Showing papers in "Tellus A in 2014"

Loss of sea ice during winter north of Svalbard

Abstract: Sea ice loss in the Arctic Ocean has up to now been strongest during summer. In contrast, the sea ice concentration north of Svalbard has experienced a larger decline during winter since 1979. The trend in winter ice area loss is close to 10% per decade, and concurrent with a 0.3°C per decade warming of the Atlantic Water entering the Arctic Ocean in this region. Simultaneously, there has been a 2°C per decade warming of winter mean surface air temperature north of Svalbard, which is 20–45% higher than observations on the west coast. Generally, the ice edge north of Svalbard has retreated towards the northeast, along the Atlantic Water pathway. By making reasonable assumptions about the Atlantic Water volume and associated heat transport, we show that the extra oceanic heat brought into the region is likely to have caused the sea ice loss. The reduced sea ice cover leads to more oceanic heat transferred to the atmosphere, suggesting that part of the atmospheric warming is driven by larger open water area. In contrast to significant trends in sea ice concentration, Atlantic Water temperature and air temperature, there is no significant temporal trend in the local winds. Thus, winds have not caused the long-term warming or sea ice loss. However, the dominant winds transport sea ice from the Arctic Ocean into the region north of Svalbard, and the local wind has influence on the year-to-year variability of the ice concentration, which correlates with surface air temperatures, ocean temperatures, as well as the local wind. Keywords: Sea ice, Atlantic Water, Svalbard, heat transport, air–ice–sea interactions (Published: 5 June 2014) Citation: Tellus A 2014, 66 , 23933, http://dx.doi.org/10.3402/tellusa.v66.23933

LakeMIP Kivu: Evaluating the representation of a large, deep tropical lake by a set of one-dimensional lake models

Abstract: The African great lakes are of utmost importance for the local economy (fishing), as well as being essential to the survival of the local people. During the past decades, these lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern. In this study, for the first time a set of one-dimensional lake models are evaluated for Lake Kivu (2.28°S; 28.98°E), East Africa. The unique limnology of this meromictic lake, with the importance of salinity and subsurface springs in a tropical high-altitude climate, presents a worthy challenge to the seven models involved in the Lake Model Intercomparison Project (LakeMIP). Meteorological observations from two automatic weather stations are used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, is used to assess the model’s performance. Simulations are performed over the freshwater layer only (60 m) and over the average lake depth (240 m), since salinity increases with depth below 60 m in Lake Kivu and some lake models do not account for the influence of salinity upon lake stratification. All models are able to reproduce the mixing seasonality in Lake Kivu, as well as the magnitude and seasonal cycle of the lake enthalpy change. Differences between the models can be ascribed to variations in the treatment of the radiative forcing and the computation of the turbulent heat fluxes. Fluctuations in wind velocity and solar radiation explain inter-annual variability of observed water column temperatures. The good agreement between the deep simulations and the observed meromictic stratification also shows that a subset of models is able to account for the salinity- and geothermal-induced effects upon deep-water stratification. Finally, based on the strengths and weaknesses discerned in this study, an informed choice of a one-dimensional lake model for a given research purpose becomes possible. Keywords: lake modelling, model intercomparison, surface–atmosphere interactions, tropical lakes, Lake Kivu (Published: 5 February 2014) Citation: Tellus A 2014, 66 , 21390, http://dx.doi.org/10.3402/tellusa.v66.21390 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

A fully coupled Mediterranean regional climate system model: design and evaluation of the ocean component for the 1980-2012 period

Abstract: A fully coupled regional climate system model (CNRM-RCSM4) dedicated to the Mediterranean region is described and evaluated using a multidecadal hindcast simulation (1980–2012) driven by global atmosphere and ocean reanalysis. CNRM-RCSM4 includes the regional representation of the atmosphere (ALADIN-Climate model), land surface (ISBA model), rivers (TRIP model) and the ocean (NEMOMED8 model), with a daily coupling by the OASIS coupler. This model aims to reproduce the regional climate system with as few constraints as possible: there is no surface salinity, temperature relaxation, or flux correction; the Black Sea budget is parameterised and river runoffs (except for the Nile) are fully coupled. The atmospheric component of CNRM-RCSM4 is evaluated in a companion paper; here, we focus on the air–sea fluxes, river discharges, surface ocean characteristics, deep water formation phenomena and the Mediterranean thermohaline circulation. Long-term stability, mean seasonal cycle, interannual variability and decadal trends are evaluated using basin-scale climatologies and in-situ measurements when available. We demonstrate that the simulation shows overall good behaviour in agreement with state-of-the-art Mediterranean RCSMs. An overestimation of the shortwave radiation and latent heat loss as well as a cold Sea Surface Temperature (SST) bias and a slight trend in the bottom layers are the primary current deficiencies. Further, CNRM-RCSM4 shows high skill in reproducing the interannual to decadal variability for air–sea fluxes, river runoffs, sea surface temperature and salinity as well as open-sea deep convection, including a realistic simulation of the Eastern Mediterranean Transient. We conclude that CNRM-RCSM4 is a mature modelling tool allowing the climate variability of the Mediterranean regional climate system to be studied and understood. It is used in hindcast and scenario modes in the HyMeX and Med-CORDEX programs. Keywords: Mediterranean Sea, regional climate system model, air-sea fluxes, hindcast simulation, interannual variability (Published: 18 November 2014) Citation: Tellus A 2014, 66 , 23967, http://dx.doi.org/10.3402/tellusa.v66.23967

Simulation of surface energy fluxes and stratification of a small boreal lake by a set of one-dimensional models

Abstract: Five one-dimensional (1D) lake models were run for the open water season in 2006 for Lake Valkea-Kotinen (Finland) using on-lake measured meteorological forcing. The model results were validated using measurements of water temperature and of eddy covariance (EC) fluxes. The surface temperature is satisfactorily simulated by all models showing slight overestimation (by 0.1–1.1°C). Both sensible and latent heat fluxes are positively biased in respect to EC data, consistent with earlier studies. However, correlation coefficients between EC-fluxes and those simulated are relatively high ranging from 0.55 to 0.74. The skill to simulate vertical temperature profiles by different models is assessed as well. It is found that the lake models underestimate the EC-derived surface drag coefficient, however providing realistic temperature profiles. It is argued that the real momentum flux from the atmosphere is larger than simulated, however it is split up between the wave development and the acceleration of lake currents. Adopting the simple parameterisation for momentum flux partitioning in one of the models showed that this mechanism can be significant. Finally, the effect of including the lake bathymetry data in k-ɛ models was the drastic overheating of water below the thermocline. This is likely to be caused by omitting the heat flux at the lake margins. Thus, the parameterisation of heat flux at the lake’s margins should be included in the models; otherwise it is recommended to neglect bathymetry effects for such small water bodies as the Lake Valkea-Kotinen. Keywords: lakes, numerical modelling, stratification, surface fluxes (Published: 16 January 2014) Citation: Tellus A 2014, 66 , 21389, http://dx.doi.org/10.3402/tellusa.v66.21389 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

A comparison of tracking methods for extreme cyclones in the Arctic basin

Abstract: Dramatic climate changes have occurred in recent decades over the Arctic region, and very noticeably in near-surface warming and reductions in sea ice extent. In a climatological sense, Arctic cyclone behaviour is linked to the distributions of lower troposphere temperature and sea ice, and hence the monitoring of storms can be seen as an important component of the analysis of Arctic climate. The analysis of cyclone behaviour, however, is not without ambiguity, and different cyclone identification algorithms can lead to divergent conclusions. Here we analyse a subset of Arctic cyclones with 10 state-of-the-art cyclone identification schemes applied to the ERA-Interim reanalysis. The subset is comprised of the five most intense (defined in terms of central pressure) Arctic cyclones for each of the 12 calendar months over the 30-yr period from 1 January 1979 to 31 March 2009. There is a considerable difference between the central pressures diagnosed by the algorithms of typically 5–10 hPa. By contrast, there is substantial agreement as to the location of the centre of these extreme storms. The cyclone tracking algorithms also display some differences in the evolution and life cycle of these storms, while overall finding them to be quite long-lived. For all but six of the 60 storms an intense tropopause polar vortex is identified within 555 km of the surface system. The results presented here highlight some significant differences between the outputs of the algorithms, and hence point to the value using multiple identification schemes in the study of cyclone behaviour. Overall, however, the algorithms reached a very robust consensus on most aspects of the behaviour of these very extreme cyclones in the Arctic basin. Keywords: Arctic cyclones, extreme storms, Arctic climate change, cyclone identification, polar amplification, sea ice (Published: 30 September 2014) Citation: Tellus A 2014, 66 , 25252, http://dx.doi.org/10.3402/tellusa.v66.25252 This publication is part of a Thematic Cluster entitled " Intercomparison of Mid-Latitude Storm Diagnostics ". Read the other papers from this thematic cluster here

Climatology of the Iberia coastal low-level wind jet: weather research forecasting model high-resolution results

Abstract: Coastal low-level jets (CLLJ) are a low-tropospheric wind feature driven by the pressure gradient produced by a sharp contrast between high temperatures over land and lower temperatures over the sea. This contrast between the cold ocean and the warm land in the summer is intensified by the impact of the coastal parallel winds on the ocean generating upwelling currents, sharpening the temperature gradient close to the coast and giving rise to strong baroclinic structures at the coast. During summertime, the Iberian Peninsula is often under the effect of the Azores High and of a thermal low pressure system inland, leading to a seasonal wind, in the west coast, called the Nortada (northerly wind). This study presents a regional climatology of the CLLJ off the west coast of the Iberian Peninsula, based on a 9 km resolution downscaling dataset, produced using the Weather Research and Forecasting (WRF) mesoscale model, forced by 19 years of ERA-Interim reanalysis (1989–2007). The simulation results show that the jet hourly frequency of occurrence in the summer is above 30% and decreases to about 10% during spring and autumn. The monthly frequencies of occurrence can reach higher values, around 40% in summer months, and reveal large inter-annual variability in all three seasons. In the summer, at a daily base, the CLLJ is present in almost 70% of the days. The CLLJ wind direction is mostly from north-northeasterly and occurs more persistently in three areas where the interaction of the jet flow with local capes and headlands is more pronounced. The coastal jets in this area occur at heights between 300 and 400 m, and its speed has a mean around 15 m/s, reaching maximum speeds of 25 m/s. Keywords: regional climate modelling, coastal wind jet, Iberian Peninsula, sea land contrast, WRF, wind energy (Published: 15 January 2014) Citation: Tellus A 2014, 66 , 22377, http://dx.doi.org/10.3402/tellusa.v66.22377

Seasonal-to-decadal predictions with the ensemble Kalman filter and the Norwegian Earth System Model: a twin experiment

Abstract: Here, we firstly demonstrate the potential of an advanced flow dependent data assimilation method for performing seasonal-to-decadal prediction and secondly, reassess the use of sea surface temperature (SST) for initialisation of these forecasts. We use the Norwegian Climate Prediction Model (NorCPM), which is based on the Norwegian Earth System Model (NorESM) and uses the deterministic ensemble Kalman filter to assimilate observations. NorESM is a fully coupled system based on the Community Earth System Model version 1, which includes an ocean, an atmosphere, a sea ice and a land model. A numerically efficient coarse resolution version of NorESM is used. We employ a twin experiment methodology to provide an upper estimate of predictability in our model framework (i.e. without considering model bias) of NorCPM that assimilates synthetic monthly SST data (EnKF-SST). The accuracy of EnKF-SST is compared to an unconstrained ensemble run (FREE) and ensemble predictions made with near perfect (i.e. microscopic SST perturbation) initial conditions (PERFECT). We perform 10 cycles, each consisting of a 10-yr assimilation phase, followed by a 10-yr prediction. The results indicate that EnKF-SST improves sea level, ice concentration, 2 m atmospheric temperature, precipitation and 3-D hydrography compared to FREE. Improvements for the hydrography are largest near the surface and are retained for longer periods at depth. Benefits in salinity are retained for longer periods compared to temperature. Near-surface improvements are largest in the tropics, while improvements at intermediate depths are found in regions of large-scale currents, regions of deep convection, and at the Mediterranean Sea outflow. However, the benefits are often small compared to PERFECT, in particular, at depth suggesting that more observations should be assimilated in addition to SST. The EnKF-SST system is also tested for standard ocean circulation indices and demonstrates decadal predictability for Atlantic overturning and sub-polar gyre circulations, and heat content in the Nordic Seas. The system beats persistence forecast and shows skill for heat content in the Nordic Seas that is close to PERFECT. Keywords: seasonal-to-decadal prediction, EnKF, NorESM, NorCPM, SST initialisation (Published: 10 March 2014) Citation: Tellus A 2014, 66 , 21074, http://dx.doi.org/10.3402/tellusa.v66.21074

Concepções indígenas de infância no Brasil

Abstract: O artigo utiliza dados recentes de pesquisas antropologicas sobre criancas indigenas para analisar aspectos recorrentes das concepcoes indigenas sobre infância e desenvolvimento infantil. Ao identificar e analisar cinco desses aspectos recorrentes, o artigo aborda questoes que vem sendo exploradas na etnologia sobre a sociabilidade e o pensamento das populacoes das Terras Baixas da America do Sul, destacando concepcoes relativas a educacao e a responsabilidade dos adultos nos cuidados com as criancas. Verifica-se que, ao contrario da a visao adultocentrica do pensamento ocidental, o pensamento indigena coloca as criancas como mediadoras entre categorias cosmologicas de grande rendimento e reconhece nelas potencialidades que as permitem ocupar espacos de sujeitos plenos e produtores de sociabilidade.

Evolution of snow and ice temperature, thickness and energy balance in Lake Orajärvi, northern Finland

Abstract: The seasonal evolution of snow and ice on Lake Orajarvi, northern Finland, was investigated for three consecutive winter seasons. Material consisting of numerical weather prediction model (HIRLAM) output, weather station observations, manual snow and ice observations, high spatial resolution snow and ice temperatures from ice mass balance buoys (SIMB), and Moderate Resolution Imaging Spectroradiometer (MODIS) lake ice surface temperature observations was gathered. A snow/ice model (HIGHTSI) was applied to simulate the evolution of the snow and ice surface energy balance, temperature profiles and thickness. The weather conditions in early winter were found critical in determining the seasonal evolution of the thickness of lake ice and snow. During the winter season (Nov.–Apr.), precipitation, longwave radiative flux and air temperature showed large inter-annual variations. The uncertainty in snow/ice model simulations originating from precipitation was investigated. The contribution of snow to ice transformation was vital for the total lake ice thickness. At the seasonal time scale, the ice bottom growth was 50–70% of the total ice growth. The SIMB is suitable for monitoring snow and ice temperatures and thicknesses. The Mean Bias Error (MBE) between the SIMB and borehole measurements was −0.7 cm for snow thicknesses and 1.7 cm for ice thickness. The temporal evolution of MODIS surface temperature (three seasons) agrees well with SIMB and HIGHTSI results (correlation coefficient, R =0.81). The HIGHTSI surface temperatures were, however, higher (2.8°C≤MBE≤3.9°C) than the MODIS observations. The development of HIRLAM by increasing its horizontal and vertical resolution and including a lake parameterisation scheme improved the atmospheric forcing for HIGHTSI, especially the relative humidity and solar radiation. Challenges remain in accurate simulation of snowfall events and total precipitation. Keywords: lake snow and ice, energy balance, lake modelling, ice mass balance buoy, MODIS, HIRLAM (Published: 14 May 2014) Citation: Tellus A 2014, 66 , 21564, http://dx.doi.org/10.3402/tellusa.v66.21564 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

Estimation of the mean depth of boreal lakes for use in numerical weather prediction and climate modelling

Abstract: Lakes influence the structure of the atmospheric boundary layer and, consequently, the local weather and local climate Their influence should be taken into account in the numerical weather prediction (NWP) and climate models through parameterisation For parameterisation, data on lake characteristics external to the model are also needed The most important parameter is the lake depth Global database of lake depth GLDB (Global Lake Database) is developed to parameterise lakes in NWP and climate modelling The main purpose of the study is to upgrade GLDB by use of indirect estimates of the mean depth for lakes in boreal zone, depending on their geological origin For this, Tectonic Plates Map, geological, geomorphologic maps and the map of Quaternary deposits were used Data from maps were processed by an innovative algorithm, resulting in 141 geological regions where lakes were considered to be of kindred origin To obtain a typical mean lake depth for each of the selected regions, statistics from GLDB were gained and analysed The main result of the study is a new version of GLDB with estimations of the typical mean lake depth included Potential users of the product are NWP and climate models Keywords: parameterisation of lakes, surface processes, physiographic data (Published: 21 March 2014) Citation: Tellus A 2014, 66, 21295, http://dxdoiorg/103402/tellusav6621295 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models" Read the other papers from this thematic cluster here

Sensitivity of simulated regional Arctic climate to the choice of coupled model domain

Abstract: The climate over the Arctic has undergone changes in recent decades. In order to evaluate the coupled response of the Arctic system to external and internal forcing, our study focuses on the estimation of regional climate variability and its dependence on large-scale atmospheric and regional ocean circulations. A global ocean–sea ice model with regionally high horizontal resolution is coupled to an atmospheric regional model and global terrestrial hydrology model. This way of coupling divides the global ocean model setup into two different domains: one coupled, where the ocean and the atmosphere are interacting, and one uncoupled, where the ocean model is driven by prescribed atmospheric forcing and runs in a so-called stand-alone mode. Therefore, selecting a specific area for the regional atmosphere implies that the ocean–atmosphere system can develop ‘freely’ in that area, whereas for the rest of the global ocean, the circulation is driven by prescribed atmospheric forcing without any feedbacks. Five different coupled setups are chosen for ensemble simulations. The choice of the coupled domains was done to estimate the influences of the Subtropical Atlantic, Eurasian and North Pacific regions on northern North Atlantic and Arctic climate. Our simulations show that the regional coupled ocean–atmosphere model is sensitive to the choice of the modelled area. The different model configurations reproduce differently both the mean climate and its variability. Only two out of five model setups were able to reproduce the Arctic climate as observed under recent climate conditions (ERA-40 Reanalysis). Evidence is found that the main source of uncertainty for Arctic climate variability and its predictability is the North Pacific. The prescription of North Pacific conditions in the regional model leads to significant correlation with observations, even if the whole North Atlantic is within the coupled model domain. However, the inclusion of the North Pacific area into the coupled system drastically changes the Arctic climate variability to a point where the Arctic Oscillation becomes an ‘internal mode’ of variability and correlations of year-to-year variability with observational data vanish. In line with previous studies, our simulations provide evidence that Arctic sea ice export is mainly due to ‘internal variability’ within the Arctic region. We conclude that the choice of model domains should be based on physical knowledge of the atmospheric and oceanic processes and not on ‘geographic’ reasons. This is particularly the case for areas like the Arctic, which has very complex feedbacks between components of the regional climate system. Keywords : regional climate model, Arctic Ocean, sea ice, downscaling, REMO, coupled model Responsible Editor: Abdel Hannachi, Stockholm University, Sweden. (Published: 3 July 2014) Citation : Tellus A 2014, 66 , 23966, http://dx.doi.org/10.3402/tellusa.v66.23966 This paper is part of the Thematic Cluster: Towards regional climate system modeling for the Baltic Sea, North Sea, Mediterranean Sea and Arctic Ocean . More papers from this issue can be found at Thematic Clusters/Special Issues

Gaussian anamorphosis in the analysis step of the EnKF: a joint state-variable/observation approach

Abstract: The analysis step of the (ensemble) Kalman filter is optimal when (1) the distribution of the background is Gaussian, (2) state variables and observations are related via a linear operator, and (3) the observational error is of additive nature and has Gaussian distribution. When these conditions are largely violated, a pre-processing step known as Gaussian anamorphosis (GA) can be applied. The objective of this procedure is to obtain state variables and observations that better fulfil the Gaussianity conditions in some sense. In this work we analyse GA from a joint perspective, paying attention to the effects of transformations in the joint state-variable/observation space. First, we study transformations for state variables and observations that are independent from each other. Then, we introduce a targeted joint transformation with the objective to obtain joint Gaussianity in the transformed space. We focus primarily in the univariate case, and briefly comment on the multivariate one. A key point of this paper is that, when (1)–(3) are violated, using the analysis step of the EnKF will not recover the exact posterior density in spite of any transformations one may perform. These transformations, however, provide approximations of different quality to the Bayesian solution of the problem. Using an example in which the Bayesian posterior can be analytically computed, we assess the quality of the analysis distributions generated after applying the EnKF analysis step in conjunction with different GA options. The value of the targeted joint transformation is particularly clear for the case when the prior is Gaussian, the marginal density for the observations is close to Gaussian, and the likelihood is a Gaussian mixture. Keywords: Gaussian anamorphosis, ensemble Kalman filter, nonlinearity, joint transformations, non-Gaussianity (Published: 26 September 2014) Citation: Tellus A 2014, 66 , 23493, http://dx.doi.org/10.3402/tellusa.v66.23493

Effect of the atmospheric heat source on the development and eastward movement of the Tibetan Plateau vortices

Abstract: Based on the final analyses data (FNL) of the Global Forecasting System of the National Centers for Environment Prediction (NCEP) and the radiosonde data over the Tibetan Plateau, evolutions of two types of the Tibetan Plateau vortices, moving-off the plateau (Type A) and dying-out on the plateau (Type B), are investigated respectively. Compared to Type B vortices, the large-scale circulations associated with Type A vortices show stronger ridge to the north of the plateau and deeper trough near the Bay of Bengal at 500 hPa, and the southwesterly flow from the trough and the northwesterly flow from the ridge converge more intensively to the east of Type A vortices. Meanwhile, at 200 hPa the divergence on the right-hand side of the upper westerly jet is just over the vortices. The convergence at 500 hPa and divergence at 200 hPa provide favourable conditions for the development and eastward motion of the vortices. The diagnoses of the potential vorticity (PV) budgets reveal that in the developing stages of the two types of vortices, the vertical distribution of the atmospheric heat source determines both their intensity and the moving direction. In the decaying stage, the maintenance and eastward movement for Type A vortices mainly depend on the convergence of the strong northwesterly and southwesterly to the east of the vortices. For Type B vortices, the vertical PV flux divergence caused by the ascending motion around the vortices reduces the intensity of the vortices and is unfavourable for their eastward motion. Keywords: Tibetan Plateau vortex, large-scale circulation, heating effect, condensation latent heating, PV budget (Published: 11 August 2014) Citation: Tellus A 2014, 66 , 24451, http://dx.doi.org/10.3402/tellusa.v66.24451

Estimating correlated observation error statistics using an ensemble transform Kalman filter

Abstract: For certain observing types, such as those that are remotely sensed, the observation errors are correlated and these correlations are state- and time-dependent In this work, we develop a method for diagnosing and incorporating spatially correlated and time-dependent observation error in an ensemble data assimilation system The method combines an ensemble transform Kalman filter with a method that uses statistical averages of background and analysis innovations to provide an estimate of the observation error covariance matrix To evaluate the performance of the method, we perform identical twin experiments using the Lorenz '96 and Kuramoto-Sivashinsky models Using our approach, a good approximation to the true observation error covariance can be recovered in cases where the initial estimate of the error covariance is incorrect Spatial observation error covariances where the length scale of the true covariance changes slowly in time can also be captured We find that using the estimated correlated observation error in the assimilation improves the analysis Keywords: data assimilation, correlated observation errors, ensemble transform Kalman filter (Published: 4 September 2014) Citation: Tellus A 2014, 66 , 23294, http://dxdoiorg/103402/tellusav6623294

Impact of partly ice-free Lake Ladoga on temperature and cloudiness in an anticyclonic winter situation - a case study using a limited area model

Abstract: At the end of January 2012, a low-level cloud from partly ice-free Lake Ladoga caused very variable 2-m temperatures in Eastern Finland. The sensitivity of the High Resolution Limited Area Model (HIRLAM) to the lake surface conditions was tested in this winter anticyclonic situation. The lake appeared to be (incorrectly) totally covered by ice when the lake surface was described with its climatology. Both parametrisation of the lake surface state by using a lake model integrated to the NWP system and objective analysis based on satellite observations independently resulted in a correct description of the partly ice-free Lake Ladoga. In these cases, HIRLAM model forecasts were able to predict cloud formation and its movement as well as 2-m temperature variations in a realistic way. Three main conclusions were drawn. First, HIRLAM could predict the effect of Lake Ladoga on local weather, when the lake surface state was known. Second, the current parametrisation methods of air–surface interactions led to a reliable result in conditions where the different physical processes (local surface processes, radiation and turbulence) were not strong, but their combined effect was important. Third, these results encourage work for a better description of the lake surface state in NWP models by fully utilising satellite observations, combined with advanced lake parametrisation and data assimilation methods. Keywords: lake-effect, data assimilation, lake ice, NWP, cold outbreaks, stable boundary layer, lake–atmosphere interaction (Published: 8 December 2014) Citation: Tellus A 2014, 66 , 23929, http://dx.doi.org/10.3402/tellusa.v66.23929 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

The sensitivity of characteristics of cyclone activity to identification procedures in tracking algorithms

Abstract: The IMILAST project (‘Intercomparison of Mid-Latitude Storm Diagnostics’) was set up to compare low-level cyclone climatologies derived from a number of objective identification algorithms. This paper is a contribution to that effort where we determine the sensitivity of three key aspects of Northern Hemisphere cyclone behaviour [namely the number of cyclones, their intensity (defined here in terms of the central pressure) and their deepening rates] to specific features in the automatic cyclone identification. The sensitivity is assessed with respect to three such features which may be thought to influence the ultimate climatology produced (namely performance in areas of complicated orography, time of the detection of a cyclone, and the representation of rapidly propagating cyclones). We make use of 13 tracking methods in this analysis. We find that the filtering of cyclones in regions where the topography exceeds 1500 m can significantly change the total number of cyclones detected by a scheme, but has little impact on the cyclone intensity distribution. More dramatically, late identification of cyclones (simulated by the truncation of the first 12 hours of cyclone life cycle) leads to a large reduction in cyclone numbers over the both continents and oceans (up to 80 and 40%, respectively). Finally, the potential splitting of the trajectories at times of the fastest propagation has a negligible climatological effect on geographical distribution of cyclone numbers. Overall, it has been found that the averaged deepening rates and averaged cyclone central pressure are rather insensitive to the specifics of the tracking procedure, being more sensitive to the data set used (as shown in previous studies) and the geographical location of a cyclone. Keywords: cyclone identification, IMILAST, cyclone life cycle, rapidly intensifying cyclones, synoptic climatology, reanalysis (Published: 12 December 2014) Citation: Tellus A 2014, 66 , 24961, http://dx.doi.org/10.3402/tellusa.v66.24961 This publication is part of a Thematic Cluster entitled " Intercomparison of Mid-Latitude Storm Diagnostics ". Read the other papers from this thematic cluster here

An algorithm based on sea-level pressure fluctuations to identify major Baltic inflow events

Abstract: Major Baltic inflows are an important process to sustain the sensitive steady state of the Baltic Sea. We introduce an algorithm to identify atmospheric variability favourable for major Baltic inflows. The algorithm is based on sea-level pressure (SLP) fields as the only parameter. Characteristic SLP pattern fluctuations include a precursory phase of 30 days and 10 days of inflow period. The algorithm identifies successfully the majority of observed major Baltic inflows between 1961 and 2010. In addition, the algorithm finds some occurrences which cannot be related to observed inflows. In these cases with favourable atmospheric conditions, inflows were precluded by contemporaneously existing saline water masses or strong freshwater supply. Moreover, the algorithm clearly identifies the stagnation periods as a lack of SLP variability favourable for MBIs. This indicates that the lack of inflows is mainly a consequence of missing atmospheric forcing during this period. The only striking inflow which is not identified by the algorithm is the event in January 2003. We demonstrate that this is due to the special evolution of SLP fields which are not comparable with any other event. Finally, the algorithm is applied to an ensemble of scenario simulations. The result indicates that the number of atmospheric events favourable for major Baltic inflows increases slightly in all scenarios. Keywords: major Baltic inflows, algorithm development, SLP variability, Baltic Sea, climate modeling (Published: 24 October 2014) Citation: Tellus A 2014, 66 , 23452, http://dx.doi.org/10.3402/tellusa.v66.23452

Prior history of Mistral and Tramontane winds modulates heavy precipitation events in southern France

Abstract: Heavy precipitation events (HPEs) are frequent in southern France in autumn. An HPE results from landward transport of low-level moisture from the Western Mediterranean: large potential instability is then released by local convergence and/or orography. In the upstream zone, the sea surface temperature (SST) undergoes significant variations at the submonthly time scale primarily driven by episodic highly energetic events of relatively cold outflows from the neighbouring mountain ranges (the Mistral and Tramontane winds). Here, we study the HPE of 22–23 September 1994 which is preceded by a strong SST cooling due to the Mistral and Tramontane winds. This case confirms that the location of the precipitation is modulated by the SST in the upstream zone. In fact, changes in latent and sensible heat fluxes due to SST changes induce pressure and stratification changes which affect the low-level dynamics. Using three companion regional climate simulations running from 1989 to 2009, this article statistically shows that anomalies in the HPEs significantly correlate with the SST anomalies in the Western Mediterranean, and hence with the prior history of Mistral and Tramontane winds. In such cases, the role of the ocean as an integrator of the effect of past wind events over one or several weeks does indeed have an impact on HPEs in southern France. Keywords: heavy precipitation events, Mediterranean, Mistral, air–sea interactions, CORDEX, atmosphere–ocean coupling (Published: 7 November 2014) Citation: Tellus A 2014, 66, 24064, http://dx.doi.org/10.3402/tellusa.v66.24064 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Multimodel estimates of the changes in the Baltic Sea ice cover during the present century

Abstract: We project changes in the annual maximum ice extent and the maximum coastal fast ice thickness in the Baltic Sea during the ongoing century. The influence of future warming on the ice conditions was assessed using the November–March Baltic coastal mean temperature as a predictor for the annual maximum ice extent (MIB), and the local freezing degree-day sum as a predictor for the fast ice thickness. Future winter temperatures were derived by adjusting observational baseline-period temperatures in accordance with temperature projections based on 28 global climate models (GCMs) participating in the Coupled Model Intercomparison Project Phase 5. Under the Representative Concentration Pathway (RCP) 4.5 scenario, the ensemble-mean trend of MIB is −6400 km 2 /10 yr, and from the 2060s onwards in a typical winter MIB remains below 80×10 3 km 2 . If the RCP8.5 scenario is realised, the corresponding estimates are −10 900 km 2 /10 yr for the trend and 60×10 3 km 2 for a typical MIB. For cold rather than typical winters, the projected rate of decrease in MIB is even faster. During the late century under RCP8.5, in 9 out of 10 yr the ice would only cover 5–20% of the total sea area. The projected trends in the mean annual maximum ice thickness are −7.6 … −3.3 cm/10 yr, depending on location and applied scenario. In the 2040s under both scenarios, and in the 2080s under RCP4.5, the ice thickness may still exceed 60 cm in the northernmost Bay of Bothnia, while elsewhere in the Gulf of Bothnia and in the Gulf of Finland, it will vary between about 10 and 40 cm. In the 2080s under RCP8.5, virtually no ice occurs outside the Bay of Bothnia. For both the ice extent and thickness, the spread among the responses based on the temperature projections of individual GCMs is considerable. Nonetheless, a robust finding is that the Baltic Sea is unlikely to become totally ice-free during this century. Keywords: sea ice extent, ice thickness, climate change, CMIP5 models, inter-model differences, inter-annual variability (Published: 4 April 2014) Citation: Tellus A 2014, 66 , 22617, http://dx.doi.org/10.3402/tellusa.v66.22617

Modelling the impact of Baltic Sea upwelling on the atmospheric boundary layer

Abstract: Coastal upwelling, with a strong sea-surface temperature (SST) signal, is extremely common in the Baltic Sea during the summer months. Although the spatial scale of upwelling is small, its high frequency of occurrence in the semi-enclosed basin may allow the SST signature to have significant feedback onto the lower atmosphere. In this paper, we develop a method to remove the signature of upwelling from SST fields, and use these modified SST fields as the lower boundary condition of an atmospheric model, allowing us to evaluate the importance of coastal upwelling to the mean summer conditions over the Baltic. The contribution of upwelling is found to be small generally, although significant locally over areas where upwelling is most common. This includes 2 m air temperature reductions of up to 2°C, wind speed reductions of up to 0.25 m s −1 , and reductions in the mean boundary-layer height of up to 100 m. High-resolution simulations with artificially enhanced upwelling suggest that upwelling may become increasingly important as winds slacken at the end of an upwelling period. Keywords: Upwelling, Air-sea interaction, Boundary-Layer Meteorology, Physical Oceanography (Published: 29 September 2014) Citation: Tellus A 2014, 66 , 24041, http://dx.doi.org/10.3402/tellusa.v66.24041

Evaluating decadal predictions of northern hemispheric cyclone frequencies

Abstract: Mid-latitudinal cyclones are a key factor for understanding regional anomalies in primary meteorological parameters such as temperature or precipitation. Extreme cyclones can produce notable impacts on human society and economy, for example, by causing enormous economic losses through wind damage. Based on 41 annually initialised (1961–2001) hindcast ensembles, this study evaluates the ability of a single-model decadal forecast system (MPI-ESM-LR) to provide skilful probabilistic three-category forecasts (enhanced, normal or decreased) of winter (ONDJFM) extra-tropical cyclone frequency over the Northern Hemisphere with lead times from 1 yr up to a decade. It is shown that these predictions exhibit some significant skill, mainly for lead times of 2–5 yr, especially over the North Atlantic and Pacific. Skill for intense cyclones is generally higher than for all detected systems. A comparison of decadal hindcasts from two different initialisation techniques indicates that initialising from reanalysis fields yields slightly better results for the first forecast winter (month 10–15), while initialisation based on an assimilation experiment provides better skill for lead times between 2 and 5 yr. The reasons and mechanisms behind this predictive skill are subject to future work. Preliminary analyses suggest a strong relationship of the model’s skill over the North Atlantic with the ability to predict upper ocean temperatures modulating lower troposphere baroclinicity for the respective area and time scales. Keywords: climate modelling, decadal predictions, probabilistic forecasts, verification, extra-tropical cyclones, MiKlip (Published: 17 April 2014) Citation: Tellus A 2014, 66 , 22830, http://dx.doi.org/10.3402/tellusa.v66.22830

Improving quantitative precipitation nowcasting with a local ensemble transform Kalman filter radar data assimilation system: observing system simulation experiments

Abstract: This study develops a Doppler radar data assimilation system, which couples the local ensemble transform Kalman filter with the Weather Research and Forecasting model. The benefits of this system to quantitative precipitation nowcasting (QPN) are evaluated with observing system simulation experiments on Typhoon Morakot (2009), which brought record-breaking rainfall and extensive damage to central and southern Taiwan. The results indicate that the assimilation of radial velocity and reflectivity observations improves the three-dimensional winds and rain-mixing ratio most significantly because of the direct relations in the observation operator. The patterns of spiral rainbands become more consistent between different ensemble members after radar data assimilation. The rainfall intensity and distribution during the 6-hour deterministic nowcast are also improved, especially for the first 3 hours. The nowcasts with and without radar data assimilation have similar evolution trends driven by synoptic-scale conditions. Furthermore, we carry out a series of sensitivity experiments to develop proper assimilation strategies, in which a mixed localisation method is proposed for the first time and found to give further QPN improvement in this typhoon case. Keywords: radar data assimilation, local ensemble transform Kalman filter, quantitative precipitation nowcasting, observing system simulation experiment, mixed localisation method (Published: 5 March 2014) Citation: Tellus A 2014, 66 , 21804, http://dx.doi.org/10.3402/tellusa.v66.21804

Etnodesenvolvimento local: autonomia cultural na era do neoliberalismo global

Abstract: O artigo discute o conceito de etnodesenvolvimento local no contexto da proposta universalista do desenvolvimento economico por um lado e do crescente reconhecimento da diversidade cultural por outro. Usando uma perspectiva antropologica, o texto analisa as variadas criticas levantadas contra o modelo hegemonico vigente de desenvolvimento junto com o surgimento de novas propostas para um etnodesenvolvimento centrado nas reivindicacoes e necessidades de determinados grupos etnicos da America Latina, com enfase nos grupos indigenas do Brasil. Propoe que a nocao de autonomia cultural esteja na base de qualquer programa de etnodesenvolvimento, ao mesmo tempo em que reconhece a necessidade do grupo etnico elaborar estrategias de interacao com os mercados regional, nacional e internacional. O texto conclui com uma breve revisao tanto dos perigos quanto dos desafios da procura de um etnodesenvolvimento ambientalmente sustentavel.

Changes in extreme sea-levels in the Baltic Sea

Abstract: In a climate change context, changes in extreme sea-levels rather than changes in the mean are of particular interest from the coastal protection point of view. In this work, extreme sea-levels in the Baltic Sea are investigated based on daily tide gauge records for the period 1916–2005 using the annual block maxima approach. Extreme events are analysed based on the generalised extreme value distribution considering both stationary and time-varying models. The likelihood ratio test is applied to select between stationary and non-stationary models for the maxima and return values are estimated from the final model. As an independent and complementary approach, quantile regression is applied for comparison with the results from the extreme value approach. The rates of change in the uppermost quantiles are in general consistent and most pronounced for the northernmost stations. Keywords: Baltic Sea, sea-level, extremes, tide gauges, quantile regression (Published: 15 April 2014) Citation: Tellus A 2014, 66 , 20921, http://dx.doi.org/10.3402/tellusa.v66.20921

Wintertime cyclone/anticyclone activity over China and its relation to upper tropospheric jets

Abstract: In this study, the wintertime cyclone/anticyclone activity and its variability over China are examined based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1948 to 2007. The climatology of the source, path and lysis regions for cyclones/anticyclones is investigated using an automatic tracking algorithm. Apparent asymmetries in source, lysis and path regions are observed between cyclones and anticyclones. The 1948–2007 data exhibit an upward trend in the annual number and a downward trend in the cyclone and anticyclone intensity. The leading empirical orthogonal function (EOF) mode of the cyclone transit counts (CTC) for the 1948–2007 period indicates an increase in cyclone activity over northeastern East Asia since the late 1970s that becomes significant in the mid-1980s. The first EOF mode of the anticyclone transit counts (ATCs) is a monopole over northern East Asia, centred west of Lake Baikal, which has increased since 1970. The CTC variability dominates the ATC variability, which corresponds well with the storm track variability. Two distinct variability modes in the upper tropospheric jets over East Asia are also observed. The first mode describes an oscillation in the subtropical jet position; the second mode describes the polar-front jet strength variation. Moreover, the second mode is closely linked to the cyclone/anticyclone activity variability. Keywords: storm tracks, cyclones/anticyclones, East Asia climate (Published: 14 January 2014) Citation: Tellus A 2014, 66 , 21889, http://dx.doi.org/10.3402/tellusa.v66.21889

Which significance test performs the best in climate simulations

Abstract: Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student’s t- test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student’s t- test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375–1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> + 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student’s t- test by the advanced techniques in most cases. Keywords: autocorrelation, temporal correlation, internal variability, climate noise, significance test, Student’s t-test (Published: 25 January 2014) Citation: Tellus A 2014, 66 , 23139, http://dx.doi.org/10.3402/tellusa.v66.23139

Impact of saltwater inflows on phosphorus cycling and eutrophication in the Baltic Sea: a 3D model study

Abstract: The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracer experiments with a three-dimensional physical model Model simulations showed that the coasts of the North West Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions where tracers from below the halocline are primarily lifted up above the halocline After 1 yr tracers are accumulated at the surface along the Swedish east coast and at the western and southern sides of Gotland Elevated concentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in the central parts of the East Gotland Basin The annual supplies of phosphorus from the deeper waters to the productive surface layers are estimated to be of the same order of magnitude as the waterborne inputs of phosphorus to the entire Baltic Sea The model results suggest that regionally the impact of these nutrients may be quite large, and the largest regional increases in surface concentrations are found after large inflows However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below the halocline to the surface waters is small because vertical transports are comparably large also during periods without major inflows Our model results suggest that phosphorus released from the sediments between 60 and 100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of the eastern Baltic Proper Keywords: Phosphorus, eutrophication, saltwater inflows, biogeochemistry, Baltic Sea, numerical modelling (Published: 21 August 2014) Citation: Tellus A 2014, 66 , 23985, http://dxdoiorg/103402/tellusav6623985

A comparison of the performance of the 3-D super-ensemble and an ensemble Kalman filter for short-range regional ocean prediction

Abstract: This study compares the ability of two approaches integrating models and data to forecast the Ligurian Sea regional oceanographic conditions in the short-term range (0–72 hours) when constrained by a common observation dataset. The post-processing 3-D super-ensemble (3DSE) algorithm, which uses observations to optimally combine multi-model forecasts into a single prediction of the oceanic variable, is first considered. The 3DSE predictive skills are compared to those of the Regional Ocean Modeling System model in which observations are assimilated through a more conventional ensemble Kalman filter (EnKF) approach. Assimilated measurements include sea surface temperature maps, and temperature and salinity subsurface observations from a fleet of five underwater gliders. Retrospective analyses are carried out to produce daily predictions during the 11-d period of the REP10 sea trial experiment. The forecast skill evaluation based on a distributed multi-sensor validation dataset indicates an overall superior performance of the EnKF, both at the surface and at depth. While the 3DSE and EnKF perform comparably well in the area spanned by the incorporated measurements, the 3DSE accuracy is found to rapidly decrease outside this area. In particular, the univariate formulation of the method combined with the absence of regular surface salinity measurements produces large errors in the 3DSE salinity forecast. On the contrary, the EnKF leads to more homogeneous forecast errors over the modelling domain for both temperature and salinity. The EnKF is found to consistently improve the predictions with respect to the control solution without assimilation and to be positively skilled when compared to the climatological estimate. For typical regional oceanographic applications with scarce subsurface observations, the lack of physical spatial and multivariate error covariances applicable to the individual model weights in the 3DSE formulation constitutes a major limitation for the performance of this multi-model-data fusion approach compared to conventional advanced data assimilation strategies. Keywords: operational oceanography, regional ocean prediction, data assimilation, ensemble Kalman filter, 3-D super-ensemble, underwater gliders, Ligurian Sea (Published: 16 January 2014) Citation: Tellus A 2014, 66 , 21640, http://dx.doi.org/10.3402/tellusa.v66.21640

An operational analysis of Lake Surface Water Temperature

Abstract: Operational analyses of Lake Surface Water Temperature (LSWT) have many potential uses including improvement of numerical weather prediction (NWP) models on regional scales. In November 2011, LSWT was included in the Met Office Operational Sea Surface Temperature and Ice Analysis (OSTIA) product, for 248 lakes globally. The OSTIA analysis procedure, which has been optimised for oceans, has also been used for the lakes in this first version of the product. Infra-red satellite observations of lakes and in situ measurements are assimilated. The satellite observations are based on retrievals optimised for Sea Surface Temperature (SST) which, although they may introduce inaccuracies into the LSWT data, are currently the only near-real-time information available. The LSWT analysis has a global root mean square difference of 1.31 K and a mean difference of 0.65 K (including a cool skin effect of 0.2 K) compared to independent data from the ESA ARC-Lake project for a 3-month period (June to August 2009). It is demonstrated that the OSTIA LSWT is an improvement over the use of climatology to capture the day-to-day variation in global lake surface temperatures. Keywords : LSWT, OSTIA, satellite, in situ, near-real-time (Published: 15 June 2014) Citation: Tellus A 2014, 66, 21247, http://dx.doi.org/10.3402/tellusa.v66.21247 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

The importance of using a high-resolution model to study the climate change on small islands: the Lesser Antilles case

Abstract: High-resolution climate change simulations over the Lesser Antilles are performed using the ALADIN-Climate regional climate model nested within the global model ARPEGE (Meteo-France). Three sets of simulations are conducted at 10 km grid spacing for reference (1971-2000) and future climate (2071-2100) under two CMIP5 scenarios (RCP4.5 and RCP8.5). With the dynamical downscaling, islands of Lesser Antilles are considered as land by the model, whereas, for the driving model, there is only sea over the domain. Temperature and precipitation change are analysed on land and on sea separately. For temperature, the warming is greater on land than on sea, especially for the minimum daily temperature (3.28C vs. 2.38C for the RCP85 scenario). For precipitation, projections are less reliable because the seasonality is not well reproduced by the model. Nevertheless, simulations exhibit the fact that projections on land differ from one island to the other and disagree with those on sea notably during the wet season. This underlines the importance of the dynamical downscaling to study the climate on small islands. Statistical downscaling has been performed on the Guadeloupe Island to study changes in extreme precipitation indices. The projections provided by the regional climate model suggest an increase in extreme rainfall events: longer dry periods, a bigger annual total precipitation, more frequent very heavy daily precipitation and a stronger 1 day maximum precipitation, whereas for the driving Global Climate Model, these trends are less intense.