Microphysical structures of stratiform clouds associated with the MJO observed during MISMO project

In this study, microphysical structures of stratiform clouds associated with the Madden-Julian Oscillation (MJO) over the equatorial Indian Ocean were investigated using videosondes. Videosonde observations were conducted as part of the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO; Yoneyama et al., 2007) from October 16 to November 27, 2006. The R/V Mirai maintained a fixed position on 0º, 80.5ºE during observation period. Tropical convections associated with the MJO has been discussed in previous studies (eg., Johnson et al., 1999; Kemball-Cook and Weare, 2001; Kikuchi and Takayabu, 2004). However, in situ microphysical observation of the clouds developed over the Indian Ocean has never conducted. We had launched seven videosondes, which give us images of precipitation particles are acquired by a CCD camera, into the stratiform clouds developed over the equatorial Indian Ocean. Particle images transmitted from videosondes were ice crystals, graupel, and aggregate (snowflakes) near and above the freezing level. The shapes of these aggregates were different from aggregations of nearly round graupel observed in the maritime stratiform clouds during TOGA-COARE (Takahashi et al., 1995) and the R/V Mirai MR04-08 cruise over the western Pacific Ocean (Suzuki et al., 2006). The number concentrations of ice crystal and graupel were greater than that observed in the maritime stratiform clouds over the western Pacific region. It was found that the stronger ice crystal formation process in the upper level of stratiform clouds was dominant over the Indian Ocean. Moreover, during the MISMO observation period, we experienced the drastic enhancement of convective activity that may be related to the onset of deep convection in the MJO after mid-November, and the large-scale upper air circulation had been dramatically changed. Before and behind that, the vertical precipitation particle distributions were greatly different. The number concentrations of ice crystal and graupel that had been observed in the latter half was greater than that observed in the first half. The backward trajectory analysis shows that the air mass from the northern hemisphere (continental) after the onset of the MJO was dominant, while the air mass from the southern hemisphere (maritime) before the MJO. This suggested that a large-scale circulation might greatly influence such microphysical features in clouds.