Gravity Wave Ducting over Antarctica P-D Pautet 1 , MJ Taylor 1 , - PowerPoint PPT Presentation

Gravity Wave Ducting over Antarctica P-D Pautet 1 , MJ Taylor 1 , and D. Murphy 2 1 Center for Atmospheric and Space Sciences (CASS), Utah State University, Logan, UT, USA 2 Australian Antarctic Division, Hobart, Australia 4 th ANGWIN Workshop – Sao Jose dos Campos - 24-26 April, 2018

Gravity Wave Propagation Questions about GW propagation: • Effects of the sources? • Effects of the background atmosphere (horizontal wind, temperature)? • Long distance propagations: percentage of ducted waves? • What about Antarctica?

Gravity Wave Equations Fundamental fluid equations (conservation of mass, momentum and energy [Fritts and Alexander, 2003]): With gravity wave solutions having the form:

Gravity Wave Dispersion Relation H is the scale height, N is the buoyancy frequency, and k, c, and u are the GW horizontal wave number, the GW horizontal phase speed, and the background wind speed Temperature T Background wind u

3 Different Cases m 2 >0 -> freely propagating wave Region with m 2 <0 bounded by evanescent regions -> possible ducted wave m 2 <0 -> Evanescent wave

Effects of the Atmospheric Background on GW Propagation (Bossert et al., 2014) AMTM keogram: short-scale GW visible the whole night Na lidar temperature and horizontal wind vertical profiles Vertical wavenumber

Is there a way to assess the background atmosphere structure with only an imager?

International ANGWIN Instrument Network

Advanced Mesospheric Temperature Mapper (AMTM) • Capability: High-resolution mapping of gravity wave intensity and temperature field at ~87 km and wave phase relationship. • Sequentially observes selected emission lines in the infrared (1.5-1.65 μ m) OH (3,1) band to derive high- quality temperature maps. • Temperature precision/pix ~1-2 K in <30 sec. • High-latitude capability as emission lines avoid auroral contamination. AMTM at South Pole PFRR Aurora + Airglow ALOMAR (69.3° N, 16.0° E) South Pole (90ºS) Data since 2011 (6 winters each site) Temperature: ratio of P 1 (2) and P 1 (4) lines

First “Frontal Event” over SP, May 21, 2010 Na OH • Phase speed: 60m/s • Δ T~10K, Δ I~100% • Trailing oscillations: Δ T~2.5K, Δ I~6% AMTM Na: Courtesy B. Williams GATS, CO

Other Frontal Events Airglow Observations Bore over Halley (76°S), 27 May 2001 (Nielsen et al., 2006) Bore over Texas (30.7°N), 14 Nov 1999 (Smith et al., 2003) Bores over São João de Cariri (7.5°S), 30 Sep (left) and 28 Dec (right), 2000 (Fechine et al., 2005)

Easy Identification Using Keograms May 4, 2013 Aug 25, 2013

Large Number of Frontal Events Observations over South Pole Total of 86 events during 4 winters (2012-15) ~ 1 per week

Yearly/Monthly Distributions

Intensity/Temperature Frontal GW Event - May 4, 2013

Cook Inlet, Alaska Tropospheric Bore California Bores are characterized by a sharp leading front followed by several adjacent trailing crests that grow in number as the front sheds energy

“Bores” or “Walls”? Taylor et al., Swenson and 1995 Espy, 1995 • Sharp front • Phase speed ~20-100m/s • Fixed pattern of trailing waves • Phase or antiphase in different • Phase shift between emissions • Trailing waves will grow in different emissions/altitudes • Freely propagating number over time • Ducted Stable layer to provide a duct (Doppler or thermal)

Fronts Horizontal Parameters < < ~ Short-period GWs over SP (Suzuki et al., 2011)

Fronts Directionality SP fronts (2012-15) SP short-period GWs (2010) Same direction as general short-period GWs population

Trailing Waves Characteristics AVG: 8.5 waves AVG: 0.95 wave/hr Previous reports: Smith et al., 2003 -> 1 event - 14 trailing waves Fechine et al., 2005 -> 64 events - 2-12 trailing waves Nielsen et al., 2006 (high latitude) -> 1 event - 12 trailing waves South Pole 2012-15 -> 86 events -> 2-24 trailing waves (15 cases >12!!!) SP events propagating for a long distance (>1000 km)

Temperature/Intensity Perturbations

Krassovsky Ratio (1) dI I 0 η = Krassovsky (1972): dT T 0 Strong dependence between the GW parameters and both the intensity and temperature perturbations, resulting in a complicated relation for |ɳ | (Hines and Tarasick, 1987; Hickey and Yu, 2005)

Krassovsky Ratio (2) Ideal ducting is predicted to result in a near-zero or 180° response, depending on chemistry and the layered species profiles (Hines and Tarasick, 1994; Snively et al., 2010), which would imply that a large number of these events (~70%) might have been ducted.

Summary South Pole (Pautet et al., 2018) • Large number of frontal events (~1 per week) observed over SP during 4 winter seasons (2012-15) • Their characteristics are similar to the general short-period gravity waves population, and they travel in the same typical direction, • But their horizontal wavelengths are in average significantly shorter, and consequently their periods are shorter as well, • The Krassovsky ratio phases associated with these events indicate that possibly ~70% are ducted, and therefore might be bores, • In this case, the large number of their trailing waves (more than in any previous observations) suggests that a significant proportion of events may have been ducted over very long trans-Antarctic distances up to several 1000s km, • Thus, revealing the presence of extensive thermal ducts at high latitudes, since Doppler ducting is unlikely due to small horizontal winds over South Pole (Hernandez et al., 1992; Portnyagin et al., 1997).

International ANGWIN Instrument Network

Multiple Bore Events over Davis • A large number (>18) mesospheric “ bore- like” wave events over Davis during 2012 and 2013 • Very few measurements of bores in Antarctica (Nielsen et al., 2006).

Example GW Frontal Events, Sep 24-25, 2012 Duration ~2.5 hours Source ? First wave parameters: Note the development of >12 wave λ x =18.5 km V x = 74.9 m/s crests, characteristic of a Mesospheric T= 4.1 min Direction = 15° Bore Event.

Results of “Bore” Measurements 2012 -2013 Example characteristics of bore events • Davis bores occurred post equinox, during the latter part of the winter season, & had high phase speeds. • All bores exhibited remarkably consistent propagation headings to ~N-NE away from Antarctic continent. • Data suggests a localized recurrent wave source. Bore directions

Bore Occurrence and MLS Temperature Structure Davis, August 2012 Reconstruction Full MLS with WN 0 & 1 reconstruction • Satellite MLS temperature analysis at Davis reveals strong mesospheric ducting present during the observed bore events. • Isothermal layer over Davis mostly due to interaction between a wave 1 structure and the background temperature gradient • Strong effect on gravity wave propagation at different sites.

Bore Occurrence and MLS Temperature Structure Davis, September 2012 Full MLS reconstruction Reconstruction with WN 0 & 1

MLS Temperature Profiles Rothera, August 2012 No isothermal layers at Rothera during same periods. Indicates longitudinal dependence of this climatological feature. Strong effect on gravity wave propagation at different sites.

Summary Davis • Surprising discovery of a large number (18+) mesospheric “frontal” or “bore - like” wave events over Davis • All Davis events were observed during post equinox period progressing towards the ~N-NE (see Table) suggesting a localized recurrent wave source • MLS temperature structure shows mesospheric duct at the time of the events • This duct is probably due to the interaction between a wavenumber 1 structure with the background • No such duct appeared over Rothera where no frontal events were observed