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Convective cloudiness in the Atlantic sector of the Arctic is considered as an atmospheric spatially self-organized convective field. Convective cloud development is usually studied as a local process reflecting the convective instability of the turbulent planetary boundary layer over a heated surface. The convective cloudiness has a different dynamical structure in high latitudes. Cloud development follows cold-air outbreaks into the areas with a relatively warm surface. As a result, the physical and morphological characteristics of clouds, such as the type of convective cloud, and their geographical localization are interrelated. It has been shown that marginal sea ice and coastal zones are the most frequently occupied by Cu hum, Cu med convective clouds, which are organized in convective rolls. Simultaneously, the open water marine areas are occupied by Cu cong, Cb, which are organized in convective cells. An intercomparison of cloud statistics using satellite data ISCCP and ground-based observations has revealed an inconsistency in the cloudiness trends in these data sources: convective cloudiness decreases in ISCCP data and increases in the groundbased observation data. In general, according to the stated hypothesis, the retreat of the sea-ice boundary may lead to an increase in the amount of convective clouds. [ABSTRACT FROM AUTHOR]

Here we advance the physical background of the energy- and flux-budget turbulence closures based on the budget equations for the turbulent kinetic and potential energies and turbulent fluxes of momentum and buoyancy, and a new relaxation equation for the turbulent dissipation time scale. The closure is designed for stratified geophysical flows from neutral to very stable and accounts for the Earth's rotation. In accordance with modern experimental evidence, the closure implies the maintaining of turbulence by the velocity shear at any gradient Richardson number Ri, and distinguishes between the two principally different regimes: 'strong turbulence' at $${Ri \ll 1}$$ typical of boundary-layer flows and characterized by the practically constant turbulent Prandtl number Pr; and 'weak turbulence' at Ri > 1 typical of the free atmosphere or deep ocean, where Pr asymptotically linearly increases with increasing Ri (which implies very strong suppression of the heat transfer compared to the momentum transfer). For use in different applications, the closure is formulated at different levels of complexity, from the local algebraic model relevant to the steady-state regime of turbulence to a hierarchy of non-local closures including simpler down-gradient models, presented in terms of the eddy viscosity and eddy conductivity, and a general non-gradient model based on prognostic equations for all the basic parameters of turbulence including turbulent fluxes. [ABSTRACT FROM AUTHOR]

A band of cooling that extends across mid-latitude Eurasia is identified in the wintertime surface air temperatures of the latest ECMWF reanalysis. This cooling is related to extreme warming around the Kara Sea through changes in the meridional temperature gradient. Surface temperatures in the Arctic have risen faster than those at lower latitudes, and as the Arctic warming increases, this north-south temperature gradient is weakened. This change in the meridional temperature gradient causes a decrease in the westerly winds that help maintain the mild European climate by transporting heat from the Atlantic. Since decreasing sea ice concentrations have been shown to be a driving factor in Arctic amplification, a singular value decomposition analysis is used to confirm the co-variability of the Arctic sea ice, including the Kara Sea, and the temperatures over the mid-latitude Eurasia. These findings suggest that decreasing sea ice concentrations can change the meridional temperature gradient and hence the large-scale atmospheric flow of the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

The meridional energy transport into high latitudes of the Northern Hemisphere is an important climate-forming factor in the Arctic. This work presents the results of calculating the meridional energy flux across 70° N based on the Integrated Global Radiosonde Archive (IGRA) data from the radio sounding of the atmosphere. The long-term mean energy flux over the period 1992-2007 in the layer from the Earth's surface to 30 hPa is 70.6 W m. The fraction of the sensible heat flux is 23.2 W m, i.e., 33% of the total energy flux; the fraction of the latent heat flux is 28.0 W m (40% of the total energy flux); the fraction of the potential energy is 20.0 W m (27%); and the fraction of the kinetic energy is 0.53 W m, i.e., less than 1% of the total energy flux. The vertical structure of the flux shows that the main energy transport into the Arctic takes place in the middle troposphere-lower stratosphere layer, whereas the energy is transported mainly out of the Arctic in the lower troposphere, which agrees well with the schematic notion about the polar circulation cell. The spatial structure of the flux shows that the key regions with a positive (directed into the Arctic) energy flux are located in the vicinity of 160° E (the northwestern part of Eurasia, Pacific sector) and 50° W (Greenland sector). The regions with a negative (directed out of the Arctic) energy flux are located near 120° W (Canadian Arctic Archipelago) and from 20° E to 90° E (Atlantic sector). In the period from 1992 to 2007, the meridional energy transport into the Arctic weakened by −0.26 W m yr. The changes were mutually correlated; namely, positive and negative energy fluxes weakened in amplitude, almost without changing their locations. [ABSTRACT FROM AUTHOR]

We give a new derivation of the familiar linear relation for the dimensionless velocity gradient in the stably stratified surface layer and provide physical and empirical grounds for its universal applicability in stationary homogeneous turbulence over the whole range of static stabilities from Ri = 0 to very large Ri. Combining this relation with the budget equation for the turbulent kinetic energy we obtain the “equilibrium formulation” of the turbulent dissipation length scale, and recommend it for use in turbulence closure models. [ABSTRACT FROM AUTHOR]

Theoretical analysis of boundary layer turbulence has suggested a feasibility of sufficiently accurate turbulence resolving simulations at relatively coarse meshes. However, large eddy simulation (LES) codes, which employ traditional eddy-viscosity turbulence closures, fail to provide adequate turbulence statistics at coarse meshes especially within a surface layer. Manual tuning of parameters in these turbulence closures may correct low order turbulence statistics but severely harms spectra of turbulence kinetic energy (TKE). For more than decade, engineering LES codes successfully employ dynamic turbulence closures. A dynamic Smagorinsky turbulence closure (DSM) has been already tried in environmental LES. The DSM is able to provide adequate turbulence statistics at coarse meshes but it is not completely consistent with the LES equations. This paper investigates applicability of an advanced dynamic mixed turbulence closure (DMM) to simulations of Ekman boundary layers of high Reynolds number flows. The DMM differs from the DSM by explicit calculation of the Leonard term in the turbulence stress tensor. The Horizontal Array Turbulence Study (HATS) field program has revealed that the Leonard term is indeed an important component of the real turbulence stress tensor. This paper presents validation of a new LES code LESNIC. The study shows that the LES code with the DMM provides rather accurate low order turbulence statistics and the TKE spectra at very coarse meshes. These coarse LES maintain more energetic small scale fluctuations of velocity especially within the surface layer. This is critically important for success of simulations. Accurate representation of higher order turbulence statistics, however, requires essentially better LES resolution. The study also shows that LES of the Ekman boundary layer cannot be directly compared with conventionally neutral atmospheric boundary layers. The depth of the boundary layer is an important scaling parameter for turbulence statistics. [ABSTRACT FROM AUTHOR]

Two types of neutral planetary boundary layer (PBL) are distinguished: truly neutral – developed against a neutrally stratified free flow, and conventionally neutral – developed against a background stable stratification. Atmospheric PBLs treated as neutral are almost always conventionally neutral. Theoretical reasoning and results from large-eddy simulation (LES) show that A and B coefficients of the Rossby-number similarity theory are not constants. The same is true for the coefficient Ch in the Rossby–Montgomery formula for the neutral boundary-layer depth h = Chu*/|f|, where u* is the friction velocity. Contrary to classical ideas, A, B and Ch depend on the ratio μN ≡ N/|f| of the free-flow Brunt–V*auml;isäl ä frequency N to the absolute value of the Coriolis parameter |f|. This new development can explain why atmospheric and LES estimates of A, B and Ch appear inconsistent. It results from neglecting the fact that atmospheric data for μN ∼ 102 were compared with LES data for μN = 0, violating an obvious requirement of similarity with respect to μN. [ABSTRACT FROM AUTHOR]