By definition of cumulonimbus cloud, at its top water droplets are transformed into ice crystals.

But for cumulonimbus calvus, content of ice crystals are meager and polar are in early stage, so the cloud tops still look round and puffy.

[4] Developing cumulonimbus calvus lose sharp outlines of the top as more water droplets transform into ice crystals.

Strong updrafts may form pileus or thin vertical stripes may protrude upwards out of the cloud.

It can form into a cumulonimbus incus, and bring more severe thunderstorms that can cause constant cloud-to-ground lightning, extensive wind damage, microbursts, hail, and occasionally a tornado.