SPECIFIC THERMO-PLUVIOMETRIC FEATURES ON THE WESTERN MOUNTAINS SIDE OF WESTERN CARPATHIANS

Transcription

1 SPECIFIC THERMO-PLUVIOMETRIC FEATURES ON THE WESTERN MOUNTAINS SIDE OF WESTERN CARPATHIANS GHEORGHE MĂHĂRA 1 Key words: thermic anomalies, convection, orographical rainfall The thermo-pluviometric specific features on the West side of the Western Carpathians are highlighted by the analysis of the data of temperature and rainfalls coming from the weather stations situated in the Gulf depressions of the Apuseni Mountains (Borod, Holod, Ineu), from those in the mountainous area (Dumbrăviţa de Codru, Huedin and Stâna de Vale), and for the plain area we considered the values of the weather station Oradea. The analysed period comprises the last 12 years ( ) for the thermic conditions and the last 30 years ( ) for the pluviometric conditions. Generalising, the thermopluviometric characteristics of the Apuseni Mountains can be extended over the entire mountain range of the Western Carpathians. Fig. 1.a - The orographical scheme of the Western side of Western Carpathians; b The mechanism of the appearance of thermic inversions in the gulf depression on the West of Western Carpathians 1 University of Oradea

2 Gheorghe Mahara 1. The multiannual monthly average temperature highlights a first thermic specific feature on the West side of the Western Carpathians, namely: the existence of some thermic inversions during the cold season in the gulf depressions with higher values in the mountainous area. This fact is illustrated in fig. 2 where, at Dumbrăviţa de Codru, situated on the Northern mountainside of the Codru-Moma Mountains at 580 m altitude, the thermic values are higher in January with 0,8 C, compared to Holod station (136 m) in the Beiuş depression and with 0,6 C in February compared to Ineu station (111 cm) in the Zarandului depression. Comparing these thermic values to the difference of altitude, there results a vertically positive thermic gradient of 0,12 0,17 C/100 m, between the gulf depressions and Codru- Moma mountains. The annual running of the monthly average temperature at Dumbrăviţa de Codru shows a higher value than the other two stations, from January to March when the situation is reversed, the depression areas having higher values, and in December the situation at the three stations is almost identical, from a thermic point of view. The positive thermic anomalies on the West side of the Apuseni Mountains are well highlighted almost every month of the year by the values of the average temperatures of the daily minimum and by the average of the absolute monthly minimum (fig. 2 b, c). At Dumbrăviţa de Codru, there were frequently recorded, in the cold season, the highest absolute minimum temperatures in the country, with a positive difference of more than 5-6 C as compared to the stations in the neighbouring depressions. Absolute Minimun Temperatures Table 1 Station Temperature C Date Dumbrăviţa de Codru - 19, Holod - 24, Ineu - 25, Another thermic feature of the area is given by the fact that, unlike classical situations when thermic inversions in the Carpathians are produced unde anti-cyclone conditions, of high atmospheric stability, on the west side of the Western Carpathians, these appear also unde cyclone conditions. Here, the positive anomalies are of a circulatory nature, associated with the topography of the land and affect all the middle altitude massifs on the West of Romania. The phenomenon is produced during the cold season when the region is affected by mobile Mediterranean cyclones coming from Southwest. These bring masses of hot subtropical air that meet, in the depression areas, a dense, cold air which they cannot replace and which remains at the basis of the Southern and South-western mountainsides. The hot air slides over the cold one at the basis and unde the cold one at the altitude, reaching the summits of the middle mountains (Pădurea Craiului, Codru-Moma, Zarand, Banatului Mountains) and generating higher temperatures than those in the depressions (fig. 1a). Should this hot air be engaged in a slight downward movement, it becomes foehn wind and raises its temperature, a fact that is highlighted by the thermic measurements at the Dumbrăviţa de Codru station, situated on the Northern mountainside of the Codru-Moma Mountains, at 590 m altitude, where, during winter, there frequently appear the absolute thermic maximums in our country (fig. 2c). 212

3 Specific thermo-pluviometric features on the Western Mountains Side of Western Carpathians Fig. 2 The thermic conditions at Dumbrăviţa de Codru, Holod and Ineu 2. Atmospheric rainfalls represent the weather parameter with the greatest variability in time and space. Following the monthly average values, we see that they are unevenly spread during the year, varying from one month to another. The variability is given by the character of the circulation of air masses, by the presence of the fronts and by the intensity of the convective processes. The least rainfalls appear between January and March because of the predominance of anticyclone barometric conditions and due to the lack of thermic convection. Starting in March, rainfalls increase progressively till June when there appears the main annual rainfall maximum with all stations. This month, the rainfalls are due to the activity of the oceanic cyclones that move at the Northern purlieus of the dorsal of the Azores anticyclone, bringing wet masses of air, fabvouring the formation of rainfalls and the intensification of convective processes. The values of rainfalls continue to decrease till September-October when the secondary minimum is recorded, and then they increase again in November-December, under the influence of the Mediterranean cyclones that cross the Pannonian Plain. From the seasonal distribution of rainfalls during the analysed period there comes another specific featurek, namely, that the pluviometric maximum does not appear in June in all cases, but it can also appear during every season, even during winter. Thus, during winter, the Mediterranean cyclones, reaching the Western sides of the mountains, produce rainfalls, determining the sudden melting of existing snow and creating huge floods. The was the case of the winter between when, between , there fell 338 mm in the Stâna de Vale region. 213

4 Gheorghe Mahara Fig. 3 Atmospheric rainfalls monthly average The annual distribution of the monthly average quantities of rainfalls is shown in fig. 3, out of which there results, generally, the increase of rainfall values together with altitude, according to the vertical pluviometric gradient whose value is 99,9 mm/100 us. There are also some exceptions regarding this increase of rainfalls together with altitude. Thus, a pluviometric specific feature is constituted by the regions barred at the West by high mountainous summits, regions situated in a so-called shadow of rainfalls where annual values are smaller than in other regions situated at the same altitude. Such a case is the Huedin depression that, although situated at 560 m altitude, has an annual average value of only 581,3 mm, being situated behind the Vlădeasa massif (1836 m), and the Borod depression, situated at 333 m, has an annual average value of 709,1 mm. Another pluviometric specific feature is given by the high frequency of torrential rainfalls during the hot season, due to the orographical convection which, together with the frontal convection, generate rainfalls of high intensity. Thus, at Stâna de Vale, on , within 24 hours, there fell 137,6 mm, and the highest intensity was at Borod where, on , within 11 minutes, there fell 15,5 mm, resulting 1,41 mm/minute. The average number of days of rainfalls increases too from the plain region, where it is of days, to days in gulf depressions and 189 days at altitudes above 1,000 m. 214

5 Specific thermo-pluviometric features on the Western Mountains Side of Western Carpathians It is to be noticed that the high frequency of the days of rainfalls (0,1 mm) appears in the months of the annual pluviometric maximum (May June) and also during winter when there persist for long the stratiform clouds that generate drizzle and little rainfalls. Rainfalls in the form of snow are analysed under four aspects: the frequency of the days of snowfalls and covered soil, the duration of the snow layer and its thickness. As for the frequency of the days of snowfalls, we notice their high number at altitudes above 1,000 m (74,2 days at Stâna de Vale), compared to that in the gulf depressions (25 35 days). Still, the number of these days oscillated between large limits from one year to another, namely 3 57 days in lower regions and days at Stâna de Vale. The frequency of days of covered soil exceeds by little the number of days of snowfalls in the lower plain regions, but, together with altitude, the difference increases, reaching a 2-1 report (fig. 4). Fig. 4 The frequeny of days of snowfalls and covered soil Distributed over months, in the period December February, the number of days of covered soil exceeds the number of days of snowfalls with every weather station. Following this phenomenon on altitude, we notice that, on the plains, the average number of days of snowfalls is higher than that of days of covered soil in March April and October November. In depressions, these months are April and October and, above 1,000 m altitude, it 215

6 Gheorghe Mahara is only in October that the frequency of the days of snowfalls is higher than that of days of covered soil. Altitude, together with the depression character of the relief with shadowy mountainsides, as well as the relatively high number of days of snowfalls at Stâna de Vale (74,2), determine a longer duration of the snow layer on the ground that, here, reaches, on an average, 148,8 days/year, the snow maintaining itself in an uniform layer starting at the end of October till the first ten days of May (fig. 4). The average thickness of the snow layer increase together with the altitude, having an oscillating character from one year to another. Fig. 5 The thickness of the snow layer over periods of ten days and the frequency of days covered soil Unlike the Banato-Crişană Plain where the snow layer can completely disappear in full winter, in gulf depressions it is maintained, with a thickness around 1 cm, starting in November, and then it increases to 6 cm/year in February, then it decrease again till the end of March. At altitudes above 1,000 m, the highest average thickness of the snow layer appears in January February and the first ten days of March, with values ranging from 82,2 82,8 cm 216

7 Specific thermo-pluviometric features on the Western Mountains Side of Western Carpathians at Stâna de Vale (fig. 5). I mention that, at this station, the maximum thickness of snow over ten days, in the analysed period, reached 194 cm in the first ten days of February The large thickness of the snow layer, as well as the high frequency of days of covered soil, make Stâna de Vale an important resort for practising winter sports. BIBLIOGRAPHY Cazacu G., Dincă Ileana, Tuinea P., Regimul precipitaţiilor atmosferice în câmpiile Mureşului şi Banatului, I.M.H. Studii şi cercetări, Meterorologie, 1976, Bucureşti. Măhăra Gh., Câmpia Crişurilor studiu fizico-geografic, în vol. Câmpia Crişurilor, Crişul Repede, Ţara Beiuşului, Edit. Ştiinţifică şi enciclopedică, 1977, Bucureşti. Măhăra Gh., Neamu Gh., Mihai Elena, Teodorescu Elena, (1970), Profil topoclimatic pe versantul vestic al Munţilor Apuseni, Lucr. St. ale Inst. Pedag. Oradea, vol. IV. Măhăra Gh., Măhăra Nadia, Regimul precipitaţiilor în zona staţiunii Stâna de Vale din Munţii Apuseni, Nymphaea-Folia Naturae Bihariae VIII-IX, Oradea, Moldovan Florin, Tipuri de vreme caracteristice pentru partea de nord-vest a României, I.N.M.H. Studii şi cercetări, 1982, Bucureşti. Neamu Gh., Mihai Elena, Măhăra Gh., Teodoreanu Elena, (1973), Profilî temperaturî rozduha zapadnom sklone zapadnîh gor, Dokl. VI Mejd Konf. Meteor. Karpat, Kiev, p , 1 fig., rez. Sorodoc C., Formarea şi evoluţia ciclonilor mediteranieni şi influenţa lor asupra timpului în R.S.R., Culegere de lucrări a I.M., 1960, Bucureşti. Stoenescu St., Unele particularităţi ale frecvenţei zilelor cu precipitaţii pe teritoriul R.S.R., Probleme de geografie, vol. VIII, 1961, Bucureşti. x x x, Geografia României, vol. I, Edit. Academiei, 1983, Bucureşti. 217