1. Most Rain is created when tiny water droplets in clouds become ice crystals and bind together. This makes them heavy enough to fall from the sky. To become ice, the water
droplets need a solid particle to crystallize around or initiate freezing; often a piece of dust. This process is called ice nucleation. Without the presence of these microscopic dust
particles or Ice Nuclei (IN), as they are known, the super-cooled (i.e. below 0˚C) water must reach ~-40˚C before it will freeze. This seems counterintuitive as we all know water
freezes below 0˚C. This is because the water we encounter in ‘everyday situations’ is full of crystalline impurities which instigate the freezing, whereas, clouds at high altitude contain
super-cooled water vapor and droplets without impurities which will be affected by the presence of IN. It is, therefore, important to study and understand the transfer of IN such as
dust, sea salt and smoke into clouds and the characteristics of the IN such as their activation properties.
Development and Testing of Airborne Atmospheric
Research Instrumentation
Alec Hammond
Research and Development Summer Placement at The Met Office
University of Kent (4th
year MPhys Student)
Introduction
What are Ice Nuclei?
Acknowledgements
My Role
What is the INC?
During my summer placement, I worked closely within a team, in particular,
focusing on the Ice Nucleus Counter (INC) which is being developed to further
understand the nature the enigmatic processes of Ice Nucleation. It has the
capacity to take airborne samples and re-create Ice Nucleation conditions
within the column (see Fig. 1). Here's how it works:
As a key part of the INC, my main task was to design and help build the Evaporator section of the INC . Each stage is broken down below.
 Main ‘science’ happens within the column chamber. This consists
of two co-centric cylinders of different diameter, one inside the
other, leaving a thin annular gap or ‘chamber’. The chamber is
super-cooled before being pumped and then drained of water
very quickly to produce a thin layer of ice on the inside walls.
 The refrigerator system controls the temperatures of the inside
and outside walls and therefore can produce a water vapour
pressure gradient between the two iced walls.
 The airflow system creates a ‘sheath’ of dry air into which the
sample air enters and descends the length of the column.
 An evaporation section at the bottom of the chamber allows
water droplets that have formed in the chamber to be removed so
as not to affect the reliability of results.
 Filters at the inlet only allow in a certain particle size range and
concentration that can then be monitored.
Identifying IN:
Step1: Sample air enters the chamber through an inlet on the outside
of the research aircraft. The inlet only allows in particles of a certain
size range and of known sample concentration. The particles are
candidates for IN.
Step 2: Conditions are set in the chamber by
adjusting the temperature of the two surface walls.
The water vapour pressure gradient between the two
walls means that naturally the water vapour, sourced
from the iced walls, will flow from the higher pressure
region to the lower pressure region (one wall to the
other).
Step 3: As the sample air is injected and travels
through the chamber, particles of the correct
crystalline structure will form ice crystals, increasing
significantly in size, while the others will pass straight
through unchanged.
Step 4: Large particle filters separate successful IN
from other particles at the chamber exit and so by
calculating the difference in the number of particles
that entered the chamber and the number that were
left unchanged upon exiting the chamber a
representation of IN concentrations within samples
may be gained.
Fig.1: Column and airflow components of the INC
Foremost, I would like to express my sincere gratitude to each and every one of the OBR team at
the Met Office, especially Joss Kent and Clare Lee, for their friendship and guidance throughout my
placement, as well as, Richard Cotton for making my role possible. I must also extend my thanks to
SEPnet for providing the funding and opportunity, without which my placement would not have been
possible. Figure 1 is credited to Richard Cotton.
This involved learning and using Computer-Aided-
Design (CAD) software and creating a true scale
model of the evaporator system from a circuit
diagram adhering to spatial and weight restrictions.
Problem Solving skills were essential to ensuring
that the design was as simple and efficient as
possible.
Design Build Test
After many design stages and the final
construction had been decided, it was possible
to have each piece made up and sent off to be
anodized. Some of the less specialised
components were made up in the lab as part of
my role with help and training from Andy Wilson
which was a great hands-on experience.
Before being built, it was possible to create
prototypes of some of the brackets and supports
using the departments 3D printer which created
parts from a durable polymer. This could be
done directly from the CAD software.
Due to the large scale and on-going work on
the INC I was unable to take part in the ‘test‘
stage of my particular piece of work. However, I
was involved in COPE – the (CO)nvective
(P)recipitation (E)xperiment, where I shadowed
the OBR team and learnt how equipment is
tested in the field and the data is analysed and
discussed post flight.
The INC aims to be functioning properly by
2015 where it’s first objective will be to measure
the effect of Saharan dust storms on the transfer
of IN to clouds and the effects of IN on cumulus
cloud evolution.
Fig. 2: Design Stage of Evaporator Section