International
Joint Project to Study Effects from Ice Initiation on Clouds and Climate
Project leader: Dr V. Phillips, INES, University of Lund, Lund, Sweden
Co-Investigator: Thomas Bjerring Kristensen, Nuclear Physics Division, Department of Physics, University of Lund, Lund, Sweden
Co-Investigator: Paul Miller, INES, University of Lund, Lund, Sweden
Co-Investigator: Trude Storelvmo, Department of Geosciences, Oslo University, Oslo, Norway
Co-Investigator: Donifan Barahona, Goddard Space Flight Center, NASA, Maryland, USA
Overview
A project entitled “Influence from Ice Nucleus (IN) Aerosols on Mixed-Phase Clouds and Climate: Lab Observations and Modeling” led by Lund University was funded in 2018 by the Swedish Research Council for Sustainable Development (‘FORMAS’; award number 2018-01795). It will last 4 years until 2022.
This joint project involves several institutions in USA, Norway and Sweden:
· Goddard Space Flight Center, NASA, in USA
· Nuclear Physics Division, Department of Physics, Lund University, in Sweden;
· Department of Geosciences, Oslo University, in Norway;
· Department of Physical Geography and Ecosystem Science, Lund University, in Sweden.
The aim is, first, to perform lab observations of the time-dependence of ice initiation by ice nucleus (IN) aerosols. Then, by applying this lab data for improved treatment of cloud glaciation, there will be investigation of the effect from environmental IN aerosols on cloud properties and lifetime, the Earth’s radiation budget and climate. The approach is to apply different types of models, one spanning a single thunderstorm and another covering the globe.
Concentrations of ice are influential for the climate partly because they determine the abundances of hydrometeors of various types and precipitation production in cold clouds. This in turn affects the Earth’s radiation budget, since the reflection of sunlight to space is largely by clouds. The net absorption of solar and longwave radiation by the climate system controls the mean surface temperature of the globe. Changes in cloud properties and extent may arise from anthropogenic changes in loadings of ice-nucleating aerosols, such as carbonaceous pollution from combustion.
In particular, changes in ice concentrations from IN aerosols may alter cloud phase, which can either by liquid-only, ice-only or a mixture of both liquid and ice. This has repercussions for the reflectiveness of clouds and hence for the Earth’s radiation budget governing the climate. The ice from IN aerosols may also drive precipitation by the ice phase in some cloud-types, such as those with cold bases.
Consequently, the project seeks to improve the way that cloud glaciation is represented in atmospheric models. Focus is given to including the hitherto overlooked time-dependence of the activity of IN aerosols in models of clouds and climate, so as to assess its impact on atmospheric radiation, temperatures and precipitation.
Finally, it is ineluctable that heterogeneous ice nucleation by IN aerosols is accompanied by other processes such as homogeneous freezing and secondary ice production. They all need to be simulated adequately if the true role of time-dependent IN aerosols is to be discerned.
Lab
experimentation
During the first half of the project, time-dependence of IN activity was observed with the Lund University Cold Stage (LUCS) at the Physics Department. Aerosol material was sampled from the real atmosphere. Then its freezing was observed by means of an array of drops in which it was immersed.
LUCS with 100 sample drops on the 40 x 40 mm stage; each drop confined to a sealed cell. Shown are drops that are still in the liquid phase and reflect the light from the circularly polarised light source (1), drops that are just undergoing freezing (2) and those fully frozen (3). From Jakobsson et al. (2022).
An automated control of the temperature of the ‘cold stage’, on which the drops rest in a ‘honeycomb’-like structure of 100 cells, enabled the freezing to be observed over long times of many hours. Frequent images like the one above and image analysis software allowed the freezing to be automatically computed.
The observations have now been published (Jacobsson et al. 2022) and the raw data is now accessible here. This publication provides a method for representing the observed time-dependence in a cloud model.
Cloud
modelling
There are many mechanisms for initiation of ice in clouds. Our aerosol-cloud model (AC) at Lund simulates each of the known and empirically quantified mechanisms. These involve the activity of aerosols (‘primary ice’) and the initiation of new ice particles by pre-existing ice precipitation (‘secondary ice production’ [SIP]).
Budget of sources of ice particles from a simulation by our AC model for a cold-based convective storm (US High Plains) influenced by breakup in ice-ice collisions (‘Breakup’), not including homogeneous freezing. Dark red is the source from ice-nucleating aerosols (IN), (‘heterogeneous ice nucleation’). From a paper by Phillips and colleagues in 2017.
AC predicts the microphysical, dynamical and radiative properties of clouds. AC represents about 11 chemical species of aerosol: several types of primary biological aerosols, non-biological insoluble organics, soluble organics, sea-salt, ammonium sulphate, mineral dust and soot.
To include time-dependence of heterogeneous ice nucleation, the lab results noted above have now been incorporated into the treatment of ice initiation by AC. Analysis of validated simulations by AC of the observed cloud cases reveals the impact from this time-dependence.
Essentially, simulations reveal that little overall impact arises from inclusion of time-dependence. This is partly due to the ubiquity of SIP, which dominates total concentrations of ice in the mixed-phase region (0 to -36 oC). In a layer-cloud case with less ice multiplication than the deep convective case over Southern England (‘APPRAISE’) in the winter of 2009, persistence of surface precipitation from growth of ice crystals is explicable in terms of mixing of solid aerosol material from the environment continually into the cloud. Time-dependence of heterogeneous ice nucleation is not needed to explain this persistence.
Bar chart in (a) showing a comparison of the budget of number of ice crystals initiated from primary and SIP processes between the control and ‘no time-dependent INP’ run for the APPRAISE case. Also shown is the source of homogeneously nucleated ice (“HOM”), total ice from all ice initiation processes (“TOTAL-ICE”), heterogeneous ice nucleation at temperatures warmer than -30 oC (‘PRIM-WARM’) and colder than -30 oC (PRIM-COLD) and various SIP mechanisms active. These are fragmentation during raindrop freezing (‘Raindrop freezing frag’), ice-ice collisions (‘Frag ice-ice collisions’) and sublimation (‘Sublimation frag’), and the HM process (‘Hallett-Mossop’). The same information is shown (b) with a pie bar chart (excluding “HOM” and “TOTAL-ICE”).
A measure of the relative activity of SIP compared to IN aerosols is given by the ice enhancement (IE) ratio, which is the ratio of the numbers of ice particles from SIP relative to IN activity. Simulations with AC showed that the classic historical plot of IE ratio as a function of cloud-top temperature from Hobbs and colleagues in their 1980 paper could be reproduced for the same region and cloud-types by AC. Tagging tracers reveal the contributions from the various SIP processes in this plot at different cloud-top temperatures. It was shown that the H-M process prevails in young ascending turrets but that breakup in ice-ice collisions generally overtakes it as convective cells grow older and approach maturity. More details are given by Waman et al. (2022).
Predicted IE ratios for total non-homogeneous ice (squares) concentration and that observed by Hobbs and colleagues in 1980 (right-pointing triangles) as a function of cloud top temperature. Also shown are the contributions to the prediction from sublimation breakup (asterisks), fragmentation during raindrop freezing (upward-pointing triangles), the Hallett-Mossop (open circles), and fragmentation in ice-ice collision (pentagrams). From Waman et al. (2022).
Advances in climate modeling
The community atmospheric model (CAM) from NCAR in USA has hitherto had a simple treatment of cloud microphysics in convective clouds, omitting dependencies of hydrometeor initiation on aerosol loadings and chemistry. This has arisen because in conventional global climate models, convective clouds are too narrow (a few km wide) to resolve explicitly and so they must be somehow treated statistically.
Conventional climate models must treat convective clouds statistically since they are too small spatially to resolve on the global numerical grid. They are usually treated by one or more representative ‘bulk’ plumes in each grid-box. From a paper in 2014 by Sherwood and colleagues.
An approach taken in the project is to embed a 1D detailed model in the statistical ‘convection scheme’ of the global model, applying elements of AC to include the linkage between aerosol properties and cloud microphysical properties in each convective plume simulated. This has now been completed using an observed storm case (MC3E) as a test-bed for the model development. Almost all the processes represented in AC for ice initiation, in terms of its dependencies on ascent, temperature and aerosol conditions, are represented. Time-dependence of heterogeneous ice nucleation is treated, along with three SIP mechanisms, in the convection scheme.
Aircraft
measurements
In the project, observed cases of clouds from three field campaigns were used to validate the AC model and cloud component of the global climate model. Aircraft flew through clouds measuring ice and drop concentrations while radar and ground-based measurements were made in both campaigns:
· MC3E (funded by DoE/NASA): Warmer-based clouds in precipitating deep convective
systems were sampled by aircraft during April 2011 over Oklahoma;
· 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);
· APPRAISE (funded by UK’s NERC): a thin supercooled layer-cloud that was long-lived, observed by aircraft and radar over Southern England.
Current
progress in project
The project was funded in late 2018. Two PhD students and two postdoctoral scholars have been hired.
Lab
observations:
The lab experiment was completed by one of the postdoctoral scholars (2019-2021) in the summer of 2021. A paper describing results was published at a journal (Jakobsson et al. 2022). These lab results have informed the improvement to our cloud model so as to represent the time-dependence of ice initiation.
Cloud
modeling:
One of the PhD students has performed a modeling study of fragmentation of ice, as a prelude to the study of the time-dependence of IN activity. Understanding the dependencies of the multiple mechanisms of ice initiation and establishing realism in their representation allows the role of IN aerosols in simulations to be assessed reliably. A paper was published at a journal, showing accurate simulation of the ice concentrations and other properties of a storm observed in the above campaign, MC3E (Waman et al. 2022).
Development
of global climate model:
Another PhD student has incorporated a version of the cloud-microphysics representation of AC into the convective cloud scheme of a global climate model. This has been done by running the climate model in a ‘single column mode’, where only one location is represented. This location has been the MC3E storm case for comparison with coincident observations. A paper is in preparation for a peer-reviewed journal. The second postdoctoral scholar is now extending this treatment to the large-scale cloud scheme of the global model.
Summary:
Having implemented the lab data of time-dependent IN activity into their respective cloud and climate models, both PhD students have performed sensitivity tests to quantify the extent of influence from its inclusion in atmospheric simulations. Papers been published describing this representation in the context of SIP, which depends partly on cloud-top temperature. A paper is now in review at a peer-reviewed journal about the impacts from time-dependence of heterogeneous ice nucleation on cloud systems for contrasting cloud types.
Additionally, various other papers have been published by Phillips in peer-reviewed journals on various topics of cloud microphysics. Each paper includes acknowledgment of support by FORMAS.
Bibliography
Jakobsson, J., Waman, D., Phillips, V. T. J., and T. Bjerring-Kristensson, 2022: Time-dependence of heterogeneous ice nucleation by ambient aerosols: laboratory observations and a formulation for models. Atmos. Chem. Phys., 22, 6717–6748
Waman, D., Patade, S., Jadav, A., Deshmukh, A., Phillips, V. T. J., Gupta, A. K., and A. Bansemer, 2022: Dependencies of four mechanisms of secondary ice production on cloud top temperature in a continental convective storm. J. Atmos. Sci., 79, 3375–3404