Despite longstanding recognition of the importance of ice nucleation and multiplication processes in clouds, both for precipitation processes as well as for the radiative impact of clouds on global climate, our understanding of heterogeneous ice nucleation mechanisms and the relevance of ice multiplication processes remains limited. The gaps in our understanding of ice formation processes hinder parameterization of ice initiation processes in numerical simulations of clouds, thus leading to uncertainties in cloud radiative properties and precipitation simulated in numerical models.
The Ice in Clouds Experiments (ICE) are a series of field campaigns investigating ice nucleation in clouds in a range of different conditions, taking advantage of improvements in observational capabilities including improved in situ probes and airborne Doppler radar and lidar. ICE-Tropical (ICE-T), will investigate ice formation in shallow, convective clouds over the eastern Caribbean.
Intellectual merits. The Wyoming Cloud Radar (WCR), together with upward- and downward-looking versions of the Wyoming Cloud Lidar (WCL) will be flown on the NSF (National Science Foundation)/NCAR (National Center for Atmospheric Research) C-130 aircraft during ICE-T and will be used to observe ice occurrence within cloud. This research will help understanding the relationship between cloud-dynamics and microphysical processes. Lidar backscatter and depolarization measurements will be used in conjunction with the PCASP (Passive Cavity Aerosol Spectrometer Probe) and FSSP-300 (Forward Scattering Spectrometer Probe) measurements to identify aerosol distributions and dust layers. The effect of dust on ice formation in clouds will be examined by contrasting ice formation and development in clouds under dusty vs. non-dusty conditions.
The dual-Doppler capability of the WCR will be used to investigate how ice is transported inside cloud, and explore the link between cloud dynamics and ice initiation/multiplication. The combination of WCR and WCL will allow the reflectivity and velocity fields observed by the radar to be unambiguously linked to the boundaries of the mixed-phase region.
Broader impacts. Radar, lidar, and in situ aerosol data collected during this investigation will facilitate analyses conducted by other ICE-T investigators. A graduate student will participate in the ICE-T field campaign and data analysis phase. Data dissemination and other broader outreach efforts will also be carried out through various phases of this project. Finally, an improved understanding of ice nucleation and multiplication mechanisms in the atmosphere will lead to improved parameterizations of ice processes in numerical models, in turn lead to a better cloud simulations in global climate models.
Our understanding of heterogeneous ice nucleation mechanisms and the relevance of ice multiplication processes in convective clouds remains limited. The gaps in our understanding of ice formation processes lead to greater uncertainties in cloud radiative properties and precipitation simulated in numerical models. The Ice in Clouds Experiments (ICE)-Tropical (ICE-T) investigated ice formation in shallow, convective clouds over the eastern Caribbean. Upward- and downward-looking Wyoming Cloud Lidar (WCL) were flown on the NSF/NCAR C-130 during ICE-T together with the Wyoming Cloud Radar (WCR, added polarimetric measurements capability in one beam) for the first time to document aerosol and cloud vertical structures, identify where ice occurs within cloud, and investigate its relation to cloud-dynamics and microphysical processes. The PCASP and FSSP-300 measurements were used to measure coarse mode aerosol (0.1 μ m to 10 μ m diameter), to better link aerosols with ice formation. The related datasets were generated and archived for others to use. Other than providing above critical observations, we combined remote sensing and in situ measurements during ICE-T to perform in-depth analyses to better understand ice generations in tropical convective clouds. Aerosol measurements from the PCASP and FSSP 300 were used together with other in situ measurements from ICE-T and other recent NSF-supported campaigns to explore the aerosol-ice nuclei-cloud nexus. A new approach was developed to use individual particle images collected by 2D-C and 2D-P probes onboard the C-130 to discriminate ice and liquid particles, which opens a new avenue for studying ice generation in convective clouds. For the first time, we can present the evolution of large liquid drops and ice crystal size distributions in convective clouds. Our ICE-T researches already contributed to 5 published journal papers with several more in the pipeline. Our key findings are: 1) FSSP300 measurements show that significant free-tropospheric large aerosols exist in the ICE-T measurements region even in the absence of dust layers. 2) WCR measurements provide unambiguous evidence of the occurrence of precipitation-sized liquid drops, which upon freezing can substitute for graupel in the Hallett-Mossop process. 3) In situ data analyses show that the rapid glaciation of tropical convective clouds sampled during ICE-T is mainly initiated by small particles. 4) High primary ice production at temperatures around -5 °C is observed during the ICE-T, which could be a necessary condition for the rapid glaciation of tropical convective clouds. 5) Biological ice nuclei are the likely candidate responsible for the observed high primary ice generation at temperatures warmer than -8°C.
