Refinement of East Mediterranean Winter Climatological Regimes from Self-organizing Map Analysis of Boundary Layer Profiles

Abstract:

The atmospheric profile is usually characterized according to seasonal or monthly averages. Since the features of the boundary layer profile may significantly vary from these averages, a thorough examination of their variability is needed. To the best of our knowledge, the characteristics of the boundary layer profile over the Israeli coastal plain were not studied systematically. High variability of synoptic groups and pressure gradient intensity occurs over the East Mediterranean (EM) during the winter. Therefore, we perform an objective unsupervised classification of the major winter regimes at Beit Dagan (32°N, 34.8°E), on the central coastal plain of Israel, by multi variable self-organizing maps (SOM) analysis. The data was 12 UTC high resolution (100 m) radiosonde measurements of wind, temperature, humidity and pressure during Dec- Feb 2007-2018. The SOM analysis yielded 30 multi-variable (wind, pressure, temperature and specific humidity) profile regimes, exhibiting the main regimes of the BL profile during winter.

An examination of the composites of the various synoptic variables: geopotential height, temperature, humidity, potential vorticity and precipitation, in the extended EM region under each profile regime revealed a strong association between the regional synoptic systems and the profile features.

Four major synoptic groups were detected:

1. Deep winter lows with strong SW to NW winds, high turbulence and precipitation.
2. Strong winter highs and Red Sea trough (RST) with an upper southerly ridge, moderate easterly wind up to 1-2 km and wind direction shear at this height. The extreme hot and dry regime is a member of this group.
3. Moderate pressure gradients under shallow lows, high to the west and RST to the east of Israel.
4. Active RST, accompanied by an upper northern trough leading to enhanced upward motion and unstable conditions. As a result, convective storms, frequently accompanied by heavy showers and thunderstorms, occur at the eastern and southern parts of Israel. The latter group offers a first objective classification of active RST.

The profile classification includes information on distinct categories of thermal stability, BL height and turbulence. The relation to synoptic conditions is consistent with previous knowledge about BL characteristics under the various synoptic groups and enables further direct classification of the active Red Sea Trough, as well as extreme winter lows or highs.

This study has revealed that the automatic objective classification of profile data from a single station is a sensitive discriminator of winter synoptic regimes in the Eastern Mediterranean. This approach is potentially valuable for assessing the predictability of local high impact weather.