Proceedings with Extended Abstracts (single PDF file) - Radio ...

TROPOPAUSE EROSION BY MOUNTAIN WAVE BREAKINGDavid A Hooper 1 and Ed Pavelin 21 Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK2 Department of Meteorology, University of Reading, PO Box 243, Reading. RG6 6BB, UKIntroductionThe tropopause represents a boundary between air masses with distinct chemical anddynamical properties. Traditionally it has been thought of as a discrete level at which theseproperties change sharply. Although this is often the case, the transition between uppertroposphericand lower-stratospheric characteristics can also be much more gradual (Hooperand Arvelius, 2000). Under such circumstances it is more appropriate to think of thetropopause as a region which can have a vertical extent of up to several kilometres. Thecurrent study demonstrates that turbulent mixing, caused by gravity wave breaking, is one ofthe mechanisms which can lead to a broadening of the tropopause region. A case study ispresented for observations made by the UK Natural Environment Research Council MSTradar at Aberystwyth.Case Study of 18th July 2001MST radar data for the altitude region 8 - 12 km on 18th July 2001 are shown in Figure 1.The crosses superimposed on each panel indicate the altitude of the tropopause derived fromthe vertical beam signal power (top panel) using an objective algorithm (Hooper andArvelius, 2000); a sharpness value (second panel) of 3 corresponds to a definite tropopause(and a narrow tropopause region), a value of 0 to an indefinite tropopause (and a broadregion), and intermediate values to intermediate sharpnesses (and depths of region). As willbe shown shortly, the changes in sharpness are more significant than those of altitude. Thevertical beam radial velocity fluctuations seen between 0600 and 1500 UT (third panel) areattributed to mountain wave activity corresponding to low-level winds (not shown) from thenorth-east (Prichard et al., 1995). Enhanced values of the beam broadening corrected verticalbeam spectral widths (fourth panel) indicate that moderate turbulence coincides with themaximum altitude reached by the mountain wave activity. This is particularly clear at around9.5 km altitude between 0600 and 1100 UT, but can still be seen, to a lesser extent, as themaximum altitude drops from around 9.5 km, at 1330 UT, to below 8 km, at 1500 UT.The fact that the mountain wave activity is not able to propagate beyond the tropopause levelis attributed to critical level absorption in the upper troposphere (Worthington and Thomas,1996). For a mountain wave, a critical level corresponds to an altitude at which thecomponent of the horizontal wind parallel to the low-level (wave generating) wind hasreduced to zero. A mountain wave cannot propagate above a critical level but will dissipateall of its energy at this level through turbulent mixing. It can be seen that the maximumaltitude reached by the mountain wave activity corresponds closely to that at which the windspeed (fifth panel) first drops to zero. The existence of a critical level can be demonstratedmore explicitly by considering the normalised projected wind (sixth panel). This is thenormalised dot product of the wind vector at each level with that at the lowest level observedby the radar (1.7 km), i.e. the cosine of the angle between the wind vectors. The maximumaltitude reached by the mountain wave activity corresponds closely to that at which thisfactor first drops to zero.46