Growth of Sobolev norms and loss of regularity in transport equations

We consider transport of a passive scalar advected by an irregular divergence free vector field. Given any non-constant initial data $\bar \rho \in H^1_$\text{loc}$({\mathbb R}^d)$,$d\geq 2$, we construct a divergence free advecting velocity field $v$ (depending on $\bar{\rho }$) for which the unique weak solution to the transport equation does not belong to $H_{\text{loc}}^1({\mathbb{R}}^d)$ for any positive positive time. The velocity field $v$ is smooth, except at one point, controlled uniformly in time, and belongs to almost every Sobolev space $W^{s,p}$ that does not embed into the Lipschitz class. The velocity field $v$ is constructed by pulling back and rescaling an initial data dependent sequence of sine/cosine shear flows on the torus. This loss of regularity result complements that in [Ann. PDE, 5(1):Paper No. 9, 19, 2019].