Abstract: 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 was built to study the mechanism governing the slumping phase of gravity current. 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 were 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.