Global importance of secondary ice production
Submitter:
Liu, Xiaohong — Texas A&M University
Area of research:
Cloud-Aerosol-Precipitation Interactions
Journal Reference:
Science
Measured ice crystal number concentrations are often orders of magnitude higher than the number concentrations of ice nucleating particles (INPs), indicating the existence of secondary ice production (SIP) in clouds. However, the SIP is poorly represented in global climate models (GCMs) and its impacts not quantified. In this study, we present the first study to quantify the global importance of SIP using a GCM (CESM2). We represent several SIP mechanisms (i.e., frozen raindrop shattering, ice–ice collisional breakup, and rime splintering) in CESM2.
Impact
SIP decreases the global annual mean liquid water path (LWP) by −14.6 g m−2 (−22%), increases the ice water path (IWP) by 8.7 g m−2 (23%), improving the model agreement with observations. SIP changes the global annual mean shortwave, longwave, and net cloud forcing by 2.1, −1.0, and 1.1 W m−2, respectively, highlighting the importance of SIP on cloud properties on the global scale.
Summary
SIP dominates the ice formation in moderately cold clouds over the Southern Ocean and in the middle levels of the tropics, where the ice multiplication reaches up to three orders of magnitude at temperatures warmer than −10°C. SIP decreases the LWP by 40–60 g m−2 at mid-to-high latitudes of both hemispheres, and increases the IWP at all latitudes. Cloud phase changes induced by SIP may have important implications for model cloud sensitivity via the cloud phase feedbacks.
Annual zonal-mean distributions of (a) liquid water path over ocean, (b) ice water path, (c) shortwave cloud forcing, and (d) longwave cloud forcing from the CESM2 model without SIP (CTL, orange) and with SIP (SIP_CNT, green), and observations (OBS, gray dashed lines).