Effects of cloud condensation nuclei and ice nucleating particles on precipitation processes and supercooled liquid in mixed-phase orographic clouds

摘要

How orographic mixed-phase clouds respond to the change incloud condensation nuclei (CCN) and ice nucleating particles (INPs) arehighly uncertain. The main snow production mechanism in warm and coldmixed-phase orographic clouds (referred to as WMOCs and CMOCs, respectively,distinguished here as those having cloud tops warmer and colder than−20 °C) could be very different. We quantify the CCN and INPimpacts on supercooled water content, cloud phases, and precipitation for aWMOC case and a CMOC case, with sensitivity tests using the same CCN and INPconcentrations between the WMOC and CMOC cases. It was found that deposition plays amore important role than riming for forming snow in the CMOC case, while the roleof riming is dominant in the WMOC case. As expected, adding CCN suppressesprecipitation, especially in WMOCs and low INPs. However, this reversesstrongly for CCN of 1000 cm and larger. We found a new mechanismthrough which CCN can invigorate mixed-phase clouds over the Sierra Nevadaand drastically intensify snow precipitation when CCNconcentrations are high (1000 cm or higher). In this situation, morewidespread shallow clouds with a greater amount of cloud water form in theCentral Valley and foothills west of the mountain range. The increased latentheat release associated with the formation of these clouds strengthens thelocal transport of moisture to the windward slope, invigorating mixed-phaseclouds over the mountains, and thereby producing higher amounts of snowprecipitation. Under all CCNconditions, increasing the INPs leads to decreased riming and mixed-phasefraction in the CMOC as a result of liquid-limited conditions, but has the opposite effects in the WMOC as a result of ice-limited conditions. However, precipitation in both cases is increased by increasingINPs due to an increase in deposition for the CMOC but enhanced riming anddeposition in the WMOC. Increasing the INPs dramatically reduces supercooledwater content and increases the cloud glaciation temperature, whileincreasing CCN has the opposite effect with much smaller significance.