And yes the upward lightening will be explained later on in the novel.
Doesn't lightening go up anyway? Or rather it does both -- up and down in cloud/ground strikes. But cloud/ground strikes are less than 30% (I believe) of all lightening anyway. The majority of lightening is more "sideways" than up/down; either occurring within a single cloud or between two clouds.
Or are you meaning the lightening goes upwards from the cloud (as if it were a ground based streamer)? In which case it will need something above it to be attracted to...