Low-tropospheric humidity inversions over Europe: spatiotemporal variability and relations to temperature inversions' occurrence

Tropospheric humidity inversions are an important component of the Earth's climate system as well as a significant factor affecting the global radiation budget and cloud formation. Their occurrence enlarges the amount of downward longwave radiation trapped near the Earth's surface and provides moisture to maintain the top of clouds from evaporation. The aim of this paper is to examine the spatiotemporal variability of the humidity inversions over Europe. For the first time, we provide also a comprehensive analysis of relations between the humidity inversions and temperature inversions over the domain considered. The study is based on data derived from the ERA-Interim reanalysis for the period 1981-2015. We have confirmed that the temporal and spatial variability of the humidity inversions is strongly related to inversion type. The mean seasonal frequency of surface-based humidity inversions (SBHI) usually does not exceed 20%, while the mean seasonal frequency of elevated humidity inversions (EHI) ranges from 5 to 60% depending on the region and season. On average, EHI are substantially deeper and stronger than SBHI. We found also that the low-tropospheric humidity inversions often occur simultaneously with the temperature inversions. Moderate positive correlations exist, however, only among the parameters (depth and strength) of the same inversion type, but not between the humidity inversions and the temperature inversions. Considering the links between EHI and ETI (elevated temperature inversions) parameters, slightly higher values of the Spearman's rank correlation coefficient (rho(s) > 0.50) are found between EHI base height and ETI base height. Nonetheless, the simultaneous occurrence of EHI and ETI usually fosters the intensity of both inversion types.