Mid-troposphere variables and their association with daily local precipitation

Transcription

1 Meteorol. Appl. 6, (1999) Mid-troposphere variables and their association with daily local precipitation N E Ruiz, W M Vargas, Departamento de Ciencias de la Atmósfera, FCEyN, Universidad de Buenos Aires, Ciudad Universitaria, Pab II, 1428 Buenos Aires, Argentina H H Ciappesoni, Servicio Meteorológico Nacional, Buenos Aires, Argentina This paper describes the meteorological variables directly affecting precipitation in the southern region of South America. Results show the relationships between daily precipitation at Ezeiza, Buenos Aires, and 500 mb dynamic variables (e. g. geopotential heights, relative geostrophic vorticity and its advection, vorticity tendency and zonal and meridional geostrophic wind components) using regional synoptic analyses from the Servicio Meteorológico Nacional of Argentina for The distribution of significant biserial correlation coefficients reveals the preferred positions for the anomalous wind components which favour the occurrence of rainfall at Ezeiza. In the same way, spatial distributions and temporal lag relations of 500 mb relative vorticity anomalies in relation to precipitation show the positions and relative movement of centres of anomalously cyclonic and anticyclonic vorticity which increase the probability of precipitation at Buenos Aires. Vorticity advection does not yield higher correlations, at least in a daily temporal scale. The geographical distribution of temporal means and standard deviations of geopotential heights and zonal and meridional wind components at the 500 mb level are also shown. 1. Introduction It is well known that mid-troposphere variables play an important role in determining surface meteorological phenomena such as precipitation. In the southern part of South America the Cordillera de los Andes forms a high meridional barrier which greatly modifies the migratory synoptic systems coming from the Pacific Ocean. Therefore, it is of interest to analyse the spatial patterns of various variables at 500 mb associated with precipitation at a given place using the regional objective analyses produced in Argentina. This may help us to understand the mechanisms involved in rainfall occurrence in this region. Some variables characterising the mid-tropospheric flow over southern South America are explored. 500 mb geopotential heights and zonal and meridional components of 500 mb geostrophic wind fields are investigated in order to relate these fields to daily local precipitation in Buenos Aires, Argentina. In the quasigeostrophic system, vorticity and its advection at upper levels are some of the elements used to diagnose (Holton, 1979). Hence, in this study the 500 mb relative and absolute vorticity fields and their advection as well as the vorticity tendency are especially examined in connection with rainfall in Buenos Aires. Buenos Aires is located in the humid region of Argentina on Río de la Plata (the River Plate), as displayed in Figure 1(a). It is the most highly populated region of the country as well as one of the more economically productive zones in agricultural terms, hence the importance of studying the synoptic climatology of precipitation in this region. 2. Data and methodology The data available for this study consist of 500 mb geopotential height fields at 12:00 GMT for the region covering Argentina (approximately S and W) for These objective analyses were obtained from the Servicio Meteorológico Nacional (National Meteorological Service) of Argentina. The system of numerical analysis was outlined by Hordij & Ciappesoni (1975) following the ideas of Bergthorsson & Döös (1955) and Cressman (1959). Part of the data set, domain, and geostrophic vorticity calculations are the same as used by Ruiz & Vargas (1998). For the period under consideration only 15 days are missing (4%). Precipitation data from three meteorological stations in Buenos Aires are used in the study: Ezeiza (34 50 S, W), Aeroparque (34 34 S, W) and Observatorio Central Buenos Aires (34 35 S, W). However, it is mainly data from Ezeiza that are presented here. The methodology is based on calculating biserial correlation fields. The biserial correlation coefficient 273

2 N E Ruiz, W M Vargas and H H Ciappesoni Figure 1. (a) Mean geopotential height field at 500 mb (gpm) for 1986 and ( b) the corresponding standard deviations (gpm). The Buenos Aires area is marked by a triangle in (a). Inset figure shows a close view of the Buenos Aires region with three meteorological stations in the area: Aeroparque, Observatorio Central Buenos Aires and Ezeiza. (Panofsky & Brier, 1965) allows precipitation to be considered as a dichotomous or binary variable (i.e. either there is event occurrence or non-occurrence). It is defined as follows: where x 1 is the mean of x when the binary variable is 1 (occurrence), x 0 is the mean of x when the binary variable is 0 (non-occurrence), S is the standard deviation of x for all the data, p and q are the empirical probabilities of precipitation and no precipitation, respectively, and z is the ordinate on the Gaussian curve at which the total area is divided into p and q. For the sample used, biserial correlation coefficients with an absolute value larger than 0.17 are statistically significant using a t-student test at the 95% level. The patterns obtained may be interpreted as anomalous configurations present when local precipitation takes place, or, conversely, when it does not. As 500 mb fields are considered to be the most reliable, 274 r bis x = x S 1 0 pq z the analysis of dynamic variables and circulation is confined to this level. 3. Geopotential heights There have been a number of studies dealing with meteorological and climatological aspects of various quantities in the Southern Hemisphere (Taljaard et al., 1969; Kao et al., 1970, Trenberth, 1979, 1981b, 1991). Most of them are based on hemispheric analyses at 00:00 GMT because the data coverage is vastly superior in the Australia New Zealand sector at that time (Trenberth, 1979). However, that is not the case for the South American sector, where data are better at 12:00 GMT. For this reason the mean geopotential height field at 500 mb (Figure 1(a)) and the corresponding standard deviations (Figure 1(b)) are presented just for comparison, even though they are calculated from only one year of data. Means are nearly zonal, but there is an indication of the Atlantic subtropical anticyclone in the eastern part over Brazil. The strongest gradient appears south of 40 S and it is more marked over the Atlantic. The standard deviation in the geopotential

3 Mid-troposphere variables and precipitation height is about 30 gpm in the tropics and progressively increases to a maximum of 140 gpm in the south of the continent. The biserial correlation field between 500 mb geopotential heights at 12:00 GMT of day i and precipitation in the 24 hours from 12:00 GMT of day i to 12:00 GMT of day i + 1 at Ezeiza is shown in Figure 2(a). Two fairly large and smooth regions of significant coefficients with opposite signs may be clearly observed. One of them, negatively correlated, is located to the west south-west of Buenos Aires over the Cordillera de los Andes, and the other one appears in the south of Brazil and its Atlantic coast. Thus, areas of anomalously low and high 500 mb geopotential heights indicate the preferred positions of the synoptic scale trough and ridge related to precipitation at Ezeiza in the following 24 hours. The gradient of correlation isopleths over the centre and east of Argentina indicates the presence of anomalous geostrophic wind from the north north-eastern direction at the 500 mb level associated with precipitation at Ezeiza. 4. Geostrophic wind components Zonal and meridional wind components are computed geostrophically using centred differences from the grid used by the Servicio Meteorológico Nacional (Ruiz & Vargas, 1998). Some distortions in wind components occur in the south-western and south-eastern borders because of the stereographic projection used. Mean zonal wind varies between 3 m s 1 in the tropical region to a maximum of 23 m s 1 at middle latitudes centred at 50 S over the Atlantic Ocean; there is evidence of the mean position of the jet stream (Figure 3(a)). Winds are westerly everywhere. The broad maximum at middle latitudes is consistent with the results of Trenberth (1981a). The maximum in the standard deviations of the geopotential heights (140 gpm) is centred at 57 S, 65 W (Figure 1(b)), poleward and somewhat south-westward of the mean jet stream, as also indicated by Trenberth (1981b). Standard deviations of zonal wind (Figure 3(b)) range from 5 m s 1 at about 25 S to a maximum of 12 m s 1 in the south of the continent. These values are larger south of 45 S than those obtained by Trenberth (1981b), the latter dealing with zonal mean variances of the east west component, but they are smaller for tropical and subtropical latitudes. Mean meridional wind exhibits a southward component east of about 60 W (Figure 4(a)) and has a northward component in the west, indicating the position of the 500 mb mean trough. Intensities are stronger over the Atlantic Ocean. Standard deviations of the meridional component Figure 2. Biserial correlation fields between precipitation ( 0.1 mm) at Ezeiza and 500 mb geopotential heights. (Figure 4(b)) are similar to those of the zonal component north of 40 S, but they increase rapidly, reaching 14 m s 1 between 50 S and 55 S in the east of the continent. This is 6 poleward of the latitude of the maximum zonally averaged standard deviation of the meridional component found by Trenberth (1981b). The maxima in the standard deviations, which are closely related to the storm track in South America (Trenberth, 1991), lies only 2 or 3 poleward of the region of strongest westerlies at 500 mb. In general, wind standard deviations south of 40 S, reflecting transient baroclinic disturbances, are higher than those obtained by Trenberth (1981b). Some differences may be partly due to the limited one-year sample and to the fact that the annual cycle is not removed. The location of the maxima reveals excellent agreement with the distribution of cyclone centres described by Taljaard (1972). Consistent relationships between 500 mb wind anomalies and precipitation at Ezeiza may be found through inspection of the corresponding biserial correlation fields. Westerly anomalies between 30 S and 35 S over 275

4 N E Ruiz, W M Vargas and H H Ciappesoni Figure 3. (a) Mean of the zonal component of the geostrophic wind at 500 mb (m s 1 ) and (b) the corresponding standard deviations. 276 Figure 4. As in Figure 3, except for the meridional component.

5 Mid-troposphere variables and precipitation the Cordillera de los Andes occur in relation to Ezeiza precipitation, while easterly anomalies over Paraguay and Brazil extending over north-western Argentina are also highly associated with precipitation (Figure 5(a)). The zero-correlation isopleth passes just over Buenos Aires, indicating the zone where the zonal wind anomaly reverses. The gradient in the correlation field from approximately 33 S, 70 W to 24 S, 53 W shows that this is the most likely direction for the anomalous zonal shear to happen which favours precipitation in Buenos Aires. North of the positive correlation centre a strong gradient also exists which is associated with a strong anticyclonic shear, as will be seen later when the relative vorticity is considered. Similarly, south of this centre a significant anomalous cyclonic centre is present. With respect to the meridional wind component (Figure 5(b)), the most striking feature is the large region with northerly wind (negatively correlated with precipitation) with its minimum correlation centre appearing in the south of the province of Buenos Aires (40 S, 62 W). This is linked to the position of the trough and to the northerly advection necessary to produce precipitation at Ezeiza. East of 55 W there is a strong gradient in the correlation isopleths associated with anomalous southerly winds over Brazil. 5. Geostrophic relative vorticity Perturbations on weather maps are associated with large values of vorticity. Therefore, 500 mb geostrophic relative vorticity is investigated as a derived dynamic variable related to precipitation. The precipitation is accumulated in different periods of 24 hours with respect to the 12:00 GMT analyses (Klein et al., 1959). The intention is to examine the existence of spatial and temporal lag relations through the distribution and relative movement of centres of anomalous cyclonic or anticyclonic vorticity. Figure 6 presents the biserial correlation fields between 500 mb relative vorticity and precipitation at Ezeiza during three different days with respect to the analysis day i: (a) in the 24-hour period one day in advance i + 1, (b) in the 24-hour period following the analysis day i (considered here as a quasi-simultaneous relationship), and (c) in the 24-hour period one day earlier i 1. An anomalous cyclonic vorticity centre between 37 S and 43 S is located west of the Cordillera de los Figure 5. Biserial correlation fields between precipitation ( 0.1 mm) at Ezeiza and (a) zonal and ( b) meridional geostrophic wind components at the 500 mb level. 277

6 N E Ruiz, W M Vargas and H H Ciappesoni Figure 6. Biserial correlation fields between 500 mb relative vorticity and precipitation ( 0.1 mm) at Ezeiza in the 24-hour period (a) one day in advance, (b) following the analysis day, and (c) one day earlier. Andes over the Pacific Ocean one day previous to the occurrence of precipitation at Ezeiza. The region north of 35 S is dominated by an anomalous anticyclonic circulation in the north-west south-east direction with two centres, the more important one being in the north of Chile at about 28 S. The cyclonic centre invades the continent on the same day as the precipitation occurs. The anticyclonic nucleus to the east of Buenos Aires is reinforced, and the zero correlation isopleth passes just over Ezeiza. Thereafter, the cyclonic vorticity centre is at the longitude of Buenos Aires, somewhat to the south once the precipitation finishes (precipitation occurred the day before). Other correlation zones are non-significant, except for the anticyclonic circulation over Brazil. Therefore, it takes one day for the anomalous cyclonic centre to cross the Cordillera (less than 100 km wide); then it advances about 2000 km the next day in relation to the occurrence of precipitation at Ezeiza. This discontinuity in the cyclonic anomaly motion is due to topographic effects. If one considers the occurrence of precipitation in a larger period, say 48 hours, the resulting pattern 278

7 Mid-troposphere variables and precipitation (Figure 7) is basically similar to that of Figure 6(b) but with stronger correlation coefficients. This means that a 500 mb relative vorticity distribution like this will produce precipitation at Ezeiza at some time during the following two days with a higher probability. Precipitation at three synoptic weather stations in Buenos Aires (Ezeiza, Aeroparque and Observatorio Central Buenos Aires) is considered to find out if local precipitation at Ezeiza is representative of the zone, at least on a daily temporal scale. The precipitation event is now defined as the occurrence of 0.1 mm of average precipitation at the three stations in a 24-hour period (from 12:00 GMT on day i to 12:00 GMT on day i + 1). The correlation map (Figure 8) shows exactly the same configuration as in Figure 6(b). Therefore, precipitation at a single station may be used to represent the region for this temporal and spatial scale, rather than the group of stations having to be used. Frequency distributions of 500 mb relative vorticity under the condition of occurrence and non-occurrence of precipitation at Ezeiza are statistically different at gridpoints (Figure 6(b)) where the correlation is shown to be higher (Ruiz & Vargas, 1998). These gridpoints of interest are at 39 S, 71 W (west of Neuquén), 33.5 S, 52 W (east of Uruguay). At the gridpoint 34.5 S, 57 W (Buenos Aires) the difference in the frequency distributions is particularly insignificant, which means that 500 mb relative vorticity at Buenos Aires does not yield any information about dry or rainy days at that place (Ruiz & Vargas, 1998). To evaluate whether the quantity of precipitation at Ezeiza is discriminated by the 500 mb vorticity intensity, joint absolute frequency distributions between these variables are calculated and are shown in Table 1 for the west of Neuquén, east of Uruguay and Buenos Aires. There exists a predominance of moderate to heavy precipitation associated with cyclonic relative vorticity over Neuquén, with the presence of anticyclonic relative vorticity to the east of Uruguay in most cases. There does not appear to be a bias in the relative vorticity when heavy rainfall takes place over Buenos Aires, just a little anticyclonic relative vorticity. 6. Vorticity advection As a conceptual tool, vorticity advection at the nondivergence level is clearly related to synoptic vertical Figure 7. Biserial correlation field between 500 mb relative vorticity and precipitation ( 0.1 mm) at Ezeiza in the following 48 hours. Figure 8. Biserial correlation field between 500 mb relative vorticity and precipitation ( 0.1 mm) in the region of Buenos Aires using data from Ezeiza, Aeroparque and Observatorio Central Buenos Aires. 279

8 N E Ruiz, W M Vargas and H H Ciappesoni Table 1. Matrix showing the joint distributions between 500 mb vorticity intensity (10 5 s 1 ) and quantity of precipitation for (a) west of Neuquén (39 S, 71 W), (b) east of Uruguay (33.5 S, 52 W), and (c) Buenos Aires (34.5 S, 57 W) (a) Precipitation (mm) Relative vorticity (10 5 s 1 ) < 6 6 to 4 4 to 2 2 to 0 0 to 2 2 to 4 4 to 6 > 6 > to to to (b) Precipitation (mm) Relative vorticity (10 5 s 1 ) < 6 6 to 4 4 to 2 2 to 0 0 to 2 2 to 4 4 to 6 > 6 > to to to (c) Precipitation (mm) Relative vorticity (10 5 s 1 ) < 6 6 to 4 4 to 2 2 to 0 0 to 2 2 to 4 4 to 6 > 6 > to to to motion and therefore to precipitation. So, 500 mb relative and absolute vorticity advection are calculated to examine their skill as potential predictors of precipitation, compared with other 500 mb variables. Biserial correlation coefficients between 500 mb relative vorticity advection and precipitation at Ezeiza (Figure 9) indicate that the anomaly of cyclonic advection, the only one that is significant, is located to the south and south-west of Buenos Aires (nearer Ezeiza than in the case of vorticity anomalies). Cyclonic vorticity advection that occurs in this region is more likely to produce rainfall; however, the coefficient magnitudes are not as strong as for vorticity itself. According to absolute vorticity advection (not shown), the resulting pattern is very similar to the previous one, but is less intense. 7. Vorticity tendency 280 Relationships between 500 mb vorticity tendency over 24 hours and the occurrence of precipitation in that period, which includes advection as well as other causes of vorticity change, show that this change at the 500 mb level is strongly connected with rainfall with a minimum biserial correlation coefficient of 0.40 occurring over the Atlantic Ocean close to the coast, south of the precipitation zone of interest (Figure 10(a)). A comparison between Figure 10(a) and Figure 9 reveals that there exist distinct mechanisms, other than vorticity advection, affecting vorticity tendency at the 500 mb level which in turn influence precipitation. It is noted, however, that in this case advection is considered an instantaneous quantity, while the tendency is a 24-hour difference. To find out how vorticity tendency anticipates precipitation, the correlation map with rainfall one day in advance is computed (Figure 10(b)). It may be observed that the stronger vorticity fall is situated in the centre of the country and over northern Patagonia one day before the precipitation event at Ezeiza. Nevertheless, the anomalous cyclonic vorticity centre is in the west of the Cordillera de los Andes (Figure 6(a)). 8. Final remarks This study has provided insight into the mid-troposphere dynamical mechanism responsible for a significant fraction of precipitation at Buenos Aires. 500 mb geopotential heights and derived variables have been

9 Mid-troposphere variables and precipitation used to characterize the mid-tropospheric flow and their link to daily precipitation. The results indicate that relative vorticity anomalies, among others, serve as good tracers to visualize dynamic features of the atmosphere that affect the occurrence of rainfall in a given locality. Although the correlation patterns are statistically significant, they explain only a fraction of the total variance of precipitation. The above results are only for the limited period of a year, but they are important as they indicate possible physical mechanisms associated with precipitation which require further study. An operational system based upon these elements will be proposed for the Forecast Office of the Servicio Meteorológico Nacional to provide objective probabilities of precipitation. These will supplement operational numerical forecasts by identifying those areas where precipitation is likely to occur on a particular day. Acknowledgements Figure 9. Biserial correlation fields between precipitation ( 0.1 mm) at Ezeiza and 500 mb relative vorticity advection. The authors acknowledge the Servicio Meteorológico Nacional of Argentina for providing all the data and computational facilities, and Dr J. H. Hordij for his Figure 10. Biserial correlation fields between 500 mb vorticity tendency and precipitation ( 0.1 mm) at Ezeiza in the 24-hour period (a) following the analysis day, and (b) one day in advance. 281

10 N E Ruiz, W M Vargas and H H Ciappesoni helpful comments. This research was partially supported by the Universidad de Buenos Aires under Grant EX274 and by the CONICET under PIP References Bergthorsson, P. & Döös, B. (1955). Numerical weather map analysis. Tellus, 7: Cressman, G. P. (1959). Operational objective analysis system. Mon. Wea. Rev., 87: Holton, J. R. (1979). An Introduction to Dynamic Meteorology, 2nd edn (Int. Geophys. Ser., Vol. 23). New York, Academic Press. Hordij, J. H. & Ciappesoni, H. H. (1975). Aplicación de métodos numéricos al análisis de alturas geopotenciales de niveles de presión. Meteorológica, 6: Kao, S.-K., Jenne, R. L. & Sagendorf, J. F. (1970). The kinetic energy of large-scale atmospheric motion in wavenumber frequency space: II. Mid-troposphere of the Southern Hemisphere. J. Atmos. Sci., 27: Klein, W. H., Lewis, B. M. & Enger, I. (1959). Objective prediction of five-day mean temperatures during winter. J. Meteorol., 16: Panofsky, H. & Brier, G. W. (1965). Some Applications of Statistics to Meteorology. Pennsylvania, The Pennsylvania State University. Ruiz, N. E. & Vargas, W. M. (1998). 500 hpa vorticity analyses over Argentina: their climatology and capacity to distinguish synoptic-scale precipitation. Theoretical and Appl. Climatol., 60: Taljaard, J. J., van Loon, H., Crutcher, H. L. & Jenne, R. L. (1969). Climate of the Upper Air: Southern Hemisphere, Vol. 1, Temperatures, Dew Points, and Heights at Selected Pressure Levels. Washington, DC, NAVAIR 50-IC-55, Chief of Naval Operations, 135 pp. Taljaard, J. J. (1972). Synoptic meteorology of the Southern Hemisphere. Meteorology of the Southern Hemisphere (Meteorol. Monogr., No. 35, C. W. Newton, editor), Amer. Meteorol. Soc Trenberth, K. E. (1979). Interannual variability of the 500 mb zonal mean flow in the Southern Hemisphere. Mon. Wea. Rev., 107: Trenberth, K. E. (1981a). Interannual variability of the Southern Hemisphere 500 mb flow: regional characteristics. Mon. Wea. Rev., 109: Trenberth, K. E. (1981b). Observed Southern Hemisphere eddy statistics at 500 mb: frequency and spatial dependence. J. Atmos. Sci., 38: Trenberth, K. E. (1982). Seasonality in Southern Hemisphere eddy statistics at 500 mb. J. Atmos. Sci., 39: Trenberth, K. E. (1991). Storm tracks in the Southern Hemisphere. J. Atmos. Sci., 48: