Towards a spacelike characterization of the null singularity inside a black hole

Black hole interiors are expected to contain singularities that have both a spacelike piece and a null piece, where the null piece occurs at the inner horizon. However, most numerical simulations use a foliation by spacelike Cauchy surfaces. How is one to (1) pick a spacelike foliation that approaches the null singularity, and (2) recognize that the singularity is null using the variables of the simulation? I propose a foliation using harmonic time slicing and a characterization by the same sort of scale invariant tetrad variables that have been used in simulations of spacelike singularities. As a proof of concept of this proposal, I analyze the properties of the interior of a Reissner-Nordstrom black hole in this foliation and using these variables. I find a crucial difference between spacelike and null singularities is the following: for spacelike singularities the scale invariant spatial triad vanishes as the singularity is approached, whereas for null singularities there is one nonvanishing spatial triad vector corresponding to the generators of the horizon.