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Multiyear assessment of ground-based Sun-tracking microwave radiometric observations in Rome, NY (USA) at millimeter and sub-millimeter wavelengths
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An alternative formulation of the Langley plot relating observed solar irradiance, extraterrestrial solar irradiance, and air mass has been suggested to potentially improve radiometer calibration accuracy. In this study, results from the traditional and alternative plotting methods are compared using both simulated and measured data. The simulations indicate that their relative accuracies depend on the time scale of the atmospheric extinction fluctuations. The two methods are found to be essentially equivalent with the measured data.
In this paper, we develop, present and evaluate a refined, statistical index of model performance. This ‘new’ measure (d r) is a reformulation of Willmott's index of agreement, which was developed in the 1980s. It (d r) is dimensionless, bounded by − 1.0 and 1.0 and, in general, more rationally related to model accuracy than are other existing indices. It also is quite flexible, making it applicable to a wide range of model‐performance problems. The two main published versions of Willmott's index as well as four other comparable dimensionless indices—proposed by Nash and Sutcliffe in 1970, Watterson in 1996, Legates and McCabe in 1999 and Mielke and Berry in 2001—are compared with the new index. Of the six, Legates and McCabe's measure is most similar to d r. Repeated calculations of all six indices, from intensive random resamplings of predicted and observed spaces, are used to show the covariation and differences between the various indices, as well as their relative efficacies. Copyright © 2011 Royal Meteorological Society
We examine the best solar submillimeter observations made on the James
Clerk Maxwell Telescope in 1991 and 1992. In these observations, the
solar disk was observed concurrently in pairs of wavelengths chosen from
350, 850, and 1200 μm. Images at all of these wavelengths show clear
limb brightening of the quiet Sun. The observations clearly resolve the
chromospheric supergranular network in active and quiet regions. The
quiet Sun is characterized by large-scale variations in brightness,
particularly the occasion of anomalously dark regions that tend to
surround active regions. Sunspots are clearly resolved, with large dark
umbrae clearly distinguished from sometimes particularly bright
penumbrae.
While radio observations of the Sun have mostly focused on active region phenomena, they also contribute unique data to our
knowledge of the quiet Sun, in particular through accurate measurements of the temperature as a function of height in the
atmosphere and through the measurement of nonthermal emissions from chromospheric and coronal heating events. Here we review
observations of the quiet Sun using radio telescopes and discuss current science problems that will be addressed with future
facilities such as the Frequency Agile Solar Radiotelescope (FASR).
Solar radio emission provides valuable information on the structure and dynamics of the solar atmosphere above the temperature
minimum. We review the background and most recent observational and theoretical results on the quiet Sun and active region
studies, covering the entire radio range from millimeter to decameter wavelengths. We examine small- and large-scale structures,
at short and long time scales, as well as synoptic aspects. Open questions and challenges for the future are also identified.
KeywordsChromosphere, quiet–Corona, quiet–Polarization, radio–Radio emission, quiet–Active regions, radio
A characterization of the antenna noise temperature due to precipitating clouds at Ku band and above is described by deriving a closed-form solution of the scalar radiative transfer equation. Following the so called Eddington approximation, the analytical model is based on the truncated expansion of unpolarized brightness temperature angular spectrum in terms of Legendre polynomials. The accuracy of the sky-noise Eddington model (SNEM) is evaluated by comparing it with an accurate numerical solution, taking into consideration a wide variability of medium optical parameters as well as a typical rain slab model. The effect of the antenna pattern for ground-based antennas is also quantified. Physically-based radiative cloud models, characterized by a vertically-inhomogeneous geometry, are also introduced. Hydrometeor optical parameters are calculated and modeled for a large set of beacon channel frequencies. Nimbostratus and cumulonimbus models are finally applied to SNEM for simulating slant-path attenuation and antenna noise temperatures for ground-based antennas. Results are compared with ITALSAT satellite receiver measurements and co-located radiometric data between 13.0 and 49.5 GHz for various rain events during 1998.
The mean radiative temperature (T
mr
) is a key function controlling the sky noise temperature in microwave receiving systems. A generalized parametric prediction (GPP) model of T
mr
for microwave slant paths in all-weather conditions is formulated and presented. The proposed GPP model is aimed at being multifrequency and surface-temperature scaled, valid for elevation angles from 5° to 90° and for frequencies ranging from 5 to 95 GHz within the three transmission windows delimited by the water vapor and the oxygen absorption peaks. The core of the GPP model is a parametrization driven by a physically based radiative transfer approach taking into account extinction, emission, and multiple scattering. The expression of T
mr
is normalized to the surface temperature of the considered site. The GPP model is verified with measurements available from the multi-instrument Italian Satellite (ITALSAT) campaign in Spino d'Adda, Milan, Italy, in 1994-1997, obtaining a fractional mean error ranging from 0.045 to 0.068. A comparison of the GPP model with the current ITU-R model shows a reduction in the root mean square error up to about 20 and 30 K, depending on the considered frequency.
Sun-tracking (ST) microwave radiometry is a technique where the Sun is used as a microwave signal source and it is here rigorously summarized. The antenna noise temperature of a ground-based microwave radiometer is measured by alternately pointing toward-the-Sun and off-the-Sun while tracking it along its diurnal ecliptic. During clear sky the brightness temperature of the Sun disk emission at K and Ka band and in the unexplored millimeter-wave frequency region at V and W band can be estimated by adopting different techniques. Using a unique dataset collected during 2015 through a ST multifrequency radiometer, the Sun brightness temperature shows a decreasing behavior with frequency with values from about 9000 K at K band down to about 6600 K at W band. In the presence of precipitating clouds the ST technique can also provide an accurate estimate of the atmospheric extinction up to about 32 dB at W band with the current radiometric system. Parametric prediction models for retrieving all-weather atmospheric extinction from ground-based microwave radiometers are then tested and their accuracy evaluated.
Sun-tracking millimeter-wave radiometry exploits the Sun as a beacon source by tracking it along its diurnal ecliptic. The atmospheric brightness temperature is measured by alternately pointing toward-the-Sun and off-the-Sun according to ad hoc switching strategy. By properly developing a retrieval algorithm, we can estimate the atmospheric path attenuation in all-weather conditions. The Langley method, based on elevation-scanning, is proposed to estimate the equivalent brightness temperature of the Sun, which is a critical step for precipitation extinction estimation. An application to available Sun-tracking radiometric measurements at V and W band in Rome (NY, USA) is shown, discussed and compared with the conventional technique using the clear-air approximation of the mean radiative temperature. Results show an appealing potential of Sun-tracking technique in order to exploit millimeter-wave radiometry for atmospheric retrievals even in a cloudy and rainy conditions.
Solar radiation and atmospheric attenuation were measured at 4.3-mm wavelength The sun was scanned with a radio telescope consisting of a 10-foot precision paraboloid antenna (6.7-minute beamwidth) and a Dicke-type radiometer. Atmospheric attenuations were determined from the change in received solar radiation with changing elevation of the sun and from direct measurements of the thermal radiation from the atmosphere. The measured attenuations at the zenith for clear skies were between 1.6 and 2.2 db. At 4.3-mm wavelength, the sun (when it is free of sunspots) appears to be a uniform disk nearly one per cent larger than its optical size. From a large number of measurements over a period of 6 months, the solar brightness temperature was found to be 7000 °K with an uncertainty of about 10 per cent. Sunspot regions are slightly brighter than the quiet areas; the largest observed enhancement is 2 per cent.
A physically-based parametric model (PPM) to predict the sky-noise temperature in all weather conditions is proposed. The proposed prediction model is based on the non-linear regression fit of numerical simulations derived from the sky-noise eddington radiative-transfer model (SNEM) in an absorbing and scattering medium such as gaseous, cloudy and rainy atmosphere. The PPM prediction method, dependent on measured path attenuation, beacon frequency, and antenna-pointing elevation angle, describes the statistical behavior of the atmospheric mean radiative temperature, which in its turn relates sky-noise temperature to slant-path attenuation. PPM validity ranges from X-to W- band and from 10 to 90 in terms of elevation angle. A comparison of the estimated PPM radio-propagation variables with corresponding ITALSAT satellite data, collected at the Spino d'Adda receiving station (Italy), is also carried out and discussed. The PPM prediction technique provides a root-mean-square retrieval error generally less than 8 K for all frequencies. Results show an improvement with respect to the current International Telecommunication Union (ITU) recommendations, especially at Q- and V-band and above, where the atmospheric multiple scattering effects cannot be disregarded.
A computer program that the NOAA Wave Propagation Laboratory (WPL)
developed to simulate the brightness temperatures (T sub b) observed by
an upward-looking microwave radiometer system along a vertical or slant
path is described. FORTRAN source code listings are included as an
appendix. WPL uses the software to calibrate radiometers, to simulate
hypothetical radiometer systems, and to develop retrieval techniques.
The program computes clear-sky T sub b from radiosonde profiles of
pressure, temperature, and humidity after extrapolating them to 0.1 mb
and interpolating between levels where necessary to approximate a
continuous atmosphere. Clear-sky attenuation is calculated from the
Liebe and Layton water vapor and oxygen absorption model, which is valid
for frequencies up to 1 THz. An optional cloud-modeling scheme permits
simulations under a variety of cloud configurations. The Rayleigh
approximation that underlies the cloud absorption algorithm restricts
its validity to nonprecipitating clouds with particle radii less than
about 100 micrometers; scattering is neglected. Hence, the cloudy T sub
b simulations are valid only for frequencies less than 100 GHz.
Sky-Noise Temperature Modelling and Prediction for Deep Space Applications from X Band To W Band
- V Mattioli
- F S Marzano
- N Pierdicca
- C Capsoni
- A Martellucci