Search

To better understand the interactions between C-band radar waves and snow, a tower-based experiment was set up in the Idaho Rocky Mountains for the period of 2021–2023. The experiment objective was to improve understanding of the sensitivity of Sentinel-1 C-band backscatter radar signals to snow. The data were collected in the time domain to measure the backscatter profile from the various snowpack and ground surface layers. The data show that scattering is present throughout the snow volume, although it is limited for low snow densities. Contrasting layer interfaces, ice features and metamorphic snow can have considerable impact on the backscatter signal. During snow melt periods, wet snow absorbs the signal and the soil backscatter becomes negligible. A comparison of the vertically integrated tower radar data with Sentinel-1 data shows that both systems have a similar temporal behavior, and both feature an increase in backscatter during the dry snow period in 2021–2022, even during weeks of nearly constant snow depth, likely due to morphological changes in the snowpack. The results demonstrate that C-band radar is sensitive to the dominant seasonal patterns in snow accumulation, but that changes in microstructure, stratigraphy, melt-freeze cycles, and snow wetness may complicate satellite-based snow depth retrievals. [ABSTRACT FROM AUTHOR]

Monitoring atmospheric water is essential for the understanding of the dynamic processes of the atmosphere and for the assessment of wave-propagation properties. Microwave radiometers, in combination with a thermal infrared channel, have the potential to fulfill these tasks. This paper is focused on the surface-based system TROWARA with microwave channels at 21.3 and 31.5 GHz. TROWARA has been used for tropospheric water measurements at Bern since 1994 together with a standard meteo station. So far, emphasis has been put on integrated water vapor (IWV) measurements, particularly for climate studies, but integrated liquid water (ILW) has been retrieved as well. We report on methodological advances with the data analysis. First, the original algorithm was replaced by a new statistical retrieval based on the simulations of TROWARA data using radiosonde profiles. Second, in a physical refinement, the cause of the variable ILW bias has been identified, and a method for its reduction to the level of 0.001 to 0.005 mm has been developed and tested. The bias is mainly a result of the variable water-vapor influence on absorption at 31 GHz. The bias correction also influences the IWV retrieval. The refined physical retrieval includes the temperature dependence of cloud absorption based on a recent dielectric model of water. The three algorithms (original, new, and refined) have been compared for two years of data. The applications of the refined algorithm are focused on physical processes, such as the development of supercooled clouds. Future advances will include precipitation measurements. Index Terms--Microwave radiometry.

The study is based on experimental work conducted in alpine snow. We made microwave radiometric and near-infrared reflectance measurements of snow slabs under different experimental conditions. We used an empirical relation to link near-infrared reflectance of snow to the specific surface area (SSA), and converted the SSA into the correlation length. From the measurements of snow radiances at 21 and 35 GHz, we derived the microwave scattering coefficient by inverting two coupled radiative transfer models (the sandwich and six-flux model). The correlation lengths found are in the same range as those determined in the literature using cold laboratory work. The technique shows great potential in the determination of the snow correlation length under field conditions. OCIS codes: 280.0280, 280.4991, 120.5630, 120.5700.

The Scanning Polarimetric Imaging Radiometer (SPIRA) is a versatile fully polarimetric imager operating at 91 GHz. It is designed for measurements of polarimetric signatures of the Earth's surface and man-made objects. SPIRA combines a method for the measurement of the complete polarization state with a relatively fast high-resolution imager, which is suitable for a range of applications. The instrument measures all four Stokes parameters simultaneously and delivers images by mechanically scanning the scene with an elevation over azimuth scanner and an offset parabolic antenna. A two-channel heterodyne receiver is used for the reception of polarized radiation. The Stokes parameters are obtained by correlating two linear orthogonal-polarization components in a broadband analog adding correlator. In this paper, we present the design of the instrument and analyze its radiometric and polarimetric characteristics. The polarimetric calibration and a method for the characterization of the polarimetric calibration device are described. First polarimetric measurements are presented and discussed. Index Terms--Four Stokes parameters, imaging, millimeterwave, passive remote sensing, polarimeter.

The crown vegetation of a deciduous forest is known to be semitransparent at low microwave frequencies, and leaf litter covering the forest soil has been recognized to have a significant impact on ground emission. The proposed approach for modeling the L-band radiative transfer through leaf litter consists of an isotropic effective medium approach for the litter permittivities, a coherent radiative transfer model for computing the coherent reflectivities from dielectric depth profiles, and an averaging procedure for computing the reflectivities determining the field-scale brightness temperatures. Evaluations were performed for the case of leaf litter on top of a conducting wire grid (litter-grid formation) and for litter on underlying soil (litter-soil formation). A model sensitivity analysis was performed with respect to parameters characterizing litter thickness variations and boundary roughness. For the litter-soil formation, the model was rather sensitive to local irregularities at the air-to-litter boundary. Modeled microwave signatures reproduced the major features of the measurements performed on a site comprising a litter-grid formation. Under dry conditions, the investigated litter layer was nearly 'invisible.' When the same litter layer was wetted, it acted as an important radiation source to be taken into account for the quantitative remote soil moisture detection of forested areas. Under certain conditions, the simulations revealed an increasing brightness when the litter is wetted prior to tile underlying soil. Further wetting of the litter-soil system then resulted in a decreasing brightness as expected for increased moisture. Such effects are important to know to avoid misleading interpretations of L-band signatures. Index Terms--Leaf litter, microwave radiometry, radiative transfer, soil moisture.

The microwave Forest Soil Moisture Experiment (FOSMEX) was performed at a deciduous forest site at the Research Centre Julich (Germany). An L- and an X-band radiometer were mounted 100 m above ground and directed to the canopy. The measurements consist of dual- and single-polarized L- and X-band data and simultaneously recorded ground moisture, temperature, and meteorological data. The canopy L-band transmissivity was estimated from a subset of the FOSMEX data, where the ground was masked with a metalized foil. For the foliage-free canopy, the reflecting foil diminished the L-band brightness by [approximately or equal to]24 K, whereas brightness increased by [approximatelyor equal to]14 K when the foil was removed from below the foliated canopy. Depending on the assumption made on the scattering albedo of the canopy, the transmissivities were between 0.2 and 0.51. Furthermore, the contribution of the foliage was quantified. Although, the evaluation revealed the semitransparency of the canopy for L-band frequencies, the brightness sensitivity with respect to ground moisture was substantially reduced for all foliation states. The effect of ground surface moisture was explored in an irrigation experiment. The L-band measurements were only affected for a few hours until the water drained through the litter layer. This emphasizes the significance of the presence of litter for soil moisture retrieval from remotely sensed L-band brightness data. The FOSMEX database serves for further testing and improving radiative transfer models used for interpreting microwave data received from future spaceborne L-band radiometers flying over areas comprising a considerable fraction of deciduous forests. Index Terms--Canopy transmissivity, forest, microwave radiometry, radiative transfer, soil moisture.

For significant surface reflection, the brightness temperature above planetary surfaces depends not only on surface temperature, emissivity, and atmospheric emission, but also on the type of bistatic scattering. For a plane-parallel atmosphere, this dependence can be specified by an effective incidence angle [[theta].sub.eff] from zenith of downwelling radiation. We obtained analytic expressions for the reflectivity and [[theta].sub.eff] for typical scattering functions such as Lambert, Lommel-Seeliger, multiple isotropic scattering (Chandrasekhar), and Peake's grass model. In all cases, [[theta].sub.eff] decreases with increasing zenith opacity (considered range: 0 to 1), and in most cases, the dependence on observation direction is small. These results are in contrast to specular reflection, where the effective incidence angle is given by the observation angle, which is, of course, independent of opacity. The dependence of terrestrial polar-region brightness temperatures on the type of bistatic scattering was studied for the Advanced Microwave Sounding Unit-A (AMSU-A). The difference between upwelling brightness temperatures calculated with diffuse and specular surface scattering is greatest for the zenith direction and for zenith opacities in the range of 0.3 to 0.6, and it decreases with increasing emissivity. A potential exists to infer a parameter [A.sub.L] describing the relative contributions of Lambertian ([A.sub.L] = 1) and specular ([A.sub.L] = 0) scattering. Some non-Lambertian scattering functions give values of [A.sub.L] > 1. For example, Lommel-Seeliger surfaces that are observed near vertical give [A.sub.L] values of about 1.2, and still larger values (~1.6) are obtained with the model of Peake. The angle dependence of AMSU-A measurements from the vicinity of Dome C, Antarctica, agrees with the Lambert model. Index Terms--Microwave signatures of Antarctic ice, microwave surface scattering.

A field experiment with an L-band radiometer at 1.4 GHz was performed from May-July 2004 at an experimental site near Zurich, Switzerland. Before the experiment started, clover grass was seeded. Thermal infrared, in situ temperature, and time-domain reflectometer (TDR) measurements were taken simultaneously with hourly radiometer measurements. This setup allowed for investigation of the microwave optical depths and mode opacities (parallel and perpendicular to the soil surface) of the clover grass canopy. Optical depths and opacities were determined by in situ analysis and remotely sensed measurements using a nonscattering radiative transfer model. Due to the canopy structure, optical depth and opacity depend on the polarization and radiometer direction, respectively. A linear relation between vegetation water-mass equivalent and polarization-averaged optical depth was observed. Furthermore, measured and modeled radiative transfer properties of the canopy were compared. The model is based on an effective-medium approach considering the vegetation components as ellipsoidal inclusions. The effect of the canopy structure on the opacities was simulated by assuming an anisotropic orientation of the vegetation components. The observed effect of modified canopy structure due to a hail event was successfully reproduced by the model. It is demonstrated that anisotropic vegetation models should be used to represent the emission properties of vegetation. The sensitivity of radiometer measurements to soil water content was investigated in terms of the fractional contribution of radiation emitted from the soil to total radiation. The fraction of soil-emitted radiation was reduced to approximately 0.3 at the most developed vegetation state. The results presented contribute toward a better understanding of the interaction between L-band radiation and vegetation canopies. Such knowledge is important for evaluating data generated from future satellite measurements. Index Terms--Microwave measurements, microwave radiometry, remote sensing, soil, soil measurements, soil moisture, vegetation.

This paper presents the first stereoscopic range measurements at a wavelength of 3.3 mm and discusses the accuracy of this new method. The synthesis of passive millimeter-wave imaging and stereoscopy combines the advantages of both principles, naturally looking high-contrast images and superior poor-weather performance (compared to visible and infrared wavelengths), as well as the passive ranging capability. Our setup using two antennas with a half-power beamwidth (HPBW) of 0.9[degrees] and a stereoscopic baseline of 1.15 m allows ranging with an accuracy of [approximately equal to] 10 % up to a distance of [approximately equal to] 300 m. The range resolution improves with increasing stereoscopic baseline, lower radiometer noise, narrower antenna beams, and higher scene contrast. For scenes with sufficient contrast, the directional resolution is considerably better than the antenna HPBW. Thus, massive oversampling of the scene in the plane of the stereoscopic baseline is required. For our setup, an oversampling factor of 36 is optimal. Since additional ranging errors result from nonstationary scenes, fast scanning imagers should be applied. Index Terms--Distance measurement, millimeter-wave imaging, millimeter-wave technology, radiometry, stereo vision.

We report on field-measured microwave emission in a period of frost penetration into a grassland soil. The measurements were recorded with a high temporal resolution using an L-band radiometer mounted on a 7-m high tower. The observation period (December 2002 to March 2003) included two cycles of soil freezing and thawing with maximum frost depth of 25 cm. In situ soil temperature and liquid water content were measured at five depths down to 45 cm. Soil moisture profiles were calculated using the COUP numerical soil water and heat model in combination with measured soil properties and meteorological data monitored at the site. The L-band radiation data clearly showed the penetration and thawing of seasonal soil frost. We calculated soil reflectivities based on in situ measured and modeled soil moisture profiles by applying a coherent radiative transfer model. The calculated reflectivities were compared with the radiometrically determined soil reflectivities, it was demonstrated that the quantitative consistency between these reflectivities was significantly improved by applying an impedance matching approach accounting for surface effects. In this particular case, the dielectric structure of the uppermost soil horizon was largely influenced by soil roughness, vegetation, and snow cover. The radiometrically measured soil reflectivities were fitted using a radiative transfer model in combination with a roughness model assuming a soil surface roughness of 25 mm. The analysis during a period of frost penetration shows coherent behavior of the soil reflectivity. Temporal oscillation of the measured L-band radiation appears to be a coherent effect. This effect has the potential to be used for estimating the frost penetration velocity. Index Terms--Dielectric measurements, frozen soil, ice, microwave radiometry, remote sensing, soil measurements, soil water content.

We report on an experiment linking microwave brightness temperatures and emissivities to the dynamics of in situ measured water content. The bare plot of 14 x 20 m was instrumented with two ground-based radiometers (1.4 and 11.4 GHz, respectively) and in situ time domain reflectometer (TDR) and temperature probes installed at five depths in three soil profiles. All data were recorded every 30 min from May to July 2002. The measured brightness temperature dynamics is a superposition of three patterns. The largest variations were caused by the changes in water content due to precipitation and the subsequent drying process. During dry periods, we observed daily oscillations of brightness temperature. Calculating effective soil temperatures based on the in situ measured temperature profiles revealed that these oscillations were partially due to changes in water content in the very topsoil. This effect was especially pronounced for the 11.4-GHz measurements, indicating that the very surface experienced the strongest changes in water content. At 1.4 GHz and horizontal polarization, the brightness temperature measurements turned out to be very sensitive to sun reflection at the ground. This reflection was visible through exceptionally high brightness temperature values at distinct times of the day. We observed pronounced hysteresis loops when comparing the emissivities of the two radiometers or the radiometric signals with the water content measured in situ at 2-cm depth. Such loops could be seen for the daily wetting and drying cycles as well as in the longer term drying and wetting behavior. This demonstrates that the gradient of the water content in the topsoil is a dynamic property depending on various time scales. These gradients affected the measurements of radiometers and TDR probes to different degrees and at different times. Despite an almost smooth surface, slight changes in the surface structure during the experiment caused a substantial variability with respect to the relationships between water content and emissivity for both radiometers. This is an indication that surface structures smaller than the wavelength may exert important effects on the emissivity of a bare soil. Index Terms--Field experiment, ground based, L-band, radiometry, soil moisture.

Over a period of 3 years a precision Sun photometer (SPM) operating between 300 and 1025 nm was calibrated four times at three different high-mountain sites in Switzerland, Germany, and the United States by means of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2[Sigma] - statistical plus systematic errors) of the calibration constants [V.sub.o]([Lambda]), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the UV-A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared spectral region. For SPM channels within strong water-vapor or ozone absorption bands a modified Langley-plot technique was used to determine [V.sub.o]([Lambda]) with a lower accuracy. Within the same period of time, we calibrated the SPM five times using irradiance standard lamps in the optical labs of the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Switzerland, and of the Remote Sensing Group of the Optical Sciences Center, University of Arizona, Tucson, Arizona. The lab calibration method requires knowledge of the extraterrestrial spectral irradiance. When we refer the standard lamp results to the World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengths. The largest disagreement (4.4%) is found for the channel centered at 610 nm. The results of these intercomparisons change significantly when the lamp results are referred to two different extraterrestrial solar irradiance spectra that have become recently available.

Volume scattering and absorption of electromagnetic radiation by a nonmagnetic, granular medium with large dielectric contrast has been achieved using an improved Born approximation. The low-frequency model is used to derive formulas and measured dielectric properties are used to obtain relevant model data of dry snow. The results reproduce Rayleigh scattering for a single particle.

This paper describes two network-analyzer (NA)-based scatterometers at 5.3 (C-band) and 35 GHz (Ka-band) as well as snowcover measurements made in the Swiss and Austrian Alps between December 1993 and January 1996. First, the setup and the mode of operation of the scatterometers are discussed. Both instruments measure the backscattering coefficients [Gamma] at hh, vv, vh, and vh polarizations and for incidence angles ranging from 0 to 70 [degrees]. The accuracy of 3 is generally better than [+ or -]1.8 dB, and the scatterometers are well suited for signature studies of natural surfaces. During the two years, we performed many backscattering measurements of natural, strongly layered snowcovers and we investigated relationships between [Gamma] and physical parameters of the snowcover. All measurements were collected in a signature catalogue. Here we report on results at 40 [degrees] incidence angle. We found that the combined use of active sensors at 5.3 and 35 GHz allows the discrimination of various snowcover situations, if multitemporal information is available. In addition, we observed a relationship of [Gamma] at 5.3 GHz with the integrated column height of liquid water and dependencies of [Gamma] at 35 GHz on the height of the dry snow, on the volumetric liquid water content at the snow surface, and on the thickness of the refrozen crust at the snow surface.

The complex, relative dielectric permittivity [Epsilon] of dry desert sand, collected at the Grand Erg Oriental in Ksar Ghilane, Tunisia, in October 1994, was measured with microwave resonators at frequencies between 0.245 and 6 GHz. The results are presented here. Whereas the real part [Epsilon][prime] is nearly independent of frequency, a monotonous decrease of the imaginary part [Epsilon][double prime] with increasing frequency is observed. The data can be fitted to a Debye relaxation spectrum with a relaxation frequency of about 0.27 GHz, leading to a nearly frequency-independent penetration depth of about 1 m over the range from 1-10 GHz. The spectrum can be explained by Maxwell-Wagner losses of semiconducting spheres embedded in a nonconducting sand medium.

Aerosol optical depth measurements by means of ground-based Sun photometry were made in Bern, Switzerland during two and a half years primarily to provide quantitative corrections for atmospheric effects in remotely sensed data in the visible and near-infrared spectral region. An investigation of the spatial variability of tropospheric aerosol was accomplished in the summer of 1994 in the Swiss Central Plain, a region often covered by a thick aerosol layer. Intercomparisons are made with two Sun photometers operated by the Swiss Meteorological Institute in Payerne (Swiss Central Plain) and Dayos (Swiss Alps, 1590 m a.s.I.). By means of an inversion technique, columnar particle size distributions were derived from the aerosol optical depth spectra. Effective radius, columnar surface area, and columnar mass were computed from the inversion results. Most of the spectra measured in Bern exhibit an Angstrom-law dependence. Consequently, the inverted size distributions are very close to power-law distributions. Data collected during a four month calibration campaign in fall 1993 at a high-mountain station in the Swiss Alps (Jungfraujoch, 3580 m) allowed us to study optical properties of stratospheric aerosol. The extinction spectra measured have shown to be still strongly influenced by remaining aerosol of the June 1991 volcanic eruptions of Mount Pinatubo. Inverted particle size distributions can be characterized by a broad monodisperse peak with a mode radius around 0.25 [[micro]meter]. Both aerosol optical depths and effective radii had not yet returned to pre-eruption values. Comparison of retrieved aerosol optical depth, columnar surface area and mass, with the values derived from lidar observations performed in Garmisch-Partenkirchen, Southern-Germany, yielded good agreement. Index Terms - Optical depth, size distribution, Sun photometry.

The relative dielectric constant, or relative permittivity, [Epsilon] of dry snow, is independent of frequency from about 1 MHz up to the microwave range of at least 10 GHz. New measurements of [Epsilon] with improved accuracy were made with a specially designed resonator operating near 1 GHz. The coaxial sensor accurately defines the sample volume whose actual mass can be determined to give the density of the snow sample. A special electronic instrument, called resometer, enabled accurate and rapid measurements under field conditions. Some 90 measurements of different kinds of dry snow (fresh, old, wind-pressed snow, depth hoar, and refrozen crusts) were made at test sites in the Swiss and Austrian Alps. The data indicate that [Epsilon] is a function of snow density only, given that the standard deviation of 0.006 from the fitted curve is just due to the expected measurement errors. The interpretation of these data in terms of physical mixing theory favors the effective medium formula of Polder and van Santen. The data allow to relate the average axial ratio X as a function of ice volume fraction. Both prolate and oblate spheroids can explain the data. Independent reasoning gives preference to oblate particles. In both cases, the axial ratio increases with increasing fraction up to a critical value of 0.33, followed by a decrease at still higher fractions. The destructive metamorphism of slowly compacting snow explains the increase of X, while the following decrease might be due to sintering. So far, no effect on [Epsilon] by a liquid-like surface layer on the ice grains at temperatures between -10 [degrees] C and 0 [degrees] C has been observed.

A semiempirical formula for the complex dielectric permittivity of leaves from different plants is found from a comparison of published measurements covering the frequency range from 1 to 100 GHz. The explicit parameters are the dry-matter fraction [m.sub.d] of the leaf and the permittivity of [Epsilon.sub.SW] of saline water with a salinity of about 1 percent. The physical part of the formula is its basis on [Epsilon.sub.SW], while the empirical part is its linearity with [m.sub.d]. The formula is applicable to fresh leaves; their [m.sub.d] values are in the range 0.1 < [m.sub.d] < 0.5. A test indicates that besides the [m.sub.d] variation and the spectral dependence the formula also describes the temperature variation correctly.

Ultraviolet light was measured at four channels (305, 311, 318, and 332 nm) with a precision filter radiometer (UV-PFR) at Arosa, Switzerland (46.78 [degrees], 9.68 [degrees], 1850 m above sea level), within the instrument trial phase of a cooperative venture of the Swiss Meteorological Institute (MeteoSwiss) and the Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center. We retrieved ozone-column density data from these direct relative irradiance measurements by adapting the Dobson standard method for all possible single-difference wavelength pairs and one double-difference pair (305/311 and 305/318) under conditions of cloud-free sky and of thin clouds (cloud optical depth [is less than] 2.5 at 500 nm). All UV-PFR retrievals exhibited excellent agreement with those of collocated Dobson and Brewer spectrophotometers for data obtained during two months in 1999. Combining the results of the error analysis and the findings of the validation, we propose to retrieve ozone-column density by using the 305/311 single difference pair and the double-difference pair. Furthermore, combining both retrievals by building the ratio of ozone-column density yields information that is relevant to data quality control. Estimates of the 305/311 pair agree with measurements by the Dobson and Brewer instruments within 1% for both the mean and the standard deviation of the differences. For the double pair these values are in a range up to 1.6%. However, this pair is less sensitive to model errors. The retrieval performance is also consistent with satellite-based data from the Earth Probe Total Ozone Mapping Spectrometer (EP-TOMS) and the Global Ozone Monitoring Experiment instrument (GOME). [C] 2001 Optical Society of America OCIS codes: 120.0120, 120.5630, 010.4950, 040.7190, 350.2460.