Synoptic climatological analysis on the appearance features of extremely low temperature days around Germany for 1971/72 to 2010/11 winters

Seasonal cycle including day-to-day fluctuations shows great variety from region to region even among the middle/higher latitudes. Thus to know such detailed seasonal cycle for each region is the common basis for deeper understanding of (1) seasonal backgrounds of extreme meteorological or climatological events and (2) that of cultural generation leading to the cultural understanding education through the “seasonal feeling”. For example, the “seasonal feeling” of the severe winter relating to the traditional event for driving the winter away (“Fasnacht”) around Germany might be greatly reflected by the intermittent appearance of the extremely low temperature events, although the winter mean temperature there is lower only by about 3~5℃than in the southern part of the Japan Islands area.

Hamaki et al.(2018, although written in Japanese) suggested, based on the case study for 2000/01 winter, that the appearance of such events with large intraseasonal variation are greatly controlled by that of the Icelandic low. Furthermore, a case study for the same winter by Miyake and Kato (EGU2020 Assembly) pointed out that the rapid seasonal increase in the appearance frequency of extremely low temperature events there as the intraseasonal variation around mid-December, although the seasonal mean the Icelandic low already appeared in mid-October.

However, the climatological appearance features of the extremely low temperature events for longer periods should be examined further. Thus we will describe the detailed synoptic climatological features for 1971/72 to 2010/11 winters, based on the NCEP/NCAR reanalysis data. In this study, the days with daily mean surface air temperature less than or equal to -7℃ are referred to as the "extremely low temperature days".

Amplitude of the intraseasonal variation of the surface air temperature around Germany increased seasonally in association with the beginning of the nearly minimum seasonal mean temperature period (around December to February), resulting in the seasonal increase in the appearance frequency of the extremely low temperature days from around December. Such features are clearly found for the 10 winters (referred to as the "typical years") of the total 40 winters.

In the typical years, amplitude of the intrasesaonal variation increased rapidly around December (although this timing is somewhat different among the typical winters) and the persistent extremely low temperature days for about a week appeared two or three cycles in a winter. According to the case study for a typical winter (e.g., 1984/85 winter), while the temperature around Germany was relatively higher at the eastward shift phase of the Icelandic low due to the strong warm air advection by the SW-ly wind, the temperature was extremely low there when the Icelandic low was weakened and retreated westward as pointed out for the case study for 2000/2001 by Hamaki et al. (2018). It is also noted that the low corresponding to a part of the equivalent barotropic wave train lined up zonally was located around Germany in the extremely low temperature phase in 1984/85 winter.