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Lightning production is described using thermodynamic hypotheses mainly according to convective activity in the atmosphere. However, existing formal thermodynamic descriptions are unable to fully explain the profound difference in the flash rate between tropical Africa and South America and between land and ocean. Aerosols are shown to be regulators of lightning in regional studies, but their influence on lightning production at the global scale is not described. I analyzed spatial patterns of the satellite annual global flash rate and simulated, annually averaged cloud condensation nuclei (CCN) distribution and found consistent positive correlation between them for land, ocean and continents. I developed a simple model of lightning production that is based solely on an aerosol hypothesis. The central premise of this model is that concentration of graupel pellets and concentration of ice crystals, which both determine flash rate, are monotonically increasing by CCN concentration up to a critical value (around 2000 cm<sup>−3</sup>). However, ice crystal concentration falls rapidly after the threshold due to lowering in the number of large cloud droplets effective for rime-splintering ice multiplication. Comparison of the model with a model of the flash rate based on thermodynamic hypotheses demonstrates that the aerosol hypothesis explains the global annual spatial distribution of lightning production consistently better over land and over oceans. My results emphasize importance of aerosols for lightning production and point to the existence of a global aerosol-lightning feedback, which affects both the climate system and the land surface.
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Stable oxygen isotope composition of atmospheric precipitation (δ<sup>18</sup>O<sub>p</sub>) was scrutinized from 39 stations distributed over Switzerland and its border zone. Monthly amount-weighted δ<sup>18</sup>O<sub>p</sub> values averaged over the 1995–2000 period showed the expected strong linear altitude dependence (−0.15 to −0.22‰ per 100 m) only during the summer season (May–September). Steeper gradients (~ −0.56 to −0.60‰ per 100 m) were observed for winter months over a low elevation belt, while hardly any altitudinal difference was seen for high elevation stations. This dichotomous pattern could be explained by the characteristically shallower vertical atmospheric mixing height during winter season and provides empirical evidence for recently simulated effects of stratified atmospheric flow on orographic precipitation isotopic ratios. This helps explain "anomalous" deflected altitudinal water isotope profiles reported from many other high relief regions. Grids and isotope distribution maps of the monthly δ<sup>18</sup>O<sub>p</sub> have been calculated over the study region for 1995–1996. The adopted interpolation method took into account both the variable mixing heights and the seasonal difference in the isotopic lapse rate and combined them with residual kriging. The presented data set allows a point estimation of δ<sup>18</sup>O<sub>p</sub> with monthly resolution. According to the test calculations executed on subsets, this biannual data set can be extended back to 1992 with maintained fidelity and, with a reduced station subset, even back to 1983 at the expense of faded reliability of the derived δ<sup>18</sup>O<sub>p</sub> estimates, mainly in the eastern part of Switzerland. Before 1983, reliable results can only be expected for the Swiss Plateau since important stations representing eastern and south-western Switzerland were not yet in operation.
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To challenge the hypothesis that equatorial waves in the lower stratosphere are essentially forced by convection, we use the LMDz atmospheric model extended to the stratosphere and compare two versions having very different convection schemes but no quasi-biennial oscillation (QBO). The two versions have realistic time mean precipitation climatologies but very different precipitation variabilities. Despite these differences, the equatorial stratospheric Kelvin waves at 50 hPa are almost identical in the two versions and quite realistic. The Rossby gravity waves are also very similar but significantly weaker than in observations. We demonstrate that this bias on the Rossby gravity waves is essentially due to a dynamical filtering occurring because the model zonal wind is systematically westward. During a westward phase of the QBO, the ERA-Interim Rossby gravity waves compare well with those in the model. <br><br> These results suggest that (i) in the model the effect of the convection scheme on the waves is in part hidden by the dynamical filtering, and (ii) the waves are produced by other sources than equatorial convection. For the Kelvin waves, this last point is illustrated by an Eliassen and Palm flux analysis, showing that in the model they come more from the subtropics and mid-latitude regions, whereas in the ERA-Interim reanalysis the sources are more equatorial. We show that non-equatorial sources are also significant in reanalysis data sets as they explain the presence of the Rossby gravity waves in the stratosphere. To illustrate this point, we identify situations with large Rossby gravity waves in the reanalysis middle stratosphere for dates selected when the stratosphere is dynamically separated from the equatorial troposphere. We refer to this process as a stratospheric reloading.
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Southern Africa is a significant source region of atmospheric pollution, yet long-term data on pollutant concentrations and properties from this region are rather limited. A recently established atmospheric measurement station in South Africa, Welgegund, is strategically situated to capture regional background concentrations, as well as emissions from the major source regions in the interior of South Africa. We measured non-refractive submicron aerosols (NR-PM<sub>1</sub>) and black carbon over a one year period in Welgegund, and investigated the seasonal and diurnal patterns of aerosol concentration levels, chemical composition, acidity and oxidation level. Based on air mass back trajectories, four distinct source regions were determined for NR-PM<sub>1</sub>. Supporting data utilised in our analysis included particle number size distributions, aerosol absorption, trace gas concentrations, meteorological variables and the flux of carbon dioxide. <br><br> The dominant submicron aerosol constituent during the dry season was organic aerosol, reflecting high contribution from savannah fires and other combustion sources. Organic aerosol concentrations were lower during the wet season, presumably due to wet deposition as well as reduced emissions from combustion sources. Sulfate concentrations were usually high and exceeded organic aerosol concentrations when air-masses were transported over regions containing major point sources. Sulfate and nitrate concentrations peaked when air masses passed over the industrial Highveld (iHV) area. In contrast, concentrations were much lower when air masses passed over the cleaner background (BG) areas. Air masses associated with the anti-cyclonic recirculation (ACBIC) source region contained largely aged OA. <br><br> Positive Matrix Factorization (PMF) analysis of aerosol mass spectra was used to characterise the organic aerosol (OA) properties. The factors identified were oxidized organic aerosols (OOA) and biomass burning organic aerosols (BBOA) in the dry season and low-volatile (LV-OOA) and semi-volatile (SV-OOA) organic aerosols in the wet season. The results highlight the importance of primary BBOA in the dry season, which represented 33% of the total OA. Aerosol acidity and its potential impact on the evolution of OOA are also discussed.
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Spatial and temporal variations of summer sea ice albedo over the Arctic are analyzed using an ensemble of historical CMIP5 model simulations. The results are compared to the CLARA-SAL product that is based on long-term satellite observations. The summer sea ice albedo varies substantially among CMIP5 models, and many models show large biases compared to the CLARA-SAL product. Single summer months show an extreme spread of ice albedo among models; July values vary between 0.3 and 0.7 for individual models. The CMIP5 ensemble mean, however, agrees relatively well in the central Arctic but shows too high ice albedo near the ice edges and coasts. In most models, the ice albedo is spatially too uniformly distributed. The summer-to-summer variations seem to be underestimated in many global models, and almost no model is able to reproduce the temporal evolution of ice albedo throughout the summer fully. While the satellite observations indicate the lowest ice albedos during August, the models show minimum values in July and substantially higher values in August. Instead, the June values are often lower in the models than in the satellite observations. This is probably due to too high surface temperatures in June, leading to an early start of the melt season and too cold temperatures in August causing an earlier refreezing in the models. The summer sea ice albedo in the CMIP5 models is strongly governed by surface temperature and snow conditions, particularly during the period of melt onset in early summer and refreezing in late summer. <br><br> The summer surface net solar radiation of the ice-covered Arctic areas is highly related to the ice albedo in the CMIP5 models. However, the impact of the ice albedo on the sea ice conditions in the CMIP5 models is not clearly visible. This indicates the importance of other Arctic and large-scale processes for the sea ice conditions.
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This study examines the occurrence and morphology of frozen-drop aggregates in thunderstorm anvils from the United States Midwest and describes the environmental conditions where they are found. In situ airborne data collected in anvils using several particle imaging and sizing probes and bulk total water instrumentation during the 2012 Deep Convective Clouds and Chemistry experiment are examined for the presence of frozen-drop aggregates. Chains of frozen drops have been only rarely reported before and are hypothesized to aggregate due to electrical forces in the clouds. They were identified in nine of the anvil cases examined to date, suggesting that they are common features in these Midwestern anvils. High concentrations of individual frozen droplets occurred on the tops and edges of one particular set of anvils, while regions closer to the center and bottom of these anvils exhibited fewer frozen drops and more frozen-drop aggregates. Bulk ice water content measurements across these anvils could only be explained by contributions from both small particles (frozen droplets) and large particles (large aggregates of frozen droplets). Dual Doppler radar analysis confirmed the presence of deep and strong (> 15 m s<sup>−1</sup>) updrafts in the parent cloud of one of the anvils. These features contrast with previous anvil measurements in tropical/maritime anvils that evidently do not exhibit the same frequency of frozen-drop aggregates.
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Sulfur dioxide (SO<sub>2</sub>) is a major atmospheric pollutant with a strong anthropogenic component mostly produced by the combustion of fossil fuel and other industrial activities. As a precursor of sulfate aerosols that affect climate, air quality, and human health, this gas needs to be monitored on a global scale. Global climate and chemistry models including aerosol processes along with their radiative effects are important tools for climate and air quality research. Validation of these models against in-situ and satellite measurements is essential to ascertain the credibility of these models and to guide model improvements. In this study, the Goddard Chemistry, Aerosol, Radiation, and Transport (GOCART) module running on-line inside the Goddard Earth Observing System version 5 (GEOS-5) model is used to simulate aerosol and SO<sub>2</sub> concentrations. Data taken in November 2010 over Frostburg, Maryland during an SO<sub>2</sub> field campaign involving ground instrumentation and aircraft are used to evaluate GEOS-5 simulated SO<sub>2</sub> concentrations. Preliminary data analysis indicated the model overestimated surface SO<sub>2</sub> concentration, which motivated the examination of the specification of SO<sub>2</sub> anthropogenic emission rates. As a result of this analysis, a revision of anthropogenic emission inventories in GEOS-5 was implemented, and the vertical placement of SO<sub>2</sub> sources was updated. Results show that these revisions improve the model agreement with observations locally and in regions outside the area of this field campaign. In particular, we use the ground-based measurements collected by the United States Environmental Protection Agency (US EPA) for the year 2010 to evaluate the revised model simulations over North America.
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Humidity inversions have a high potential importance in the Arctic climate system, especially for cloud formation and maintenance, in wide spatial and temporal scales. Here we investigate the climatology and characteristics of humidity inversions in the Arctic, including their spatial and temporal variability, sensitivity to the methodology applied and differences from the Antarctic humidity inversions. The study is based on data of the Integrated Global Radiosonde Archive (IGRA) from 36 Arctic stations between the years 2000 and 2009. The results indicate that humidity inversions are present on multiple levels nearly all the time in the Arctic atmosphere. Almost half (48%) of the humidity inversions were found at least partly within the same vertical layer with temperature inversions, whereas the existence of the other half may, at least partly, be linked to uneven vertical distribution of horizontal moisture transport. A high atmospheric surface pressure was found to increase the humidity inversion occurrence, whereas relationships between humidity inversion properties and cloud cover were generally relatively weak, although for some inversion properties they were systematic. For example, humidity inversions occurred slightly more often and were deeper under clear sky than in overcast conditions for almost all stations. The statistics of Arctic humidity inversion properties, especially inversion strength, depth and base height, proved to be very sensitive to the instruments and methodology applied. For example, the median strength of the strongest inversion in a profile was twice as large as the median of all Arctic inversions. The most striking difference between the Arctic and Antarctic humidity inversions was the much larger range of the seasonal cycle of inversion properties in the Arctic. Our results offer a baseline for validation of weather prediction and climate models and also encourage further studies on humidity inversions due to the vital, but so far poorly understood, role of humidity inversions in Arctic cloud processes.
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Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High Altitude and Long Range Research Aircraft (HALO) in November 2010. The data are used to quantify the influence of surface albedo variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.
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HONO is a major precursor for OH radicals in early mornings. Its formation has been mainly attributed to the heterogeneous hydrolysis of NO<sub>2</sub> on surfaces such as soot, glass, mineral oxides and aerosol surfaces. In particular, dust events which are loaded with mineral oxide aerosols have been associated with higher HONO concentrations in the gas phase. In order to understand the mechanism of reactions related to this process, samples during dusty and non-dusty days were collected between October 2009 and April 2011. Based on HYSPLIT backward trajectories, data were divided between wind trajectories originating from Arabian or African deserts. In this study an increase of HONO levels was observed during dusty days. The increase in the acidic gas concentrations was accompanied by an increase in the PM nitrate and sulfate ion concentrations. During high relative humidity (African dusty days), it is proposed that the mechanism of NO<sub>2</sub> hydrolysis predominates whereas during Arabian dusty days, where the air is relatively dry, a synergistic mechanism of adsorption and reaction between NO<sub>2</sub> and SO<sub>2</sub> on dust particles to produce HONO and sulfate in the particle phase is suggested. This study implies that the NO<sub>x</sub> reactivity on mineral oxide surfaces leads to a higher mixing level of OH. An increase in the sulfate forming capacity could account for the underestimation of sulfates in aerosols when the reactive uptake of SO<sub>2</sub> alone is considered.
