COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN

Size: px

Start display at page:

Download "COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN"

Coleen Day
5 years ago
Views:

1 RISCURI ŞI CATASTROFE, NR. XV, VOL. 18, NR. 1/2016 COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN M.C. HUSTIU 1 ABSTRACT. Cold and heat waves in the Barlad Plateau. Among the meteorological phenomena with severe impact upon individual humans, upon society and the environment one distinguishes cold and heat waves. The geographic location of the Bârlad Plateau east of the mountainous barrier of the Eastern Carpathians and in a region where main pressure centers travel along the year facilitates the occurrence of cold and heat waves. Such events were analysed, for the interval , both as the frequency of massive warming and cooling situations, and their duration taking as a reference intervals of at least 5 consecutive days when in any month thermal anomalies are recorded, either positive or negative. Key words: cold, heat wave, freezing nights 1. INTRODUCTION As with canicular days, there are no universally-accepted definitions neither for a cold wave, nor for a heat wave. As such, the temperature values that the inhabitants of a region consider to be normal, may be extreme in another region if in the latter those values deviate largely from the multiannual daily means. Recently, with relation to a WHO methodology, the WMO makes a recommendation to which we subscribe: Heat or hot weather that lasts for several days, often referred to as a heatwave, is a pervasive natural hazard that can exact a heavy toll on human systems, affecting health, livelihoods and infrastructure. Natural systems can also be severely affected by the impacts sustained beyond the duration of a heatwave. Although there is no universally acceptable definition of heatwaves (Perkins and Alexander, 2013;Robinson, 2001), they are understood to be periods of unusually hot and dry or hot and humid weather that have a subtle onset and cessation, a duration of at least two to three days and a discernible impact on human activities (WMO-WHO, Heatwaves and Health, 2015). As a result, this new take on heat waves is fully adequate for thermal hazards impacting upon 1 National Meteorological Admistration, Bucharest, Romania, hustiumihaita@yahoo.com, mihaita.hustiu@meteoromania.ro 31

2 M. C. HUSTIU people and at the same time it does not require that strict criteria of timespan and threshold have to be met. The warming trend of climate is obvious in the region we analysed herein and in the last years it is manifesting itself by frequent weather extremes mostly of thermal nature. The last winters were generally warm, with short intervals of excessive drops in temperature and heavy snowfall. The geographic location of the Bârlad Plateau the south-east of Europe (fig.1), in a region where main pressure centers travel along the year and whose activity drives the entire mechanism of atmospheric circulation is a cause for strong deviations in the temporal and spatial evolutions of climatic factors, from the multiannual means which are being taken as the norm. They constitute aperiodic variations of the climate, whose intensity amplifies or diminishes in direct relation to the conditions of the underlying surface, and those of the low landscape especially. Fig. 1. Hypsometric map and relief units of Plateau of Barlad 32

COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013 2. DATA AND METHODS.

To highlight the cold/heat waves having a climatc hazard character, (Bălteanu, Şerban, 2005), we have considered the local climate to wit, the mean temperature derived from the

3 COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN DATA AND METHODS. HWDI/CWDI (Heat/Cold Wave Duration Index) deviations of maximum/minimum temperature of at least 5 degrees from the multiannual means, lasting at least 5 days. To highlight the cold/heat waves having a climatc hazard character, (Bălteanu, Şerban, 2005), we have considered the local climate to wit, the mean temperature derived from the minimum values in January (fig. 2 - a) (minimum values of ºC) and the mean temperature derived from the maximum values in July (maximum values of ºC) (fig. 2 - b) a) b) Fig. 2. Mean temperature derived from the minimum value (a in January) and the mean temperature (b in July) 33

4 M. C. HUSTIU 3. RESULTS AND DISCUSSION The absolute, extreme temperatures To get the limits of the temperature variations, data has been extracted from the database of the National Meteorological Administration in Romania (NMA): the absolute maximum temperatures in the cold season (October to March) and the absolute minimum temperatures in the warm season (April to September). The lowest absolute minimum temperatures occur when arctic air masses pass over Romania, and during radiative cooling in an anticyclonic context. During the analysed interval ( ), the absolute minimum temperatures occurred in months with the largest monthly temperature deviations specifically, in January 1963 and January 1985, when the minimum values of air temperature got as low as ºC at Bârlad (on January 18, 1963); ºC at Tecuci (on January 13, 1985) and ºC at Bacău (on January 18, 1963) (table 1). Table 1. Minimum monthly temperatures (extremes) at stations within Bârlad Plateau, between 1961 and Staţia X XI XII I II III 27/79 28/93 26/02 13/85 13/69 01/85 Tecuci -9, ,9-27, ,2 27/88 26/93 28/96 18/63 09/76 01/86 Bârlad -7,4-18,4-21,2-25, ,4 27/79 26/93 26/02 18/63 09/76 01/86 Bacău -7,8-21,4-23,4-30, ,5 Source: The archive database of NMA, in Bucharest Since the beginning of measurements, the absolute minimum temperatures were: ºC at Vaslui (on January 15, 1911) and -30.5ºC at Bârlad (on January 25, 1942). In the warmest years of the 21 st Century, the absolute maximum temperatures measured in the same period have surpassed the previous top values of 39.7 ºC at Bârlad (on July 30, 1936) and 39.1 ºC at Vaslui (on July 5, 2000). In 2007, these maximum temperatures were recorded: 39.9 ºC at Bârlad on July 23; 39.1 ºC at Tecuci on July, 20 (tabel 2); 39.7 ºC at Negreşti on July 24; 40.3 ºC at Bacău on July 24, and the absolute top value for Bârlad Plateau was 40.8 ºC, recorded at Vaslui on July 19, In August of 2012 these outstanding values of absolute temperature were surpasses anew, so that the new ones were: 40.4 ºC at Tecuci; 39.8ºC at Bârlad (a tenth of degree lower than the previous top value in 2007), on August 8, and 42.5ºC were recorded at Bacău the same day. 34

5 COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN Table 2. Maximum monthly temperatures (extremes) at stations within the Bârlad Plateau, between 1961 and 2010 Staţia IV V VI VII VIII IX 06/98 17/69 26/07 20/07 25/07 27/87 Tecuci /98 17/69 26/07 23/07 25/07 27/87 Bârlad /98 16/69 26/07 24/07 24/07 18/86 Bacău Source: The archive database of NMA, in Bucharest Aperiodic variations of temperature in January and July, and the trend in its evolution. From the graphs representing the variations - and the trend of evolution of the mean temperature in January, one can see that the coldest month in the analysed interval was January, 1963 (fig.3) when the negative deviations in Bârlad Plateau varied between 8 and 10 degrees. During that month, the mean temperature values were C at Vaslui, C at Huşi, and C at Bârlad. The analysis of graphs depicting the variation and trend of the deviations from the July norm a month which is taken as representative for the warm season (in fact, the warmest month) shows a trend of acclerated increase in the last two decades. During those years the deviation was positive, with the notable maximums of July, 2007 when it was C (fig.3) and of July, 2012 when the mean deviation was C. 5,50 3,50 1,50-0,50 6,48 3,74-2,50-4,50-2,59-6,50-8,50-8,09 ianuarie iulie Liniară (ianuarie) Liniară (iulie) Fig. 3. Average monthly mean deviations and the linerar trend (January and July) in Barlad Plateau ( ) 35

6 M. C. HUSTIU Regarding the comparison between the evolution of mean deviations of temperature in January and July, one can see a larger amplitude of deviations in January. The linear trend of evolution in analyzed period ( ) of the mean temperature in July and January shows significant increase, of about C Similarly, from the analysis of the variation and trend of evolution of the mean temperatures during July and January representative months for the warm and cold season, respectively we can conclude that the warming trend spotted for the mean annual temperature is mostly caused in a higher degree by the stronger warming trend during winter and summer, than during transition seasons. Cold waves By its position in front of the mountainous obstacle of the Romanian Carpathians, the Bârlad Plateau is frequently affected by cold waves during the cold season of the year. Looking at the entire country, its eastern, south-eastern and southern regions have distinguished themselves in the 20 th Century by the highest frequency of massive temperature drops (minimum temperatures -30 ºC), that is 53 cases which amount to 47.3% of the total (Bogdan, 1998, Bogdan, Niculescu, 1999). Knowing the minimum temperatures during the analysed interval, it was possible to compute the frequency of cooling episodes/temperature drops -10 ºC in the Barlad Plateau, judging by value thresholds. As for the intensity of cooling episodes (deviation from normal), the leading months are January, 1963 and February, During the analysed period there were identified 41 intervals with remarkable low temperature(s). The most intense and frequent cold waves have occurred in the coldest months as well the winers of 1963, 1969, 1985, 1987 and The cold waves with the highest frequency were those ranging in 7 to 14 days (53.7% of total cases), followed by those ranging in 5 to 7 days, and lastly by the cold waves longer than 14 days (Table 3) Table 3. The frequency of cold waves within Bârlad Plateau, by their duration ( ) Duration < 5 days days days > 14 days Num. cases % Source: Processed data from the archive database of NMA, in Bucharest. The longest cold wave 33 days has occurred during the winter of 1987, when the nights were extremely cold ( -10 ºC) and the days were especially 36

7 COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN wintry ( 0 ºC) over the entire analysed geographic area. This cold wave was not very intense, since the minimum temperature has dropped below -25 ºC in a single instance, and below -15 ºC it has dropped 58 times. The next cold wave, by duration was that of January-February 2012, when for almost a month (27 days) the temperatures were below climatological means of that time of the year. The most intense cold waves have occurred during the winters of 1963 and In the winter of 1963 there were 3 episodes of excessive cooling, when in two instances the minimum temperatures have dropped below -30 ºC, in 11 instances they have dropped below -25 ºC, and below -15 ºC in 90 instances. During the winter of 1985, although there were two episodes only of excessive cooling, their duration was much longer (January 1 22, and February 8 March 1). That winter there were 5 instances only of temperature below -25 ºC, none with values below -30 ºC; however, in 118 instances (Fig.4) the minimum temperatures were between -15 ºC and -25 ºC Fig. 4. The evolution of minimum temperatures by value thresholds (total number of cases at 5 stations in and around the Bârlad Plateau) ( ) 0 As for the time of year that these cold waves have occurred, the highest frequency was in January with almost 50% of the total cases (Table 4). February comes in second, with 24,4% of the total, yet in the last two decades this frequency has increased during December >-25 too, amounting to 22% over the entire ,9 C interval. Table 4. The Linear frequency ( ,9 C) of cold waves in Bârlad Plateau, Linear (-10-14,9 C) by month of occurrence ( ) 37

8 M. C. HUSTIU Month November December January February March Num. cases % 2,4 22,0 48,8 24,4 2,4 Source: Processed data from the archive database of NMA, in Bucharest. During the analysed interval there were cold waves at the beginning and the end of the cold season as well. As such, the earliest cold wave has occurred in the last two thirds of November 1993, when the temperature dropped to -20 ºC at Tecuci, ºC grade at Bârlad, ºC at Bacău and ºC at Iaşi. The latest cold wave has occurred between March 3 and 16, 1987 when the lowest minimum temperatures were ºC at Tecuci, ºC at Bârlad, ºC grade at Bacău and ºC at Iaşi. Heat waves The Moldova Plateau is located in the temperate zone as well as in a continental area subject to multiple climatic influences, over which air masses travel that originate in the tropical zone, a situation that derives into positive deviations from the climatologic means which at times get an unique status or establish themselves as climatic records (Bogdan, 1992, 1994; Bogdan, Niculescu, 1999). Such deviations are the result of the large, aperiodic variations of the climate and as such they appear to be isolated or singular, therefore the name of positive thermal singularities. The positive thermal singularities are the manifestation of heat waves, namely of warm air of a tropical and continental nature (or of warm, maritime and tropical air which gets above Moldova Plateau already continentalised and devoid o precipitation/precipitable water), generated by continental highs that develop either in the south-east of Europe, or in the south-west of Asia, or in the Black Sea basin, or in the Balkan Peninsula, or iin the north-west of Africa etc. At times, such positive thermal singularities are caused by warm air masses of a tropical nature that reside at the margin of oceanic cyclones that extend over the central and eastern areas of Europe; or, southern air masses tag along retrograde cyclones that cover the southern parts of Moldova (Clima RPR, I, 1962). When the high pressure systems persist, the local insolation processes intensify that further contribute together with tropical air advections to the increase in warming and drying, thus increasing the values of subsequent positive thermal singularities. The literature on this subject contains frequent links to the strongest warming instances, that are often (yet, not exclusively) presented as absolute maximum temperatures, and also links to their consequences. Judging by the mean temperatures of the warmest months (July and August), the most intense warming instances are those 25 C; by the absolute maximum temperatures (be them monthly or annual), they are those surpassing 30 C (tropical 38

9 COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN days) and higher ( 35 C, canicular days), and judging by the nocturnal minima, they are those 20 C (tropical nights). In the last century, in the Barlad Plateau there were frequent situations when the absolute maximum temperatures surpassed 30 C in the warmest months and even from May to September. However, not every tropical day may be the manifestation of a strong warming. This depends on the persistence of anticyclones that drive the frequency of tropical days and that of warming processes. For the warm season, the most dangerous event that southern circulations cause quite often in the geographic area of Romania is represented by the heat waves, and canicular weather respectively which in Romania occur under the effect of North-African High's ridge, yet such bouts may be also governed by the hot pulses less frequent of the Arabic Low. As such, it appears of high concern to note the fact that, due to the climate specifics of each geographical region, one cannot formulate an universally-valid definition of heat wave. In Romania, the weather is said to be canicular when during the temperature reaches or rises above 35 o C by day and when it does not get below 20 o C at night. On the other hand, heat and heat waves are also frequently defined by parameters that relate to thermal stress or feels like temperature, whose formula include the value of relative humidity along with that of the measured temperature. As with massive cooling in meteorological terminology, a new term has been established heat wave or canicular heat wave. Since The World Meteorological Organization recommends a 5 degrees deviation, for at least 5 days, to label an interval as a heat wave, for the Bârlad Plateau maximum temperatures were considered those 33 C. As a result, for the analysed region canicular heat waves were considered those intervals when the maximum temperatures have surpassed 33 0 C for 5 days in a row. During the period of interest, 32 intervals with heat wave were identified (temperature 33 C more than 5 days) Until 1987, this meteo-climatic situation has had a insignificant frequency, and its extent was in the south of the region only. During the last two decades, the canicular heat waves have become more and more common (Fig. 5), noting that between 2000 and 2012 especially the warmest summers were recorded, since the beginning of meteorological observations. The warmest summers in Romania and Bârlad Plateau were those of 2007 and 2012, when new absolute temperature peaks were reached and surpassed, for this part of the country. During the summer of 2007 there were two canicular heat waves of medium duration (8 10 days) yet maximum intensity, when at the above mentioned stations 32 instances of maximum temperatures above 38 C were recorded, and 40 cases of values above 35 C. 39

10 M. C. HUSTIU The summer of 2012 remains the warmest summer in the history of meteorological observations, with 4 canicular heat waves; the one in July having a long duration (17 days) yet medium intensity, and those in August having a shorter duration (5 7 days) yet large intensity, when new absolute records were reached. During this summer, in the Bârlad Plateau the largest number of canicular days was recorded (228 cases in 2012, to 153 cases in 2007), which shows the persistence of canicular heat waves for a long time (Fig. 5), but the number of days with the maximum temperature over 38 C was a bit lower than that of 2007 (29 days in 2012, to 32 days in 2007) T max > 38 C Tmax 33 34,9 C T max ,9 C T max >33 (caniculă) Linear (T max >33 (caniculă)) Fig. 5. The evolution of maximum temperatures by value thresholds (total number of cases at 5 stations in and around the Bârlad Plateau) ( ) The aperiodic variability of freezing nights ( -10 ºC) The mean annual number of such nights, when the minimum air temperature gets as low as -10 ºC at most varies over the Bârlad Plateau between 15 and 20, while it can surpass 20 nights both on the upper and medium Bârlad River and in the valleys of the rivers Berheci and Tutova, where the landscape is markedly low. At Tecuci the mean of this number is 14.7, at Bârlad 15.2, at Vaslui it increases at 19.2, at Huşi it is 18.7, while at Negreşti 22. The aperiodic evolution of such nights shows a considerable decline over the analysed period, of approximately 23 % (Fig.6), a process caused by the warming of climate over the past two decades. 40

11 anul Nr. zile COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN anul Tecuci Bârlad Vaslui Negreşti huşi Linear (Bârlad) Fig. 6. The aperiodic variability of freezing nights ( -10 ºC) in January in the Barlad Plateau ( ) The aperiodic variability of the number of tropical days The mean annual number of tropical days over the analysed interval surpasses 25 days in the southern areas and is days only at altitudes over 300 m(etres) in the Central Moldavian Plateau and Tutova Hills. At Tecuci there were 28 days, at Bârlad 23, at Vaslui 25, at Negreşti 21, while at Huşi 26 days. The frequency of tropical days varies significantly from a year to the next. As such, over the analysed interval the maximum monthly number of tropical days was recorded in August, 1992 and July, 2012 months when at Tecuci there were 27 such days Tec uc i Bârlad Vaslui Negreşti Huşi Linear (Bârlad) Fig. 7. The aperiodic variability of the number of tropical days 3 0 C in July in the Barlad Plateau ( ) 41

12 M. C. HUSTIU By a multiannual analysis, the trend of increase in the number of tropical days is obvious in all summer months. Specifically, in July and August when the largest number of such days is recorded, the increase over the last 50 years s-a produs was the double of the initial number (an increase of almost 100%). At Bârlad, both in July (Fig. 7) and August the mean number of such days has increased from days in the 1960s, to days at the present CONCLUSIONS The trends that we have identified (as increase) in the annual frequency of the number of days with heat waves and especially of days with canicular heat waves (suggested in this study by the increase in the number of days with maximum temperature 33 ºC) reflects the climatic change noticed in the analysed region. The canicular heat waves have extended over the entire region and have increased exponentially after In these conditions, a doubling of the number of tropical days was noted; a special weight carries the fact that, while at the middle of the last century there were almost no tropical nights, at the present their number is both in July and August The rate of decrease in the annual mean number of days with cold waves and that of instances with extreme minimum temperatures ( -10 ºC) during the cold waves in the cold season are lower. However, an obvious decrease is noted in the number of total freeezing nights. REFERENCES 1. Bălteanu D. Mihaela, Şerban (2005), Modificările globale ale mediului. O evaluare interdisciplinară a incertitudinilor, Edit. CNI Coresi, Bucureşti. 2. Bogdan Octavia, Niculescu Elena, (1999), Riscurile climatice din România, Institutul de Geografie al Academiei Române, Bucureşti. 3. ***WMO-No.1142, World Meteorological Organization and World Health Organization, 2015, Heatwaves and Health:Guidance on Warning-System Development, 107 pp. 42

AN EXCESSIVELY FROSTY MONTH - JANUARY 1963

AN EXCESSIVELY FROSTY MONTH - JANUARY 1963 ELENA NICULESCU 1 Key-words: January 1963, negative thermal imblances, Romania Against the continental-temperate climatic background of Romania, the presence