The a/c domain pattern of tetragonal PbZr0.10Ti0.90O3 thin films under tensile misfit strain is investigated by piezoresponse force microscopy and X-ray diffraction. The results show a hierarchical ordering of the dense a/c domain structure into larger superstructures. The latter exhibit a preferred orientation and occasionally form distinct patterns such as flux closure loops of the net polarization. Additionally, the residual strain is measured with reciprocal space maps and the a-domain fraction is determined by theoretical calculations and correlated with results from rocking curve scans.