36 research outputs found
Gravity Waves in the Lower Atmosphere in Mountainous Regions and the Role of the Tropopause
Die Hintergrundbedingungen in der AtmosphÀre sind selten konstant und variieren
sowohl zeitlich als auch rÀumlich. Das beeinflusst die Ausbreitung von atmosphÀrischen
Schwerewellen und deren Implus- und Energietransport in der AtmosphÀre. Im Bereich
der unteren AtmosphÀre (TroposphÀre, untere StratosphÀre) finden sich ausgeprÀgte
Dichte- und TemperaturÀnderungen vor allem in der unteren TroposphÀre und an
der Tropopause. Gebirgswellen breiten sich daher nicht nur, wie von der zweidimensionalen und linearen hydrostatischen Lösung der Bewegungsgleichungen beschrieben, vertikal direkt ĂŒber dem Gebirge aus. Sie können an der Grenzschichtinversion in der
unteren TroposphÀre gefangen sein, wobei sie dann als GrenzflÀchenwellen bezeichnet
werden. Anhand von idealisierten Simulationen
wird in dieser Arbeit gezeigt, dass Gebirgswellen auch an der Tropopauseninversion
gefangen sein können, wenn die InversionsstĂ€rke, welche durch die Ănderung der potentiellen Temperatur ĂŒber die Tropopause hinweg beschrieben wird, ausreichend
groĂ ist. Damit wird die GĂŒltigkeit der linearen AbschĂ€tzung zum Auftreten von
gefangenen Wellen auch fĂŒr die Tropopauseninversion bestĂ€tigt. Im Falles eines mit
der Höhe konstanten Windes, muss die StÀrke der Tropopauseninversion aufgrund
der stabiler geschichteten StratosphĂ€re doppelt so groĂ sein wie fĂŒr eine Inversion in
der unteren TroposphÀre. Wenn höhere Windgeschwindigkeiten vorherrschen, muss
die Tropopauseninversion sogar noch stÀrker sein. Die horizontale WellenlÀnge der
GrenzflÀchenwellen reduziert sich mit zunehmender StabilitÀt oberhalb der Inversion.
Des Weiteren zeigen die Simulationen, dass reflektierten Wellen stromabwÀrts des
Gebirges in der TroposphÀre existieren, obwohl der Scorer Parameter nicht mit der
Höhe abnimmt. Ein mit der Höhe abnehmender Scorer Parameter ist die Voraussetzung
fĂŒr den klassischen Fall von gefangenen Wellen in der TroposphĂ€re. Die Amplituden der
reflektierten Wellen in der TroposphĂ€re gröĂer, wenn anstatt des normalen Ăbergangs von troposphĂ€rischer zu stratosphĂ€rischer StabilitĂ€t an der Tropopause eine Tropopauseninversion vorhanden ist. DarĂŒber hinaus vergröĂert sich die horizontale WellenlĂ€nge
der sich ausbreitenden Wellen, der GrenzflÀchenwellen und der reflektierten Wellen bei
höheren Windgeschwindigkeiten
Airborne measurements and large-eddy simulations of small-scale gravity waves at the tropopause inversion layer over Scandinavia
Gravity waves are an important coupling mechanism in the atmosphere. Measurements by two research aircraft during a mountain wave event over Scandinavia in 2016 revealed changes of the horizontal scales in the vertical velocity field and of momentum fluxes in the vicinity of the tropopause inversion. Idealized simulations revealed the presence of interfacial waves. They are found downstream of the mountain peaks, meaning that they horizontally transport momentum/energy away from their source
Multilevel Cloud Structures over Svalbard
The presented picture of the month is a superposition of spaceborne lidar observations and high-resolution temperature fields of the ECMWF Integrated Forecast System (IFS). It displays complex tropospheric and stratospheric clouds in the Arctic winter of 2015/16. Near the end of December 2015, the unusual northeastward propagation of warm and humid subtropical air masses as far north as 80N lifted the tropopause by more than 3 km in 24 h and cooled the stratosphere on a large scale. A widespread formation of thick cirrus clouds near the tropopause and of synoptic-scale polar stratospheric clouds (PSCs) occurred as the temperature dropped below the thresholds for the existence of cloud particles. Additionally, mountain waves were excited by the strong flow at the western edge of the ridge across Svalbard, leading to the formation of mesoscale ice PSCs. The most recent IFS cycle using a horizontal resolution of 8 km globally reproduces the large-scale and mesoscale flow features and leads to a remarkable agreement with the wave structure revealed by the spaceborne observations
Airborne coherent wind lidar measurements of the momentum flux profile from orographically induced gravity waves
In the course of the GW-LCYCLE II campaign, conducted in Jan/Feb 2016 from Kiruna, Sweden, coherent Doppler wind lidar (2â”mâDWL) measurements were performed from the DLR Falcon aircraft to investigate gravity waves induced by flow across the Scandinavian Alps. During a mountain wave event on 28 January 2016, a novel momentum flux (MF) scan pattern with fore and aft propagating laser beams was applied to the 2â”mâDWL. This allows us to measure the vertical wind and the horizontal wind along the flight track simultaneously with a high horizontal resolution of â800âm and hence enables us to derive the horizontal MF profile for a broad wavelength spectrum from a few hundred meters to several hundred kilometers. The functionality of this method and the corresponding retrieval algorithm is validated using a comparison against in situ wind data measured by the High Altitude and Long Range (HALO) aircraft which was also deployed in Kiruna for the POLSTRACC (Polar Stratosphere in a Changing Climate) campaign. Based on that, the systematic and random error of the wind speeds retrieved from the 2â”mâDWL observations are determined. Further, the measurements performed on that day are used to reveal significant changes in the horizontal wavelengths of the vertical wind speed and of the leg-averaged momentum fluxes in the tropopause inversion layer (TIL) region, which are likely to be induced by interfacial waves as recently presented by Gisinger et al. (2020).</p
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 (âMFtrack) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN mâ1 and 1500 m, respectively. Their maxima were phase-shifted to the maximum low-level forcing by â 8 h. Small-scale waves (λx â 20â30 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 â8 h. Small-scale waves (λx â 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 12â15 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
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Gravity waves excited during a minor sudden stratospheric warming
An exceptionally deep upper-air sounding launched from Kiruna airport (67.82ââN, 20.33ââE) on 30 January 2016 stimulated the current investigation of internal gravity waves excited during a minor sudden stratospheric warming (SSW) in the Arctic winter 2015/16. The analysis of the radiosonde profile revealed large kinetic and potential energies in the upper stratosphere without any simultaneous enhancement of upper tropospheric and lower stratospheric values. Upward-propagating inertia-gravity waves in the upper stratosphere and downward-propagating modes in the lower stratosphere indicated a region of gravity wave generation in the stratosphere. Two-dimensional wavelet analysis was applied to vertical time series of temperature fluctuations in order to determine the vertical propagation direction of the stratospheric gravity waves in 1-hourly high-resolution meteorological analyses and short-term forecasts. The separation of upward- and downward-propagating waves provided further evidence for a stratospheric source of gravity waves. The scale-dependent decomposition of the flow into a balanced component and inertia-gravity waves showed that coherent wave packets preferentially occurred at the inner edge of the Arctic polar vortex where a sub-vortex formed during the minor SSW
Gravity-Wave-Driven Seasonal Variability of Temperature Differences Between ECMWF IFS and Rayleigh Lidar Measurements in the Lee of the Southern Andes
Long-term high-resolution temperature data of the Compact Rayleigh Autonomous Lidar (CORAL) is used to evaluate temperature and gravity wave (GW) activity in ECMWF Integrated Forecasting System (IFS) over RĂo Grande (53.79°S, 67.75°W), which is a hot spot of stratospheric GWs in winter. Seasonal and altitudinal variations of the temperature differences between the IFS and lidar are studied for 2018 with a uniform IFS version. Moreover, interannual variations are considered taking into account updated IFS versions. We find monthly mean temperature differences ±10 K) and increase with altitude. We relate this seasonal variability to middle atmosphere GW activity. In the upper stratosphere and lower mesosphere, the observed temperature differences result from both GW amplitude and phase differences. The IFS captures the seasonal cycle of GW potential energy (Ep) well, but underestimates Ep in the middle atmosphere. Experimental IFS simulations without damping by the model sponge for May and August 2018 show an increase in the monthly mean Ep above 45 km from only â10% of the Ep derived from the lidar measurements to 26% and 42%, respectively. GWs not resolved in the IFS are likely explaining the remaining underestimation of the Ep
Sensitivity of Mountain Wave Drag Estimates on Separation Methods and Proposed Improvements
Internal gravity waves (GWs) are ubiquitous in the atmosphere, making significant contributions to the mesoscale motions. Since the majority of their spectrum is unresolved in global circulation models, their effects need to be parameterized. In recent decades GWs have been increasingly studied in high-resolution simulations, which, unlike direct observations, allow us to explore full spatio-temporal variations of the resolved wave field. In our study we analyze and refine a traditional method for GW analysis in a high-resolution simulation on a regional domain around the Drake Passage. We show that GW momentum drag estimates based on the Gaussian high-pass filter method applied to separate GW perturbations from the background are sensitive to the choice of a cutoff parameter. The impact of the cutoff parameter is higher for horizontal fluxes of horizontal momentum, which indicates higher sensitivity for horizontally propagating waves. Two modified methods, which choose the parameter value from spectral information, are proposed. The dynamically determined cutoff is mostly higher than the traditional cutoff values around 500 km, leading to larger GW fluxes and drag, and varies with time and altitude. The differences between the traditional and the modified methods are especially pronounced during events with significant drag contributions from horizontal momentum fluxes