云凝结核浓度对不同弗罗德数下形成的地形云和降水的影响

Effects of CCN concentration on orographic clouds and precipitation formed with different Froude numbers

中文关键词:  云凝结核(CCN)  地形云和降水  焚风效应

英文关键词:Cloud condensation nuclei (CCNs)  Orographic clouds and precipitation  Foehn effect

中文摘要:

      云凝结核（CCN）对云和降水的影响除与其物理化学性质密切相关外，还受到气象条件的影响，但此类研究较少。文中基于WRF中尺度数值模式，引入了表征大气层流速、层结稳定度和地形关系的湿弗罗德（F_w）数，研究揭示了CCN浓度的变化对不同F_w下形成的地形云和降水的影响。研究表明，当F_w≤1，接近临界流时，地形阻挡起主要作用，地形抬升和重力波作用主要发生在迎风坡一侧，主要形成层状云和向上游传播的浅对流波状云，降水主要发生在靠近山顶的迎风坡一侧。在此种情况下，CCN浓度升高对地形云和降水影响较小，当CCN浓度由100 cm^-3增至1000 cm^-3时，云滴含水量增大，但雨水含量减小，说明云粒子向降水粒子的转化效率降低，CCN浓度升高抑制了暖雨过程。但在云发展后期，云滴被上升气流带至高层形成过冷云滴，与雪粒子发生碰并形成霰粒子，使冰相物理过程有所增强。CCN浓度升高可导致20 h累积降水量减少10—15 mm，约减小7%—8%；当F_w>1时，CCN浓度升高会导致20 h地形云累积降水量减小超过50%，最大达到96%，导致地形云几乎不产生降水，而且降水量峰值位置向山顶后移动5—10 km。研究表明，降水显著减小的原因不仅与CCN浓度升高有关，过山气流产生的背风坡焚风效应也起了非常重要的作用。由于CCN浓度升高形成了大量云滴粒子，使雨滴形成效率显著降低，不能形成降雨的大量云滴被强过山气流快速带至下游背风坡区，由于背风坡下坡气流的绝热加热形成的焚风效应很显著，导致云滴和雨滴快速蒸发，使降水显著减小。这一结果可以解释在落基山脉、以色列及中国华山发现的地形降水减小30%—50%的现象，说明气象环境条件在气溶胶影响降水中起重要作用，污染气溶胶与背风坡焚风效应产生的叠加效应可造成地形云降水显著减小。

英文摘要:

      The effect of cloud condensation nuclei (CCNs) on clouds and precipitation is not only determined by the physical and chemical properties of CCNs, but also strongly influenced by atmospheric conditions such as the airflow speed and stability as well as orographic properties. Investigations of the later are relatively few. We conducted a series of numerical experiments using WRF model with an ideal bell-shaped topography. The wet Froude number (F_w) was also introduced to represent the relationship among airflow speed, stability and mountain height. Effects of CCN concentration variation on orographic clouds and precipitation formation under different F_w were investigated. The results show that when the wet Froude number F_w ≤ 1, which indicates that the airflow is near-critical flow and the terrain block plays a major role, the orographic dynamic lifting and mountain waves primarily occur over the windward side of the mountain, and the dominant orographic clouds are stratiform and shallow convective clouds traveling upstream. Precipitation is primarily produced in the upstream region near the crest of the mountain and the effect of CCN concentration on precipitation is relatively small under this condition. When the CCN number concentration increases from 100 cm^-3 to 1000 cm^-3, cloud water content increases but rainwater content decreases, indicating that the conversion rate from cloud droplets to rain drops decreases. Thereby, the increase in CCNs primarily suppresses the warm rain process. However, during the late stage of cloud development, the cloud droplets can be lifted to higher levels and become supercooled droplets that collide with snow particles and form graupel particles, and thus increase ice process in clouds. The overall effect of the increased CCN number concentration on rainfall is relatively small, the accumulation rainfall decreases by about 10-15 mm, which accounts for about 7%-8% of the total precipitation. When F_w> 1, the orographic clouds induced by the dynamic lifting form primarily at the terrain crest, and mountain waves form mainly over the leeward side and propagate toward the region downstream of the crest, producing quasi-stable shallow convective wave clouds in the downstream region. The increase in CCN number concentration induces decreases in the accumulative rainfall by more than 50% in 20 h, and the maximum decrease can even reach 96%, leading to almost no rainfall by orographic clouds. Moreover, the area of rainfall peak shifts downward by around 5-10 km. This study found that the prominent decrease in precipitation is not only associated with the increase in CCN concentration, but also closely related to the foehn effect formed at the leeward of the mountain. Due to the increase in CCN concentration, a large amount of small cloud droplets form, which suppresses the warm rain process in clouds. These small droplets are brought to the leeside of the mountain by strong mountain airflow and evaporate quickly due to the foehn effect induced by descending adiabatic warming, leading to rapid decrease in precipitation. The above results can be used to explain the obvious reduction in orographic precipitation by 30%-50% at mountainous areas of the Rocky Mountains, Israel, and Mt. Hua near Xi'an in central China. This study indicates that the meteorological condition can play an important role in precipitation suppression induced by aerosols. The effects caused by pollutions and the foehn could prominently reduce orographic precipitation.