Meteorol. Appl. 6, (1999)

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1 Meteorol. Appl. 6, (1999) Meteorological situations associated with significant temperature falls in Buenos Aires: an application to the daily consumption of residential natural gas Gustavo Escobar and Susana Bischoff, Departamento Ciencias de la Atmósfera, Universidad de Buenos Aires, Ciudad Universitaria Pab. II, (1428) Buenos Aires, Argentina The synoptic situations associated with significant temperature decreases in Buenos Aires are analysed. Knowledge of these situations for the winter season is very useful for the community, especially when they are related to the consumption of natural gas, which is one of the most important sources of energy in Argentina. The rotated principal component method is used to classify the synoptic situations. The significant temperature decreases in winter during the period occur in five modes at 1000 hpa and three modes at 500 hpa. The relationship between the two levels identifies two dominant patterns. The first has a strong high-pressure system with a northwest southeast axis and a maximum strength near 80 W, 40 S; this pattern is related to south-southeast winds over Buenos Aires. The second type shows a high-pressure system at 105 W, 48 S and a minimum near 110 W, S; this kind of pattern is associated with persistent blocking situations in the South Pacific which result in persistent winds from the south over Buenos Aires. In addition, a relationship between such situations and the resulting increases in the demand for natural gas in residential areas is shown. This will help in the forecasting of such events in the future. 1. Introduction Significant temperature decreases (STDs) are very important owing to their impact on society. When such an event occurs in Buenos Aires, the atmosphere has special characteristics, mostly linked to the passage of synoptic disturbances over this region. These perturbations could be cold fronts moving mainly from the south-west. Another possible cause could be cyclonic waves generated along the littoral fluvial region, which then progress towards the south-east. In the case of the cold fronts the temperature decrease tends to be more abrupt than during cyclogenesis events. The locations of Buenos Aires City and the littoral fluvial region are shown in Figure 1. In some cases Buenos Aires is under the influence of anomalous circulation patterns which last for periods longer than the average synoptic timescale. For example, blocking situations, depending on their geographic location, can be linked to weather characterised by a sequence of cold air systems moving over central Argentina (Berbery & Lozano, 1991). In particular, the city of Buenos Aires, from May to September, suffers from STDs which are followed by many days of persistently low temperatures. Under such circumstances, the surge in energy demand of various kinds adversely affects the energy supply. The basic sources of energy are electricity and natural gas, and the best way to ensure their adequate supply is to be able to predict such an onset with sufficient anticipation. A review of the bibliography on this local problem shows a number of papers on synoptic meteorology but very few on synoptic climatology. An example of the former is Scian (1970), in which the synoptic situations are analysed for the first fortnight of June 1967, when extreme minimum temperatures were observed throughout most of the country. With regard to the synoptic climatology studies, Hoffmann (1971) determined among other things the mean monthly frequency of the different frontal systems over northern Argentina, as well as the daily variations in surface temperature and vapour pressure resulting from air mass changes. Also Rusticucci & Vargas (1995) studied the cold and heat waves over Argentina by considering synoptic situations at the surface. Using Lund s objective method and data for the period they found three basic patterns linked to cold waves in winter. In the case of applied meteorology, van den Berg (1994) shows a strong link between the natural gas consumption in the Netherlands and various meteorological 253

2 G Escobar and S Bischoff data was provided by the Departamento de Ciencias de la Atmósfera, Universidad de Buenos Aires (Department of Atmospheric Sciences, University of Buenos Aires) and the Servicio Meteorológico Nacional, Argentino (Argentine National Weather Service). Information about daily residential use of natural gas in Buenos Aires was provided by Metrogas SA. The data corresponds to the months of June, July, August and September, for the years 1993, 1994 and 1995, this being the period when natural gas use is critically linked to meteorological conditions. 3. Methodology The magnitude of temperature decrease during the passage of a cold front over the city of Buenos Aires is mostly determined by the track and origin of the cold air masses, as well as the prevailing weather conditions on the day before the occurrence of an STD. The impact on the temperature clearly depends on whether the mean temperature for the previous day is close to, above or below the mean for the period. Figure 1. Location of Buenos Aires City (BA, indicated by a dot) and the littoral fluvial region (LFR, indicated by a dashed line). variables. Furthermore, in the natural gas demand forecast, he includes the influence of wind strength and cloud cover. Because different circulation patterns can lead to STDs, it is convenient to first characterise the behaviour of the atmosphere at the time of such extreme events. Thus the aim of this study is to determine which synoptic situations are linked to STDs in Buenos Aires. In consequence, an objective classification of 1000 and 500 hpa geopotential heights fields is carried out. Furthermore, examples are included to describe the relationship between such situations and the resulting increases in natural gas demand in residential areas of Buenos Aires. 2. Data Daily temperature maxima and minima, from the Observatory Central Buenos Aires station (OCBA 34 S, 58 W) are used. These correspond to the months of May to September, given that this is the period with the largest STD occurrence between 1961 and Furthermore, daily (12 UTC) geopotential height fields at 1000 and 500 hpa from the ECWMF medium range forecast analysis are used. The area under study is bounded between 20 and 55 S, and 40 and 120 W, using a grid, for the period The 254 If the air mass track is continental (i.e. on a south to north direction, over land) the characteristic Antarctic signature of the air mass will be maintained as it passes over the cold continental surface, particularly in midwinter. When it reaches the area under study, the temperature difference will be significant. If the prevailing temperatures before the arrival of the cold air are above the mean, the thermal contrast will be enhanced. Therefore to define the occurrence of an STD it is necessary to consider not only the daily temperature difference but also the mean daily temperatures for the previous and subsequent days. Given these considerations, a simple criterion was defined in order to identify the occurrence of such events. Thus a statistical analysis of daily mean temperature and daily temperature difference for the OCBA dataset was carried out; this leads to the definition of a threshold value for STDs. After the criterion for case selection was determined, an objective classification was carried out of the synoptic situations at 1000 and 500 hpa associated with STDs. There are a variety of statistical methods available to classify synoptic patterns (Barry & Perry, 1973). Such methods include the Lund and Blasing correlations, McQuitty s simple enchainment, and principal component analysis (PCA) (Green & Carol, 1978). The PCA method was applied in this study. One of the advantages of this method, in comparison to the others, is that the model patterns obtained are orthogonal, allowing the separation of different prominent pressure system modes.

3 4. Situation sampling Table 1 shows the average of the daily mean temperatures (T m ) and the standard deviation of the daily temperature changes (σ( T m )) for the OCBA station. The STD threshold was defined according to these results. The event selection was determined according to the following conditions: (a) Mean daily temperature decrease is greater than 4 C. As can be seen in Table 1 the standard deviation of the daily temperature change is approximately 2.7 C for the sampled months. With a temperature decrease of 4 C, an STD event can be safely identified. (b) Mean daily temperature on the day before the temperature decrease is above the monthly mean value. (c) Mean daily temperature on the day the temperature decrease is below the monthly mean value. Table 1 shows how the monthly mean temperature varies from one month to the next. The most important differences are observed during May and September. With the criteria defined in (a), (b) and (c) it is possible to determine in most cases an STD resulting from the passage of a synoptic perturbation. The random selection and study of synoptic situations, which fitted the imposed conditions, successfully tested their validity. Using the criteria described above, the synoptic situations at 1000 and 500 hpa were used to select 58 events for analysis. Significant temperature falls in Buenos Aires Analysis of the geopotential height at 1000 hpa yielded five basic patterns that describe more than 80% of the total variance. All of them show pressure patterns that result in southerly surface winds over Buenos Aires. To evaluate if the obtained patterns represent real situations, the factor loadings time series (Green & Carol, 1978) for each of the principle components were analysed. Basically, factor loadings show the relationship between the theoretical patterns (principle components) and real synoptic situations. This means that the closer the factor loading is to 1, the better the pattern represents the real situation. The following figures show the theoretical patterns and actual synoptic situations which have a factor loading greater than 0.8 with each of the patterns. (a) Pattern 1 (Figure 2) explains 29.9% of the total variance and represents the most frequent synoptic situation leading to STD occurrence in Buenos Aires. This pattern has an intense post-frontal anticyclone, which moves into the continent mainly from the south-west, leading to south/south-easterly winds over Buenos Aires. This high-pressure centre is linked to the Pacific anticyclone located at 40 S, 85 W. This feature can be very intense over a large part of the Southern Pacific region. (b) Pattern 2 (Figure 3) describes 15.3% of the total variance, and shows a high-pressure belt on a westsouthwest to east-northeast axis, with the highest value at 45 S, 105 W. This pattern leads to weak southerly winds. Such a pattern can be related to blocking situations such that subsequent migratory systems flow over the continent on a southwesterly track until the blocking weakens. (c) Pattern 3 (Figure 4) explains 10.3% of the total variance. It shows a strong trough with the axis located at 90 W, with an associated low-pressure system at approximately 50 S. Two high-pressure centres can also be observed: one north of Malvinas Islands, which produces southeasterly flow over 5. Classification 5.1. Geopotential height at 1000 hpa Table 1. The average daily temperature (T m ) and standard deviation of the daily temperature change (σ( T m ), T m = T m i T m i 1, i: day) and for the OCBA station. May September. Period: 1961/1990 Months T m σ( T m ) May June July August September Figure 2. First principal component at 1000 hpa (top) and actual map for 12 UTC on 28 September 1983 (bottom). 255

4 G Escobar and S Bischoff Figure 3. Second principal component at 1000 hpa (top) and actual map for 12 UTC on 11 August 1986 (bottom). Figure 5. Fourth principal component at 1000 hpa (top) and actual map for 12 UTC on 22 August 1985 (bottom). Buenos Aires, and the other further away at 35 S, 110 W. (d) Pattern 4 (Figure 5) explains 12.1% of the total variance, with a high-pressure ridge on a southwest northeast axis which covers most of the country. A post-frontal anticyclone is located over Buenos Aires. Such a pattern results in locally clear skies and little wind. Consequently, these clear skies lead to a radiative loss overnight, resulting in extreme low temperatures next day. This pattern also shows a low-pressure system at 40 S, 100 W and a high-pressure one at 65 S, 100 W. (e) Pattern 5 (Figure 6) explains 12.5% of the total variance. This pattern has an intense anticyclone located at 30 S, 90 W, with a trough along the 70 W meridian. Such a surface pressure distribution represents the passage of a cold front over Buenos Aires, and an STD occurs owing to cold advection from the south Geopotential height at 500 hpa 90% of the total variance can be explained at 500 hpa with only three principal components. Figure 4. Third principal component at 1000 hpa (top) and actual map for 12 UTC on 12 September 1984 (bottom). 256 (a) Pattern 1 (Figure 7) with 36.8% of the total variance yields a deep trough on the northwest southeast axis with a closed system at about 40 S, 80 W. Such a pattern results in a south-westerly flow over the Buenos Aires region. (b) Pattern 2 (Figure 8) describes 23.7% of the total variance. There is a high-pressure system at 50 S, 100 W and a low-pressure one at 35 S, 110 W. The effect of these two systems is a westerly flow over central and northern Argentina, while in the south the winds are southerly. (c) Pattern 3 (Figure 9) explaining 23.1% of total variance has a major long-wave trough along a

5 Significant temperature falls in Buenos Aires Figure 6. Fifth principal component at 1000 hpa (top) and actual map for 12 UTC on 10 September 1985 (bottom). Figure 7. First principal component at 500 hpa (top) and actual map for 12 UTC on 30 August 1981 (bottom). northwest southeast axis. There also is a shortwave trough at 60 W with maximum cyclonic vorticity towards north-eastern Uruguay Relationships between surface and upper-level patterns The method used in section 5 was adapted and applied to the study of these events. Figure 12 shows the changes in the daily use of natural gas (cubic metres) in family homes as a function of daily mean temperature on the previous day and the In order to determine possible relationships between the surface and upper-level patterns, factor loadings time series for each of the principal components were analysed at each level. It was thus possible to identify a very good correlation between the factor loadings time series of pattern 1 at 1000 hpa and the factor loadings time series of pattern 1 at 500 hpa; the same applies to pattern 2 at the two levels. This means that these pairs of patterns (from 1000 and 500 hpa) represent the most frequent situations in the whole middle troposphere and support significant temperature falls in Buenos Aires. Figures 10 and 11 show the real situations related to both patterns. 6. Application to the daily natural gas consumption The use of natural gas in family homes shows an important dependence on the local weather, particularly air temperature and wind strength. As expected there is surge in demand during the winter months and cold spells. To help improve the forecast of natural gas demand, an objective classification was made of synoptic situations the day before large surges in demand. Figure 8. Second principal component at 500 hpa (top) and actual map for 12 UTC on 11 August 1986 (bottom). 257

6 G Escobar and S Bischoff Figure 9. Third principal component at 500 hpa (top) and actual map for 12 UTC on 20 June 1983 (bottom). Figure 11. As Figure 10 for 12 UTC on 12 June daily temperature change. The period under study includes the months of June to September for 1993, 1994 and It can be seen that the largest changes (greater than two million cubic metres) in the daily demand of natural gas occur when the fall in temperature is greater than 4 C and the mean temperature on the previous day is between about 14 C and 18 C. A secondary maximum can also be observed when the mean temperature on the previous day is in the range C and the temperature drops by 2 C. Thus the synoptic situations used for the study were chosen according to the above data. The synoptic situations were obtained using the same datasets as in section 5. Figure 10. Actual maps for 30 August 1981 (12 UTC) at 1000 hpa (top) and 500 hpa (bottom). 258 Figure 12. Daily change of gas consumption (cubic metres) in family homes as a function of daily mean temperature on the previous day and the daily temperature change. Period: June September for 1993, 1994 and Shaded areas indicate positive values and dashed lines negative values.

7 Significant temperature falls in Buenos Aires Figure 13. First principal components (top) and actual weather map for 12 UTC on 29 August 1981 (bottom). Figure 14. Third principal components (top) and actual weather map for12 UTC on 19 June 1983 (bottom). Five basic model patterns were obtained which could explain more than 90% of the total variance. Three of them are the most similar to those presented in section 5, considering the whole meteorological dataset. Figures 13, 14 and 15 show the three principal components with the corresponding real situations that yield the best correlation. The main difference is due to a lag in the systems, given that the analysed situations correspond to the pre-std day. (a) Pattern 1 (Figure 13), representing 30.3% of the total variance, when there is the largest surge in gas demand (i.e. temperature of C followed by a 4 C or larger decrease in temperature). The day before the STD event, the cold front moves over the region of interest, ahead of the post-frontal anticyclone. (b) Pattern 3 (Figure 14), representing 26.6% of total variance is similar to pattern 5, shown in section 4.2. It shows a post-frontal anticyclone moving over the continent with a more prominent zonal component, albeit at a more northern position than pattern 1. (c) Pattern 4 (Figure 15), representing 7% of total variance, shows migratory systems moving with a more significant meridional component. This behaviour could be due to the deep pressure minimum located at 35 S, 110 W, which hinders the perturbations normal zonal flow. In summary, the fact that these model patterns are coincident with those determined in the previous sections points to the great dependence between weather changes and the variability in the natural gas demand in residential areas. Furthermore, this coincidence remains after the STD definition is modified. Figure 15. Fourth principal component (top) and actual weather map for 12 UTC on 10 June 1986 (bottom). 259

8 G Escobar and S Bischoff 7. Conclusions Given its geographic location, Buenos Aires is subject to weather phenomena that have a strong impact on surface temperature variability. Within this variability, and in particular for the period May September, significant temperature decreases are an important feature, which may result in low temperatures for a number of subsequent days. These events are linked to the passage of cold fronts from the south-west and the occurrence of cyclonic waves in the Littoral region. Significant temperature decreases (STDs) were defined as those leading to a decrease in the mean daily temperature of at least 4 C as well as fulfilling other conditions described in section 4. The method of objective classification using principal component analysis was used to identify synoptic patterns linked to the STD events. Five model patterns were identified at the 1000 hpa level and three at 500 hpa. The relationship between the two levels helped identify two dominant situations: (a) A strong post-frontal anticyclone with a northwest southeast axis, with maximum strength near 80 W, and 40 S. At 500 hpa this anticyclone shows a significant wedge over the Pacific, on the same axis as the surface anticyclone. (b) An ample high-pressure zone extending from the South Pacific to Central Argentina. Two pressure maxima are observed, a particularly strong one located at 105 W, 45 S, and a somewhat weaker one located over the province of Buenos Aires. At the upper level a pressure maximum is located at 105 W, 48 S and a minimum near 110 W and S. Such a pattern tends to be linked to persistent blocking situations in the South Pacific, leading to a sequence of days of low temperatures in Buenos Aires. The methodology was applied to forecast major surges in natural gas demand for households in Buenos Aires. The results obtained show a strong link between these surges and the occurrence of STDs. These results will allow both the forecasting of such demand surges as well as the improvement of the forecasts of these very same events. Acknowledgement The authors wish to thank the University of Buenos Aires for the financial support given through the research fellowships and subsidy UBA Res(Cs) 1411/94 EX035. This research was also supported by project code 862/OC-AR PMT-PICT0519(BID). References Barry, R. G. & Perry, A. H. (1973). Synoptic Climatology. Methods and Applications. Methuen & Co. Ltd, 555 pp. Berbery, E. H. & Alfaro Lozano, L. (1991). Características regionales de alturas persistentes en los océanos Atlántico y Pacífico sur. Anales del Sexto Congreso Argentino de Meteorología. Septiembre de 1991, Green, P. E. & Carol (1978). Analysing Multivariate Data. The Dryden Press. Illinois, USA, 519 pp. Hoffmann, J. A. J. (1971). Frentes, masas de aire y precipitaciones en el norte Argentino. Meteorologica, 2: Rusticucci, M. & Vargas, W. (1995). Synoptic situations related to spells of extreme temperatures over Argentina. Meteorol. Appl., 2: Scian, B. (1970). Situación sinóptica asociada a las temperaturas extremas observadas en junio de Meteorológica, 1: van den Berg, W. D. (1994). The role of various weather parameters and the worst-case forecasts in prediction of gas sales. Meteorol. Appl., 1: