Study of the subgrid scale parameterization of dust emissions caused by gentle local terrain under the neutrally stratified atmosphere

Dust emissions caused by local terrain have an important effect on forecast precision of dust model.Based on the existing research results of narrow-pipe effect and downslide effect as well as the speed-up equation of wind flowing over hills the subgrid-scale parameterizations of surface wind velocities associated with both narrow-pipe effect and downslide effect were firstly studied on conditions of neutral stratification and local gentle terrain, then the subgrid-scale-parameterization of dust emissions were also proposed based on equations for flat terrain in which friction velocity of flat terrain was replaced with that of complex one.At last three sets of ideal experiments were applied to test the narrow-pipe effect, downslide effect and superposition of both effects.The experiments show that as to narrow-pipe effect,the velocity in the narrow pipe saltation flux and the emissions will increase (decrease) when wind flows in from the wide (narrow) entrance and flow out from the narrow(wide) exit of the pipe.While as to the downslide effect,the velocity saltation flux and the emissions increase, too and the increasing amounts at hill foot are bigger than those at hillside.As far as the superposition of both effects is concerned,when wind flows in from the wide end and flow out from the narrow end,the velocity saltation flux and the emissions also increase with the velocity at hill foot bigger than that at hillside and the saltation flux associated with big-sized grains of sand bigger than those associated with small-sized grains.On the contrary, when wind flows in from the narrow end and flow out from the wide end,there exists a threshold value for the ratio of exits width to entrance's.When beyond it,the velocity decreases otherwise the velocity increases.Furthermore,the saltation flux will decrease except for the case in which there arc small-sized grains Locatcd on hill foot.