  This paper is a case study of horizontal atmospheric rolls that formed over
the Black Sea on 16 August 2007. The rolls were discovered in WRF modeling
results for a mesoscale cyclone that originated over the sea on 15 August 2007.
The roll formation mechanisms, such as Rayleigh-Benard convective instability,
dynamic instability, advection and stretching of vertical velocity field
inhomogeneities, are considered. It is shown that indeed convective instability
played an important role in the roll formation but dynamic instability did not
occur. In order to distinguish other possible mechanisms of the roll formation
numerical experiments were performed. In these experiments sea surface
temperature in the initial conditions was decreased in order to prevent
convective instability. Even though convective instability was suppressed
roll-like structures still appeared in the modeling results, although their
height and circulation velocity were smaller than in the control run. It was
found that these structures were caused by advection and stretching of vertical
velocity field inhomogeneities (i.e. small areas of strong updrafts or
downdrafts).
