On the importance of synoptic classification methods with respect to environmental phenomena

One of the most important goals of synoptic climatology is to analyse the relationships between atmospheric circulation and surface environmental conditions. Since manual (subjective) classification might reflect differently environmental phenomena as compared to computer assisted (semi-objective) classification, a comparison between both classifications was performed for the Eastern Mediterranean (EM) for 10 years. The overall frequencies of the 19 recurring synoptic types characterising the circulation regime over the EM are similar for both methods (correlation coefficient = 0.96, p-value < 0.0001). Nevertheless, comparing the classifications on a day-to-day basis for the three most common types showed a large degree of disagreement between the two. For synoptic types with a deep horizontal pressure gradient both classifications yielded the highest agreement (over 50%), improving the consistency between both classifications. Yarnal's (1993) 'environment-to-circulation' approach was tested on three surface environment processes: air pollutants, desert dust intrusions, and flash floods. Our results indicate that the weak pressure gradients associated with high ozone levels make the classification more difficult for both methods. Regarding dust outbreaks, classifications point on the importance of the cold cyclone location rather than its pressure gradient. As for flash floods, the flow pattern at the surface level is insufficient to predict atmospheric conditions prone for their occurrence, suggesting that upper air data is an essential factor for determining such highly convective events. Finally, two new approaches to evaluate the quality of classifications are presented, demonstrated on two frequent circulation systems persisting over the EM. A qualitative approach compares the composite mean sea level pressure maps from each classification, while a quantitative approach compares the resultant winds at a central site. This study demonstrates that comparing different synoptic classification methodologies and different environmental applications can lead to additional and valuable insights on the interaction between the environment and synoptic-scale circulation.