Atmosphere, Volume 7, Issue 7 (July 2016) – 11 articles

Influences of different computational grid resolutions on modeled ambient benzene concentrations from open burning were assessed in this study. The CALPUFF (California Puff Mesoscale Dispersion Model) was applied to simulate maximum ground level concentration over the modeling domain of 100 × 100 km². Meteorological data of the year 2014 was simulated from the Weather Research and Forecasting (WRF) model. Four different grid resolutions were tested including 0.75 km, 1 km, 2 km and 3 km resolutions. Predicted values of the maximum 24-h average concentrations obtained from the finest grid resolution (0.75 km) were set as reference values. In total, there were 1089 receptors used as reference locations for comparison of the results from different computational grid resolutions. Comparative results revealed that the larger the grid resolution, the higher the over-prediction of the results. Nevertheless, it was found that increasing the grid resolution from the finest resolution (0.75 km) to coarser resolutions (1 km, 2 km and 3 km) resulted in reduction of computational time by approximately 66%, 97% and >99% as compared with the reference grid resolution, respectively. Results revealed that the grid resolution of 1 km is the most appropriate resolution with regard to both accuracy of predicted data and acceptable computational time for the model simulation of the open burning source. Full article

On 21 September 2012 the entire sky was covered by contrails over the Gulf of Lyon (NW of the Mediterranean basin). These clouds were well recorded by ground observers as well as by Meteosat imagery. The atmospheric characteristics at the levels where these anthropic clouds formed are analyzed by performing a WRF simulation in the area where Meteosat recorded contrail clouds. According to the vertical profiles of temperature and the relative humidity respect to the ice (RHI), the environmental condition favors that the water vapor exhaust emitted by the aircraft engines reaches the deposition point and form crystal clouds, which spread out because the temperature remained below 230 K and the RHI was higher than 70% during the whole episode. Full article

Temperature and water vapor profiles from the Korea Meteorological Administration (KMA) and the United Kingdom Met Office (UKMO) Unified Model (UM) data assimilation systems and from reanalysis fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) were assessed using collocated radiosonde observations from the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) for January–December 2012. The motivation was to examine the overall performance of data assimilation outputs. The difference statistics of the collocated model outputs versus the radiosonde observations indicated a good agreement for the temperature, amongst datasets, while less agreement was found for the relative humidity. A comparison of the UM outputs from the UKMO and KMA revealed that they are similar to each other. The introduction of the new version of UM into the KMA in May 2012 resulted in an improved analysis performance, particularly for the moisture field. On the other hand, ECMWF reanalysis data showed slightly reduced performance for relative humidity compared with the UM, with a significant humid bias in the upper troposphere. ECMWF reanalysis temperature fields showed nearly the same performance as the two UM analyses. The root mean square differences (RMSDs) of the relative humidity for the three models were larger for more humid conditions, suggesting that humidity forecasts are less reliable under these conditions. Full article

Ozone concentrations have been increasing in the Guadalajara Metropolitan Area (GMA) in Mexico. To help devise efficient mitigation measures, we investigated the ozone formation regime by a chemical transport model (CTM) system WRF-CMAQ. The CTM system was validated by field measurement data of ground-level volatile organic compounds (VOC) and vertical profiles of ozone in GMA as well as in the Mexico City Metropolitan Area (MCMA). By conducting CTM simulations with modified emission rates of VOC and nitrogen oxides (NO_x), the ozone formation regime in GMA was found to lie between VOC-sensitive and NO_x-sensitive regimes. The result is consistent with the relatively large VOC/NO_x emission ratio in GMA compared to that in MCMA where the ozone formation regime is in the VOC-sensitive regime. Full article

Lightning electromagnetic fields in the presence of conducting (grounded) structure having a height of 60 m and a square cross-section of 40 m × 40 m within about 100 m of the observation point are analyzed using the 3D finite-difference time-domain (FDTD) method. Influence of the conducting structure on the two orthogonal components of magnetic field is analyzed, and resultant errors in the estimated lightning azimuth are evaluated. Influences of ground conductivity and lightning current waveshape parameters are also examined. When the azimuth vector passes through the center of conducting structure diagonally (e.g., azimuth angle is 45°) or parallel to its walls (e.g., azimuth angle is 0°), the presence of conducting structure equally influences H_x and H_y, so that H_x/H_y is the same as in the absence of structure. Therefore, no azimuth error occurs in those configurations. When the conducting structure is not located on the azimuth vector, the structure influences H_x and H_y differently, with the resultant direction finding error being greater when the structure is located closer to the observation point. Full article

The natural wetlands of the Tibetan Plateau (TP) are considered to be an important natural source of methane (CH₄) to the atmosphere. The long-term variation in CH₄ associated with climate change and wetland loss is still largely unknown. From 1950 to 2010, CH₄ emissions over the TP were analyzed using a model framework that integrates CH4MOD_wetland, TOPMODEL, and TEM models. Our simulation revealed a total increase of 15% in CH₄ fluxes, from 6.1 g m⁻² year⁻¹ to 7.0 g m⁻² year⁻¹. This change was primarily induced by increases in temperature and precipitation. Although climate change has accelerated CH₄ fluxes, the total amount of regional CH₄ emissions decreased by approximately 20% (0.06 Tg—i.e., from 0.28 Tg in the 1950s to 0.22 Tg in the 2000s), due to the loss of 1.41 million ha of wetland. Spatially, both CH₄ fluxes and regional CH₄ emissions showed a decreasing trend from the southeast to the northwest of the study area. Lower CH₄ emissions occurred in the northwestern Plateau, while the highest emissions occurred in the eastern edge. Overall, our results highlighted the fact that wetland loss decreased the CH₄ emissions by approximately 20%, even though climate change has accelerated the overall CH₄ emission rates over the last six decades. Full article

To obtain a better understanding of carbon cycle and accurate climate prediction models, highly accurate and temporal resolution observation of atmospheric CO₂ is necessary. Differential absorption LIDAR (DIAL) remote sensing is a promising technology to detect atmospheric CO₂. However, the traditional DIAL system is the dual-wavelength DIAL (DW-DIAL), which has strict requirements for wavelength accuracy and stability. Moreover, for on-line and off-line wavelengths, the system’s optical efficiency and the change of atmospheric parameters are assumed to be the same in the DW-DIAL system. This assumption inevitably produces measurement errors, especially under rapid aerosol changes. In this study, a multi-wavelength DIAL (MW-DIAL) is proposed to map atmospheric CO₂ concentration. The MW-DIAL conducts inversion with one on-line and multiple off-line wavelengths. Multiple concentrations of CO₂ are then obtained through difference processing between the single on-line and each of the off-line wavelengths. In addition, the least square method is adopted to optimize inversion results. Consequently, the inversion concentration of CO₂ in the MW-DIAL system is found to be the weighted average of the multiple concentrations. Simulation analysis and laboratory experiments were conducted to evaluate the inversion precision of MW-DIAL. For comparison, traditional DW-DIAL simulations were also conducted. Simulation analysis demonstrated that, given the drifting wavelengths of the laser, the detection accuracy of CO₂ when using MW-DIAL is higher than that when using DW-DIAL, especially when the drift is large. A laboratory experiment was also performed to verify the simulation analysis. Full article

The estimation of ambient particulate matter with diameter less than 10 µm (PM₁₀) at high spatial resolution is currently quite limited in China. In order to make the distribution of PM₁₀ more accessible to relevant departments and scientific research institutions, a semi-physical geographically weighted regression (GWR) model was established in this study to estimate nationwide mass concentrations of PM₁₀ using easily available MODIS AOD and NCEP Reanalysis meteorological parameters. The results demonstrated that applying physics-based corrections could remarkably improve the quality of the dataset for better model performance with the adjusted R² between PM₁₀ and AOD increasing from 0.08 to 0.43, and the fitted results explained approximately 81% of the variability in the corresponding PM₁₀ mass concentrations. Annual average PM₁₀ concentrations estimated by the semi-physical GWR model indicated that many residential regions suffer from severe particle pollution. Moreover, the deviation in estimation, which primarily results from the frequent changes in elevation, the spatially heterogeneous distribution of monitoring sites, and the limitations of AOD retrieval algorithm, was acceptable. Therefore, the semi-physical GWR model provides us with an effective and efficient method to estimate PM₁₀ at large scale. The results could offer reasonable estimations of health impacts and provide guidance on emission control strategies in China. Full article

Both research-grade and operational numerical weather prediction models perform simulations with horizontal grid spacings as fine as 1 km, and their multi-scale terrain data have become increasingly important for high-resolution model forecasting. This study focused on the influence of multi-scale surface databases of topographical height and land use on the modeling of atmospheric circulation in a megacity. The default data were the global 30S United States Geographic Survey terrain data set and Moderate Resolution Imaging Spectroradiometer land-use data. The capacity for topographical expression under the combined scale effect was evaluated against observational data. The experiments showed that surface input data using finer resolutions for the Weather Research and Forecasting model with 1-km resolution gave better topographical expression and meteorological reproduction in a megacity and agreed with observational data in the fields of temperature and relative humidity, but precipitation values were not sensitive to the surface input data when verified against a suite of observational data including, but not limited to, ground-based instruments. The results indicated that the use of high-resolution databases improved the local atmospheric circulation in a megacity and that a fine-scale model was sensitive to the resolution of the surface input data whereas a coarse-scale model was less sensitive to it. Full article

The atmospheric seasonal cycle of the North Atlantic region is dominated by meridional movements of the circulation systems: from the tropics, where the West African Monsoon and extreme tropical weather events take place, to the extratropics, where the circulation is dominated by seasonal changes in the jetstream and extratropical cyclones. Climate variability over the North Atlantic is controlled by various mechanisms. Atmospheric internal variability plays a crucial role in the mid-latitudes. However, El Niño-Southern Oscillation (ENSO) is still the main source of predictability in this region situated far away from the Pacific. Although the ENSO influence over tropical and extra-tropical areas is related to different physical mechanisms, in both regions this teleconnection seems to be non-stationary in time and modulated by multidecadal changes of the mean flow. Nowadays, long observational records (greater than 100 years) and modeling projects (e.g., CMIP) permit detecting non-stationarities in the influence of ENSO over the Atlantic basin, and further analyzing its potential mechanisms. The present article reviews the ENSO influence over the Atlantic region, paying special attention to the stability of this teleconnection over time and the possible modulators. Evidence is given that the ENSO–Atlantic teleconnection is weak over the North Atlantic. In this regard, the multidecadal ocean variability seems to modulate the presence of teleconnections, which can lead to important impacts of ENSO and to open windows of opportunity for seasonal predictability. Full article

SO₂ and H₂S are the two most important gas-phase sulfur species emitted by volcanoes, with a global amount from non-explosive emissions of the order 10 Tg-S/yr. These gases are readily oxidized forming SO₄²⁻ aerosols, which effectively scatter the incoming solar radiation and cool the surface. They also perturb atmospheric chemistry by enhancing the NO_x to HNO₃ heterogeneous conversion via hydrolysis on the aerosol surface of N₂O₅ and Br-Cl nitrates. This reduces formation of tropospheric O₃ and the OH to HO₂ ratio, thus limiting the oxidation of CH₄ and increasing its lifetime. In addition to this tropospheric chemistry perturbation, there is also an impact on the NO_x heterogeneous chemistry in the lower stratosphere, due to vertical transport of volcanic SO₂ up to the tropical tropopause layer. Furthermore, the stratospheric O₃ formation and loss, as well as the NO_x budget, may be slightly affected by the additional amount of upward diffused solar radiation and consequent increase of photolysis rates. Two multi-decadal time-slice runs of a climate-chemistry-aerosol model have been designed for studying these chemical-radiative effects. A tropopause mean global net radiative flux change (RF) of −0.23 W·m⁻² is calculated (including direct and indirect aerosol effects) with a 14% increase of the global mean sulfate aerosol optical depth. A 5–15 ppt NO_x decrease is found in the mid-troposphere subtropics and mid-latitudes and also from pole to pole in the lower stratosphere. The tropospheric NO_x perturbation triggers a column O₃ decrease of 0.5–1.5 DU and a 1.1% increase of the CH₄ lifetime. The surface cooling induced by solar radiation scattering by the volcanic aerosols induces a tropospheric stabilization with reduced updraft velocities that produce ice supersaturation conditions in the upper troposphere. A global mean 0.9% decrease of the cirrus ice optical depth is calculated with an indirect RF of −0.08 W·m⁻². Full article