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Meteorology of clouds and climate

The science of climate and climate change involves study of the interaction between clouds, atmospheric radiation and aerosol particles. Researchers at the department are investigating how the environment controls cloud glaciation, precipitation, lightning and other cloud properties. Improved ways of treating clouds in global climate models that predict climate change are being developed.

Ice Initiation by Fragmentation in Contrasting Cloud-Types

In a recent project, laboratory observations of a new type of fragmentation involving freezing raindrops were done, enabling its representation in atmospheric models. By simulating observed cloud systems the role of this and other types of fragmentation in nature by numerical simulation of observed clouds were evaluated. The four broad types of cloud to be studied are warm- and cold-based convective and stratiform clouds. The way that cloud types organise the mechanism of ice initiation was elucidated.

Warm and Cold Processes of Precipitation

There are two processes known for production of precipitation:

  • Coalescence of cloud-droplets to form rain (‘warm rain process’), which may freeze at subzero levels or may fall out as ‘warm’ precipitation
  • growth of ice crystals to form snow (‘ice crystal process’), which may rime to form graupel/hail or may fall out as ‘cold’ precipitation.  

The problem is that in mixed-phase clouds (supercooled liquid and ice) in principal both mechanisms can co-exist, and it is unclear which of the two prevail. Research at this department has been investigating the balance between both precipitation mechanisms, with theoratical and numerical modeling approaches. We have predicted the separate contributions of precipitation from the two processes in our existing high-resolution model of clouds. We have discovered that the ice crystal process produces most of the precipitation at middle latitudes, but that the warm rain process prevails in the tropics.

Effects from Bioaerosols on Clouds

Clouds consist of vast numbers of cloud droplets and ice crystals suspended in the air. Each cloud droplet and each initial ice crystal is initiated by an aerosol particle. The cloud properties are controlled by the aerosol conditions of the environment.
Primary biological aerosol particle (PBAPs) have a distinctive role because they initiate both cloud droplets and, at remarkably warm sub-zero temperatures, ice crystals. There has been a hypothesis that emissions of PBAPs can influence precipitation on the local scale.

In a recent project, to investigate such effects, the separate contributions of ice nucleation by five broad types of PBAPs were represented in an atmospheric model of clouds such as bacteria and fungal particles. This was achieved by a field trip to the Amazon rainforest where aerosol particles were sampled. Their ice nucleation was observed in laboratories in France, Germany and Sweden.

Next-Generation Modeling of Cloud Feedbacks and Climate Change using AI

The source of most of the uncertainty in prediction of climate change is the treatment of clouds in global models. Clouds control the climate sensitivity to any forcing, such as anthropogenic emissions of greenhouse gases.

A new technology for prediction of climate change is being pioneered at INES. The coarse representations of clouds in a global model are replaced by our detailed cloud scheme accelerated by AI, using deep learning (convolutional neural networks).    

A motivation is for cloud-radiation feedbacks, which dominate the climate sensitivity to greenhouse gas emissions, to be assessed more reliably. Implications for long-term policy decisions about renewable energy sources in Sweden are to be assessed.

Effects from Ice Initiation on Clouds and Climate

Concentrations of ice determine the abundances of hydrometeors of various types and precipitation production in cold clouds.  This in turn affects the Earth’s radiation budget and hence the climate, since the reflection of sunlight to space is largely by clouds.  The net absorption of solar and longwave radiation by the climate system controls climate and climate change. Cloud properties and extent, and cloud phase (ice-only, mixed-phase, or liquid-only)may be somehow altered by anthropogenic changes in loadings of ice-nucleating aerosols, such as carbonaceous pollution.

A current project at INES is improving how cloud glaciation is represented in atmospheric models. With laboratory experiments at Lund, the hitherto overlooked time-dependence of the activity of IN aerosols in models of clouds and climate. This enables its impact on atmospheric radiation, temperatures and precipitation to be assessed.

The rationale of the project is 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 evaluated.

Effects from Bioaerosols on Layer-clouds and Climate with AI

Mixed-phase layer-clouds can exist entirely at such warm subzero temperatures where biological ice nucleation can prevail. They can generate their precipitation either by the ice phase or by coalescence of cloud-droplets, as quantified in another past project. The ice crystal process of precipitation production involves INPs initiating crystals that then grow to become snow, which may either rime to form graupel or may melt as rain.   Any precipitation removes condensate from the cloud, determining the cloud lifetime and microphysical properties.  Thus, there is reason to expect that mixed-phase clouds can be sensitive in their properties to loadings of bioaerosols in the environment.

A current project at INES is evaluating how the radiative and microphysical properties and extent of layer-clouds over mountains in USA can respond to fluctuations in loadings of these five types of PBAPs. The approach will be first to boost the realism of the model by performing field observations of breakup in graupel-snow collisions, a key alternative pathway for ice initiation. Next, an observed case of layer-clouds will be simulated, to study how initiation of ice by PBAPs controls precipitation and cloud extent, potentially also affecting the regional climate. 

Environmental Influences on the Lightning of Warm-based Deep Convection

Lightning arises from the separation of charge in ice-ice collisions in the presence of supercooled cloud-liquid.  The extent and polarity of charging is governed by the sizes of colliding particles, and by the mass content of supercooled cloud-liquid aloft in any thunderstorm. Thus, the frequency and properties of lightning are determined by the microphysical regime of the thunderstorm and by environmental conditions of instability and aerosols.

A current project at INES is to accelerate the intricate computations of lightning propagation, enabling the cloud/electrification model to simulate warm-based thunderstorms. How the environment controls the frequency and properties of lightning in the tropics will be elucidated.