# Rainfall Erosivity Main Features and Their Associated Synoptic Conditions in North-Eastern Romania
## 3 Results and Discussions
### Synoptic Analysis of Days with Heavy Rainfalls  
Overall, it is widely known that the most prone synoptic conditions favorable to the occurrence of heavy rainfall are associated with the development of well-organized convective systems as a direct consequence of increased atmospheric instability. The atmospheric instability leading to heavy precipitation could be specific for the contact between different types of air masses, represented by various types of frontal systems, or  
Figure 3: Monthly evolution of the average number of hours with light (**a**), moderate (**b**), and heavy (**c**) intensity of precipitation (2013–2019) in northeastern Romania. manifests within a single unstable air mass. In such conditions, daytime radiative heating of the surface can augment the tropospheric lapse rate, thereby generating robust updrafts, particularly when the middle and upper layers of the troposphere are cold and dominated by upper-level troughs or cut-off low systems (25,60). The general image of the heavy rainfall events was built based on the days recording at least one hourly interval of precipitation amount exceeding 5 mm. In this way, based on the two weather stations, between 2013 and 2019, we summed up a total number of 63 independent days. The composite maps of these days highlight, at sea level, a synoptic pattern that can be considered typical for days with increased atmospheric instability, especially in north-eastern part of Romania (22). Firstly, the composite maps indicate a ridge structure extending over the European continent from the Azores region to northeastern Europe, while a low-pressure area prevails in the south-eastern part of the continent, imposing an easterly atmospheric circulation in the Black Sea region, including the region of Romania (Figure 4a). These sea-level conditions are also associated with an upper level structure characterized by a long-wave upper trough that can sustain cold air at this level, imposing atmospheric instability on lower levels (Figure 4b). This general overview of the synoptic conditions conducive to heavy rainfall in the study region was facilitated by self-organizing maps (SOMs) of all 63 days selected for analysis. The application of this method has enabled the identification of three major weather patterns types that favor intense rainfalls. The prevalent weather pattern associated with heavy rainfall (Type 1, accounting for 44.4% of heavy rainfall days) is defined by a strong zonal westerly flow in the upper atmosphere, combined with a relatively weak pressure gradient and mild cyclonic conditions at the surface (Figure 5). This pattern is most characteristic of the warm season, particularly between May and September, with peak occurrence in June and July, when it serves as the primary synoptic driver of heavy rainfall events. During September, this weather pattern is responsible for the highest hourly precipitation totals of approximately 20 mm, driven by infrequent but exceptionally intense rainfall events that are typical of this month (Figure 6). Figure 4: Composite maps of sea-level pressure (**a**) and 500 hPa geopotential height (**b**) at continental scale for days with heavy precipitation (2013–2019) in the central part of the Moldavian Plateau. Atmospheric conditions under Type 1 are marked by moderate CAPE values, indicating high instability capable to support convective activity. The 700 hPa wind vector, oriented from the south-west, leads to the same direction of storm cell movement (Figure 7). This is likely to increase the likelihood of intense soil erosion on west-facing slopes. The daily mean precipitation for Type 1 events averages around 10 mm (Figure 8), with a clear spatial gradient showing higher amounts in the northern parts of northeastern Romania compared to the south. Figure 5: Self-organizing maps (SOM) based on sea level pressure (hpa) and 500 hPa geopotential height (white lines) describing the main synoptic patterns associated with the days with heavy intensity precipitation (2013–2019) in the Moldavian Plateau.