Potential-vorticity regimes over the Eastern Mediterranean and their relation to local boundary layer profiles

Abstract

The occurrence and passage of synoptic-scale systems modulate the local boundary-layer (BL) profile. In the Eastern Mediterranean (EM), a detailed clustering of the winter profiles over Beit-Dagan, at the Israeli central coastal plain, showed direct links to winter highs, lows and Red Sea troughs and further enabled the identification of the active RST, a longstanding challenge to objectively identify. Since high resolution radiosondes profile data at Beit Dagan is available for the recent 20 years, and sometimes suffers lack of data, its application as synoptic tool is limited. Objective synoptic classification during longer periods is needed. Our research  investigates the synoptic regimes according to upper tropospheric PV during the winter months (DJF 2011-2021). We utilize the self-organizing map (SOM) clustering method and the ERA5 reanalysis data to achieve this aim. Various domains, SOM parameters, quantization and topographical errors, standard deviations of each SOM class, and gradual size of maps were tuned and inspected respectively to select the final map. The synoptic regimes are later related to the boundary layer profile variability. The relation between the PV classes and the variability of the BL profile is found according to the frequencies of the PV classes under each BL profile class. The ageostrophic balance next to the surface effect the BL profile. To include this important factor, extended synoptic classification, according to multi variable clustering of PV and 1000 hPa geopotential height (gph) was devised. SOM training and projection on the BL profile classes were accordingly preformed. SOM clustering of 320K isentropic surface potential vorticity (PV) data presented 4X4 classes. Two PV classes relate to high PV (> 2 PVU) over the EM: the first presents wide northerly trough and the second a thinner trough with a north-easterly axis towards Israel due to anticyclonic shear. Most of the other classes present low PV values (< 2 PVU) over the EM relating to southerly wide ridge or anticyclonic wave breaking propagating to the east of the EM. Strong or weak PV activity over the EM is related to some of the BL profile classes (few classes with relatively high frequency (> 20%)). Under mild PV activity which is related to mild surface pressure gradients, no strong relation is found. Multi-variable SOM clustering of gph and PV presented 4X5 classes which follow the variability of surface winter lows, highs and active Red Sea troughs. The active Red Sea trough relates to the north easterly relatively narrow PV stream. The main PV classes of the 4X4 single variable SOM classification resemble those of the combined (PV + gph) classification. The multi-variable clustering somewhat improves the indication of the BL profile classes. Better indication between BL profile pattern and strong winter highs is obtained. This work suggests a new approach to inspect the co variability of synoptic regimes over the EM with various meteorological variables (beyond the BL profile) including examination of trends and persistence of each synoptic regime.