High Resolution Dynamics Limb Sounder measurements of gravity wave activity in the 2006 Arctic stratosphere

Observations from the High Resolution Dynamics Limb Sounder (HIRDLS) instrument on NASA's Aura satellite are used to quantify gravity wave momentum fluxes in the middle atmosphere. The period around the 2006 Arctic sudden stratospheric warming (SSW) is investigated, during which a substantial elevation of the stratopause occurred. Analysis of the HIRDLS results, together with analysis of European Centre for Medium-Range Weather Forecasting zonal winds, provide direct evidence of wind filtering of the gravity wave spectrum during this period. This confirms previous hypotheses from model studies and further contributes to our understanding of the effects of gravity wave driving on the winter polar stratopause.

Use of a portable topographic mapping millimetre wave radar at an active lava flow

A ground-based millimetre wave radar, AVTIS (All-weather Volcano Topography Imaging Sensor), has been developed for topographic monitoring. The instrument is portable and capable of measurements over ranges up to similar to 7 km through cloud and at night. In April and May 2005, AVTIS was deployed at Arenal Volcano, Costa Rica, in order to determine topographic changes associated with the advance of a lava flow. This is the first reported application of mm-wave radar technology to the measurement of lava flux rates. Three topographic data sets of the flow were acquired from observation distances of similar to 3 km over an eight day period, during which the flow front was detected to have advanced similar to 200 m. Topographic differences between the data sets indicated a flow thickness of similar to 10 m, and a dense rock equivalent lava flux of similar to 0.20 +/- 0.08 m(3) s(-1).

Multi-mode approximations to wave scattering by an uneven bed

Approximations to the scattering of linear surface gravity waves on water of varying quiescent depth are Investigated by means of a variational approach. Previous authors have used wave modes associated with the constant depth case to approximate the velocity potential, leading to a system of coupled differential equations. Here it is shown that a transformation of the dependent variables results in a much simplified differential equation system which in turn leads to a new multi-mode 'mild-slope' approximation. Further, the effect of adding a bed mode is examined and clarified. A systematic analytic method is presented for evaluating inner products that arise and numerical experiments for two-dimensional scattering are used to examine the performance of the new approximations.