THE PROBLEM OF THE NOWCASTING IN THE RIVER PLATA BASIN,

THE PROBLEM OF THE NOWCASTING IN THE RIVER PLATA BASIN

José M. Nuñez Servicio Meteorológico Nacional , Conicet
Norberto L. Di Lorenzo Dto.Cs. de la Atmósfera ,FCEyN , Universidad de Buenos Aires
Gorki Jover Servicio Meteorológico de la Armada ,República Argentina

1. INTRODUCCION

Regularly , Buenos Aires province northeast is affected by storms of variable intensity . In particular , during Spring and Summer months .
For this reason is necessary to typify them , to have at one’s disposal techniques for its detection , and forecast its evolution if it were possible . Normally , storm that appear in the region are associated with frontal activity , upper system , squall line and air masses .
Storm developement studies have been done in synoptic scale and mesoscale, using cyclogenetical probability and of precipitation indeces in determining its skill degress, Ciappesoni et all (1994) . In general , the problem of Nowcasting was stated for the region by Nuñez et all (1991) .
Our purpose is to analyze conduct and characteristics of the main precipitation processes above mentioned .

2. CASE STUDY SELECTION (SQUALL LINE ANALISIS ON 16 OCTOBER 1993)

2.1 Synoptic - aerological situation

On 16 October 1993 at 0900 Z a prefrontal trough that is on the line Mendoza , South Cordoba , San Luis and Necochea (Buenos Aires province) slightly push forward . At the same time a cold front is over Bahía Blanca (South Buenos Aires province ), At 1200 Z the front and the trough get together on the line San Juan , Mercedes (San Luis) and Necochea .
At 1500 Z there are storms in Junín ,Bolívar and Tandil . At 15.15 HOA(Argentine Oficial Hour ) precipitation begin to be recorded in the Buenos Aires Metropolitan Region , (San Miguel ,Castelar and Ezeiza ) .
Inestability index obtained from radiosonde data at 0900 Z , 10.16.93 not have storm condition . Indices K , Showalter and Total Totals show them the day after , at 0900 Z on 10.17.93 when compared with the previous data, table 1 .
In not any case index pointed out severe storm conditions and hail ; notwithstanding some of the cells of the analyzed line showed such caracteristics .

Table 1

Instability indices	10/16/93 Date	10/17/93 Date
K	-7,5	27,5
Sh	9,7	0,3
LI	5,8	7,2
FM	3,3	5,1
TT	32,0	45,5
Sweat	-227,0	--------

Instability indices values , K (Whiting ), Sh (Showalter) , LI(Lifted index)

FM(Fawbush-Miller),TT(TotalTotals) , Sweat (Severe Weather Threat)

received from radiosonde at 0900 Z on 16 and 17 October 1993 .

2.2. From pluviometric field

Two of the stations that recorded the storm precipitation were:
Castelar INTA , lat : 34^o 40’ S and long. : 58^o 30’ W
Buenos Aires , lat : 34^o 35’ S and long. : 58^o 29’ W

From them precipitation maxima , between 15,15 HOA (Argentine Oficial Hour) and 18,45 HOA is given in table 2

TABLE 2

Station \ Time min	5	10	15	20	30	45	60	120
Castelar INTA mm	7,7	11,3	14,4	15,4	17,2	20,3	20,5	38,5
Buenos Aires mm	6,4	10,2	16	16,4	16,9	17,6	18,3	35,7

Precipitation maxima each 5 minutes of two Meteorological Station from Boletín Climatológico SMN 1993

3. AVAILABLE EQUIPMENT

Radar M33 , S band was employed . Located at Ezeiza Airport , lat.: 34^o50’S and long.:58^o32’ W
Furthermore , Ezeiza radiosonde data , and the National Weather Service pluviometric network .

4. 16 OCTOBER 1993 STORM ANALYSIS ACCORDING RADAR DATA

Although starting at 15:00 HOA storms were detected by weather radar , only at 16:30 HOA .
At 16:36 HOA squall line shape is observe ( see fig.1) and later on at 18:36 HOA this is totaly developed and intensified.

Figure 1.: M33 Radar echo images at 16:36 at 18:36 HOA . 10-16-93 . Circles pointed out 50 Km distance

General behavior of the line could be determined through analysis of all disposable material between 1630 HOA and 2030 HOA , each 5 min . The system was divided in two areas for movements study. First included the line principal part and the second cell of anomalous movement
According to Quinteros et all (1984), in this region , for 22 analized cases general characteristics are the following :
SW to EN movement direction , concave or convex forms ; center of gravity mean velocity of the line is 55,4 Km/h and its mean displacement direction 280^o , mean longitude of 290 Km , 52,6 Km maximun mean wide and 8.350 Km² of mean area .
16 october 1993 line had 400 Km longitude and 60Km wide maximun . Mean displacement velocity was 37Km/h , NE direction ; 58 Km/h southeastmost and 40 Km/h northwest, counterclockwise .
For the most part of the storm life’s cycle , three high reflectivity centers wereactive.The anomalous behavior of a cell displaced from behind of tline is showed in Fig.2 , with 75 Km/h velocity and Az . 305^o

Figure 2 : Position of anomalous cell 10-16-93 storm

Figure 3 show time variation of ocupied area by storm echoes , where greater values are recorded between 1725 and 1810 HOaccordingly to greater precipitation intensity values (see table 2) .

Figure 3 : Radar echo total area variation on 10-16-93

During this period an area a storm 52.000 Km² was covered by the line .
Mean precipitation area (ATI) computed for 25 dBz level was 69.538,8 Km²/h , the mean rain rate (R) was 3,23 mm/h and the mean precipitation total quantity (P) was 12,92 mm.
These and Table 2 values shows that the storm was of moderate intensity but widely spread . Intense convective activity , hail included ,come to pass in some isolated points with no inflence over mean values .
It showed a mean displacement velocity of 37 Km/h over continent and disminished to 27 Km/h over River Plate basin

5. NOWCASTING ANALYSIS

At first glance general aspect of the storm allows to watch squall line movement from SW to EN with a mean velocity of 30 Km . For this experience and for Nowcasting, Cross Correlation method was used , Austin and Bellon (1974), being applied to divers sectors selected by structure of reflectivity field analyzed, and defined by 2 AZ values and 2 in range, for each one .

Several results were observed.Most of them with high values of the g correlation coeficient ,(see table 3) . For different sectors , 1,2,8 and 9 columns show different moments with respect to the beginning of the storm , distinct movement analysis intervals and different speed and dircetion values .
For the 8 analyzed cases , former three belong to propagation effects , next two displacement and to the last three the greastest weight concerns to the displacement .
Its is to be noted 6^th case showing a value of 0,780 instead of 1 . This suggests an evolutive effect .

TABLE 3 Results of cross correlation thecniques application to the squall line different sector on 10-16-93

Hi (HOA)	Dt min	R₁ Km	R₂ Km	Az₁ deg	Az₂ deg	g	Velocity Km/min	Direction deg
16.30	10	50	150	35	70	0.535	0.707	261.87
17.15	15	100	200	0	50	0.447	5.6	207.8
17.40	20	100	200	45	90	0.336	7.5	270.0
17.00	25	100	200	0	45	0.820	1.75	190.0
17.00	45	100	200	0	45	0.483	0.92	190.0
16.30	10	50	50	0	90	0.780	0	-------
17.15	10	100	200	0	90	0.412	4.7	214.7
18.05	10	100	200	0	90	0.904	2.5	210.7

H_i: Initial Hour (Start time) , Dt : Analized time interval , R_i : Radar minimun distance from analized sector

R₂ : Radar maximun distance from analysed sector , AZ₁ : Minimun azimuth from analysed sector

AZ₂: maximun azimuth from analyzed sector , g : Cross-correlation coeficient

6. CONCLUSIONS

The squall line show a main mass with 3 high reflectivity centers which move quasi as whole .
The squall line analyzed is a typical one of our region but greater than mean value , with SW to NE movement direction .
Particularities detected are :

- When the main mass of the line pass from the continent to de la Plata estuary slow down it velocity from 37 to 27 Km/h . Intensifying reflectivity values and high reflectivity center areas .

-Anomalous movement cells is presented , Az :305^O and high velocity : 75 Km/h

-With a mean velocity of 32 Km/h , southeastermost comes up to 58 Km/h and northwest to 40 Km/h , clear counterclockwise .

-It is necessary to discriminate the results according to the moment in the life’s cycle of squall line and it forming .

-Differnt moments and places chosen give distinct results , depending on displacement and propagation that take place in the respective sector .

-Up to now we do not think it suitable to forecast future cells location , at intervals longer than 30 min .

Acknowledgments
The autors appreciate cooperation of Mr. Kyu Hyung Kyung who implemented part of the sofware for radar image analisys.

References

1. Austin G.L. and A. Bellon , 1974 : The use of digital weather radar records for short-term precipitation forecasting , Quart. J.R. Met.Soc. ,100 ,658-664 .

2. Ciappesoni HH. , J.M.Nuñez , M.A. Giachino , M.A. Gan and N.Arai ,1994 : Some characteristics of the life cycles of extratropical cyclogenes in Southern South America .Int.Symp. on life Cycles of Extr. Cyclones . June 1994 , Bergen , Norway.

3. Nuñez J.M. ,M.A. Giachino , M.E. Saluzzi and N.L.Di Lorenzo,1991 : The problem of the Nowcasting in the River Plata Basin , PartI: Measurement and Tratament of Data by Radar and Employment of numerical Analysis .
25 th Int, Radar Conf. , AMS June 1991 , Paris , France .

4. Quinteros C.Y. , N.L. Di Lorenzo and E.A.Caimi ,1984 : Ecos en linea dentro del alcance del radar M33 .
3^er Congreso Brasileño de Meteorología , Belo Horizonte , Brasil ,1984

5. Servicio Meteorológico Nacional (SMN) , Boletín Climatológico , octubre 1993 ,vol V , N^O10 p.p. 46-50 .
Buenos Aires , Argentina

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