THE UTILIZATION OF SATELLITE PRODUCTS FOR MONITORING FOG OVER ROMANIA

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1 THE UTILIZATION OF SATELLITE PRODUCTS FOR MONITORING FOG OVER ROMANIA Gabriela Bancila, Viorica Dima, Florinela Georgescu National Meteorological Administration, Bucharest, ROMANIA Abstract The last cold season ( ) was, most of the time, particularly foggy for the Romanian territory and neighbourhoods also. The main cause for the persistence of this phenomenon was the long time presence of high pressure surface fields over our country, sustained by structures of ridge at the upper levels. This evolution of the atmospheric pressure generated: - persistent fog that results in problems concerning the flight and highway traffic - white frost affecting the trees branches, electric cables etc. The purposes of the present study was to analyse the synoptic configuration during foggy intervals, to establish the origin and stratification of involved air masses and to investigate the different fog types of interest in the analysed periods. To reach these goals were used the available soundings and the ECMWF numerical model analyses. A special accent concerned the importance of EUMETSAT (Meteosat 9) satellite products in issuing very short range forecasts and in identification and monitoring this phenomenon (fog - day and night) using particularly the RGB Composite NIR 1.6, VIS 0.8, VIS 0.6, RGB Composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8; RGB Composite VIS 0.8, NIR1.6, IR 3.9r and RGB Composite IR 12.0 IR10.8, IR 10.8 IR 8.7, IR INTRODUCTION Fog is defined as an obscurity in the surface layers of the atmosphere which is caused by a suspension of water droplets, with or without smoke particles, and which is defined by international agreement as being associated with visibility less than 1000 meters [Peterssen, 1940]. The most dominant type is radiation fog, which is produced over a land area when radiational cooling reduces the air temperature to its dewpoint. The development of radiation fog is primarily controlled by a balance between radiative cooling, which encourages fog, and turbulence, which inhibits it. Colder ground temperatures and moist soils may help aid in strengthening the inversion even more, so it is important to know the temperature and moisture conditions of the soil. Also, it is known that the highest occurrence of dense fog to be during the winter when there is a minimum of solar radiation. Dense fog typically formed during the early morning hours when temperatures are on average at their daily minimum [Peterssen, 1940, Kurz, 1998, Triplet and Roche, 1986]. Advection fog, the second most common, is caused by the flow of moist, warm air over a cold surface, and the consequent cooling of that air below its dewpoint. One of the conditions necessary to produce advection fog is a strong moisture and thermal gradient between the boundary layer and the surface. The dewpoints that advected in might be several degrees higher than the ambient air temperature, so the fog is denser over the areas that had good radiational cooling prior to the advection of higher dewpoints [Peterssen, 1940, Stefan, 2004, Malardel, 2005]. Precipitation fog is a type of evaporation fog that happens when relatively warm rain or snow falls through cool, almost saturated air, and evaporation from the precipitation saturates the cool air. It can turn dense, persist for a long time, and may extend over large areas. Although it is mostly associated with warm fronts, it can occur with slow cold fronts or stationary fronts as well, hence the name frontal fog is also used. Another evaporation fog, the most localized form, is created by cold air passing over much warmer and moist land and it often causes freezing fog. This paper will only limit to the above listed types of fog and to the derivatives they imply, considered as representative to the cold season over the Romanian territory.

2 The typical pressure structures that cause the fog to occur over the Romanian territory can be classified by five large categories, as follows: 1.The active Mediterranean cyclones, engaged on trans-balkan trajectories are, as a rule, responsible for the type of fog known as advection fog. This type of fog, either pre-frontal, or of warm sector, is generated by the prevailingly southern circulation over our country, made up of two components. On the one hand, there is the southern circulation, of a Mediterranean type, developed on the ascending slope of the trough corresponding to the cyclone slowly moving over the Pannonian Plain, a cyclone that does not affect the Baragan, but only the western areas, Banat and Crisana [Bordei-Ion et al, 1983]. On the other hand, the southern, south-eastern circulation component over eastern Romania, owed to the western sector of an East-European high, which imposes, after all, a trajectory noninvasive for Romania to the trans-pannonian Mediterranean cyclone. Because of the described synoptic circumstance, the warm, moist air advected by the cyclone above the pre-existing cold anticyclonic air generates fog, usually short lasting. However, if prior to this warm, southern air advection, a stable air mass had existed favouring severe radiative cooling of the ground, the socalled advective-radiative fog occurs, that may persist days on end. The start cause is, in that case, the radiative cooling with the stable air mass. Further, the fog persistency is caused by warm air advection that preserves the thermal inversion [Kurz, 1998, Malardel, 2005]. 2. Another category of pressure relief shapes determining fog to occur in Romania during the cold season is the ridge of the Azores High, expanded as far east as over our country. Under such circumstances, the oceanic air gradually turn continental and cools at its base, becoming stable, thus able to generate dense and persistent radiation fogs, less in the southern areas of the country, but frequently in the western and central ones [Danut, 2004]. 3. A third pressure pattern favouring fog occurrence consists in anticyclonic east and east-european structures, with ridge support in the altitude, penetrating above Romania from the Black Sea. The high relief of the Carpathians causes the anticyclone to stagnate (not necessarily an east-european one, but possibly originating above Turkey also, or of any other origin, on condition it had evolved over the Black Sea) on the outer side of the Carpathian chain, implicitly triggering sedimentation of the cold air, with thermal inversions occurrence. The natural result is the fog emergence, transformed intro stratiform clouds as is lifts up [Dima, 2002]. Function of the moment when this configuration becomes a fact, several different situations become possible: if the air advected by the anticyclone is very cold and dry, the fog occurrence probability is smaller, but if the advection occurs in the former part of the cold season, when the Black Sea is warmer, fog occurs there, and is advected according to the circulation, towards the south, and sometimes- the east of the country [Pinelis, 2004]. Thus, when the advected air mass is relatively warm and moist and the anticyclone is in no way connected to the polar ice cap, probability increases for the occurrence of fog, stratiform clouds and drizzle [Kurz, 1998, Triplet and Roche, 1986]. In that case, the extra-carpathian areas encounter higher frequency of fog and stratiform clouds, whereas the interior of the Carpathian arch remains sunny. 4. Another type of pressure centres distribution causing fog occurrence in Romania is characterized by the existence of a central-european high pressure nucleus, emerged either from an Azores ridge, or even from the withdrawing East-European High. Those stable anticyclones cause typical radiation fog to occur, because of the subsidence (sedimentation) inversions they generate. In such cases, the radiative cooling is very strong because of the mostly sunny sky, and the descending air along the whole tropospheric air column generates adiabatic compression of the air, causing it to warm and favouring temperature inversions, sometimes at ground surface, some other times in the altitude. These are the structures that generate radiation fog the most often, able to persist days or weeks on end in wintertime. 5. One last circumstance that favours fog occurrence on the Romanian territory resides in the existence above the country of a barometric swamp, i.e. a barometric saddle, characterized by virtually no pressure gradients, meaning weak circulation of the air. Such a situation is also encountered when in the high northern latitudes the air circulation is intense, whereas in the low

3 latitudes the pressure field is relatively uniform, without notable gradients, most often slightly below average. Under such circumstances, the fog that occurs is of a mixed type, and the residual moisture of a dying cyclonic activity is the starting factor, on the background of weak circulation favouring fog persistency. The described phenomenon is the object of this paper because, in the cold season , in Romania there were no less than 66 days with fog reported (43.42%). Of all those, the situations described in detail further were selected function of the aimed type of fog, so as to exemplify the way how the numerical models and the satellite imagery were used in the operational activity of the National Weather Forecasting Centre. THE FOG EVENT OF DECEMBER 2007 In December 2007, the mean monthly temperature in Romania was below the normal values, the precipitation regime displayed deficit in the western part of the country and excess in the east and south-east of the territory. As a result, the ground was covered by snow layer, thicker in the centre of the country and in the extra-carpathian areas and fog was reported, especially in these areas, were the snow cover was deeper. In the third decade, the expansion and persistency of the phenomenon were remarkable, and the fog was associated with rime accretion. In that interval, the pressure pattern in Romania was markedly anticyclonic. Synoptic, mesoscale and aerological analysis From 19 to 22 December, the Scandinavian High centered in mid-month over Jutland made a descent in the latitude towards the geographic area of our country, preserving the 1043 hpa central pressure. This was the pressure centre of interest for Romania until 24 December. Further, the expansion of the East European High towards central-southern and south-eastern Europe prevailed, along with the emergence of the high pressure belt, with the Azores ridge developing over the south-west of the continent. At the level of Romania, the surface pressure reached 1040 hpa on those days (28 and 29 December), favouring subsidence and radiation fog occurrence (figure1, left). Figure 1: Sea-level pressure and temperature at 850 hpa, ECMWF numerical model analysis, 28 (left) and (right), 00 UTC During the closing days of the month, the cyclonic activity enhanced over the central Mediterranean, the Icelandic Low also advanced over the continent, and the high pressure belt got weaker, however preserving rather high pressure values, of hpa over Romania (figure 1, right). As regards the upper-air situations, throughout the interval chosen for analysis ( ), isohypses took a ridge-type shape, with average values of 560 hpa over the area of Romania (figure 2). Figure 2: Geopotential height and temperature at 500 hpa, ECMWF numerical model analysis, 28 (left) and (right), 00 UTC

4 In the low troposphere, the air mass existing at the level of our country was characterized by temperatures of -1 2ºC, at the level of 850 hpa and of -5-2ºC at the level of 1000 hpa (figure 3), which pictured a marked temperature inversion, responsible for the persisting fog from the low areas. Only in the higher elevation areas, isotherms suggested positive temperatures at ground level (figure 3, right). Figure 3: Temperature at 850 hpa (left), 925 hpa (centre) and 1000 hpa (right), ALADIN numerical model analysis, , UTC The 10-m wind was nearly calm, blowing with only m/s throughout the considered interval, which ensured light propagation to the air cooled through radiation, from ground surface towards the slightly higher layers, which, however, did not induce turbulence enough to prevent fog occurrence (figure 4). Figure 4: 2-m wind, ALADIN numerical model, analysis, , UTC (left) and , UTC (right) Of the ALADIN limited area model products, the 2-m humidity field also supported high probability for the occurrence of fog - a phenomenon otherwise very difficult to forecast (figure 5). Figure 5: 2-m relative humidity, ALADIN numerical model, analysis, , UTC; , UTC; , UTC; , UTC Upper air soundings indicated inversion up towards m, ascertaining the fog and its persistency. Enhanced cooling induced by the radiation fog in the lower levels and the slight humidity increase, both the specific and the relative one, were also noticeable, along with the light wind speed in the layer near the ground (figure 6). Figure 6: Upper air soundings, Bucharest, , 00 UTC (left) and 12 UTC (right)

5 The satellite imagery discloses in turn the presence of the fog in the geographic area of Romania in the interval of interest, each combination of channels having the role to highlight specifics and peculiarities connected to fog monitoring during the day and night (figure 7, 8, 9 and 10). Figure 7: RGB Composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8; , UTC (left) and , UTC (right) Figure 8: Fog Depth Color Enhancement IR 3.9 IR 10.8; , UTC (left) and , UTC (right) Figure 9: RGB Composite NIR 1.6, VIS 0.8, VIS 0.6; , UTC (left) and , UTC (right) Figure 10: RGB Composite VIS 0.8, NIR1.6, IR 3.9r; , UTC (left) and , UTC (right) As a conclusion, the analysed phenomenon behaved in a classical way, fully justified, as every condition imposed by such evolution was fulfilled. THE FOG EVENT OF JANUARY 2008 The fog episode that comprised the days of 10, 11 and 12 January was actually part of a longer spell with fog present over the Romanian territory, i.e January.

6 January 2008 was characterized in our country by generally scanty precipitation amounts, except for the areas in the south and south-east, where they displayed excess. On the first days of the year (1-5 January) it snowed heavily in Romania and snow layer accumulated in most regions, however deeper in the south and south-east of Romania. In the second decade of the month, the weather was dull in the extra-carpathian areas and fog occurred widely, associated with rime, persisting locally. During the third decade the thermal regime was slightly above the multiannual means and the ensemble circulation initially became zonal, ceasing to favour fog occurrence in the area of Romania. Synoptic, mesoscale and aerological analysis In the 8-9 January 2008 interval, the ridge of the East European High advanced towards the Balkan Peninsula, causing a pressure rise of 7 8 hpa at the level of Romania, and the modification of the isobars shape: from a north-to-south trough to a east-to-west ridge. On 10 January, the Icelandic Low, active in the extreme north-west of the continent started descending in the latitude, inducing a slightly south-western circulation component at the level of Central Europe also. Accordingly, the east- European ridge settled slightly more to the south, Romanian now finding itself in the front, ascending part of the ridge. On 11 January the situation maintained (figure 11, left ), then, on 12 January the pressure gradient became more intense over our country, that was, at that time positioned in the contact area between the two pressure formations however, still in an anticyclonic regime and the air circulation from the south-south-west was ever more intense (figure 11, right). Figure 11: Sea-level pressure and temperature at 850 hpa, ECMWF numerical model analysis, 11 (left) and (right), 00 UTC During the interval of interest, the same south-western circulation was noticeable in the geopotential field, over central and central-southern Europe, whereas a south-west-to-north-east ridge captured interest over Romania (figure 12). Figure 12: Geopotential height and temperature at 500 hpa, ECMWF numerical model analysis, 11 (left) and (right), 00 UTC As regards the thermal stratification of the air mass over Romania from 10 to 12 January, temperature maps at the levels of 850, 925 and 1000 hpa (figure 13), disclose the emergence and persistency of a temperature inversion very much developed in the vertical. This was mainly owed to the advective factor of the circulation component, which transported warm and moist air from the Mediterranean towards the Romanian territory, but also to the radiative factor induced by the prevailing high pressure regime.

7 Figure 13: Temperature at 850 hpa (left), 925 hpa (centre) and 1000 hpa (right, ALADIN numerical model analysis, , UTC It is therefore obvious that in the mentioned interval (10-12 January), fog was frequent and particularly persistent in the central regions of Romania, where the pre-existing snow layer was very deep, as revealed by the annexed satellite imagery (figure 14 and 15 for monitoring fog during the night time; figure 16 and 17 for monitoring fog during the day time). Figure 14: RGB Composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8; , UTC (left) and , UTC (right) Figure 15: Fog Depth Color Enhancement IR 3.9 IR 10.8; , UTC (left) and , UTC (right) Figure 16: RGB Composite NIR 1.6, VIS 0.8, VIS 0.6; , UTC (left) and , UTC (right)

8 Figure 17: RGB Composite VIS 0.8, NIR1.6, IR 3.9r ; , UTC (left); , UTC (center) and , UTC (right) The upper air sounding performed for the area of Bucharest, where the snow layer still was 20 cm deep, pointed at the penetration of the warm air mass and its gradual generalization along the tropospheric column, determining inversions at more levels, which was specific to the advective processes that also induced fog in the low layers (figure 18). Figure 18: Upper air soundings, Bucharest, , 00 UTC; , 12 UTC; , 00 UTC; , 12 UTC Conclusions The highest frequency of foggy days during the last cold season, justifies this study which was oriented mostly to emphasize the importance of fog forecasting and monitoring with the products available in operational activity. The newest satellite products (Meteosat 9 RGB Composite) used in the operational activity of the National Weather Forecasting Centre, make now possible a good fog monitoring during the day and night also, like in the other Eumetsat member states. The satellite imagery allows, as the paper emphasizes, a very good separation between fog, clouds, snow and vegetation areas. REFERENCES Bordei-Ion E., Cordoneanu E., Banciu D. (1983): Anticiclonul eurasiatic de iarnă şi evoluţia vremii în R.S.România, Indrumar pentru centrele de prognoză meteorologică, editat I.N.M.H., Bucuresti. Danut, P. (2004): Tipuri de ceata Impact asupra zonei Banat-Crisana, Lucrare atestare meteorolog I Dima, V. (2002): Impactul Carpaţilor şi al Mării Negre asupra diferitelor tipuri de circulaţie a aerului în zona României. Fenomene mezometeorologice induse, editat A.N.M.-C.N.P.M., Bucureşti. Kurz M. (1998): Synoptic Meteorology, Deutscher Wetterdienst, Offenbach am Main: 44-49, , Malardel, S., (2005): Fondamentaux de Meteorologie, A l ecole du temps, Editions Cepadues: 28-39, , , 535 Petterssen S, (1940): Weather analysis and forecasting, McGraw-Hill Book Company, First Editon: Pinelis S., (2004): Procesele de condensare producerea ceturilor si precipitatiilor, Lucrare atestare meteorologi Stefan S., (2004): Fizica atmosferei, vremea si clima, Editura Universitatii din Bucuresti, Triplet, J.P. and Roche, G.,(1986): Meteorologie generale, Meteo-France, 3-e Edition, *** ***