Ice in the coldest clouds on Earth
Lead supervisor: Ben Murray; Co-supervisors: Liane Benning, John Plane
Enquiries: B.J.Murray(at)Leeds.ac.uk
Figure 1. Mesospheric clouds. When observed by eye they are often referred to as noctilucent clouds. The clouds are at ~83 km and illuminated by the sun which is below the horizon. The lower atmosphere is in shadow. Picture taken by Ben Murray near Stockholm, July 2009.
Overview
The high latitude summer mesopause region at around 82-90 km is the coldest natural environment on Earth. Temperatures have been recorded on occasions below 100 K and are persistently below the ice frost point of 150 K. Despite very dry conditions, the temperatures are sufficiently low that clouds can form. These clouds are visible to a ground level observer during twilight (see Figure 1) when the lower atmosphere is in shadow; and space based observations show they are they are present during the whole summer season in both hemispheres.
It has been suggested that mesospheric clouds are a useful indicator of a changing climate and atmosphere since mesospheric temperatures and water vapour are both very sensitive to anthropogenic emission. For example, half of the water vapour in this region originates from the oxidation of methane and methane emissions have increased by more than a factor of two since pre-industrial levels. Noctilucent clouds were first reported by experienced atmospheric observers in the summer of 1886 and it seems that they simply did not exist in the preceding decades. Since then their brightness and occurrence frequency has increased (see Figure 2).
In recent years there has been increasing interest in interpreting changes in these clouds (e.g. Thomas et al, EOS, 2010), but in order to do this we need a thorough understanding of the mechanism through which they form. At present there is significant controversy over the mechanism through which the cloud particles form and also how they evolve and change with time. For example, recent work (Murray, JASTP, 2010) suggests amorphous ice (solid condensed water which is non-crystalline – see link) should form in the mesosphere, but satellite data (Hervig, JGR, 2010) suggests that the ice is dominantly crystalline. The phase of ice is important, because amorphous ice has a larger vapour pressure than crystalline ice and the transformation of one phase to the other in the mesosphere could potentially lead to substantial changes in cloud properties.
Proposed research
The aim of the student’s project will be to quantify the transformation of amorphous ice to crystalline ice under the extreme conditions of the high latitude summer mesopause and also determine the structure of the crystalline ice which forms.
The student will use a newly developed cold and humidity controlled stage which is coupled to an X-ray diffractometer. This stage has been developed by Dr Tamsin Malkin (post-doc in Ben Murray’s group) for work on the phase of ice which forms under tropospheric conditions, but is also capable of mesospheric temperatures. We have recently made the discovery that a well established form of low temperature crystalline ice does not have the crystal structure that it was thought to have for the past 70 years. This metastable crystalline ice is thought to form throughout the Earth’s atmosphere including in the mesosphere and was thought to have a cubic crystal structure, but we showed it in fact has a stacking disordered structure. This work has received favourable reviews at the Proceedings of the National Academy of Science of the United States.
Figure 2. Satellite data for mesospheric cloud frequency. The 11 year cycle is linked to the solar cycle, but the general trend is for increasing frequency of PMC occurrence, especially at high latitudes. Reproduced from Shettle et al. GRL, 2009
In light of this new discovery, we need to reassess the structure of ice which is likely to form in the summer mesosphere. This will involve making amorphous ice via deposition of water vapour onto a cold surface and then monitoring the diffraction pattern over time in order to quantify the rate of conversion. We will use the computer model developed by Dr Malkin to quantify the structure of the material which forms and gain insight into the transformation mechanisms. In the literature it is stated that amorphous ice coexists with cubic ice, our recent results suggest this interpretation is incorrect and the student will examine this in detail.
The student will also examine the transformation of the metastable stacking disordered ice to stable hexagonal ice under mesospheric conditions. Optical measurements of mesospheric clouds suggest strongly aspherical ice particles, which is consistent with hexagonal ice. However, the limited existing data suggests that hexagonal ice should not form.
This data will provide the underpinning understanding for improving microphysical models of mesospheric clouds. The existing CARMA mesospheric cloud model used by Murray and Jensen (JASTP, 2010) will be used to explore the impact of the student’s findings in collaboration with NASA (Eric Jensen) and the University of Colorado (Chuck Bardeen) which would involve travelling to the United States.
Year 1: Become an expert in X-ray diffraction and use of the cold stage. Make amorphous ice and quantify the temperature dependent rate of transition to crystalline ice.
Year 2: Use a model already developed in order to quantify the structure of ice and gain insight to the mechanism of phase transformation. Examine the metastable (stacking disordered ice) to stable hexagonal phase transition.
Year 3: Work with Eric Jensen and Chuck Bardeen with the CARMA model – implement the phase transformation rates to quantify the potential amorphous to crystalline (Bergeron-Findeisen like) transformation in mesospheric clouds.
Year 4 (half year): Write up thesis and research papers where appropriate.
For more information on Ben Murray’s research go to his web pages.