Self-organizing map classification of the boundary layer profile – a refinement of Eastern Mediterranean winter synoptic regimes

Abstract

<p>The boundary layer (BL) profile over the coastal plain of Israel, Eastern Mediterranean (EM),<br>varies considerably during winter. Although, in the context of air pollution, the<br>characteristics of the BL height (BLH) was intensively investigated, a quantitative<br>classification of the BL profile regimes has not been performed. Here, we seek to reveal the<br>dominant, recurring regimes of the BL profiles, their quantitative characteristics and links to<br>regional synoptic-scale patterns.<br>An objective unsupervised classification of winter BL radiosonde profiles is performed for<br>the first time by multi-parameter self-organizing map (SOM) analysis. The analysis uses high-<br>resolution, 12-UTC data of wind, temperature, humidity and pressure measurements during<br>Dec-Feb 2007-2018, and yields 30 distinct profile regimes.<br>Composite analysis using ERA5 reanalysis suggests strong association between the profile<br>regimes and synoptic weather systems and highlights four groups: 1. Deep winter cyclones<br>with strong westerly wind and precipitation; 2. Strong surface anticyclones and Red Sea<br>troughs (RST) with a mid-tropospheric ridge, moderate dry easterly wind and extreme<br>temperatures. 3. Moderate pressure gradients under shallow cyclones, anticyclone to the<br>west and RST to the east of Israel. 4. Active RSTs, accompanied by upper-tropospheric<br>trough/cutoff low and heavy precipitation. For the first time, general objective classification<br>observes the active RST without requiring specific criteria.<br>Consistent with previous knowledge, the new classification exhibits distinct categories of<br>thermal stability, BLH and turbulence. Importantly, we show that the automatic objective<br>classification of profile data from a single station can be a sensitive discriminator of winter<br>synoptic regimes in the EM, and therefore explains the variability of the BL profile. It<br>facilitates the study of the interaction between the BL and the free troposphere and may<br>improve the prediction of air pollution or future BL profile regimes based on long time series<br>from historical data or climate models.</p>