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1.
Agriculture crop residue burning in tropics is an important source of atmospheric aerosols and monitoring their long-range transport is an important element in climate change studies. Synchronous measurements using micro-pulsed lidar, MICROTOPS-II sun photometer, multi-filter rotating shadow band radiometer (MFRSR) on aerosol optical depth and ground reaching solar irradiance were carried at an urban location in central region of India. Aerosol backscatter profiles obtained from micro-pulse lidar showed elevated aerosol layers up to ～3 km on certain days during October 2007. Satellite data observations on aerosol properties suggested transport of particles from agriculture crop residue burning in Indo-Gangetic Plains over large regions. Radiative forcing of aerosols estimated from SBDART model with input information on aerosol chemical properties, aerosol optical depth and single scattering albedo and broadband solar irradiance measurements using MFRSR showed good correlation (R=0.98).  相似文献   

2.
Reflectance measurements of both the visible and infrared bands of passive remote sensing sensors are widely used to retrieve aerosol optical depth(AOD) information. This is performed commonly for data obtained over both ocean and land, and these measurements allow for the off line development of a lookup table using radiative transfer models. Owing to molecular and aerosol effects, the reflected light received by the sensor is usually highly polarized. The linear polarization effect may be up to 100%, and the polarization factor of a sensor optical system will change the total intensity as well as the polarization status of the signal reaching the detector. The detector response will be different when the incident light polarization status changes, even if the total intensity remains constant. However, if the polarization calibration is neglected, it will cause obvious errors in the aerosol data retrieval. This is especially true for aerosol optical depth retrieval over an ocean. This measurement relies directly on the reflectance output of the sensor. Cases involving land surfaces are not discussed herein because the inhomogeneous properties conceal the error due to polarization. Taking the 550 and 860 nm bands as examples, the difference between the real top-of-atmosphere(TOA) reflectance and the reflectance reaching the detector is calculated using three different sensor polarization standards according to the Sea-viewing Wide Field-of-view Sensor(Sea Wi FS) and Moderate Resolution Imaging Spectroradiometer(MODIS) standards. The differences in AOD retrieval are also demonstrated using the lookup table developed previously from a vector radiative transfer code. The results reveal that under a normal situation in which the AOD is 0.15, the maximum AOD retrieval error could reach 0.04 in 550 nm but only 0.02 in 860 nm for the dust aerosol model. For the soot aerosol model, the maximum AOD retrieval error is 0.1 in 550 nm and 0.12 in 860 nm, indicating that the lack of polarization calibration will lead to large errors in aerosol retrieval over an ocean.  相似文献   

3.
In this paper we describe the results of a research campaign dedicated to the studies of aerosol optical properties in different regions of both the open Baltic Sea and its coastal areas. During the campaign we carried out simultaneous measurements of aerosol optical depth at 4 stations with the use of the hand-held Microtops II sun photometers. The studies were complemented with aerosol data provided by the MODIS. In order to obtain the full picture of aerosol situation over the study area, we added to our analyses the air mass back-trajectories at various altitudes as well as wind fields. Such complex information facilitated proper conclusions regarding aerosol optical depth and Ångström exponent for the four locations and discussion of the changes of aerosol properties with distance and with changes of meteorological factors. We also show that the Microtops II sun photometers are reliable instruments for field campaigns. They are easy to operate and provide good quality results.  相似文献   

4.
Fernald前向积分法能否用于机载大气探测激光雷达气溶胶后向散射系数的反演一直是一个有争议的课题.本文利用青岛机载大气探测激光雷达实测数据、国外机载大气探测激光雷达实测数据和机载大气探测激光雷达模拟数据,对Fernald前向积分法应用于不同高度的机载大气探测激光雷达气溶胶后向散射系数反演的误差进行了定量分析,分析结果表明:飞机的飞行高度在3.5 km左右,标定值存在20%的误差时,离地面2 km的高度范围内反演得到的气溶胶后向散射系数的相对误差在12%以内,但在标定点附近相对误差可达20%;飞机飞行高度在7 km左右,当标定值存在100%的误差时,反演得到的气溶胶后向散射系数的相对误差大都在10%~15%之间,标定值存在400%的误差时,反演得到的气溶胶后向散射系数的相对误差大部分在15%~50%之间.本文从理论上对Fernald前向积分法应用于机载大气探测激光雷达气溶胶后向散射系数反演出现负值的原因进行了探讨.研究表明:Fernald前向积分法能够较准确地反演出中高空探测(4.5 km以上)机载大气探测激光雷达气溶胶后向散射系数,但应用于低空探测(4.5 km以下)机载大气探测激光雷达气溶胶后向散射系数反演时,反演误差较大甚至反演结果会出现负值.  相似文献   

5.
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6.
The error-propagation characteristics of an implicit time integration algorithm in pseudodynamic testing are examined. It is shown that the implicit algorithm is superior to explicit integration algorithms in terms of experimental error amplification. The influence of systematic experimental errors is studied and methods for controlling these errors are examined. In spite of the fact that the implicit algorithm is unconditionally stable, it is shown that the integration time interval in a pseudodynamic test is limited by the calibration range of the electronic hardware as well as the degree of participation of the higher modes. Furthermore, the tolerance for experimental errors decreases as the integration time interval increases.  相似文献   

7.
We present a study on the retrieval sensitivity of the column-averaged dry-air mole fraction of CO2 (XCO2) for the Chinese carbon dioxide observation satellite (TanSat) with a full physical forward model and the optimal estimation technique. The forward model is based on the vector linearized discrete ordinate radiative transfer model (VLIDORT) and considers surface reflectance, gas absorption, and the scattering of air molecules, aerosol particles, and cloud particles. XCO2 retrieval errors from synthetic TanSat measurements show solar zenith angle (SZA), albedo dependence with values varying from 0.3 to 1 ppm for bright land surface in nadir mode and 2 to 8 ppm for dark surfaces like snow. The use of glint mode over dark oceans significantly improves the CO2 information retrieved. The aerosol type and profile are more important than the aerosol optical depth, and underestimation of aerosol plume height will introduce a bias of 1.5 ppm in XCO2. The systematic errors due to radiometric calibration are also estimated using a forward model simulation approach.  相似文献   

8.
Continuous measurements of solar spectral radiation using the Multi-filter Rotating Shadow Band Radiometer (MFRSR) are performed at the Actinometric Station of the National Observatory of Athens (ASNOA). The present study utilizes three clear-sky days of continuous observations, from local sunrise to local sunset, in order to investigate the daily variation of the radiation components (diffuse, global and direct-beam) as well as their ratios (diffuse-to-global, DGR, and diffuse-to-direct-beam, DDR) under different atmospheric conditions. Both ratios have received great scientific interest, especially for investigating solar irradiance modifications under various atmospheric conditions, aerosol load and optical properties. Apart from this, the present study shows that the DDR can constitute a measure of atmospheric turbidity when it is determined at longer wavelengths, while the DGR cannot. The effect of the solar zenith angle (SZA) on both ratios is significant at the shorter wavelengths with varying sensitivities depending on the aerosol field and sun elevation. The present study confirms the results obtained by previous solar irradiance measurements in Athens and also those computed via radiative transfer codes and sheds light on the scientific knowledge of the use of spectral DDR as an atmospheric turbidity index.  相似文献   

9.
Summary The phenomenon of the green flash at sunset (or sunrise) is reviewed. Several possible mechanisms that have been put forth to explain the green flash are discussed. A quantitative model for the phenomenon is then described, which includes parameters that are representative for a polar atmosphere (low humidity and small aerosol optical depth). It is supposed that the primary mechanism responsible for causing the green flash are natural molecular disperion, and, the filtering action imposed by the atmosphere for low elevation angles. Results from the model indicate that a green rim of vertical extent 0.15 milliradians would appear at the upper limb of the sun during sunset or sunrise. The theoretical results are compared with observations of a green flash made at wintertime in interior Alaska.  相似文献   

10.
Pyrgeometers are used to measure longwave terrestrial radiation. Regular pyrgeometer calibration against an internationally recognized standard is required in order to measure the longwave radiation consistently at different sites around the globe. At present, there is no internationally recognized standard to calibrate pyrgeometers. A well-characterized blackbody is, however, an accepted approach. This paper describes a method of establishing a precise blackbody reference and using it to calibrate a group of four transfer reference pyrgeometers. The group is then deployed outdoors to evaluate the precision of the blackbody calibration. The results from the outdoor data shows that the percentage mean-square-error of each transfer reference pyrgeometer is 0.12%, 0.07%, 0.46%, and 0.10% with a resultant percentage root-mean-square of 0.43%. The errors are calculated with respect to the average of the irradiance readings of the transfer reference pyrgeometers. To minimize the number of transfer reference pyrgeometers and to allow more space for calibrating test pyrgeometers, a sub-set of the transfer reference pyrgeometers is then used to calibrate a test pyrgeometer outdoors. The calibration of the test pyrgeometer resulted in reducing its error from +4.00% to ±0.32% with respect to the irradiance measured by the sub-set of the transfer reference pyrgeometers. The outdoor calibration method can minimize the calibration cost resulting from using the lengthy and costly blackbody calibration because many pyrgeometers can be calibrated at the same time.Appendix A shows a diagram that describes the paper's concept.  相似文献   

11.
This paper presents the results of the studies of aerosol optical properties measured using lidars and sun photometers. We describe two case studies of the combined measurements made in two coastal zones in Crete in 2006 and in Rozewie on the Baltic Sea in 2009. The combination of lidar and sun photometer measurements provides comprehensive information on both the total aerosol optical thickness in the entire atmosphere as well as the vertical structure of aerosol optical properties. Combination of such information with air mass back-trajectories and data collected at stations located on the route of air masses provides complete picture of the aerosol variations in the study area both vertically and horizontally. We show that such combined studies are especially important in the coastal areas where depending on air mass advection directions and altitudes the influence of fine or coarse mode (in this case possibly sea-salt) particles on the vertical structure of aerosol optical properties is an important issue to consider.  相似文献   

12.
Continuous measurements of solar spectral radiation using the multifilter rotating shadow band radiometer (MFRSR-7) are performed at the Actinometric Station of the National Observatory of Athens (ASNOA). The present study utilizes 4 days of continuous observations, from local sunrise to sunset, in order to investigate the daily variation of the radiation components (diffuse and global) as well as their ratio (diffuse-to-global irradiance ratio, DGR) under different atmospheric conditions. DGR has received a great scientific interest, as well as the respective diffuse-to-direct-beam ratio especially for investigating solar irradiance modifications under different atmospheric conditions, aerosol load and optical properties. Apart from this, the present study shows that the DGR can also constitute a powerful tool for cloud screening, i.e. for removing perturbed data due to cloud contamination from automated sun scanning radiometers. The relationship between DGR at a specific wavelength with the respective ratio for the whole MFRSR band (300–1100 nm) is found to exhibit a curvature; this curvature is strongly modified when perturbed irradiance data (possibly caused by clouds) occur. Even though the perturbed data can also be easily identified from the diurnal irradiance variation, the present study is the first to show the effect of perturbed solar spectral data on the DGR.  相似文献   

13.
The Bohai Sea is a semi-enclosed inland sea with case-2 waters near the coast. A comprehensive set of optical data was collected during three cruises in June, August, and September 2005 in the Bohai Sea. The vertical profile measurements, such as chlorophyll concentration, water turbidity, downwelling irradiance, and diffuse attenuation coefficient, showed that the Bohai Sea was vertically stratified with a relative clear upper layer superimposed on a turbid lower layer. The upper layer was found to correspond to the euphotic zone and the deep chlorophyll maximum (DCM) occurs at the base of this layer. By tuning a semi-analytical model (24 and 25) for the Bohai Sea, we developed a method to derive water inherent optical properties and the depth of DCM from above-surface measurements. Assuming a ‘fake’ bottom in the stratified water, this new method retrieves the ‘fake’ bottom depth, which is highly correlated with the DCM depth. The average relative error between derived and measured values is 33.9% for phytoplankton absorption at 440 nm, 25.6% for colored detrital matter (detritus plus gelbstoff) absorption at 440 nm, and 24.2% for the DCM depth. This modified method can retrieve water inherent optical properties and monitor the depth of DCM in the Bohai Sea, and the method is also applicable to other stratified waters.  相似文献   

14.

In this paper we have analyzed aerosol distribution over the Tibetan Plateau by using the global monthly mean satellite data of Stratospheric Aerosol and Gas Experiment II (SAGE II). The results are as follows: (1) Stratospheric aerosol optical depth can oscillate in the four seasons. It means that the aerosol optical depth is the thickest in winter and a little thinner in spring and the thinnest in summer and then a little thicker in autumn. We have found that the oscillation is caused by the oscillation of tropopause in different seasons. (2) Stratospheric aerosol comes mainly from sprays of volcano. After eruption of Mount Pinatubo aerosol optical depth in stratosphere over the Tibetan Plateau increases 10 times compared with before. (3) The characteristic of aerosol vertical distribution over the Tibetan Plateau is that there is an extremely high value at the altitude of 70 hPa. The most interesting thing is that the extremely high value can oscillate between 50 hPa and 100 hPa. We have verified that the oscillation is a unique characteristic over the Tibetan Plateau by comparing it with South China and North China. Then the radiative forcing and regional climate response over the Tibetan Plateau of aerosol are investigated. We have discovered such things as followed by: (1) The radiative forcing is positive because the parameterized aerosol optical depth is less than 0.14 which is the optical depth of the uniform background boundary aerosol layer. It is 0–3 W/m2 in January and 0–4 W/m2 in April and less than 3 W/m2 in July and 3–6 W/m2 in October. (2) Ground temperature rises 0.1–0.2 K in October which is the biggest increasing magnitude, and 0.01–0.02 in July which is the smallest one. It rises about 0.05-0.01 K in January and April. (3) Air temperature near the earth’s surface and the one at the altitude of 500 hPa rise too, but the increasing magnitude is less than the former one.

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15.
Adaptive time stepping with embedded error control is applied to the mixed form of Richards equation. It is the first mathematically based adaptive scheme applied to this form of Richards equation. The key to the method is the approximation of the local truncation error of the scheme in terms of the pressure head, although, to enforce mass conservation, the principal time approximation is based on the moisture content. The time stepping scheme is closely related to an implicit Thomas–Gladwell approximation and is unconditionally stable and second-order accurate. Numerical trials demonstrate that the new algorithm fully automates stepsize selection and robustly constrains temporal discretisation errors given a user tolerance. The adaptive mechanism is shown to improve the performance of the non-linear solver, providing accurate initial solution estimates for the iterative process. Furthermore, the stepsize variation patterns reflect the adequacy of the spatial discretisation, here accomplished by linear finite elements. When sufficiently dense spatial grids are used, the time step varies smoothly, while excessively coarse grids induce stepsize oscillations.  相似文献   

16.
UV-B solar irradiance and meteorological variables were measured at the C.I.B.A. site (Low Atmosphere Research Laboratory), University of Valladolid, Spain, between January 2003 and March 2006. Calculated Ångström turbidity and aerosol optical thickness values were evaluated from the direct downward irradiance, surface pressure, air temperature and relative humidity values. Monthly turbidity β values for an average year showed minimum values in winter and maximum values in summer. The obtained values are according to an aerosol standard atmosphere between mean and clean continental model. UV-B model calculations were performed using a radiative transfer tropospheric model, TUV 4.1a; measured and calculated UV-B irradiance values were compared in order to establish possible values of single scattering albedo. The results show that calculated single scattering albedo values are according to an aerosol standard atmosphere between mean and polluted continental model.  相似文献   

17.
In this paper, we derive analytical expressions for one‐way and two‐way kinematical parameters in elliptical tilted transverse isotropy media. We show that the homogeneous elliptical tilted transverse isotropy models result in hyperbolic moveout with a reflection point sideslip x0, which can be considered as an additional traveltime parameter for one‐way wave propagation. For homogeneous elliptical tilted transverse isotropy models we show that the inversion of one‐way traveltime parameters suffers from the ambiguity for large tilts. It is shown that the accuracy of the inversion is sensitive to the error in x0. We also derive and invert the traveltime parameters for a vertically heterogeneous elliptical tilted transverse isotropy model with a tilt gradually changing with depth. The a priori knowledge of parameter δ is very important for inversion. The wrong choise of this parameter results in significant errors in inverted model parameters.  相似文献   

18.
This paper, based on a real world case study (Limmat aquifer, Switzerland), compares inverse groundwater flow models calibrated with specified numbers of monitoring head locations. These models are updated in real time with the ensemble Kalman filter (EnKF) and the prediction improvement is assessed in relation to the amount of monitoring locations used for calibration and updating. The prediction errors of the models calibrated in transient state are smaller if the amount of monitoring locations used for the calibration is larger. For highly dynamic groundwater flow systems a transient calibration is recommended as a model calibrated in steady state can lead to worse results than a noncalibrated model with a well-chosen uniform conductivity. The model predictions can be improved further with the assimilation of new measurement data from on-line sensors with the EnKF. Within all the studied models the reduction of 1-day hydraulic head prediction error (in terms of mean absolute error [MAE]) with EnKF lies between 31% (assimilation of head data from 5 locations) and 72% (assimilation of head data from 85 locations). The largest prediction improvements are expected for models that were calibrated with only a limited amount of historical information. It is worthwhile to update the model even with few monitoring locations as it seems that the error reduction with EnKF decreases exponentially with the amount of monitoring locations used. These results prove the feasibility of data assimilation with EnKF also for a real world case and show that improved predictions of groundwater levels can be obtained.  相似文献   

19.
Solar irradiance, skylight, and aureole intensities were measured early in spring 1981 in Kensington, Australia. The aerosol optical thickness, the aureole intensities, and the scattering phase function at 5° suggest that scattering was due to aerosols of very small sizes. The estimates of the weighted aerosol mean square radius,p 2, seem to lead to the same conclusion.  相似文献   

20.
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