Katrina Bossert

The Deep Propagating Gravity Wave Experiment (DEEPWAVE): An Airborne and Ground-Based Exploration of Gravity Wave Propagation and Effects from Their Sources throughout the Lower and Middle Atmosphere

AbstractThe Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropson...

Quantifying gravity wave momentum fluxes with Mesosphere Temperature Mappers and correlative instrumentation

An Advanced Mesosphere Temperature Mapper and other instruments at the Arctic Lidar Observatory for Middle Atmosphere Research in Norway (69.3°N) and at Logan and Bear Lake Observatory in Utah (42°N) are used to demonstrate a new method for quantifying gravity wave (GW) pseudo-momentum fluxes accompanying spatially and temporally localized GW packets. The method improves on previous airglow techniques by employing direct characterization of the GW temperature perturbations averaged over the OH airglow layer and correlative wind and temperature measurements to define the intrinsic GW properties with high confidence. These methods are applied to two events, each of which involves superpositions of GWs having various scales and character. In each case, small-scale GWs were found to achieve transient, but very large, momentum fluxes with magnitudes varying from ~60 to 940 m2 s−2, which are ~1–2 decades larger than mean values. Quantification of the spatial and temporal variations of GW amplitudes and pseudo-momentum fluxes may also enable assessments of the total pseudo-momentum accompanying individual GW packets and of the potential for secondary GW generation that arises from GW localization. We expect that the use of this method will yield key insights into the statistical forcing of the mesosphere and lower thermosphere by GWs, the importance of infrequent large-amplitude events, and their effects on GW spectral evolution with altitude.

Momentum flux estimates accompanying multiscale gravity waves over Mount Cook, New Zealand, on 13 July 2014 during the DEEPWAVE campaign

Observations performed with a Rayleigh lidar and an Advanced Mesosphere Temperature Mapper aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V research aircraft on 13 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) measurement program revealed a large-amplitude, multiscale gravity wave (GW) environment extending from ~20 to 90 km on flight tracks over Mount Cook, New Zealand. Data from four successive flight tracks are employed here to assess the characteristics and variability of the larger- and smaller-scale GWs, including their spatial scales, amplitudes, phase speeds, and momentum fluxes. On each flight, a large-scale mountain wave (MW) having a horizontal wavelength ~200–300 km was observed. Smaller-scale GWs over the island appeared to correlate within the warmer phase of this large-scale MW. This analysis reveals that momentum fluxes accompanying small-scale MWs and propagating GWs significantly exceed those of the large-scale MW and the mean values typical for these altitudes, with maxima for the various small-scale events in the range ~20–105 m2 s−2.

Large-amplitude mesospheric response to an orographic wave generated over the Southern Ocean Auckland Islands (50.7°S) during the DEEPWAVE project

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) project was conducted over New Zealand and the surrounding regions during June and July 2014, to more fully understand the generation, propagation, and effects of atmospheric gravity waves. A large suite of instruments collected data from the ground to the upper atmosphere (~100 km), with several new remote-sensing instruments operating on board the NSF Gulfstream V (GV) research aircraft, which was the central measurement platform of the project. On 14 July, during one of the research flights (research flight 23), a spectacular event was observed as the GV flew in the lee of the sub-Antarctic Auckland Islands (50.7°S). An apparent “ship wave” pattern was imaged in the OH layer (at ~83.5 km) by the Utah State University Advanced Mesospheric Temperature Mapper and evolved significantly over four successive passes spanning more than 4 h. The waves were associated with orographic forcing generated by relatively strong (15–20m/s) near-surface wind flowing over the rugged island topography. The mountain wave had an amplitude T′~ 10 K, a dominant horizontal wavelength ~40 km, achieved a momentum flux exceeding 300m s , and eventually exhibited instability and breaking at the OH altitude. This case of deep mountain wave propagation demonstrates the potential for strong responses in the mesosphere arising from a small source under suitable propagation conditions and suggests that such cases may be more common than previously believed.

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

New measurements at the ALOMAR observatory in northern Norway (69°N, 16°E) using the Weber sodium lidar and the Advanced Mesospheric Temperature Mapper (AMTM) allow for a comprehensive investigation of a gravity wave (GW) event on 22 and 23 January 2012 and the complex and varying propagation environment in which the GW was observed. These observational techniques provide insight into the altitude ranges over which a GW may be evanescent or propagating and enable a clear distinction in specific cases. Weber sodium lidar measurements provide estimates of background temperature, wind, and stability profiles at altitudes from ~78 to 105 km. Detailed AMTM temperature maps of GWs in the OH emission layer together with lidar measurements quantify estimates of the observed and intrinsic GW parameters centered near 87 km. Lidar measurements of sodium densities also allow more precise identification of GW phase structures extending over a broad altitude range. We find for this particular event that the extent of evanescent regions versus regions allowing GW propagation can vary largely over a period of hours and significantly change the range of altitudes over which a GW can propagate.

Does Strong Tropospheric Forcing Cause Large‐Amplitude Mesospheric Gravity Waves? A DEEPWAVE Case Study

The DEEPWAVE (deep-propagating wave experiment) campaign was designed for an airborne and ground-based exploration of gravity waves from their tropospheric sources up to their dissipation at high altitudes. It was performed in and around New Zealand from 24 May till 27 July 2014, being the first comprehensive field campaign of this kind. A variety of airborne instruments was deployed onboard the research aircraft NSF/NCAR Gulfstream V (GV) and the DLR Falcon. Additionally, ground-based measurements were conducted at different sites across the southern island of New Zealand, including the DLR Rayleigh lidar located at Lauder (45.04 S, 169.68 E). We focus on the intensive observing period (IOP) 10 on the 4 July 2014, when strong WSW winds of about 40 m/s at 700 hPa provided intense forcing conditions for mountain waves. At tropopause level, the horizontal wind exceeded 50 m/s and favored the vertical propagation of gravity waves into the stratosphere. The DLR Rayleigh Lidar measured temperature fluctuations with peak-to-peak amplitudes of about 20 K in the mesosphere (60 km to 80 km MSL) over a period of more than 10 hours. Two research flights were conducted by the DLR Falcon (Falcon Flight 04 and 05) during this period with straight transects (Mt. Aspiring 2a) over New Zealand´s Alps at three different flight-levels around the tropopause (approx. 11 km MSL). These research flights were coordinated with the GV (Research Flight 16) where the largest mountain wave amplitudes at flight-level (approx. 13 km MSL) were measured during DEEPWAVE. Additionally a first analysis of Falcons in-situ flight-level data revealed amplitudes in the vertical wind larger than 4 m/s at all altitudes in the vicinity of the highest peaks of the Southern Alps. Here, we present a comprehensive picture of the gravity wave characteristics and propagation properties during this interesting gravity wave event. We use the airborne observations combined with a comprehensive set of ground-based measurements consisting of 13 radiosoundings (1.5 hourly interval) together with the DLR Rayleigh lidar. To cover the altitude range from the troposphere to the mesosphere, high-resolution (1 hourly) ECMWF analyses and forecasts are used to estimate the propagation conditions of the excited mountain waves. The goal of our investigation is to find out whether the large amplitude mesospheric gravity waves are caused by the strong tropospheric forcing.

Dynamics of Orographic Gravity Waves Observed in the Mesosphere over the Auckland Islands during the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

AbstractOn 14 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE), aircraft remote sensing instruments detected large-amplitude gravity wave oscillations within mesospheric airglow and sodium layers at altitudes z ~ 78–83 km downstream of the Auckland Islands, located ~1000 km south of Christchurch, New Zealand. A high-altitude reanalysis and a three-dimensional Fourier gravity wave model are used to investigate the dynamics of this event. At 0700 UTC when the first observations were made, surface flow across the islands’ terrain generated linear three-dimensional wave fields that propagated rapidly to z ~ 78 km, where intense breaking occurred in a narrow layer beneath a zero-wind region at z ~ 83 km. In the following hours, the altitude of weak winds descended under the influence of a large-amplitude migrating semidiurnal tide, leading to intense breaking of these wave fields in subsequent observations starting at 1000 UTC. The linear Fourier model constrained by upstream reanalysis...

Numerical modeling of a multiscale gravity wave event and its airglow signatures over Mount Cook, New Zealand, during the DEEPWAVE campaign

A 2D nonlinear compressible model is used to simulate a large amplitude, multiscale mountain wave event over Mt. Cook, NZ, observed as part of the DEEPWAVE campaign, and to investigate its observable signatures in the hydroxyl (OH) layer. The campaign observed the presence of a λx=200km mountain wave as part of the 22nd research flight with amplitudes of >20K in the upper stratosphere that decayed rapidly at airglow heights. Advanced Mesospheric Temperature Mapper (AMTM) showed the presence of small-scale (25-28km) waves within the warm phase of the large mountain wave. The simulation results show rapid breaking above 70km altitude, with the preferential formation of almost-stationary vortical instabilities within the warm phase front of the mountain wave. An OH airglow model is used to identify the presence of small-scale wave-like structures generated in situ by the breaking of the mountain wave that are consistent with those seen in the observations. While it is easy to interpret these feature as waves in OH airglow data, a considerable fraction of the features are in fact instabilities and vortex structures. Simulations suggest that a combination of a large westward perturbation velocity and shear, in combination with strong perturbation temperature gradients, cause both dynamic and convective instability conditions to be met particularly where the wave wind is maximized and the temperature gradient simultaneously minimized. This leads to the inevitable breaking and subsequent generation of smaller scale waves and instabilities which appear most prominent within the warm phase front of the mountain wave.

Momentum Flux Spectra of a Mountain Wave Event Over New Zealand

During the Deep Propagating Gravity Wave Experiment (DEEPWAVE) 13 July 2014 research flight over the South Island of New Zealand, a multiscale spectrum of mountain waves (MWs) was observed. High‐resolution measurements of sodium densities were available from ~70 to 100 km for the duration of this flight. A comprehensive technique is presented for obtaining temperature perturbations, T′, from sodium mixing ratios over a range of altitudes, and these T′ were used to calculate the momentum flux (MF) spectra with respect to horizontal wavelengths, λH, for each flight segment. Spectral analysis revealed MWs with spectral power centered at λH of ~80, 120, and 220 km. The temperature amplitudes of these MWs varied between the four cross‐mountain flight legs occurring between 6:10UT and 9:10UT. The average spectral T′ amplitudes near 80 km in altitude ranged from 7–13 K for the 220 km λH MW and 4–8 K for the smaller λH MWs. These amplitudes decayed significantly up to 90 km, where a critical level for MWs was present. The average MF per unit mass near 80 km in altitude ranged from ~13 to 60 m²/s² across the varying spectra over the duration of the research flight and decayed to ~0 by 88 km in altitude. These MFs are large compared to zonal means and highlight the importance of MWs in the momentum budget of the mesosphere and lower thermosphere at times when they reach these altitudes.