Ice Crystal Shapes in Midlatitude Cirrus Clouds Derived from Hydrometeor Videosonde (HYVIS) Observations

抄録

 This study investigates the frequency of the occurrence of ice crystal habits in midlatitude cirrus clouds that were primarily associated with warm or stationary fronts within synoptic-scale lows. The measurements were performed with a balloonborne hydrometeor videosonde (HYVIS). The predominant types were single bullets at temperatures ranging from -60° to -20°C. Plate-type crystals were dominant at temperatures warmer than -20°C, whereas column or bullet rosette crystals became dominant at temperatures colder than -60°C. The distributions of the ice crystal habits derived from the HYVIS observations were consistent with the results of recent laboratory and field experiments, although the dependency of the ice crystal habits on ice supersaturation was not characterized in this study because only limited accurate humidity data were available under low temperature conditions.
 The size dependency of the distributions of the axis ratios of column and bullet crystals tended to decrease with increasing crystal dimension. We found no clear temperature dependency of the axis ratios of columnar crystal shapes. The area ratios of the six classified crystal habits were found to be comparable with previously reported ratios. The fact that the area ratio decreased with increasing dimension was apparent for all the crystal types. Polynomial curves fit to plots of area ratio versus maximum crystal dimension for each crystal type evidenced patterns different from those reported in previous studies. When the conventional power-law relationships between cross-sectional area and dimension reported in a previous study were applied to the HYVIS data, we found that the power-law relationships could overestimate the measured cross-sectional area integrated over the 250 m height interval between the cloud base and the cloud top by 10 %-80 %. The degree of overestimation was highly variable between individual cases.