Influences of source conditions on mountain wave penetration into the stratosphere and mesosphere

We present atmospheric gravity wave (GW) measurements obtained by a Rayleigh/Raman lidar at Lauder, New Zealand (45∘ S, 170∘ E) during and after the DEEPWAVE campaign. GW activity and characteristics are derived from 557 hours of high-resolution lidar data recorded between June and November 2014 in an altitude range between 28 and 76 km. In this period, strong GW activity occurred in sporadic intervals lasting a few days. Enhanced stratospheric GW potential energy density is detected during periods with high tropospheric wind speeds perpendicular to New Zealand's Southern Alps. These enhancements are associated with the occurrence of quasi-stationary GW (mountain waves). Surprisingly, the largest response in the mesosphere is observed for conditions with low to moderate lower tropospheric wind speeds (2–12 m/s). On the other hand, large-amplitude mountain waves excited by strong tropospheric forcings often do not reach mesospheric altitudes, either due to wave breaking and dissipation in the stratosphere or refraction away from New Zealand

Mountain Wave Propagation under Transient Tropospheric Forcing: A DEEPWAVE Case Study

The impact of transient tropospheric forcing on the deep vertical mountain wave propagation is investigated by a unique combination of in-situ and remote-sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a cos2 shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere, lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile as well as the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes (−MFtrack) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m−1 and 1500 m, respectively. Their maxima were phase-shifted to the maximum low-level forcing by ≈ 8 h. Small-scale waves (λx ≈ 20–30 km) were continuously forced and their flux values depended on wave attenuation by breaking and reflection in the UTLS region

Mountain-Wave Propagation under Transient Tropospheric Forcing: A DEEPWAVE Case Study

The impact of transient tropospheric forcing on the deep vertical mountain-wave propagation is investigated by a unique combination of in situ and remote sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a cos2 shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere and lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile, and the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes (-MFtrack) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m-1 and 1500 m, respectively. Their maxima were phase shifted to the maximum low-level forcing by ≈8 h. Small-scale waves (λx ≈ 20 - 30 km) were continuously forced, and their flux values depended on wave attenuation by breaking and reflection in the UTLS region. Only maximum flow over the envelope of the mountain range favored the excitation of longer waves that propagated deeply into the mesosphere. Their long propagation time caused a retarded enhancement of observed mesospheric gravity wave activity about 12–15 h after their observation in the UTLS. For the UTLS, we further compared observed and simulated MFtrack with fluxes of 2D quasi-steady runs. UTLS momentum fluxes seem to be reproducible by individual quasi-steady 2D runs, except for the flux enhancement during the early decelerating forcing phase

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

On 4 July 2014, during the Deep Propagating Gravity Wave Experiment (DEEPWAVE), strong low-level horizontal winds of up to 35 m s−1 over the Southern Alps, New Zealand, caused the excitation of gravity waves having the largest vertical energy fluxes of the whole campaign (38 W m−2). At the same time, large-amplitude mesospheric gravity waves were detected by the Temperature Lidar for Middle Atmospheric Research (TELMA) located at Lauder (45.0°S, 169.7°E), New Zealand. The coincidence of these two events leads to the question of whether the mesospheric gravity waves were generated by the strong tropospheric forcing. To answer this, an extensive data set is analyzed, comprising TELMA, in situ aircraft measurements, radiosondes, wind lidar measurements aboard the DLR Falcon as well as Rayleigh lidar and advanced mesospheric temperature mapper measurements aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V. These measurements are further complemented by limited area simulations using a numerical weather prediction model. This unique data set confirms that strong tropospheric forcing can cause large-amplitude gravity waves in the mesosphere, and that three essential ingredients are required to achieve this: first, nearly linear propagation across the tropopause; second, leakage through the stratospheric wind minimum; and third, amplification in the polar night jet. Stationary gravity waves were detected in all atmospheric layers up to the mesosphere with horizontal wavelengths between 20 and 100 km. The complete coverage of our data set from troposphere to mesosphere proved to be valuable to identify the processes involved in deep gravity wave propagation

Combination of Lidar and Model Data for Studying Deep Gravity Wave Propagation

The paper presents a feasible method to complement ground-based middle atmospheric Rayleigh lidar temperature observations with numerical simulations in the lower stratosphere and troposphere to study gravity waves. Validated mesoscale numerical simulations are utilized to complement the temperature below 30-km altitude. For this purpose, high-temporal-resolution output of the numerical results was interpolated on the position of the lidar in the lee of the Scandinavian mountain range. Two wintertime cases of orographically induced gravity waves are analyzed. Wave parameters are derived using a wavelet analysis of the combined dataset throughout the entire altitude range from the troposphere to the mesosphere. Although similar in the tropospheric forcings, both cases differ in vertical propagation. The combined dataset reveals stratospheric wave breaking for one case, whereas the mountain waves in the other case could propagate up to about 40-km altitude. The lidar observations reveal an interaction of the vertically propagating gravity waves with the stratopause, leading to a stratopause descent in both cases

Long-term lidar observations of wintertime gravity wave activity over northern Sweden

This paper presents an analysis of gravity wave activity over northern Sweden as deduced from 18 years of wintertime lidar measurements at Esrange (68� N, 21� E). Gravity wave potential energy density (GWPED) was used to characterize the strength of gravity waves in the altitude regions 30–40 km and 40–50 km. The obtained values exceed previous observations reported in the literature. This is suggested to be due to Esrange’s location downwind of the Scandinavian mountain range and due to differences in the various methods that are currently used to retrieve gravity wave parameters. The analysis method restricted the identification of the dominating vertical wavelengths to a range from 2 to 13 km. No preference was found for any wavelength in this window. Monthly mean values of GWPED show that most of the gravity waves’ energy dissipates well below the stratopause and that higher altitude regions show only small dissipation rates of GWPED. Our analysis does not reproduce the previously reported negative trend in gravity wave activity over Esrange. The observed interannual variability of GWPED is connected to the occurrence of stratospheric warmings with generally lower wintertime mean GWPED during years with major stratospheric warmings.A bimodal GWPED occurrence frequency indicates that gravity wave activity at Esrange is affected by both ubiquitouswave sources and orographic forcing

Evaluation of methods for gravity wave extraction from middle atmospheric lidar temperature measurements

This study evaluates commonly used methods of extracting gravity wave induced temperature perturbations from lidar measurements. The spectral response of these methods is characterized with the help of a synthetic dataset with known temperature perturbations added to a realistic background temperature profile. The simulations are carried out with the background temperature being either constant or varying in time to evaluate the sensitivity to temperature perturbations not caused by gravity waves. The different methods are applied to lidar measurements over new Zealand and the performance of the algorithms is evaluated. We find that the Butterworth filter performs best if gravity waves over a wide range of periods are to be extracted from lidar temperature measurements. The running mean method gives good results if only gravity waves with short periods are to be analyzed

Lidar-Messungen von Schwerewellen in der mittleren Atmosphäre während der DEEPWAVE-Kampagne in Neuseeland

Im Rahmen des DEEP propagating WAVE experiment over New Zealand wurden zwischen Juni und November 2014 umfangreiche Lidar-Messungen mit einem bodengebundenen Rayleigh-Lidar über Lauder (45S, 170E) durchgeführt. Der Ort zeichnet sich durch seine Lage außerhalb des Polarwirbels sowie östlich der neuseeländischen Alpen aus. Letztere stellen eine Quelle für orographische Schwerewellen dar. Schwerewellen, die sich horizontal und vertikal über große Distanzen ausbreiten, sind durch die Impulsdeposition in der mittleren Atmosphäre von großer Bedeutung für die globale Zirkulation. In Modellen sind sie jedoch bisher nur unzureichend parametrisiert. Die Untersuchung von Schwerewellen mit einer Vielzahl von Instrumenten war das Ziel der DEEPWAVE-Kampagne. Wir haben ein leistungsfähiges Rayleigh-Lidar entwickelt, das Messungen in einem Höhenbereich zwischen 20 und 90 km mit hoher vertikaler und zeitlicher Auflösung erlaubt. Mit einer geeigneten Filterung werden Signaturen von Schwerewellen aus den Temperaturprofilen extrahiert. Die vertikale Wellenlänge und die Periode einzelner Wellen werden mit Hilfe von spektralen Analysemethoden bestimmt. Wir präsentieren eine Auswahl von Einzelfällen und geben einen Überblick der thermischen Struktur und der Schwerewellenaktivität während des gesamten Messzeitraums, der sowohl den Winter- als auch den Übergang zum Sommerzustand umfasst

Characteristics of gravity waves observed by Rayleigh lidar in the middle atmosphere above New Zealand

The Temperature Lidar for Middle Atmosphere research (TELMA) made atmospheric soundings at Lauder, New Zealand, over a period of 5 months in southern winter 2014. Strong gravity wave (GW) activity occurred in sporadic intervals lasting a few days. Large-amplitude mountain waves were detected during periods with high tropospheric wind speeds perpendicular to the Southern Alps. During the strongest event (29 July to 1 August) mountain wave amplitudes in excess of 15 K occurred at 40 km altitude, resulting in GW potential energy density enhancements exceeding one order of magnitude. The quasi-stationary large-amplitude waves often break and dissipate in the stratosphere and do not reach mesospheric altitudes. Secondary GWs radiated in this process account for a significant shift in the wave spectrum toward smaller scales. In addition, a substantial number of small-scale secondary GWs are generated as a result of propagating primary GWs interacting with tides. In the mesosphere, largest mountain wave amplitudes occur during conditions of weak to moderate tropospheric forcing (wind speeds in the range 2-12 m/s) and sufficiently strong stratospheric wind speeds (> 10 m/s) to allow vertical propagation