Piya, you may be confusing two different things because their abbreviations are similar. Alternating & direct current are AC & DC respectively. Lightning is essentially DC as explained by @FireMadeFlesh.
Alternating current (AC) comes out of electrical outlets. DC comes out of batteries. AC doesn’t normally occur in nature.
Analog & digital, on the other hand, are two different approaches to transmitting or recording information—it doesn’t apply to lightning (if I have to pick one, I’d call it analog)
But analog-to-digital converters are often called AD converters or ADCs—confusing by its similarity to AC / DC.