The relation between atmospheric composition and fronts

Application deadline: 3 February 2017

Introduction:

The concept of an atmospheric front as a boundary between two air masses is an old one in meteorology, but has been largely superseded by the modern picture of a front as a transition zone between airstreams that move vertically as well as horizontally. One would expect therefore that a gradual change would be seen in the concentration of trace gases as one traversed a front. This is not what we observe: sharp gradients in constituents such as ozone and carbon monoxide are often observed near fronts, as shown in the figure which was compiled from measurements made on a research aircraft around a warm front. This suggests that small-scale mixing, which acts to smooth out gradients, is either absent or does not have enough time to act as the airstreams come together. The problem is relevant to atmospheric chemistry – there can be chemical consequences of mixing two air masses – and to atmospheric dynamics, especially in the mixing of stratospheric air into the troposphere in the vicinity of fronts.

A related question concerns the very sharp vertical gradients in humidity found in descending airstreams, which show up in VHF radar profiles as multiple frontal bands (Lawson et al 2013).

Project Summary:

This project will explore the creation and maintenance of sharp and multiple gradients of chemical species (including water vapour) near fronts. To start with, the project will examine data from the FAAM aircraft in the vicinity of fronts, exploiting a dataset which has not been fully examined for this aspect. It is likely that this will lead to opportunities to gather further data with FAAM during future field campaigns.

Alongside the experimental data, the project will use the WRF (Weather Research and Forecasting) model for simulations of fronts, to determine whether the model is able to reproduce the observed structure and to use its parameterisations to calculate the degree of mixing near fronts. Thus, the student will gain experience with analysis of field data and numerical modelling, as well as tackling an interesting problem in atmospheric physics.

References:

S. Bethan, G. Vaughan, C. Gerbig, A. Volz-Thomas, H. Richer and D. A. Tiddeman. Chemical air mass differences near fronts. J. Geophys. Res., 103, 13413-434, 1998

J. Lawson, D. M. Schultz, G. Vaughan and D. Kirshbaum. Multiple bands near fronts in VHF wind-profiling radar and radiosonde data. Atmos. Sci. Lett., DOI: 10.1002/asl2.4316 May 2013

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