Tropospheric temperature inversions seem to be an important feature of climate, as well as a significant factor affecting air quality and fog formation. The aim of this article is to investigate the temporal and spatial variability of surface-based inversions (SBIs) over Europe. It is based on data derived from the ERA-Interim reanalysis for the period 1981-2015. The study examines diurnal, seasonal and multiannual variability of temperature inversions based on their frequency, depth and strength. These three parameters are characterized by strong temporal variability -diurnal and seasonal, as well as strong spatial differentiation. It has been confirmed that the energy budget is the key factor responsible for diurnal, and partly seasonal variability of SBI occurrence. Its negative values lead to an intense cooling of active surface and initiate the formation of the radiative inversions, which are the most common type of inversion occurring over the mainland of Europe. Temperature inversions usually form shortly after dusk, increasing gradually their depth and strength until sunrise. At 0000 Universal Time Coordinated (UTC), SBI frequency attains higher values in the summer than in the winter over the predominant part of Europe. However, the inversion layers occurring then are noticeably shallower and weaker. Surface type strongly affects SBI properties. This is mostly marked in terms of the distinct differentiation between marine and land areas as well as the impact of vast glaciers located across Greenland and western part of Iceland. Moreover, the distribution of SBI depth and strength is shaped by atmospheric circulation. For instance, their higher values occur over Eastern Europe in the winter, which is associated with the influence of a seasonal high pressure system found over Russia. Taking into account multiannual variability, SBI parameters, primarily depth and strength, exhibit the most significant negative changes in the winter.KEY WORDS surface-based inversions; ERA-Interim reanalysis; vertical temperature profile

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Tropospheric temperature inversions are thought to be an important feature of climate as well as a significant factor affecting air quality and low‐level cloud formation. The aim of this study is to investigate the temporal and spatial variability of the tropospheric temperature inversions, in particular so‐called elevated inversions, over Europe. The analysis is based on data gained from ERA‐Interim reanalysis for the period 1981–2015. The data consist of air temperature, and geopotential height from the entire vertical cross‐section of the troposphere, that is, from 1,000 to 100 hPa. The study examines the temporal (intra‐ and inter‐annual) variability of the temperature inversions based on their frequency, base height, depth, and strength. The analysis conducted revealed that the temperature inversions are a common phenomenon occurring in the lower troposphere. Their temporal and spatial variability is, however, determined by the inversion type. Surface‐based inversions (SBI) indicate a clear diurnal cycle, while the day–night variability of elevated inversions (EI) is far less pronounced. Two main regions of the most frequent EI occurrence may be distinguished. These are: (a) a marine area west of the Iberian Peninsula and (b) Eastern Europe. Both of them are located in areas which are under the influence of extensive high‐pressure systems—the permanent Azores High and semipermanent Siberian High, respectively. The development of EI should be therefore attributed to the large‐scale subsidence and adiabatic heating of air parcels. EI are also quite common over the other parts of the Atlantic Ocean, which is closely linked to the development of marine inversions. SBI tend to be stronger than EI—the mean seasonal inversion strength is usually substantially higher for SBI. In turn, EI reach higher values of the mean seasonal inversion depth as compared with SBI.

Despite the fact that tropospheric temperature inversions are thought to be an important feature of climate as well as a significant factor affecting air quality, low-level cloud formation, and the radiation budget of the Earth, a quantitative assessment of their representation in atmospheric reanalyses is yet missing. Here, we provide new evidence of the occurrence of low-tropospheric temperature inversions and associated uncertainties in their parameters existing among reanalyses produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) and upper-air soundings for Europe covering the period 2001–2010. The reanalyses utilized here include (1) surface-input reanalyses represented by ERA-20C and CERA-20C as well as (2) full-input reanalyses represented by ERA-Interim and ERA5. The upper-air soundings were derived from the Integrated Global Radiosonde Archive (IGRA), version 2. The data consists mainly of air temperature and geopotential height from the model levels (ModLev) and pressure levels (PresLev) of ECMWF reanalyses. The results show that the frequency of surface-based inversions (SBI) and elevated inversions (EI) is largely in agreement among the reanalyses. The quality of their representation depends, however, on the inversion type, season, and region. Over the vast majority of IGRA upper-air stations, SBI frequency is overestimated and EI frequency is underestimated by ECMWF reanalyses. Substantially larger uncertainties arise from the selection between the data of ModLev and PresLev of the reanalyses—the differences in the frequency of the temperature inversions are particularly large for summertime SBI suggesting that PresLev are not capable of resolving the main features of shallow and weak SBI.

Severe wind events are often related to the occurrence of mesoscale convective systems with arch-shaped radar reflectivity, i.e., a bow echo. In this research, the kinematic and thermodynamic conditions associated with 91 bow echo cases which occurred in the warm season (i.e., from early April until late September) in Poland (2007-2014) were analyzed. The environmental conditions were determined primarily based on the upper air soundings, and additionally on data obtained from ERA-Interim reanalysis. The results indicate that there is a relatively wide range of shear and instability environments associated with bow echoes over Poland. The identified cases occurred both in weakly forced environments, and as well developed in dynamic synoptic patterns with low instability. We have also found cases with strong instability and significantly increased shear values. The combination of a moist boundary layer and steep mid-tropospheric lapse rate usually resulted in moderate to high CAPE values for identified bow echo cases. The median of surface-based CAPE was equal to 1594 J/kg (Mean Layer CAPE = 1038 J/kg) for soundings, and to 1622 J/kg (Mean Layer CAPE = 1275 J/kg) for ERA-Interim. Bow echo environments also showed significantly increased potential for strong downdrafts and damaging outflow winds (the median Downdraft CAPE reached 849 J/kg for soundings and 734 J/kg for ERA-Interim). Bow echoes were usually associated with the occurrence of strong air flow in the troposphere. The presence of a jet stream in the middle and upper troposphere contributed to the development of increased vertical wind shear values. The median of 0-6-km shear exceeded 15 m/s, whereas for 0-3-km shear, it was approximately equal to 12.5 m/s and to 7 m/s for 0-1-km shear. 1 Introduction Mesoscale convective systems (MCSs) can pose a significant risk to human life and health, as well as huge losses in the economy. Every year across Europe, several thousand destructive wind, tornado, hail, or heavy rain events cause temporary disorganization of life. These phenomena are frequently connected with the movement of strong meso-β-scale convective systems with arch-shaped radar reflectivity, i.e., bow echo. According to Klimowski et al. (2003), at least 29% of all severe wind reports recorded in the USA (Northern High Plains) during the warm seasons of 1996-1999 were caused by the activity of convective systems with a bow echo (24% of fatal/deadly nontornadic convective wind storms in the USA from 1998 to 2007 (all seasons)-Schoen and Ashley 2011). Gatzen (2013), in turn, pointed out that 58% of severe wind reports (≥ 26 m/s) in Germany were related to a bow echo (for the warm season between 1997 and 2011). Research on the spatial and temporal variability of bow echo occurrence focused primarily on the area of the USA and Central Europe. They included both warm season