Development of negative dipoles in a stratiform cloud layer in a Okinawa “Baiu” MCS system

In a wet stratiform cloud layer extending from a Baiu convective system over Okinawa, videosonde data showed a large negative dipole near the melting level in a background positive space charge by ice crystals. An intense radar echo extended to higher levels above the bright band and snowflake formation was active through the whole cloud depth in the negative dipole case. This is in strong contrast to the abrupt decrease of echo intensity above the bright band and the limited snowflake formation near the melting level in the absence of a negative dipole. In MCS scattered convective cells may form such special stratiform cloud layers. It is suggested that a new charge separation process occurs between graupel and ice crystals in the layer slightly above the melting layer. Negative graupel falling from above, possibly being from the convective region of a MCS, changes sign through collision with newly formed ice crystals in this narrow layer. Positive graupel and negative ice crystals are thus formed. The riming electrification process (Takahashi, 1978 and others) may be the main charge separation mechanism. With a mesoscale updraft, negative ice crystals are lifted and some coagulate, forming negative snowflakes. Positive graupel, falling against an updraft, will also adhere, forming positive snowflakes. A negative dipole, negative above by ice crystals and ice crystal-composed snowflakes, and positive below by graupel-composed snowflakes, will thus develop near the melting level. This highly negative dipole may be related to the “Sprite”-form negative dipole observed in the thick stratiform cloud layer of a Mesoscale Convective System (MCS).