School of Earth and Environment

Climate and Atmospheric Science (ICAS) PhD Projects

Formation of ice particles in convective clouds.

Supervisors: Professor Alan Blyth and Dr Steven Dobbie

The problem of how to explain the formation and growth of ice particles in clouds remains one of the major problems in cloud physics. Ice particles affect all important characteristics of clouds such as precipitation formation, thunderstorm electrification, interaction with radiation and latent heat release. One of the largest uncertainties in climate models stems from not knowing enough about the formation and behaviour of ice particles in clouds. The problem is challenging, not only because of the difficulty of measuring the ice nuclei (IN) and cloud particles, but because of the complex dynamics of clouds. It is difficult to identify ice nuclei that initiate ice crystals in a cloud due to a lack of information regarding their chemical composition and activation spectrum, and the multiple mechanisms by which the nucleation events might occur. Identifying where the first ice originates is difficult observationally, because initial concentrations are low and the small sizes and near spherical shapes of ice crystals makes them difficult to distinguish from water droplets.  In addition, far fewer ice nuclei are observed than ice crystals, and especially so in maritime clouds. Laboratory experiments and observations in clouds suggest that secondary ice production (ice multiplication processes) may act under certain conditions that can sometimes explain the large number of ice crystals. These poorly understood processes of ice initiation and secondary ice production in clouds produce large uncertainties in our ability to predict climate change.

It is expected that the student will use data gathered in the NERC-funded Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) project  which took place mainly in frontal clouds that formed over the UK during the last few years. Some convective clouds were also studied. The overall goal of APPRAISE-Clouds is to determine how ice forms in clouds and in particular how ice formation is determined by the properties of aerosol entering into the cloud. The specific objectives are:

  1. To determine the nucleating ability of specific ice nuclei and the initiation and development of ice in mixed phase clouds.
  2. To determine how aerosol particles control the cloud microphysics, preciptiation and dynamics in mixed phase clouds. The flights were made with the FAAM BAe 146 aircraft and most of them  were along a radial of the Chilbolton radar (photos  below). Aerosol measurements were made in the University of Manchester aerosol container at Chilbolton.  Standard instruments were on board the aircraft to measure  meteorological parameters, such as temperature, air pressure, humidity and wind.  The aircraft was also equipped with state-of-the-art instruments to measure properties of the aerosol and cloud particles; i.e. type, size and concentration.

FAAM BAe aircraft. Note instruments under the wing.  Big radar dish (25-m across) at Chilbolton.

The Leeds group is part of the UK APPRAISE consortium led by Professor Tom Choularton, University of Manchester. There is considerable expertise in Leeds in collaboration with Manchester in making measurements of ice particles in clouds (e.g. in the Ice and Precipitation Initiation in Cumulus project) and in modelling the clouds (e.g. in the Modelling the Modification of Convective Clouds due to Anthropogenic Aerosol project). There is also expertise in laboratory work.

This project will involve analysis and synthesis of data gathered with the aircraft and radars during APPRAISE to address the question of ice formation and development in mixed-phase clouds.  An example of the type of data is shown below, extracted from a paper submitted to ACP  by Crosier et al. 2010.

Total particle mass (left axes, black lines) and number (right axes, blue lines) from the CDP and 2DS as a function of distance/time from CFARR. Data from the 2DS are separated into round and irregular categories. In-situ temperature from the de-iced Rosemount sensor is also shown. Data are from the BAe146 aircraft along the 253oradial during constant altitude run R2. Also shown is the reflectivity from the 3 GHz CAMRa RHI scan (12:19:29 – 12:21:29) along the 253o radial.

A new cloud resolving model with bin microphysics developed by Zhiqiang Cui as part of APPRAISE will be available to use for interpreting the data and helping to understand the
processes of ice initiation and development.

There may be an opportunity to participate in the US project ICE-T to be held in maritime clouds in the Caribbean (see: http://www.eol.ucar.edu/~dcrogers/Ice-Init).  Flights will be made with the NCAR C-130 aircraft through the many clouds that develop in the area. Two flight patterns will be used: a) to fly through many clouds at a variety of levels; and b) to fly through the tops of individual clouds as they ascend. There will be collaboration with Drs Sonia Lasher-Trapp at Purdue University and Andy Heymsfield, NCAR.

There will also be an opportunity to apply to use NCAS flight hours for making new flights in UK convective clouds in collaboration with the Met Office to test new idea emerging from the research.