UK ICE-D

The goal of this research is 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 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) is 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 will make 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 will be 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 improve parametrizations.

Who might benefit from this research?

1. International academic community in the field of atmospheric science and
land/ocean/atmosphere Earth-system science. In particular scientists
involved in aerosols, cloud physics, aerosol-cloud interactions,
mesoscale and synoptic-scale weather systems, hurricanes and global
climate models. These scientists will benefit because of the knowledge
gained on the nucleation of ice particles in clouds and in particular
the role of Saharan desert dust on ice nucleation.

2. The Public Sector, Business and the General Public
will all benefit from the improved forecasts of high-impact weather and
the reduction in uncertainty in global climate model predictions as
explained below.

3. The main beneficiary of knowledge arising from this research will be
the Met Office. The ultimate benefits mentioned below will come via the
improved MO forecasts.

How might they benefit from this research?

The lack of understanding of the fundamental processes
in clouds causes uncertainty in global climate models. That in turn
leads to political indecision which potentially has catastrophic
consequences with immeasurable economic and societal impact on a global
scale. Governments need to potentially make detailed plans for increased
flooding, stronger wind storms, severe drought, changes in farming
practices, etc. This can only be done by providing better forecasts on a
global scale on decadal or longer time scales. At the same time,
improvements are also urgently needed for forecasts of high-impact
weather events on smaller scales so that local governments, businesses,
and people can make informed decisions. Although uncertainty in the
models is undoubtedly being reduced as processes are understood and
quantified, models are improved and computing capabilities are
increasing, there remains considerable uncertainty in cloud processes.
The nucleation of ice and the chain of physical processes that follow
are among the most important.

1. There is significant uncertainty in prediction by global climate
models of temperature change and also the amount and distribution of
precipitation throughout the globe especially because of the role of
aerosol-cloud interactions. 2. Numerical Weather Predictions (NWP)
models, particularly of convective storms that produce high-impact
weather, such as flash flooding, strong wind gusts, tornadoes, etc, and
extra-tropical weather systems. The ice process is crucial for the
production of precipitation in deep convective clouds, for example. It
can also influence the dynamics of clouds and weather system through
diabatic processes. In addition, convective clouds in the UK ICE-D
project domain off the west coast of Africa can lead to the formation of
the convective clusters that lead to tropical storms and hurricanes in
the Tropical Atlantic. Therefore, lack of understanding of the ice
processes in these clouds in particular influences model predictions of
hurricanes.

The research proposed in UK ICE-D will address the issue of improving
these global climate and regional weather-scale forecasts by
understanding and quantifying the above processes and working with the
Met Office (a major beneficiary) to improve the parameterisation within
the models. The results from the project will also be useful to other
modelling centres. The benefit to governments and society will be
immense by providing longer advanced warnings as mentioned above.

Improved global-scale and regional-scale forecasts of high-impact
weather events on all time scales will provide much greater certainty
which will allow informed decisions to be made as mentioned above.
Improved representation of the ice nucleation process and subsequent ice
cloud evolution in climate models will help to reduce the uncertainty
related to aerosol-cloud interactions recognised in these models.