  We consider a vertical cavity composed of two chambers separated by a
retractable thermally insulated thin membrane. The upper and lower chambers are
filled with an incompressible Boussinesq fluid and maintained at temperatures
$T_2$ and $T_1>T_2$, respectively by two separate heaters. Upon retraction of
the membrane, the two fluid masses form an unstably stratified configuration
with cold and heavy fluid overlying a warmer and lighter fluid and separated by
a non-free interface across which there is a jump in the density. The aim of
this paper is to determine the threshold conditions for convection onset and
associated fluid flow patterns induced by this discontinuous density
stratification. We find that the discontinuity of the density profile leads to
the appearance of temperature perturbation iso-contours that have a lens shape
instead of the classical oval shape and that the mixing is confined to near the
location of the density jump with stagnant and isothermal fluid away from the
discontinuity. We derive the flux conditions at the discontinuity interface and
show that it acts like a heat sink for both the lower and upper fluid layers.
Moreover, we put forth the dependence of the stability threshold parameters for
convection onset on the locus of the density discontinuity. The experimental
set-up suggested in the paper can be used to test the predictions of the
present theory.
