NDT: Tephigrams and Sonde Data

Ray Flavell has given a thorough account of how you can examine radiosonde data to look for evidence of ducts. Given the very small changes in refractive index required by the Booker-Gordon theory, you might think that sonde information is of no use in studying NDT. This may or may not be right - but for the time being, you don't have much else to go on, besides MST radar studies. As there are few MST radars around the world, and relatively many more stations which collect sonde data, it is worth seeing what you can obtain.

The classic way of presenting upper air soundings is in a tephigram, although you may well come across the more basic plot of

Wet and dry bulb temperatures with height

The raw data is usually expressed against atmospheric pressure, which is of limited direct use in radio propagation, so here I have estimated height in its place (there are several different ways of doing this, with differing accuracies). This tells you one useful piece of information - the approximate height of the tropopause, where temperatures stabilise instead of continuing to fall, in this case about 10 km. The dashed line shows wet bulb, and the solid line dry bulb temperatures.

Next, we should look at the radio refractive index, which can be computed using the formulae given by Flavell (and elsewhere). So here is a plot of

Refractive Index (N) with atmospheric pressure

At least this suggests that you will be out of luck looking for ducts! Once again, I would rather examine

Refractive Index (N) with height

This shows how low the refractive index becomes (i.e. close to 1.0) at higher altitudes, and how it changes most at lower altitudes. Whilst Booker and Gordon may only want us to look for very small differences, aren't they going to be less likely up towards the tropopause? So, one final plot of interest is

Change in Refractive Index (dN) with height

The wild oscillations at lower altitudes are artefacts which should perhaps be removed by smoothing. Notice though how dN gets very low by the level of the tropopause, and then slowly falls in the lower stratosphere. If you want to, you can estimate all sorts of other interesting measures from upper air soundings, including the Brunt-Vaisailla Buoyancy Frequency. We'll delve into that in future revisions of these pages.

Recent work, including that by Newell and others (Nature 398:316-319; "Ubiquity of quasi-horizontal layers in the troposphere", RE Newell, V Thouret, JYN Cho, P Stoller, A Marenco & HG Smit, 1999) calls ordinary sonde data into question. They described data collected from sensors fitted to a number of commercial aircraft, which provides good evidence for distinct tropospheric layers of 1 km and less, which are generally missing from ordinary atmospheric soundings, because of the relatively coarse nature of sonde data. Although capable of transmitting data every 30-100 m, most soundings are only reported at 1-2.5 km height intervals. Aircraft data (from the MOZAIC project and NASA PEM missions) show tropospheric layers which are thinner than those height intervals, but which may contain high or low ozone levels, and moist or dry air. Predicted profiles of heating rates suggest that these layers can account for instability above the boundary layer, and turbulence in otherwise clear air. They are thus a fascinating potential source of forward scatter of radio waves. Further work clearly needs to be done.

NDT Inner Sanctum

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Last updated 5 Apr 1999
Howard Oakley
Mail howard@quercus.demon.co.uk