I've personally no experience painting scorched ground, however I'd probably go about it the same way you would a standard base, I'd be inclined to use some of the generic grass tufts, the kind of thing that
GW and many other retailers find, maybe some of the
GW texture paints that give a cracked ground appearance (if you want to go that far). Then I'd use something like this smoke pigment from AK interactive
http://ak-interactive.com/v2/product/smoke/ could give you the effect that you're looking for