Showing papers on "Zenith published in 1995"

Volcanic sulfur dioxide measurements from the total ozone mapping spectrometer instruments

Abstract: The total ozone mapping spectrometer (TOMS), first flown on the Nimbus 7 satellite, has delivered an unanticipated set of unique information about volcanic plumes because of its contiguous spatial mapping and use of UV wavelengths. The accuracies of TOMS sulfur dioxide retrievals, volcanic plume masses, and eruption totals under low-latitude conditions are evaluated using radiative transfer simulations and error analysis. The retrieval algorithm is a simultaneous solution of the absorption optical depth equations including ozone and sulfur dioxide at the four shortest TOMS wavelengths and an empirical correction based on background condition residuals. The retrieval algorithm reproduces model stratospheric sulfur dioxide plume amounts within ±10% over most central scan angles and moderate solar zenith angles if no aerosols or ash are present. The errors grow to 30% under large solar zenith angle conditions. Volcanic ash and sulfate aerosols in the plume in moderate optical depths (0.3) produce an overestimation of the sulfur dioxide by 15–25% depending on particle size and composition. Retrievals of tropospheric volcanic plumes are affected by the reflectivity of the underlying surface or clouds. The precision of individual TOMS SO2 soundings is limited by data quantization to ±6 Dobson units. The accuracy is independent of most instrument calibration errors but depends linearly on relative SO2 absorption cross-section errors at the TOMS wavelengths. Volcanic plume mass estimates are dependent on correction of background offsets integrated over the plume area. The errors vary with plume mass and area, thus are highly individual. In general, they are least for moderate size, compact plumes. Estimates of the total mass of explosively erupted sulfur dioxide depend on extrapolation of a series of daily plume masses backward to the time of the eruption. Errors of 15–30% are not unusual. Effusive eruption total mass estimates are more uncertain due to difficulties in separating new from old sulfur dioxide in daily observations.

A reliable and efficient two-stream algorithm for spherical radiative transfer: Documentation of accuracy in realistic layered media

Abstract: We present a fast and well documented two-stream algorithm for radiative transfer and particle transport in vertically inhomogeneous, layered media. The physical processes considered are internal production (emission), scattering, absorption, and Lambertian reflection at the lower boundary. The medium may be forced by internal sources as well as by parallel or uniform incidence at the top boundary. This two-stream algorithm is based on a general purpose multi-stream discrete ordinate algorithm released previously. It incorporates all the advanced features of this well-tested and unconditionally stable algorithm, and includes two new features: (i) corrections for spherical geometry, and (ii) an efficient treatment of internal sources that vary rapidly with depth. It may be used to compute fluxes, flux divergences and mean intensities (actinic fluxes) at any depth in the medium. We have used the numerical code to investigate the accuracy of the two-stream approximation in vertically inhomogeneous media. In particular, computations of photodissociation and warming/cooling rates and surface fluxes of ultraviolet and visible radiation for clear, cloudy and aerosol-loaded atmospheres are presented and compared with results from multi-stream computations. The O3 +hv → O(1D) + O2 and O3 +hv → O(3P) + O2 photodissociation rates were considered for solar zenith angles between 0.0–70.0° and surface albedos in the range 0.0–1.0. For small and moderate values of the solar zenith angle and the surface albedo the error made by the two-stream approximation is generally smaller, <10%, than the combined uncertainty in cross sections and quantum yields. Surface ultraviolet and visible fluxes were calculated for the same range of solar zenith angles and surface albedos as the photodissociation rates. It was found that surface ultraviolet and visible fluxes may be calculated by the two-stream approximation with 10% error or less for solar zenith angles less than 60.0° and surface albedos less than 0.5. For large solar zenith angles and/or large surface albedos, conditions typical at high latitudes, the error made by the two-stream approximation may become appreciable, i.e. 20% or more for the photodissociation rates in the lower stratosphere and for ultraviolet and visible surface fluxes for large surface albedos. The two-stream approximation agrees well with multi-stream results for computation of warming/cooling rates except for layers containing cloud and aerosol particles where errors up to 10% may occur. The numerical code provides a fast, well-tested and robust two-stream radiative transfer program that can be used as a ‘software tool’ by aeronomers, atmospheric physicists and chemists, climate modellers, meteorologists, photobiologists and others concerned with radiation or particle transport problems. Copies of the FORTRAN77 program are available to interested users.

Optical scintillations and fade statistics for a satellite-communication system

Abstract: Estimates of the scintillation index, fractional fade time, expected number of fades, and mean duration of fade time associated with a propagating Gaussian-beam wave are developed for uplink and downlink laser satellite-communication channels. Estimates for the spot size of the beam at the satellite or the ground or airborne receiver are also provided. Weak-fluctuation theory based on the log-normal model is applicable for intensity fluctuations near the optical axis of the beam provided that the zenith angle is not too large, generally not exceeding 60°. However, there is an increase in scintillations that occurs with increasing pointing error at any zenith angle, particularly for uplink channels. Large off-axis scintillations are of particular significance because they imply that small pointing errors can cause serious degradation in the communication-channel reliability. Off-axis scintillations increase more rapidly for larger-diameter beams and, in some cases, can lead to a radial saturation effect for pointing errors less than 1 µrad off the optical beam axis.

Rotational Raman scattering and the ring effect in zenith‐sky spectra

Abstract: The technique of zenith-sky spectroscopy is widely used to measure the vertical columns of O3, NO2, OClO and BrO in the atmosphere. In this paper, a model to simulate the effect of rotational Raman scattering by O2 and N2 on zenith-sky spectra is presented. The model is used to calculate the Raman-scattering cross-section for zenith-sky measurements and this cross-section is shown to correspond closely to the measured Ring cross-section, supporting the case that Raman scattering is the major cause of the Ring effect. Raman scattering is also shown to reduce the depths of structured molecular absorptions in scattered light spectra, leading to a general underestimation of the slant columns of molecules measured by zenith-sky spectroscopy which can be significant in some cases. This effect varies with solar zenith angle, so will affect particularly attempts to retrieve the vertical profile of an absorber from the variation of slant column with zenith angle. The calculated Ring cross-section is used to infer the proportion of multiply-scattered light which enters a zenith-sky spectrometer at twilight, and thus to estimate the magnitude of the corresponding underestimation of measured slant columns.

Geographical differences in the UV measured by intercompared spectroradiometers

Abstract: Five UV spectroradiometers representative of the types used in monitoring programs on several continents were intercompared at the Fraunhofer Institute for Atmospheric Environmental Research (IFU), Garmisch-Partenkirchen, Southern Germany, during a campaign in August 1994. Global spectral irradiances between 290 and l410 nm were measured over a range of solar zenith angles from 30{degrees} to 80{degrees}. Scans were synchronized to enable useful comparisons to be made under changing weather conditions, which included overcast, partly cloudy, and clear skies. No exchange of data was allowed between participating groups until after the campaign. At wavelengths longer than 310 nm, the spectra generally agreed to within {plus_minus}5%. At a wavelengths shorter than 3l10 nm, differences between instruments were larger, especially at larger solar zenith angles. Causes of differences are discussed. For all instruments, deviations in erythemally weighted irradiances were always less than 7% from the mean. The agreement between measurement systems is sufficient to allow an investigation of geographical differences in UV, under all observing conditions. UV doses measured at sites in the southern hemisphere are systematically larger than those measured at the corresponding northern latitudes. During the summer months the daily doses at the South Pole exceed those at mid-latitudes in the Northernmore » Hemisphere. Further investigations must be performed to establish a global UV-climatology. 8 refs., 5 figs., 2 tabs.« less

Instrument self-shading in underwater optical measurements: experimental data

Abstract: Self-shading error of in-water optical measurements has been experimentally estimated for upwelling radiance and irradiance measurements taken just below the water surface. Radiance and irradiance data have been collected with fiber optics that terminated with 1°, 18°, and 2π optics housed in the center of a disk that simulated the size of the instrument. Analysis of measurements taken at 500, 600, and 640 nm in lake waters have shown errors ranging from a few percent up to several tens of percent as a function of the size of the radiometer, the absorption coefficient of the medium, the Sun zenith, and the atmospheric turbidity. Comparisons between experimental and theoretical errors, the latter computed according to a scheme suggested by other authors, have shown absolute differences generally lower than 5% for radiances and lower than 3% for irradiances. Analysis of radiance measurements taken with 1° and 18° fields of view have not shown appreciable differences in the self-shading error. This finding suggests that correction schemes for self-shading error developed for narrow-field-of-view radiance measurements could also be applied to measurements taken with relatively larger fields of view.

On the influence of tropospheric clouds on zenith‐scattered‐light measurements of stratospheric species

Abstract: Examples for the influence of tropospheric clouds on the ground-based measurement of stratospheric species using the DOAS-technique (Differential Optical Absorption Spectroscopy) are reported. At Camborne/Great Britain (50.216°N, 5.316°W) on Sept. 11–15, 1994, episodic enhancement of absorption lines of O4, H2O, O3 and NO2 were observed in coincidence with tropospheric clouds being in the instrumental field of view (1.1° full angle). At a solar zenith angle (SZA) of 88°, absorption enhancements up to roughly a factor of 3 were detected for the tropospheric species O4 and H2O and the tropospheric fractions of the total column of O3 and NO2. The additional absorptions in the visible spectral range are probably caused by multiple Mie-scattering in tropospheric clouds. For our conditions, a tropospheric light path enhancement (TLPE) of 135±40 km can be inferred, being largely independent of SZA. This observation has several important implications for the atmospheric radiative transport, which are briefly discussed.

Bidirectional reflectance of oceanic waters: A comparison of modeled and measured upward radiance fields

Abstract: The bidirectional reflectance of oceanic waters is conveniently described in a normalized way by forming the ratio of the upwelling irradiance E(sub u) to any upwelling radiance L(sub u)(theta prime, phi). This ratio, Q (theta prime, theta(sub 0), (phi(sub 0) - phi), where theta prime, phi are the nadir and azimuth angles for the upward radiance and theta(sub 0), phi(sub 0) are the zenith and azimuth angles of the Sun, has been determined from measurements at sea and computed via Monte Carlo simulations using the inherent optical properties measured in the field and appropriate boundary conditions (clear sky, no wind, varying Sun angle). Experimental ad computed Q values are in excellent agreement. This successful comparison confirms the importance of the bidirectional character of ocean reflectance, already pointed out from a purely numerical approach without field validation, and corroborates the extended range of the Q variations. The later point is of importance when interpreting the marine signals detected by an ocean color satellite-borne sensor. The validation is extended by considering the historical data for the radiance distributions in Lake Pend Oreille determined at various depths. The closure issue in ocean optics is examined by solving the direct problem of radiative transfer and through a model-data comparison in terms of radiance field.

The displacement of the sun from the galactic plane using IRAS and faust source counts

Abstract: I determine the displacement of the Sun from the Galactic plane by interpreting IRAS point-source counts at 12 and 25 microns in the Galactic polar caps using the latest version of the SKY model for the point-source sky (Cohen 1994). A value of solar zenith = 15.5 +/- 0.7 pc north of the plane provides the best match to the ensemble of useful IRAS data. Shallow K counts in the north Galactic pole are also best fitted by this offset, while limited FAUST far-ultraviolet counts at 1660 A near the same pole favor a value near 14 pc. Combining the many IRAS determinations with the few FAUST values suggests that a value of solar zenith = 15.0 +/- 0.5 pc (internal error only) would satisfy these high-latitude sets of data in both wavelength regimes, within the context of the SKY model.

Zenith-angle dependence of VHF specular reflection echoes in the lower atmosphere

Abstract: We studied the characteristics of specular echoes reflected from stratified layers in the troposphere and lower stratosphere. In particular, we observed echoes at antenna-beam zenith angles, θ, from 0 ° to 28 ° in steps of 2 °. When the radar measurements were averaged over about 30 min, the zenith angle dependence of the echo power normalized by the vertical power, S (θ) , was generally the same for sufficiently intense reflection echoes. That is, the echo power was largest in the vertical direction, decreased to about − 10 dB at 6 °, and then gradually decreased to a constant level between − 15 and − 25 dB at θ ≥ 20 °. This constant level is interpreted as the isotropic turbulence scattering level S i . The width of S (θ) was significantly broader than expected for specular reflection from a perfectly horizontal layer. In order to explain this broadening, we developed two numerical models that describe statistically the slope of a reflection layer that has been distorted by vertical gravity-wave motions. With realistic gravity-wave spectra, the shape of S (θ) for θ = 0–6 ° was successfully explained. However, from 8 ° to 18 ° the observed S (θ) was enhanced by as much as 7 dB over the model. From the observations we showed that all of the reflection echoes, including the enhanced echoes at θ = 8–18 °, are probably due to the same process. Then we showed that the discrepancy with the model may be the result of our neglect of the horizontal component of gravity-wave motions, which was done in order to constrain the number of calculations.

Ozone and NO2 air‐mass factors for zenith‐sky spectrometers: Intercomparison of calculations with different radiative transfer models

Abstract: Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith-sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.

Midlatitude observations of the diurnal variation of stratospheric BrO

Abstract: The diurnal variation of BrO through sunrise and sunset has been measured above Cambridge, England, (52°N) during March and April 1994 using zenith sky spectroscopy. The measured BrO slant column at 90° solar zenith angle (SZA), relative to that at 80° SZA, is typically 1.5×1014 cm−2 but varies significantly from day to day. The average variation of BrO slant column with SZA through sunrise and sunset is in good agreement with predictions from a photochemical model, coupled to a radiative transfer model. The zenith sky measurements are consistent with in situ measurements of BrO concentrations and indicate that the inorganic bromine content of the stratosphere is 18–24 parts per trillion by volume (pptv).

Ozone measurements by zenith-sky spectrometers: An evaluation of errors in air-mass factors calculated by radiative transfer models

Abstract: Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are presented and discussed. Causes and size of errors in AMFs of ozone in the visible are evaluated. Errors can be caused by approximations in the calculation (intensity-weight approximation, ignoring the finite field of view of the instrument); by approximation in the scheme of the calculation (single scattering, ignoring refraction); or by variable geophysical parameters (vertical profile of constituents). These relative errors in AMF cause identical relative errors in vertical columns of ozone deduced from measurements by zenith-sky spectrometers. The mean of the relative errors of ozone AMFs due to using one set of AMFs for all seasons and locations is ±2.4% when averaged over the commonly used range of solar zenith angles.

Rayleigh lidar detection of aerosol echoes from noctilucent cloud altitudes at the Arctic Circle

Abstract: During 3 out of 16 observations runs in July and August 1993 the Rayleigh Lidar at the Andoya Rocket Range (69°N, 16°E) in Northern Norway detected aerosol echoes from noctilucent cloud altitudes on July 28, August 7, and August 9. The geometric elevation of the center of the Sun was from +1.3° to −4.5° during aerosol detection. These three events differed significantly in peak signal strength, altitude, cloud layer shape, altitude integrated signal, and temporal evolution. Aerosol echoes were seen from the altitude range 81 to 87 km. The strongest aerosol event showed a peak backscatter ratio of 240 at 83.2 km altitude equivalent to the molecular (Rayleigh) scattering signal from 41.5 km. The weakest event had a peak backscatter ratio of 7 at 84.8 km with a Rayleigh equivalent altitude of 73.3 km. The zenith optical thickness of the aerosol layers varied by approximately two orders of magnitude. Detection times ranged from longer than 5 hours to as short as 15 minutes. The temporal evolution during the events suggests that single clouds were drifting through the laser beam which has a diameter of approximately 4 m at 85 km altitude. All events occurred before local midnight and the gross temporal evolution is compatible with tidal models for the diurnal variation of the visibility in PMCs and NLCs although there is considerably more structure in the lidar data than predictable by such a model. The estimated zenith optical thickness is within the bounds of microphysical NLC models.

Inferring the thermal-infrared hemispheric emission from a sparsely-vegetated surface by directional measurements

Abstract: The thermal-infrared (longwave) emission from a vegetated terrain is generally anisotropic, i.e., the emission temperature varies with the view direction. If a directional measurement of temperature is considered to be equal to the effective temperature of the hemispheric emission, then the estimate of the latter can be significantly in error. The view-direction (zenith angle theta(sub eq) at which the emission equivalence does hold is determined in our modeling study. In a two-temperature field-of-view (soil and plants), theta(sub eq) falls in a narrow range depending on plant density and canopy architecture. Theta(sub eq) does not depend on soil and (uniform) plant temperatures nor on their ratio, even though the pattern of emission vs. the view direction depends crucially on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimuth) with horizontal facets, theta(sub eq) ranges from 48 to 53 deg depending on the optical density of the vertical elements alone. When plant elements are modeled as small spheres, theta(sub eq) lies between 53 to 57 deg (for the same values of the canopy optical density). Only for horizontal leaves (a truly planophile canopy) is the temperature measured from any direction equal to the temperature of the hemispheric emission. When the emission temperature changes with optical depth within the canopy at a specified rate, theta(sub eq) depends to some extent on that rate. For practically any sparsely vegetated surface, a directional measurement at the zenith angle of 50 deg offers an appropriate evaluation of the hemispheric emission, since the error in the estimate will, at most, only slightly exceed 1% (around 4 W/sq m). Estimates of the hemispheric emission through a nadir measurement, on the other hand, can be in error in some cases by about 10%, i.e., on the order of 40 W/sq m.

Impact of aerosols and clouds on the troposphere and stratosphere radiation field with application to twilight photochemistry at 20 km

Abstract: A series of coordinated investigations of polar stratospheric chemistry, radiation, and dynamics has resolved and generated many questions regarding ozone depletion in the Earth's atmosphere. Photodissociation plays a key role, and the radiation field models utilized have often been singled out as possible sources of uncertainty. Mean intensities (source functions) are compared for three independent models: discrete-ordinate, integral equation, and Monte Carlo simulation. It was found that the results differ by <10% for clear, cloudy, and aerosol conditions over an altitude range of 0–50 km, wavelength interval of 175–800 nm and a wide range of solar zenith angles. It is shown that in the presence of Pinatubolike aerosol concentrations, twilight photodissociation rates within an aerosol layer centered at 20 km can be reduced by more than an order of magnitude, whereas for solar zenith angles <60°, photodissociation rate changes are a few percent or less. Assessment of the effects of the aerosols on twilight stratospheric chemistry within the aerosol layer shows that radiation field perturbations on the homogeneous photochemistry are small and that heterogeneous processes are the major source of the altered photochemistry.

Indian MST radar 2. First scientific results in ST mode

Abstract: Clear air radars operating in the VHF range provide excellent information on middle atmospheric structure and dynamics with fine height and time resolutions. One such radar is installed at Gadanki, a tropical station in India. Experiments were carried out using the ST mode of this newly established MST radar to study the atmospheric wind field, characteristics of atmospheric stable layers, and clear air turbulence over this tropical station. Atmospheric stable layers are observed at various heights in the troposphere and lower stratosphere. Multilayer structures are observed near the tropopause and in the lower stratosphere. The range-time-intensity (RTI) maps for the zenith beam show that once these structures are formed, they are seen to last for more than an hour, indicating their large horizontal extent. The model computations of radar signal-to-noise ratio (SNR) for zenith beam, using simultaneous radiosonde observations taken at Madras, show a gross agreement with the observed SNR. However, the model SNR profiles do not show the fine structure observed by the radar, the limitation of the model profiles being the lower height resolution of the radiosonde measurements. The refractivity turbulence structure constant C2n is determined using SNR for 20° off-zenith beams pointed in east, west, north, and south directions. Profiles of C2n for the four oblique beams are found to agree within 10 dB, indicating that the intensity of the turbulence for the same range bin, within the volume scanned by the radar, is the same. The parameter C2n is also computed using meteorological parameters and compared with radar C2n. The observed and model C2n profiles are found to agree within 5 dB. Radar C2n profiles are found to show large diurnal and day-to-day variability. The results of an experiment conducted to determine the effect of transmitted pulse length on the received signal spectral width show that the wind shear effect is important for oblique beams and for longer pulse lengths, where as the beam-broadening effect is important for both oblique and vertical beams for all pulse lengths. Various turbulence parameters are determined using the observed spectral width after correcting for these effects.

Neutron measurements in near-Earth orbit with COMPTEL

Abstract: The fast neutron flux in near-Earth orbit has been measured with the COMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For this measurement one of COMPTEL's seven liquid scintillator modules was used as an uncollimated neutron detector with threshold of 12.8 MeV. The measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigidity and 3° to 177° in spacecraft geocenter zenith angle. One of the measurements occurred near the minimum of the deepest Forbush decrease ever observed by ground-level neutron monitors. After correction for solar modulation, the total flux is well fitted by separable functions in rigidity and zenith angle. With the spacecraft pointed near the nadir the flux is consistent with balloon measurements of the atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extremes of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important than its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measurements.

Comparison of airborne and surface spectral bidirectional reflectance factors, spectral hemispherical reflectance and spectral vegetation indices

Abstract: Six sets of airborne advanced solid-state array spectroradiometer (ASAS) and ground modular multiband radiometer (MMR) remotely sensed bidirectional measurements acquired over one First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment site in 1987 and 1989 were compared for the following parameters: bidirectional spectral reflectance factor, spectral hemispherical reflectance, simple ratio, and normalized difference vegetation index. ASAS at-sensor radiances were atmospherically corrected and converted to surface reflectance factors. Selected MMR bands were simulated with ASAS data, and hemispherical reflectance for both ground and airborne data sets was estimated using data collected only in or close to the solar principal plane. The shapes of the reflectance factor response curves (as a function of view zenith angle) were strongly affected by solar zenith angle and compared well between instruments. Off-nadir anisotropy in reflectance factors was comparable between sensors, while actual values of reflectance factors differed 2–35% relative between instruments in the green channel and 0–38% relative in the red channel. For the data set giving the overall closest agreement, ASAS reflectance factors differed from MMR values by 5–17% relative (0.3–1.8% absolute) in the green and 5–10% relative (0.2–0.5% absolute) in the red. These differences showed no correlation with solar zenith angle. Some of the differences were arbitrarily introduced by the 5° offset in view zenith angles (except nadir) between the two instruments. Other differences were caused by the discrepancy in solar zenith angle for some of the sets, variable deviations from the solar principal plane, hotspot and associated shadowing effects not consistently recorded by both instruments, and sampled ground area variations. Estimates of hemispherical reflectance compared very well between sensors, with differences of only 3–14% relative (0.3–1.6% absolute) in the green and 11–27% relative (0.8–2.4% absolute) in the red. For the ASAS data, using atmospherically corrected reflectance factors (instead of at-sensor radiances) significantly increased the values of the spectral vegetation indices (SVIs). Off-nadir anisotropy for the SVIs derived from both sensors' data sets was less than that observed for the bidirectional reflectance factors, and NDVI off-nadir deviations were much less than those of the simple ratio (SR). Large differences in the values of SR and NDVI between sensors indicate SVIs calculated from broadband (MMR) versus narrowband (ASAS) data are not comparable.

On radar interferometric techniques in the situation of volume scatter

Abstract: A volume scatter situation is assumed, and results expected from radar interferometric techniques are derived. It is shown that fictitious scattering points will appear, with the property that the points for different Doppler frequencies lie along a straight line. This line is not generally in the direction of the wind; only if the scatter is isotropic in azimuth will the line coincide with the wind direction. In the presence of turbulence the effective scattering points still lie along a straight line. However, the scattering point for a particular Doppler frequency will be closer to the zenith than it would be in the absence of turbulence. The results apply if smoothed cross spectra are used. If the cross spectra are not smoothed, the expected effects are harder to predict.

Spaced antenna and interferometric velocity measurements with MF and VHF radars

Abstract: The spaced antenna and interferometry techniques are applied to data collected on two MF radars (at Christchurch, New Zealand, and at Scott Base, in the Antarctic) and one VHF radar (the MU radar at Shigaraki, Japan). Velocity results obtained by the two techniques are compared as functions of characteristics such as the temporal scale and anisotropy of the scatter. In general, there was good agreement between the two techniques; however, when the temporal scale is small (indicating turbulent scatter) there was less agreement. The interferometry technique did not appear to be influenced greatly by anisotropy in the scatter. The interferometry technique was found to be affected by averaging of the Doppler spectra and by the use of an extra receiver. It is suggested that less averaging of the spectra results in increased random noise components in the spectra, which in the phase spectra are interpreted as scattering from a greater range of zenith angles. It was found that the use of three receivers rather than four also lead to scattering that appeared to originate from a greater range of zenith angles. These effects lead to lower interferometric velocity estimates that are more biased towards full correlation analysis (FCA) true velocities.

Drifting F-Layer Patches over the Magnetic Pole.

Abstract: Establishment of a polar observatory at Eureka, Canada in 1991 has permitted studies of F-layer ionization patches drifting over the north magnetic pole, using a multi-channel photometer and a digital ionosonde. Optical characteristics of two typical patches are shown. A case study of a sequence of several patches observed on February 17, 1993 is reported. Their optical intensities as determined from 630 and 558 nm meridian photometer scans averaged 200 R and 50 R respectively above airglow background. Their drift velocities through zenith, as measured by a digital ionosonde (CADI), averaged about 0.3 km/s while their bulk motion as seen optically was somewhat greater. The first 3 patches of the sequence, from 0945 to 1010 UT, moved generally in an antisunward direction while three immediately following, from 1015 to 1100 UT, had a velocity component in the dawn to dusk direction with a velocity vector rotated a full 90° from the earlier motions.

The Effect of Tropospheric Propagation Delay Errors in Airborne GPS Precise Positioning

Abstract: When operating in an airborne environment, test results have shown that ambiguity resolution is particularly sensitive to errors in the tropospheric delay models applied to the carrier phase observations. Since the aircraft is at a higher altitude than the ground-based reference station, the model must accurately represent the relative tropospheric delay caused by the altitude difference. In kinematic applications, the zenith tropospheric delay can be determined with prediction models such as Saastamoinen’s using pressure, temperature, and humidity measurements. This zenith delay is then mapped to other elevation angles using mapping functions such as those of Ifadis or Niell. This paper highlights the performance of several widely used tropospheric delay models, including the model currently proposed for the FAA’s WAAS. The accuracy of this model is assessed by (1) comparisons with ray tracing through an extensive set of radiosonde data, covering different latitudes, and (2) analyzing position solutions and the carrier phase observation residuals of GPS flight tests. We conclude that (1) the tropospheric delay error is mainly due to the inaccuracy of the zenith delay determination, and (2) a combination of a zenith delay model with the Niell or Ifadis mapping functions yields improved solutions, as compared to the currently proposed WAAS model.

Effect of molecular anisotropy on backscattered ultraviolet radiance.

Abstract: The effect of molecular anisotropy on backscattered UV (BUV) radiances is computed by accounting for it in both Rayleigh optical thickness and the scattering-phase matrix. If the effect of molecular anisotropy is included only in the optical thickness and not in the phase matrix, then for high sun (θ0 ~ 0°), the nadir radiance (I0) leaving the top of the atmosphere is approximately 1.8% higher than the radiance (Iop) computed with the effect included in the phase matrix. For very low sun (θ0 > 80°), I0 is approximately 2.3% lower than Iop. For off-nadir radiances the relative increase (decrease) depends on both the local zenith angle as well as the azimuth angle. Also, an increase in the surface reflectivity decreases the effect of molecular anisotropy on the upwelling radiances. Exclusion of the anisotropy factor in the Rayleigh-phase matrix has very little effect (<1%) on ozone retrieval from the BUV-type instruments. This is because of the ratio technique used in the retrieval algorithm, which practically cancels out the anisotropy effect.

Calculation of leaf area index and other canopy indices from gap fraction: a manual for the laicalc software

Abstract: LAICALC is a computer program that calculates leaf area index (LAI) and other canopy indices from gap fraction measurements. Gap fraction, the proportion of unobstructed sky for a range of zenith angles, can be measured by a variety of techniques, including hemispherical photography and optical sensors. Various canopy indices can be calculated from gap fraction based on inversion models. The LAICALC software, written in the "C" programming language and available on PC and UNIX platforms, performs five main calculations: 1) effective leaf area index (LAIeff), 2) leaf angle distribution, 3) mean tilt angle, 4) extinction coefficients as a function of zenith angle, and 5) skyview factor. A flexible configuration file specifies the format of input data, the calculations to be reported, and the format of output. Thus, LAICALC serves as a general-purpose gap fraction analysis program, useful for convenient calculation of canopy indices.

Study of seasonal evolution of tree emission using zenith-looking microwave radiometers

Abstract: The seasonal evolution of microwave emission of different species of deciduous and coniferous trees was studied using ground-based zenith-looking radiometers at 1.4 GHz and 9.5 GHz. The tree brightness temperature follows the annual cycle in phase with growth and loss of leaves. The X-band radiometric data are more sensitive to the changes in leaf volume and leaf water content.

The Swiss field-goniometer system (FIGOS)

Abstract: The reflectance characteristics of most natural objects vary with illumination and viewing geometry, i.e. expose a non-Lambertian behaviour. New sensor systems are capable of viewing targets quasi-simultaneously from nadir and different off-nadir positions. In regard to radiometric corrections the Lambertian assumption has to be overcome by detailed knowledge of the bidirectional reflectance distribution function (BRDF) of targets at the Earth's surface. In order to obtain bidirectional reflectance factor (BRF) data of naturally illuminated targets a transportable field-goniometer system (FIGOS) has been developed. It is operated together with a GER-3700 spectroradiometer. The goniometer consists of an azimuth full-circle and a zenith semi-arc of 2 m radius each. It enables one to observe a target from any desired viewing direction. First measurements are taken from a plane meadow under different solar zenith angles over the hemisphere in a resolution of 15/spl deg/ and 30/spl deg/ in zenith and azimuth direction, respectively. A Spectralon panel is measured at the beginning and end of a hemispherical data set. As the position of the Sun and the atmospheric conditions cannot be assumed constant over the measurement period of about 18 minutes, the global solar irradiance is monitored simultaneously to the BRF-data acquisition. The obtained results clearly show the non-Lambertian reflectance characteristics of the meadow.