  Anomalous scaling in the statistics of an active scalar in homogeneous
turbulent convection is studied using a dynamical shell model. We extend
refined similarity ideas for homogeneous and isotropic turbulence to
homogeneous turbulent convection and attribute the origin of the anomalous
scaling to variations of the entropy transfer rate. We verify the consequences
and thus the validity of our hypothesis by showing that the conditional
statistics of the active scalar and the velocity at fixed values of entropy
transfer rate are not anomalous but have simple scaling with exponents given by
dimensional considerations, and that the intermittency corrections are given by
the scaling exponents of the moments of the entropy transfer rate.
