  The previous, high angular resolution 225 GHz ($\sim$1.3 mm) continuum
observations on the transitional disk DM Tau have resolved an outer ring at
20-120 au radii that is weakly azimuthally asymmetric. We aimed to examine dust
growth and filtration in the outer ring. We performed the $\sim$0$''$.06
($\sim$8.7 au) resolution Karl G. Jansky Very Large Array (JVLA) 40-48 GHz
($\sim$7 mm; Q band) continuum observations and the complementary observations
at lower frequencies. In addition, we analyzed the archival JVLA observations
that were taken since 2010. Intriguingly, the Q band image resolved the
azimuthally highly asymmetric, knotty dust emission sources close to the inner
edge of the outer ring. Fitting the 8-700 GHz spectral energy distribution
(SED) with two dust components indicates that the maximum grain size in these
knotty dust emission sources is likely $\gtrsim$300 $\mu$m while it is
$\lesssim$50 $\mu$m in the rest of the ring. These results may be explained by
trapping of inward migrating grown dust close to the ring inner edge. The exact
mechanism for developing the azimuthal asymmetry has not yet been identified,
which may be due to planet-disk interaction that might also be responsible for
the creation of the dust cavity and pressure bump, or the fluid instabilities
and vortex formation due to shear motions. Finally, we remark that the
asymmetries in DM Tau are hard to diagnose from the $\gtrsim$225 GHz
observations owing to a high optical depth at the ring. In other words, the
apparent symmetric or asymmetric morphology of the transitional disks may be
related to the optical depths of those disks at the observing frequency.
