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The distribution of stations network is not uniform in Iran. The number of stations is more in densely populated areas and less around a large part of the central areas of Iran. Also, these stations have been operating for various periods. Therefore, in situ observation cannot lead to robust results and conclusions for the climate model evaluation and climate change studies. In recent years, reanalysis products have been considered as pseudo-observations, especially for climate studies and weather forecasting. The evaluation of reanalysis products is fundamental and critical because of the uncertainty due to the interpolation and to the data assimilation and forecasting models. In this paper, the reanalysis data of CFS-v2, MERRA-2, and ERA-5 climate databases are evaluated, using the data of 143 meteorological stations over the period 1991-2020. Reanalysis data are compared using Pearson's correlation coefficient (R), root mean square error (RMSE), normalized mean square error (NRMSE), Nash-Sutcliffe efficiency coefficient (NSC), bias index (BIAS), mean absolute error (MAE) and Kling-Gupta efficient (KGE), in the 6 regions representative of climate diversity in Iran. Also, the seasonal accuracy, annual trend, and probability distribution of the reanalyzed data were analyzed in comparison with the observations. The results show that CFS-v2, MERRA-2, and ERA-5 have high accuracy for estimating the temperature in Iran. In general, based on all statistical indicators and in all regions, ERA-5 is the most accurate; MERRA-2 and CFS-v2 were ranked second and third, with MAE values of 1.39, 1.71, and 2.077, respectively. In the spring, for the hot cluster (southwest), CFS-v2 data; in the summer, for the hot (southwest) and very cold (northwest) clusters, MERAA-2 data; and in autumn and winter, for the warm coastal cluster (coasts of Oman Sea and Persian Gulf), MERAA-2 data; are most accurate. In terms of annual trends, in the very cold cluster (northwest), CFS-v2 data; in the hot cluster (southwest), MERRA-2 data; and in other places, the data of ERA-5 have the closest trend to the trend of observations. [ABSTRACT FROM AUTHOR]

Atmosphere‐only experiments are widely used to investigate climate feedbacks simulated in more computationally expensive fully coupled global climate model simulations. We confirm that this remains a valid approach by comparing the radiative feedbacks and forcing between coupled and atmosphere‐only simulations for the latest models taking part in the 6th phase of the Coupled Model Intercomparison Project (CMIP6). For global‐mean cloud feedbacks, we find a better than previously known correspondence between these experiments, which applies even to the response of individual cloud properties (amount, altitude, and optical depth) and holds even when considering atmosphere‐only simulations of only 1 yr duration. For regional cloud feedbacks, the correspondence between the two experiments is generally present at every geographic location except for the tropical Pacific but takes longer experiments to reveal. For the lapse rate and surface albedo feedbacks, the correspondence between the two experiments is weaker due to the non‐uniform warming pattern and loss of sea ice in the coupled experiment. For the across‐model relationship between 4xCO2 effective radiative forcing and feedback, we find a different behavior across experiments in CMIP6 than in CMIP5, casting doubt on the physical significance of previous results that highlighted an anti‐correlation between the two quantities. Overall, these results confirm the utility of atmosphere‐only experiments particularly to study cloud feedbacks, which are the dominant source of inter‐model spread in climate sensitivity. Key Points: Cloud radiative feedbacks in atmosphere‐only and coupled simulations are highly correlated across both CMIP5 and CMIP6 modelsThis correlation extends to cloud property feedbacks and the regional distribution of cloud feedbacksAtmosphere‐only experiments need only be run for 1 yr to capture the inter‐model spread of global‐mean coupled cloud feedbacks [ABSTRACT FROM AUTHOR]

Based on the zero-dimensional box model Module Efficiently Calculating the Chemistry of the Atmosphere/Chemistry As A Box model Application (CAABA/MECCA-3.72f), an OH airglow model was developed to derive night-time number densities of atomic oxygen ([ O(3P) ]) and atomic hydrogen ([H]) in the mesopause region (∼75 –100 km). The profiles of [ O(3P) ] and [H] were calculated from OH airglow emissions measured at 2.0 µ m by the Sounding of the Atmosphere using Broadband Emission Radiography (SABER) instrument on board NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The two target species were used to initialize the OH airglow model, which was empirically adjusted to fit four different OH airglow emissions observed by the satellite/instrument configuration TIMED/SABER at 2.0 µ m and at 1.6 µ m as well as measurements by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument on board the Environmental Satellite (ENVISAT) of the transitions OH(6-2) and OH(3-1). Comparisons between the "best-fit model" obtained here and the satellite measurements suggest that deactivation of vibrationally excited OH(ν) via OH(ν≥7) + O2 might favour relaxation to OH(ν′≤5) + O2 by multi-quantum quenching. It is further indicated that the deactivation pathway to OH(ν′=ν-5) + O2 dominates. The results also provide general support of the recently proposed mechanism OH(ν) + O(3P) → OH(0≤ν′≤ν-5) + O(1D) but suggest slower rates of OH(ν=8,7,6,5) + O(3P) , partly disagreeing with laboratory experiments. Additionally, deactivation to OH(ν′=ν-5) + O(1D) might be preferred. The profiles of [ O(3P) ] and [H] derived here are plausible between 80 and 95 km but should be regarded as an upper limit. The values of [ O(3P) ] obtained in this study agree with the corresponding TIMED/SABER values between 80 and 85 km but are larger from 85 to 95 km due to different relaxation assumptions of OH(ν) + O(3P). The [H] profile found here is generally larger than TIMED/SABER [H] by about 50 % from 80 to 95 km, which is primarily attributed to our faster OH(ν=8) + O2 rate. [ABSTRACT FROM AUTHOR]

A hierarchy of idealized monsoons with increased degrees of complexity is built using an intermediate model with simplified physics and idealized land–sea geometry. This monsoon hierarchy helps formulate a basic understanding about the distribution of the surface equivalent potential temperature θe, which proves to provide a general guide on the monsoon rainfall. The zonally uniform monsoon in the simplest aquaplanet simulations is explained by a linearized model of the meridional distribution of θe, which is driven by the seasonally varying solar insolation and damped by both the monsoon overturning circulation and the local negative feedback. The heat capacities of the surface and the atmosphere give rise to an intrinsic time scale that causes the monsoon migration to lag behind the sun and reduces the monsoon extent and intensity. Monsoons with a zonally confined continent can be understood based on the zonally uniform monsoon by considering the ocean influence on the land through the westerly jet advection, which reduces the monsoon extent and induces zonal asymmetry. Monsoon responses to more realistic factors such as land geometry, albedo, and ocean heat flux are consistently predicted by their impacts on the surface θe distribution. The soil moisture effect, however, does not fully fit into the surface θe argument and provides additional control on monsoon rainfall by inducing regional circulation and rainfall patterns. [ABSTRACT FROM AUTHOR]

Characterizing and understanding the basic functioning of the Mediterranean Sea in terms of heat and salt redistribution within the basin is a crucial issue to predict its evolution. Here we quantify and analyze the heat and salt transfers using a simple box model consisting of four layers in the vertical for each of the two (western and eastern) basins. Namely, we box-average 14 regional simulations of the Med-CORDEX ensemble plus a regional and a global reanalysis, computing for each of them the heat and salt exchanges between layers. First, we analyze in detail the mechanisms behind heat and salt redistribution at different time scales from the outputs of a single simulation (NEMOMED8). We show that in the western basin the transfer between layer 1 (0-150 m) and layer 2 (150-600 m) is upwards for most models both for heat and salt, while in the eastern basin both transfers are downwards. A feature common to both basins is that the transports are smaller in summer than in winter due to the enhanced stratification, which dampen the mixing between layers. From the comparison of the 16 simulations we observe that the spread between models is much larger than the ensemble average for the salt transfer and for the heat transfer between layer 1 and layer 2. At lower layers (below 600 m) there is a set of models showing a good agreement between them, while others are not correlated with any other. The mechanisms behind the ensemble spread are not straightforward. First, to have a coarse resolution prevents the model to correctly represent the heat and salt redistribution in the basin. Second, those models with a very different initial stratification also show a very different redistribution, especially at intermediate and deep layers. Finally, the assimilation of data seems to perturb the heat and salt redistribution. Besides this, the differences among regional models that share similar spatial resolution and initial conditions are induced by more subtle mechanisms which depend on the variable and process analyzed. In order to reduce the uncertainties in the Mediterranean regional climate projections further modelling studies and better observational datasets are needed to constrain the main sources of discrepancies among models. In the absence of those, an ensemble modelling approach as the one followed in the Med-CORDEX initiative seems to be the best solution to evaluate model uncertainties into the future climate projections. [ABSTRACT FROM AUTHOR]

Particulate nitrate (pNO-3 ) is an important component of secondary aerosols in urban areas. Therefore, it is critical to explore its formation mechanism to assist with the planning of haze abatement strategies. Here we report vertical measurements of NOx and O3 by in situ instruments on a movable carriage on a tower during a winter heavy-haze episode (18 to 20 December 2016) in urban Beijing, China. Based on the box model simulation at different heights, we found that pNO-3 formation via N2O5 heterogeneous uptake was negligible at ground level due to N2O5 concentrations of near zero controlled by high NO emissions and NO concentration. In contrast, the contribution from N2O5 uptake was large at high altitudes (e.g., > 150 m), which was supported by the lower total oxidant (NO2 CO3) level at high altitudes than at ground level. Modeling results show the specific case that the nighttime integrated production of pNO-3 for the high-altitude air mass above urban Beijing was estimated to be 50 μgm-3 and enhanced the surface-layer pNO-3 the next morning by 28 μgm-3 through vertical mixing. Sensitivity tests suggested that the nocturnal NOx loss by NO3-N2O5 chemistry was maximized once the N2O5 uptake coefficient was over 2×10-3 on polluted days with Sa at 3000 μm2 cm-3 in wintertime. The case study provided a chance to highlight the fact that pNO-3 formation via N2O5 heterogeneous hydrolysis may be an important source of particulate nitrate in the urban airshed during wintertime. [ABSTRACT FROM AUTHOR]

A unified approach to Multiple and single State Dependent Parameter modelling, termed Extended State Dependent Parameters (ESDP) modelling, of nonlinear dynamic systems described by time-varying dynamic models applied to ARX or transfer-function model structures. Crucially, the approach proposes an effective model structure identification method using a novel Information Criterion (IC) taking into account model complexity in terms of the number of states involved. In ESDP, model structure involves not only the model orders, but also selection of the states driving the parameters, which effectively prevents the use of most current IC. This leads to a powerful methodology for investigating nonlinear systems building on the Data-Based Mechanistic (DBM) philosophy of Young and expanding the applications of the existing DBM methods. The methodologies presented are tested and demonstrated on both simulated data and on high frequency hydrological observations, showing how structure identification leads to discovery of dynamic relationships between system variables. [ABSTRACT FROM AUTHOR]

The Background Error Analysis Testbed (BEATBOX) is a new data assimilation framework for box models. Based on the BOX Model eXtension (BOXMOX) to the Kinetic Pre-Processor (KPP), this framework allows users to conduct performance evaluations of data assimilation experiments, sensitivity analyses, and detailed chemical scheme diagnostics from an observation simulation system experiment (OSSE) point of view. The BEATBOX framework incorporates an observation simulator and a data assimilation system with the possibility of choosing ensemble, adjoint, or combined sensitivities. A user-friendly, Python-based interface allows for the tuning of many parameters for atmospheric chemistry and data assimilation research as well as for educational purposes, for example observation error, model covariances, ensemble size, perturbation distribution in the initial conditions, and so on. In this work, the testbed is described and two case studies are presented to illustrate the design of a typical OSSE experiment, data assimilation experiments, a sensitivity analysis, and a method for diagnosing model errors. BEATBOX is released as an open source tool for the atmospheric chemistry and data assimilation communities. [ABSTRACT FROM AUTHOR]

We present a brief review of the recent investigations on gravity currents in horizontal channels with non-rectangular cross-section area (such as triangle, $$\bigvee $$ -valley, circle/semi-circle, trapezoid) which occur in nature (e.g., rivers) and constructed environment (tunnels, reservoirs, canals). To be specific, we discuss the propagation of a gravity current (GC) in a horizontal channel along the horizontal coordinate x, with gravity g acting in the $$-z$$ direction, and y the horizontal-lateral coordinate. The bottom and top of the channel are at $$z=0,H$$ . The 'standard' problem is concerned with 2D flow in a channel with rectangular (or laterally unbounded) cross-section area (CSA). Recent investigations have successfully extended the standard knowledge to the channels of CSA given by the quite general $$-f_1(z)\le y \le f_2(z)$$ for $$0 \le z \le H$$ . This includes the practical $$\bigvee $$ -valley, triangle, circle/semi-circle and trapezoid; these geometries may be in 'up' or 'down' setting with respect to gravity, e.g., $$\bigtriangleup $$ and $$\bigtriangledown $$ . The major objective of the extended theory is to predict the height of the interface $$z=h(x,t)$$ and the velocity (averaged over the CSA) u( x, t), where t is time; the prediction includes the speed and position of the nose $$u_N(t), x_N(t)$$ . We show that the motion is governed by a set of simplified equations, called 'model,' that provides versatile and insightful solutions and trends. The emphasis in on a high-Reynolds-number current whose motion is dominated by buoyancy-inertia balance; in particular a GC released from a lock, which also contains general effects such as front and internal jumps (shocks), and reflected bore. We discuss two-layer, one-layer, and box models; Boussinesq and non-Boussinesq systems; compositional and particle-driven cases; and the effect of stratification of the ambient fluid. The models are self-contained, and admit realistic initial and boundary conditions. The governing equations are amenable to analytical solutions in some special circumstances. Some salient features of the buoyancy-viscous regime, and the estimate for the length at which transition to this regime takes place, are also presented. Some experimental support to the theory, and open questions for further investigations, are also mentioned. The major conclusions are (1) The CSA geometry has significant influence on the motion of the GC; and (2) The new theory is a useful, very significant, extension of the standard two-dimensional GC problem. The standard current is just a particular case, $$f_{1,2} =$$ constants, among many other covered by the new theory . [ABSTRACT FROM AUTHOR]

Clastic-biogenic varves are widely used for reconstructing past climate: in boreal environments, the accumulation of minerogenic clasts on the lake floor is generally considered a proxy for past variations in spring floods reflecting previous winter conditions. However, the physical mechanisms behind this winter climate sensitivity and the influence of catchment type on the varve formation are not fully investigated. Here, we present a winter climate record inferred from the clastic laminae of three lakes located on the region of fine-grained tills in Eastern Finland spanning from AD 1890 to 1990. The minerogenic varve data are compared with instrumental meteorological and hydrological time series in order to investigate the physical link between winter and spring climate and minerogenic matter accumulation. Our analyses reveal that the climate-catchment mechanisms operating in the region of fine-grained tills differ crucially from previously described climate catchment interactions on sand moraine-dominated catchments in Finland, where the maximum river discharge in spring controls the clastic lamina formation. However, in contrast to earlier boreal varve records from Central Finland, the clastic lamina formation in the studied region correlates negatively with spring temperatures and winter precipitation. This could be an artefact of varying catchment dynamics but also related to the regional climate. The lakes surrounded by catchments characterized by fine-grained tills are more sensitive to cold and dry winters. The differences in the sensitivity of varve characteristics to climate, highlights the importance of understanding the catchment dynamics in detail in order to better understand climatic forcing. [ABSTRACT FROM AUTHOR]

Peroxyacetic acid (PAA, CH 3 C(O)OOH) plays an important role in atmospheric chemistry, serving as reactive oxidant and affecting radical recycling. However, previous studies revealed an obvious gap between modelled and observed concentrations of atmospheric PAA, which may be partly ascribed to the uncertainty in the kinetics and mechanism of OH-oxidation. In this study, we measured the rate constant of OH radical reaction with PAA ( k PAA+OH ) and investigated the products in order to develop a more robust atmospheric PAA chemistry. Using the relative rates technique and employing toluene and meta-xylene as reference compounds, the k PAA+OH was determined to be (9.4–11.9) × 10 −12 cm 3 molecule −1 s −1 at 298 K and 1 atm, which is about (2.5–3.2) times larger than that parameter used in Master Chemical Mechanism v3.3.1 (MCM v3.3.1) (3.70 × 10 −12 cm 3 molecule −1 s −1 ). Incorporation of a box model and MCM v3.3.1 with revised PAA chemistry represented a better simulation of atmospheric PAA observed during Wangdu Campaign 2014, a rural site in North China Plain. It is found that OH-oxidation is an important sink of atmospheric PAA in this rural area, accounting for ∼30% of the total loss. Moreover, the major terminal products of PAA–OH reaction were identified as formaldehyde (HCHO) and formic acid (HC(O)OH). The modelled results show that both primary and secondary chemistry play an important role in the large HCHO and HC(O)OH formation under experimental conditions. There should exist the channel of methyl H-abstraction for PAA–OH reaction, which may also provide routes to HCHO and HC(O)OH formation. [ABSTRACT FROM AUTHOR]

Isoprene oxidation schemes vary greatly among gas-phase chemical mechanisms, with potentially significant ramifications for air quality modeling and interpretation of satellite observations in biogenic-rich regions. In this study, in situ observations from the 2013 SENEX mission are combined with a constrained 0-D photochemical box model to evaluate isoprene chemistry among five commonly used gas-phase chemical mechanisms: CB05, CB6r2, MCMv3.2, MCMv3.3.1, and a recent version of GEOS-Chem. Mechanisms are evaluated and inter-compared with respect to formaldehyde (HCHO), a high-yield product of isoprene oxidation. Though underestimated by all considered mechanisms, observed HCHO mixing ratios are best reproduced by MCMv3.3.1 (normalized mean bias = −15%), followed by GEOS-Chem (−17%), MCMv3.2 (−25%), CB6r2 (−32%) and CB05 (−33%). Inter-comparison of HCHO production rates reveals that major restructuring of the isoprene oxidation scheme in the Carbon Bond mechanism increases HCHO production by only ∼5% in CB6r2 relative to CB05, while further refinement of the complex isoprene scheme in the Master Chemical Mechanism increases HCHO production by ∼16% in MCMv3.3.1 relative to MCMv3.2. The GEOS-Chem mechanism provides a good approximation of the explicit isoprene chemistry in MCMv3.3.1 and generally reproduces the magnitude and source distribution of HCHO production rates. We analytically derive improvements to the isoprene scheme in CB6r2 and incorporate these changes into a new mechanism called CB6r2-UMD, which is designed to preserve computational efficiency. The CB6r2-UMD mechanism mimics production of HCHO in MCMv3.3.1 and demonstrates good agreement with observed mixing ratios from SENEX (−14%). Improved simulation of HCHO also impacts modeled ozone: at ∼0.3 ppb NO, the ozone production rate increases ∼3% between CB6r2 and CB6r2-UMD, and rises another ∼4% when HCHO is constrained to match observations. [ABSTRACT FROM AUTHOR]

Features of the North Atlantic thermohaline circulation response to periodic, stochastic, and instantaneous forcing are studied using a four-box model. The present-day circulation is shown to be characterized by a stable quasi-periodic oscillatory mode that manifests itself as the Atlantic Multidecadal Oscillation. The thermohaline catastrophe is unlikely in the modern climate epoch. [ABSTRACT FROM AUTHOR]

Recently, several Lagrangian microphysical models have been developed which use a large number of (computational) particles to represent a cloud. In particular, the collision process leading to coalescence of cloud droplets or aggregation of ice crystals is implemented differently in various models. Three existing implementations are reviewed and extended, and their performance is evaluated by a comparison with well-established analytical and bin model solutions. In this first step of rigorous evaluation, box model simulations, with collection/aggregation being the only process considered, have been performed for the three well-known kernels of Golovin, Long and Hall. Besides numerical parameters, like the time step and the number of simulation particles (SIPs) used, the details of how the initial SIP ensemble is created from a prescribed analytically defined size distribution is crucial for the performance of the algorithms. Using a constant weight technique, as done in previous studies, greatly underestimates the quality of the algorithms. Using better initialisation techniques considerably reduces the number of required SIPs to obtain realistic results. From the box model results, recommendations for the collection/aggregation implementation in higher dimensional model setups are derived. Suitable algorithms are equally relevant to treating the warm rain process and aggregation in cirrus. [ABSTRACT FROM AUTHOR]

This paper describes the development and implementation of an extendable aqueous-phase chemistry option (AQCHEM - KMT(I)) for the Community Multiscale Air Quality (CMAQ) modeling system, version 5.1. Here, the Kinetic PreProcessor (KPP), version 2.2.3, is used to generate a Rosenbrock solver (Rodas3) to integrate the stiff system of ordinary differential equations (ODEs) that describe the mass transfer, chemical kinetics, and scavenging processes of CMAQ clouds. CMAQ's standard cloud chemistry module (AQCHEM) is structurally limited to the treatment of a simple chemical mechanism. This work advances our ability to test and implement more sophisticated aqueous chemical mechanisms in CMAQ and further investigate the impacts of microphysical parameters on cloud chemistry. Box model cloud chemistry simulations were performed to choose efficient solver and tolerance settings, evaluate the implementation of the KPP solver, and assess the direct impacts of alternative solver and kinetic mass transfer on predicted concentrations for a range of scenarios. Month-long CMAQ simulations for winter and summer periods over the US reveal the changes in model predictions due to these cloud module updates within the full chemical transport model. While monthly average CMAQ predictions are not drastically altered between AQCHEM and AQCHEM - KMT, hourly concentration differences can be significant. With added in-cloud secondary organic aerosol (SOA) formation from biogenic epoxides (AQCHEM - KMTI), normalized mean error and bias statistics are slightly improved for 2-methyltetrols and 2-methylglyceric acid at the Research Triangle Park measurement site in North Carolina during the Southern Oxidant and Aerosol Study (SOAS) period. The added in-cloud chemistry leads to a monthly average increase of 11-18% in "cloud" SOA at the surface in the eastern United States for June 2013. [ABSTRACT FROM AUTHOR]

Appropriately treating riverine freshwater discharge into the oceans in Earth system models is a challenging problem. Commonly, the river runoff is discharged into the ocean models with zero salinity and arbitrarily distributed either horizontally or vertically over several grid cells. Those approaches entirely neglect estuarine physical processes that modify river inputs before they reach the open ocean. In order to realistically represent riverine freshwater inputs in Earth system models, a physically based Estuary Box Model (EBM) is developed to parameterize the mixing processes in estuaries. The EBM represents the estuary exchange circulation with a two-layer box structure. It takes as input the river volume flux from the land surface model and the subsurface salinity at the estuary mouth from the ocean model. It delivers the estuarine outflow salinity and net volume flux into and out of the estuary to the ocean model. An offline test of the EBM forced with observed conditions for the Columbia River system shows good agreement with observations of outflow salinity and high-resolution simulations of the exchange flow volume flux. To illustrate the practicality of use of the EBM in an Earth system model, the EBM is implemented for all coastal grid cells with river runoff in the Community Earth System Model (CESM). Compared to the standard version of CESM, which treats runoff as an augmentation to precipitation, the EBM increases sea surface salinity and reduces stratification near river mouths. The EBM also leads to significant regional and remote changes in CESM ocean surface salinities. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO) simulation in atmospheric chemistry models is frequently used for source-receptor analysis, emission inversion, interpretation of observations, and chemical forecasting due to its computational efficiency and ability to quantitatively link simulated CO burdens to sources. While several methods exist for modelling CO source attribution, most are inappropriate for regions where the CO budget is dominated by secondary production rather than direct emissions. Here, we introduce a major update to the linear CO-only capability in the GEOS-Chem chemical transport model that for the first time allows source-region tagging of secondary CO produced from oxidation of non-methane volatile organic compounds. Our updates also remove fundamental inconsistencies between the CO-only simulation and the standard full chemistry simulation by using consistent CO production rates in both. We find that relative to the standard chemistry simulation, CO in the original CO-only simulation was overestimated by more than 100 ppb in the model surface layer and underestimated in outflow regions. The improved CO-only simulation largely resolves these discrepancies by improving both the magnitude and location of secondary production. Despite large differences between the original and improved simulations, however, model evaluation with the global dataset used to benchmark GEOS-Chem shows negligible change to the model's ability to match the observations. This suggests that the current GEOS-Chem benchmark is not well suited to evaluate model changes in regions influenced by biogenic emissions and chemistry, and expanding the dataset to include observations from biogenic source regions (including those from recent aircraft campaigns) should be a priority for the GEOS-Chem community. Using Australasia as a case study, we show that the new ability to geographically tag secondary CO production provides significant added value for interpreting observations and model results in regions where primary CO emissions are low. Secondary production dominates the CO budget across much of the world, especially in the southern hemisphere, and we recommend future model-observation and multi-model comparisons implement this capability to provide a more complete understanding of CO sources and their variability. [ABSTRACT FROM AUTHOR]

This study discusses how much of the biases in top-of-atmosphere (TOA) radiation and clouds can be removed by parameter tuning in the present-day simulation of a climate model in the Coupled Model Inter-comparison Project phase 5 (CMIP5) generation. We used a low-resolution version of the Model for Interdisciplinary Research on Climate version 5 (MIROC5) Atmosphere-Ocean General Circulation Model (AOGCM) and compared the output of a perturbed parameter ensemble (PPE) experiment in the pre-industrial control setting with satellite observation data. The model biases and the parametric uncertainty of the biases are evaluated with respect to TOA radiation and clouds. We used the output of the PPE experiment without flux adjustment, which is consistent with the experimental design of the CMIP5. The results indicate that removing or changing the sign of the biases by parameter tuning alone is difficult. Especially, the cooling bias of the shortwave cloud radiative effect in low latitudes could not be removed, neither in the zonal mean nor at each latitude--longitude grid point. The bias was related to the overestimation of both cloud amount and cloud optical thickness, which could not be removed by the parameter tuning either. However, they could be alleviated by tuning parameters such as the maximum cumulus updraft velocity at the cloud base. On the other hand, the bias of the shortwave cloud radiative effect in the Arctic was sensitive to parameter tuning. It could be removed by tuning such parameters as albedo of ice and snow both in the zonal mean and at each grid point. The obtained results illustrate the benefit of PPE experiments which provide useful information regarding effectiveness and limitations of parameter tuning. [ABSTRACT FROM AUTHOR]

Perfluoroalkyl carboxylic acids (PFCAs) are environmental contaminants that are highly persistent, bioaccumulative, and have been detected along with their atmospheric precursors far from emissions sources. The importance of precursor emissions as an indirect source of PFCAs to the environment is uncertain. Modeling studies have used degradation mechanisms of differing complexities to estimate the atmospheric production of PFCAs, and these differing mechanisms lead to quantitatively different yields of PFCAs under differing atmospheric conditions. We evaluate PFCA formation with the most complete degradation mechanism to date, to our knowledge, using a box model analysis to simulate the atmospheric chemical fate of fluorotelomer precursors to long-chain PFCAs. In particular, we examine the variability in PFCA formation in different chemical environments, and estimate the uncertainty in PFCA formation due to reaction rate constants. We calculate long-chain PFCA formation theoretical maximum yields for the degradation of fluorotelomer precursor species at a representative sample of atmospheric conditions from a three-dimensional chemical transport model, and estimate uncertainties in such calculations for urban, ocean, and Arctic conditions using polynomial chaos methods. We find that atmospheric conditions farther from pollution sources have both higher capacities to form long-chain PFCAs and higher uncertainties in those capacities. Our calculations of theoretical maximum yields indicate that under typical Northern Hemisphere conditions, less than 10% of emitted precursor may reach long-chain PFCA end products. This results in a possible upper bound of 2-50 t year-1 of long-chain PFCA (depending on quantity of emitted precursor) produced in the atmosphere via degradation of fluorotelomer products. However, transport to highyield areas could result in higher yields. While the atmosphere is a potentially growing source of long-chain PFCAs in the Arctic, oceanic transport and interactions between the atmosphere and ocean may be relatively more important pathways to the Arctic for long-chain PFCAs. [ABSTRACT FROM AUTHOR]

To investigate the influence of a river discharging outside a bay on circulation in the bay, we carried out current and salinity measurements from mooring systems and hydrographic observations in Matsushima Bay, Japan, and off the Naruse River, which discharges outside the bay. Previously, enhancement of horizontal circulation in the bay induced by increased freshwater input from the Naruse River was reported to have degraded the seedling yield of wild Pacific oysters in the bay, but the freshwater inflow from the river was not directly measured. Our hydrographic observations in Katsugigaura Strait, approximately 3 km southwest of the Naruse River mouth, detected freshwater derived from the river. The mooring data revealed that freshwater discharged by the river flowed into Matsushima Bay via the strait and that the freshwater transport increased when the river was in flood. The inflow through straits other than Katsugigaura was estimated by a box model analysis to be 26–145 m 3 s −1 under normal river discharge conditions, and it decreased to 6 m 3 s −1 during flood conditions. During flood events, the salt and water budgets in the bay were maintained by the horizontal circulation: inflow occurred mainly via Katsugigaura Strait, and outflow was mainly via other straits. [ABSTRACT FROM AUTHOR]

Two advanced models for forecasting the output power of photovoltaic plants are discussed in details: a black-box Takagi-Sugeno fuzzy model and a physically inspired, semiparametric statistical model (Generalized Additive Model, GAM) based on smoothing splines. The structure of the two models, their strengths and weaknesses, are presented. The models performance is thoroughly compared with the performance of a simple linear model tested under the frame of the European Cooperation in Science and Technology (COST) Action “Weather Intelligence for Renewable Energies”, as a benchmark used also in the forecasting exercise reported in Sperati et al. Energies 8 (2015) 9594. The models are used to forecasting the output power at time horizons of 1–72 h ahead. The data used during the COST competition are used here as input. The present study extends beyond the traditional evaluation of overall model accuracy. Detailed influences of seasonal effects, sun elevation angle and solar irradiance level upon the models performance are assessed. While the accuracy of the simple linear model is not entirely bad, it differs in important details from the two advanced forecasting models. The results show that a moderate, carefully chosen increase in model structure complexity can improve the predictive performance. Suitable penalty on model complexity can help both to enforce parsimony and improve practical forecasting abilities, to a certain extent. The physically inspired GAM comes out as the best performing model. [ABSTRACT FROM AUTHOR]

Estimation of horizontal salt transfer from the central part of the Black Sea to the continental slope is carried out based on numerical modeling. A box hydrodynamic model of the Black Sea based on the POM model is used. Horizontal transport caused by seasonal variability of vertical velocity in the central part of the Black Sea and horizontal turbulent diffusion is considered. In this work we demonstrate that, in the 0- to 30-m layer on average for the year, there is an outflow of salty water under the impact of both factors. In the surface layer, the transport caused by seasonal variability contributes significantly (30%). In the 80- to 150-m layer on average over the year, the waters from the continental slope arrive into the central part of the basin; they are characterized by higher salinity due to the inflow of the Marmara Sea waters across the Bosporus Strait. [ABSTRACT FROM AUTHOR]

Both atmospheric deposition and benthic remineralisation influence the marine nitrogen cycle, and hence ultimately also marine primary production. The biological and biogeochemical relations in the eastern tropical South Pacific (ETSP) among nitrogen deposition, benthic denitrification and phosphorus regeneration are analysed in a prognostic box model of the oxygen, nitrogen and phosphorus cycles in the ETSP. Atmospheric nitrogen deposition (≈ 1:5TgNyr-1 for the years 2000-2009) is offset by half in the model by reduced N2 fixation, with the other half transported out of the model domain. Model- and data-based benthic denitrification in our model domain are responsible for losses of 0.19 and 1.0 TgNyr-1, respectively, and both trigger nitrogen fixation, partly compensating for the NO-3 loss. Modeland data-based estimates of enhanced phosphate release via sedimentary phosphorus regeneration under suboxic conditions are 0.062 and 0.11 TgPyr-1, respectively. Since phosphate is the ultimate limiting nutrient in the model, even very small additional phosphate inputs stimulate primary production and subsequent export production and NO-3 loss in the oxygen minimum zone (OMZ). A sensitivity analysis of the local response to both atmospheric deposition and benthic remineralisation indicates dominant stabilising feedbacks in the ETSP, which tend to keep a balanced nitrogen inventory; i.e. nitrogen input by atmospheric deposition is counteracted by decreasing nitrogen fixation; NO-3 loss via benthic denitrification is partly compensated for by increased nitrogen fixation; enhanced nitrogen fixation stimulated by phosphate regeneration is partly counteracted by stronger water-column denitrification. Even though the water column in our model domain acts as a NO-3 source, the ETSP including benthic denitrification might be a NO-3 sink. [ABSTRACT FROM AUTHOR]

Determining the mixing height using a tracer can improve the information obtained using traditional techniques. Here we provide an improved box model based on radon progeny measurements, which considers the vertical entrainment of residual layers and the variability in the soil radon exhalation rate. The potential issues in using progeny instead of radon have been solved from both a theoretical and experimental perspective; furthermore, the instrumental efficiency and the counting scheme have been included in the model. The applicability range of the box model has been defined by comparing radon-derived estimates with sodar and lidar data. Three intervals have been analyzed ('near-stable', 'transition' and 'turbulent'), and different processes have been characterized. We describe a preliminary application case performed in Rome, Italy, while case studies will be required to determine the range limits that can be applied in any circumstances. [ABSTRACT FROM AUTHOR]

The article discusses the impact of coniferous and deciduous vegetation in northern New England on the mean residence times (MRTs) of various metals. Topics include a box model for the estimation of metal residence in the organic horizons of both major metals and trace metals, the soil properties of secondary growth forest stands at mountain sites, and the projected impacts of climate change on forests in the region.

Firewood, coal, dung cake, kerosene and Liquefied Petroleum Gas (LPG) are the most commonly used fuels for cooking and heating purposes in developing countries. Combustion of such fuels leads to high concentrations of pollutants in the indoor environment. Polycyclic aromatic hydrocarbons (PAHs) are emitted during residential combustion, and have carcinogenic and genotoxic properties. In this study, data from an experimental setup for estimation of emission rate of particulate-bound PAHs generated during combustion of such fuels are used. PAHs emission rates were used in a probabilistic single box model for prediction of time average particle associated indoor B(a)P equivalent (B(a)P eq ) concentrations of PAHs resulting from typical Indian cooking. Model parameters such as fuel consumption rate, stove power and cooking time were also evaluated experimentally. Particle bound B(a)P eq PAH emission factor was found to be highest (0.96 mg kg −1 ) for dung cake, and lowest for LPG (0.48 mg kg −1 ) among tested fuels. The time averaged B(a)P equivalent concentrations in indoor environment were found to be 0.82, 0.45, 0.87, 0.30, and 0.14 μg m −3 for firewood, coal, dung cake, kerosene and LPG respectively. Results reveal that there was higher B(a)P equivalent concentration during combustion of biomass (dung cake, fire wood) as compared to fossil fuels (coal) and non-solid fuels (kerosene, LPG). Predicted time averaged indoor air B(a)P eq concentrations of PAHs were found to be much higher than the WHO indoor air guideline values for all tested fuels. [ABSTRACT FROM AUTHOR]

Using the regional climate model ALARO-0, the Royal Meteorological Institute of Belgium and Ghent University have performed two simulations of the past observed climate within the framework of the Coordinated Regional Climate Downscaling Experiment (CORDEX). The ERAInterim reanalysis was used to drive the model for the period 1979-2010 on the EURO-CORDEX domain with two horizontal resolutions, 0.11 and 0.44°. ALARO-0 is characterised by the new microphysics scheme 3MT, which allows for a better representation of convective precipitation. In Kotlarski et al. (2014) several metrics assessing the performance in representing seasonal mean near-surface air temperature and precipitation are defined and the corresponding scores are calculated for an ensemble of models for different regions and seasons for the period 1989--2008. Of special interest within this ensemble is the ARPEGE model by the Centre National de Recherches Météorologiques (CNRM), which shares a large amount of core code with ALARO-0. Results show that ALARO-0 is capable of representing the European climate in an acceptable way as most of the ALARO-0 scores lie within the existing ensemble. However, for near-surface air temperature, some large biases, which are often also found in the ARPEGE results, persist. For precipitation, on the other hand, the ALARO-0 model produces some of the best scores within the ensemble and no clear resemblance to ARPEGE is found, which is attributed to the inclusion of 3MT. Additionally, a jackknife procedure is applied to the ALARO-0 results in order to test whether the scores are robust, meaning independent of the period used to calculate them. Periods of 20 years are sampled from the 32-year simulation and used to construct the 95% confidence interval for each score. For most scores, these intervals are very small compared to the total ensemble spread, implying that model differences in the scores are significant. [ABSTRACT FROM AUTHOR]

We discuss a mathematical model of migration channel based on the truncated Waring distribution. The truncated Waring distribution is obtained for a more general model of motion of substance through a channel containing finite number of boxes. The model is applied then for case of migrants moving through a channel consisting of finite number of countries or cities. The number of migrants in the channel strongly depends on the number of migrants that enter the channel through the country of entrance. It is shown that if the final destination country is very popular then large percentage of migrants may concentrate there. [ABSTRACT FROM AUTHOR]

Urban air pollution has become a major concern over the past decades. One of the largest conurbations in Sub-Saharan Africa is developed around the cities of Johannesburg and Pretoria (Jhb-Pta megacity). In this study, a photochemical box model with a detailed representation of ozone (O3) formation chemistry was used to investigate the state of current air quality and photochemical processes in the Jhb-Pta megacity, as well as scenarios that could possibly mitigate air pollution. Results indicated that the Jhb-Pta megacity is within a VOC-limited (or NOx-saturated) regime. Major sources of NOx include transport from the Mpumalanga Highveld and local traffic emissions. O3 levels in the Jhb-Pta megacity will be more effectively reduced if VOC (volatile organic compound) emissions are decreased. A reduction of NOx emissions leads to an increase in O3 because of a decrease in titration through the reaction with NO. The same effect was observed in various cities worldwide where O3 levels increased when NOx emissions were reduced during emission control strategies. The effect of reducing vehicular emissions in the Jhb-Pta megacity on the production of O3 was also investigated. A significant increase of approximately 23 ppb O3 was observed when emissions of VOCs, NOx and CO were reduced by changing from Euro-0 to Euro-3 vehicles. It is therefore recommended that VOC emissions are decreased together with the implementation of Euro-3 and cleaner vehicles in the future. [ABSTRACT FROM AUTHOR]

Isolated power plants with well characterized emissions serve as an ideal test case of methods to estimate emissions using satellite data. In this study we evaluate the Exponentially-Modified Gaussian (EMG) method and the box model method based on mass balance for estimating known NO x emissions from satellite retrievals made by the Ozone Monitoring Instrument (OMI). We consider 29 power plants in the USA which have large NO x plumes that do not overlap with other sources and which have emissions data from the Continuous Emission Monitoring System (CEMS). This enables us to identify constraints required by the methods, such as which wind data to use and how to calculate background values. We found that the lifetimes estimated by the methods are too short to be representative of the chemical lifetime. Instead, we introduce a separate lifetime parameter to account for the discrepancy between estimates using real data and those that theory would predict. In terms of emissions, the EMG method required averages from multiple years to give accurate results, whereas the box model method gave accurate results for individual ozone seasons. [ABSTRACT FROM AUTHOR]

Nighttime concentrations of the gas phase nitrate radical (NO 3 ) were successfully measured during a four week field campaign in an arid urban location, Reno Nevada, using long-path Differential Optical Absorbance Spectrometry (DOAS). While typical concentrations of NO 3 ranged from 5 to 20 ppt, elevated concentrations were observed during a wildfire event. Horizontal mixing in the free troposphere was considerable because the sampling site was above the stable nocturnal boundary layer every night and this justified a box modeling approach. Process analysis of box model simulations showed NO 3 accounted for approximately half of the loss of internal olefins, 60% of the isoprene loss, and 85% of the α-pinene loss during the nighttime hours during a typical night of the field study. The NO 3 + aldehyde reactions were not as important as anticipated. On a polluted night impacted by wildfires upwind of the sampling location, NO 3 reactions were more important. Model simulations overpredicted NO 2 concentrations for both case studies and inorganic chemistry was the biggest influence on NO 3 concentrations and on nitric acid formation. The overprediction may be due to additional NO 2 loss processes that were not included in the box model, as deposition and N 2 O 5 uptake had no significant effect on NO 2 levels. [ABSTRACT FROM AUTHOR]

Summary Bosten Lake in the southeast of Yanqi Catchment, China, supports the downstream agricultural and natural environments. Over the last few decades the intensive agricultural activities in Yanqi Catchment resulted in decreased lake levels and deteriorated lake water quality. A two-box model is constructed to understand the evolution of lake level and salinity between 1958 and 2008. The two-box model of the lake indicates that the evaporation does have the same trend as the observed lake area and the annual average evaporation agrees with the value obtained from the Penman–Monteith approach. To achieve a correct salt balance, the ratio of outflow concentration and average lake concentration has to be around 0.7. This is due to the incomplete mixing of the lake caused by short-circuiting between tributary inflow and the main outflow via the pump stations abstracting water from the lake. This short-circuiting is investigated in more detail by a 1D numerical flow and transport model of the lake calibrated with observations of lake level and lake concentrations. The distributed model reproduces the correct time-varying outflow concentration. It is used for the assessment of two basic management options: increasing river discharge (by water saving irrigation, reduction of phreatic evaporation or reduction of agricultural area) and diverting saline drainage water to the desert. Increasing river discharge to the lake by 20% reduces the east basin salt concentration by 0.55 kg/m 3 , while capturing all the drainage water and discharging it to depressions instead of the lake reduces the east basin salt concentration by 0.63 kg/m 3 . A combination of increasing river inflow and decreasing drainage salt flux is sufficient to bring future lake TDS below the required 1 kg/m 3 , to keep a lake level that sustains the lake ecosystem, and to supply more water for downstream development and ecosystem rehabilitation. [ABSTRACT FROM AUTHOR]

Within the framework of the Dust Observations for Models (DO4Models) project, the performance of three commonly used dust emission schemes is investigated in this paper using a box model environment. We constrain the model with field data (surface and dust particle properties as well as meteorological parameters) obtained from a dry lake bed with a crusted surface in Botswana during a 3 month period in 2011. Our box model results suggest that all schemes fail to reproduce the observed horizontal dust flux. They overestimate the magnitude of the flux by several orders of magnitude. The discrepancy is much smaller for the vertical dust emission flux, albeit still overestimated by up to an order of magnitude. The key parameter for this mismatch is the surface crusting which limits the availability of erosive material, even at higher wind speeds. The second-most important parameter is the soil size distribution. Direct dust entrainment was inferred to be important for several dust events, which explains the smaller gap between modelled and measured vertical dust fluxes. We conclude that both features, crusted surfaces and direct entrainment, need to be incorporated into dust emission schemes in order to represent the entire spectra of source processes. We also conclude that soil moisture exerts a key control on the threshold shear velocity and hence the emission threshold of dust in the model. In the field, the state of the crust is the controlling mechanism for dust emission. Although the crust is related to the soil moisture content to some extent, we are not as yet able to deduce a robust correlation between state of crust and soil moisture. [ABSTRACT FROM AUTHOR]

The capacity of earth system models (ESMs) to make reliable projections of future atmospheric CO2 and climate is strongly dependent on the ability of the land surface model to adequately simulate the land carbon (C) cycle. Defining 'adequate' performance of the land model requires an understanding of the contributions of climate model and land model errors to the land C cycle. Here, a benchmarking framework is applied based on significant, observed characteristics of the land C cycle for the contemporary period, for which sufficient evaluation data are available, to test the ability of the JSBACH land surface component of the Max Planck Institute Earth System Model (MPI-ESM) to simulate land C trends. Particular attention is given to the role of potential effects caused by climate biases, and therefore investigation is made of the results of model configurations in which JSBACH is interactively 'coupled' to atmosphere and ocean components and of an 'uncoupled' configuration, where JSBACH is driven by reconstructed meteorology. The ability of JSBACH to simulate the observed phase of phenology and seasonal C fluxes is not strongly affected by climate biases. Contrarily, noticeable differences in the simulated gross primary productivity and land C stocks emerge between coupled and uncoupled configurations, leading to significant differences in the decadal terrestrial C balance and its sensitivity to climate. These differences are strongly controlled by climate biases of the MPI-ESM, in particular those affecting soil moisture. To effectively characterize model performance, the potential effects of climate biases on the land C dynamics need to be considered during the development and calibration of land surface models. [ABSTRACT FROM AUTHOR]

Nitrate and its precursor (gaseous HNO3) in China are generally overestimated by current chemical transport models in comparison to in-situ observations. In this study, we used an observation-based box model and in-situ measurements at a rural site in southern China to investigate possible missing sinks of nitrate. We found that a heterogeneous reaction of HNO3 to NOx on soot can better balance the NOx/HNO3 chemistry in models and improve model performance with regard to nitrate, particularly where an additional HONO source (the heterogeneous reaction of NO2 on soot) is incorporated into current models. Through a series of sensitivity simulations, the uptake coefficients of heterogeneous reactions were suggested to be 3.0 × 10-3 and 1 × 10-4 for HNO3 and HONO over southern China in fall, respectively. A 3-D simulation with the suggested uptake coefficients further confirmed that heterogeneous reactions significantly decreased the nitrate concentrations (PNO3) in southern China, by up to 10 μg m-3 (50%-80% of simulated PNO3) in polluted regions (e.g., the Yangtze River). In contrast to nitrate and HNO3, NOx concentrations in China were enhanced, which partly explained the underestimation of NO2 in current models compared to satellite observations. [ABSTRACT FROM AUTHOR]

Column densities from satellite retrievals can provide valuable information for estimating emissions and chemical lifetimes objectively across the globe. To better understand the uncertainties associated with these estimates, we test four methods using simulated column densities from a point source: a box model approach, a 2D Gaussian fit, an Inverse Radius fit and an Exponentially-Modified Gaussian fit. The model results were simulated using the WRF and CAMx models for the year 2005, for a single point source outside Atlanta in Georgia, USA with specified emissions and three chemical scenarios: no chemical reactions, 12 h chemical lifetime and 1 h chemical lifetime. No other sources were included in the simulations. We find that the box model provides reliable estimates irrespective of plume speed and plume direction, if the plume speed and the chemical lifetime are known accurately. The 2D Gaussian fit was found to be sensitive to plume speed and direction, and requires omnidirectional dispersion in order to have a decent fit. However, the 2D Gaussian fit is only an approximate fit to the data, and the discrepancies mean that the results are dependent on the geographical domain used for the optimization. An Inverse Radius fit is introduced to correct this issue, which is found to provide improved emissions and lifetime estimates. The Exponentially-Modified Gaussian fit also gave improved estimates. It is however dependent on accurate plume rotation such that reported chemical lifetimes with this method could be significantly underestimated. [ABSTRACT FROM AUTHOR]

The purpose of this paper was to investigate formaldehyde emission behavior of building materials from on-site measurements of air phase concentration at material surface used as input data of a box model to estimate the indoor air pollution of a newly built classroom. The relevance of this approach was explored using CFD modeling. In this box model, the contribution of building materials to indoor air pollution was estimated with two parameters: the convective mass transfer coefficient in the material/air boundary layer and the on-site measurements of gas phase concentration at material surfaces. An experimental method based on an emission test chamber was developed to quantify this convective mass transfer coefficient. The on-site measurement of gas phase concentration at material surface was measured by coupling a home-made sampler to SPME. First results had shown an accurate estimation of indoor formaldehyde concentration in this classroom by using a simple box model. [ABSTRACT FROM AUTHOR]

Two years of radon-222 observations collected at L'Aquila (Italy) in the atmospheric surface layer during 2004-2006 were analyzed in correlation with meteorological data and other atmospheric tracers. A box model was developed to better understand the mechanisms of diurnal and seasonal variability of the tracer and to indirectly assess the magnitude of the monthly averaged radon soil flux in the L'Aquila measurement site. The model was successfully validated with measurements, with a 0.8 average correlation coefficient between hourly values for the whole period of radon observations. Measurements taken during March 2009 were analyzed to find possible signs of perturbation due to the ongoing seismic activity that would have reached its peak on the 6 April 2009 destructive earthquake. Contrary to the professed (and unpublished) dramatic increases of radon activity unofficially announced to the inhabitants at that time, the study presented here shows that no radon activity increase took place in L'Aquila with respect to a previous 'seismically unperturbed' year (same month with similar meteorological conditions), but that an average 30 % decrease was experienced. This conclusion is reached from a direct comparison of observed data and also as a result of the previously validated radon box model constrained by actual meteorological data, from which an indirect estimate of a 17 % reduction of the radon soil flux is obtained. [ABSTRACT FROM AUTHOR]

A box model of the inter-hemispheric Atlantic meridional overturning circulation is developed, including a variable pycnocline depth for the tropical and subtropical regions. The circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface. The model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere, and to changes in freshwater transport in the ocean. These feedbacks are closely connected with the stability properties of the meridional overturning circulation, in particular in response to freshwater perturbations. A separate box is used for representing the region north of the Antarctic circumpolar current in the Atlantic sector. The density difference between this region and the north of the basin is then used for scaling the downwelling in the north. These choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models, and therefore suggest that the southernmost part of the Atlantic Ocean north of the Drake Passage is of fundamental importance for the stability of the meridional overturning circulation. With this configuration, the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing. The role of the freshwater transport by the overturning circulation ( M) as a stability indicator is discussed. It is investigated under which conditions its sign at the latitude of the southern tip of Africa can provide information on the existence of a second, permanently shut down, state of the overturning circulation in the box model. M will be an adequate indicator of the existence of multiple equilibria only if salt-advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies. M is a perfect indicator if feedbacks other than salt-advection are negligible. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Third order susceptibilities in supercooled liquids and the "box model" theory versus experiments" in the June 27, 2014 issue.

A response from the author of the article "Third order susceptibilities in supercooled liquids and the "box model" theory versus experiments" in the June 27, 2014 issue is presented.

The article focuses on the application of Monte Carlo methods in Climate Science. Topics discussed include the box model approach which involve the use of related things such as atmospheric gases, salinity and particulate levels, the application of forcing method and feedback, the probability modeling and the Markov Chain Monte Carlo method.

In situ airborne measurements of OH and HO2 with the HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy) instrument were performed in the summertime upper troposphere across Europe during the HOOVER 2 (HOx OVer EuRope) campaign in July 2007. Complementary measurements of trace gas species and photolysis frequencies were conducted to obtain a broad data set, which has been used to quantify the significant HOx sources and sinks. In this study we compare the in situ measurement of OH and HO2 with simulated mixing ratios from the constrained box model CAABA/MECCA (Chemistry As A Box Model Application/Module Efficiently Calculating the Chemistry of the Atmosphere), and the global circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry Model). The constrained box model reproduces the observed OH and HO2 mixing ratios with better agreement (obs/mod median 98% OH, 96% HO2) than the global model (median 76% OH, 59% HO2). The observations and the computed HOx sources and sinks are used to identify deviations between the models and their impacts on the calculated HOx budget. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) and hydroperoxyl (HO2) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NOx conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere- Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO + NO2) and other inorganic species were used to constrain a zerodimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70±0.31) for isoprene mixing ratios between 1-2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years. [ABSTRACT FROM AUTHOR]

Nitryl chloride (ClNO2) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO2 is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N2O5) on chloride‐containing aerosol, with a production yield, ϕ(ClNO2), defined as the moles of ClNO2 produced relative to N2O5 lost. The ϕ(ClNO2) has been increasingly incorporated into 3‐D chemical models where it is parameterized based on laboratory‐derived kinetics and currently accepted aqueous‐phase formation mechanism. This parameterization models ϕ(ClNO2) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individual ϕ(ClNO2) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field‐based ϕ(ClNO2) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory‐based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence of ϕ(ClNO2) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous‐phase competition reactions for the nitronium ion intermediate and/or direct ClNO2 loss mechanisms. Further laboratory studies of ClNO2 formation and the impacts of aerosol water, sulfate, organics, and ClNO2 aqueous‐phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robust ϕ(ClNO2) parameterization. Key Points: Box modeling of wintertime measurements over the eastern United States provides the first determinations of ClNO2 yields (ϕ(ClNO2)) from aircraftThe median ϕ(ClNO2) value derived from the box model is overpredicted by 75‐84% by the current laboratory‐based ϕ(ClNO2) parameterizationModeled and parameterized ϕ(ClNO2) values show opposite trends with aerosol water, suggesting an incorrectly parameterized dependence [ABSTRACT FROM AUTHOR]

We studied dynamical balances in magnetorotational instability (MRI) turbulence with a net vertical field in the shearing box model of disks. Analyzing the turbulence dynamics in Fourier (-)space, we identified three types of active modes that define the turbulence characteristics. These modes have lengths similar to the box size, i.e., lie in the small wavenumber region in Fourier space labeled “the vital area” and are (i) the channel mode, uniform in the disk plane with the smallest vertical wavenumber; (ii) the zonal flow mode, azimuthally and vertically uniform with the smallest radial wavenumber; and (iii) the rest (parasitic) modes. The rest modes comprise those harmonics in the vital area whose energies reach more than 50% of the maximum spectral energy. The rest modes individually are not so significant compared to the channel and zonal flow modes; however, the combined action of their multitude is dominant over these two modes. These three mode types are governed by the interplay of the linear and nonlinear processes, leading to their interdependent dynamics. The linear processes consist of disk flow nonmodality modified classical MRI with a net vertical field. The main nonlinear process is the transfer of modes over wavevector angles in Fourier space—the transverse cascade. The channel mode exhibits episodic bursts supplied by linear MRI growth, while the nonlinear processes mostly oppose this, draining the channel energy and redistributing it to the rest modes. As for the zonal flow, it does not have a linear source and is fed by nonlinear interactions of the rest modes. [ABSTRACT FROM AUTHOR]

Data obtained from long-term sediment trap experiments in the Indian Ocean were analysed in conjunction with satellite-derived observations and results obtained from a box model to study the influence of primary production and the ballast effect on organic carbon flux into the deep sea. The results are used to better understand the associated impacts on the CO2 uptake of the organic carbon pump. In line with other findings derived from global data sets, our results suggest that a preferential export of organic matter in slower-sinking particles reduces the transfer efficiency of exported organic matter in high-productive systems compared with low-productive regions. The resulting enhanced respiration of organic matter maintains the high nutrient availability in the surface ocean and thus the high productivity during the summer and winter bloom in the Arabian Sea. In turn, mineral ballast is essential for the transport of organic matter and nutrients into the deep sea. The additional lithogenic ballast effect can increase organic carbon fluxes by 60 %. Our model results indicate that lithogenic ballast enhances the CO2 uptake of the organic carbon pump by increasing the amount of nutrients utilised by the organic carbon pump to bind CO2. By enhancing the export of organic matter into the deep sea, the ballast effect increases the residence time of these nutrients in the ocean. They lose the attached CO2 if they are introduced into the surface ocean at higher latitude, where the lack of light prevents photosynthesis in winter. Considering the impact of the lithogenic ballast effect on the organic carbon export into the deep sea and the enhanced mobilisation of lithogenic matter due to land-use changes, it is assumed that humans influence the CO2 uptake of the organic carbon pump, which might hold relevance for the discussion about the anthropogenic CO2 uptake of the ocean. [ABSTRACT FROM AUTHOR]