Asymmetry of Day-to-Day Temperature Difference: Climatology and Mechanisms

Substantial progress has been made in analyses of temperature means and extremes, while much less has been done in understanding short-term (intraseasonal and synoptic-scale) variability of temperature. One particular aspect of short-term temperature variability is day-to-day temperature difference (DTD). Large DTDs negatively affect human health and impact also animals and plants, hence posing one of the many weather-related risks to society.

Distribution of DTD in central Europe is known to be asymmetrical: in winter, large temperature increases prevail over large decreases and small decreases prevail over small increases. The opposite holds for summer: large temperature decreases prevail over large increases and small increases prevail over small decreases. However, mechanisms causing asymmetry of DTD, and of large day-to-day temperature rises and drops in particular, have only been hypothesized but not investigated in sufficient detail on scales larger than local.

This contribution presents climatology of DTD, with focus on its skewness, for daily minimum (Tmin) and maximum temperature (Tmax) over Europe in individual seasons. We compare station data (ECA&D) with ERA-5 reanalysis. The magnitude of DTD is largest in winter and smallest in summer, and grows from the coast to the continental interior. Skewness of DTD for Tmax is negative over most of Europe in summer, while in winter, there is a tendency for positive skewness of Tmin DTD to occur in the north and for negative skewness to occur in the south and over the British Isles. Kurtosis of DTD is everywhere and in all seasons larger than Gaussian; i.e., DTD distributions have heavy tails.

Both datasets agree one with another to a reasonable degree. This allows us to study the mechanisms behind DTD asymmetry in the ERA-5 data. 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 Tmax drops than increases in summer, while warm front passages cause Tmin 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; the objectivized classification after Jenkinson-Collison is used for this purpose. Anticyclonic types 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.