  Here, we propose a conceptual framework of Aeolian sediment transport
initiation that includes the role of turbulence. Upon increasing the wind shear
stress $\tau$ above a threshold value $\tau^\prime_t$, particles resting at the
bed surface begin to rock in their pockets because the largest turbulent
fluctuations of the instantaneous wind velocity above its mean value
$\overline{u}$ induce fluid torques that exceed resisting torques. Upon a
slight further increase of $\tau$, rocking turns into a rolling regime (i.e.,
rolling threshold $\tau_t\simeq\tau^\prime_t$) provided that the ratio between
the integral time scale $T_i\propto\delta/\overline{u}$ (where $\delta$ is the
boundary layer thickness) and the time $T_e\propto\sqrt{d/[(1-1/s)g]}$ required
for entrainment (where $d$ is the particle diameter and $s$ the
particle-air-density ratio) is sufficiently large. Rolling then evolves into
mean-wind-sustained saltation transport provided that the mean wind is able to
compensate energy losses from particle-bed rebounds. However, when $T_i/T_e$ is
too small, the threshold ratio scales as
$\tau_t/\tau^\prime_t\propto{T}_e/T_i\propto{s}d^2/\delta^2$, consistent with
experiments. Because $\delta/d$ controls $T_i/T_e$ and the relative amplitude
of turbulent wind velocity fluctuations, we qualitatively predict that Aeolian
sediment transport in natural atmospheres can be initiated under weaker
(potentially much weaker) winds than in wind tunnels, consistent with indirect
observational evidence on Earth and Mars.
