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

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.