Weather prediction and climate simulations need reliable parameterizations of turbulent fluxes in the stable surface layer. Especially in these conditions, the uncertainties of such parametrizations ...are still large. Most of them rely on the Monin‐Obukhov similarity theory (MOST), for which universal stability functions (SFs) represent important ingredients. The SFs are nonlinear, if so, a numerical iteration of the MOST equations is required. Moreover, presently available SFs are significantly different at large stability. To simplify the calculations, a non‐iterative parametrization of fluxes is derived and corresponding bulk transfer coefficients for momentum and heat for a package of five pairs of state‐of‐the‐art SFs are proposed. For the first time, a parametrization of the related transfer coefficients is derived in a universal framework for all package members. The new parametrizations provide a basis for a cheap systematic study of the impact of surface layer turbulent fluxes in weather prediction and climate models.
Plain Language Summary
Results of weather forecast, present‐day climate simulations, and future climate projections depend among other factors on the interaction between the atmosphere and the underlying sea‐ice, the land, and the ocean. In numerical weather prediction and climate models, some of these interactions are accounted for by transport coefficients describing the turbulent exchange of momentum, heat, and humidity. Currently used transfer coefficients have, however, large uncertainties in flow regimes being typical for cold nights and seasons, but especially in the polar regions. Furthermore, their determination is numerically complex. It is obvious that progress could be achieved when the transfer coefficients would be given by simple mathematical formula in frames of an economic computational scheme. Such a new universal, so‐called non‐iterative parametrization scheme is derived for a package of transfer coefficients. The derivation is based on the Monin‐Obukhov similarity theory, which is well accepted in the scientific community. The new scheme provides a basis for a cheap systematic study of the impact of near‐surface turbulence and of the related transports of momentum, heat, and humidity in models.
Key Points
A non‐iterative universal parameterization of surface layer turbulent fluxes is derived using Monin‐Obukhov similarity theory
Bulk transfer coefficients are given, which are based on five pairs of state‐of‐the‐art surface layer stability functions
The new parametrizations provide a basis for a cheap study of the impact of surface layer turbulent fluxes in numerical weather prediction and climate models
We examine the simulated Arctic sea ice drift speed for
the period 2003–2014 in the coupled Arctic regional climate model
HIRHAM–NAOSIM 2.0. In particular, we evaluate the dependency of the drift
...speed on the near-surface wind speed and sea ice conditions. Considering the
seasonal cycle of the Arctic basin averaged drift speed, the model
reproduces the summer–autumn drift speed well but significantly
overestimates the winter–spring drift speed, compared to satellite-derived
observations. Also, the model does not capture the observed seasonal phase
lag between drift and wind speed, but the simulated drift speed is more in
phase with the near-surface wind. The model calculates a realistic negative
correlation between drift speed and ice thickness and between drift speed
and ice concentration during summer–autumn when the ice concentration is
relatively low, but the correlation is weaker than observed. A daily
grid-scale diagnostic indicates that the model reproduces the observed
positive correlation between drift and wind speed. The strongest impact of
wind changes on drift speed occurs for high and moderate wind speeds, with a
low impact for rather calm conditions. The correlation under low-wind
conditions is overestimated in the simulations compared to
observation/reanalysis data. A sensitivity experiment demonstrates the
significant effects of sea ice form drag from floe edges included by an
improved parameterization of the transfer coefficients for momentum and heat
over sea ice. However, this does not improve the agreement of the modeled
drift speed / wind speed ratio with observations based on reanalysis data for
wind and remote sensing data for sea ice drift. An improvement might be
achieved by tuning parameters that are not well established by observations.
Sea ice leads play an important role in energy exchange between the ocean and atmosphere in polar regions, and therefore must be considered in weather and climate models. As sea ice leads are not ...explicitly resolved in such models, lead‐averaged surface heat flux is of considerable interest for the parameterization of energy exchange. Measurements and numerical studies have established that the lead‐averaged surface heat flux depends not only on meteorological parameters, but also on lead width. Nonetheless, few studies to date have investigated the dependency of surface heat flux on lead width. Most findings on that dependency are based on observations with lead widths smaller than a few hundred meters, but leads can have widths from a few meters to several kilometers. In this parameter study, we present the results of three series of large‐eddy simulations of turbulent exchange processes above leads. We varied the lead width and air temperature, as well as the roughness length. As this study focused on conditions without background wind, ice‐breeze circulation occurred, and was the main driver of the adjustment of surface heat flux. A previous large‐eddy simulation study with uncommonly large roughness length found that lead‐averaged surface heat flux exhibited a distinct maximum at lead widths of about 3 km, while our results show the largest heat fluxes for the smallest leads simulated (lead width of 50 m). At more realistic roughness lengths, we observed monotonously increasing heat fluxes with increasing lead width. Further, new scaling laws for the ice‐breeze circulation are proposed.
Plain Language Summary
In polar sea ice, often channel‐like openings appear due to ocean and atmosphere currents. These openings are like open windows, where huge amount of heat is transported from ocean to atmosphere. The width of the leads range from a few meters up to more than 10 km. Leads cover only a small area of the polar sea ice, but play an important role in the climate system. Generally, the amount of heat transport depends on wind speed and temperature of air and water. Further, measurements and computational model studies have indicated, that the efficiency of heat transport depends strongly on the width of a lead. Nonetheless, few studies to date have investigated the dependency on lead width. Most findings on that are based on observations for leads smaller than a few hundred meters. In this study, we present results of computer simulations with a so‐called large eddy simulation model, which captures the small scale turbulent processes explicitly. We varied the lead width from 50 m to 25 km for the special meteorological conditions without background wind and found remarkably different results compared to a former study on that topic with same type of computational model.
Key Points
Large eddy simulation study on the convection above sea ice leads of variable width under zero background wind conditions
Lead averaged surface heat flux depends strongly on lead width and is controlled by ice breeze circulation
Our findings differ remarkably from results of a former large eddy simulations study on that topic
Realistic modeling of polar sea ice dynamics and atmospheric processes over sea ice needs a detailed representation of the near‐surface atmospheric fluxes of momentum. In this study, parametrizations ...of neutral drag coefficients mostly used in different general circulation models are compared with a recently developed parametrization including the impact of sea ice morphology. The new parametrization, using the sea ice and melt pond fraction as governing parameters, accounts for the effect of form drag caused by edges at leads, melt ponds, and floes. Based on remote sensing data of ice and melt pond fraction, it is shown that during Arctic summer the traditionally used drag coefficients differ from the new ones by a factor 0.5–1.2. The geographic distribution of drag coefficients obtained from both parametrizations is very different. Differences are due to a nonlinear and non‐monotonic dependence of drag coefficients on sea ice concentration in the new parametrization.
Key points
Surface drag distribution in polar regions is not correctly reproduced by GCMs
Parametrizations of drag coefficients over sea ice should account for melt ponds
Remote sensing data of sea ice can be used to determine drag coefficients
The closure problem for the convective turbulence of the shear-free and low to moderate wind atmospheric boundary layer is considered. Non-Gaussian parameterizations are developed for fourth-order ...moments based on a two-scale mass-flux approach. With this approach the ballistic stirring of fluid by coherent structures is taken into account and the differences in the horizontal scales and spacing of the velocity and temperature fields are recognized.
Turbulent heat transport over inhomogeneous surfaces with sharp temperature discontinuities is investigated with a focus on the flow over leads in sea ice. The main goal consists in the development ...of a turbulence closure for a microscale atmospheric model resolving the integrated effect of plumes emanated from leads, but not the individual convective eddies. To this end, 10 runs are carried out with a large eddy simulation (LES) model simulating the flow over leads for springtime atmospheric conditions with near‐neutral inflow and a strong capping inversion. It is found that leads contribute to the stabilizing of the polar atmospheric boundary layer (ABL) and that strong countergradient fluxes of heat exist outside a core region of the plumes. These findings form the basis for the development of the new closure. It uses a new scaling with the internal ABL height and the characteristic vertical velocity for the plume region as the main governing parameters. Results of a microscale model obtained with the new closure agree well with the LES for variable meteorological forcing in case of lead orthogonal flow and for a fixed ABL height and lead width. The good agreement concerns especially the plume inclination, temperature, and heat fluxes as well as the relative contributions of gradient and countergradient transport of heat. A future more general closure should account, for example, for variable lead widths and wind directions. Results of the microscale model could be used to derive a future parameterization of the lead effect in large‐scale models.
The Millionshchikov hypothesis of quasi-normal distribution of fourth-order moments fails for convective conditions where the probability density functions of temperature and vertical velocity ...fluctuations are skewed. This is shown for aircraft and large-eddy simulation (LES) data, and new closures for fourth-order moments that take the skewness into account are suggested. These new closures are in very good agreement with the data.
Gryanik examines vorticity bands and zonal energy of barotropic beta-plane turbulence driven by small-scale stochastic forcing into regimes dominated by quasi-periodic zonal jets. A simple kinetic ...model of multijet beta-plane flows is proposed that explains the shape of the coherent part of zonal energy spectra and asymmetry between westward and eastward jets generated by vorticity bands, and quantifies the zonal wavenumber of jets in terms of the ratio of zonal enstrophy to zonal energy.
The turbulence closure problem for convective boundary layers is considered with the chief aim to advance the understanding and modeling of nonlocal transport due to large-scale semiorganized ...structures.
In this study self-organized periodic coherent vortex structures arising in geophysical turbulent flows at low Rossby number are investigated by developing a conceptual model based on an analytical ...theory of von Kármán vortex streets affected by stratification and differential rotation. In the framework of a quasi-geostrophic (QG) two-layer beta-plane model vortex streets with three different types of vertical structures (barotropic, upper layer and hetonic) are analysed using the point vortex approximation. The streets are found to be exact solutions of the potential vorticity equation and to be characterized by four non-dimensional parameters. Von Kármán streets are semi-localized solutions which form a bridge between vortex pairs (limit of symmetric dilute streets) and two parallel vortex sheets (limit of dense streets). On the beta-plane QG von Kármán streets can only move to the east, i.e. with a speed outside the range of speeds of Rossby waves, so that a dynamical asymmetry in the zonal direction is introduced. A complete classification on a diagram of states shows that critical bounds exist in the parameter space, prescribing for example a maximum distance between vortex rows beyond which no QG vortex streets can be found. Typically a fast and a slow vortex street with different flow structures are found in the region of existence. As a function of distance between vortex rows baroclinic QG vortex streets show a characteristic non-monotonic speed behaviour at scales of the order of the baroclinic Rossby radius. A wide region of possible existence of QG von Kármán streets is found in atmospheric, oceanic and planetary conditions as well as in rotating tank experiments. The theory can be applied to describe the coherent part of turbulent baroclinic intermittent zonal jet-like and frontal flows and provides a scaling for such flows.
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