(dB/km) (1) where: : specific attenuation (dB/km) within the cloud c : cloud liquid water specific attenuation coefficient ((dB/km)/(g/m3)) Kl : liquid water density in the cloud or fog (g/m3) M frequency (GHz) f: : cloud liquid water temperature (K). T At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant. The liquid water density in fog is typically about 0.05 g/m3 for medium fog (visibility of the order of 300 m) and 0.5 g/m3 for thick fog (visibility of the order of 50 m). 2 Cloud liquid water specific attenuation coefficient A mathematical model based on Rayleigh scattering, which uses a double-Debye model for the dielectric permittivity ( ) of water, can be used to calculate the value of for frequencies up to f Kl 200 GHz: (dB/km)/(g/m3) (2) where is the frequency (GHz), and: f (3) The complex dielectric permittivity of water is given by: (4) (5) where: = 77.66 + 103.3 ( 1) (6) – 0 = 0.0671 (7) 1 = 3.52 (8) 2 = 300 / (9) T and is the liquid water temperature (K). T The principal relaxation frequency, and secondary relaxation frequency, are: fp, fs, = 20.20 146 ( 1) + 316 ( 1)2 (GHz) (10) fp – – – = 39.8fp (GHz) (11) fs 3 Cloud attenuation along slant paths If local measured data of the total columnar content of cloud liquid water, (kg/m2 or, equivalently, mm), is not available, the method in § 3.1 should be used to predict the L cloud attenuation along slant paths. This prediction method is based on ERA-40 data where the total columnar content of cloud liquid water reduced to a fixed temperature of 273.15 K, (kg/m2 or, Lred equivalently, mm), is used. If local measured data of the total columnar content of cloud liquid water, is available from other L, sources, e.g. from ground radiometric measurements, Earth observation products or meteorological
(dB/km) (1) where: c : specific attenuation (dB/km) within the cloud Kl : cloud liquid water specific attenuation coefficient ((dB/km)/(g/m3)) M : liquid water density in the cloud or fog (g/m3) f: frequency (GHz) T : cloud liquid water temperature (K). At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant. The liquid water density in fog is typically about 0.05 g/m3 for medium fog (visibility of the order of 300 m) and 0.5 g/m3 for thick fog (visibility of the order of 50 m). 2 Cloud liquid water specific attenuation coefficient A mathematical model based on Rayleigh scattering, which uses a double-Debye model for the dielectric permittivity ( f ) of water, can be used to calculate the value of Kl for frequencies up to 200 GHz: (dB/km)/(g/m3) (2) where f is the frequency (GHz), and: (3) The complex dielectric permittivity of water is given by: (4) (5) where: 0 = 77.66 + 103.3 ( – 1) (6) 1 = 0.0671 (7) 2 = 3.52 (8) = 300 / T (9) and T is the liquid water temperature (K). The principal relaxation frequency, fp, and secondary relaxation frequency, fs, are: fp = 20.20 – 146 ( – 1) + 316 ( – 1)2 (GHz) (10) fs = 39.8fp (GHz) (11) 3 Cloud attenuation along slant paths If local measured data of the total columnar content of cloud liquid water, L (kg/m2 or, equivalently, mm), is not available, the method in § 3.1 should be used to predict the cloud attenuation along slant paths. This prediction method is based on ERA-40 data where the total columnar content of cloud liquid water reduced to a fixed temperature of 273.15 K, Lred (kg/m2 or, equivalently, mm), is used. If local measured data of the total columnar content of cloud liquid water, L, is available from other sources, e.g. from ground radiometric measurements, Earth observation products or meteorological
R-REC-P.840-7-201712-I!!MSW-E.docx