Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part II: Results from the Vorcore Campaign in Antarctica

Abstract

The stratospheric gravity wave field in the Southern Hemisphere is investigated by analyzing observations collected by 27 long-duration balloons that flew between September 2005 and February 2006 over Antarctica and the Southern Ocean. The analysis is based on the methods introduced by Boccara et al. in a companion paper. Special attention is given to deriving information useful to gravity wave drag parameterizations employed in atmospheric general circulation models. The balloon dataset is used to map the geographic variability of gravity wave momentum fluxes in the lower stratosphere. This flux distribution is found to be very heterogeneous with the largest time-averaged value (28 mPa) observed above the Antarctic Peninsula. This value exceeds by a factor of ϳ10 the overall mean momentum flux measured during the balloon campaign. Zonal momentum fluxes were predominantly westward, whereas meridional momentum fluxes were equally northward and southward. A local enhancement of southward flux is nevertheless observed above Adélie Land and is attributed to waves generated by katabatic winds, for which the signature is otherwise rather small in the balloon observations. When zonal averages are performed, oceanic momentum fluxes are found to be of similar magnitude to continental values (2.5-3 mPa), stressing the importance of nonorographic gravity waves over oceans. Last, gravity wave intermittency is investigated. Mountain waves appear to be significantly more sporadic than waves observed above the ocean.

Upper atmospheric gravity wave details revealed in nightglow satellite imagery

“…Processes related to jet imbalance and wind shear instability associated with the strong, rapidly evolving jet streams of intensifying cold fronts are well-known sources for gravity waves (32,44). Although typically weaker than their orographic cousins, the aggregate contribution of these gravity waves to the global distribution of energy and momentum flux may be comparable (45). However, the exact mechanisms responsible for wave launching are not well understood and are difficult to evaluate without a robust observational database.…”

Sensitivity study for mesoscale simulations of gravity waves above Antarctica during Vorcore

“…Simulated fluxes plotted with the full resolution (Figure 4(a)) compare well with those plotted with a coarse resolution and using only times when balloon observations were available (Figure 4(c)). Although this test is only for a short specific period, it suggests that the maps shown in Hertzog et al (2008) are also representative.…”

“…The large-scale wake of enhanced momentum fluxes has implications for the understanding of the non-orographic wave sources: in such a case, momentum fluxes 1000 km away from mountain ranges, although naturally considered as non-orographic (as in Hertzog et al, 2008), may in fact be linked to the orography and should perhaps be included in the parametrizations of orographic gravity-wave drag. As suggested by Figures 4 and 5, this wake extends further downstream with height, implying a smearing of sources as we increase altitude.…”

“…These measurements give access to the intrinsic frequencies of gravity waves, providing a more direct estimation of momentum fluxes than methods based on Eulerian measurements . Hertzog et al (2008) have produced maps of the momentum fluxes above Antarctica during spring, estimation of the intermittency of the wave field, and estimation of the relative importance of orographic and non-orographic waves. In particular, it was shown that waves identified as non-orographic are generally of much smaller amplitude than orographic waves, yet because they are less intermittent and exist over a wider domain, their overall contribution to upward momentum flux is comparable to that of orographic waves.…”

Gravity wave reflection and its influence on the consistency of temperature- and wind-based momentum fluxes simulated above Typhoon Ewiniar

“…The assumption has been used for the momentumflux estimations of the observed gravity waves not only by the satellites but by other measurement methods as well (e.g., Espy et al, 2004;Hertzog et al, 2008;Li et al, 2011). However, in the middle atmosphere downward propagation of gravity waves occurs by reflection of the upward propagating waves due to the changes in the vertical wavenumber with height (e.g., Hines and Reddy, 1967;Gossard and Hooke, 1975) or from the in situ wave sources in the upper level (e.g., Holton and Alexander, 1999;Zhou et al, 2002;Chun and Kim, 2008).…”