The vertical temperature profile in the lowest 1000 m of the atmosphere determines a number of important physical processes and meteorological phenomena such as high concentrations of anthropogenic air pollutants in urban areas. Long-term monitoring of temperature profiles at high vertical and temporal resolution has only become feasible only recently with the introduction of affordable angular scanning microwave temperature profile radiometers. In this study, we analyzed 2 years of continuous temperature profile measurements with the MTP-5HE instrument in the urbanized coastal Bergen valley, Norway. The data have a 10 min temporal and 50 m vertical spatial resolution, thus, constituting a unique data set for a microclimatic characterization of the atmosphere in this high-latitude valley. We studied a 2 year record of ground-based (G-) and elevated (E-) inversions, their dynamics and connection to large-scale circulations, and the links between the urban air quality and the temperature inversions. G-inversions are commonly observed during wintertime nocturnal hours. The local topographic features allow for the frequent occurrence of G-inversions, even during strong winds of up to 16 m/s. E-inversions exist mostly during spring and summer and only during unusual synoptic circulation with large-scale warm air advection directly above the valley. Events with high air pollution, identified based on measurements of NO 2 and particulate matter concentrations, are highly dependent on the existence of G-inversions. Meteorological models poorly capture both G-inversions and E-inversions reducing their utility for the assessment of urban air quality and local weather forecasts.

Abstract. Urban air quality is one of the most prominent environmental concerns for modern city residents and authorities. Accurate monitoring of air quality is difficult due to intrinsic urban landscape heterogeneity and superposition of multiple polluting sources. Existing approaches often do not provide the necessary spatial details and peak concentrations of pollutants, especially at larger distances from monitoring stations. A more advanced integrated approach is needed. This study presents a very high-resolution air quality assessment with the Parallelized Large-Eddy Simulation Model (PALM), capitalising on local measurements. This fully three-dimensional primitive-equation hydrodynamical model resolves both structural details of the complex urban surface and turbulent eddies larger than 10 m in size. We ran a set of 27 meteorological weather scenarios in order to assess the dispersion of pollutants in Bergen, a middle-sized Norwegian city embedded in a coastal valley. This set of scenarios represents typically observed weather conditions with high air pollution from nitrogen dioxide (NO2) and particulate matter (PM2.5). The modelling methodology helped to identify pathways and patterns of air pollution caused by the three main local air pollution sources in the city. These are road vehicle traffic, domestic house heating with wood-burning fireplaces and ships docked in the harbour area next to the city centre. The study produced vulnerability maps, highlighting the most impacted districts for each weather and emission scenario. Overall, the largest contribution to air pollution over inhabited areas in Bergen was caused by road traffic emissions for NO2 and wood-burning fireplaces for PM2.5 pollution. The effect of emission from ships in the port was mostly restricted to the areas close to the harbour and moderate in comparison. However, the results have contributed to implementation of measures to reduce emissions from ships in Bergen harbour, including provision of shore power.

Abstract. Street-level urban air pollution is a challenging concern for modern urban societies. Pollution dispersion models assume that the concentrations decrease monotonically with raising wind speed. This convenient assumption breaks down when applied to flows with local recirculations such as those found in topographically complex coastal areas. This study looks at a practically important and sufficiently common case of air pollution in a coastal valley city. Here, the observed concentrations are determined by the interaction between large-scale topographically forced and local-scale breeze-like recirculations.Analysis of a long observational dataset in Bergen, Norway, revealed that the most extreme cases of recurring wintertime air pollution episodes were accompanied by increased large-scale wind speeds above the valley. Contrary to the theoretical assumption and intuitive expectations, the maximum NO 2 concentrations were not found for the lowest 10 m ERA-Interim wind speeds but in situations with wind speeds of 3 m s −1 . To explain this phenomenon, we investigated empirical relationships between the large-scale forcing and the local wind and air quality parameters. We conducted 16 large-eddy simulation (LES) experiments with the Parallelised Large-Eddy Simulation Model (PALM) for atmospheric and oceanic flows. The LES accounted for the realistic relief and coastal configuration as well as for the large-scale forcing and local surface condition heterogeneity in Bergen. They revealed that emerging local breeze-like circulations strongly enhance the urban ventilation and dispersion of the air pollutants in situations with weak largescale winds. Slightly stronger large-scale winds, however, can counteract these local recirculations, leading to enhanced surface air stagnation.Furthermore, this study looks at the concrete impact of the relative configuration of warmer water bodies in the city and the major transport corridor. We found that a relatively small local water body acted as a barrier for the horizontal transport of air pollutants from the largest street in the valley and along the valley bottom, transporting them vertically instead and hence diluting them. We found that the stable stratification accumulates the street-level pollution from the transport corridor in shallow air pockets near the surface. The polluted air pockets are transported by the local recirculations to other less polluted areas with only slow dilution. This combination of relatively long distance and complex transport paths together with weak dispersion is not sufficiently resolved in classical air pollution models. The findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

Transient perturbations of subionospheric very low frequency (VLF) radiowave signals provide new evidence for lightning‐induced electron precipitation (LEP) events involving short (<1 s) bursts of >1 MeV electrons from the earth's inner radiation belt at L ≤1.8. The signal amplitude changes are attributed to increased absorption in the earth‐ionosphere waveguide and/or alterations of the waveguide mode structure due to localized secondary ionization enhancements produced in the nighttime lower ionosphere and the mesosphere by the precipitating electrons. The otherwise stably trapped electrons are believed to be scattered in pitch angle during cyclotron resonant interactions in the magnetosphere with the lightning‐generated whistler waves. That some precipitation bursts consist partly of MeV electrons is suggested by (i) confinement of the perturbed subionospheric signal path to low magnetic latitudes (L ≤1.8), for which corresponding electron energies for gyroresonance with typical whistler‐wave frequencies in the magnetosphere are >1 MeV, and (ii) the temporal signatures of the perturbation events, which often exhibit an unusually rapid initial recovery (time constant of τ <1 s) followed by further recovery at rates believed characteristic of less energetic events (τ ∼5–20 s). The latter is interpreted as a manifestation of the rapid variation with altitude of the effective loss rate for excess ionization over an exceptionally wide range of mesospheric altitudes (40–70 km) penetrated by the >1 MeV electrons.

Expected occurrence characteristics of lightning‐induced electron precipitation (LEP) events at longitudes of the western (110° W) versus eastern (71° W) United States are considered from the point of view of available trapped particle flux at the edge of the loss cone. Considering published data on nighttime fluxes of >68 keV electrons observed at L ≃ 2.5, and for “direct” precipitation into the northern hemisphere induced by northern hemisphere lightning, the occurrence rate and flux levels are expected to be a factor of 20–200 higher in the west than in the east, assuming no significant variation in lightning source activity with longitude. Again assuming lightning sources in the north, it is predicted that at 71° W, “mirrored” precipitation into the southern hemisphere would involve precipitation fluxes 30–300 times higher than “direct” precipitation into the northern hemisphere. However, at 110° W and again assuming lightning in the north, southern hemisphere precipitation would tend to be limited to that small fraction of particles that were initially scattered into the northern loss cone and that were then backscattered from the northern atmosphere so as to reach the south. Preliminary experimental investigation of these predictions is based on observations of lightning‐associated perturbations of two geographically separate subionospheric VLF/LF signal paths, one (48.5 kHz) originating at Silver Creek, Nebraska, and observed at Stanford, California, and the other (28.5 kHz) originating at Aguadilla, Peurto Rico, and observed at Lake Mistissini, Quebec. The association of the characteristic VLF signal perturbations with lightning is generally evidenced by simultaneous (within ∼1 s) observation of single or multiple radio atmospherics. In most cases, high‐resolution measurements of event signatures reveal a ∼0.5–1 s delay between the atmospheric and event onset, as well as an ∼1‐s onset duration, consistent with theoretical predictions of a test particle model of the gyroresonant whistler‐particle interaction. The data, considered in the light of previous observations in the southern hemisphere, provide qualitative support for several of the predictions based on considerations of the trapped flux level near the loss cone, in particular the prediction of comparable rates in the north at 110° W and the south at 71° W, and the prediction of substantially larger rates in the south than in the north near 71° W.