Marian Klein

Polarimetric scanning radiometer C and X band microwave observations during SMEX03

Soil Moisture Experiments 2003 (SMEX03) was the second in a series of field campaigns using the NOAA Polarimetric Scanning Radiometer (PSR/CX) designed to validate brightness temperature data and soil moisture retrieval algorithms for the Advanced Microwave Scanning Radiometer on the Aqua satellite. Data from the TRMM Microwave Imager were also used for X-band comparisons. The study was conducted in different climate/vegetation regions of the US (Alabama, Georgia, Oklahoma). In the current investigation, more than one hundred flightlines of PSR/CX data were extensively processed to produce gridded brightness temperature products for the four study regions. Variations associated with soil moisture were not as large as hoped for due to the lack of significant rainfall in Oklahoma. Observations obtained over Alabama include a wide range of soil moisture and vegetation conditions. Comparisons were made between the PSR and AMSR for all sites

Interference mitigation in passive microwave radiometry

Relentless development of the microwave spectrum for telecommunications and other active services enhances the risk of anthropogenic interference to the passive Earth Exploration Satellite Service (EESS). While spectral allocation remains the primary basis for avoiding interference between the passive and active services, it is also prudent to consider the use of interference mitigation technology for passive microwave remote sensing, especially for spectral regions wherein primary EESS allocation is non-negotiable. Accordingly, we consider several means of detecting and correcting for anthropogenic interference in passive microwave imagery, including spectral subbanding, polarization detection, polarimetric detection, and azimuthal detection. A spectral subband technique applicable to either narrow-band and/or window channels is demonstrated with C-band data obtained using the NOAA Polarimetric Scanning Radiometer (PSR) airborne imaging system. The technique provides very good rejection of strong interference, and is readily applicable for implementation on future airborne and spaceborne passive microwave sensors.

Assessment of EOS Aqua AMSR-E Arctic Sea Ice Concentrations Using Landsat-7 and Airborne Microwave Imagery

An assessment of Advanced Microwave Scanning Radiometer Earth Observing System (AMSR-E) sea ice concentrations under winter conditions using ice concentrations derived from Landsat-7 Enhanced Thematic Mapper Plus (ETM+) imagery obtained during the March 2003 Arctic sea ice validation field campaign is presented. The National Oceanic and Atmospheric Administration Environmental Technology Laboratorys Airborne Polarimetric Scanning Radiometer Measurements, which were made from the National Aeronautics and Space Administration P 3B aircraft during the campaign, were used primarily as a diagnostic tool to understand the comparative results and to suggest improvements to the AMSR-E ice concentration algorithm. Based on the AMSR-E/ETM+ comparisons, a good overall agreement with little bias (~1%) for areas of first year and young sea ice was found. Areas of new ice production result in a negative bias of about 5% in the AMSR-E ice concentration retrievals, with a root mean square error of 8%. Some areas of deep snow also resulted in an underestimate of the ice concentration (~10%). For all ice types combined and for the full range of ice concentrations, the bias ranged from 0% to 3%, and the rms errors ranged from 1% to 7%, depending on the region. The new-ice and deep-snow biases are expected to be reduced through an adjustment of the new-ice and ice-type C algorithm tie points

Microwave Signatures of Snow on Sea Ice: Observations

Part of the Earth Observing System Aqua Advanced Microwave Scanning Radiometer (AMSR-E) Arctic sea ice validation campaign in March 2003 was dedicated to the validation of snow depth on sea ice and ice temperature products. The difficulty with validating these two variables is that neither can currently be measured other than in situ. For this reason, two aircraft flights on March 13 and 19, 2003, were dedicated to these products, and flight lines were coordinated with in situ measurements of snow and sea ice physical properties. One flight was in the vicinity of Barrow, AK, covering Elson Lagoon and the adjacent Chukchi and Beaufort Seas. The other flight was farther north in the Beaufort Sea (about 73degN, 147.5degW) and was coordinated with a Navy ice camp. The results confirm the AMSR-E snow depth algorithm and its coefficients for first-year ice when it is relatively smooth. For rough first-year ice and for multiyear ice, there is still a relationship between the spectral gradient ratio of 19 and 37 GHz, but a different set of algorithm coefficients is necessary. Comparisons using other AMSR-E channels did not provide a clear signature of sea ice characteristics and, hence, could not provide guidance for the choice of algorithm coefficients. The limited comparison of in situ snow-ice interface and surface temperatures with 6-GHz brightness temperatures, which are used for the retrieval of ice temperature, shows that the 6-GHz temperature is correlated with the snow-ice interface temperature to only a limited extent. For strong temperature gradients within the snow layer, it is clear that the 6-GHz temperature is a weighted average of the entire snow layer

A calibration method for fully polarimetric microwave radiometers

A technique for absolute end-to-end calibration of a fully polarimetric microwave radiometer is presented. The technique is based on the tripolarimetric calibration technique of Gasiewski and Kunkee, but is extended to provide a means of calibrating all four Stokes parameters. The extension is facilitated using a biaxial phase-retarding microwave plate to provide a precisely known fourth Stokes signal from the Gasiewski-Kunkee (GK) linearly polarized standard. The relations needed to determine the Stokes vector produced by the augmented standard are presented, and the effects of nonidealities in the various components are discussed. The application of the extended standard to determining the complete set of radiometer constants (the calibration matrix elements) for the National Oceanic and Atmospheric Administration Polarimetric Scanning Radiometer in a laboratory environment is illustrated. A calibration matrix inversion technique and error analysis are described, as well. The uncertainties associated with practical implementation of the fully polarimetric standard for spaceborne wind vector measurements are discussed relative to error thresholds anticipated for wind vector retrieval from the U.S. National Polar-Orbiting Environmental Satellite System.

Measurement of Low Amounts of Precipitable Water Vapor Using Ground-Based Millimeterwave Radiometry

Abstract Extremely dry conditions characterized by amounts of precipitable water vapor (PWV) as low as 1–2 mm commonly occur in high-latitude regions during the winter months. While such dry atmospheres carry only a few percent of the latent heat energy compared to tropical atmospheres, the effects of low vapor amounts on the polar radiation budget—both directly through modulation of longwave radiation and indirectly through the formation of clouds—are considerable. Accurate measurements of PWV during such dry conditions are needed to improve polar radiation models for use in understanding and predicting change in the climatically sensitive polar regions. To this end, the strong water-vapor absorption line at 183.310 GHz provides a unique means of measuring low amounts of PWV. Weighting function analysis, forward model calculations based upon a 7-yr radiosonde dataset, and retrieval simulations consistently predict that radiometric measurements made using several millimeter-wavelength (MMW) channels near ...

Airborne radio-frequency interference studies at C-band using a digital receiver

Corruption of C-band microwave brightness observations by radio-frequency interference (RFI) has been reported in recent data from orbiting radiometers; methods for mitigating these effects are of great importance for the design of future spaceborne microwave radiometers. One approach that has been suggested involves the use of multiple subchannels at C-band as opposed to a single channel; the use of multiple subchannels allows RFI to be detected and mitigated by analyzing relationships among subchannel brightnesses. While this approach has been utilized in previous airborne measurements, demonstrations of the RFI mitigation performance achieved have been difficult to obtain. To address this issue, an enhanced airborne system for observing radio-frequency interference effects on C-band microwave radiometers was developed, and is described in this paper. The system includes a traditional microwave radiometer with four C-band subchannels, so that RFI removal is possible using a subchannel mitigation algorithm. In addition, the system includes a digital receiver with the capability of providing high temporal and spectral resolution observations of interference. This high-resolution data allows improved understanding of RFI sources to be obtained, and also allows analysis of subchannel mitigation algorithm performance. Observations using the system in a test flight near Wallops Island, VA are described. Results show the four subchannel approach generally to be effective in mitigating the observed RFI sources, although examples are also illustrated using the digital receiver data to demonstrate failure of this approach. While studies of the digital receiver data alone could be performed to demonstrate further improvements in RFI mitigation, issues with this initial dataset limit the extent of such studies. Nevertheless, the results obtained still demonstrate qualitatively the improved RFI mitigation that can be achieved in brightness observations through the use of digital receivers

Nadir sensitivity of passive millimeter and submillimeter wave channels to clear air temperature and water vapor variations

The upwelling microwave-to-submillimeter wave brightness temperature observed from above the Earths atmosphere is sensitive to parameters such as pressure, temperature, water vapor, and hydrometeor content, and this sensitivity has been successfully used for passive vertical sounding of temperature and water vapor profiles. To determine optimal satellite observation strategies for future passive microwave instruments operating at frequencies above those now used, a study of the potential clear-air vertical sounding capabilities of all significant microwave oxygen and water vapor absorption lines in the frequency range from approximately 10 to 1000 GHz has been performed. The study is based on a second-order statistical climatological model covering four seasons, three latitudinal zones, and altitudes up to ∼70 km. The climatological model was developed by comparing data from three sources: the Upper Atmosphere Research Satellite Halogen Occultation Experiment (UARS HALOE) instrument, the TIROS Operational Vertical Sounder (TOVS) Initial Guess Retrieval radiosonde set, and the NOAA advanced microwave sounder unit (AMSU) radiosonde set. The Liebe MPM87 absorption model is used for water vapor and oxygen absorption and considers the effects of ozone and isotope absorption. Variations in the vertical sounding capabilities due to statistical variations of water vapor and temperature with latitude and season around each line are considered, and useful channel sets for geostationary microwave vertical sounding are suggested.

The Ground-Based Scanning Radiometer: A Powerful Tool for Study of the Arctic Atmosphere

Measurements of water vapor and clouds in the polar regions are difficult, because conventional instruments show little sensitivity (~1.3 K/mm) to low amounts. On the other hand, millimeter- and submillimeter-wavelength radiometry offers greatly enhanced sensitivity (up to 51.4 K/mm, depending upon frequency). For this reason, the National Oceanic and Atmospheric Administrations Physical Science Division designed a new instrument, the Ground-Based Scanning Radiometer (GSR), for continuous and unattended observations at millimeter and submillimeter wavelengths (50-380 GHz). The GSR was deployed for the first time during the Arctic winter radiometric experiment in March-April 2004. In this paper, we discuss the GSR calibration procedure, which allows for accurate measurements during clear and cloudy skies. Error-budget analysis and comparison with independent measurements show an absolute accuracy on the order of 1-2 K. Examples of multifrequency and multiangle GSR observations are illustrated, representing a valuable new data set for the study of water vapor, clouds, and atmospheric absorption models in the Arctic.

Microwave Signatures of Snow on Sea Ice: Modeling

Accurate knowledge of snow-depth distribution over sea ice is critical for polar climate studies. Current snow-depth-over-sea-ice retrieval algorithms do not sufficiently account for variations in snow and ice physical properties that can affect the accuracy of retrievals. For this reason, airborne microwave observations were coordinated with ground-based measurements of snow depth and snow properties in the vicinity of Barrow, AK, in March 2003. In this paper, the effects of snowpack properties and ice conditions on microwave signatures are examined using detailed surface-based measurements and airborne observations in conjunction with a thermal microwave-emission model. A comparison of the Microwave Emission Model of Layered Snowpacks (MEMLS) simulations with detailed snowpack and ice data from stakes along the Elson Lagoon and the Beaufort Sea and radiometer data taken from low-level flights using a Polarimetric Scanning Radiometer (PSR-A) shows that MEMLS can be used to simulate snow on sea ice and is a useful tool for understanding the limitations of the snow-depth algorithm. Analysis of radiance data taken over the Elson Lagoon and the Beaufort Sea using MEMLS suggests that the radiometric differences between the two locations are due to the differences in sea-ice emissivity. Furthermore, measured brightness temperatures suggest that the current snow-depth retrieval algorithm is sufficient for areas of smooth first-year sea ice, whereas new algorithm coefficients are needed for rough first-year sea ice. Snowpack grain size and density remain an unresolved issue for snow-depth retrievals using passive-microwave radiances