1-3hit |
Fumiaki MINEMATSU Yoichi SUZUKI Masashi KAMEI Kazuyoshi SHOGEN
The rain attenuation in the 12-GHz band and one-minute-rain rate were measured in Tokyo over a four-year period (2000-2003). The statistical characteristics of this data are presented. The one-minute-rain rates at 0.01% of time percentage of year in Tokyo and Osaka are compared to other past and recent values. The comparison of measured rain attenuation in the 12-GHz band in Tokyo and Osaka with prediction by ITU-R methods is conducted. The root-mean-square prediction error of rain attenuation for the prediction by ITU-R Rec.P.618 is evaluated. Convective rain cell effects can be seen in the scatter diagram of one-minute-rain rate and rain attenuation. However, it is found that the effect is not properly accounted for by the slant path length adjustment factor of P.618-8. A reliable rain attenuation prediction requires some revisions of the slant path length adjustment factor with taking local weather characteristics into account for the P.618-8.
Yasuyuki MAEKAWA Nion Sock CHANG Akira MIYAZAKI
Ice depolarization characteristics are discussed using cross-polarization discrimination (XPD) observations of the CS-2 beacon signal (19.45GHz, right-hand circular polarization, elevation angle of 49.5) in the stratus type rainfall events, which show a clear bright band in the simultaneous X-band radar observations. Both amplitude and phase of the mean ice depolarizations are deduced in each rainfall event by subtracting theoretical rain depolarizations from the observed values. In spite of the difference in rainfall rates on the ground, the inferred depolarizations indicate much the same amplitude and phase as those directly obtained in pure ice depolarization events without appreciable rain depolarizations. The origin of the ice depolarizations in the stratus type events, as well as in the pure ice events, seems to be ice crystals near the cloud top which are not very much concerned with the ground rainfall rates. Compared with the radar measurements above the bright band, the ice depolarizations are approximately proportional to the vertical length of the ice region at least up to 3km above the bright band. This result yields the equivalent "specific depolarization" per unit path length: |Ci|610-3km-1 (44dB in XPD) for the mean ice depolarizations in each event. Using this coefficient, the ice effects (XPD), which refer to the deviations of the observed depolarizations from the theoretical rain depolarization, are well described as a function of the height ratio of the ice region to the rain region in the stratus type events. Finally, the ice effects (XPD) are calculated against vertical lengths of the ice region in the case of specific rain heights of 2-4 km. These calculations are performed for various rainfall rates of 2-15mm/h in view of ground-based rain observations.
Yasuyuki MAEKAWA Nion Sock CHANG Akira MIYAZAKI
Observations of rain depolarization characteristics were conducted using the CS-2 and CS-3 beacon signals (19.45GHz, circular polarization, elevation angle=49.5) during seven years of 1986-1992 at Neyagawa, Osaka. The mean cross-polar phase relative to the co-polar phase of each rainfall event is distributed in a comparatively wide range from -100 to -150. This large variation is suggested to be caused by the difference of raindrop size distribution (DSD) in addition to that of rain intensity. The effects of DSD are examined by rain attenuation statistics for specific months, together with direct measurements of raindrop diameters on the ground for several rainfall events. Compared with representative DSD models, the effects of the Joss-drizzle type with relatively small raindrops primarily appear in "Baiu (Tsuyu)" period, while the effects of the Marshall-Palmer type which represents a standard type are enhanced in "Shurin (Akisame)" period. On the other hand, the effects of the Joss-thunderstorm type with comparatively large raindrops do not indicate a very clear seasonal variation. Possible improvements of XPD performed by differential phase shifters are generally found to be lower than 10dB for the rain depolarization due to the effect of residual differential attenuation after the cancellation of differential phase shift. Such XPD improvements are, however, very sensitive to the type of DSD, and it is suggested that the improvements are at least greater than 6dB for the Joss-drizzle type, whereas they are less than 6dB for the Marshall-Palmer and Joss-thunderstorm types. The effects of the XPD improvements are thus related to rainfall types, i.e., the type of DSD, and the improvements are considerably dependent upon the seasons in which each rainfall type frequently appears.