Introduction to the Boundary Layer Profiling at Halley:

Acoustic Radar, Kites and Blimps

 

 

Phil Anderson, 1st December, 2001.

 

The Halley monostatic acoustic radar, or Sodar, has operated sporadically at the British Antarctic Survey's Halley Station since 1986. This introduction aims to explain what the Bleeping Bucket is, what the resulting data show, and why we are running this equipment today.

 

 

 Looking north-east from Halley's Simpson building: the Sodar antenna on a snow mound and barchan snow dunes

 

Brief History of Atmospheric Acoustics.

 

The idea of probing the atmosphere with sound has been around for over a century. Before aircraft or balloon borne electronic telemetry, studies of the atmosphere were limited to observations from the ground or very occasional low level profiling using clockwork mechanisms lifted by kites. Remote sensing of the atmosphere, where information can be gleaned from a distance, was essentially a human activity, and indeed for many activities, the human eye and ear acting as detectors, together with the brain as an integrated data processor still provide a benchmark in radiation sensitivity, dynamic range, compactness and energy efficiency (Schwiesow, 1986).

 

Active acoustic remote sensing was first attempted around 1875 by Tyndall using a fog horn and an observer (more exactly a listener) with an ear horn to amplify the return signal. This was an attempt to get echoes from atmospheric features in the horizontal; aside from notes on the technique, I know of no actual results from these studies.

 

 

 

 

 

 

Sound propagation was known to be affected by conditions within the lower atmosphere, and the occurrence of acoustic shadows has been well documented. These are areas around a loud sound source which are silent, despite the source being audible further a field, gun fire or battle being the usual source. The outcome of some of the land battles of the American civil war are believed to have been influenced by such shadows, when signal for flanking engagement failed due to the inaudible sound of gun fire.  Such shadows are mainly due to the combination of wind shear and temperature profile.

 

Little progress was made with atmospheric acoustic radar research until the 1960s. This is slightly surprising, as ASDIC (British) or SONAR (American) acoustic techniques in water advanced remarkably during the 50s and 60s, for submarine tracking. Gilman et al. , in 1946, had measured order of magnitude larger echo return than expected, and was the first to use the term Sodar ; they suggested that turbulent microstructure acting on a varying temperature field was responsible for enhancing the scattering. As Neff (1986) then states, "A hiatus in Sodar development followed, until [] the scattering experiments of Kalistratova (1959)", which lead to an improved calculation of scattering cross section by Monin.

 

Atmospheric acoustic back scattering is generally inaudible, but experience showed that the return signal is easily detectable using the electronic amplification methods and an acoustic dish. The typical 1960s Sodar was a large speaker pointing downwards,  directed at a parabolic dish to focus the transmission beam and amplify rthe return signal. Side lobes to the main beam are a problem, with a significant off-axis signal and the solution then, as now, was to use an acoustic wall to absorb the side lobes. In the 1960s, this was often just a wall of straw bails around the dish. The design was simple and successful, and the Halley Sodar antenna is remarkably similar to these early designs, albeit with a more technical baffle.

 

The very first Sodars detected the strength of the return echo and displayed the record on an oscilliscope; time lapse photographs of the trace then gave a permanent record of backscatter profile. By the 1960's, facsimile recorders, similar to depth sounders on ships, were used, with a time/height record being produced over a few hours. Initial Sodar research was aimed at confirming theoretical scattering theory, which in turn would validated turbulence theory. After the initial flourish of academic interest in the 1960s, the 1970s saw commercial systems being built, with bucket style antennae, whilst the academic community started work on wind profiling, using the Doppler shift in the signal return from a slightly tilted Sodar. This was also very successful, and the resulting commercial Doppler profilers of the 1980s had three bucket style antennae, two at an angle (North and West say) and one pointing vertically. The Halley bucket frame actually has the holes pre-drilled to allow off-vertical antenna operation.

 

From the 1980s, various developments were achieved, including the radio-acoustic sounding systems (RASS), which measures co-located temperature profiles with the wind profiles, smaller portable systems (mini-Sodar) which maintained vertical range whilst increasing resolution, and phased array systems for multiple beam Doppler. Acoustic scattering research per se was relatively moribund: the theoretical groups based in Moscow were having difficulties maintaining research during the collapse of the Soviet Union, the technical groups in the U.S. were busy commercialising RASS, and the European groups were using the new cheap profilers to look at flow down valleys, over hills, next to airports and generally doing applied science.

 

Such was the state of Sodar research at the beginning of the 1980s. Monostatic non-Doppler sounders were well understood, wind profilers were available commercially, and RASS was the hot research topic in atmospheric acoustics.

 

Acoustic Radars come to Halley

 

In 1983,  Dr. John King joined BAS, seconded from the Met Office research group at Cardington, Bedfordshire, in order to organise an Antarctic boundary layer experiment at the Survey's Halley research station. Halley is an ideal site for such work: the ice shelf is very flat, snow is remarkably smooth, the wind is predominantly from the east at the site, and the winter months provide periods of stable atmospheric stratification with minimal diurnal influence from the sun.

 

This was the first of The Stable Antarctic Boundary Layer Experiments: STABLE I, (King and Anderson, 1988).

 

The main aim of the study was to make co-located measurements of heat and momentum flux, with vertical profiles of wind speed and temperature, in order to parameterise the relation between the profile data and the fluxes for global circulation models. Published results from these and similar Halley boundary layer field studies may be found in the references section. The initial STABLE experiment included a 30 m mast carrying an array of turbulence probes, small anemometers and aspirated thermometers, whilst in a nearby undisturbed area, radiation meters and snow thermometers measured other components of the heat flux. In addition to all this, a monostatic Sodar was included.

 

The turbulence probes measured the heat fluxes directly, and this was compared to the vertical profiles of wind speed and temperature. He hoped to understand the so called flux/profile relationships for conditions which are seen often in the UK at night, but are still "settling down" by the time the sun returns in the morning. Such conditions are also sensitive to hills, buildings, surface slope and the roughness of the surface. Halley would be an ideal site for such work, being flat, smooth and with an uninterrupted "fetch". The Sodar was going to measure "the height of the stable boundary layer". This height, above which turbulence becomes small, was known to be visible on Sodar echoes as a thick dark line...at least in mid-latitudes and in the Northern hemisphere. Boundary layer height is often used in equations of turbulence theory, and the Sodar was simply an easy way to measure it.

 

By 1985, I had been employed, and was busy putting together the instruments, loggers and a data archiving system, ready for wintering in1986. Despite a physics background, I had no academic knowledge of meteorology, not much more of fluid dynamics, but picked up scraps of micrometeorology as the need arose, such as eddy correlation techniques and buoyancy waves. My 1986 winter was spent as often as not, in a small wooden caboose which was stuck away from the main base in order to reduce wind flow distortion. Inside the caboose was kept warm and moderately humid, more to keep the equipment running well than for my own comfort. Outside to the south, a few mast were all there were to break the flat vista of the ice shelf, and in winter, only darkness. It felt fairly remote.

 

Communications in those days were limited, but we had a facsimile satellite link and a weekly data sched. Compressed summary data would be sent back to the UK for quality checks, and analysis using the more power computing facilities available in Cambridge . All seemed fine, until I was asked for an estimation of boundary layer depth. What this might be I was not sure, but further instructions from Cambridge clarified the situation; they wanted to know, "the height of the dark band on the Sodar". I then asked, quite reasonably I thought, "which dark band?"....

 

The two digital images are Halley Sodar backscatter echogrammes that have been taken more recently, but have been processed to be similar to the facsimile images from 1986. The left hand image, from Boxing Day, 2000, at the height of the austral summer shows a "typical" stable boundary layer echogram: a strong surface echo with a well defined upper level. Such images are typical at Halley in mid-summer during "night-time" when the sun angle is small.

 

The lower plot is more typical of the winter months, when surface cooling over many days has generated a deep and complex stable layer at the surface, whilst above the atmosphere is still stable but to a lesser degree. The echogram is complex, with multiple layers splitting and reforming. It was then apparent that the "depth" of the boundary layer is not well defined under such conditions.

 

 

 

 

The wintertime Sodar backscatter echogrammes from Halley are notable for their complexity and texture. Not only layering, but other stability induced effects can be seen, such as internal gravity waves, Kelvin-Helmholtz instabilities and surface solitary waves. Wave effects are well known under stable atmospheric conditions, and indeed part of the STABLE I  instrument suite was to study their propagation across the ice shelf. But the cause of the layered structure was less clear. Such echoes had been seen previously, notably in Boulder Colorado, where Sodar research was an active part of NOAA research groups, and some ideas posited as to their mechanism, but question remains today after nearly 20 years. Why is the Sodar chart complex?

 

We know something about what the data imply: acoustic reflections come from areas of turbulence. But turbulence is literally a great mixer: why then doesn't the turbulence blur out the features? How can an initially smooth or homogenous atmosphere become layered. How is it maintained? What are the bulk differences between a strong echo region and a weak one? We still don't know. The more we study the phenomenon, and the more we search for a theoretical understanding of the effects, the more peculiar the data appear.

 

Since 1986, monostatic Sodar studies at Halley have concentrated on the complexity of acoustic scattering. There has been no real progress to turbulence or acoustic scattering theory since the 1960's: we lack data. We have years of Sodar data, but very little associated high resolution profiles. It is at this gap in our field data that the present tethered profiling studies are aimed.

 

Rest of Document Still Under Construction

References:

 

References for these web pages

 

Gilman, G.W., Coxhead, H.B. and Willis, F.H. (1946): Reflection of sound signals in the Troposphere. Journal of the Acoustical Society of America. 18 274-283

 

Schwieson, R. L. 1986. A Comparitive Overview of Active Remote-Sensing Techniques. In: Probing the Atmospheric Boundary Layer, D.H. Lenschow Ed.

 

Tyndall, J. 1875: Selected Works of John Tyndall: Sound. D. Appleton, New York, N.Y., 306-320

 

Acoustic Radars

 

Anderson, P.S. (1996): Comparison of Internal Gravity Wave Events Recorded by SODAR and Microbarograph over an Antarctic Ice Shelf. Proceedings of the 8th International Symposium on Acoustic Remote Sensing, Moscow, Russia.

 

Anderson, P.S. (1997): Very Stable Boundary Layer Dislocations and Layering. 12th Symposium on Boundary Layers and Turbulence, Vancouver.158-159

 

Anderson, P.S. (In Press): Analysis of acoustic Dot Echo Signature over an Antarctic Ice Shelf:  The Possible Remote Sensing of Antarctic Petrels. Antarctic Science

 

Culf, A.D. (1989): Acoustic Sounding of the Atmospheric Boundary Layer at Halley, Antarctica. Antarctic Science, 1, 363-372

 

Hipkin, V. J, Anderson, P.S. and Mobbs, S.D. (1997): Layered Structure in the Atmospheric Boundary Layer. 12th Symposium on Boundary Layers and Turbulence, Vancouver. 160-161

 

STABLE publications on Flux/profile relationships.

 

Anderson, P.S. (1993): Evidence for an Antarctic winter coastal polynya. Antarct. Sci. 5(2) 221‑226.

 

Anderson, P.S. and Varley, M.J. (1995): Antarctic Observations of an Atmospheric Mixed Layer overlying an Extreme Surface Temperature Gradient: Surface Momentum Flux Dislocations. Presented at IUGG, Boulder, Co.

 

King, J.C. and Anderson, P.S. (1988): Installation and Performance of the STABLE Instrumentation at Halley. British Antarctic Survey Bulletin, 79, 65-77

 

King, J.C., Mobbs, S.D. Rees, J.M. Anderson, P.S. and Culf, A.D. (1989) The Stable Antarctic Boundary Layer Experiment at Halley Station. Weather, 44, 398-405

 

King, J.C and Anderson, P.S. (1994): Heat and Water Vapour Fluxes and Scalar Roughness Lengths over an Antarctic Ice Shelf. Boundary-Layer Meteorology, 69, 101-121.

 

King, J.C., Anderson, P.S., Smith, M.C. and Mobbs, S.D. (1996): The surface energy and mass balance at Halley, Antarctica during winter. Journal of Geophysical Results 101 D14 19,119-19,128

 

King, J.C. and Anderson, P.S. (1999): A humidity climatology for Halley, Antarctica, based on frost-point hygrometer readings. Antarctic Science, 11, 100-104.

 

Mann, G. W., Anderson, P.S. and Mobbs, S.D. (2000): Profile measurements of blowing snow at Halley, Antarctica. Journal of Geophysical Results 105 D19 24,491-24,508.

 

King, J.C., Anderson, P.S. and Mann, G.W.  (2001): The seasonal cycle of sublimation at Halley J. Glaciol. 47, No. 156 1-8

 

King, J.C., Connolley, W.M. and Derbyshire, S.H. (2001): Sensitivity of modelled Antarctic climate to surface and boundary layer flux parametrisations. Q.J.R.Met.Soc. 127, 779-794.  [50% ICD6]

 

 

Internal Waves and Solitons

 

Rees, J.M., McConnell, I. Anderson, P.S. and King, J.C. (1994): Observations of Internal Gravity Waves Over an Antarctic Ice Shelf using a Microbarograph Array. In: Castro,I and Rocklifte, N (Eds.) Proceedings of the 4th IMA conference on stably-stratified flows. Oxford University Press.

 

 

Rees, J.M, .Denholm-Price, J.C.W, King, J.C. and Anderson, P.S. (2000)  A Climatological Study of Internal Gravity Waves in the Atmospheric Boundary Layer Overlying the Brunt Ice Shelf, Antarctica. J. Atmos Science 57 511-526

 

Air/snow interaction

 

Morris, E.M., Anderson, P.S., Bader, H-P, Weilenmann, P. and Blight, C. (1994): Modelling of Mass and Energy Exchange over Polar Snow Using the DAISY Model. Proceedings IAMAP Symposium, Yokohama, July 1993. IAHS Press.

 

Morris, E.M., Anderson, P.S., King, J.C., Robb, C., Peel, D.A. and Doake, C.S.M., 1994: Energy and mass balance studies on the Uranus Glacier, Final Report for Climate, Sea Level Change and the Implications for Europe, contract No. ENSV-CT91_0051.

 

Technical papers

 

Anderson, P.S., Mobbs, S.D., King, J.C. McConnell, I., and Rees, J.M. (1992): A microbarograph for internal gravity wave studies in Antarctica. Antarct. Sci, 4(2), 241-248.

 

Anderson, P.S. (1994): A Method for Rescaling Humidity Sensors at Temperatures Well Below Freezing.  Journal of Atmospheric and Oceanic Technology. 11 1388-1391

 

Anderson, P.S. (1995). Mechanism for the behaviour of hydroactive materials used in humidity sensors. Journal of Atmospheric and Oceanic Technology.

 

Anderson, P.S. (1995): Automated snow measurements of surface energy balance and snow accumulation at Uranus Glacier, Antarctica. Presented at EGS, Hamburg

 

Anderson, P.S. (1995): A method for estimating surface temperature, snow surface heat flux,and accumulation time series over snow, suitable for high latitude, low power automatic weather stations. Presented at EGS, Hamburg

 

Anderson, P.S. (1996): Reply to comments on AA method for rescaling humidity instruments at temperatures well below freezing@. J. Atmos. Oceanic Technol., 13 913-914

 

Boundary Layer Chemistry