Videosonde observations of supercooled cloud droplet layers at the tops of wintertime stratiform clouds in northern Japan

Supercooled cloud droplets (SCDs) are widely present at the temperature range below 0°C. SCDs are closely related with nucleation and growth of ice particles and radiation, and therefore play important roles in precipitation formation and radiation transfer processes. We performed in-situ observation of cloud and ice crystal particles using hydrometeor videosondes in February 2011 at Rikubetsu in inland Hokkaido of Japan. During the observation period, SCD layers were found in three of five videosondes released in non-frontal clouds. Characteristics of SCD layers in the non-frontal clouds were examined.

Radar observation showed that the clouds with SCD layers had stratiform characteristics with the maximum radar reflectivity around 30 dBZ and the horizontal extents were tens of kilometers. Cloud-top heights estimated from videosonde images and radiosonde data were about 3000 m for the three cases. Triple SCD layers were present in a cloud at 11 February while single SCD layers were at 25 and 27 February. A common feature of the cases was that SCD layers were present at the tops of clouds. The thicknesses of cloud-top SCD layers were between 60 m and 230 m and the temperature ranged from -21°C to -25.5°C. The potential temperature profiles were close to the absolutely neutral stratification around the cloud-top SCD layers. Strong stable and very dry layers were present right above the cloud-top SCD layers.

Although number of observation cases is small, SCDs were present at high frequency at the tops of non-frontal stratiform clouds formed in northern Japan. As compared to the Arctic clouds with long-lasting cloud-top SCD layers, the present cases were shorter in the lifetimes and were characterized by dry profiles right above the SCD layers.