ice types, an emissivity difference (vertically polarized emissivity difference between 10.65 and 18.7 GHz) was devised to differentiate between first-year sea ice and multiyear sea ice. A comparison with the ice status information obtained from Cold Regions Research and Engineering Laboratory buoy measurements demonstrates that the method can separate first-year ice from multiyear ice, implying that this technique enables us to obtain instantaneous and pixel-level ice-type information from space-based passive microwave measurements.OAIID:RECH_ACHV_DSTSH_NO:T201724861RECH_ACHV_FG:RR00200001ADJUST_YN:EMP_ID:A002329CITE_RATE:3.318DEPT_NM:지구환경과학부EMAIL:[email protected]_YN:YY
Long-term arctic snow/ice interface temperature from special sensor for microwave imager measurements
No full textThe Arctic sea ice region is the most visible area experiencing global warming-induced climate change. However, long-term measurements of climate-related variables have been limited to a small number of variables such as the sea ice concentration, extent, and area. In this study, we attempt to produce a long-term temperature record for the Arctic sea ice region using Special Sensor for Microwave Imager (SSM/I) Fundamental Climate Data Record (FCDR) data. For that, we developed an algorithm to retrieve the wintertime snow/ice interface temperature (SIIT) over the Arctic Ocean by counting the effect of the snow/ice volume scattering and ice surface roughness on the apparent emissivity (the total effect is referred to as the correction factor). A regression equation was devised to predict the correction factor from SSM/I brightness temperatures (TBs) only and then applied to SSM/I 19.4 GHz TB to estimate the SIIT. The obtained temperatures were validated against collocated Cold Regions Research and Engineering Laboratory (CRREL) ice mass balance (IMB) drifting buoy-measured temperatures at zero ice depth. It is shown that the SSM/I retrievals are in good agreement with the drifting buoy measurements, with a correlation coefficient of 0.95, bias of 0.1 K, and root-mean-square error of 1.48 K on a daily time scale. By applying the algorithm to 24-year (1988-2011) SSM/I FCDR data, we were able to produce the winter-time temperature at the sea ice surface for the 24-year period.OAIID:RECH_ACHV_DSTSH_NO:T201831737RECH_ACHV_FG:RR00200001ADJUST_YN:EMP_ID:A002329CITE_RATE:3.406DEPT_NM:지구환경과학부EMAIL:[email protected]_YN:YY
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