Cloud attenuation at millimeter wavelengths

Abstract

Abslmct-Total atmospheric attenuation under conditions of cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. The attenuations were actually inferred from extinction measurements using the sun as a source. Two hundred and ten sets of data were collected at each frequency under conditions of complete cloud cover; 131 sets of data were collected under conditions of partial cloud cover. Each set of data consisted of measurements made at 29 elevation angles from 1" to 20". The angle dependence of the attenuation was examined. For most cloud conditions the attenuation was shown to be proportional to the slant path distance through the absorbing atmosphere. For elevation angles above about 8', a flat earth approximation is valid and the slant path distance is proportional to the cosecant of the elevation angle. For low elevation angles the slant path distance is a function of the effective earth radius and the effective height of the attenuating atmosphere, in addition to the elevation angle. A statistical technique for determining the radius and height is described. A zenith attenuation was extrapolated from each set of data. The humidity dependence of the attenuation was examined next. A linear regression of zenith attenuation as a function of surface absolute humidity was performed. Correlation coefficients were of the order of 0.7 to 0.8. The attenuations measured under conditions of both complete and partial cloud cover were compared with previously obtained clear sky attenuations and found to be consistently higher. Finally the frequency dependence of the cloud attenuation was studied. It was assumed that the cloud attenuation has a frequency dependence similar to that of fog. From this dependence, in conjunction with the 35 GHz data, attenuations under conditions of cloud cover could be estimated for frequencies up to 100 GHz in the window regions of the spectrum. Finally, with these results, an algorithm for estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived.

Influence of clouds on attenuation of electromagnetic waves

“…Attenuations were measured at both frequencies simultaneously and then correlated with the only meaningful meteorological parameter that could be readily determined, the surface absolute humidity. In [12], it was mentioned that cloud attenuation was primarily due to absorption by the cloud droplets; scattering losses were secondary. For clouds at an altitude above the zero degree isotherms, absorption by ice particle is negligible because the imaginary component of the index of refraction of ice is very small.…”

“…For clouds at an altitude above the zero degree isotherms, absorption by ice particle is negligible because the imaginary component of the index of refraction of ice is very small. A linear regression of zenith attenuation as a function of surface absolute humidity was performed in [12]. The frequency dependence of the cloud attenuation was studied.…”

“…In [12], total atmospheric attenuation under conditions of cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. Attenuations were measured at both frequencies simultaneously and then correlated with the only meaningful meteorological parameter that could be readily determined, the surface absolute humidity.…”

“…An algorithm estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived. The following expression for the zenith attenuation for complete cloud cover was presented in [12]:…”

“…This factor must be considered when analysing rain attenuation and cloud attenuation. The cloud attenuation in various regions was analysed in [1,2,[7][8][9][10][11][12]. The attenuation of radio waves due to rain under Lithuanian climatic conditions was analysed in [13][14][15][16].…”