  According to field observations, the atmospheric boundary layer is usually
unstably stratified before a dust and sandstorm, the particle-laden turbulent
gravity current with an extremely high Reynolds number. In this paper, an
inviscid model is built to study the mechanism governing the slumping phase of
gravity current, and it is shown that the dimensionless current front speed,
the Froude number, decreases when the current fluid or the ambient medium or
both fluids are unstably stratified. In spite of the density interface mixing,
the relation between the front speed and the front height described by the
inviscid model agrees with the numerical simulation results, where the
lock-exchange gravity currents with different initial lock heights are
calculated for different unstable stratification cases. Furthermore, the
velocity increments obtained by field observations at the sandstorm fronts are
satisfactorily consistent with the evaluations of the model, suggesting that
the inviscid mechanism makes contribution to such high Reynolds number
turbulent flows.
