Changes in the large‐scale thermodynamic instability and connection with rain shower frequency over Romania: verification of the Clausius–Clapeyron scaling

Abstract

The goal of this article is twofold: first, changes in the spring and summer thermodynamic instability over the North Atlantic European region and the connection with frequency of rain showers at 81 locations across Romania for the period 1961–2010 are analysed; second, the relationship between hourly precipitation extremes and air temperature with respect to the validity of Clausius–Clapeyron (CC) scaling is investigated. The Best Lifted Index (lftx4) has been selected to quantify the thermodynamic instability. The mechanisms responsible for the rain showers' variability in Romania were analysed using the canonical correlation analysis (CCA). Results show that the regional distribution of the lftx4 changes is quite similar in the two seasons, with a transition towards thermodynamic stability over most of Mediterranean regions – central Europe and instability in northern Europe, Black Sea, North Atlantic (summer), southern North Atlantic (spring). The frequency of rain showers in Romania exhibits a significant increasing trend in both seasons, which is in agreement with the increase in frequency of Cumulonimbus clouds. In contrast, the frequency of rainfall days does not exhibit any significant trend that shows a shift in the nature of precipitation towards more showers. The CCA shows that the optimum large-scale mechanisms responsible for this behaviour are given by the first CCA pair with the lftx4 pattern represented by a dipole structure with a nucleus of thermodynamic instability centred over Romania. The dynamic factor is an additional ingredient for spring, as precipitable water is for summer. The extreme hourly rainfall intensities reveal a similar behaviour over the entire country: the 90th percentile shows dependence close to the CC relation for all temperatures; the 99th and 99.9th percentiles exhibit rates close to double the CC rate for temperatures between ∼10 and ∼22 °C and negative scaling rates for higher temperatures. The daily intensities have a less clearly defined scaling behaviour.