Fresnel scattering model for the specular echoes observed by VHF radar

Gage, K. S.; Balsley, B. B.; Green, J. L.

doi:10.1029/rs016i006p01447

Cited by 66 publications

(30 citation statements)

References 19 publications

(16 reference statements)

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“…This type of situation could arise either due to isolated intense turbulent patch advecting through the off-zenith beam or due to tilt of the echoing layers. Gage et al (1981a, b), Gage (1986) and Hobbs and Reid (2000) suggested that this type of situation could arise due to the passage of atmospheric waves over the radar site and associated fluctuation in vertical and oblique beam echo power. If the layers are horizontally stratified in both east-west (E-W) or north-south (N-S) directions, the power is expected to be equal between the two symmetric beams.…”

Section: Tilting Of Atmospheric Layers and Their Generation Mechanismsmentioning

confidence: 99%

Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

et al. 2004

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Abstract. Simultaneous observations made on four days using the MST radar and GPS-sonde at Gadanki (13.5 • N, 79.2 • E), a tropical station in India, are presented to address the aspect sensitivity of radar backscatters observed at different heights. The observations show that wherever stability parameter N 2 is high, vertical shear of horizontal wind is low and Richardson number (R i ) is high, the aspect sensitivity is high indicating that the aspect sensitive radar backscatters are due to thermal structures in the atmosphere. Such a case can be seen very clearly in the upper troposphere and lower stratosphere. At some heights, where N 2 is high, R i is high, but shears are relatively weak, the aspect sensitivity is found to almost disappear, indicating that some amount of shear provides favorable conditions for causing aspect sensitivity. Aspect sensitivity does not occur at all where N 2 is low or negative and R i is low in spite of wind shear being either high or low, indicating that the regions are well mixed and hence turbulent. The study also shows a power difference in the symmetric beams. A case study on this aspect suggests that this asymmetry is due to the tilting of layers by the action of atmospheric waves. There is indication that these waves are generated through Kelvin-Helmholtz-instability (KHI).

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Section: Tilting Of Atmospheric Layers and Their Generation Mechanismsmentioning

confidence: 99%

Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

et al. 2004

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“…Details of these mechanisms are described by, for example, Gage and Balsley (1980), Gage et al (1981Gage et al ( , 1985, Hocking (1985), and Röttger and Larsen (1990). For monostatic radars, backscatter arises from refractive index variation n at the radar half wavelength (i.e.…”

Section: Observationsmentioning

confidence: 99%

Derivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway

Rapp

Schrön

et al. 2016

Ann. Geophys.

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Abstract. We present the derivation of turbulent energy dissipation rate ε from a total of 522 days of observations with the Middle Atmosphere Alomar Radar SYstem (MAARSY) mesosphere-stratosphere-troposphere (MST) radar running tropospheric experiments during the period of 2010-2013 as well as with balloon-borne radiosondes based on a campaign in the summer 2013. Spectral widths are converted to ε after the removal of the broadening effects due to the finite beam width of the radar. With the simultaneous in situ measurements of ε with balloon-borne radiosondes at the MAARSY radar site, we compare the ε values derived from both techniques and reach an encouraging agreement between them. Using all the radar data available, we present a preliminary climatology of atmospheric turbulence in the UTLS (upper troposphere and lower stratosphere) region above the MAARSY site showing a variability of more than 5 orders of magnitude inherent in turbulent energy dissipation rates. The derived ε values reveal a log-normal distribution with a negative skewness, and the ε profiles show an increase with height which is also the case for each individual month. Atmospheric turbulence based on our radar measurements reveals a seasonal variation but no clear diurnal variation in the UTLS region. Comparison of ε with the gradient Richardson number Ri shows that only 1.7 % of all the data with turbulence occur under the condition of Ri < 1 and that the values of ε under the condition of Ri < 1 are significantly larger than those under Ri > 1. Further, there is a roughly negative correlation between ε and Ri that is independent of the scale dependence of Ri. Turbulence under active dynamical conditions (velocity of horizontal wind U > 10 m s −1 ) is significantly stronger than under quiet conditions (U < 10 m s −1 ). Last but not least, the derived ε values are compared with the corresponding vertical shears of background wind velocity showing a linear relation with a corresponding correlation coefficient r = 58 % well above the 99.9 % significance level. This implies that wind shears play an important role in the turbulence generation in the troposphere and lower stratosphere (through the Kelvin-Helmholtz instability).

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“…The first is the Fresnel scattering model for highly aspect-sensitive radar scatter, as mentioned above. This model is expressed by Gage et al (1981Gage et al ( , 1985, in terms of a normalised power reflection coefficient |ρ| 2 / r which we will refer to as Fresnel scatter:…”

Section: Radar Scattering Modelsmentioning

confidence: 99%

“…Gage and co-workers went on to develop a theory of "Fresnel scatter" which explained the radar echoes in terms of partial reflection from a series of sharp gradients in refractive index, extended horizontally to cover several times the Fresnel radius within the radar beam. This theory was able to account for both a dependence of echo strength on static stability (the mean vertical gradient of refractive index, M, is related to static stability), for strong aspect sensitivity, and for an observed proportionality to the length of the radar pulse used to measure the scatter strength (Gage and Green, 1978;Gage et al, 1981Gage et al, , 1985. Doviak and Zrnic (1984), however, showed that it was not necessary for the reflecting structures to be as extensive as supposed in Gage and co-worker's theory, only that the correlation length of the irregularities should be comparable to the radius of the first Fresnel zone.…”

Section: Introductionmentioning

confidence: 99%

Fresnel scatter revisited – comparison of 50 MHz radar and radiosondes in the Arctic, the Tropics and Antarctica

et al. 2010

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Abstract. High-resolution radiosondes and calibrated radars operating close to 50 MHz, are used to examine the relationship between the strength of radar scatter and refractive index gradient. Three radars are used, in Kiruna in Arctic Sweden, at Gadanki in southern India and at the Swedish/Finnish base Wasa/Aboa in Queen Maud Land, Antarctica. Calibration is accomplished using the daily variation of galactic noise measured at each site. Proportionality between radar scatter strength and the square of the mean gradient of potential refractive index, M 2 , is found in the upper troposphere and lower stratosphere at all three sites, confirming previously reported results from many VHF radars. If the radar scatter is interpreted as Fresnel scatter, the constant of proportionality between radar scatter and M 2 is found to be the same, within the calibration uncertainties, for all three radars. The radiosondes show evidence of distinct layering with sharp gradients, extending over 10s of kilometers horizontally, but the scatter is found to be two orders of magnitude weaker than would be expected from true Fresnel scatter from such layers. Using radar reflectivities resolved to a few 100 ms, we show that this is due to strong temporal variability in the scattering conditions, possibly due to undulations of the scattering layers. The constancy of the radar scatter -M 2 relationship between the different sites suggests an unexpected uniformity in these perturbations between very different regions of the globe.

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Fresnel scattering model for the specular echoes observed by VHF radar

Cited by 66 publications

References 19 publications

Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

Derivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway

Fresnel scatter revisited – comparison of 50 MHz radar and radiosondes in the Arctic, the Tropics and Antarctica

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