I don't think that there are specific rules, but I would combine the rules for falling and damage from falling objects.
You jump down on the enemy, and if you make the (DC 15) acrobatics check, you take 1d6 non-lethal and 1d6 lethal damage. You fall prone.
If you make the attack roll, you deal normal weapon damage, plus 3d6 as you are a medium-sized falling object. (I was going to suggest a charge from above type attack, but then I thought charge and fall don't compare. A fall is much faster.)
If you miss, you just fall prone at the feet of your opponent. Splat.
And if you don't make the acrobatics check... well, I think it's up to the GM. No attack if it's a heroic style campaign, I guess, but I like it more cinematic, so you may attack anyway, albeit with a penalty.
Edit: If you are playing a monk of at least 4th level, you don't fall prone. Instead, you land next to your opponent and you can use your flurry of blows next round.
I suppose it acts as a charge, with a bonus for having the High Ground.
Unless you just jump on top of the target, in which case you and it both simply take the falling damage.
There are rules on crushing damage. Specifically a table detailing the weight descending on an individual and the corresponding wait. Its a weird kind of downward bullrush. The result of the check would add further damage in addition to your own weight to whoever you're landing on. Back few pages of complete warrior for more details.