Mechanisms governing asymmetry of day-to-day temperature changes

The focus of this contribution is on one particular aspect of short-term atmospheric variability, namely, the day-to-day temperature difference (DTD). Large DTDs negatively affect human health and impact also animals and plants, hence posing risk to society and asking for being thoroughly investigated.

Similar to other climate variables, risks are related to tails of DTD distributions, that is, to large day-to-day temperature drops and rises. It is known that the distribution of DTDT changes in central Europe is asymmetrical: large temperature increases prevail over large decreases in winter, and vice versa in summer. The opposite is true for small DTDT changes: in winter, small decreases prevail over small increases. However, unlike for temperature, mechanisms causing large DTDs and their asymmetry have only been hypothesized but not investigated in sufficient detail on scales larger than local.

In this contribution, we analyze two mechanisms that have been hypothesized to stay behind the asymmetry of DTD distributions. The first mechanism is passages of atmospheric fronts, which contribute to the asymmetry in large DTD changes: cold front passages are responsible for more frequent and stronger temperature drops than increases in summer, while warm front passages cause temperature increases to occur more often than drops in winter. To this end, we employ objective frontal analysis on the European scale. The second mechanism consists in circulation conditions conducive to radiative warming in summer or radiative cooling in winter, which govern the asymmetry in small DTD changes. The circulation conditions are described by classifications of atmospheric circulation patterns performed separately for individual gridpoints; anticyclonic types and types with warm advection in south and southwest direction contribute to the DTD asymmetry in summer, while anticyclonic types and types with cold northerly and northeasterly advection contribute to the DTD asymmetry in winter.