PNNL

Abstract

How orographic mixed-phase clouds respond to the change of cloud condensation nuclei (CCN) and ice nucleating particles (INPs) are highly uncertain. The main snow production mechanism in warm and cold mixed-phase orographic clouds (referred to as WMOC and CMOC, respectively, distinguished here as those having cloud tops warmer and colder than -20°C) could be very different. We quantify the CCN and INP impacts on supercooled water content, cloud phases and precipitation for a WMOC and a CMOC case with a set of sensitivity tests. It is found that deposition plays a more important role than riming for forming snow in the CMOC, while the role of riming is dominant in the WMOC case. As expected, adding CCN suppresses precipitation especially in WMOC and low INP. However, this reverses strongly for CCN > 1000 cm-3. We find a new mechanism through which CCN can invigorate mixed-phase clouds over the Sierra Nevada Mountains and drastically intensify snow precipitation when CCN concentrations are high (1000 cm-3 or higher). In this situation, more widespread shallow clouds with greater amount of cloud water form in the valley and foothills, which changes the local circulation through more latent heat release that transports more moisture to the windward slope, leading to much more invigorated mixed-phase clouds over the mountains that produce higher amounts of snow precipitation. Increasing INPs leads to decreased riming and mixed-phase fraction in the CMOC but opposite in the WMOC, as a result of liquid-limited and ice-limited conditions, respectively. However, it increases precipitation in both cases due to an increase of deposition for the CMOC but enhanced riming and deposition in the WMOC. Increasing INPs dramatically reduces supercooled water content and increases the cloud glaciation temperature, while increasing CCN has the opposite effects with much smaller significance