Turbulentne strukture većih dimenzija u Ekmanovom graničnom sloju

The Ekman boundary layer (EBL) is a non-stratifi ed turbulent layer of fluid in a rotated frame of reference. The EBL comprises two sub-layers, namely, the surface sub-layer, where small scale well-developed turbulence dominates, and the core sub-layer, where large scale self-organized turbulence dominates. This study reports self-organization of large scale turbulence in the EBL as simulated with the large-eddy simulation (LES) model LESNIC. The simulations were conducted in a large domain (144 km in the cross-flow direction, which is an equivalent to about 50 EBL depths) to resolve statistically signifi cant number of the largest self-organized eddies. Analysis revealed that the latitude of the LES domain and, unexpectedly, the direction of the geostrophic wind forcing control the self-organization, turbulence scales, evolution and the quasi steady-state averaged vertical profi les in the EBL. The LES demonstrated destabilization of the EBL turbulence and its mean structure by the horizontal component of the Coriolis force. Visualisations of the EBL disclosed existence of quasi-regular large scale turbulent structures composed of counter-rotating vortices when the geostrophic fl ow was set from East to West. The corresponding structures are absent in the EBL when the geostrophic fl ow was set in the opposite (i.e. West to East) direction. These results fi nally resolve the long-standing controversy between the Leibovich-Lele and the Lilly-Brown instability mechanisms acting in the EBL. The LES demonstrated that the Lilly-Brown mechanism, which involves the vertical component of the Coriolis force, is working in the polar EBL where its impact is nevertheless rather small. The Leibovich-Lele mechanism, which involves the horizontal component of the Coriolis force, acts in low latitudes where it completely alters the turbulent structure of the EBL.Ekmanov granični sloj (EGS) je nestratificirani turbulentni sloj u rotirajućem fluidu. EGS se učestalo opaža i u atmosferi i u oceanu. Sastoji se od dva podsloja: od površinskog,u kojem dominira dobro razvijena turbulencija male skale, i središnjeg, u kojem dominira samoorganizirana turbulencija veće skale. Ova studija prikazuje samoorganiziranuturbulenciju veće skale u EGS-u, koja je simulirana modelom velikihx vrtloga (LES)nazvanim LESNIC. Kako bi se odredio statistički značajan broj najvećih samoorganiziranih vrtloga, simulacije su rađene na velikoj domeni (144 km u smjeru okomitom na strujanje, što približno odgovara pedeseterostrukoj debljini EGS-a). Analiza je pokazala dazemljopisna širina LES domene i, neočekivano, smjer forsirajućeg geostrofičkog vjetra kontroliraju samoorganiziranost, skale turbulencije, razvoj i kvazistacionarno stanjeusrednjenih vertikalnih profila u EGS-u. LES rezultati su pokazali destabilizaciju EGSturbulencije i njene srednje strukture s horizontalnom komponentom Coriolisove sile.Vizualizacija EGS-a je otkrila postojanja skoro pravilnih turbulentnih struktura velikeskale, koje su se u slučaju promjene geostrofičkog vjetra od istočnog u zapadni, sastojaleod vrloga sa suprotnom rotacijom. Odgovarajuće strukture nisu bile prisutne u EGS-uukoliko je vjetar mijenjan u suprotnom smjeru (od zapada prema istoku). Ovi rezultati ukonačnici rješavaju dugo prisutnu polemiku između Leibovich-Leleovog i Lilly-Brownovogmehanizma nestabilnosti, koji djeluju u EGS-u. LES simulacije pokazuju da Lilly-Brownovmehanizam, koji uključuje vertikalnu komponentu Coriolisove sile, vrijedi u polarnomEGS-u, gdje je njegov utjecaj ipak malen. Leibovich-Leleov mehanizam, koji uključujehorizontalnu komponentu Coriolisove sile, djeluje na nižim zemljopisnim širinama, gdjeu potpunosti mijenja turbulentnu strukturu EGS

Turbulentne strukture većih dimenzija u Ekmanovom graničnom sloju

The Ekman boundary layer (EBL) is a non-stratifi ed turbulent layer of fluid in a rotated frame of reference. The EBL comprises two sub-layers, namely, the surface sub-layer, where small scale well-developed turbulence dominates, and the core sub-layer, where large scale self-organized turbulence dominates. This study reports self-organization of large scale turbulence in the EBL as simulated with the large-eddy simulation (LES) model LESNIC. The simulations were conducted in a large domain (144 km in the cross-flow direction, which is an equivalent to about 50 EBL depths) to resolve statistically signifi cant number of the largest self-organized eddies. Analysis revealed that the latitude of the LES domain and, unexpectedly, the direction of the geostrophic wind forcing control the self-organization, turbulence scales, evolution and the quasi steady-state averaged vertical profi les in the EBL. The LES demonstrated destabilization of the EBL turbulence and its mean structure by the horizontal component of the Coriolis force. Visualisations of the EBL disclosed existence of quasi-regular large scale turbulent structures composed of counter-rotating vortices when the geostrophic fl ow was set from East to West. The corresponding structures are absent in the EBL when the geostrophic fl ow was set in the opposite (i.e. West to East) direction. These results fi nally resolve the long-standing controversy between the Leibovich-Lele and the Lilly-Brown instability mechanisms acting in the EBL. The LES demonstrated that the Lilly-Brown mechanism, which involves the vertical component of the Coriolis force, is working in the polar EBL where its impact is nevertheless rather small. The Leibovich-Lele mechanism, which involves the horizontal component of the Coriolis force, acts in low latitudes where it completely alters the turbulent structure of the EBL.Ekmanov granični sloj (EGS) je nestratificirani turbulentni sloj u rotirajućem fluidu. EGS se učestalo opaža i u atmosferi i u oceanu. Sastoji se od dva podsloja: od površinskog,u kojem dominira dobro razvijena turbulencija male skale, i središnjeg, u kojem dominira samoorganizirana turbulencija veće skale. Ova studija prikazuje samoorganiziranuturbulenciju veće skale u EGS-u, koja je simulirana modelom velikihx vrtloga (LES)nazvanim LESNIC. Kako bi se odredio statistički značajan broj najvećih samoorganiziranih vrtloga, simulacije su rađene na velikoj domeni (144 km u smjeru okomitom na strujanje, što približno odgovara pedeseterostrukoj debljini EGS-a). Analiza je pokazala dazemljopisna širina LES domene i, neočekivano, smjer forsirajućeg geostrofičkog vjetra kontroliraju samoorganiziranost, skale turbulencije, razvoj i kvazistacionarno stanjeusrednjenih vertikalnih profila u EGS-u. LES rezultati su pokazali destabilizaciju EGSturbulencije i njene srednje strukture s horizontalnom komponentom Coriolisove sile.Vizualizacija EGS-a je otkrila postojanja skoro pravilnih turbulentnih struktura velikeskale, koje su se u slučaju promjene geostrofičkog vjetra od istočnog u zapadni, sastojaleod vrloga sa suprotnom rotacijom. Odgovarajuće strukture nisu bile prisutne u EGS-uukoliko je vjetar mijenjan u suprotnom smjeru (od zapada prema istoku). Ovi rezultati ukonačnici rješavaju dugo prisutnu polemiku između Leibovich-Leleovog i Lilly-Brownovogmehanizma nestabilnosti, koji djeluju u EGS-u. LES simulacije pokazuju da Lilly-Brownovmehanizam, koji uključuje vertikalnu komponentu Coriolisove sile, vrijedi u polarnomEGS-u, gdje je njegov utjecaj ipak malen. Leibovich-Leleov mehanizam, koji uključujehorizontalnu komponentu Coriolisove sile, djeluje na nižim zemljopisnim širinama, gdjeu potpunosti mijenja turbulentnu strukturu EGS

Wind Climate in Kongsfjorden, Svalbard, and Attribution of Leading Wind Driving Mechanisms through Turbulence-Resolving Simulations

This paper presents analysis of wind climate of the Kongsfjorden-Kongsvegen valley, Svalbard. The Kongsfjorden-Kongsvegen valley is relatively densely covered with meteorological observations, which facilitate joint statistical analysis of the turbulent surface layer structure and the structure of the higher atmospheric layers. Wind direction diagrams reveal strong wind channeled in the surface layer up to 300 m to 500 m. The probability analysis links strong wind channeling and cold temperature anomalies in the surface layer. To explain these links, previous studies suggested the katabatic wind flow mechanism as the leading driver responsible for the observed wind climatology. In this paper, idealized turbulence-resolving simulations are used to distinct between different wind driving mechanisms. The simulations were performed with the real surface topography at resolution of about 60 m. These simulations resolve the obstacle-induced turbulence and the turbulence in the non-stratified boundary layer core. The simulations suggest the leading roles of the thermal land-sea breeze circulation and the mechanical wind channeling in the modulation of the valley winds. The characteristic signatures of the developed down-slope gravity-accelerated flow, that is, the katabatic wind, were found to be of lesser significance under typical meteorological conditions in the valley

Vertical structure of recent arctic warming from observed data and reanalysis products

The final publication is available at Springer via http://dx.doi.org/10.1007/s10584-011-0192-8Spatiotemporal patterns of recent (1979–2008) air temperature trends are evaluated using three reanalysis datasets and radiosonde data. Our analysis demonstrates large discrepancies between the reanalysis datasets, possibly due to differences in the data assimilation procedures as well as sparseness and inhomogeneity of high-latitude observations. We test the robustness of Arctic tropospheric warming based on the ERA-40 dataset. ERA-40 Arctic atmosphere temperatures tend to be closer to the observed ones in terms of root mean square error compare to other reanalysis products used in the article. However, changes in the ERA-40 data assimilation procedure produce unphysical jumps in atmospheric temperatures, which may be the likely reason for the elevated tropospheric warming trend in 1979-2002. NCEP/NCAR Reanalysis show that the near-surface upward temperature trend over the same period is greater than the tropospheric trend, which is consistent with direct radiosonde observations and inconsistent with ERA-40 results. A change of sign in the winter temperature trend from negative to positive in the late 1980s is documented in the upper troposphere/lower stratosphere with a maximum over the Canadian Arctic, based on radiosonde data. This change from cooling to warming tendency is associated with weakening of the stratospheric polar vortex and shift of its center toward the Siberian coast and possibly can be explained by the changes in the dynamics of the Arctic Oscillation. This temporal pattern is consistent with multi-decadal variations of key Arctic climate parameters like, for example, surface air temperature and oceanic freshwater content. Elucidating the mechanisms behind these changes will be critical to understanding the complex nature of high-latitude variability and its impact on global climate change.acceptedVersio

Scaling the Decay of Turbulence Kinetic Energy in the Free-Convective Boundary Layer

We investigate the scaling for decaying turbulence kinetic energy (TKE) in the free-convective boundary layer, from the time the surface heat flux starts decaying, until a few hours after it has vanished. We conduct a set of large-eddy simulation experiments, consider various initial convective situations, and prescribe realistic decays of the surface heat flux over a wide range of time scales. We find that the TKE time evolution is dictated by the decaying magnitude of the surface heat flux up to 0.7 τ approximately, where τ is the prescribed duration from maximum to zero surface heat flux. During the time period starting at zero surface heat flux, we search for potential power-law scaling by examining the log–log presentation of TKE as a function of time. First, we find that the description of the decay highly depends on whether the time origin is defined as the time when the surface heat flux starts decaying (traditional scaling framework), or the time when it vanishes (proposed new scaling framework). Second, when varying τ, the results plotted in the traditional scaling framework indicate variations in the power-law decay rates over several orders of magnitude. In the new scaling framework, however, we find a unique decay exponent in the order of 1, independent of the initial convective condition, and independent of τ, giving support for the proposed scaling framework