Tropospheric wet path-delay measurements

A dual-channel microwave radiometer measuring the sky brightness temperature at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer (IRSH) measuring the ratio of the radiation from the sun at the wavelengths 931 and 880 nm, and radiosondes have been used simultaneously to determine the excess path length due to water vapor (wet path delay) of radio waves propagating through the troposphere. By a least squares fit of the measured parameters from the microwave radiometer and the infrared spectral hygrometer, respectively, to the wet path delay calculated from the radiosonde profiles, the following root mean square (rms) differences of the wet path delay in the zenith direction were obtained: infrared spectral hygrometer-radiosondes, 1.1 cm; microwave radiometer-radiosondes, 0.7 cm; and 0.5 cm for a selected group of "good weather" data. The wet path delay was also calculated from surface meteorological measurements alone and the rms difference compared with corresponding radiosonde data was 2.0 cm in the zenith direction.     

Forward model studies of water vapor using scanning microwave radiometers, global positioning system, and radiosondes during the cloudiness intercomparison experiment

Brightness temperatures computed from five absorption models and radiosonde observations were analyzed by comparing them with measurements from three microwave radiometers at 23.8 and 31.4 GHz. Data were obtained during the Cloudiness Inter-Comparison Experiment at the U.S. Department of Energy's Atmospheric Radiation Measurement Program's (ARM) site in North-Central Oklahoma in 2003. The radiometers were calibrated using two procedures, the so-called instantaneous "tipcal" method and an automatic self-calibration algorithm. Measurements from the radiometers were in agreement, with less than a 0.4-K rms difference during clear skies, when the instantaneous method was applied. Brightness temperatures from the radiometer and the radiosonde showed a bias difference of less than 0.69 K when the most recent absorption models were considered. Precipitable water vapor (PWV) computed from the radiometers were also compared to the PWV derived from a Global Positioning System station that operates at the ARM site. The instruments agree to within 0.1 cm in PWV retrieval.     

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.     

Measurements of Earth-Space Attenuation at 230 GHz

Measurements of attenuation at 230 GHz through the total atmosphere due to the presence of oxygen and water vapor molecules, clouds, and rain are presented and discussed. The measurements were carried out using a specially designed superhetrodyne receiver mounted on a sun tracker. Simultaneons measurements were also carried out at 13 GHz. For a measuring site close to sea level at Holmdel, NJ, the "clear-sky" zenith attenuation was found to be given by A (dB) = 0.35 rho, where rho was the measured ground water vapor density in g/m/sup 3/. When the ground temperature was below about 7/spl deg/C, most cloud and overcast gave < 0.5 -dB attenuation whereas with a ground temperature greater than 13/spl deg/C, cloud attenuation was 8-10 times greater. Calculations of zenith attenuation in the 230-GHz atmospheric window were also made using the Gross analytic line shape, Schulze-Tolbert empirical line shape, and an empirically modified Gross line shape. These calculations were based on determinations of water vapor density and temperature made at the measurement site, and on radiosonde measurements made at a distance of 80 km away. Measured and calculated results are graphically compared. It is concluded that either the modified Gross line shape or the Schulze-Tolbert line shape gives conservative estimates of zenith attenuation at 230 GHz for clear days, while the Gross line shape gives fair agreement with measured results.     

Atmospheric water vapor retrieval by means of both a GPS networkand a microwave radiometer during an experimental campaign in Cagliari,Italy, in 1999

Considers the remote sensing of atmospheric integrated precipitable water vapor (IPWV) using a Global Positioning System (GPS) network in the Mediterranean area during the whole year 1999. The assessment of IPWV retrieval has been carried out by means of an experimental campaign conducted in the proximity of the city of Cagliari, Italy, located on the southern coast of Sardinia in the Mediterranean Sea. The authors have compared GPS estimates with co-located measurements from a microwave dual-channel water vapor radiometer (WVR) collected in nonprecipitating conditions and with observations from radiosondes released four times a day from the Cagliari-Elmas Airport, approximately 15 km away from the GPS and WVR site. The long-term comparison has shown a fairly good agreement among the two remote sensors and the radiosonde observations, with a higher correlation between GPS and WVR     

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.     

Microwave attenuation measurements in satellite-ground links: thepotential of spectral analysis for water vapor profiles retrieval

The authors address the problem of estimating vertical profiles of atmospheric water vapor by means of attenuation measurements simultaneously made at different frequencies along a vertical satellite-ground link. The operating frequencies selected are those around the spectral absorption lines of water vapor at 22.235 GHz. A simulation is presented of multifrequency attenuation measurements, based on an atmospheric propagation model and on radiosonde data providing true profiles of temperature, pressure, and water vapor. The results indicate that such multifrequency measurements are correlated to variations of the vertical profiles of water vapor. It is therefore expected that vertical detail of such profiles depends on number and position of the frequencies utilized     

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.      

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。     

Statistical temperature profile retrievals in clear-air usingpassive 118-GHz O2 observations

Linear statistical temperature profile retrievals from nadiral passive 118-GHz O₂ spectra are demonstrated using time- and space-coincident microwave observations and radiosonde profiles. Separate retrievals are demonstrated for winter and summer midlatitude conditions in clear air over land; observations during both day and night are included. The retrieval operator is a linear-statistical minimum mean-squared-error estimator. A purely statistical retrieval operator circumvents the effects of radiative transfer model uncertainties and biases in either the radiosonde or Millimeter-wave Temperature Sounder data. The retrieved profile rms errors are ~0.7-1.2 K for either the winter or summer tropospheres     

热红外遥感中大气透过率的研究(一):大气透过率模式的构建

大气透过率是热红外遥感中的一个重要参数。通过辐射传输模型MODTRAN模拟热红外波段的大气透过率,构建了基于大气模型、气溶胶模型、水汽量、能见度和观测天顶角等5个因素的大气透过率查找表,分析了不同参数对热红外大气透过率光谱曲线的影响,通过方差分析确定了影响大气透过率的关键因子,针对不同类型的气溶胶模型,构建了基于水汽量、能见度和观测天顶角的常用卫星传感器热红外通道的大气透过率经验模式,解决了卫星热红外遥感中大气透过率精确计算的问题。     

Comparison of MM5 integrated water vapor with microwave radiometer, GPS, and radiosonde measurements

A large dataset of concurrent integrated precipitable water vapor (IPWV) estimates from ground-based microwave radiometers (MWRs), global positioning system (GPS) ground-receivers, and radiosonde observations (RAOBs) has been collected in five different sites in Central Italy. Both MWRs and GPS have shown a capability of accurate and continuous water vapor monitoring. These data are used to study the seasonal and spatial variability of IPWV. A comparison of these data with the IPWV field produced operationally by the nonhydrostatic Mesoscale Model (MM5), running at the University of L'Aquila/Center of Excellence (CETEMPS) is performed in order to find either model shortcomings and to corroborate the IPWV behavior highlighted by the measurements. Both measurements and model outputs span over a period of about one year allowing for a systematic statistical analysis for all the examined stations. The statistical analysis shows a good agreement between GPS and MWR data, whereas discrepancies are found between RAOBs and the other techniques. The IPWV shows the largest diurnal variability, approximately 3%, during the summer season. An overall good agreement is found between the forecasted and observed IPWV. The related statistical parameters show a very low bias (0.001 cm) with a good correlation coefficient (0.939). On the other hands, the seasonal analyses highlight a few discrepancies, mostly due to the MM5 difficulties in correctly forecasting the diurnal cycle.     

Performance characteristics of a 300-GHz radiometer and some atmospheric attenuation measurements

A 300-GHz Dicke-type superheterodyne radiometer receiver was used for measurements of atmospheric attenuation of electromagnetic waves over an open path at frequencies near 300 GHz. The average measured values of horizontal attenuation at 304 GHz and 316 GHz, presumably due to atmospheric water vapor absorption, were, respectively, 3.35 dB/km and 5.55 dB/km per g/m³of water vapor density. Absorption variations at 304 GHz with respect to water vapor density were shown in the measured results. The variation of the effective zenith sky temperature with respect to atmospheric water vapor density was also determined. The minimum detectable temperature difference(Delta T)_{min}, was obtained by measuring the rms value of noise in the receiver output. The best value achieved was3.16degK. Based on this result, the receiver noise figure and the mixer conversion loss were determined indirectly. The results were 31.4 dB and 22.9 dB, respectively. A blackbody radiation source served to calibrate the radiometer.     

利用RTKLIB提取对流层延迟方法与精度评估

全球卫星导航系统可以进行精确的水汽估算,能够成功地应用于天气预报中,比如数值天气预报模型.利用精密单点定位技术提取天顶对流层延迟,采用RTKLIB开源软件进行静态精密单点定位解算并提取天顶对流层延迟估值,并与国际GNSS服务IGS提供的参考值进行比较,评估其对流层解算精度.选取中国3个IGS观测站数据进行试验,结果表明...     

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.     

60 km瑞利多普勒激光雷达及其风场探测

临近空间风场的探测,在大气动力学研究和提高数值天气预报的准确性,以及航空航天保障等方面具有重要意义。研制基于瑞利散射双边缘技术的60 km多普勒激光雷达用于临近空间大气风场的测量。激光雷达主要分为垂直指向测量系统和两台斜指向测量系统。工作波长355 nm,探测距离15~60 km。为验证系统的可靠性和积累风场观测数据,于2014年下半年进行了外场实验,并与当地的探空气球数据进行对比,结果显示60 km瑞利多普勒激光雷达风场测量数据与探孔气球数据具有良好的一致性。     

Attenuation and emission of the atmosphere at 3.3 mm

Four empirical relationships are presented between the 3.3-mm attenuation determined from observations of the sun and 1) radiosonde measurements of the total amount of precipitable water in the atmosphere, 2) surface measurements of absolute humidity, 3) IR spectral hygrometer observations of the sun, and 4) observations of differential atmospheric emission at 3.3 mm. Fluctuations in atmospheric emission at 3.3 mm during good weather do not put any limit on the sensitivity of a dual-beam observing system using a receiver whose rms noise fluctuations in an output bandwidth of 0.25 Hz areapprox0.5degK.     

Retrieval of tropospheric water vapor scale height from horizontalturbulence structure

A scale height of the vertical water vapor distribution in the troposphere is shown to be related to the rate at which the total integrated water vapor (IWV) decorrelates with horizontal separation. This relationship is based on the departure from simple Kolmogorov behavior of the turbulence structure of the IWV as the horizontal separation becomes a significant fraction of the scale height of the three dimensional (3D) turbulence. The relationship is demonstrated by comparisons between direct measurements of the vertical water vapor distribution, by radiosondes, and coincident estimates of the horizontal turbulence structure, using the TOPEX Microwave Radiometer (TMR). This provides a new method by which to resolve some of the vertical structure of lower tropospheric water vapor from space. The turbulence structure estimator is applied to a larger body of TMR data to produce a sequence of images describing the dynamics of water vapor scale height across the tropical Pacific Ocean. The cyclical evolution of a basin scale east/west ridge of water vapor with high scale height near 5° north latitude is detected which is consistent with other observations of the Madden and Julian Oscillation. The general technique presented should be applicable to many other existing data sets which image the horizontal distribution of IWV, e.g., those of the Defense Meteorological Satellite Program's special sensor microwave/imagers     

Retrieval of Atmospheric Water Vapor Density With Fine Spatial Resolution Using Three-Dimensional Tomographic Inversion of Microwave Brightness Temperatures Measured by a Network of Scanning Compact Radiometers

Quantitative precipitation forecasting is currently limited by the paucity of observations on sufficiently fine temporal and spatial scales. Three-dimensional water vapor fields can be retrieved with improved spatial coverage from measurements obtained using a network of scanning microwave radiometers. To investigate this potential, an observation system simulation experiment was performed in which synthetic examples of retrievals using a network of radiometers were compared with results from the Weather Research and Forecasting model at a grid scale of 500 m. These comparisons show that the 3-D water vapor field can be retrieved with an accuracy of better than 15%-20%. A ground-based demonstration network of three compact microwave radiometers was deployed at the Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma. Results using these network measurements demonstrated the first retrieval of the 3-D water vapor field in the troposphere at fine spatial and temporal resolutions.     

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