Modeling and measurement of rainfall by ground-based multispectral microwave radiometry

The potential of ground-based multispectral microwave radiometers in retrieving rainfall parameters is investigated by coupling physically oriented models and retrieval methods with a large set of experimental data. Measured data come from rain events that occurred in the USA at Boulder, Colorado, and at the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site in Lamont, OK. Rain cloud models are specified to characterize both nonraining clouds, stratiform and convective rainfall. Brightness temperature numerical simulations are performed for a set of frequencies from 20 to 60 GHz at zenith angle, representing the channels currently deployed on a commercially available ground-based radiometric system. Results are illustrated in terms of comparisons between measurements and model data in order to show that the observed radiometric signatures can be attributed to rainfall scattering and absorption. A new statistical inversion algorithm, trained by synthetic data and based on principal component analysis is also developed to classify the meteorological background, to identify the rain regime, and to retrieve rain rate from passive radiometric observations. Rain rate estimate comparisons with simultaneous rain gauge data and rain effect mitigation methods are also discussed.     

Modifications to the Water Vapor Continuum in the Microwave Suggested by Ground-Based 150-GHz Observations

Ground-based observations from two different radiometers are used to evaluate commonly used microwave/millimeter-wave propagation models at 150 GHz. This frequency has strong sensitivity to changes in precipitable water vapor (PWV) and cloud liquid water. The observations were collected near Hesselbach, Germany, as part of the Atmospheric Radiation Measurement program's support of the General Observing Period and the Convective and Orographic Precipitation Study. The observations from the two radiometers agree well with each other, with a slope of 0.993 and a mean bias of 0.12 K. The observations demonstrate that the relative sensitivity of the different absorption models to PWV in clear-sky conditions at 150 GHz is significant and that four models differ significantly from the observed brightness temperature. These models were modified to get agreement with the 150-GHz observations, where the PWV ranged from 0.35 to 2.88 cm. The models were modified by adjusting the strength of the foreign- and self-broadened water vapor continuum coefficients, where the magnitude was model dependent. In all cases, the adjustment to the two components of the water vapor continuum was in opposite directions (i.e., increasing the contribution from the foreign-broadened component while decreasing contribution from the self-broadened component or vice versa). While the original models had significant disagreements relative to each other, the resulting modified models show much better agreement relative to each other throughout the microwave spectrum. The modified models were evaluated using independent observations at 31.4 GHz.     

Comparison of Ground-Based Millimeter-Wave Observations and Simulations in the Arctic Winter

During the Radiative Heating in Underexplored Bands Campaign (RHUBC), held in February–March 2007, three millimeter-wave radiometers were operated at the Atmospheric Radiation Measurement Program's site in Barrow, Alaska. These radiometers contain several channels located around the strong 183.31-GHz water vapor line, which is crucial for ground-based water-vapor measurements in very dry conditions, typical of the Arctic. Simultaneous radiosonde observations were carried out during conditions with very low integrated-water-vapor (IWV) content ( $≪ 2 hbox{mm}$). Observations from the three instruments are compared, accounting for their different design characteristics. The overall agreement during RHUBC among the three instruments and between instruments and forward model is discussed quantitatively. In general, the instrument cross-validation performed for sets of channel pairs showed agreement within the total expected uncertainty. The consistency between instruments allows the determination of the IWV to within around 2% for these dry conditions. Comparisons between these data sets and forward-model simulations using radiosondes as input show spectral features in the brightness-temperature residuals, indicating some degree of inconsistency between the instruments and the forward model. The most likely cause of forward-model error is systematic errors in the radiosonde humidity profiles.      

The measurement of atmospheric water vapor: radiometer comparisonand spatial variations

Two water vapor radiometer (WVR) experiments were conducted to evaluate whether such instruments are both suitable and necessary to correct for propagation effects that are induced by precipitable water vapor (PWV) on signals from the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). WVRs are suitable for these corrections if they provide wet path delays to better than 0.5 cm. They are needed if spatial variations of PWV result in complicated, direction-dependent propagation effects that are too complex to be parameterized in the GPS or VLBI geodetic solution. In the first experiment, the suitability of radiometers were addressed by comparing six WVRs at Stapleton International Airport in Denver, Colorado, for two weeks. The second experiment addressed the question whether radiometers are needed for the detection of inhomogeneities in the wet delay. Three JPL D-series radiometers were operated at three sites in Colorado approximately 50 km apart. The WVRs simultaneously sampled PWV at different azimuths and elevations in search of spatial variations of PWV     

A comparison of ground-based and satellite-borne microwaveradiometric observations in the Great Plains

The authors compare ground-based and the special sensor microwave/imager (SSM/I) brightness temperatures at 19 and 37 GHz in the Northern and the Southern Great Plains. The comparison was conducted to examine season-related differences in plot-scale and satellite footprint-scale brightness temperatures at these frequencies. The ground-based observations were from the three Radiobrightness Energy Balance Experiments (REBEXs), viz., REBEX-1, REBEX-4, and REBEX-5. REBEX-1 and REBEX-4 were conducted near Sioux Falls, SD, in fall and winter 1992-93, and in summer 1996, respectively. REBEX-5 was conducted near Lamont, OK, during summer 1997 as part of the Southern Great Plains Hydrology Experiment-1997 (SGP'97). The instantaneous fields of view (FOV) of the ground-based radiometers were only a few meters compared to those of the SSM/I, which were several tens of kilometers. The REBEX and the SSM/I brightness temperatures are moderately correlated at both the 19 and 37 GHz. They match well during winter when there was uniform snow cover over the SSM/I footprint. During spring, summer, and fall, REBEX brightness temperatures at the grass-site were on average 18 K higher than the SSM/I brightness temperatures because the SSM/I footprint included nearby agricultural fields in summer and predominantly bare soil in fall and spring. During summer, REBEX-4 brightness temperatures at the bare soil site were on average 10 K cooler than the SSM/I brightness temperatures. In effect, the REBEX grass and bare soil brightness temperatures bracket the SSM/I observations with the SSM/I brightness temperatures lying closest to those of the bare soil     

Atmospheric moisture measurements: a microwaveradiometer-radiosonde comparison

The NASA/Goddard Space Flight Center Crustal Dynamics Project microwave Water Vapor Radiometer (WVR-J03) is used to measure the thermal emission of the sky at three frequencies (20.7, 22.2, and 31.4 GHz). Measurements were taken during the Atmospheric Moisture Intercomparison Study (ATMIS) held at Wallops Island, VA during April 1989. These measurements were compared with brightness temperatures inferred from measurements from VAISALA radiosonde packages launched every 3 h during the experiment period. An error analysis for the radiosonde-inferred brightness temperatures was performed, assuming reasonable random uncertainties for the pressure, temperature, and humidity measurements and propagating these uncertainties through the analysis algorithm. Two different water vapor emission models were used for the derivation of the brightness temperatures from the radiosonde measurements. Differences between the two models increase as the moisture content increases and vary as a function of frequency     

A three-year study of radio wave propagating delays due to tropospheric water vapor

Measurements of the radio emission from the sky at 21.0 and 31.4 GHz have been made from May 1980 to April 1983 at the Onsala Space Observatory on the west coast of Sweden. A total of 483 such observations have been used to calculate the water vapor induced propagation delay of radiowaves penetrating the troposphere. A comparison with simultaneous radiosonde launches made at Gothenburg-Landvetter Airport (37 km away from Ousala) gives a root mean square (rms) difference of 1.3 cm in the zenith direction and 1.1 cm for a certain group of stable weather data. The zenith wet path delays are also calculated with three different models based on traditional meteorological observations at the surface. The lowest rms difference obtained by using a model and compare the results with radiosonde data at the same site was 2.1 cm. A third site on the Swedish west coast was compared with the other two by using old monthly means of radiosonde data together with the three models. The offsets between the models and the radiosonde/water vapor radiometer (WVR) data are dependent on the model, on the site, and on the time of the year.     

Thermal emission from a layered medium bounded by a slightly roughinterface

The small perturbation method (SPM) is applied to study thermal emission from a layered medium bounded by a slightly rough interface. Brightness temperatures are calculated to second order in surface height, including both specular reflection coefficient corrections and incoherent Bragg scatter terms. Unlike the homogeneous medium case, in which the SPM applied for emission predictions produces an expansion in surface slope, the theory remains a small height expansion, and convergence of the series is shown to depend on properties of the layered medium. Results from this theory can be applied in studies of soil moisture, sea ice, or sea surface remote sensing and buried object detection with microwave radiometers     

Validation of ASTER/TIR standard atmospheric correction using water surfaces

The standard atmospheric correction algorithm for the five thermal infrared (TIR) bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is based on radiative transfer calculation using the MODTRAN code. Atmospheric profiles input to MODTRAN are extracted from either the Global Data Assimilation System (GDAS) product or the Naval Research Laboratory (NRL) climatology model. The present study provides validation results of this algorithm. First, in situ lake surface temperatures measured in 13 vicarious calibration (VC) experiments were compared with surface temperatures retrieved from ASTER data. As the results, the mean bias was 0.8 and 1.8 K for GDAS and NRL, respectively. The NRL model performed worse than GDAS for four experiments at Salton Sea, CA, probably because the model was not suitable for this site, which has typically higher surface temperature and humidity than other VC sites. Next, the algorithm was validated based on the max-min difference (MMD) of water surface emissivity retrieved from each of 163 scenes acquired globally. As a result, the algorithm error increased quadratically with the precipitable water vapor (PWV) content of the atmosphere, and the expected MMD error was 0.049 and 0.067 for GDAS and NRL, respectively, with a PWV of 3 cm, where 0.05 on MMD is roughly corresponding to -0.8 or +2.3 K on the retrieved surface temperature error. The algorithm performance degraded markedly when the surface temperature exceeded about 25/spl deg/C, particularly for NRL. Consequently, GDAS performs better than NRL as expected, while both will perform less well for humid conditions.     

ACTS propagation experiment: experiment design, calibration, anddata preparation and archival

The National Aeronautics and Space Administration Advanced Communications Technology Satellite (ACTS) propagation experiment was designed to obtain slant-path attenuation statistics for locations within the United States and Canada for use in the design of low-margin Ka-band satellite communication systems. Experimenters at seven different locations have collected propagation data for move than two years. The propagation terminals used for the experiment were identical. A single preprocessing program was used by the experimenters to provide for automatic calibration, generation of attenuation histograms, and data archival. In this paper, the calibration procedures are described mid estimates given for measurement accuracy. ACTS provided beacons at 20.2 and 27.5 GHz for use in making attenuation measurements. In addition to the beacon receivers, each ACTS propagation terminal has two total power radiometers with center frequencies at the beacon frequencies. The radiometers are used to establish the beacon signal reference levels needed for calculating beacon attenuation values. For the combined radiometer and beacon measurement system, the attenuation measurement error was less than a maximum of 1.0 dB and was generally less than 0.3 dB. The dynamic range for attenuation measurement varied from site to site depending on location relative to the peak of the satellite beacon antenna pattern. For locations within the continental United States, the dynamic range was better than 20 dB     

Effect of scattering on radiometer measurements of attenuation in rain

Calculations have been made that take into account the effect of scattering by rain when computing attenuation from aerial temperatures measured with radiometers. Results for the frequency range 37?110 GHz show that attenuation can be significantly underestimated if it is assumed to arise solely from absorption.     

基于改进射线描迹法的对流层斜延迟估计

该文针对传统对流层延迟模型和射线描迹法在估计对流层延迟方面的局限性,如效率低、成本高、精度受地表参数和探空数据限制等不足,提出一种基于改进射线描迹法的对流层斜延迟估计方法。该方法结合中纬度大气模式气象参数公式和UNB3m气象参数模型,改进了射线描迹法中折射率剖面的计算,克服了气象数据对射线描迹法的限制。选取亚洲地区10个站点2012年的气象数据,分别采用改进射线描迹法和传统对流层延迟模型估计各个站天顶方向至 高度角区间15个方向的对流层斜延迟,并与基于探空数据获取的对流层斜延迟真值进行比较,计算结果表明该方法的估计精度优于传统对流层延迟模型,为非气象数据情况下对流层斜延迟实时估计提供了新的思路。     

WVR-GPS comparison measurements and calibration of the 20-32 GHz tropospheric water vapor absorption model

Collocated measurements of opacity (from water vapor radiometer brightness temperatures) and wet path delay (from ground-based tracking of global positioning satellites) are used to constrain the model of atmospheric water vapor absorption in the 20-32 GHz band. A differential approach is presented in which the slope of opacity-versus-wet delay data is used as the absorption model constraint. This technique minimizes the effects of radiometric calibration errors and oxygen model uncertainties in the derivation of a best-fit vapor absorption model. A total of approximately five months of data was obtained from two experiment sites. At the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, three independent water vapor radiometers (WVRs) provided near-continuous opacity measurements over the interval July-September 1998. At the NASA/Goldstone tracking station in the California desert two WVRs; obtained opacity data over the September-October 1997 interval. At both sites a Global Positioning Satellite (GPS) receiver and surface barometer obtained the data required for deriving the zenith wet delays over the same time frames. Measured values of the opacity-versus-wet delay slope parameter were obtained at four WVR frequencies (20.7, 22.2, 23.8, and 31.4 GHz) and compared with predictions of four candidate absorption models referenced in the literature. With one exception, all three models provide agreement within 5% of the opacity-versus-wet delay slope measurements at all WVR frequencies at both sites. One model provides agreement for all channels at both sites to the 2-3% level. This absorption model accuracy level represents a significant improvement over that attainable using radiosondes.     

Wet Reflectors in Millimeter-Wave Radiometry-Expenrment and Theory

Microwave radiometers are often required to operate under all weather conditions, including those under which an exposed surface of the antenna system becomes wetted by rain. Here we discuss the special case of a wetted flat reflector, with energy at 20.60 and 31.65 GHz linearly and orthogonally polarized at an incidence angle of 45°. The brightness temperatures produced by the wet reflector are computed from conventional theory, and are measured for various thicknesses of water layer. Both theory and experiment show that the brightness temperatures produced by the wet reflector have the same behavior throughout a range of water-layer thicknesses. The impact of these results on design of radiometers for research and operational application is discussed.     

被动微波遥感大气校正

以星载微波辐射计AMSR-E的频段设置和亮温数据为参考,利用一年的大气廓线数据,对与过境时间相匹配的AMSR-E亮温数据进行了大气校正,并通过计算校正前后的微波植被指数(MVI)分析了大气校正的有效性;最后选择一天的AMSR-E亮温数据,利用现有的大气参数产品进行大气校正.校正过程中,以求取地表发射率为中间过程.结果显示,能够实现全天时全天候条件下的星载微波辐射计数据的大气校正,算法成功有效,有潜力进行推广.     

Surface-Based Passive Microwave Observations of Sea Ice in the Bering and Greenland Seas

Measurements of brightness temperatures of sea ice were carried out during both the MIZEX field experiments in 1983, the first in Februrary in the Bering Sea and the second in June/July in the northern Greenland Sea. In the Bering Sea thin growing sea ice types from black ice to 400-mm-thick snow-covered floes were investigated. Brightness temperatures increased with ice thickness up to 100 mm from values of 100 K for open water to as high as 250 K (e= 0.97) for thick ice, and a moderate dependence on snow thickness was found. In the Greenland Sea thick first-year and multiyear ice types were studied. Brightness temperatures were quite variable depending on the daily melt-freeze cycle superimposed on the seasonal warming, ranging from near blackbody values for melting conditions to multiyear-like spectra when the surface layers refroze. The melt season was sufficiently advanced, however, that first-year and multiyear ice could not be differentiated radiometrically.     

Optimum Strategies and Performance for the Remote Sensing of Path-Delay using Ground-Based Microwave Radiometers

Computer simulations have been used to study the accuracy with which excess radio-propagation path delay due to atmospheric water vapor can be determined using a microwave remote-sensing technique. A number of strategies were investigated for the remote sensing of path delay with the objective of providing a broad foundation for the development and use of water-vapor radiometers (WVR's) for geodetic applications. A data base of nearly 10 000 radiosondes from a variety of climatological regions was used for the study. Strategies were judged by their "retrieval performance" on this data base; i.e., by the rms difference between simulations of "retrieved" path delay and "true" path delay, at the zenith, averaged over a radiosonde data base. An observing approach using the frequency set 20.7/22.2/31.4 GHz was found to be close to optimum. A statistical retrieval approach using retrieval coefficients stratified for clear and cloudy weather and for individual location was found to offer a substantial improvement over the use of a single all-weather set of retrieval coefficients. It was found that a reasonably well optimized WVR, with an estimated calibration uncertainty of 0.5 K, can achieve an overall retrieval performance of: 0.27 cm, clear; 0.51 cm, cloudy; and 0.38 cm, all weather average. The weather-averaged retrieval performance for individual locations was found to vary by no more than 14 percent from the average for all locations, in spite of the fact that the mean path delay for these locations ranged from 5 to 26 cm.     

基于微波高光谱技术的数据仿真及参数反演研究

发展高光谱微波辐射计对于提升大气参数反演精度具有重要意义。利用微波辐射传输模型mpm93以及BP 神经网络方法分别构建正演上行辐射亮温和反演大气温度廓线的模型,并研究了晴空条件下高光谱微波辐射计反演大气温度廓线的精度。54～58 GHz、64～68 GHz 在氧气吸收波段选取80 个通道作为高光谱通道,基于2015 年5～12 月昆明的探空资料进行正、反演仿真实验。选取微波成像仪/ 探测仪(SSMIS)的9 个温度探测通道进行对比实验,评估分析反演效果。实验结果表明:在大气3～10 km 高度范围内,高光谱通道的反演精度较SSMIS 提高了0.3 ～0.6 K;在0～3 km 高度范围内,反演精度提高了1 K。     

陆面温度的反演算法和大气订正的影响

提出了由卫星辐射测量反演陆面温度的算法,可在不需要预先给定通道的地面比辐射率的情况下,同时确定地面温度和比辐射率。同时,为了了解实际应用这种算法的可能性,在一系列陆面温度反演的模拟试验中,考察分析了在大气温度和湿度廓线有各种不同的误差时大气订正对陆面温度反演精度的影响。结果表明,在大气证正中应用的大气温度和湿度的误差不超过通常由探空和卫星遥感得到的大气温度和湿度的误差时,反演得到的地面温度仍可具有     

Multifrequency Measurements of the Effects of Soil Moisture, Soil Texture, And Surface Roughness

An experiment on remote sensing of soil moisture content was conducted over bare fields with microwave radiometers at the frequencies of 1.4, 5, and 10.7 GHz, during July-September of 1981. Three bare fields with different surface roughnesses and soil textures were prepared for the experiment. Ground-truth acquisition of soil temperatures and moisture contents for 5 layers down to the depths of 15 cm was made concurrently with radiometric measurements. The experimental results show that the effect of surface roughness is to increase the soil's brightness temperature and to reduce the slope of regression between brightness temperature and moisture content. The slopes of regression for soils with different textures are found to be comparable and the effect of soil texture is reflected in the difference of regression line intercepts at brightness-temperature axis. The result is consistent with laboratory measurement of soil's dielectric permittivity. Measurements on wet smooth bare fields give lower brightness temperatures at 5 than at 1.4 GHz. This phenomenon is not expected from current radiative transfer theory, using laboratory measurements of the relationship between dielectric permittivity and moisture content for different soil-water mixtures at frequencies of <5 GHz.