  We search for meridional variation in the abundance of CH$_3$D relative to
CH$_4$ on Titan using near-IR spectra obtained with NIRSPAO at Keck, which have
a photon-limited signal-to-noise ratio of $\sim$50. Our observations can rule
out a larger than 10% variation in the column of CH$_3$D below 50 km. The
preferential condensation of the heavy isotopologues will fractionate methane
by reducing CH$_3$D in the remaining vapor, and therefore these observations
place limits on the amount of condensation that occurs in the troposphere.
While previous estimates of CH$_3$D fractionation on Titan have estimated an
upper limit of -6 per mil, assuming a solid condensate, we consider more recent
laboratory data for the equilibrium fractionation over liquid methane, and use
a Rayleigh distillation model to calculate fractionation in an ascending parcel
of air that is following a moist adiabat. We find that deep, precipitating
convection can enhance the fractionation of the remaining methane vapor by -10
to -40 per mil, depending on the final temperature of the rising parcel. By
relating fractionation of our reference parcel model to the pressure level
where the moist adiabat achieves the required temperature, we argue that the
measured methane fractionation constrains the outflow level for a deep
convective event. Observations with a factor of at least 4 to 6 times larger
signal-to-noise are required to detect this amount of fractionation, depending
on the altitude range over which the outflow from deep convection occurs.
