  The multiscale flow structure in the solar convection zone - the coexistence
of such features as the granules, mesogranules, supergranules and giant cells -
has not yet been properly understood. Here, the possible role of one physical
factor - variations in the thermal diffusivity - in the formation of a
multiscale convection structure is investigated. Thermal convection in a plane
horizontal fluid layer is numerically simulated. The temperature dependence of
thermal diffusivity is chosen so as to produce a sharp kink in the static
temperature profile near the upper layer boundary. As a result, the magnitude
of the (negative) static temperature gradient dTs/dz, being small over the most
part of the layer thickness, reaches large values in a thin boundary sublayer.
To identify the structures on different scales, we apply a smoothing procedure,
computational-homology techniques and spectral processing to the temperature
field. The flow is found to be a superposition of three cellular structures
with three different characteristic scales. The largest (first-scale)
convection cells, with central upflows, fill the whole layer thickness; most of
these cells are divided by bridges, or isthmuses, into a few smaller
(second-scale) ones, which are localised in the upper portion of the layer;
finally, there are numerous tiny (third-scale) features that dot the horizontal
sections of the layer located near its upper boundary and exhibit a tendency of
gathering in the intercellular lanes of the first-scale cells. The third-scale
cellular structures are advected by the first-scale and second-scale convective
flows. The spatial spectrum of the flow does not directly indicate the presence
of the second-scale and third-scale structures; however, they can be selected
by using our processing techniques. On the whole, the simulated flow pattern
qualitatively resembles that observed on the Sun.
