Graupel in the different developing stages of Baiu monsoon clouds observed by videosondes

Abstract

Graupel is generally observed as a cone-like shape in clouds, as a result of riming. However, nearly spherical graupel is also observed in clouds. In this study, we focused on the shape of graupel and evaluated it quantitatively using particle images obtained from videosonde observations. Videosonde soundings were conducted as part of a campaign observation by a C-band polarimetric radar (COBRA) that was synchronized with videosondes during the Baiu monsoon rainy season in 2008 and 2009 in Okinawa, Japan. Four typical cloud types (a shallow convective cloud, two deep convective clouds and a cloud that is in the process of transitioning from convective to stratiform) were chosen for the analysis. The flattening of graupel (f) and ratios of the graupel cross sections to the ellipsoid-assumed cross sections (r) were calculated on the basis of the graupel shapes observed between 0°C and −10°C. In the cases of shallow and deep convective clouds, f was smaller than that of the case with the transition cloud. Moreover, it was found that r decreased and f increased as the cloud type changed from convective to stratiform. This indicated that nearly spherical graupel were more likely to be observed in shallow convective clouds and irregularly shaped graupel in stratiform clouds. The results suggest that the spherical graupel derived its origin from frozen particles lifted by updrafts in the convective clouds, which was quite different from the formation of conical graupel that originated from ice crystals.