On the right track, just have to use the index for both variables...
index_liq = (M_CP==2); % index to find where M_CP ==2
plot(Radius(index_liq),cldTT1(index_liq)) % plot matching records
NB: You don't need the temporary variable to plot; it may well be convenient to create it for later use if doing a lot with it, but doing so instead of just using the logical addressing vector does create a copy in memory of the same data as already have (and the temperature variable along with it or any other variables of same condition). Since this is a pretty-decent-sized variable, that could have ramifications if done indiscriminantly.
