the minimum temperatures in the winter 2006/07 in slovenian frost hollows and cold basins Matej Ogrin Department of Geography, University of Ljubljana, Aškerčeva 2, SI-1000 Ljubljana, Slovenia e-mail: matej.ogrin@siol.net Izvirni znanstveni članek COBISS 1.01 Abstract The members of Slovenian Meteorological Forum, Department of Geography at University of Ljubljana and Slovenian Forestry Institute started to measure temperatures in Slovenian frost hollows and cold basins in 2004. The measurements, which improved during the period 2004-2006, continued also in the winter 2006-2007, all together, in more than 30 frost hollows and cold basins Alpine, Dinaridic and even Submediterranean areas. Although the winter 2006/2007 was very mild, minimum temperatures in frost hollow Hribarice fell below - 35 °C. Key words: minimum temperatures, frost hollows, cold basins, winter2006/2007, nocturnal cooling, Slovenia. MINIMALNE TEMPERATURE V SLOVENSKIH MRAZIŠČIH V ZIMI 2006/07 Izvleček Člani Slovenskega meteorološkega foruma, Oddelka za geografijo Univerze v Ljubljani in Gozdarskega inštituta Slovenije smo v letu 2004 pričeli z meritvami v slovenskih mraziščih. Mreža meritev se je v obdobju 2004-2006 razširila, z meritvami pa smo nadaljevali tudi v zimi 2006/2007. Meritve so potekale v več kot 30 mraziščih dinarskega, alpskega in tudi submediteranskega sveta. Kljub mili zimi 2006/2007, so temperature v mrazišču Hribarice padle pod - 35 °C. Ključne besede: minimalne temperature, mrazišča, zima 2006/2007, nočno ohlajanje, Slovenija. 1. INTRODUCTION In the last years, researches of temperature conditions in Slovenian frost hollows and cold basins have brought new knowledge on this field. Previously, most of the researches in Slovenia focused on the researching of ecological conditions in frost hollows: Gams (1972, 1974), Zavadlav (1974), Majdič (1973), Ogrin, Krevs (1995), or they focused on air pollution-related problems in cold air pools in valleys and basins: Petkovšek, Rakovec (1975), Petkovšek (1971). The last researches focused more on topographical climatological conditions (Ogrin 2000). As a further step, researches continued in successful collaboration between Department of Geography (Ogrin D., Ogrin M.) at University of Ljubljana, Slovenian Forestry Institute (Sinjur I.) and Slovenian Meteorological Forum (Vertačnik G., Ortar J., Gustinčič M., Debevc K., Trošt A., Logar T.) and they brought us to conclusions that temperature in some Alpine frost hollows during winter time quite often fall below - 30 °C, in some extreme cases even below - 40 °C (Ogrin, Ogrin 2005; Sinjur, Ogrin 2006; Sinjur et al. 2007). In the winters of 2004/2005 and 2005/2006, temperatures below - 40 °C were measured twice. On January 25, 2006, the lowest temperature ever recorded in Slovenia was measured. On the Lepa Komna Alpine karst plateau, in the frost hollow Mrzla Komna the temperature fell to - 41,7 °C. In other Alpine countries, research of temperature conditions in some Alpine frost hollows such as Grunloch /Austria/ also went on in last years (Whiteman et al. 2004). Very known Alpine frost hollows are also Funtensee /Germany/, Glattalp /Switzerland/, Lago di Fosses and Campo Lussi /both Italy/. 2. METHODOLOGY The measurements from the period 2004-2006 continued also in the winter 2006-2007 in more than 30 frost hollows and cold basins in the Julian Alps, the Karavanke mountains, the Pohorje, Dinaric karst plateaus Snežnik and Trnovski gozd, the Dinaric region of Notranjska and, also, in the Slovenian Istria. We used analogous Thermo Schneider alcohol minimum thermometers and Dallas semiconductor digital thermometers which are also data loggers. They are named also i-buttons. Analogous minimum thermometers were used in the cold basins of the Notranjska region. According to the provider, the instrument error is ± 0.3 °C and the resolution 0.5 °C. The range of measurement is - 40 °C to + 40 °C. In Alpine frost hollows, where temperatures can fall much lower, we used similar thermometers with a measurement range down to -50 °C or even - 60 °C. The advantage of alcohol minimum thermometers is a small error in comparison to digital thermometers, especially at temperatures below - 30 °C. The disadvantage is that we can only measure current temperature and minimum temperature in one period, so the data cannot be stored and we cannot see the dynamics of temperature changes in time. Digital thermometers we used are also data loggers, so they could give us the insight into the processes of nocturnal cooling and the formation or degradation of cold air pool. Their disadvantage is a relatively big error of measurements. At - 30 °C, the error is from - 0.7 to + 1.2 °C and at - 40 °C, the error rises to the interval from - 0.9 to + 1.6 °C. The minimum limit value for the measurement range is - 40 °C, which is the biggest disadvantage of digital thermometers. In some higher frost hollows, the measurements were carried out with both kinds of thermometers. The resolution of digital thermometers can be set to 0.1 or 0.5 °C. In most cases, we chose the resolution of 0.5 °C, because lower resolution enables more memory for the data logger and a longer period of measurements, which was set on 85 days. All the measurements were taken from 1.5 to about 3 m above the surface. In the lowlands, we had no problems with the setting of the height of measurements because snow cover during the winter 2006/2007 was very poor and, except for a day or two in the last decade of March 2007, it did not exceed 50 cm. The only exception was in the frost hollow Najen in the Planica valley where the snow cover was deeper (about 1 m) during the winter. There we put the thermometer about 2.5 m above the ground. In the mountains, problems with snow cover were bigger. In some parts, snow cover was deeper than 2 m, so from the experience of previous winters, we had to prepare a special equipment to keep the thermometer and the shield high enough. We prepared special wooden or metal poles, which were set on the basic carriers. The height of the measurements changed daily due to the changing of the snow cover height. In some cases, wind accumulation brought also additional snow which caused even more problems. Nevertheless, thermometers were almost all the time at least 1,5 m above the surface. Figure 1: A radiation shield for a digital thermometer (Photo: I. Sinjur). Figure 2: A radiation shieldfor an alcohol thermometer (Photo: M. Ogrin). The research focused on temperature minima, which usually appear during morning hours. If conditions are not ideal, they can appear also earlier in the night. When measuring temperature, it is also very important what kind of radiation shield we use. The World Meteorological Organization declared a Stevenson screen as a reference standard for mea- suring temperature. For temperature measurements in the mountains and at many different locations we did not use Stevenson screens, the reason for this being financial and logistical problems, but we made adequate low cost radiation shields. As for nocturnal temperatures, minimum temperatures in the Stevenson screen were 0.3 °C higher than in our shields for digital thermometers (Sinjur, Vertačnik 2007), and the temperatures measured with our alcohol thermometer in a special radiation shield were 0.8 °C lower than in the reference Stevenson screen. 3. RESULTS 3.1 Weather in the winter 2006/2007 The article describes the period from December 1, 2006, to March 31, 2007. We took a period of meteorological winter and also March, which is also considered as a winter month due to low average temperatures. Table 1, showing comparison of the average temperature between December to March on Kredarica (2515 m) and Krvavec (1740 m), confirms this statement. These are the highest meteorological stations in the Slovenian mountains. Kredarica lies in the central part of the Julian Alps in the northwestern part of Slovenia, while Krvavec is located in the Kamnik and Savinja Alps in the northern part of Slovenia. Table 1: Average monthly temperatures (°C) on Krvavec (1740 m) and on Kredarica (2515 m) during the reference period 1971-2000. T nvg. (1971-2000) Dec ( C) Tvg. (1971-2000) March (-C) Kredarica (2515 m) - 6.0 - 6.6 Krvavec (1740 m) - 2.6 - 2.1 Source: Oddelek za geografijo 2006. The winter 2006/2007 was extremely mild in the Alps and consequently it was also a 'green' winter, especially in lowlands. In Slovenia, December was very warm at first. Even on Kredarica, the compact snow cover disappeared and the ground was only partly covered with snow. This happened for the very first time since the beginning of measurements on Kredarica in 1955. On December 9, after a cold front had passed Slovenia, the snow line dropped below 1000 m. Afterwards a short colder period began and it lasted for five days. The first clear night after the cold front was the night on December 12. Then the conditions for intensive nocturnal cooling in December 2006 were the best in most of the frost hollows. On December 15, warm advection began and warm weather lasted until December 18. On December 18, the temperatures fell again and this was a start of a mainly dry type of weather in the mountains which lasted until December 30. Only on the first days, the temperatures were around the average, but soon warm advection began and above 1500 m the temperatures were around or above 0 °C. Since December 9, there was permanent snow cover in the Slovenian Alps until the late spring in May or June 2007. Snow cover is very important for temperature conditions in frost hollows and cold basins because of special physical characteristics. White surface of snow reflects most of incoming solar radiation and emits almost as blackbody in the long-wavelength infrared part of spectrum. Consequently, in clear and sunny weather the equilibrium temperature on snow surface is much lower than on other types of ground. This explains unusually low daily temperatures in clear and calm weather. Beside this, very low thermal conductivity minimizes heat flux from the ground to the atmosphere, which enables very quick cooling of the air during evening hours and extremely low nocturnal temperatures. Average December temperatures in the mountains were 2 to 3 °C above the average in the reference period (this means the period 1971-2000). In Rateče (865 m), which lies in the Upper Sava valley on the border between the Karavanke mountains and the Julian Alps in the very northwestern part of Slovenia, the average December temperature was - 0,7 °C, on Vogel (1515 m) in the southern part of the Julian Alps it was 1.2 °C, on Krvavec (1740 m) 0.6 °C and on Kredarica (2515 m) - 3.1 °C (ARSO 2007). January 2007 was, relatively, the warmest month of the winter, which means that the positive anomaly from the average temperature was the biggest. From January 1 to January 21, the weather was very warm. During this period southwestern winds prevailed. On January 22, very unsettled weather with frequent precipitation began and the temperatures went slowly lower and lower. On January 24, after a cold front had passed Slovenia, winter weather began and lasted until the last days of January. On January 26 and 27, good conditions for nocturnal cooling in the frost hollows and cold basins appeared again. Although the last week of January was much colder than the previous ones, the average January temperatures were very much above the average. On Kredarica, the average daily temperature was below the average for only six days and the average January temperature was - 3.6 °C, which is 3.6 °C warmer than the average of the reference period. In Rateče, the average January temperature was 0.1 °C, which is 4 °C warmer than the average of the reference period. On Krvavec, January was 2.9 °C warmer than the average (ARSO 2007). Table 1: The average monthly temperatures (°C) for the period December 2006 - March 2007 at some Slovenian mountain stations and means of the reference period 1971-2000 (in the brackets). T Dec 2006 avg. T Jan 2007 avg. T Feb 2007 avg. T Mar 2007 avg. T Dec 2006 - Mar 2007 avg. Rateče (864 m) - 0.7 (- 3.1) 0.1 (- 3.9) 0.7 (- 2.1) 3.1 (1.4) 0.8 (- 1.9) Kredarica (2515 m) - 3.1 (- 6.0) - 3.6 (- 7.2) - 5.5 (- 8) - 5.7 (- 6.6) - 4.5 (- 7.0) Krvavec (1740 m) 0.6 (- 2.6) - 0.9 (- 3.8) - 18 (- 4) - 0.9 (- 2.1) - 0.8 (- 3.1) Vogel (1510 m) 1.2 0.6 - 0.2 0.4 0.5 Source: ARSO 2006. In February 2007, mild weather continued. During the whole month not even a single strong cold front passed Slovenia. However, there was a lot of unsettled weather, which is also called 'Atlantic' type of weather. The positive anomaly of temperature was smaller than in January, but it was still high. In Rateče, the average February temperature was 0.7 °C, which is 2.8 °C warmer than the average. On Kredarica, the average temperature in February 2007 was - 5.5 °C, which is 2.5 °C above the average, and on Krvavec the February temperature was 2.2 °C higher than the average (ARSO 2007). Due to the unsettled weather with frequent precipitation, conditions for nocturnal cooling in the frost hollows and cold basins were unfavourable and the lowest temperatures did not fall as low as in January or December. Additionally, temperature extremes did not appear on the same date, which also proves the changeability of the weather. The lowest temperatures appeared on February 4, 5, 17, 18 and 20. Early spring weather started already in March and lasted until March 19. Until this date, the temperatures were similar to the average of April. On March 19, a strong cold front passed Slovenia and in most places in the lowlands, the highest snow cover of the winter 2006/2007 was measured. From 10 to 40 cm of snow fell in most of the Slovenian interior lowlands. In most of the Slovenian frost hollows and cold basins, the best conditions for nocturnal cooling in March appeared in the morning of March 22. In general, March 2007 was warmer than the average, although the positive anomaly was smaller than in the period December 2006 - February 2007. In Rateče, the positive anomaly of average March temperature was 1.7 °C, on Krvavec 1.2 °C and on Kredarica only 0.9 °C (ARSO 2007). Table 2: Monthly air temperature (°C) anomaly for the period December 2006 - March 2007 from corresponding means of the reference period 1971-2000. Anomaly (°C) Dec 2006 Anomaly (°C) Jan 2007 Anomaly (°C) Feb 2007 Anomaly (°C) Mar 2007 Average anomaly (°C) Rateče (864 m) 2.4 4 2.8 1.7 2.7 Kredarica (2515 m) 2.9 3.6 2.5 0.9 2.5 Krvavec (1740 m) 2 2.9 2.2 1.2 2.1 Average 2.4 3.5 2.5 1.3 2.4 Source: ARSO 2006. We see that during the period of research the average monthly temperatures were much higher than usually. On average, the temperatures in this period were 2.4 °C higher than in normal winters. Table 3: Monthly air temperature (°C) and its anomaly from the corresponding means of the reference period 1971-2000 in Rateče and on Kredarica. Ti O o