The UK Ice in Clouds Experiment -- Dust (UK ICE-D)
The UK ICE-D project was funded by the Natural Environement Research Council (NERC) with the grant references: NE/M00340X/1 and NE/M001954/1. These were led by Professor Alan Blyth (University of Leeds) and Professor Thomas William Choularton (The University of Manchester)
The goal of this research was to determine how desert dust affects the nucleation of ice particles in convective and layer clouds and the subsequent development of precipitation and glaciation of the clouds. Dust is believed to be a critical aerosol particle in the Earth system mainly because the dust particles themselves, and particles that are chemically and possibly biologically modified as they are transported from their source, are believed to be the most important ice nuclei in a global sense and because dust particles are transported to many parts of the globe. Predicting the initiation and subsequent evolution of the size distribution of ice particles in clouds from a distribution of aerosol particles is one of the most important problems in atmospheric science. It is fundamental to the NERC high-level strategy objective ``Understand and predict how our planet works'', because the lack of understanding of the processes causes uncertainty in the way global models treat the interaction of radiation with ice and mixed-phase clouds and the development of precipitation. They also cause uncertainty in Numerical Weather Prediction (NWP) models, which is concerned with the NERC strategy objective ``Resilience to environmental hazards''. The proposed research aims was to tackle this problem by making measurements of aerosols and cloud particles close to one of the largest sources of desert dust in the world. The measurements are difficult to make, which is why such detailed measurements have never been made in this region before. It is possible to do this now because the instruments are capable of determining the chemical and physical properties of aerosol particles, the aircraft cloud physics instruments can detect small ice particles, and there is a mobile dual-polarisation radar.
The UK Ice in Clouds Experiment -- Dust (UK ICE-D) was part of the US-UK aircraft and ground-based project based in Cape Verde off the coast of Senegal, Africa to be held in 2015 (mainly UK) and 2016 (mainly US). Measurements will be made in the environment around the clouds to characterise the aerosol particles and their ability to act as ice nuclei and cloud condensation nuclei, and within the clouds to determine the influence of the particles on the cloud properties. Convective clouds will be measured as a priority, but layer clouds will also be targeted. Observations will be made when dust is present in high concentrations at appropriate altitudes and when almost no dust is present. The availability of the US and UK aircraft has inadvertently provided a unique opportunity to maximise the sampling statistics of the clouds. The location and time was selected from climatology studies because dust concentrations are often large and convective and layer clouds also occur frequently. In addition, the convective clouds in the region are known to be important since they can form clusters that lead to storms and hurricanes in the Tropical Atlantic.
Specifically, UK ICE-D made measurements on days with and without the presence of dust of the following:
* Aerosol particles on the ground with the instruments in the aerosol container at Cape Verde and with the BAe 146 aircraft;
* Cloud droplets, supercooled raindrops, the first ice particles and development of ice and precipitation particles with the aircraft;
* The altitudes of the supercooled raindrops, the location and time of the first precipitation echoes, and the radial air motions using the radar;
* The thermodynamics and dynamics of the clouds and their environment with the aircraft and to some extent the radar.
Model results were compared with the observations of the initiation temperatures and rates of growth and development. A spectrum of models ranging from climate through regional forecast models to explicit cloud physics process-based models, will be used as forecasting tools and as tools to interpret the data and to develop or
1. Characterise the chemical and physical properties of aerosol particles and determine the activation properties of CCN and IN.
2. Given an initial well-characterised aerosol distribution, can we predict the number concentration of ice particles that will be produced in the clouds through primary nucleation? Specifically, determine if primary ice particles first form as a result of freezing of supercooled raindrops.
3. Determine the physical processes responsible for the production of warm rain and the rates of growth. E.g. is the process dependent on entrainment and mixing or determined by straightforward autoconversion or by giant CCN.
4. Determine whether the HM secondary ice formation process is critical to the glaciation of the convective clouds and if so (likely) the effect of dust on the process.
5. Determine the influence of dust on the physical properties of the convective and layer clouds (e.g.\ dynamics, entrainment, liquid water content distribution, development of precipitation) and if the effects can be represented with models of all scales.
6. Use the new observations to test and improve the ability of regional NWP, global NWP and climate models to accurately simulate the properties of clouds and their environment. In particular, determine if the new prognostic treatment of aerosol through its effect on ice nucleation and cloud droplet number outperforms simpler non-prognostic treatments.
|Keywords:||desert dust, ice nucleation, clouds, aerosol, cloud condensation nuclei|
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