On 25 February 1999, due to a winter monsoon after a cyclonic storm, orographic snow clouds formed under conditions of weak cold advection on the western side of the central mountain range of Japan. In this study, the Ka-band Doppler radar and vehicle-mounted microwave radiometer and 2D-Grey imaging probe were used to obtain unique datasets for analyzing the spatial distributions of microphysical structures of the snow clouds at the windward slope. The liquid water path, number concentration of snow particles (0.1–6.4 mm diameter), and precipitation rate were found to be correlated with altitude. The greater concentration of larger particles tended to appear up the slope. The echo top was at about 2.5 km (−30 dBZ), and the relatively strong echo region (>−3 dBZ) appeared at 5 km up the slope and extended nearly parallel to the slope. According to the echo pattern, the ice water path increased with terrain height and reached the maximum intensity at about 14 km up the slope. These observations provide indirect evidence that terrain-induced updrafts lead to the generation and growth of supercooled cloud droplets and indicate that the riming process plays an important role in the growth of snow particles at higher altitudes. In this paper, it is confirmed that the abundance of supercooled liquid water (SLW) during intensified monsoon flow is due to larger water production rates caused by higher vertical velocities induced by topography. Furthermore, it can be shown that small-scale terrains enhance localized updrafts embedded within the larger-scale flow and have noticeable impact on SLW cloud distribution.
The collection efficiency of a hydrometeor videosonde (HYVIS) for dry snow particles is re-evaluated. This is estimated from comparisons with simultaneous aircraft data taken from a warm frontal cloud. The observed cloud is relatively uniform below an altitude of 5.5 km, where the standard deviation of ice particle concentrations is less than 40%. Comparing both size spectra measured with the balloonborne HYVIS and the airborne two-dimensional optical array probe within comparable size ranges (300 to 1500 mm) reveals that the collection efficiency of the HYVIS for dry snow particles is approximately 0.5, with an uncertainty of 50%, assuming that the airborne probe provides reference concentrations in these size ranges.
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