On the Intermittency of Gravity Wave Momentum Flux in the Stratosphere

Abstract

In this article, long-duration balloon and spaceborne observations, and mesoscale numerical simulations are used to study the intermittency of gravity waves in the lower stratosphere above Antarctica and the Southern Ocean; namely, the characteristics of the gravity wave momentum-flux probability density functions (pdfs) obtained with these three datasets are described. The pdfs consistently exhibit long tails associated with the occurrence of rare and large-amplitude events. The pdf tails are even longer above mountains than above oceanic areas, which is in agreement with previous studies of gravity wave intermittency in this region. It is moreover found that these rare, large-amplitude events represent the main contribution to the total momentum flux during the winter regime of the stratospheric circulation. In contrast, the wave intermittency significantly decreases when stratospheric easterlies develop in late spring and summer. It is also shown that, except above mountainous areas in winter, the momentum-flux pdfs tend to behave like lognormal distributions. Monte Carlo simulations are undertaken to examine the role played by critical levels in influencing the shape of momentum-flux pdfs. In particular, the study finds that the lognormal shape may result from the propagation of a wave spectrum into a varying background wind field that generates the occurrence of frequent critical levels.

Global distributions of overlapping gravity waves in HIRDLS data

“…In particular, these suggest that the large momentum flux signal of the monsoon appear to be primarily due to variations in the number of Ls waves, whilst the wellstudied Andes hotspot represents an actual reduction relative to the annual mean in the number of observed waves, and consequently a massively increased momentum flux per wave packet. The latter is consistent with previous observations of high wave intermittency in this region (Hertzog et al, 2012;.…”

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

“…The gravity wave activity related to the weak inflow and outflow circulations is somewhat reduced, and there is a shift from the central Mediterranean to the western Mediterranean and to the Canary Islands west of Africa. Overall, these strong changes show the strongly intermittent nature particularly of orographically generated gravity waves (e.g., Eckermann and Preusse, 1999;Jiang et al, 2002;Hertzog et al, 2008Hertzog et al, , 2012Wright et al, 2013), but obviously also jet-related gravity wave sources show strong day-to-day variability, as could be the case over the North Atlantic. Of course, it is difficult to provide reliable estimates of intermittency time scales because the observations are limited by the daily sampling patterns of the satellite instruments.…”

Interannual variability in the gravity wave drag – vertical coupling and possible climate links

“…From a methodological point of view we must also note that GWs and their effects are handicapped by the use of monthly mean data because the GWs are very intermittent in the atmosphere (e.g., Hertzog et al, 2012;Wright and Gille, 2013), and also in CMAM, the OGWD shows large daily (and shorter, not shown) variability. Therefore, for example, the monthly mean values may be hiding 1 order stronger intermittent drag values.…”