Vertical profiles of CCN concentrations and microphysical structures of shallow warm clouds in western Japan

Shallow warm clouds with top heights of 2 ~ 3km were investigated with an instrumented aircraft (B200T) and several ground-based remote sensors in western Japan. Shallow warm clouds showed a wide range of ability for producing drizzle and rain drops. Usually the ability depends on number concentrations of cloud droplets; clouds forming in maritime air mass with less CCN tend to easily produce precipitation and vise versa. Even in southerly wind (supposed to be in maritime air mass), typical number concentrations of cloud droplets are 400 ~ 600 droplets/cc and not purely maritime, but polluted to some extent. Typical microphysical structures of clouds that hardly precipitate are 1) cloud droplet number concentrations decrease with height from 600 to 400 droplets/cc, 2) droplet sizes are confined to smaller than 30 microns. On the other hand, microphysical structures of clouds that most efficiently produced precipitation were 1) cloud droplet concentrations are rather high 500 ~ 600 droplets/cc at lower levels, but very low (several tens droplets/cc) at upper and middle levels. Drizzles initiated through collision-coalescence among large droplets grew by accretion of small cloud droplets in lower parts of clouds and produced radar-detectable precipitation. Back trajectory analysis showed that air at lower part of clouds came from over the sea near Japan Islands whereas air at middle and upper parts came from over the sea far away from Japan Islands. The very low concentrations of CCN at middle and upper parts may be also attributed to long life of the cloud system.