No abstract

No abstract

Turbulent rotors in the lower troposphere are usually associated with high‐amplitude lee waves. Two types of rotors have been observed. The first type, often visible as harmless‐looking cumulus or cumulus fractus lines paralleling the mountain range, comprises a well‐defined circulation under the crests of resonant mountain waves. This type of rotor contains moderate or severe turbulence and is often confined below the height of a frequently‐observed upstream, near‐mountain‐top inversion. A second, less common, rotor type extends much higher than the upstream inversion. This type has been observed to contain severe or extreme turbulence, and is thought to be associated with a high‐amplitude mountain‐wave system resembling a hydraulic jump. Both rotor types present a hazard to aviation, although the second type of rotor is far more dangerous. We present high‐resolution two‐dimensional simulations of two distinct rotor types, which reveal dramatic differences in internal structure and turbulence intensity. The evolution of the lee‐side horizontal vorticity provides insight into the formation mechanism involved. Horizontal vorticity within the initial upstream inversion is modified due to baroclinic generation as the flow spills down the lee slope. The magnitude of the modified horizontal vorticity is regulated by shear in the initial upstream inversion. At the same time, horizontal vorticity of the opposite sign forms in a shallow surface layer. The type of rotor that forms depends on the sign of the dominant horizontal vorticity as near‐surface flow separation occurs along the lee slope. The simulations point to the vital role of an upstream near‐mountain‐top inversion, its deformation in the lee of the mountain, and the initial vertical shear within the inversion in rotor formation.

No abstract

No abstract

No abstract

The generation and organization of mesoscale convective vortices (MCVs) is a recurring theme in midlatitude and tropical meteorology during the warm season. In this work a simulation of a finite-length idealized convective line in a westerly shear environment is investigated in the absence of ambient vertical vorticity. An asymmetry in average vertical vorticity forms rapidly at early times in the present simulation. This study focuses on the formation and organization of vertical vorticity at these early simulation times. Previous simulations suggest that tilting of either ambient or storm-generated horizontal vorticity is the primary mechanism responsible for the formation, organization, and maintenance of MCVs. This study confirms recent work regarding the generation of vertical vorticity at early times in the simulation. A Lagrangian budget analysis of the vertical vorticity equation, however, shows that vorticity convergence becomes a comparable, and at times dominant, mechanism for the enhancement and long-term organization of vertical vorticity early in the simulation. Despite differences in the initial ambient horizontal vorticity, hodograph, and convective available potential energy, the Lagrangian budget analysis in the present midlatitude case is consistent with the Lagrangian budget results of a previous tropical squall line simulation. The study of idealized convective lines in midlatitude environmental conditions therefore provide valuable insight into understanding vertical vorticity production in tropical squall lines and their potential relevance to tropical cyclogenesis.

This paper contains a collection of English translations of twenty-one of Hans Ertel's papers on geophysical fluid dynamics. The selected papers were originally published between 1942 and 1970 in either German or Spanish. This collection includes the four classic 1942 papers on vorticity and potential vorticity conservation principles and also papers on generalized conservation relations, hydrodynamic commutation formulas, Clebsch and Weber transformations, and isogons and isotachs in two-dimensional flows.