Effects Of Terrain Forcing, Gravity Wave And Aerosols On Orographic Clouds And Precipitation

Orographic precipitation produced by orographic clouds is worldwide concerned and studied since it is the main resource of many important glacier and river, and closely related to agricultural production and human life.The formation of orographic clouds depends not only on airflow speed, air stratification, terrain height, water vapor etc, but on CCN (IN) number concentration. The human-made emissions to atmosphere rapidly increase with the increase of human activities. Whether the increase of aerosols could induce the change of formation, evolution, and precipitation distribution of orographic clouds is directly associated with resource of glacier and rivers.The relationship between terrain dynamic forcing, mountain wave, CCN number concentration and the orographic clouds and precipitation and formation mechanism were investigated in a conditionally unstable moisture atmosphere on May1,2009by using WRF model and wet Froude(Fw) number. The results show that under condition of topographic height of2km, half width of10km, when unstable airflow gradually increases from2.5to25m/s, the wet Froude number increases from0.19to1.81. The terrain block have a major role when Fw≤1, the orographic clouds induced by terrain dynamic lifting form at windward side, and mountain wave is mainly formed over windward side and propagate toward upstream region, which forms stratiform clouds at first and then evolves as quasi-stable shallow convective wave clouds. The maximum precipitation occurs at terrain crest near windward side. When Fw is too small, no precipitation is produced. When Fw>1, the orographic clouds induced by terrain dynamic lifting primarily form at terrain crest, and mountain wave is mainly formed over leeward side and propagate toward downstream region of terrain crest, and produces quasi-stable shallow convective wave clouds in downstream region. In addition, in weak and conditionally unstable moisture atmosphere, orographic precipitation is mainly produced by warm microphysical process induced by terrain lifting and mountain wave clouds cannot form precipitation.Orographic clouds in this study are also found to be quite sensitive to cloud condensation nuclei (CCN) number concentration (NCCN).Two cases of CCN number concentration are used, one is Nccn=100/cm3, representing clean case, and another is Nccn=1000/cm3, pollution case. The results show that under conditions of different Fw, the impact of high NCCN on orograhpic clouds and precipitation are different. The variation of CCN number concentration has a little effect on the orographic clouds when Fw《1. Under this situation, high CCN number may not cause the increase of the total cloud hydrometeor content. With increase of Fw, the effect of CCN increases. When Fw≤l and NCCN increase from100/cm3to1000/cm3, the cloud droplets increase, and rain drops decrease. In the late stage of clouds, graupel particles also increase. Due to the increase of CCN concentration, the formation of many cloud droplets leads to the low conversion rate to rain particles. This may suppress warm rain process. In the late stage of clouds, the many cloud droplets are transported into high levels and enhance the collision-coalescence process between snow and cloud droplets, and induce the increase of graupel particles. However, when Fw>1, the increase of CCN concentration may decrease both warm rain and cold rain processes.CCN number concentration has important effects on orographic precipitation under larger Fw. The increase of CCN concentration almost has no effects on precipitation when Fw《1. When Fw≤1, the increase CCN concentration from100/cm3to1000/cm3can decrease the accumulated total precipitation about10-15mm in20simulation hours, reducing about7-8%; When Fw>l,CCN concentration increase from100/cm3to1000/cm3, the accumulated total precipitation can decrease more than50%, the maximum value can reach96%in20hours, and can make clouds produce no rain.The increase of CCN also has impacts on cloud dynamics. When Fw≤1, the increase of CCN concentration can enhance airflow and updraft in the leeward region, and when Fw>1, the increase of CCN concentration can suppress the cloud dynamics and airflow.