  The aim of this paper is to contribute to the understanding and to model the
processes controlling the amplitude of the wind of Rayleigh-Benard convection.
We analyze results from direct simulation of an L/H = 4 aspect-ratio domain
with periodic sidewalls at Ra = 1e5; 1e6; 1e7; 1e8 and at Pr = 1 by decomposing
independent realizations into wind and fluctuations. It is shown that deep
inside the thermal boundary layer, horizontal heat-fuxes exceed the average
vertical heat-fux by a factor 3 due to the interaction between the wind and the
mean temperature field. These large horizontal heat-fluxes are responsible for
spatial temperature differences that drive the wind by creating pressure
gradients. The wall fluxes and turbulent mixing in the bulk provide damping.
Using the DNS results to parameterise the unclosed terms, a simple model
capturing the essential processes governing the wind structure is derived. The
model consists of two coupled differential equations for wind velocity and
temperature amplitude. The equations indicate that the formation of a wind
structure is inevitable due to the positive feedback resulting from the
interaction between the wind and temperature field. Furthermore, the wind
velocity is largely determined by the turbulence in the bulk rather than by the
wall-shear stress. The model reproduces the Ra dependence of wind Reynolds
number and temperature amplitude.
