  We study positive streamers in air propagating along polycarbonate dielectric
plates with and without small-scale surface profiles.
  The streamer development was documented using light-sensitive high-speed
cameras and a photo-multiplier tube, and the experimental results were compared
with 2D fluid streamer simulations.
  Two profiles were tested, one with 0.5 mm deep semi-circular corrugations and
one with 0.5 mm deep rectangular corrugations.
  A non-profiled surface was used as a reference.
  Both experiments and simulations show that the surface profiles lead to
significantly slower surface streamers, and also reduce their length.
  The rectangular-cut profile obstructs the surface streamer more than the
semi-circular profile.
  We find quantitative agreement between simulations and experiments.
  For the surface with rectangular grooves, the simulations also reveal a
complex propagation mechanism where new positive streamers re-ignite inside the
surface profile corrugations.
  The results are of importance for technological applications involving
streamers and solid dielectrics.
