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

Abstract

Abstract. How orographic mixed-phase clouds respond to the change in 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 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 INP impacts on supercooled water content, cloud phases, and precipitation for a WMOC case and a CMOC case, with sensitivity tests using the same CCN and INP concentrations between the WMOC and CMOC cases. It was found that deposition plays a more important role than riming for forming snow in the CMOC case, while the role of riming is dominant in the WMOC case. As expected, adding CCN suppresses precipitation, especially in WMOCs and low INPs. However, this reverses strongly for CCN of 1000 cm −3 and larger. We found a new mechanism through which CCN can invigorate mixed-phase clouds over the Sierra Nevada and drastically intensify snow precipitation when CCN concentrations are high (1000 cm −3 or higher). In this situation, more widespread shallow clouds with a greater amount of cloud water form in the Central Valley and foothills west of the mountain range. The increased latent heat release associated with the formation of these clouds strengthens the local transport of moisture to the windward slope, invigorating mixed-phase clouds over the mountains, and thereby producing higher amounts of snow precipitation. Under all CCN conditions, increasing the INPs leads to decreased riming and mixed-phase fraction 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 increasing INPs due to an increase in deposition for the CMOC but enhanced riming and deposition in the WMOC. Increasing the INPs dramatically reduces supercooled water content and increases the cloud glaciation temperature, while increasing CCN has the opposite effect with much smaller significance.

Partitioning the primary ice formation modes in large eddy simulations of mixed-phase clouds

“…Ansmann et al (2009) observed altocumulus clouds which almost always had liquid water at cloud top, suggesting deposition nucleation plays little role. This has been supported by observations in cases of lee-wave clouds (Field et al, 2012) and stratiform clouds (De Boer et al, 2011;Westbrook and Illingworth, 2011), suggesting either immersion or contact freezing dominates ice production.…”

Aerosols and Their Impact on Radiation, Clouds, Precipitation, and Severe Weather Events

“…Note that IN form a very small fraction of aerosol particles, and their concentrations are typically over five orders of magnitude less than CCN concentrations. Studies have shown that the IN effect on supercooled water and cloud phase is much more significant than the CCN effect (Fan, Leung, Rosenfeld, & DeMott, 2017).…”

“…If the cloud base is high, increasing CCN often suppresses precipitation because smaller droplets reduce riming, which is especially the case for stratiform mixed-phase clouds (Fan et al, 2012A). For orographic mixed-phase clouds, increasing CCN generally suppresses precipitation (Jirak & Cotton, 2006;Rosenfeld & Givati, 2006;Lynn, Khain, Rosenfeld, & Woodley, 2007), but the reverse could happen under extremely polluted conditions due to an invigoration mechanism for orographic mixed-phase clouds (Fan et al, 2017). Other studies show that CCN may not have a significant effect on the total precipitation; rather, they shift precipitation from the windward to leeward slope, a so-called spillover effect Saleeby, Cotton, & Fuller, 2011;Saleeby, Cotton, Lowenthal, & Messina, 2013 For DCC with low cloud bases and weak wind shear, precipitation is intensified by increasing aerosols due to the aerosol invigoration effect (Rosenfeld et al, 2008;Fan et al, 2009Li et al, 2011;Yang & Li, 2014).…”

Characteristics of Ice Nucleating Particles in and Around California Winter Storms

“…Events like these, both the multiyear drought and subsequent flooding, highlight the importance of understanding the controlling mechanisms for precipitation in this region and improving our ability to forecast changes to these mechanisms. While the availability of condensable water vapor is crucial to the intense rain and snow events associated with ARs (Ralph et al, 2013), the role of aerosol particles, especially those capable of initiating ice crystal formation, is less well understood and has been the focus of recent study (Ault et al, 2011;Creamean et al, 2013;Creamean et al, 2015;Fan et al, 2014;Fan et al, 2017;Martin et al, 2019;Rosenfeld et al, 2013).…”