Classifications of atmospheric circulation patterns as a tool for explaining asymmetry of day-to-day temperature difference

Classification of atmospheric circulation patterns has been a major powerful tool in synoptic climatology for a long time. Recently, its generalization has been introduced, which consists in developing geographically sliding classifications, i.e., independent classifications centred over individual gridpoints of a regular grid.

We employ such a tool in an attempt to explain the asymmetry of day-to-day temperature differences (DTDs). DTD is defined simply as a difference of daily temperature between two consecutive days. DTD in Europe is asymmetric: its skewness is negative over most of Europe in summer, while in winter, there is a tendency for a positive skewness to occur in the north and for negative skewness to occur in the south and over the British Isles.

We employ the ERA5 reanalysis as a major data source of both circulation and temperature data and the ECA&D station database for verification of temperature skewness in ERA5. Daily maximum temperature in summer and daily minimum temperature in winter are analyzed. Atmospheric circulation is characterized by sea level pressure, which is subject to classification by the Jenkinson-Collison (JC) method at all gridpoints over the European continent with spatial resolution of 2.5° x 2.5°. The JC method is based on types pre-defined by the strength, direction, and vorticity of geostrophic flow. We utilize its versions with 27 types (full version) and 11 types (with 8 directional types, two vorticity-based types and one undetermined for a weak flow). Under each type and at every gridpoint, we count small negative and small positive DTDs (small DTDs defined approximately as central 50% of its distribution). Types with the largest difference between small positive and small negative DTDs (i.e., with the largest asymmetry in small DTDs) are then identified.

A general behaviour, characteristic for the majority of European landmass, can be summarized as follows: Anticyclonic types, types with weak flow, and types with warm advection from south to southwest directions contribute to the asymmetry of Tmax DTD in summer, while anticyclonic types and types with cold northerly to northeasterly advection contribute to the Tmin DTD asymmetry in winter. Nevertheless, under specific conditions (upwind or leeward side of mountains, seashore, valley), any of the 11 JC types can occur among the three that most support the DTD asymmetry.