Cloud on the ground reducing the visibility to less than 1000m is Fog.
Thus - 27015 0700 FG OVC000
In a TAF or METAR, you will not get cloud layers reported above the base of the layer that is OVC (perhaps except CB but I would have to check that). The reason is twofold;
a) When making an observation, it is impossible for the observer to see through an overcast layer and see wht is beyond - if they could then it is not overcast; and
b) When making an approach as you descend, you are interested in when you will break cloud with a posibility of seeing the ground - that would not happen by descending through a layer that is above an overcast layer - hense it is not useful information (unless it is a CB?).
When delaing with enroute reports and forecasts (F215) then there can be several overcast layers reported - you may want to cruise between them!
Regards,
DFC