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Volkmar Wirth
,
Christof Appenzeller
, and
Martin Juckes

Abstract

The quasi-horizontal roll-up of unstable stratospheric intrusions into isolated vortices is known to result in specific structures on satellite water vapor images that are characterized by intermingling dark and light filaments. The current paper investigates how these features are generated and how they relate to partly similar features found on concurrent maps of the tropopause height or potential vorticity (PV). The roll-up of a stratospheric intrusion is simulated numerically with an idealized quasigeostrophic model, which focuses on the dynamics induced by anomalies in the height of the tropopause. The upper-tropospheric adiabatic vertical wind is calculated explicitly and is used to simulate water vapor images in the model. These images show qualitatively the same characteristic features as observed. They are generated through a combination of horizontal advection of initial moisture anomalies and the creation of additional moisture anomalies resulting from the upper-tropospheric vertical air motion. The latter is, in turn, induced by the quasi-horizontal motion of the tropopause anomaly. It is suggested that a substantial portion of the spiral-like structures on the water vapor images is likely to reflect the vertical wind induced by the evolution of the intrusion itself. When the tropopause is defined through a fairly low value of PV, it may acquire similar spiraling structures, as it is being advected almost like a passive tracer. On the other hand, for the dynamically active core part of the intrusion, which is located at higher values of PV, one may expect an evolution leading to more compact vortex cores and less structure overall.

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Günther Zängl
,
Joseph Egger
, and
Volkmar Wirth

Abstract

The Penn State–NCAR mesoscale model MM5 is used to simulate and better understand the wind observations in the Kali Gandaki Valley reported in the first part of this paper. The Kali Gandaki River originates in Nepal near Tibet, flows southward through the Mustang Basin, crosses the Himalayas in a gorge, and descends to the lowlands of Nepal. Extremely strong diurnal upvalley flow in the gorge and the basin alternates with rather weak drainage flow in the night. As proposed in Part I, the Mustang Basin and the Tibetan Plateau can be considered as an elevated heat source driving the upvalley flow during the day. However, the extreme strength of the diurnal upvalley winds and the order-of-magnitude asymmetry between day and night cannot be explained with a simple plateau circulation theory.

The model is successful in simulating almost all aspects of the observations. The simulations strongly suggest that the observed acceleration of the upvalley winds near the entrance to the Mustang Basin is linked to a supercritical-like flow pattern. Gravity waves induced by the ridges protruding into the valley appear to contribute to this flow structure. Humidity is found to be essential for simulating the strength of the observed day–night asymmetry because of its impact on the boundary layer structure above the Himalayan foothills, especially due to the evaporation of rain. In addition, advection of relatively stable air from the foreland into the basin is important for the formation of the gravity waves and also explains part of the asymmetry. The Plateau of Tibet appears to have a small but positive impact on the flow speeds in the valley.

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Volkmar Wirth
and
Timothy J. Dunkerton

Abstract

This paper provides a unified perspective on the dynamics of hurricane- and monsoonlike vortices by identifying them as specific limiting cases of a more general flow system. This more general system is defined as stationary axisymmetric balanced flow of a stably stratified non-Boussinesq atmosphere on the f plane. The model is based on the primitive equations assuming gradient wind balance in the radial momentum equation. The flow is forced by heating in the vortex center, which is implemented as relaxation toward a specified equilibrium temperature Te . The flow is dissipated through surface friction, and it is assumed to be almost inviscid in the interior. The heating is assumed supercritical, which means that Te does not allow a regular thermal equilibrium solution with zero surface wind, and which gives rise to a cross-vortex secondary circulation. Numerical solutions are obtained using time stepping to a steady state, where at each step the Eliassen secondary circulation is diagnosed as part of the solution strategy.

Reality and regularity of the solution is discussed, putting this work in relation to previous work. Scaling analysis suggests that for a given geometry, essential vortex properties are controlled by the ratio F = αT /cD , where αT is the rate of thermal relaxation and cD quantifies the strength of surface friction for a given surface wind. For large F, the temperature is close to Te and the vortex shows properties that can be associated with a hurricane including strong cyclonic surface winds. On the other hand, for small F, the vortex shows properties that can be associated with a monsoon; that is, the surface winds are small and the secondary circulation keeps the temperature significantly away from Te . The scaling analysis is verified by numerical solutions spanning a wide range of the parameter space. It is shown how the two limiting cases correspond with the respective approximate semianalytical theories presented previously. The results imply an important role of αT for hurricane formation.

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Volkmar Wirth
and
Timothy J. Dunkerton

Abstract

This paper investigates the occurrence, formation, and maintenance of eyes in idealized axisymmetric balanced vortices with diabatic forcing. Two key elements of the model setup are temperature relaxation toward a specified equilibrium temperature Te and Ekman pumping from a turbulent boundary layer. Furthermore, the flow is assumed to be almost inviscid in the interior. The model does not attempt any closure for moist convection. Previous work by the authors has shown that there is a continuous transition from monsoonlike vortices to hurricane-like vortices. This transition is governed by the ratio F = αT /cD , where αT is the thermal relaxation rate and cD the surface drag coefficient.

An eye is defined in terms of the vertical wind with maximum upwelling occurring at some finite radius rather than at the origin. It is possible to obtain an eye even though Te maximizes at the origin, that is, even though Te does not directly predispose upwelling at some finite radius. The occurrence of an eye is controlled by F, and the transition between vortices without any eye and vortices with a clearly defined eye is rather sudden. These results are robust with respect to the amplitude of the forcing or the specific shape of Te . The key role of F is corroborated through a systematic nondimensionalization. In a steady-state hurricane-like vortex, mechanical forcing from Ekman pumping maximizes at some finite radius and is instrumental for the maintenance of an eyelike secondary circulation. On the other hand, eye formation during spinup is a purely inviscid process. The results imply that eye formation is a robust and general feature in vortices with strong diabatic forcing.

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Andre R. Erler
and
Volkmar Wirth

Abstract

The tropopause inversion layer (TIL) is a region of enhanced static stability just above the WMO-defined thermal tropopause. It is a ubiquitous feature in midlatitudes and is well characterized by observations. However, it is still lacking a satisfactory theoretical explanation.

This study utilizes adiabatic baroclinic life cycle experiments to investigate dynamical mechanisms that lead to TIL formation. As the baroclinic wave grows, a strong TIL forms above anticyclonic anomalies, while no TIL is found above cyclonic anomalies; this is consistent with previous results. However, during the early growth phase there is no TIL in the global or zonal average: positive and negative anomalies cancel out exactly. The zonal and global mean TIL only emerges during the mature stage of the life cycle, after the onset of wave breaking. The TIL predominantly occurs equatorward of the jet and the vertical structure bears resemblance to the TIL in midlatitudes; there is no equivalent to the subpolar TIL. Life cycles without significant wave breaking develop neither a global nor a zonal mean TIL. No global mean TIL is found in any life cycle if the dynamical tropopause definition is used.

In addition, a new mechanism of dynamical TIL formation is presented, suggesting that the TIL in the global and zonal mean is linked to a strongly skewed distribution of relative vorticity after wave breaking.

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Marius Levin Thomas
and
Volkmar Wirth

Abstract

Banner clouds are clouds in the lee of steep mountains or sharp ridges on otherwise cloud-free days. Previous studies investigated various aspects of banner cloud formation in numerical simulations, most of which were based on idealized orography and a neutrally stratified ambient atmosphere. The present study extends these simulations in two important directions by 1) examining the impact of various types of orography ranging from an idealized pyramid to the realistic orography of Mount Matterhorn and 2) accounting for an ambient atmosphere that turns from neutral to stably stratified below the mountain summit. Not surprisingly, realistic orography introduces asymmetries in the spanwise direction. At the same time, banner cloud occurrence remains associated with a coherent area of strong uplift, although this region does not have to be located exclusively in the lee of the mountain any longer. In the case of Mount Matterhorn with a westerly ambient flow, a large fraction of air parcels rises along the southern face of the mountain, before they reach the lee and are lifted into the banner cloud. The presence of a shallow boundary layer with its top below the mountain summit introduces more complex behavior compared to a neutrally stratified boundary layer; in particular, it introduces a dependence on wind speed, because strong wind is associated with strong turbulence that is able to raise the boundary layer height and, thus, facilitates the formation of a banner cloud.

Open access
Paolo Ghinassi
,
Georgios Fragkoulidis
, and
Volkmar Wirth

Abstract

Upper-tropospheric Rossby wave packets (RWPs) are important dynamical features, because they are often associated with weather systems and sometimes act as precursors to high-impact weather. The present work introduces a novel diagnostic to identify RWPs and to quantify their amplitude. It is based on the local finite-amplitude wave activity (LWA) of Huang and Nakamura, which is generalized to the primitive equations in isentropic coordinates. The new diagnostic is applied to a specific episode containing large-amplitude RWPs and compared with a more traditional diagnostic based on the envelope of the meridional wind. In this case, LWA provides a more coherent picture of the RWPs and their zonal propagation. This difference in performance is demonstrated more explicitly in the framework of an idealized barotropic model simulation, where LWA is able to follow an RWP into its fully nonlinear stage, including cutoff formation and wave breaking, while the envelope diagnostic yields reduced amplitudes in such situations.

Open access
Gabriel Wolf
and
Volkmar Wirth

Abstract

It has been suggested that upper-tropospheric Rossby wave packets propagating along the midlatitude waveguide may play a role for triggering severe weather. This motivates the search for robust methods to detect and track Rossby wave packets and to diagnose their properties. In the framework of several observed cases, this paper compares different methods that have been proposed for these tasks, with an emphasis on horizontal propagation and on a particular formulation of a wave activity flux previously suggested by Takaya and Nakamura. The utility of this flux is compromised by the semigeostrophic nature of upper-tropospheric Rossby waves, but this problem can partly be overcome by a semigeostrophic coordinate transformation. The wave activity flux allows one to obtain information from a single snapshot about the meridional propagation, in particular propagation from or into polar and subtropical latitudes, as well as about the onset of wave breaking. This helps to clarify the dynamics of individual wave packets in cases where other, more conventional methods provide ambiguous or even misleading information. In some cases, the “true dynamics” of the Rossby wave packet turns out to be more complex than apparent from the more conventional diagnostics, and this may have important implications for the predictability of the wave packet.

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Paolo Ghinassi
,
Marlene Baumgart
,
Franziska Teubler
,
Michael Riemer
, and
Volkmar Wirth

Abstract

Recently, the authors proposed a novel diagnostic to quantify the amplitude of Rossby wave packets. This diagnostic extends the local finite-amplitude wave activity (LWA) of N. Nakamura and collaborators to the primitive-equations framework and combines it with a zonal filter to remove the phase dependence. In the present work, this diagnostic is used to investigate the dynamics of upper-tropospheric Rossby wave packets, with a particular focus on distinguishing between conservative dynamics and nonconservative processes. For this purpose, a budget equation for filtered LWA is derived and its utility is tested in a hierarchy of models. Idealized simulations with a barotropic and a dry primitive-equation model confirm the ability of the LWA diagnostic to identify nonconservative local sources or sinks of wave activity. In addition, the LWA budget is applied to forecast data for an episode in which the amplitude of an upper-tropospheric Rossby wave packet was poorly represented. The analysis attributes deficiencies in the Rossby wave packet amplitude to the misrepresentation of diabatic processes and illuminates the importance of the upper-level divergent outflow as a source for the error in the wave packet amplitude.

Open access
Joseph Egger
,
Sapta Bajrachaya
,
Ute Egger
,
Richard Heinrich
,
Joachim Reuder
,
Pancha Shayka
,
Hilbert Wendt
, and
Volkmar Wirth

Abstract

The diurnal wind system of the Kali Gandaki Valley in Nepal was explored in September and October 1998 in a field campaign using pilot balloons as the main observational tool. This valley connects the Plateau of Tibet with the Indian plains. The river crosses the Himalayas forming the deepest valley on Earth. Intense upvalley winds blow up this valley during the day. Observations were made along the river at various spots selected between the exit point from the Himalayas and the source close to the Plateau of Tibet. The strongest upvalley winds were found between Marpha and Chuksang with typical speeds of 15–20 m s−1. The upvalley wind sets in first at the ground but an upvalley wind layer of 1000–2000-m depth forms quickly after the onset. This deep inflow layer persists up to Lo Manthang, a town located a few kilometers south of the Plateau of Tibet. Deceleration in the late afternoon and evening also appears to commence near the ground. Weak drainage flow forms late in the night. The causes of these phenomena are discussed.

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