Albin J. Gasiewski

High-resolution passive polarimetric microwave mapping of ocean surface wind vector fields

The retrieval of ocean surface wind fields in both one and two dimensions is demonstrated using passive polarimetric microwave imagery obtained from a conical-scanning airborne polarimeter. The retrieval method is based on an empirical geophysical model function (GMF) for ocean surface thermal emission and an adaptive maximum likelihood (ML) wind vector estimator. Data for the GMF were obtained using the polarimetric scanning radiometer/digital (PSR/D) on the NASA P-3 aircraft during the Labrador Sea Deep Convection Experiment in 1997. To develop the GMF, a number of buoy overflights and GPS dropsondes were used, out of which a GMF of 10.7, 18.7, and 37.0 GHz azimuthal harmonics for the first three Stokes parameters was constructed for the SSM/I incident angle of 53.1/spl deg/. The data show repeatable azimuthal harmonic coefficient amplitudes of /spl sim/2-3 K peak-to-peak, with a 100% increase in harmonic amplitudes as the frequency is increased from 10.7 to 37 GHz. The GMF is consistent with and extends the results of two independent studies of SSM/I data and also provides a model for the third Stokes parameter over wind speeds up to 20 m/s. The aircraft data show that the polarimetric channels are much less susceptible to geophysical noise associated with maritime convection than the first two Stokes parameters. The polarimetric measurement technique used in the PSR/D also demonstrates the viability of digital correlation radiometry for aircraft or satellite measurements of the full Stokes vector. The ML retrieval algorithm incorporates the additional information on wind direction available from multiple looks and polarimetric channels in a straightforward manner and accommodates the reduced SNRs of the first two Stokes parameters in the presence of convection by weighting these channels by their inverse SNR.

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

An Airborne Millimeter-Wave Imaging Radiometer for Cloud, Precipitation, and Atmospheric Water Vapor Studies

Abstract A six-channel airborne total-power Millimeter-wave Imaging Radiometer (MIR) was recently built to provide measurements of atmospheric water vapor, clouds, and precipitation. The instrument is a cross-track scanner that has a 3-dB beamwidth of 3.5° and an angular swath of 100°. It measures radiation at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz. The inclusion of the 220-GHz receiver makes this instrument unique; no other instrument has made atmospheric radiation measurements using this combination of frequencies. The temperature sensitivities ΔT, based on the actual flight data with a 6.8-ms integration time, are found to be 0.44, 0.44, 1.31, 1.30. 1.02, and 1.07 K. The instrument has two external calibration loads maintained at the temperatures of 330 and 250 K (the ambient temperature at an aircraft altitude of 20 km). These calibration load temperatures are monitored precisely so that the radiometric measurements of the instrument could be made to better than 1 K o...

Numerical sensitivity analysis of passive EHF and SMMW channels to tropospheric water vapor, clouds, and precipitation

Potential uses of specific extremely high frequency (EHF) and submillimeter-wave (SMMW) channels at 90, 166, 283, 220, 325, 340, and 410 GHz for passive spaceborne remote sensing of the troposphere and lower stratosphere are investigated using an iterative numerical radiative transfer model. Collectively, these channels offer potential for high spatial resolution imaging using diffraction-limited apertures of practical size, along with the ability to profile water vapor, map precipitation beneath optically opaque cloud cover, and to measure nonprecipitating cloud (e.g. cirrus) parameters. A passive airborne imaging instrument for tropospheric meteorological sensing at 90, 150, 183+or-1, 3, 7, 220, and 325+or-1, 3, 9 GHz, called the Millimeter-wave Imaging Radiometer (MIR), is described. >

Calibration and applications of polarization-correlating radiometers

Hardware and associated software for nonmechanical rotation of the polarization basis of a dual linearly polarized cross-correlating radiometer are demonstrated. The technique requires precise measurement of two orthogonal-mode antenna temperatures along with cross-correlation of the two orthogonal-mode field amplitudes. Collectively, these are the first three Stokes parameters. A polarized blackbody load was developed for accurate calibration of the orthogonal-mode and cross-correlating channels. Using the hardware, rotation of the radiometer antennas polarization basis by a matrix transformation was demonstrated by 91.65 GHz near-Brewster-angle scans of a polarizing water surface. The experimental results demonstrate the viability of the rotation technique and suggest practical calibration schemes for airborne and spaceborne polarimetric radiometers. Application of the rotation technique in mechanically scanned polarization-sensitive imaging radiometers will eliminate polarization coupling inherent in conventional fixed-feed dual-polarization scanner designs. >

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.

Polarized microwave emission from water waves

Partially polarimetric measurements of thermal emission from a striated water surface at 91.65 GHz illustrate the potential for remote sensing of water wave direction by passive microwave radiometry. The three Stokes parameter measurements were made using a precision polarimetric radiometer trained on a rotatable water wave tank at several elevation angles from near nadir to near grazing. The polarimetric measurements are well corroborated by calculations using a tilted-facet geometrical optics model for the water surface emission and scattering. Multiple scattering of the incident background radiation is incorporated for observation angles approaching grazing. The downwelling background brightness is computed by using an atmospheric radiative transfer model. We show that azimuthal brightness variations in the third Stokes parameter are in phase quadrature with the first and second modified Stokes parameters. For observation angles near ∼60°–70° from nadir the first three parameters have particularly large azimuthal brightness variations and thus have significant potential for measuring ocean wave direction. Moreover, the azimuthal brightness variations caused by water waves are not negligible for many passive microwave atmospheric sounding and surface remote sensing purposes, even at nadir. A range of elevation angles resulting in minimal azimuthal variations is identified.

Ground-Based Millimeter- and Submillimeter-Wave Observations of Low Vapor and Liquid Water Contents

Ground-based observations at millimeter (mm) and submillimeter (submm) wavelengths were collected at the atmospheric radiation measurement program site at Barrow, AK, during the Arctic winter by a new 25-channel radiometer. A weighting function analysis is presented to demonstrate the enhanced sensitivity of mm- and submm-wave (50-400 GHz) radiometers to low vapor and liquid water contents with respect to conventional instruments such as the ones operating at centimeter (cm) wavelengths (20-30 GHz). In addition, based on measurements, we carried out a quantitative analysis of mm- and submm-wavelength sensitivity, yielding improvement factors from 1.5 to 69 for precipitable water vapor (PWV) and 3 to 4 for liquid water path (LWP) when compared to 20-30 GHz radiometers. Furthermore, using a simulated data set, we evaluate the effect of hydrometeor scattering: given the conditions occurring during the experiment, the scattering contribution is within the instrumental noise for most, but not all, of the considered channels. With the same data set, we demonstrate that in the dry conditions of the Arctic, a simple linear regression yields satisfactory results when applied on selected mm- and submm-wave channels. For a dual-channel combination, the expected accuracy is ~0.23 (0.007) mm for PWV (LWP), when using mm- and submm-wavelengths, whereas it is 0.37 (0.012) mm using cm-wave channels. When the retrieval is applied to real observations, the accuracy is found in agreement with theoretical expectations.

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

Simulation of passive microwave wind direction signatures over the ocean using an asymmetric‐wave geometrical optics model

The geophysical nature of polarized azimuthal microwave brightness temperature signatures over the open ocean and their relation to surface wind-driven anisotropies are investigated. The broadband nature of previously observed wind-direction signatures suggests the predominance of a frequency-independent thermal emission mechanism involving scattering and emission from asymmetric ocean gravity waves and ocean foam. To verify this hypothesis, an asymmetric-wave Monte Carlo ocean surface model and a geometrical optics electromagnetic emission model of the upwelling radiation field were developed. The model incorporates the effects of ocean wave asymmetry, ocean foam, and multiple geometric scattering. Model calculations compare favorably with the results of a previous study that used data from the Special Sensor Microwave Imager (SSM/I) at 19 and 37 GHz and at wind speeds of 7.9 and 12.2 m/s and at 53° incidence. The good agreement between the satellite data and the asymmetric-wave geometrical optics model supports the hypothesis that azimuthal brightness temperature signatures at wind speeds characteristic of moderate-to-heavy wind conditions are to a great extent caused by ocean wave asymmetry and foam.