Two-Wavelength (0.86cm and 3.2cm) Radar Observation of Microphysical Process in Stratiform Precipitation

Abstract

Strong attenuation of millimeter wave due to precipitation particles is the most serious problem when we use K-band (0.86cm wavelength) radar for the observation of the precipitation.In this paper, we calculated the attenuation of K-, X-(3.2cm) and C-band (5.6cm) waves based on a model of precipitation, for three precipitation intensities, i, e., heavy (32mm/hr), ordinate (4mm/hr) and weak (0.5mm/hr) precipitations. And, we surveyed any possibility of the observation with weak affection of the attenuation using the bands. The observation of the precipitation was executed in Part II.The results show that the attenuation coefficient is larger in the melting layer by one order or two orders than in the lower raindrop region.The results also indicate that the affection of the size distribution to the attenuation abruptly increases as the attenuation is enhanced.The attenuations of X-and C-band waves are small enough to be neglected except for the heavy precipitation.In the case of heavy precipitation, K-band wave is severely attenuated, especially in the melting layer, so that the signals cannot be detected by the radar receiver.In the cases of ordinary and weak precipitations, however, we can obtain true radar reflectivity factor of K-band wave by corrections of the attenuation. In addition, the correction method using the radar reflectivity factor of the X-band radar was investigated.We can mention conclusively that after the correction the precipitations weaker than about 4*7mm/hr can be safely observed within an error of 2dB by the twowavelength radar technique using K-and X-band radars in the raindrop region.In the melting layer, the correction method can be utilized without large error when the precipitation is weaker than about 2*4mm/hr.

Two-Wavelength (0.86cm and 3.2cm) Radar Observation of Microphysical Process in Stratiform Precipitation

“…Combining the present results with those obtained by Yokoyama et al (1984Yokoyama et al ( , 1985, we can conclude that the size distribution changes equivalent to coalescence predominantly occur through the whole melting layer. Activation of coalescence is due probably to the surface wetness, the wide spread fallspeed spectra and the large cross-sections of the particles.…”

“…Activation of coalescence is due probably to the surface wetness, the wide spread fallspeed spectra and the large cross-sections of the particles. The conclusion described in (ii) is also supported by direct sampling observation (Yokoyama et al, 1985) carried out at Mt. Fuji.…”

“…Actually, the raindrop size distributions measured on the ground by Willis (1984) and Yokoyama et al (1985) have very frequently the shapes that the number density in the diameter range of 0.1*0.3 cm exceeds the MP distribution.…”

“…Ohtake (1969) and Yokoyama et al (1985) measured the size distribution change of the precipitation particles by direct sampling at different altitudes along mountain slopes. However, conclusions of the two papers are opposite ; the former concluded that the size distribution change through the melting layer is rather minor and that a similarity of size distribution is kept during the fall, while the latter demonstrated that the size distribution change due to coalescence of the melting snowflakes is well noticeable in the layer.…”

“…An inevitable barrier is a severe attenuation of K-band wave by the precipitation particles. The barrier, however, can be overcome by means of a correction method proposed by Yokoyama (1985) (hereafter Part I) if the precipitation is relatively weak.…”