Limited-area Atmospheric Model Research Articles

Assessing the impact of Argo floats temperature measurements on the numerical weather prediction forecast skill

The forecast skill of numerical weather prediction (NWP) models relies, among other factors such as the prediction itself and the assimilation scheme, on the accuracy of the observations utilized in the assimilation systems for the production of initial and boundary conditions. One of the most crucial parameters in weather forecasting is the sea surface temperature (SST). In the majority of NWP models, the initial and lower boundary conditions involve gridded (SST) analyses which consist of data obtained by buoys, ships and satellites. The main aim of this study is to integrate Argo temperature measurements in gridded SST analyses and to assess their impact on the forecast skill of a limited area atmospheric model. Argo floats are “state-of-the-art” oceanographic instruments producing high-quality temperature profiles for the ice-free ocean. In this study, Argo temperatures are incorporated into gridded SST fields without applying any smoothing method in order to directly assess the impact of Argo temperatures on numerical weather prediction. Their impact is assessed under intense weather cyclonic conditions at the Mediterranean Sea by performing two sensitivity simulations either incorporating or not Argo temperatures into gridded SST fields used in the generation of the initial and lower boundary conditions. The results indicate that the inclusion of Argo-measured near-surface temperatures in the lower boundary condition modifies the surface heat fluxes, thus affecting mean sea level pressure and precipitation. In particular, an overall improvement of the precipitation forecast skill up to 3% has been demonstrated. Moreover, the incorporation of Argo temperatures affects the simulated track and intensity of the cyclone over the Balkan Peninsula.

Assessment of the role of sea surface fluxes on eastern Mediterranean explosive cyclogenesis with the aid of the limited-area model COSMO.GR

Explosive cyclogenesis, being characterised by complex non adiabatic dynamics at low levels, is difficult to be predicted by operational models, especially in closed maritime regions such as the Mediterranean Sea. The objective of this study is to examine the sensitivity of the limited-area atmospheric model COSMO.GR, which runs operationally at the Hellenic National Meteorological Service (HNMS) as a forecast tool, in effectively simulating the deepening rate and to further explore the role of sea surface heat fluxes for two cases of explosive cyclogenesis in the eastern Mediterranean. Model runs were performed for six different values of the sea roughness model parameter, being proven, among others, to significantly affect the evolution of sea surface heat fluxes. The simulation justified the crucial role of sea surface heat fluxes in explosive cyclogenesis in the eastern Mediterranean by influencing the cyclone deepening rates and the respective minimum values of mean seal level pressure. The smaller values of the sea roughness parameter as compared to the default model value (equal to 20), leads to significant enhancement of the simulated sea surface heat fluxes and in turn produces higher deepening rates during the explosive deepening period. For both cases, the evaluation of the simulated mean sea level pressure values against observations demonstrated that values 10 and 1 better simulated the evolution of the mean sea level pressure along the cyclonic tracks. The results of this study act as a motivation to test the role of specific parameters in operational models associated with diabatic processes towards the improvement of local forecasts, mainly during severe phenomena.

Numerical modeling analysis of the mesoscale environment conducive to two tornado events using the COSMO.Gr model over Greece

The COnsortium for Small-scale MOdeling (COSMO) was formed in October 1998, and its general goal is to develop, improve and maintain a non-hydrostatic limited-area atmospheric model. The COSMO model has been designed both for operational numerical weather prediction (NWP) as well as various scientific applications on the meso-β and meso-γ scale. Two tornado case studies were selected to investigate the ability of COSMO model to depict the characteristics of severe convective weather, which favoured the development of the associated storms.The first tornado (TR01) occurred, close to Ag. Ilias village, 8 Km north-western of Aitoliko city over western Greece on February 7, 2013, while the second tornado (TR02) was developed close to Palio Katramio village, 8 Km southern from Xanthi city over northern Greece on November 25, 2015. Although both tornadoes had a short lifetime, they caused significant damages. The COSMO.GR atmospheric model was initialized with analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The resulting numerical products with spatial resolution of 0.02° (∼2km) over the geographical domain of Greece depicted very well the severe convective conditions close to tornadoes formation. The Energy Helicity Index (EHI) diagnostic variable in both numerical simulations showed a gradual increase of values closing to the location and time of the tornadogenesis. Similar to EHI, the storm relative helicity (SRH) spatio-temporal analysis followed a gradual increase prior to the tornadogenesis events and was reduced after them.

Physical Processes Controlling the Tide in the Tropical Lower Atmosphere Investigated Using a Comprehensive Numerical Model

Abstract The lower-atmospheric circulation in the tropics is strongly influenced by large-scale daily variations referred to as atmospheric solar tides. Most earlier studies have used simplified linear theory to explain daily variations in the tropics. The present study employs a comprehensive limited-area atmospheric model and revisits some longstanding issues related to atmospheric tidal dynamics. The tides in the tropical lower atmosphere are realistically simulated in the control experiment with a near-global (75°S–75°N) version of the model. Sensitivity experiments with different aspects of the solar heating suppressed showed that the semidiurnal (S2) tide near the surface can be attributed roughly equally to stratospheric and tropospheric direct solar heating and that the diurnal (S1) tide is excited almost entirely by tropospheric direct solar heating as well as solar heating of Earth’s surface. Linear theory with forcing only by direct radiative heating predicts the phase of the S2 barometric oscillation should be ~0910 LT versus the ~0945 LT phase seen in low-latitude observations. The roles of (i) convective and latent heating and (ii) mechanical dissipation, in determining the S2 phase, are assessed in the model. It is found that the former effect delays the phase by ~25 min and the latter by ~5 min; these two effects together explain the observed phase. When the model is run in limited-area domains comparable to those used in typical regional climate studies the S2, but not S1, tide is found to be significantly weaker than observed, even using atmospheric reanalysis data to drive the lateral boundaries.

Two-Dimensional Evaluation of ATHAM-Fluidity, a Nonhydrostatic Atmospheric Model Using Mixed Continuous/Discontinuous Finite Elements and Anisotropic Grid Optimization

AbstractThis paper presents the first attempt to apply the compressible nonhydrostatic Active Tracer High-Resolution Atmospheric Model–Fluidity (ATHAM-Fluidity) solver to a series of idealized atmospheric test cases. ATHAM-Fluidity uses a hybrid finite-element discretization where pressure is solved on a continuous second-order grid while momentum and scalars are computed on a first-order discontinuous grid (also known as ). ATHAM-Fluidity operates on two- and three-dimensional unstructured meshes, using triangular or tetrahedral elements, respectively, with the possibility to employ an anisotropic mesh optimization algorithm for automatic grid refinement and coarsening during run time. The solver is evaluated using two-dimensional-only dry idealized test cases covering a wide range of atmospheric applications. The first three cases, representative of atmospheric convection, reveal the ability of ATHAM-Fluidity to accurately simulate the evolution of large-scale flow features in neutral atmospheres at rest. Grid convergence without adaptivity as well as the performances of the Hermite–Weighted Essentially Nonoscillatory (Hermite-WENO) slope limiter are discussed. These cases are also used to test the grid optimization algorithm implemented in ATHAM-Fluidity. Adaptivity can result in up to a sixfold decrease in computational time and a fivefold decrease in total element number for the same finest resolution. However, substantial discrepancies are found between the uniform and adapted grid results, thus suggesting the necessity to improve the reliability of the approach. In the last three cases, corresponding to atmospheric gravity waves with and without orography, the model ability to capture the amplitude and propagation of weak stationary waves is demonstrated. This work constitutes the first step toward the development of a new comprehensive limited area atmospheric model.

Performance evaluation of Wavewatch III in the Mediterranean Sea

For this study, we analyzed the performance of the wave model Wavewatch III® forced by a limited area atmospheric model in the Mediterranean Sea. The simulation results have been compared to buoy measurements through single point statistical indicators such as normalized bias and symmetrically normalized root mean square error. A performance evaluation of the growth-dissipation source terms and their reference parameterizations was carried out on seventeen case studies corresponding to storms in the Northern Tyrrhenian Sea and off the Mediterranean Spanish coast. The source terms introduced by Ardhuin et al. (2010) proved to be the best overall choice, although they led to an overestimation in the significant wave height under calm conditions and to an underestimation under severe conditions. A sensitivity analysis in the parameter space was performed within the neighborhood of the reference parameterization of Ardhuin et al. (2010), and a calibration was carried out to reduce the overall positive bias in the significant wave height. Furthermore, to investigate the effect from the wind forcing resolution, wind data with different resolutions was used in a sensitivity analysis. Because mesoscale features are relevant to the overall Mediterranean wave dynamics, we carried out a further investigation into the impact of the resolution on a different set of ten case studies characterized by strong mesoscale patterns. A comparison of the simulations with the measurements using single point statistical indicators shows that the high resolution results are affected by the so-called double penalty effect, although in some cases, they apparently provide a better qualitative description of the event. Finally, a hindcast covering 32 years (from 1979 to 2010) was developed using a reference parameterization from Ardhuin et al. (2010) and its calibrated variant. An analysis of the performance of the calibrated parameterization on the hindcast dataset reveals that it performs better than the reference parameterization over a wide range of wave heights, in seas that range from calm to moderate, whereas it increases the tendency to underestimate the significant wave height under severe conditions.

Hydrometeorological multi-model ensemble simulations of the 4 November 2011 flash flood event in Genoa, Italy, in the framework of the DRIHM project

Abstract. The e-Science environment developed in the framework of the EU-funded DRIHM project was used to demonstrate its ability to provide relevant, meaningful hydrometeorological forecasts. This was illustrated for the tragic case of 4 November 2011, when Genoa, Italy, was flooded as the result of heavy, convective precipitation that inundated the Bisagno catchment. The Meteorological Model Bridge (MMB), an innovative software component developed within the DRIHM project for the interoperability of meteorological and hydrological models, is a key component of the DRIHM e-Science environment. The MMB allowed three different rainfall-discharge models (DRiFt, RIBS and HBV) to be driven by four mesoscale limited-area atmospheric models (WRF-NMM, WRF-ARW, Meso-NH and AROME) and a downscaling algorithm (RainFARM) in a seamless fashion. In addition to this multi-model configuration, some of the models were run in probabilistic mode, thus giving a comprehensive account of modelling errors and a very large amount of likely hydrometeorological scenarios (> 1500). The multi-model approach proved to be necessary because, whilst various aspects of the event were successfully simulated by different models, none of the models reproduced all of these aspects correctly. It was shown that the resulting set of simulations helped identify key atmospheric processes responsible for the large rainfall accumulations over the Bisagno basin. The DRIHM e-Science environment facilitated an evaluation of the sensitivity to atmospheric and hydrological modelling errors. This showed that both had a significant impact on predicted discharges, the former being larger than the latter. Finally, the usefulness of the set of hydrometeorological simulations was assessed from a flash flood early-warning perspective.

Medicanes in an ocean–atmosphere coupled regional climate model

Abstract. So-called medicanes (Mediterranean hurricanes) are meso-scale, marine, and warm-core Mediterranean cyclones that exhibit some similarities to tropical cyclones. The strong cyclonic winds associated with medicanes threaten the highly populated coastal areas around the Mediterranean basin. To reduce the risk of casualties and overall negative impacts, it is important to improve the understanding of medicanes with the use of numerical models. In this study, we employ an atmospheric limited-area model (COSMO-CLM) coupled with a one-dimensional ocean model (1-D NEMO-MED12) to simulate medicanes. The aim of this study is to assess the robustness of the coupled model in simulating these extreme events. For this purpose, 11 historical medicane events are simulated using the atmosphere-only model, COSMO-CLM, and coupled model, with different setups (horizontal atmospheric grid spacings of 0.44, 0.22, and 0.08°; with/without spectral nudging, and an ocean grid spacing of 1/12°). The results show that at high resolution, the coupled model is able to not only simulate most of medicane events but also improve the track length, core temperature, and wind speed of simulated medicanes compared to the atmosphere-only simulations. The results suggest that the coupled model is more proficient for systemic and detailed studies of historical medicane events, and that this model can be an effective tool for future projections.

Oceanic Forcing of Antarctic Climate Change: A Study Using a Stretched-Grid Atmospheric General Circulation Model

Abstract A variable-resolution atmospheric general circulation model (AGCM) is used for climate change projections over the Antarctic. The present-day simulation uses prescribed observed sea surface conditions, while a set of five simulations for the end of the twenty-first century (2070–99) under the Special Report on Emissions Scenarios (SRES) A1B scenario uses sea surface condition anomalies from selected coupled ocean–atmosphere climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3). Analysis of the results shows that the prescribed sea surface condition anomalies have a very strong influence on the simulated climate change on the Antarctic continent, largely dominating the direct effect of the prescribed greenhouse gas concentration changes in the AGCM simulations. Complementary simulations with idealized forcings confirm these results. An analysis of circulation changes using self-organizing maps shows that the simulated climate change on regional scales is not principally caused by shifts of the frequencies of the dominant circulation patterns, except for precipitation changes in some coastal regions. The study illustrates that in some respects the use of bias-corrected sea surface boundary conditions in climate projections with a variable-resolution atmospheric general circulation model has some distinct advantages over the use of limited-area atmospheric circulation models directly forced by generally biased coupled climate model output.

Downscaling ocean conditions: Experiments with a quasi-geostrophic model

The predictability of small-scale ocean variability, given the time history of the associated large-scales, is investigated using a quasi-geostrophic model of two wind-driven gyres separated by an unstable, mid-ocean jet. Motivated by the recent theoretical study of Henshaw et al. (2003), we propose a straightforward method for assimilating information on the large-scale in order to recover the small-scale details of the quasi-geostrophic circulation. The similarity of this method to the spectral nudging of limited area atmospheric models is discussed. Results from the spectral nudging of the quasi-geostrophic model, and an independent multivariate regression-based approach, show that important features of the ocean circulation, including the position of the meandering mid-ocean jet and the associated pinch-off eddies, can be recovered from the time history of a small number of large-scale modes. We next propose a hybrid approach for assimilating both the large-scales and additional observed time series from a limited number of locations that alone are too sparse to recover the small scales using traditional assimilation techniques. The hybrid approach improved significantly the recovery of the small-scales. The results highlight the importance of the coupling between length scales in downscaling applications, and the value of assimilating limited point observations after the large-scales have been set correctly. The application of the hybrid and spectral nudging to practical ocean forecasting, and projecting changes in ocean conditions on climate time scales, is discussed briefly.

Overlapping Interests: The Impact of Geographic Coordinate Assumptions on Limited-Area Atmospheric Model Simulations

AbstractThere is growing use of limited-area models (LAMs) for high-resolution (<10 km) applications, for which consistent mapping of input terrestrial and meteorological datasets is critical for accurate simulations. The geographic coordinate systems of most input datasets are based on spheroid-shaped (i.e., elliptical) Earth models, while LAMs generally assume a perfectly sphere-shaped Earth. This distinction is often neglected during preprocessing, when input data are remapped to LAM domains, leading to geolocation discrepancies that can exceed 20 km at midlatitudes.A variety of terrestrial (topography and land use) input dataset configurations is employed to explore the impact of Earth model assumptions on a series of 1-km LAM simulations over Colorado. For the same terrestrial datasets, the ~20-km geolocation discrepancy between spheroidal-versus-spherical Earth models over the domain leads to simulated differences in near-surface and midtropospheric air temperature, humidity, and wind speed that are larger and more widespread than those due to using different topography and land use datasets altogether but not changing the Earth model. Simulated differences are caused by the shift of static fields with respect to boundary conditions, and altered Coriolis forcing and topographic gradients.The sensitivity of high-resolution LAM simulations to Earth model assumptions emphasizes the importance for users to ensure terrestrial and meteorological input data are consistently mapped during preprocessing (i.e., datasets share a common geographic coordinate system before remapping to the LAM domain). Concurrently, the modeling community should update preprocessing systems to make sure input data are correctly mapped for all global and limited-area simulation domains.

Urbanization and climate change impacts on future urban flooding in Can Tho city, Vietnam

Abstract. Urban development increases flood risk in cities due to local changes in hydrological and hydrometeorological conditions that increase flood hazard, as well as to urban concentrations that increase the vulnerability. The relationship between the increasing urban runoff and flooding due to increased imperviousness is better perceived than that between the cyclic impact of urban growth and the urban rainfall via microclimatic changes. The large-scale, global impacts due to climate variability and change could compound these risks. We present the case of a typical third world city – Can Tho (the biggest city in Mekong River Delta, Vietnam) – faced with multiple future challenges, namely: (i) the likely effect of climate change-driven sea level rise, (ii) an expected increase of river runoff due to climate change as estimated by the Vietnamese government, (iii) increased urban runoff driven by imperviousness, and (iv) enhancement of extreme rainfall due to urban growth-driven, microclimatic change (urban heat islands). A set of model simulations were used to construct future scenarios, combining these influences. Urban growth of the city was projected up to year 2100 based on historical growth patterns, using a land use simulation model (Dinamica EGO). A dynamic limited-area atmospheric model (WRF), coupled with a detailed land surface model with vegetation parameterization (Noah LSM), was employed in controlled numerical experiments to estimate the anticipated changes in extreme rainfall patterns due to urban heat island effect. Finally, a 1-D/2-D coupled urban-drainage/flooding model (SWMM-Brezo) was used to simulate storm-sewer surcharge and surface inundation to establish the increase in the flood hazard resulting from the changes. The results show that under the combined scenario of significant change in river level (due to climate-driven sea level rise and increase of flow in the Mekong) and "business as usual" urbanization, the flooding of Can Tho could increase significantly. The worst case may occur if a sea level rise of 100 cm and the flow from upstream happen together with high-development scenarios. The relative contribution of causes of flooding are significantly different at various locations; therefore, detailed research on adaptation are necessary for future investments to be effective.

Analysis of a two-year meteorological dataset produced on Italian territory with a coupling procedure between a limited area atmospheric model and a sequential MSG-SEVIRI LST assimilation scheme

The sensible and latent heat exchanges at the interface between soil and atmosphere are fundamental components of the surface energy budget and constitute the lower boundary condition for atmospheric numerical models. These models often represent such phenomena in very simplified ways or employ a detailed description of these dynamics that require complex parameterization, difficult or impossible to retrieve with sufficient accuracy. The characterization of such variables on large areas is possible only through remote-sensing techniques by monitoring indirectly related variables, with particular reference to the land surface temperature (LST). In this work, the Regional Atmospheric Modeling System (RAMS), a limited area atmospheric model, was coupled with the 1-D LST variational assimilation scheme Assimilation Code for Hydrologic-Atmospheric Budget (ACHAB), which is able to provide optimal estimates of the surface energy budget. The source of the LST maps for this work was Meteosat Second Generation-Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI) imagery. A 2 year-long meteorological dataset (2005–2006) was produced on the Italian territory using this coupling framework. The purpose of the experiment was to evaluate how much the assimilation of satellite-derived LST in a simple surface energy budget model is able to affect the dynamics of the troposphere on the study area, considering both summer and winter. Evaluations of the results of the coupling framework were carried out by comparing with both observations of the ground sensors network and the atmospheric soundings available in the study period. The analysis showed significant improvements in terms of reconstructions of the surface and atmospheric states during summer, while a general worsening of the performances of the atmospheric model was observed in winter months.

Application of Boyd’s Periodization and Relaxation Method in a Spectral Atmospheric Limited-Area Model. Part II: Accuracy Analysis and Detailed Study of the Operational Impact

Abstract Spectral limited-area models face a particular challenge at their lateral boundaries: the fields need to be made periodic. Boyd proposed a windowing-based method to improve the periodization and relaxation. In a companion paper, the implementation of this windowing method in the operational semi-implicit semi-Lagrangian spectral HARMONIE system was described and some first reproducibility tests, comparing this method to the old existing one, were presented. The present paper provides an in-depth study of the impact of this method for different configurations of the implementation. This is carried out in three steps in well-controlled experimental setups of increasing complexity. First, different aspects of Boyd’s method are analyzed in an idealized perfect-model test using a representative 1D shallow-water model. Second, the implementation is tested in an adiabatic 3D numerical weather prediction (NWP) model with perfect-model experiments. Finally, the impact of using Boyd’s method in a more operational-like NWP context is investigated as well. The presented tests show that, while the implementation of Boyd’s method is neutral in terms of scores, it is superior to the existing spline method in the case of strong dynamical forcings at the lateral boundaries.

The 1-way on-line coupled atmospheric chemistry model system MECO(n) – Part 1: Description of the limited-area atmospheric chemistry model COSMO/MESSy

Abstract. The numerical weather prediction model of the Consortium for Small Scale Modelling (COSMO), maintained by the German weather service (DWD), is connected with the Modular Earth Submodel System (MESSy). This effort is undertaken in preparation of a new, limited-area atmospheric chemistry model. Limited-area models require lateral boundary conditions for all prognostic variables. Therefore the quality of a regional chemistry model is expected to improve, if boundary conditions for the chemical constituents are provided by the driving model in consistence with the meteorological boundary conditions. The new developed model is as consistent as possible, with respect to atmospheric chemistry and related processes, with a previously developed global atmospheric chemistry general circulation model: the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. The combined system constitutes a new research tool, bridging the global to the meso-γ scale for atmospheric chemistry research. MESSy provides the infrastructure and includes, among others, the process and diagnostic submodels for atmospheric chemistry simulations. Furthermore, MESSy is highly flexible allowing model setups with tailor made complexity, depending on the scientific question. Here, the connection of the MESSy infrastructure to the COSMO model is documented and also the code changes required for the generalisation of regular MESSy submodels. Moreover, previously published prototype submodels for simplified tracer studies are generalised to be plugged-in and used in the global and the limited-area model. They are used to evaluate the TRACER interface implementation in the new COSMO/MESSy model system and the tracer transport characteristics, an important prerequisite for future atmospheric chemistry applications. A supplementary document with further details on the technical implementation of the MESSy interface into COSMO with a complete list of modifications to the COSMO code is provided.

A new approach to resolving climate-cryosphere relations: Downscaling climate dynamics to glacier-scale mass and energy balance without statistical scale linking

[1] We present a novel combination of methods to quantify the local mass response of mountain glaciers to large-scale circulation. Previously, such multiscale approaches bypassed the mesoscale processes of the mountain-induced atmospheric flow, by statistical transfer functions or subgrid parameterizations, and included simplified glacier mass balance (MB) models. Here we show, on the basis of Kilimanjaro (East Africa) as a test case, that a limited area atmospheric model (LAM) and a process-resolving MB model can be linked without statistical corrections at their interface. This is evident from robust energy and MB patterns at the glacier surface, regardless of whether the MB model is forced by (1) in situ meteorological measurements or (2) uncorrected output from the high-resolution LAM grid over the glacier area. The latter is achieved by multiple grid nesting in the land-atmosphere-ocean domain of the LAM. Since this setup resolves the mesoscale process space, we also show the potential to increase knowledge of how dynamical, thermodynamic, and microphysical phenomena of the mountain-induced flow affect glacier MB. All these results are encouraging for future research because they demonstrate that a dynamical system, which operates on very different space-time scales, can be quantified in a fully physical way, if dynamic meteorology and glaciology are exploited in a complementary sense. This will enhance the process understanding of forward problems (glacier response to climate forcing) and backward problems (climate signal extraction from past extents of mountain glaciers).

Soil moisture impacts on convective indices and precipitation over complex terrain

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields.

Influence of coupling on atmosphere, sea ice and ocean regional models in the Ross Sea sector, Antarctica

Air–sea ice–ocean interactions in the Ross Sea sector form dense waters that feed the global thermohaline circulation. In this paper, we develop the new limited-area ocean–sea ice–atmosphere coupled model TANGO to simulate the Ross Sea sector. TANGO is built up by coupling the atmospheric limited-area model MAR to a regional configuration of the ocean–sea ice model NEMO. A method is then developed to identify the mechanisms by which local coupling affects the simulations. TANGO is shown to simulate realistic sea ice properties and atmospheric surface temperatures. These skills are mostly related to the skills of the stand alone atmospheric and oceanic models used to build TANGO. Nonetheless, air temperatures over ocean and winter sea ice thickness are found to be slightly improved in coupled simulations as compared to standard stand alone ones. Local atmosphere ocean feedbacks over the open ocean are found to significantly influence ocean temperature and salinity. In a stand alone ocean configuration, the dry and cold air produces an ocean cooling through sensible and latent heat loss. In a coupled configuration, the atmosphere is in turn moistened and warmed by the ocean; sensible and latent heat loss is therefore reduced as compared to the stand alone simulations. The atmosphere is found to be less sensitive to local feedbacks than the ocean. Effects of local feedbacks are increased in the coastal area because of the presence of sea ice. It is suggested that slow heat conduction within sea ice could amplify the feedbacks. These local feedbacks result in less sea ice production in polynyas in coupled mode, with a subsequent reduction in deep water formation.

Land-atmosphere interactions in an high resolution atmospheric simulation coupled with a surface data assimilation scheme

Abstract. A valid tool for the retrieving of the turbulent fluxes that characterize the surface energy budget is constituted by the remote sensing of land surface states. In this study sequences of satellite-derived observations (from SEVIRI sensors aboard the Meteosat Second Generation) of Land Surface Temperature have been used as input in a data assimilation scheme in order to retrieve parameters that describe energy balance at the ground surface in the Tuscany region, in central Italy, during summer 2005. A parsimonious 1-D multiscale variational assimilation procedure has been followed, that requires also near surface meteorological observations. A simplified model of the surface energy balance that includes such assimilation scheme has been coupled with the limited area atmospheric model RAMS, in order to improve in the latter the accuracy of the energy budget at the surface. The coupling has been realized replacing the assimilation scheme products, in terms of surface turbulent fluxes and temperature and humidity states during the meteorological simulation. Comparisons between meteorological model results with and without coupling with the assimilation scheme are discussed, both in terms of reconstruction of surface variables and of vertical characterization of the lower atmosphere. In particular, the effects of the coupling on the moisture feedback between surface and atmosphere are considered and estimates of the precipitation recycling ratio are provided. The results of the coupling experiment showed improvements in the reconstruction of the surface states by the atmospheric model and considerable influence on the atmospheric dynamics.

Low frequency Mediterranean sea level variability: The contribution of atmospheric pressure and wind

We examine 44 yr (1958–2001) of extensively validated model data with the aim of characterizing the contribution of atmospheric pressure and wind to low frequency (the seasonal cycle and lower) Mediterranean sea level variability. The sea level data set was produced within the HIPOCAS Project framework, by means of a long-term barotropic run of the HAMSOM model, with a 1/4° × 1/6° spatial resolution. The atmospheric pressure and wind fields used to force the 44-yr ocean hindcast were produced by dynamical downscaling (1/2° × 1/2°) from the NCEP/NCAR global reanalysis, using the atmospheric limited-area model REMO. Results show that during the examined period, the effect of the mechanical atmospheric forcing has been in the sense of lowering sea level at a mean rate of − 0.6 mm/yr, mainly due to an increase of atmospheric pressure over the region. This effect concentrates in winter (− 1.3 mm/yr), while in summer the obtained trends (− 0.2 mm/yr) are not significant. The marked seasonality of the trends can be explained in terms of the seasonal trends of the North Atlantic Oscillation (NAO) and in particular of the Mediterranean Oscillation Index (MOI). The contribution of the mechanical atmospheric forcing to the observed sea level cycle is not very large in magnitude (2 cm amplitude) and is offset from the steric cycle by about 6 months, then reducing the amplitude of the annual cycle when fitting a harmonic function to tide gauge data. Finally, a modal decomposition of the detrended, deseasoned mechanical atmospheric contribution shows a basin-wide leading EOF (66% of variance explained) that implies the existence of a related flow exchange through Gibraltar. The amplitude of the leading EOF is clearly related to the MOI and less related to the NAO.