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We introduce an analytical model that describes the vertical structure of Ekman boundary layer flows coupled to the Monin-Obukhov Similarity Theory (MOST) surface layer representation, which is valid for conventionally neutral (CNBL) and stable (SBL) atmospheric conditions. The model is based on a self-similar profile of horizontal stress for both CNBL and SBL flows that merges the classic 3/2 power law profile with a MOST-consistent stress profile in the surface layer. The velocity profiles are then obtained from the Ekman momentum balance equation. The same stress model is used to derive a new self-consistent Geostrophic Drag Law (GDL). We determine the ABL height (h) using an equilibrium boundary layer height model and parameterize the surface heat flux for quasi-steady SBL flows as a function of a prescribed surface temperature cooling rate. The ABL height and GDL equations can then be solved together to obtain the friction velocity (u ∗) and the cross-isobaric angle ( α 0 ) as a function of known input parameters such as the Geostrophic wind speed and surface roughness (z 0) . We show that the model predictions agree well with simulation data from the literature and newly generated Large Eddy Simulations (LES). These results indicate that the proposed model provides an efficient and relatively accurate self-consistent approach for predicting the mean wind velocity distribution in CNBL and SBL flows. [ABSTRACT FROM AUTHOR]

The Arctic is undergoing rapid changes due to global warming, including the expansion of the marginal ice zone (MIZ), a zone of mixed ice and open water surfaces. To predict the atmospheric interaction with these surfaces, a critical process in climate models, this paper examines a simplified theoretical framework to non-dimensionalize the dynamics of the atmospheric boundary layer (ABL) over a mixed ice-water surface (MIZ–ABL). A heterogeneity Richardson number, Ri h , is proposed to account for the difference in temperature between the ice and water surface in relation to the synoptic pressure gradient forcing. With the wind angle relative to the ice-water interface, α , this framework hypothesizes that these two dimensionless numbers, regardless of individual dimensional variables (surface temperature and geostrophic wind speed) are sufficient to predict the MIZ–ABL dynamics. To test this framework, large-eddy simulations were employed over half-ice and half-water surfaces, with varying surface temperatures and geostrophic wind velocities. While the surface heat fluxes over ice, water, and the aggregate surface seem to be captured reasonably well by α and Ri h , the mean wind and turbulent kinetic energy (TKE) profiles were not, suggesting that not only the difference in stability between the two surface, but also the individual stabilities over each surface influence the dynamics. The wind angle had a significant impact on the results, both in terms of heat fluxes at the surface, turbulent and dispersive fluxes in the MIZ–ABL, and the structure of the secondary circulations. When wind blows perpendicular to the water-ice interface, internal boundary layers are favoured except at the highest Ri h simulated. For cases with wind parallel to the interface, large rolls parallel to the shore emerge. The paper raises at least as many questions as it answers, highlighting the need for further studies of the MIZ–ABL. [ABSTRACT FROM AUTHOR]

Large-eddy simulations (LESs) of inversion-capped neutral atmospheric boundary layers (ABLs) are augmented to earlier small-domain LESs of a sparse forest and field observation to evaluate the budgets of all non-negligible resolved-scale Reynolds stress components. The focus is on the atmospheric surface layer comprised of the roughness sublayer (RSL) in and above horizontally homogeneous forests and the inertial sublayer (ISL) above the RSL over flat terrain. The greater LES domain and ABL depths result in greater depths of both the RSL and the ISL. A key result is that in the upper portions of the canopy and above, pressure redistribution is a major sink of normal stress in the horizontal direction with mean shear production as a major source, whereas in the horizontal direction absent of mean shear production and in the vertical direction, pressure redistributions are major sources of normal stresses. In the lower portions of the canopy where mean shear production and turbulent transport are much reduced, pressure redistributions are major sources of horizontal velocity variances but a major sink of vertical velocity variance. Pressure transport is a greater source of vertical velocity variance than turbulent transport from the ground level to just under the treetops where it transitions to a major sink up to about 1.5 times the canopy height. This greater significance of pressure transport over turbulent transport increases with increased vegetation area index (VAI). The impact of increased value of geostrophic wind speed is negligible compared to that of increased value of VAI on enhancing normalized budget terms in the vicinity of treetops. [ABSTRACT FROM AUTHOR]

An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets (LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa (2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall. The precipitation rate reached a maximum during 20–21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs (SLLJs) at 850–700 hPa and boundary-layer jets (BLJs) at ∼950 hPa. The coupling of the SLLJ and BLJ provided strong mid- and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on 21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex (or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale. [ABSTRACT FROM AUTHOR]

This study presents the results of large-eddy simulation (LES) of the evening transition in the atmospheric boundary layer in the case of free convection and in the presence of geostrophic wind. The turbulent kinetic energy (TKE) balance and its components are analyzed. It is shown that within the transition, periods of fast and slow decay can be distinguished. The differences in TKE anisotropy between these two periods are demonstrated. During the fast decay period, the majority of the energy within the vertical component is consumed due to inertial movement of the thermals after the cease of convection. This is followed by the TKE redistribution into large-scale horizontal components, which leads to the formation of quasi-horizontal turbulence, where the TKE dissipation is significantly slower in comparison to the isotropic state. It is shown that one-dimensional boundary-layer model, in which turbulent fluxes are parameterized by means of a two-equation closure, is not able to reproduce evening transition dynamics observed in LES. In particular, the use of the gradient approximation in the one-dimensional model leads to the preservation of the convective distribution of the heat flux along the vertical during the transition period and additional TKE generation in the boundary layer due to the action of buoyancy forces. The use of the phenomenological equation for the dissipation rate leads to decreased TKE decay rate during the fast decay period and increased TKE decay rate during the slow decay period. Possible approaches toward modification of the Reynolds-averaged Navier–Stokes (RANS) closures in order to correctly reproduce transition periods of the atmospheric boundary layer are discussed. [ABSTRACT FROM AUTHOR]

The patterns of respiratory virus illness are expressed differently between temperate and tropical climates. Tropical outbreaks often peak in wet seasons. Temperate outbreaks typically peak during the winter. The prevailing causal hypotheses focus on sunlight, temperature and humidity variations. Yet no consistent factors have been identified to sufficiently explain seasonal virus emergence and decline at any latitude. Here we demonstrate close connections among global-scale atmospheric circulations, IgE antibody enhancement through seasonal pollen inhalation, and respiratory virus patterns at any populated latitude, with a focus on the US. Pollens emerge each Spring, and the renewed IgE titers in the population are argued to terminate each winter peak of respiratory illness. Globally circulated airborne viruses are postulated to subsequently deposit across the Southern US during lower zonal geostrophic winds each late Summer. This seasonally refreshed viral load is postulated to trigger a new influenza outbreak, once the existing IgE antibodies diminish to a critical value each Fall. Our study offers a new and consistent explanation for the seasonal diminishment of respiratory viral illnesses in temperate climates, the subdued seasonal signature in the tropics, the annually circulated virus phenotypes, and the northerly migration of influenza across the US every year. Our integrated geospatial and IgE hypothesis provides a new perspective for prediction, mitigation and prevention of the outbreak and spread of seasonal respiratory viruses including Covid-19 pandemic. [ABSTRACT FROM AUTHOR]

We examined the characteristics of nocturnal low-level wind maximum (NLWM) at a topographically complex site in Boseong, South Korea. The study site is located on the southern coast of the Korean peninsula. Although the area within a few kilometers of the site is flat, it is surrounded by mountains and faces the ocean on the southeast side. We defined NLWM as the lowest wind speed maximum within the lowest 300 m of the atmosphere that is at least 2 m s−1 greater than the next minimum above during nighttime. Climatology of NLWMs was derived from a 4-year half-hourly database of wind profiles (11 levels) obtained from a 300-m tower. The half-hourly data showed that NLWM occurred on approximately 18% of the nights. NLWMs were typically situated at 40 m above the ground. Half of the NLWMs had a speed of 3–5 m s−1, and its direction was predominantly west-northwesterly regardless of the height or season. The occurrence of NLWM showed a maximum in October and a minimum in July. The cause of NLWM at this site was discussed. It was found that the NLWM at this site occurred in the presence of valley wind forcing on nights with weak northerly geostrophic winds at 850 hPa. [ABSTRACT FROM AUTHOR]

Five planetary-boundary-layer parametrizations of the Weather Research and Forecasting model are compared with respect to their ability to simulate the very stable and the weakly stable regimes of the stable boundary layer. This is performed for single column models where the large-scale mechanical forcing is represented by geostrophic wind speeds ranging from 0.5 to 12 m s - 1 . The performance of the models is assessed by contrasting the relationships they produce between the turbulence velocity scale and the mean wind speed, between potential temperature gradient and the mean wind speed, and between the flux and gradient Richardson numbers. The level-2.5 Mellor–Yamada–Nakanishi–Niino parametrization simulates the very stable regime the best, mainly because its heat eddy diffusivity decreases with respect to the momentum eddy diffusivity as the stability increases, while the same is not true for the other parametrizations considered. [ABSTRACT FROM AUTHOR]

Observations of two typical contrasting weakly stable and very stable boundary layers from the winter at Dome C station, Antarctica, are used as a benchmark for two centimetre-scale-resolution large-eddy simulations. By taking the Antarctic winter, the effects of the diurnal cycle are eliminated, enabling the study of the long-lived steady stable boundary layer. With its homogeneous, flat snow surface, and extreme stabilities, the location is a natural laboratory for studies on the long-lived stable boundary layer. The two simulations differ only in the imposed geostrophic wind speed, which is identified as the main deciding factor for the resulting regime. In general, a good correspondence is found between the observed and simulated profiles of mean wind speed and temperature. Discrepancies in the temperature profiles are likely due to the exclusion of radiative transfer in the current simulations. The extreme stabilities result in a considerable contrast between the stable boundary layer at the Dome C site and that found at typical mid-latitudes. The boundary-layer height is found to range from approximately 50 m to just 5 m in the most extreme case. Remarkably, heating of the boundary layer by subsidence may result in thermal equilibrium of the boundary layer in which the associated heating is balanced by the turbulent cooling towards the surface. Using centimetre-scale resolutions, accurate large-eddy simulations of the extreme stabilities encountered in Antarctica appear to be possible. However, future simulations should aim to include radiative transfer and sub-surface heat transport to increase the degree of realism of these types of simulations. [ABSTRACT FROM AUTHOR]

Horizontal velocity spirals with a clockwise rotation (downward looking) rate of 1.7° m−1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation, the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10−4 m2 s−1 on average. An eddy diffusivity of 2870 m2 s−1 is required for dynamic balance by estimating the residual term. This obtained value of 10−4 m2 s−1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m2 s−1, respectively, which supports a closed budget for velocity rotation. [ABSTRACT FROM AUTHOR]

The turbulence generated by wind shear is described at grey-zone resolutions using a theoretical neutral boundary layer based on atmospheric conditions constructed from measurements from the CASES-99 field campaign. Six-metre-resolution large-eddy simulations (LES) are performed to access the "true" resolved turbulence for two cases, corresponding to a forcing of the boundary layer by zonal geostrophic wind speeds of 10ms-1 and 20ms-1. The LES fields are subject to a coarse-graining procedure in order to compute turbulence diagnostics in the grey zone, with the robustness and weakness of various averaging procedures tested, for which simple top-hat averaging is found to be both suitable and accurate. In addition, the "true" resolved and subgrid-scale fluxes, variances, turbulent kinetic energy and production terms are quantified on various scales. The grey zone of turbulence is defined as the range of scales where 10-90% of turbulence is resolved, which here ranges from resolutions of 25-800m (0.03<Δx/h<1, where Δx is the horizontal resolution, and h is the boundary-layer height). The subgrid/resolved partitioning of the variances of the velocity components depends on the geostrophic wind speed, which is not the case for the momentum-flux partitioning. Dynamic production terms show that fine-scale turbulence is isotropic (Δx/h<0.03) and is quasi one-directional, oriented in the direction of the geostrophic wind vector at the mesoscale (Δx/h>1). The turbulence parametrizations, which are tested in the Méso-NH model by running simulations at resolutions from the LES scale to the mesoscale, fail to produce the correct turbulence regardless of resolution. [ABSTRACT FROM AUTHOR]

In this paper, we consider the compressible models of magnetohydrodynamic flows giving rise to a variety of mathematical problems in many areas. We derive a rigorous quasi-geostrophic equation governed by magnetic field from the rotational compressible magnetohydrodynamic flows with the well-prepared initial data. It is a first derivation of quasi-geostrophic equation governed by the magnetic field, and the tool is based on the relative entropy method. This paper covers two results: the existence of the unique local strong solution of quasi-geostrophic equation with the good regularity and the derivation of a quasi-geostrophic equation. [ABSTRACT FROM AUTHOR]

The performance of an atmospheric single-column model (SCM) is studied systematically for stably-stratified conditions. To this end, 11 years (2005-2015) of daily SCM simulations were compared to observations from the Cabauw observatory, The Netherlands. Each individual clear-sky night was classified in terms of the ambient geostrophic wind speed with a $$1\hbox { m} \hbox { s}^{-1}$$ bin-width. Nights with overcast conditions were filtered out by selecting only those nights with an average net radiation of less than $$-\,30\hbox { W }\hbox {m}^{-2}$$ . A similar procedure was applied to the observational dataset. A comparison of observed and modelled ensemble-averaged profiles of wind speed and potential temperature and time series of turbulent fluxes showed that the model represents the dynamics of the nocturnal boundary layer (NBL) at Cabauw very well for a broad range of mechanical forcing conditions. No obvious difference in model performance was found between near-neutral and strongly-stratified conditions. Furthermore, observed NBL regime transitions are represented in a natural way. The reference model version performs much better than a model version that applies excessive vertical mixing as is done in several (global) operational models. Model sensitivity runs showed that for weak-wind conditions the inversion strength depends much more on details of the land-atmosphere coupling than on the turbulent mixing. The presented results indicate that in principle the physical parametrizations of large-scale atmospheric models are sufficiently equipped for modelling stably-stratified conditions for a wide range of forcing conditions. [ABSTRACT FROM AUTHOR]

We prove a weak-strong uniqueness result for the semi-geostrophic system with constant Coriolis force. The main assumptions on the strong solution are the boundedness of the velocity field as well as the uniform convexity of the Legendre-Fenchel transform of the modified pressure. We give several examples where our results apply, including some classical solutions on the 2-dimensional torus, and the 'stationary' solutions for 3DSG (for which the total wind velocity is zero but the pressure may be time-dependent). [ABSTRACT FROM AUTHOR]

A theoretical problem of linear stationary disturbances of the background geostrophic flow of a stratified rotating medium (atmosphere) that are induced by inhomogeneities of the gravitational field is considered. There is a common belief that such inhomogeneities may only somewhat deform (distort) the state of hydrostatic equilibrium, but cannot affect the dynamics of the flow in the atmosphere. Generally, the problem statement is different for the processes over a solid surface and a water surface, because a water surface (the lower boundary condition for the atmosphere) is deformed by inhomogeneities of the gravitational field. The problem of disturbances over a water surface has been considered in recent papers of the authors; in this paper, the results are developed and significantly revised. The emphasis is on disturbances over a flat horizontal surface, which were not examined before. From the analytical solutions, it follows that the influence of inhomogeneities of the gravitational field on the atmospheric flows may be significant in some cases. Physical generation mechanisms of disturbances are analyzed. [ABSTRACT FROM AUTHOR]

For mid-latitude Rossby waves (RWs) in the atmosphere, the expression for the energy flux for use in a model diagnosis, and without relying on a Fourier analysis or a ray theory, has previously been derived using quasi-geostrophic equations and is singular at the equator. By investigating the analytical solution of both equatorial and mid-latitude waves, the authors derive an exact universal expression for the energy flux which is able to indicate the direction of the group velocity at all latitudes for linear shallow water waves. This is achieved by introducing a streamfunction as given by the inversion equation of Ertel's potential vorticity, a novel aspect for considering the energy flux. For ease of diagnosis from a model, an approximate version of the universal expression is explored and illustrated for a forced/dissipative equatorial basin mode simulated by a single-layer oceanic model that includes both mid-latitude RWs and equatorial waves. Equatorial Kelvin Waves (KWs) propagate eastward along the equator, are partially redirected poleward as coastal KWs at the eastern boundary of the basin, and then shed mid-latitude RWs that propagate westward into the basin interior. The connection of the equatorial and coastal waveguides has been successfully illustrated by the approximate expression of the group-velocity-based energy flux of the present study. This will allow for tropical-extratropical interactions in oceanic and atmospheric model outputs to be diagnosed in terms of an energy cycle in a future study. [ABSTRACT FROM AUTHOR]

An overview on the helicity of the velocity field and the role played by this concept in modern research in the field of geophysical fluid dynamics and dynamic meteorology is given. Different (both previously known in the literature and first presented) formulations of the equation of helicity balance in atmospheric motions (including those with allowance for effects of air compressibility and Earth's rotation) are brought together. Equations and relationships are given which are valid in different approximations accepted in dynamic meteorology: Boussinesq approximation, quasi-static approximation, and quasi-geostrophic approximation. Emphasis is placed on the analysis of helicity budget in large-scale quasi-geostrophic systems of motion; a formula for the helicity flux across the upper boundary of the nonlinear Ekman boundary layer is given, and this flux is shown to be exactly compensated for by the helicity destruction inside the Ekman boundary layer. [ABSTRACT FROM AUTHOR]

A set of absolute geostrophic current (AGC) data for the period January 2004 to December 2012 are calculated using the P-vector method based on monthly gridded Argo profiles in the world tropical oceans. The AGCs agree well with altimeter geostrophic currents, Ocean Surface Current Analysis-Real time currents, and moored current-meter measurements at 10-m depth, based on which the classical Sverdrup circulation theory is evaluated. Calculations have shown that errors of wind stress calculation, AGC transport, and depth ranges of vertical integration cannot explain non-Sverdrup transport, which is mainly in the subtropical western ocean basins and equatorial currents near the Equator in each ocean basin (except the North Indian Ocean, where the circulation is dominated by monsoons). The identified non-Sverdrup transport is thereby robust and attributed to the joint effect of baroclinicity and relief of the bottom (JEBAR) and mesoscale eddy nonlinearity. [ABSTRACT FROM AUTHOR]

Absolute geostrophic currents in the North Pacific Ocean are calculated using the P-vector method and gridded Argo profiling data from January 2004 to December 2012. Three-dimensional structures and seasonal variability of meridional heat transport (MHT) and meridional salt transport (MST) are analyzed. The results show that geostrophic and Ekman components are generally opposite in sign, with the southward geostrophic component dominating in the subtropics and the northward Ekman component dominating in the tropics. In combination with the net surface heat flux and the MST through the Bering Strait, the MHT and MST of the western boundary currents (WBCs) are estimated for the first time. The results suggest that the WBCs are of great importance in maintaining the heat and salt balance of the North Pacific. The total interior MHT and MST in the tropics show nearly the same seasonal variability as that of the Ekman components, consistent with the variability of zonal wind stress. The geostrophic MHT in the tropics is mainly concentrated in the upper layers, while MST with large amplitude and annual variation can extend much deeper. This suggests that shallow processes dominate MHT in the North Pacific, while MST can be affected by deep ocean circulation. In the extratropical ocean, both MHT and MST are weak. However, there is relatively large and irregular seasonal variability of geostrophic MST, suggesting the importance of the geostrophic circulation in the MST of that area. [ABSTRACT FROM AUTHOR]

The North Equatorial Countercurrent (NECC) is an important zonal flow in the upper circulation of the tropical Pacific Ocean, which plays a vital role in the heat budget of the western Pacific warm pool. Using satellite-derived data of ocean surface currents and sea surface heights (SSHs) from 1992 to 2011, the seasonal variation of the surface NECC in the western tropical Pacific Ocean was investigated. It was found that the intensity (INT) and axis position (Y CM ) of the surface NECC exhibit strikingly different seasonal fluctuations in the upstream (128°-136°E) and downstream (145°-160°E) regions. Of the two regions, the seasonal cycle of the upstream NECC shows the greater interannual variability. Its INT and Y are greatly influenced by variations of the Mindanao Eddy, Mindanao Dome (MD), and equatorial Rossby waves to its south. Both INT and Y CM also show semiannual signals induced by the combined effects of equatorial Rossby waves from the Central Pacific and local wind forcing in the western Pacific Ocean. In the downstream region, the variability of the NECC is affected by SSH anomalies in the MD and the central equatorial Pacific Ocean. Those in the MD region are especially important in modulating the Y of the downstream NECC. In addition to the SSH-related geostrophic flow, zonal Ekman flow driven by meridional wind stress also plays a role, having considerable impact on INT variability of the surface NECC. The contrasting features of the variability of the NECC in the upstream and downstream regions reflect the high complexity of regional ocean dynamics. [ABSTRACT FROM AUTHOR]

An objective classification scheme of atmospheric circulation, in which daily circulation is determined by the strength, direction, and vorticity of geostrophic flow, has been applied to the atmosphere over Serbia for the time period 1961-2010. The results for the sea level and isobaric level of 500 hPa for winter and summer are presented. The 26 circulation types (eight pure direction, 16 hybrid, cyclonic, and anticyclonic types) are determined and described. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The relative frequencies of the circulation types, and the relationship between the precipitation and circulation types at three stations on a seasonal time scale are analyzed. The anticyclonic weather type dominates in winter (18.93 %) and summer (18.70 %), followed by the northeasterly type (16.65 %) in summer, and the cyclonic type (12.83 %) in winter. The cyclonic types (C and hybrid) have a higher than average probability of rain at all stations. Conversely, the anticyclonic types are associated with a lower than average probability and intensity of rainfall. [ABSTRACT FROM AUTHOR]

Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both 'neutral' flows, and 'conventionally neutral' flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5 $$^{\circ }$$ . This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers. [ABSTRACT FROM AUTHOR]

Persistent strong and weak East Asian trough (EAT) cases during boreal winter are captured by projecting 12 UTC geostrophic vorticity onto that of a seasonal strong EAT. The persistent cases are investigated in terms of seasonal, intraseasonal, and synoptic temporal scales. The evolution of persistent strong and weak EAT cases displays characteristics of intraseasonal temporal variation. The onset and decay of strong (weak) cases is associated with the passage of an intraseasonal negative (positive) 500 hPa height anomaly from Northeast Asia to the midlatitude central Pacific, through the region of the EAT. The onset and decay stages of persistent strong (weak) EAT cases occur in conjunction with a seasonal stronger-than-normal (weaker-than-normal) EAT. In addition, it is noted that the difference between the number of strong and weak cases exhibits significant covariability with the strength of the seasonal EAT. Meanwhile the seasonal EAT also plays a role in the development and decay of persistent cases. The contributions of dynamic and thermodynamic processes during the evolution of persistent strong and weak EAT cases are investigated. Since the forcing of horizontal temperature advection and that of adiabatic and diabatic processes are likely to offset each other, dynamic processes make a more important contribution to the evolution of the EAT than do thermodynamic processes. Furthermore, horizontal absolute vorticity advection is dominant in the dynamic processes and is caused mainly by the advection of synoptic and intraseasonal vorticity by seasonal wind. The seasonal wind transports positive vorticity to (away from) the EAT, which results in a remarkable strengthening (weakening) of the EAT during the onset (decay) stage for persistent strong EAT cases. [ABSTRACT FROM AUTHOR]

Despite the importance of dust aerosol in the Earth system, state-of-the-art models show a large variety for North African dust emission. This study presents a systematic evaluation of dust emitting-winds in 30 years of the historical model simulation with the UK Met Office Earth-system model HadGEM2-ES for the Coupled Model Intercomparison Project Phase 5. Isolating the effect of winds on dust emission and using an automated detection for nocturnal low-level jets (NLLJs) allow an in-depth evaluation of the model performance for dust emission from a meteorological perspective. The findings highlight that NLLJs are a key driver for dust emission in HadGEM2-ES in terms of occurrence frequency and strength. The annually and spatially averaged occurrence frequency of NLLJs is similar in HadGEM2-ES and ERA-Interim from the European Centre for Medium-Range Weather Forecasts. Compared to ERA-Interim, a stronger pressure ridge over northern Africa in winter and the southward displaced heat low in summer result in differences in location and strength of NLLJs. Particularly the larger geostrophic winds associated with the stronger ridge have a strengthening effect on NLLJs over parts of West Africa in winter. Stronger NLLJs in summer may rather result from an artificially increased mixing coefficient under stable stratification that is weaker in HadGEM2-ES. NLLJs in the Bodélé Depression are affected by stronger synoptic-scale pressure gradients in HadGEM2-ES. Wintertime geostrophic winds can even be so strong that the associated vertical wind shear prevents the formation of NLLJs. These results call for further model improvements in the synoptic-scale dynamics and the physical parametrization of the nocturnal stable boundary layer to better represent dust-emitting processes in the atmospheric model. The new approach could be used for identifying systematic behavior in other models with respect to meteorological processes for dust emission. This would help to improve dust emission simulations and contribute to decreasing the currently large uncertainty in climate change projections with respect to dust aerosol. [ABSTRACT FROM AUTHOR]

The original method is presented for parameterization of surface wind-driven currents in the Black Sea using drifters, altimetry, and wind data. Buoys velocities for the cases with small time variability of the altimetry-derived geostrophic currents were parameterized as a function of the amplitude coefficient b and rotation angle a between wind and residual current direction at drifter drogue depth ∼ 15 m. The results show that the variability of a and b for different cases is very high: angle a changes from 10 to 90° to the right, amplitude coefficient b varies from 0.01 to 0.03. At the same time, it is shown that amplitude coefficient b and rotation angle a on 15 m depth are not independent parameters and there is a robust dependence between them. Using quasi-Ekman theory and the revealed dependence, we can determine parameterization for wind-driven currents exactly on the surface. As a result, we found that wind-driven currents at z = 0 m are directed on b = 13° to the right of wind direction with amplitudes equal to k = 2.8 % of wind velocity. It is important that these parameters did not depend on Ekman layer depth and can be used under different ocean conditions. This parameterization is used to compute the total surface currents and determine the trajectories of oil slick propagation observed on satellite images. Comparison shows that the presented method can be successfully used to predict floating objects trajectories basing on the satellite data. [ABSTRACT FROM AUTHOR]

Peña et al. (: Boundary-Layer Meteorology, 150, 69-89) present valuable experimental material on wind profiles in the atmospheric boundary layer (ABL) and the adjacent free troposphere by quantifying the baroclinic influence. The main profile cases presented (10 in all) have a certain degree of redundancy and under-represent unstable conditions near the surface. One of the profiles appears to be close to the ideal conditions assumed in standard ABL profile formulations and could be used for their assessment. The near-ideal profile, as well as the vast majority of measurements presented, disproves mainstream ideal-case profile formulations that have a mid-ABL super-geostrophic wind-speed maximum: there is no trace of such a maximum. Moreover, several profiles suggest an influence of regional-scale upwind surface discontinuities due to a large forest of area ca. 50 km $$^{2}$$ , which can explain some of the special profile behaviour at Høvsøre presented in detail in previous articles with measurements from that site. [ABSTRACT FROM AUTHOR]

The structure of baroclinic waves in a geostrophic flow with a constant vertical shear (Eady model) is very consistent with that of atmospheric vortex formations. This paper proposes an approach to describing the generation of these waves by initial perturbations of potential vorticity (PV). Within the framework of the suggested approach, the solution to the initial-value problem for a quasi-geostrophic form of the PV transfer equation is represented as a sum of the wave and vortex components with zero and nonzero PV, respectively. A set of ordinary differential equations with the right-hand side dependent on the vertical PV distribution is formulated using Green functions for the amplitude of the wave component (amplitude of excited baroclinic waves). The solution provides a simple description of the resonant and quasi-resonant baroclinic-wave excitation effects under which the wave amplitude grows according to the linear or logarithmic laws. These types of excitation take place for singular and discontinuous initial PV distributions if the frequencies of the wave and vortex components coincide. Smooth distributions generate finite-amplitude waves. [ABSTRACT FROM AUTHOR]

In order to accommodate general initial data, an appropriately relaxed notion of renormalized Lagrangian solutions for the Semi-Geostrophic system in physical space is introduced. This is shown to be consistent with previous notions, generalizing them. A weak stability result is obtained first, followed by a general existence result whose proof employs said stability and approximating solutions with regular initial data. The renormalization property ensures the return from physical to dual space; as consequences we get conservation of Hamiltonian energy and some weak time-regularity of solutions. [ABSTRACT FROM AUTHOR]

A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System (ARPS) model. Based on high resolution spatial and temporal data, a detailed analysis of the structural features and propagation mechanisms of the squall line was conducted. The dynamic and thermodynamic structural characteristics and their causes were analyzed in detail. Unbalanced flows were found to play a key role in initiating gravity waves during the squall line's development. The spread and development of the gravity waves were sustained by convection in the wave-CISK process. The squall line's propagation and development mainly relied on the combined effect of gravity waves at the midlevel and cold outflow along the gust front. New cells were continuously forced by the cold pool outflow and were enhanced and lifted by the intense upward motion. At a particular phase, the new cells merged with the updraft of the gravity waves, leading to an intense updraft that strengthened the squall line. [ABSTRACT FROM AUTHOR]

Mean structures of the North Pacific subtropical gyre are investigated using Argo float observations and several wind stress products, with a particular focus on the gyre axis, which is defined as a large-scale boundary between eastward motions on the northern part of the subtropical gyre and westward motions on the southern part. Structures of the gyre axis are different between the regions east and west of about 180°. In the west, the gyre axis is affected by the strong Kuroshio Extension jet, showing discrepancy with features expected from the Sverdrup balance and ventilated thermocline theory. In the east, depth-integrated geostrophic circulation displays the two gyre axes at about 25°N and 30°N in association with a local cyclonic wind stress curl anomaly. It is expected from the Sverdrup balance that the two gyre axes would extend to the west across the subtropical gyre, but the actual geostrophic circulation exhibits only one gyre axis around 30°N to the west. The vertical structure of the subtropical gyre exhibits a remarkable northward shift of the gyre axis with depth in the east, consistent with the southward decrease in the thickness of density layers around the gyre axis, while the northward shift is less obvious in the west, where the gyre axis lies along the southern flank of the Kuroshio Extension. The gyre shift is relatively smaller in the central mode water (CMW) layer due to the horizontal uniformity of the CMW, suggesting a dynamic effect of the CMW on large-scale circulations of the subtropical gyre. [ABSTRACT FROM AUTHOR]

A new method for the calculating propagation of brackish waters from the Ob-Yenisei estuary in the Kara Sea is proposed. This method is based on satellite altimetry measurements and meteorological data. Surface currents in the upper layer are estimated as the sum of geostrophic and parameterized wind-driven transport. Geostrophic velocities are calculated using altimetry-derived sea-level anomalies and mean dynamic topography. The method has been used previously to calculate surface currents in the Black Sea [7, 12]. In this paper it has been successfully verified on the basis of comparisons with field observations of surface salinity and satellite images of sea-surface chlorophyll in the Kara Sea. [ABSTRACT FROM AUTHOR]

How would typhoon activity over the western North Pacific change for various scenarios of future global warming? Using the model projections of the Coupled Model Intercomparison Project phase 3 (CMIP 3) under the SRES A1B scenario, we generated summer (September) ice-free Arctic conditions, also referred to as Blue Arctic conditions, and then used the corresponding monthly sea surface temperature (SST) and a set of CO concentrations to drive an AGCM model to simulate the resulting changes in background conditions affecting typhoon activity over the western North Pacific. Our results show that, during typhoon season (June to October), atmospheric and ocean circulations over the western North Pacific would be significantly different from the present circulations. Changes in the vertical shear of zonal wind and outgoing longwave radiation (OLR) in the western North Pacific are favorable for westward and northward shift, respectively, of the location of typhoon genesis. Moreover, changes in the above fields over the key area may be conducive to less frequent typhoons. In addition, the tropical cyclone genesis potential index (GPI) over the western North Pacific would decrease (increase) east (west) of 150°E (140°E). [ABSTRACT FROM AUTHOR]

The NCEP/NCAR R1 reanalysis data are employed to investigate the impact of forced and inertial instability in the lower troposphere over the Arabian Sea on the onset process of Indian summer monsoon (ISM), and to reveal the important role of zonal advection of zonal geostrophic momentum played in the forced unstable convection. Results show that during the ISM onset the zero absolute vorticity contour ( η= 0) shifts northward due to the strong cross-equatorial pressure gradient in the lower troposphere over southern Arabian Sea. Thus a region with negative absolute vorticity is generated near the equator in the northern hemisphere, manifesting the evident free inertial instability. When a southerly passes through this region, under the influence of friction a lower convergence that facilitates the convection flourishing at the lower latitudes appears to the north of zero absolute vorticity contour. However, owing to such a traditional inertial instability, the convection is confined near the equator which does not have direct influence on the ISM onset. On the contrary in the region to the north of the zero absolute vorticity contour and to the south of the low pressure center near the surface, although the atmosphere there is inertially stable, the lower westerly jet can develop and bring on the apparent zonal advection of zonal geostrophic momentum. Both theoretical study and diagnosing analysis present that such a zonal advection of geostrophic momentum is closely associated with the zonal asymmetric distribution of meridional land-sea thermal contrast, which induces a convergence center near and further north of the westerly jet in the lower troposphere over the southwestern coast of the Indian Peninsula, providing a favorable lower circulation for the ISM onset. It illustrates that the development of convection over the Arabian Sea in late spring and early summer is not only due to the frictional inertial instability but also strongly affected by the zonal asymmetric distribution of land-sea thermal contrast. Moreover, before the ISM onset due to the eastward development of the South Asian High (SAH) in the upper troposphere, high potential vorticity is transported to the region over the Arabian Sea. Then a local trumpet-shaped stream field is generated to cause the evident upper divergence-pumping effect which favors the ISM onset. When the upper divergence is vertically coupled with the lower convergence resulted from the aforementioned forced unstable convection development near the southwestern coast of Indian Peninsula, the atmospheric baroclinic unstable development is stimulated and the ISM onset is triggered. [ABSTRACT FROM AUTHOR]

In a recent study of trends and low frequency variability of extra-tropical cyclone activity in the ensemble of Twentieth Century Reanalyses, we concluded that 'For the North Atlantic-European region and southeast Australia, the 20CR cyclone trends are in agreement with trends in geostrophic wind extremes derived from in-situ surface pressure observations'. This conclusion has been challenged by Krueger et al. (Clim Dyn, submitted, ), because a recent study (doi:, by the same lead author) comparing annual 95th percentiles (P95) of geostrophic wind speed (geo-wind) derived from surface pressure observations and from the 20CR found that '20CR-geostrophic storminess deviates to a large extent from the observation-based curve' in the period prior to 1950. In this reply, we show that our conclusion is valid; and we clarify that several factors contribute to the reported inconsistencies between the 20CR and observation-based geo-wind extremes. These include the choice of index that is used to represent the temporal variation of extremes (e.g., annual vs. seasonal percentiles), the use of different sampling intervals (6-hourly vs. 3-hourly), and the presence of very large errors in the observations that were not identified, corrected, or excluded in any of the previous studies of observation-based geo-wind extremes. We show that the time series of consecutive seasonal P95 geo-winds derived from the observations and from 20CR are in good agreement back to about 1893, with some deviation earlier when the observations (especially digitized data) remain limited and are more uncertain. We find that the correlation between the 20CR and observation-based geo-wind extremes (P95) time series for the full 134-year record is highly significant statistically, with and without the correction or exclusion of the newly identified erroneous SLP values. The agreement between 20CR and observations is further improved after the correction or exclusion of these erroneous values. [ABSTRACT FROM AUTHOR]

We present an analysis of data from a nearly 1-year measurement campaign performed at Høvsøre, Denmark, a coastal farmland area where the terrain is flat. Within the easterly sector upstream of the site, the terrain is nearly homogenous. This topography and conditions provide a good basis for the analysis of vertical wind-speed profiles under a wide range of atmospheric stability, turbulence, and forcing conditions. One of the objectives of the campaign was to serve as a benchmark for flow over flat terrain models. The observations consist of combined wind lidar and sonic anemometer measurements at a meteorological mast. The sonic measurements cover the first 100 m and the wind lidar measures above 100 m every 50 m in the vertical. Results of the analysis of observations of the horizontal wind-speed components in the range 10-1200 m and surface turbulence fluxes are illustrated in detail, combined with forcing conditions derived from mesoscale model simulations. Ten different cases are presented. The observed wind profiles approach well the simulated gradient and geostrophic winds close to the simulated boundary-layer height during both barotropic and baroclinic conditions, respectively, except for a low-level jet case, as expected. The simulated winds are also presented for completeness and show good agreement with the measurements, generally underpredicting the turning of the wind in both barotropic and baroclinic cases. [ABSTRACT FROM AUTHOR]

Combined measurements of satellite altimeters make it possible to determine anomalies of the sea level of the Black Sea on a regular grid with a high spatial resolution 1/8°. In this work arrays of total geostrophic velocities of currents in the Black Sea basin are retrieved and compared with drifter measurements of current velocities for 1999-2007. The comparison is performed both for the whole array of drifter measurements (∼110000 measurements) and individually for each drifter. To retrieve the velocities, two different arrays of mean dynamic topography (MDT) are used: synthetic and climatic mean dynamic topography. The comparison results demonstrate that using synthetic MDT is preferable for calculating geostrophic velocities. Velocities calculated by from satellite altimetry data agree with velocities obtained by in-situ data. [ABSTRACT FROM AUTHOR]

An objective cyclone tracking algorithm is applied to twentieth century reanalysis (20CR) 6-hourly mean sea level pressure fields for the period 1871-2010 to infer historical trends and variability in extra-tropical cyclone activity. The tracking algorithm is applied both to the ensemble-mean analyses and to each of the 56 ensemble members individually. The ensemble-mean analyses are found to be unsuitable for accurately determining cyclone statistics. However, pooled cyclone statistics obtained by averaging statistics from individual members generally agree well with statistics from the NCEP-NCAR reanalyses for 1951-2010, although 20CR shows somewhat weaker cyclone activity over land and stronger activity over oceans. Both reanalyses show similar cyclone trend patterns in the northern hemisphere (NH) over 1951-2010. Homogenized pooled cyclone statistics are analyzed for trends and variability. Conclusions account for identified inhomogeneities, which occurred before 1949 in the NH and between 1951 and 1985 in the southern hemisphere (SH). Cyclone activity is estimated to have increased slightly over the period 1871-2010 in the NH. More substantial increases are seen in the SH. Notable regional and seasonal variations in trends are evident, as is profound decadal or longer scale variability. For example, the NH increases occur mainly in the mid-latitude Pacific and high-latitude Atlantic regions. For the North Atlantic-European region and southeast Australia, the 20CR cyclone trends are in agreement with trends in geostrophic wind extremes derived from in-situ surface pressure observations. European trends are also consistent with trends in the mean duration of wet spells derived from rain gauge data in Europe. [ABSTRACT FROM AUTHOR]

The present study describes a new method for statistical-dynamical downscaling that combines two different approaches, namely, a set of patterns simulated with a numerical flow model and a transformation function used to process both calculated data and measurements at a reference station. The combined method produces wind roses and wind speed histograms at an arbitrary location in the model domain. The inflow wind direction represented the key parameter to define a set of wind field simulations. The other two inflow parameters, namely, thermal stratification and geostrophic wind speed, were derived from corresponding averaged soundings. The results showed that in the Czech Republic, there are areas where wind roses are deformed by the surrounding terrain. The deformations occur in relatively shallow and wide valleys, and they are more sensitive to the inflow wind direction. Calculated wind roses are compared to corresponding observations at 22 synoptic stations. The most frequent wind direction sector in simulations agreed with measurements at 17 stations. The resulting error in frequency in that sector was under 5 % at 10 stations. In general, the main features of the wind roses are modelled well, even at a relatively large distance from the reference station. However, better performance was achieved for smaller distances between reference station and the site. In further studies, a more extensive set of flow patterns with reduced intervals of thermal stratification and wind speed will likely improve calculated wind roses. [ABSTRACT FROM AUTHOR]

A 24-h simulation with the Advanced Regional Prediction System (ARPS) nonhydrostatic model is performed for the heavy snowfall event of 3-4 February 1998 along the eastern coast of Korean Peninsula; the results are used to understand the snowfall process, including why the precipitation maxima formed along the Yeongdong coastal region rather than over the mountain slope and ridge top during. The numerical simulation with a 4-km horizontal grid spacing and 43 levels reproduces very well the narrow snowband located off the eastern Korean coast, away from, instead of over, the Yeongdong coastal mountain range. The general evolution of the snowband agrees quite well with radar observations, while the water-equivalent precipitation amount agrees reasonably well with radar precipitation estimate. The simulation results clearly show that the snow band developed due to the lifting by a coastal front that developed because of the damming of cold air against the eastern slope of the coastal mountain range. The damming was enhanced by the advection of cold air by a low-level mountain-parallel jet from the north, formed due to geostrophic adjustment as the on-shore upslope air was decelerated by the mountain blocking. As the onshore flow weakened later due to synoptic-scale flow pattern change, the cold front propagated off shore and the precipitation dissipated. [ABSTRACT FROM AUTHOR]

We introduce a simple, physically consistent method to predict nocturnal wind and temperature profiles from external forcing parameters such as the geostrophic wind. As an indicator of the radiative 'forcing' the net longwave radiative cooling is used as a proxy. Surface fluxes are expressed in terms of these parameters by coupling an Ekman model to a rudimentary surface energy balance. Additionally the model assumes validity of Monin-Obukhov similarity in order to predict near-surface wind and temperature profiles up to a height equal to the Obukhov length. The predictions are validated against an independent dataset that covers 11-years of observations at Cabauw, The Netherlands. It is shown that the characteristic profiles in response to external forcings are well-captured by the conceptual model. For this period the observational climatology is in close agreement with ECMWF re-analysis data. As such, the conceptual model provides an alternative tool to giving a first-order estimate of the nocturnal wind and temperature profile near the surface in cases when advanced numerical or observational infrastructure is not available. [ABSTRACT FROM AUTHOR]

A method of initializing tropical cyclones in high-resolution numerical models is developed by modifying a data assimilation system, the NRL atmospheric variational data assimilation system (NAVDAS), which was designed for general mesoscale weather prediction using a three-dimensional variational (3DVAR) analysis with intermittent updates. The method includes the following three upgrades to overcome difficulties resulting from tropical cyclone initialization with the NAVDAS analysis. First, synthetic observation soundings are generated on 9 vertical levels at 49 points for strong storms ( v > 23.1 m s) and 41 points for weak storms around each cyclone center to supplement the observations used by the analysis. Secondly, a vortex relocation method for nested grids is developed to correct the cyclone position in the background fields of the analysis for each nested mesh. Lastly, the 3DVAR analysis is modified to gradually reduce the horizontal length scale and geostrophic coupling constraint near the center of a tropical cyclone for minimizing the problems introduced by improper covariances and coupling constraint used in the analysis. The synthetic observations significantly improve the intensity and structure of the analysis and the track forecast. The vortex relocation significantly improves the first guess background, avoiding the large analysis corrections that would be needed to correct cyclone position, and reducing the imbalance introduced by such large analysis increments. The modifications to the analysis length scale and geostrophic coupling constraint successfully improve the inner core analysis, providing a tighter circulation, and reducing the underestimate of the mass field gradient. Among the three upgrades, the vortex relocation provides the largest improvement to the tropical cyclone initialization and forecast. [ABSTRACT FROM AUTHOR]

The article offers information regarding regions rich in nutrients with food grounds for zooplankton and structures in the ocean favorable for fishery. It mentions that favorable conditions formed in the subarctic frontal zones between cold and warm currents and in the periphery of mesoscale eddies with feeding resource of pelagic fish, are locations of fishing grounds. It discusses the Lagrangian coherent structures in the ocean and distinguishes it from favorable conditions for fishery. It determines the geostrophic velocity field was calculated from the data of satellite altimetry.

Wind climate in Northwest Europe is subject to long-term persistence (LTP), also called the Hurst phenomenon. Ignorance of LTP causes underestimation of climatic variability. The quantification of multi-year variability is important for the assessment of the uncertainty of future multi-year wind yields. Estimating LTP requires long homogeneous time series. Such series of wind observations are rare, but annual mean geostrophic wind speed ( U) can be used instead. This study demonstrates a method to estimate the 10-year aggregated mean U for the near and the far future and its uncertainty in Northwest Europe. Time series of U were derived from daily sea level pressure from the European Climate Assessment Dataset. Minor inhomogeneities cannot be ruled out, but were shown to hardly affect the estimated Hurst exponent $$(\hat{H})$$. A maximum likelihood method was adjusted to remove the biases in $$\hat{H}$$. The geostrophic wind speed over the North Sea, the British Isles and along the Scandinavian coast are characterised by statistically significant H between 0.58 and 0.74, ( H = 0.5 implies no LTP). The additional affect of the parameter uncertainty is estimated in a Bayesian way and is highly dependent on the record length. The assessment of structural changes in future wind fields requires general circulation models. An ensemble of seventeen simulations (ESSENCE) with one single climate model (ECHAM5/MPI-OM) was used to evaluate structural trends and LTP. The estimated $$\hat{H}$$ in the ESSENCE simulations are generally close to 0.5 and not significant. Significant trends in U are found over large parts of the investigated domain, but the trends are small compared to the multi-year variability. Large decreasing trends are found in the vicinity of Iceland and increasing trends near the Greenland coast. This is likely related to the sea ice retreat within the ESSENCE simulations and the associated change in surface temperature gradients. [ABSTRACT FROM AUTHOR]

This study documents the temporal and spatial variability of surface wind conditions over the Norwegian county of Finnmark and the coupling of local surface winds to the larger-scale atmospheric circulation, represented by the mean sea level pressure field. At locations along the northern coast, thermally driven offshore winds from the south dominate, especially during the cold season. During the warm season, downward mixing of westerly overlying winds becomes more important as the stability of the boundary layer stratification decreases. In the western part, locations are situated in valleys, resulting in two opposing along-channel dominant wind directions. Seasonal changes in atmospheric conditions are reflected in a reversal of the dominant wind direction, with a component towards the coast during the cold season. At all locations, a clear separation between different prevailing surface wind directions in each season can be achieved based exclusively on local mean sea level geostrophic wind direction. This allows statistical downscaling of the prevailing surface wind conditions from lower-resolution simulations of the surface pressure field and may improve local wind forecasts over complex terrain. [ABSTRACT FROM AUTHOR]

This study analyzes extremes of geostrophic wind speeds derived from sub-daily surface pressure observations at 13 sites in the European region from the Iberian peninsula to Scandinavia for the period from 1878 or later to 2007. It extends previous studies on storminess conditions in the Northeast (NE) Atlantic-European region. It also briefly discusses the relationship between storminess and the North Atlantic Oscillation (NAO). The results show that storminess conditions in the region from the Northeast Atlantic to western Europe have undergone substantial decadal or longer time scale fluctuations, with considerable seasonal and regional differences (especially between winter and summer, and between the British Isles-North Sea area and other parts of the region). In the North Sea and the Alps areas, there has been a notable increase in the occurrence frequency of strong geostrophic winds from the mid to the late twentieth century. The results also show that, in the cold season (December-March), the NAO-storminess relationship is significantly positive in the north-central part of this region, but negative in the south-southeastern part. [ABSTRACT FROM AUTHOR]

The precipitation distributions associated with two landfalling tropical cyclones (TCs) during extratropical transition (ET) were examined in this study. Their distinction is that the bulk of precipitation fell to the left of the TC track in one TC and to the right in the other. The analyses indicate that, for the TC Haima (2004) case, accompanied by the approach of a deep midlatitude trough throughout the depth of the troposphere, the warm and moist air advection by the southeasterly flow north of TC was favorable for warm advection and frontogenesis to the northwest of the TC. Due to the steepening of equivalent potential temperature ( θ), the air-parcel uplift along the θ surface, in collaboration with thermally direct circulation related to frontogenesis, led to enhanced precipitation northwest of the TC. In contrast, for TC Matsa (2005) embedded within a moister environment, a weak midlatitude trough was situated at the mid-upper level. The convection was triggered by the conditional instability at the lower level and then sustained by dynamic forcing at the mid-upper level so that the heavy precipitation occurred to the northeast of TC. For the two TC cases, the precipitation enhancement was also linked to the upper-level anomalous divergence associated with the jet-related forcing on the right side of the jet entrance. From the quasigeostrophic perspective, the advection of geostrophic absolute vorticity by the thermal wind most likely served as an indication reflecting the displacement of the vertical motion relative to the center of the TC. [ABSTRACT FROM AUTHOR]

Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat flux from the atmosphere at the water surface is about 77 W m on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input into the ice-upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased) open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive feedback is regarded as the ice-ocean albedo feedback, and explains in part the large interannual variability of the ice cover in the ice melt season. [ABSTRACT FROM AUTHOR]

We present the main results from the second model intercomparison within the GEWEX (Global Energy and Water cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today's numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled near-surface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identified. [ABSTRACT FROM AUTHOR]

Both wind turning with height and ageostrophic flow in a stably stratified atmospheric boundary layer are analyzed using a three-parameter turbulence model. For a quasi-steady state of the boundary layer, the cross-isobaric flow is determined only by turbulent stress at the surface in the direction of geostrophic wind. The 'operative' prediction models, in which the first-order turbulence closure schemes are used, tend to overestimate the boundary-layer depth and underestimate the angle between the surface and geostrophic winds when compared to 'research' models (schemes of high-level turbulence closure). The true value of the angle between the surface and geostrophic winds is significant for the presentation of a large-scale flow. A nocturnal low-level jet is a mesoscale phenomenon reflected in data obtained from measurements in a stably stratified atmospheric boundary layer. It is found that such jets are of great importance in transporting humidity, momentum, and air pollution. In this study, the difference between jet flows over a homogeneous underlying surface and over a spatially localized large-scale aerodynamic roughness is shown. [ABSTRACT FROM AUTHOR]