郭欣,郭学良,付丹红. 2017. 云凝结核浓度对不同Froude数下形成的地形云和降水的影响[J]. 气象学报, ():-, doi:10.11676/qxxb2017.014
云凝结核浓度对不同Froude数下形成的地形云和降水的影响
Effect of CCN concentration on orographic clouds and precipitation formed with different Froude numbers
投稿时间:2016-08-02  最后修改时间:2016-09-21
DOI:10.11676/qxxb2017.014
中文关键词:  云凝结核(CCN)地形云和降水 焚风效应 钟形地形
英文关键词:Cloud condensation nucleus (CCN), Orographic clouds and precipitation, foehn effect, Bell-shaped mountain
基金项目:
作者单位E-mail
郭欣 北京市人工影响天气办公室 503680547@qq.com 
郭学良 中国气象科学研究院 guoxl@mail.iap.ac.cn 
付丹红 中国科学院大气物理研究所 fudanhong@mail.iap.ac.cn 
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中文摘要:
      云凝结核(CCN)对云和降水的影响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。这篇文章基于WRF中尺度数值模式,引入了表征大气层流速度、层结稳定度和地形之间关系的湿Froude( )数,研究揭示了CCN浓度的变化对不同 下形成的地形云和降水的影响。研究表明,当 ≤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浓度增加对地形云和降水影响较小,当CCN浓度由100/cm3增至1000/cm3时,云滴含水量增加,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增加抑制了暖雨过程。但在云发展后期,云滴被上升气流带到高层形成过冷云滴,与雪粒子发生碰并形成霰粒子,使冰相物理过程有所加强。CCN增加可导致20小时累积降水量减少10-15mm,减小约7-8%左右;当 >1时,CCN浓度增加会导致20小时地形云累积降水量减小达到50%以上,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5-10km左右。研究表明,降水显著减小的原因不仅与CCN增加有关,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增加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到下游背风坡区,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 nucleus (CCN) on clouds and precipitation is not only determined by the physical and chemical properties of CCN, but also strongly influenced by atmospheric conditions such as airflow speed and stability as well as orographic properties. The later investigation is relatively few. We conducted a serious of numerical simulation experiments with an ideally bell-shaped topography using WRF model, and also introducing the wet Froude number (Fw) to represent the relationship among airflow speed, stability and mountain height, and investigated the effect of CCN concentration variation on orographic clouds and precipitation formed by different .The results show that when the wet Froude number Fw ≤1, in which the airflow is near critical flow and the terrain block plays a major role, the terrain 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 toward upstream. The precipitation is primarily produced in the upstream region near crest of the mountain and the effect of CCN concentration on precipitation is relatively small under this condition. When CCN number concentration increases from 100 cm-3 to 1000 cm3, cloudwater content increases, and rainwater content decreases, indicating that the conversion rate from cloud droplets to rain drops decreases, thus, the increase of CCN primarily suppresses the warm rain process. However, during the later stage of cloud development, the cloud droplets can be lifted to higher levels and become supercooled droplets which collide with snow particles and form graupel particles, and increase ice process in clouds. The overall effect of the increased CCN number concentration on rainfall is relatively small, the accumulation rainfall decreases about 10-15 mm, lessens about 7-8 %. When Fw> 1, the orographic clouds induced by terrain 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 of CCN number concentration induces the accumulation rainfall in 20 hours to decrease more than 50%, and even reaches 96% in maximum, leading orographic clouds almost not to produce rain. Moreover, the rainfall peak shifts around 5-10 km downward. The investigation found that the prominent decrease of precipitation is not only associated with the increase of CCN concentration, but closely related to the foehn effect formed at the leeward slop. Due to the increase of CCN concentration, a large amount of small cloud droplets form, which suppresses warm rain process in clouds. These small droplets are taken into the leeward slop region by strong mountain airflow and evaporate quickly due to foehn effect induced by descending adiabatic warming, leading to the rapid decrease of precipitation. The results can be used to explain the obvious reduction of orographic precipitation by 30%-50% discovered at mountain areas of the Rocky Mountains, Israel, and Mt. Hua near Xi’an in central China, indicating that the meteorological condition can play an important role in precipitation suppression induced by aerosols. The additive effects caused by pollutions and foehn effect could prominently reduce orographic precipitation.
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