中文摘要：

中间层顶部是大气层温度最低的区域，过饱和水汽凝结成冰晶（尘埃）颗粒，进而吸附电子形成非均匀密度分布层，造成雷达波的强烈回波．对此现象起因的研究曾引起众多探讨但尚无定论．这里以外大气层下落的微陨石颗粒和以中层底部随气流上升的烟尘颗粒为凝结核，根据尘埃颗粒与水汽分子的碰撞吸附效应建立颗粒的生长模型，研究变质量的颗粒在重力和中性气流拖曳力作用下的运动和生长过程．结果表明，颗粒半径随时间线性增长，颗粒的运动速度与半径之间存在复杂的关系，对于半径在一定范围的初始凝结核，颗粒在运动过程中可以反弹，即速度方向逆转。根据通量守恒计算了尘埃颗粒的粒子数密度随高度的分布，结果表明，由于初始凝结核大小具有一定的分布以及存在反弹效应，边界层附近可形成陡峭的密度分布（小尺度密度结构）．该结构有利于形成强的雷达回波．

英文摘要：

Mesopause is the region of the lowest temperature in the atmosphere, where supersaturated water vapor condenses into ice （dust） particulates that absorb electrons and result in the non-uniform density layer, causing strong echoes of radar signals. This phenomenon has sparkled a great number of studies but is still not fully understood. Based on the effect of eollisional attachment of water vapor on the surface of the dust grains, a model of dust growth was developed, in which the grains of micrometeoroid showering from the outer atmosphere and the grains ascending with the upward neutral wind from the bottom of the mesosphere serve as the nucleation cores. The motion of the ice grains under the gravity and the neutral drag forces was studied. It is shown that the radius of the ice grains increases linearly with time. The velocity of the grains exhibits a complicated relation with the radius. For a certain range of the radius of the initial condensation core, the grain bounces at a certain height with its velocity reversing in direction. Based on the flux conservation, the variation of the number density of the grains with respect to the height is numerically calculated. The results show that a steep density distribution （i. e. , a small-scale density structure） appears near the boundary layer because of the bounce effect as well as the effect of the finite spread of the radius of the initial condensation core. The so-formed small scale structure is a possible explanation of the formation of strong radar echoes.