Ekman Friction and the Formation of Upper Tropospheric Zonal Flows
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n Friction and the Formation of Upper Tropospheric Zonal Flows M. V. Kalashnika, b, * a
Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, 109017 Russia bNPO Taifun, Obninsk, 249038 Russia *e-mail: [email protected] Received March 3, 2020; revised May 12, 2020; accepted June 3, 2020
Abstract—The influence of the Ekman friction on the dynamics of zonal flows (ZFs) has been studied within the framework of a quasigeostrophic model of the atmosphere with two horizontal boundaries (the underlying surface and the tropopause). It is assumed that these flows have zero potential vorticity and are caused by specified buoyancy distributions at the boundaries. It is shown that, in the case of periodic distributions, the oppositely directed vertical velocity profile of ZFs transforms into a unidirectional profile with a maximum velocity at the upper boundary and zero velocity at the lower boundary. During this transformation, the velocity at the upper boundary increases; i.e., the upper tropospheric ZFs intensify due to the Ekman friction. A similar intensification occurs also in the case of initial distributions of buoyancy of the frontal type, which induce a system of two oppositely directed jet flows located in the upper and lower halves of the atmospheric layer. Due to the Ekman friction, the axial velocity of the lower flow drops to zero and the velocity of the upper flow, gradually covering the entire troposphere, doubles. The resulting flow is a jet pressed against the upper boundary, which may be considered a prototype of a western upper tropospheric jet flow. The important structural features of such a jet, which are established within the framework of a complete nongeostrophic model, are associated with horizontal jet asymmetry and the formation of fronts (discontinuity surfaces) adjacent to the upper boundary. Keywords: Ekman boundary layer, bottom friction, upper tropospheric jet flows, surface geostrophic model DOI: 10.1134/S0001433820050059
1. INTRODUCTION In geophysical hydrodynamics, the atmosphere is conventionally represented as a layer of stratified rotating fluid which is enclosed between two horizontal boundaries: the underlying surface and the tropopause [1–4]. Friction against the underlying surface (land surface) leads to the formation of the Ekman boundary layer, which significantly affects the dynamics and stability of atmospheric flows due to the Ekman pumping mechanism [5–9]. It is believed that this mechanism results in damped motions in the free atmosphere. However, this is not the case, because motions damp only in the vicinity of the surface. In the vicinity of the upper boundary (tropopause), the velocity is not affected by the Ekman friction; it is determined by the meridional distribution of temperature. This may be associated with the existence of intensive upper tropospheric jet flows at heights of 8– 10 km (the tropopause level), which were first discovered by military pilots at the end of World War II [10, 11]. The maximum velocity of jet flows may reach 10
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