Different Graupel Formations in Different Precipitating Clouds Observed by Videosondes

Ice particles in a cloud play an important role in the evolution of precipitation. Graupel, one of a variety of solid hydrometeors in the cloud, is known to be a heavily rimed particle. Graupel is generally observed as a cone-like or irregular shape, as a result of riming. However, nearly spherical graupel is sometimes observed in the cloud, especially just above the freezing level. In this study, we focused on the shape of graupel and evaluated it quantitatively using particle images obtained from videosonde observations.

Videosonde was developed by Takahashi (1990) and improved by Suzuki et al. (2012). It is a balloon-borne special radiosonde to be launched into precipitating clouds, which has a charge couple diode (CCD) camera and a strobe to capture the particle images in the air and to provide information on particle size and shape. One of the advantages of our videosonde is its ability to capture images of precipitation particles as they are suspended in the air. Because our videosonde can obtain particle images without contact, it can measure particles neither bouncing nor destroyed.

We carried out the videosonde observation at Bengkulu (3.86ºS, 102.3ºE), the southwestern coast of Sumatra Island, Indonesia from November 24 to December 15, 2015 for the better understanding of microphysical precipitation processes of tropical convection, which were conducted as a part of Pre-YMC field campaign. It was a pilot study of the Years of the Maritime Continent (YMC). Videosondes were launched into three different rain systems; 1) the typical convective, 2) the typical stratiform clouds, and 3) the upper thick stratiform cloud with a shallow convection in the low level. The quantitative evaluation of graupel shape obtained from the videosondes showed the different graupel formations in the different rain systems. In the case of the upper thick stratiform with a shallow convection, a lot of ice crystals in the upper layer worked as embryos, and formed a lot of graupel as a result of the riming of supercooled droplets that provided from the shallow convection, which was different from the typical stratiform case that no graupel was observed. In the case of typical convection case, the videosonde observed spherical graupel just above the freezing level, of which aspect ratio was larger than that in the former case. It suggested that frozen drops as embryos formed spherical graupel uplifted by the strong updraft in the convective cloud, which was different from the irregular-shaped graupel formation.