Atmospheric Boundary Layer Research


Atmosphere-Ice Exchange Processes

1. Resumé

The surface of the Antarctic continent is characterised by net radiative cooling throughout much of the year. This cooling is largely balanced by the turbulent transport of heat from the atmosphere towards the surface, generating a persistent surface temperature inversion over the ice sheets. Over the ice slopes, the surface inversion drives the katabatic drainage circulation which dominates the low-level wind field over the continent and exerts important controls over the hemispheric circulation.

A detailed understanding of fluxes of energy and water vapour at the atmosphere-ice boundary is required for modelling the behaviour of the ice sheets and the atmosphere. This project aims to improve our knowledge of the spatial and temporal variation of such fluxes, to develop parameterisation schemes for use in large-scale models and to investigate the mesoscale atmospheric circulations which result from the observed surface cooling. These circulations do not simply respond to surface forcing but are, in themselves, an important mechanism for transferring energy from the atmosphere to the surface. A new development in this project will be studies aimed at looking at the vertical transport of energy and momentum within the lowest kilometre of the atmosphere as well as in the near-surface layer.

In this project, we shall attempt to improve our understanding of exchange processes at the atmosphere-ice boundary through a number of field projects and model studies. The field projects will build on the boundary layer dynamics group's proven ability to make high-quality micrometeorological measurements under Antarctic conditions.

2. Scientific background

The stable atmospheric boundary layer over the Antarctic ice sheets has much in common with the nocturnal boundary layer observed in mid latitudes (e.g. Nieuwstadt, 1984), but there are also significant differences (King, 1993) which result from its climatological setting. Observations of the Antarctic boundary layer are limited and, in recent years, the Survey's STABLE and STABLE-II experiments at Halley (King, 1990; King and Anderson, 1994) have contributed greatly to the information available. We now have a basic understanding of the dynamics of the boundary layer over a relatively simple site - a flat ice shelf.

Even at this simple site, however, there are phenomena which we do not fully understand yet. The STABLE results demonstrated that it was not always possible to make accurate estimates of surface fluxes from data in the surface layer alone and that information on the full boundary layer structure was required. Profile measurements during STABLE were limited to those obtained from a 32 m mast; in future work these need to be extended to at least 100 m using in situ or remote sensing techniques. Scalar roughness lengths for temperature and humidity calculated from the STABLE-II data are surprisingly large. Accurate estimates of these quantities are required to parameterize surface fluxes of heat and water vapour in GCMs. It is thus essential to understand why such large values were obtained and whether it is appropriate to use these values in parameterisations.

Measurements of the climatological values of surface fluxes have been made at a very limited number of Antarctic stations and it is not yet possible to produce a reliable picture of their spatial distribution. Recent studies using AWSs (Stearns and Weidner, 1993) have demonstrated that very large local spatial variations exist and these will need to be accounted for when producing areal averages. Errors in surface temperatures predicted by GCMs are likely to result from incorrect simulation of the surface energy budget (Connolley and Cattle, 1994) and it is essential to have accurate, Antarctic-wide, surface flux climatologies available for investigating shortcomings in model performance. We shall contribute to this goal by embarking on a programme of surface flux measurements at remote sites, using the experience gained during the STABLE experiments to make these measurements.

Persistent cooling of the sloping surfaces which characterise much of the Antarctic ice sheets drives the well-known katabatic circulation. This results in the outflow from the continent of cold air at low levels and the generation of a low-level easterly vortex over the ice sheets. It has recently been recognised (Egger, 1985; James, 1989) that this katabatic flow exerts strong controls over the Southern Hemisphere broad scale atmospheric circulation. It also plays a major role in maintaining the thermal balance of the Antarctic atmosphere (Dalu et al, 1993) and is important in the formation of coastal polynyas (e.g. Kurtz and Bromwich, 1985). The continental scale katabatic flow pattern has been simulated with some success using rather simple models (Parish and Bromwich, 1987). However, it is clear that such models do not always produce adequate regional predictions because boundary-layer processes are only crudely parameterised. In particular, attention needs to be given to the rate at which overlying air is entrained into the cold katabatic layer (Manins and Sawford, 1979). We shall address some of these outstanding questions through a programme of mesoscale measurements in the Coats Land region. This will complement the mesoscale modelling studies being carried out in collaboration with Queen Mary and Westfield College.

3. Research and methodology

We intend to undertake the following investigations during the review period:

Surface flux measurements at Halley.

We will build on the work started during STABLE and STABLE-II by initiating a long-term programme of surface flux measurements at Halley. The aim of this work will be to determine definitive seasonal average values of heat and water vapour fluxes, to study interannual variability of the fluxes and to conduct further process studies motivated by the results of STABLE-II - e.g. the effects of radiative flux divergence on surface-layer profiles. The DAISY and SNTHERM models will be used to analyse the data and investigate the effect of different mathematical representations of the various physical processes. These fundamental studies will assist in the development of improved surface flux parameterisations for use in models at all scales including GCMs.

Surface flux measurements at "remote" sites.

Very little information exists on the spatial variation of surface fluxes over the Antarctic ice sheets. We propose to equip remote platforms such as AWSs and Upper Atmospheric Science Division's Automatic Geophysical Observatories (AGOs) with suitable sensors for making such measurements, building on experience gained during the first quinquennium. This study is closely linked to the mesoscale flow investigations described below.

Blowing snow processes.

Studies initiated in collaboration with Dr S D Mobbs, University of Leeds, during STABLE-II have pointed to the potential importance of blowing snow as a mechanism for evaporation and redistribution of snowfall. We intend to continue this fruitful collaboration. Further work will mainly involve modelling and analysis of data already collected. However, there will be a possibility to carry out minor observational projects, possibly linked to chemistry studies under ICD7.

The structure of the atmospheric boundary layer at Halley.

STABLE and STABLE-II revealed much about the structure of the lowest few metres of the boundary layer at Halley. In order to understand how the boundary layer controls mesoscale flows, it will be necessary to extend our measurements to greater heights. In the first instance, we intend to develop and apply the techniques required to do this (SODAR, tethersondes, etc.) at Halley before moving on to other locations. This will enable us to exploit the regular radiosonde measurements from Halley to the full. Our collaboration with Dr J.M. Rees' group (University of Sheffield) on the effects of gravity waves in the Antarctic boundary layer will continue under this activity.

Mesoscale flow patterns in Coats Land.

Coats Land is typical of the coastal regions of East Antarctica and offers an ideal location to study the development of katabatic flows originating on the high plateau, accelerating down the steep coastal slopes and then weakening as they flow out over the surrounding ice shelves. Such flows are responsible for maintaining perennial coastal polynyas and thus play a central role in the formation and drift of sea ice. In 95/96, we intend to set up a chain of AWSs from Halley stretching inland to AGO-A77 in order to study these flows. Further measurements of the vertical structure will be made during the summer season. Additional information on the flow structure will be inferred from surface temperature patterns revealed by AVHRR IR imagery from ARIES.

Modelling mesoscale flows.

The interpretation of mesoscale field experiments can be greatly aided by a programme of well-planned model experiments. We have already embarked on a programme of such experiments in collaboration with Professor B. Atkinson (Queen Mary and Westfield College) and we would see this collaboration continuing. Further collaborations may be sought with the University of Reading (JCMM) and the Hadley Centre (limited area climate modelling - see ICD1). A mesoscale snow model will be developed and tested using data from the Coats Land experiment.

Surface fluxes over sea ice.

The growth and decay of sea ice is driven by energy fluxes at the atmosphere-ocean interface. Very little is known about the values of these fluxes and how they vary in space and time. The problem is central to Antarctic climatology and it is important that the group should be involved. However, our lack of experience in ship-borne measurements makes it desirable to approach this work as a collaborative venture. We have already investigated the possibility of collaboration with UMIST. The long lead time involved in obtaining dedicated ship time and in securing funding for a university partner means that this project is unlikely to become "active" until toward the end of the review period. Coastal polynyas are known to be important regions for sea-ice production. Predominantly offshore winds maintain an area of open water and the large flux of energy from the ocean to the atmosphere is balanced by latent heat released as sea-ice forms. In association with project ICD5, we intend to study the energy and sea-ice budgets of the large polynya which forms at the Ronne Ice Front. This study will involve the use of meteorological data from coastal AWSs and satellite-derived SST and ice extent measurements obtained from ARIES.

4. Wider implications

These studies will improve our understanding of the coupling between the atmospheric, cryospheric and oceanic elements of the Antarctic climate system. They will assist in the formulation of improved parameterisations for surface fluxes and will thus contribute directly to the improvement of GCM predictions of Antarctic and global climate.

5. References

Connolley, W.M. and H. Cattle. 1994. The Antarctic climate of the UKMO Unified Model. Ant. Sci. 6, 115-122.

Dalu, G.A., M. Baldi, M.D. Moran, C. Nardone and L. Sbano. 1993. Climatic atmospheric outflow at the rim of the Antarctic continent. J. Geophys. Res. 98, 12,955-12,960.

Egger, J. 1985. Slope winds and the axisymmetric circulation over Antarctica. J. Atmos. Sci. 42, 1859-1867.

James, I.N. 1989. The Antarctic drainage flow: implications for hemispheric flow on the southern hemisphere. Ant. Sci. 1, 279-290.

King, J.C. 1990. Some measurements of turbulence over an antarctic ice shelf. Q. J. R. Meteorol. Soc. 116, 379-400.

King, J.C. and P.S. Anderson. 1994. Heat and water vapour fluxes and scalar roughness lengths over an Antarctic ice shelf. Bound. Lay. Meteorol. 69, 101-121.

Kurtz, D.D. and D.H. Bromwich. 1985. A recurring, atmospherically forced polynya in Terra Nova Bay. In Oceanology of the Antarctic continental shelf. Ed. Jacobs, S.S. American Geophysical Union, Washington, 177-201. (Antarctic Research Series, 43).

Manins, P.C. and B.L. Sawford. 1979. A model of katabatic winds. J. Atmos. Sci. 36, 619-630.

Nieuwstadt, F.T.M. 1984. The turbulent structure of the stable,nocturnal boundary layer. J. Atmos. Sci. 41, 2202-2216.

Parish, T.R. and D.H. Bromwich. 1987. The surface wind field over the Antarctic ice sheets. Nature 328, 51-54.

Stearns, C.R. and G.A. Weidner. 1993. Sensible and latent heat flux estimates in Antarctica. In Antarctic Meteorology and Climatology: studies based on automatic weather stations. Eds. Bromwich, D.H. and C.R. Stearns. American Geophysical Union, Washington, 109-138. (Antarctic Research Series, 61)