Is Upper-Level Dynamic Forcing Essential for Heavy Rain in the Levant?

This study assesses the effects of upper-level configuration on heavy rain episodes over the Levant. The dynamic upper-level forcings are attributed to ageostrophic effects. One is related to meandering jet ahead of troughs and the second to acceleration near both ends of straight jets. We collected 23 rainy episodes contained in four rainy months. The rain analysis was done on the eastern coast of the Mediterranean, and the synoptic analysis covers the domain 15–45° N, 12–45° E. The data were retrieved from ERA5 reanalysis, with 0.25° × 0.25° resolution. A subjective analysis revealed that the rain episodes are associated with three configurations, the two aforementioned and an additional, under upper trough, without upper-level divergence. In fourteen cases, the region was found ahead of trough and in only one at the end of a straight jet. In the remaining eight cases, upper trough was located over the region, implying an absence of upper-level support for rain formation. These configurations are exemplified by case studies and by composite maps. Most rain events occurred when an upper trough dominated the Levant, situated upstream (west) of a surface Cyprus Low (CL), with both contributing to rain formation. The CL persisted in the cases of the third type, in spite of an absence of upper-level support, due to surface-induced cyclogenesis. The two most frequent configurations are apparently similar but differ in the rain-producing factors. The one for the “ahead of” is upper-level dynamic, and for the “under trough”—lower level dynamic and enhanced static instability.