Water isotopologues as tracers of mixed-phase cloud processes at the Pi Chamber laboratory

Solar radiation modification (SRM) techniques such as mixed-phase cloud thinning (MCT), cirrus cloud thinning (CCT), and stratospheric aerosol injection (SAI) all include introducing foreign material into the troposphere and stratosphere, and can have unexpected effects on clouds spatially removed from the intended targets. It has been shown that mixed-phase clouds affect precipitation patterns over land, which in turn has tangible effects on agriculture. Mixed-phase clouds are generated at the Pi Chamber facility at Michigan Technological University by setting a temperature difference between the top and bottom plate of the chamber to create a convective cell, and then injecting aerosols or ice nucleating particles (INP). Using our own water isotope instruments and the cloud probes at the chamber, we will study the isotopic response of water vapor and condensate (H2O, HDO, H218O) in these mixed-phase clouds at varying glaciation fractions. We aim to better understand the growth and evolution of mixed-phase clouds in the presence of SRM material, in particular: the glaciation altitude of mixed-phase clouds, how processed SAI aerosols transported to the poles might affect mixed-phase clouds in that region, and the secondary effects of sedimenting particles used in CCT on mixed-phase clouds. We will also vary the amount and type of INP to study at what point precipitation might become suppressed. Ice deposition growth via the Wegner-Bergeron-Findeisen (WBF) process, which encourages the rapid growth and then sedimentation of ice crystals at the expense of liquid droplets, produces a strong isotopic signal that we will use to probe cloud microphysics and provide constraints on models. Early results from a bin-resolved microphysics model (BRIMM) show that our instruments are sensitive enough to see the expected isotopic signature from the WBF process. While only small modifications are needed to our previously field-tested flight and lab spectrometers to allow integration into the chamber (ChiWIS-airborne used in StratoClim and ACCLIP, and ChiWIS-lab used in IsoCloud), a method to best discriminate particles from vapor, without disturbing the cloud to be studied, is required. We explain design constraints and present early engineering test results.