International Joint Project to Study Ice Initiation by Fragmentation in Contrasting Cloud-Types

Project leader: Dr V. Phillips, University of Lund, Sweden

Co-Investigator: Paul Connolly, University of Manchester, UK

Co-Investigator: Aaron Bansemer, National Center for Atmospheric Research (NCAR), USA

Funding Agency: US Department of Energy

Overview

A project entitled “Organisation of Diverse Mechanisms of Secondary Ice Production among Basic Convective and Stratiform Cloud-types” led by Lund University was funded in Autumn 2018 by US Department of Energy (DoE). The project is about 4 years in duration.

This joint project involves three institutions in USA, UK and Sweden:

· National Center for Atmospheric Research (NCAR) in Boulder, Colorado, USA;

· School of Earth, Atmospheric and Environmental Sciences, University of Manchester, in UK;

· Department of Physical Geography and Ecosystem Science, Lund University, in Sweden.

The aim is to perform lab observations of a new type of fragmentation involving freezing rain and to investigate the role of this and other types of fragmentation in nature by numerical simulation of observed clouds. The four broad types of cloud to be studied are warm- and cold-based convective and stratiform clouds.

Cumulonimbus cloud (upper) and nimbostratus layer-cloud (lower).

Lab experimentation with freezing raindrops

In the project, fragmentation of supercooled raindrops (5 mm), each freezing on impact with a larger ice target (6 mm), was observed in a cold box by a postdoctoral scholar at Manchester University. The box was within a much larger cold chamber, namely the Manchester Ice Cloud Chamber (MICC), in which subzero temperature could be controlled.

Splashing of the drops on impact during retraction of the liquid on the glass surface was observed. Typically about 30% of the splash drops were seen to be frozen between -4 and -12 ^oC in the experiment.

View from above of the drop (5 mm) falling onto the ice target (6 mm) fixed to a horizontal glass surface. Secondary drops are shown in (a)-(c) (red arrows) from the impact. The use of a polarizing filter ((e) to (f)) on the camera and circularly polarized light reveals how some of them freeze (white arrow). From James et al. (2021).

The representation of the freezing probability for this ‘Mode 2’ of raindrop-freezing fragmentation in our cloud model noted below has been revised accordingly. This was the first experimental confirmation of this completely new mechanism of secondary ice production, proposed theoretically by Phillips et al. (2018). More details are given by James et al. (2021).

Field observations of snow

During the project, extra DoE funds enabled a Masters student at Lund University to be supported during summer 2022. The student’s project involved observing fragmentation in graupel-snow collisions in northern Sweden. In view of the inherent difficulties in creating realistic snowflakes in the laboratory, the student constructed a portable probe to be deployed outdoors on the ground during snowfall events at Vindeln near the Arctic circle in February 2022.

Plan of the chamber for observing breakup in collisions between falling snowflakes and graupel/hail particles, approximated by ice spheres (left panel). Also shown is a photo of the chamber during field observations (middle panel) and viewed from above with the lid removed (right panel). From Gautam (2023).

The observations were stratified in terms of collision kinetic energy (CKE) and the theoretical formulation of this type of breakup was re-fitted with the new data. This revealed more fragmentation for dendritic snowflakes than for the previous version of the formulation, which had been based on early published observations.

Numbers of fragments per collision as a function of CKE for days with (a) dendritic and (b) non-dendritic habits, from observations (blue diamonds) and from the new re-fitted formulation (orange stars). Also shown is the original unmodified formulation (green stars).

Since fragmentation is limited by the available energy, exact realism of the ice-ice collisions is not needed, provided that the fragmentation is related somehow to the initial CKE. More details are given in a paper in review currently at a journal (Gautam et al. 2023).

Cloud modelling

The aerosol-cloud model (AC) predicts microphysical and dynamical properties of clouds. AC represents 7 chemical species of aerosol: primary biological aerosols, non-biological insoluble organics, soluble organics, sea-salt, ammonium sulphate, mineral dust and soot. Interstitial and immersed/embedded components of each aerosol species are predicted in cloud and precipitation. The model predicts active IN and cloud condensation nuclei (CCN) concentrations from the chemistry, sizes and loading of aerosols.

In the project the model was upgraded to include the observations noted above of fragmentation of ice in the lab at Manchester, sublimational breakup and breakup in graupel-snow collisions.

Simulations of four observed cases of cloud systems are now being analysed both by tagging tracers and sensitivity tests in the PhD student’s project. This is elucidating hypotheses about how the four represented SIP mechanisms are organized by contrasting cloud-types, and how their relative activities are determined by environmental aerosol and thermodynamic conditions.

Aircraft measurements

Observed cases of clouds shown above have been used to validate the AC model. Aircraft flew through clouds measuring ice and drop concentrations while radar and ground-based measurements. National Center for Atmospheric Research in USA will help to provide the data from past field campaigns:

· MC3E (funded by DoE/NASA): Warmer-based clouds in precipitating deep convective

systems were sampled by aircraft during April 2011 over Oklahoma;

· GO-AMAZON (funded by DoE): Aerosol conditions outside clouds were measured by groundsites

and by aircraft (G-1). Lightning was observed with the Lightning detection Network (LINET);

· ACAPEX (funded by DoE): An ACAPEX case near California will be simulated. Aircraft sampled orographic cloud in the Sierra Nevada mountains in winter 2015, characterizing aerosol conditions (IN/CCN counters).

Progress in Project

The project was funded by DoE in late 2018. The funded phase has now ended, and there is now a no-cost extension of the project until summer 2023.

A PhD student was hired at Lund University in August 2019. One paper was published in 2021 about sublimational breakup (Deshmukh et al. 2021). The student is now in the final year of the PhD course, preparing a two-part paper about the cloud modeling results for the dependencies of SIP mechanisms on cloud-type.

The lab experiments at Manchester were completed in 2020 and described in a published paper (James et al. 2021). Also, a paper is now in review at a peer-reviewed journal describing the field observations of breakup in graupel-snow collisions from a trip to Vindeln in northern Sweden by another student (Gautam et al. 2023), in addition to their Masters dissertation held at Lund University.

During the DoE project, the AC model was updated with the new treatments of the above SIP processes incorporating observations from the project. Related tagging tracers were included to track the corresponding sources of cloud-ice and snow from these processes, in addition to better treatment of lightning. Size-dependence of graupel bulk density was predicted with a separate microphysical model.

In addition, several papers on topics of cloud-microphysics have been published by Phillips with the support of the DoE award, as summarised in scientific technical reports to DoE. One of these was a study of lightning, with a focus on the role of SIP and causes of cloud-to-ground strike polarity (Phillips et al. 2020; Phillips and Patade 2022).

Bibliography

Deshmukh, A., Phillips, V. T. J., Bansemer, A., Patade, S., and D. Waman, 2022: New empirical formulation for the sublimational breakup of graupel and dendritic snow. J. Atmos. Sci., 79, 317-336

Gautam, M., Waman, D., Patade, S., Deshmukh, A., Phillips, V. T. J., Jackowicz-Korczynski, M., Smith, P., and A. Bansemer: “Fragmentation in Graupel-Snow Collisions: New Formulation from Field Observations”. Nature Communications, in review (2023).

James, R. L., Phillips, V. T. J., and P. J. Connolly, 2021: Secondary ice production during the break-up of freezing water drops on impact with ice particles. Atmos. Chem. Phys., 21, 18519–18530

Phillips, V. T. J., S. Patade, J. Gutierrez and A. Bansemer, 2018: Secondary ice production by fragmentation of freezing of drops: formulation and theory. J. Atmos. Sci., 75, 3031-3070

Phillips, V. T. J., Formenton, M., Karlsson, L., Kanawade, V., Sun, J., Barthe, C., Pinty, J.-P., Detwiler, A., Lyu, W., and S. Tessendorf:, 2020: Multiple environmental influences on the lightning of cold-based continental convection. Part I: Description and validation of model. J. Atmos. Sci., 77, 3999–4024

Phillips, V. T. J., and S. Patade, 2022: Multiple environmental influences on the lightning of cold-based continental cumulonimbus clouds. Part II: sensitivity tests for its charge structure and land-ocean contrast. J. Atmos. Sci., 79, 263–300