  This paper is devoted to the global (in time) regularity problem for a family
of active scalar equations with fractional dissipation. Each component of the
velocity field $u$ is determined by the active scalar $\theta$ through
$\mathcal{R} \Lambda^{-1} P(\Lambda) \theta$ where $\mathcal{R}$ denotes a
Riesz transform, $\Lambda=(-\Delta)^{1/2}$ and $P(\Lambda)$ represents a family
of Fourier multiplier operators. The 2D Navier-Stokes vorticity equations
correspond to the special case $P(\Lambda)=I$ while the surface
quasi-geostrophic (SQG) equation to $P(\Lambda) =\Lambda$. We obtain the global
regularity for a class of equations for which $P(\Lambda)$ and the fractional
power of the dissipative Laplacian are required to satisfy an explicit
condition. In particular, the active scalar equations with any fractional
dissipation and with $P(\Lambda)= (\log(I-\Delta))^\gamma$ for any $\gamma>0$
are globally regular.
