A numerical study on the contribution of melting layer composed by coated ice spheres to the upwelling brightness temperatures in TMI channels using VDISORT algorithm

To examine the influence of bright band on the retrieval of precipitation rate, the performance of melting layer composed by coated ice precipitable particles on the satellite-based measurement of polarized microwave brightness temperatures is discussed in this article by a vector discrete ordinate radiative transfer model. After comparing the simulated brightness temperatures in different TMI channels with and without the melting layer, we conclude that: (1) The melting layer composed by liquid-coated ice spheres weakens the upwelling microwave brightness temperatures because of the absorption/emission effect caused by the liquid coat. This effect is more conspicuous in middle and high frequency channels (19, 37 and 85 GHz) but, in 85 GHz channel, with the increase of rain rate, the multi-scattering can weaken its effect. (2) In a specific frequency, the horizontally polarized brightness temperature is more severely weakened by the melting layer than the vertically polarized. With the “cold” background (ocean surface, for example), this character is more conspicuous than that with a warm background. That is to say, the inner structure of a cloud system is easier to be detected under a cold background. Only in the 85 GHz frequency and when the rain rate is larger than 4 mm/h can we find that the vertically polarized brightness temperature is more severely weakened than the horizontally polarized one. (3) The melting layer with the assumption of coated ice spheres can change the difference of brightness temperatures between the vertically and horizontally polarized channels in the same frequency. In general, the value of such difference with the assumption of melting layer is larger than that without it. With a warm background, this value is negative and only in middle frequency (37 GHz), it is both stable and conspicuous.     

Microwave scattering properties of sand particles: Application to the simulation of microwave radiances over sandstorms

The single-scattering properties of sand/dust particles assumed to be ellipsoids are computed from the discrete dipole approximation (DDA) method at microwave frequencies 6.9-89.0 GHz in comparison with the corresponding Lorenz-Mie solutions. It is found that the single-scattering properties of sand particles are strongly sensitive to the shapes of the particles. The bulk scattering properties of sandstorms composed of spherical or nonspherical particles are investigated by averaging the single-scattering properties of these particles over log-normal particle size distributions. Furthermore, a vector radiative transfer model is used to simulate microwave radiances. The microwave brightness temperatures in the vertical polarization model are essentially not sensitive to sand particle habit, whereas microwave brightness temperature polarization differences are influenced by particle habit. It is shown that microwave brightness temperatures and brightness temperature polarization differences may be useful for estimating the effective particle sizes and mass loading of sandstorms.     

Microwave radiative transfer in scattering atmosphere: Effects of complex permittivity of ice spherical crystals

This paper presents the effects of ice particle's complex permittivity uncertainties on the scattering properties and upwelling brightness temperatures of cloudy atmospheres at the Advanced Microwave Sounding Unit-B (AMSU-B) channel frequencies of 89, 150 and 183 GHz. We investigated the mean deviations of ice particle's optical efficiencies and asymmetry parameters due to the uncertainties in the real and imaginary parts of its complex permittivities. We assumed that the true values of ice particle's permittivity are, respectively, within ±20% for the imaginary part and ±5% for the real part of the permittivity values given by the model of Hufford. Microwave radiative transfer calculations were performed to estimate the absolute errors of brightness temperatures due to uncertainties in ice particle's permittivities. Ice particles were taken to be spherical and their diameters were chosen in the range of 40-4000 μm. Gamma-size distribution was employed in computing volume scattering properties and the effective diameters were 70, 100 and 150 μm with an effective variance being 0.25. We found that ±20% uncertainty in the imaginary part of ice particle's permittivity resulted in only about 10% mean deviations in the absorption efficiencies at the three AMSU-B channel frequencies. However, an uncertainty of ±5% in the real part resulted in more than 15% mean deviations in both scattering and extinction efficiencies, especially significant at the frequency of 183 GHz. The absolute variations of the emerging brightness temperature from the cloudy atmosphere due to uncertainties in the permittivity were found to be more than 1 K, which is already significant compared with the sensitivities achieved with today's technology for millimeter wave radiometers.     

Conversion issues between microwave radiance and brightness temperature

Microwave radiances are usually converted into brightness temperatures for data assimilation and retrievals. The Rayleigh-Jeans approximation has been believed to be a good approximation for the conversion at low frequencies, but inaccurate at high frequencies. However, the simplified radiative transfer models under the Rayleigh-Jeans approximation (hereafter referred as BT-RTE) have been successfully applied in radiance simulations for frequencies below 183 GHz, which has somewhat puzzled the radiative transfer community. This paper clarifies the confusion. In addition, the conversion formula for the third and the fourth Stokes components are derived.Simulations for a polarized sensor, the Special Sensor Microwave Imager and Sounder, show that the BT-RTE is generally accurate. Results for a polarimetric sensor, WINDSAT, show that the third and the fourth Stokes radiances should be converted using the exact conversion formula given in this study rather than using a direct Planck function conversion.     

180–425 GHz low noise SIS waveguide receivers employing tuned Nb/AIOx/Nb tunnel junctions

We report recent results on a 20% reduced height 270–425 GHz SIS waveguide receiver employing a 0.49 µm² Nb/AlO _x /Nb tunnel junction. A 50% operating bandwidth is achieved by using a RF compensated junction mounted in a two-tuner reduced height waveguide mixer block. The junction uses an end-loaded tuning stub with two quarter-wave transformer sections. We demonstrate that the receiver can be tuned to give 0–2 dB of conversion gain and 50–80% quantum efficiency over parts of it's operating range. The measured instantaneous bandwidth of the receiver is 25 GHz which ensures virtually perfect double sideband mixer response. Best noise temperatures are typically obtained with a mixer conversion loss of 0.5 to 1.5 dB giving uncorrected receiver and mixer noise temperatures of 50K and 42K respectively at 300 and 400 GHz. The measured double sideband receiver noise temperature is less than 100K from 270 GHz to 425 GHz with a best value of 48K at 376 GHz, within a factor of five of the quantum limit. The 270–425 GHz receiver has a full 1 GHz IF passband and has been successfully installed at the Caltech Submillimeter Observatory in Hawaii. Preliminary tests of a similar junction design in a full height 230 GHz mixer block indicate large conversion gain and receiver noise temperatures below 50K DSB from 200–300 GHz. Best operation is again achieved with the mixer tuned for 0.5–1.5 dB conversion loss which at 258 GHz resulted in receiver and mixer noise temperature of 34K and 27K respectively.     

Self-broadened half-widths and self-induced line shifts for water vapor transitions in the 3.2-17.76 μm spectral region via complex Robert-Bonamy theory

Water vapor in the Earth’s atmosphere is being studied in the 3.2-17.76 μm spectral region by the atmospheric infrared sounder (AIRS) on Aqua, the troposphere emission spectrometer (TES) and the high-resolution dynamics limb sounder (HIRDLS) on Aura, both part of the NASA EOS mission. However, the lack of sufficient data on spectral parameters will hamper the prospect of accurate retrievals of temperature and concentration profiles. The spectral parameters for thousands of water vapor transitions are required, which are hard to determine by measurements alone. As reported previously [Can. J. Chem. 82 (2004) 1013-1027], of the 10 602 measurements of H₂O self-broadening half-widths only 440 intercomparisons with more than 3 data points have estimated uncertainty less than 10%. In this work, we have employed the mean relative thermal velocity approximation of the complex implementation of Robert-Bonamy formalism to obtain the self-broadened half-widths and self-induced line shifts for 5442 water vapor transitions in the 3.2-17.76 μm region. The calculations are compared with the measurement database and trends in the half-widths and line shifts are studied.     

A 210–280 GHz sis heterodyne receiver for the James Clerk Maxwell Telescope part I: Design and performance

We describe the design and performance of a 210–280 GHz SIS heterodyne receiver built for use on the Maxwell Telescope. The mixer utilises a lead alloy SIS tunnel junction, mounted in 41 reduced height rectangular waveguide, and is tuned with a backshort in 21 reduced height guide. The receiver has a receiver noise temperature of <200K (DSB) across the RF band from 210–270 GHz, with a best noise temperature measured in the laboratory of 113K (DSB) at 231 GHz. A prototype version of this receiver was successfully operated on the telescope in May 1989. By direct intercalibration with a Schottky diode receiver we deduced a best receiver noise temperature of 140K (DSB) at 245 GHz. Discrepancies between this figure and that derived from broad band thermal load calibration are discussed in the accompanying paper (Little et al., 1992, this issue).     

A 650-GHz Band SIS Receiver for Balloon-Borne Limb-Emission Sounder

A superconducting low-noise receiver has been developed for atmospheric observations in the 650-GHz band. A waveguide-type tunerless mixer mount was designed based on one for the 200-GHz band. Two niobium SIS (superconductor-insulator-superconductor) junctions were connected by a tuning inductance to cancel the junction capacitance. We designed the R_nC_j product to be 8 and the current density to be 5.5 kA/cm². The measured receiver noise temperature in DSB was 126-259 K in the frequency range of 618-660 GHz at an IF of 5.2 GHz, and that in the IF band (5-7 GHz) was 126-167 K at 621 GHz. Direct detection measurements using a Fourier transform spectrometer (FTS) showed the frequency response of the SIS mixer to be in the range of about 500-700 GHz. The fractional bandwidth was about 14%. The SIS receiver will be installed in a balloon-borne limb-emission sounder that will be launched from Sanriku Balloon Center in Japan.     

Retrieval of ozone profile from ground-based measurements with polarization: A synthetic study

We perform a retrieval based on optimal estimation theory to retrieve the vertical distribution of ozone from simulated spectra in the Huggins bands. The model atmosphere includes scattering by aerosol as well as Rayleigh scattering. The virtual instrument is ground-based and zenith-viewing. Using this algorithm, we show that it is possible to retrieve the ozone profile provided that the spectral resolution is at least 0.2 nm and the signal to noise ratio greater than 500. Our synthetic retrievals suggest that if we are able to measure the Stokes parameters Q, U and V with accuracy comparable to that of the intensity, the information contained in the measurements, and therefore the inversion, will improve. Furthermore, we find that the measurement of the full Stokes vector from the ground-based instrument will especially enhance the retrieval of tropospheric ozone. Utilizing concepts from information theory, our arguments are confirmed by increases in the degrees of freedom and the Shannon information content in the simulated measurements.     

Comparison of phase behavior between water soluble and insoluble surfactants at the air-water interface

The surface phase behavior of 2-hydroxyethyl myristate (2-HEM) has been studied in Langmuir monolayers by measuring surface pressure (π)-area (A) isotherms with a film balance and observing monolayer morphology with a Brewster angle microscope (BAM). These results are compared with the phase behavior of 2-hydroxyethyl laurate (2-HEL) in Gibbs monolayers studied by measuring π-time (t) curves and observing monolayer morphology. The π-A isotherms of 2-HEM show a first-order phase transition from a liquid expanded (LE) phase to a liquid condensed (LC) phase in the temperature range between 5 and 35 °C whereas the π-t curves of 2-HEL represent a similar phase transition in the temperature range between 2 and 25 °C. The critical surface pressure, π_c necessary for the phase transitions increases with increasing temperature in both the cases. The LC domains formed in 2-HEM show circular shapes, which are independent of the temperature. In contrast, the circular domains having stripe texture formed at lower temperatures show a shape transition to fingering domains with uniform brightness at 15 °C. The amphiphile, 2-HEM having 13-carbon chain has higher line tension than 2-HEL that has 11-carbon chain as tail. Thus, for 2-HEM, this high line tension always dominates over other factors giving rise to circular domains at the all studied temperatures.     

A novel approach for simultaneous millimeter wave generation and high bit rate data transmission for Radio over Fiber (RoF) systems

We propose a novel method for simultaneous transmission of OC-192 (9.95328 Gbps) digital data and 60 GHz RF generation in a Standard Single Mode Fiber (SSMF) link utilizing Stimulated Brillouin Scattering (SBS). The system comprises of a 1550 nm DFB Laser diode, Mach Zehnder modulator (MZM), 50 km SSMF and Optical receiver. The receiver includes laser diode for optical pump, a regenerator for data retrieval and a RF bandpass filter for RF generation. This system requires minimum number of RF and optical components for the generation of 60 GHz RF. The remotely generated 60 GHz RF signal may be used for wireless transmission of data. The entire link is simulated in Optisystem software to analyze the system performance.     

Lineshape parameters for water vapor in the 3.2-17.76 μm region for atmospheric applications

Several NASA EOS instruments, the atmospheric infrared sounder (AIRS) on Aqua, and the tropospheric emission spectrometer (TES) and the high-resolution dynamics limb sounder (HIRDLS) on AURA, will be measuring water vapor in the Earth’s atmosphere in the 3.2-17.76 μm spectral region. In order to do retrievals of temperature and concentration profiles, the spectral parameters for many thousands of water vapor transitions must be known. Currently the largest uncertainty in these data is associated with the pressure-broadened half-width. To help ameliorate this situation, complex Robert-Bonamy calculations were made to determine N₂-, O₂-, and air-broadened half-widths and line shifts for 5442 transitions of the principal isotopologue of water vapor for the 11 vibrational bands in this region. The intermolecular potential is a sum of electrostatic terms (dipole-quadrupole and quadrupole-quadrupole), isotropic induction and London dispersion terms, and the atom-atom potential expanded to eighth order. The parameters are adjusted as described in Gamache and Hartmann [J. Quant. Spectrosc. Radiat. Transfer 83 (2004) 119]. Calculations were made at 225 and 296 K in order to determine the temperature dependence of the half-widths. When possible the data are compared with measurements. The average percent difference between the measured and calculated half-widths is −1.97, 2.6, and −1.55 for N₂-, O₂-, and air-broadening of water vapor, respectively. The agreement for the line shifts is less satisfactory. It is clear from this work that the calculations will benefit from a comprehensive adjustment of the intermolecular potential.     

The sensitivity of radiative transfer calculations to the changes in the HITRAN database from 1982 to 2004

Over the last quarter century, improvements in the determination of the spectroscopic characteristics of the infrared-active trace species have enhanced our ability to retrieve quantitative distributions of temperatures, clouds, and abundances for various trace species within the Earth's atmosphere. These improvements have also allowed for refinements in the estimates of climatic effects attributed to changes in the Earth's atmospheric composition. Modeling efforts, however, have frequently experienced significant delays in assimilating improved spectroscopic information. Such is the case for highly parameterized models, where considerable effort is typically required to incorporate any revisions. Thus, a line-by-line radiative transfer model has been used to investigate the magnitude of the effects resulting from modifications to the spectroscopic information. Calculations from this line-by-line model have demonstrated that recent modifications to the HITRAN (High Resolution Transmission) line parameters, the continuum formulation, and the CO₂ line-mixing formulation can significantly affect the interpretation of the high spectral resolution radiance and brightness temperature retrievals. For certain moderate-resolution satellite remote sensing channels, modifications to these spectroscopic parameters and formulations have shown the capacity to induce changes in the calculated radiances equivalent to brightness temperature differences of 1-2 K. Model calculations have further shown that modifications of the spectroscopic characteristics tend to have a modest effect on the determination of spectrally integrated radiances, fluxes, and radiative forcing estimates, with the largest differences being of order 1 W m⁻² for the total thermal infrared fluxes, and of order 2-3% of the calculated radiative forcing at the tropopause attributed to the combined doubling of CO₂, N₂O, and CH₄. The results from this investigation are intended to function as a guide to differentiate between cases where older parameterizations provide acceptable results, within specified accuracy bounds, and cases where upgrades to the latest spectroscopic database are necessary.     

Polarization difference due to nonrandomly oriented ice particles at millimeter/submillimeter waveband

This paper presents polarized signature due to oriented circular columnar and planar ice crystals at millimeter/submillimeter (mm/sub-mm) waveband. DDSCAT 6.1 and RT4 code package are employed for scattering properties and radiative transfer simulations, respectively, at the three estimated window frequencies (150, 220 and 340 GHz) of FengYun-4 (FY-4). We use empirical formulas to describe realistic sizes of planar and columnar particles and assume that ice particles are in Gamma-size distribution in this study. A “resonance” feature of polarized signals as a function of median mass diameter is notably found for horizontally oriented columns and blunt plates at the frequency of 340 GHz; however, there is no promising resonance characteristic for horizontally aligned plates with empirical sizes at the three window channels of FY-4. The position of the resonance peak is related to particle aspect ratio, frequency and ice water path (IWP), and it moves to a shorter median mass diameter when the particle aspect ratio decreases or IWP in clouds increases. Considering that particle canting angle distribution (Gaussian distribution in this study), polarization difference, as well as the brightness temperature difference between clear and cloudy sky, decreases rapidly when particles gradually change from horizontally oriented to randomly oriented. The upwelling brightness temperature is insensitive to particle size and shape but sensitive to particle orientation, the difference of brightness temperature between horizontal and random orientation up to 6 K, whereas polarized signature is quite sensitive to particle microphysics as well as orientation; polarized measurements thereby could benefit retrieval of cloud microphysical parameters.     

A lownoise 665 GHz SIS quasi-particle waveguide receiver

We report recent results on a 565–690 GHz SIS heterodyne receiver employing a 0.36µm² Nb/AlO _x /Nb SIS tunnel junction with high quality circular non-contacting backshort and E-plane tuners in a full height waveguide mount. No resonant tuning structures have been incorporated in the junction design at this time, even though such structures are expected to help the performance of the receiver. The receiver operates to at least the gap frequency of Niobium, 680 GHz. Typical receiver noise temperatures from 565–690 GHz range from 160K to 230K with a best value of 185K DSB at 648 GHz. With the mixer cooled from 4.3K to 2K the measured receiver noise temperatures decreased by approximately 15%, giving roughly 180K DSB from 660 to 680 GHz. The receiver has a full 1 GHz IF passband and has been successfully installed at the Caltech Submillimeter Observatory in Hawaii.     

A radiative transfer model for an idealized and non-scattering atmosphere and its application for ground-based remote sensing

Inversion of tropospheric profiles from ground-based microwave measurements requires a simple and accurate model for calculating the brightness temperatures as received by the radiometer. In the first part, an analytic solution of the radiative transfer equation is derived for an exponentially decaying absorption coefficient and a linear temperature gradient. Based on the obtained analytic expressions, a discretized radiative transfer scheme is developed in the second part. The new scheme incorporates the generic behavior of the atmosphere with the effect that brightness temperatures can be modeled more accurately and with fewer grid points compared to commonly used radiative transfer schemes. The brightness temperature modeling accuracy was improved by a factor of six. The results suggest that the model could be employed for the retrieval of temperature and humidity profiles.     

Surface-emitted THz generation using a compact ultrashort pulse fiber amplifier at 1060 nm

Generation of InAs-surface-emitted terahertz radiation by application of an ultrashort pulse 1060 nm parabolic fiber amplifier source is reported for the first time. The fiber amplifier delivers 100 fs pulses at a repetition rate of 75 MHz and an average power of maximum 12 W. This new excitation laser for surface-emitters generates high brightness broadband THz radiation ranging from 100 GHz to over 2.5 THz. THz detection is demonstrated based on two-photon absorption at low-temperature-grown GaAs dipole receivers.     

Oxidation temperature dependent properties of MgO thin film on alumina

The magnesium oxide thin films were prepared by thermal oxidation (in air) of vacuum evaporated magnesium thin film on alumina. It was found that oxidation temperature (623 K, 675 K and 723 K) and thickness (103 nm and 546 nm) dependent effects were prominently manifested in the surface morphology. Electrical and microwave properties (8-12 GHz) of the MgO thin films were also carried out. X-ray diffraction showed orientation along (2 0 0) and (2 2 0) directions. Flowerlike morphology was observed from SEM and flake like morphology for films of higher thickness oxidized at higher temperatures. The magnesium oxide thin film showed NTC behavior. Microwave transmittance was found to increase with increase in oxidation temperature but was lower than alumina. Frequency and oxidation temperature dependent microwave permittivity was obtained. The microwave dielectric constant varied in the range 8.3-15.3.     

Temperature dependence of pressure broadening of the N = 1− fine structure oxygen line at 118.75 GHz

The absorption profile of the N = 1− fine structure line of oxygen was recorded by a resonator spectrometer at a frequency range of 110-130 GHz at atmospheric pressure and different temperatures ranging from −21 °C up to +22 °C. Analysis of the observed line shape allowed determination of the temperature dependence of the line pressure broadening. The measured value of the temperature exponent is n = 0.74(5) for self-broadening. Consistency of the measurements is supported by simultaneous measurements of the line intensity, the line mixing parameter and the line center frequency, and by comparison of obtained values with previously known data.     

Non-Voigt line-shape effects on retrievals of atmospheric ozone: Collisionally isolated lines

This paper addresses the question of errors in retrievals of vertical profiles of ozone from atmospheric spectra caused by assuming that the absorption lines have pure Voigt line shapes. The case of collisionally isolated transitions (no line mixing) is treated by considering only the effects of the speed dependence (SD) of the pressure broadening. The case of O₃ retrievals from a sequence of limb transmission spectra is first treated theoretically. The results show that the influence of SD is very small, leading to changes in the residuals and in the retrieved O₃ mixing ratios smaller than 1%. These findings are then confirmed by treating a series of spectra recorded by the ACE-FTS instrument. Similar exercises are also made for other observation techniques by treating simulated or measured limb and nadir emission spectra as well as ground-based solar and in-situ laser transmission data. All lead to the general conclusion that SD (and Dicke narrowing) can be neglected in retrievals of ozone amounts from recorded atmospheric spectra. Indeed, the biases caused in the ozone profiles by the use of pure Voigt line shapes still remain significantly smaller than the total error/uncertainty from other sources such as improper line intensities and widths, uncertainty in the instrument function, errors in the pressure and temperature profiles and so forth.