Synoptic evolution associated with the January 18, 2003  mesoscale convective system.

Juan Ruiz and Celeste Saulo (CIMA/UBA)

Table of Contents:

1- Introduction

2- Observed precipitation.

3- Map showing the location of the upper air selected stations.

4- Pilot Balloons.

    4.1 Argentina

    4.2 Bolivia

    4.3 Brazil

    4.4 Paraguay 

5- Radiosonde observations.

6- Synoptic discussion.

    6.1 Pre-genesis stage.

    6.2 Genesis stage.

    6.3 Maturity stage.

    6.4 Dissipating stage.

7- More plots from the GDAS dataset.

Introduction:

This analysis has mainly been carried out using the GDAS analysis (1deg horizontal resolution and 26 vertical levels) and is included in this page to provide a guiadance to check key patterns that could be well or bad represented by the models run for this intercomparison.

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Observed precipitation:  (http://www.cpc.noaa.gov/products/precip/realtime/retro.html):

-17 January 2003

-18 January 2003

-19 January 2003

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Map showing the location of the upper air selected stations:

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Pilot Balloons:

Available.

Not available.

ARGENTINA
Date	Hour (UTC)	Córodba	Chamical	Santiago	Resistencia	P. Guanacos	Salta	J. V. Gonzalez	Paraná	Tostado
17 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
18 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
19 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15

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BOLIVIA
Date	Hour (UTC)	Santa Cruz	Cobija	Uyuni
17 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
18 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
19 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00

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BRAZIL
Date	Hour (UTC)	Dourados	Vilenha	Rio Branco
17 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
18 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
19 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00

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PARAGUAY
Date	Hour (UTC)	Asunción	Mariscal Estigarribia
17 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
18 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00
	21:15
19 January	00:00
	03:00
	06:00
	09:30
	11:30
	15:00
	18:00

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Radiosonde Obserbations:

Available.

Not available.

Date	Hour (UTC)	Santiago	Córdoba	Resistencia	Chamical
17 January	06:00
	12:00
	18:00
18 January	06:00
	12:00
	18:00
19 January	06:00
	12:00
	18:00

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Synoptic Discussion:

Pre-Genesis Stage (from 00 UTC to 18 UTC, 17 January 2003):

At dawn a NW-SE oriented cold front was observed over south of Buenos Aires, moving towards the northeast (Fig 1). Convection associated with this front was weak and not organized. Temperature contrast increased across the frontal zone during the day due to clear skies ahead of the frontal zone over northern and central Argentina (Fig 2). The strong heating also contributed to the development of a low level cyclonic circulation over northwestern argentina as can be seen in the 1000 hPa. analysis (Fig 3).
Low level circulation was from the north over northern and central Argentina, Paraguay and Bolivia. The strongest winds were located in a narrow stream just ahead of the frontal zone where wind speeds exceeded 16 m/s (Fig 4). This stream met Bonner's criterion over Santiago del Estero and Cordoba at 06 and 12 UTC (Fig 5 & 6). Warm advection and large moisture convergence (values exceeding 50 g/kg/day see Fig 7 ) were superimposed with this narrow jet stream at 850 hPa.. However, no significant warm advection was observed over the area were the MCS began to develop a few hours later (Fig 8). Equivalent potential temperature values at 850 hPa. were high in a narrow tongue latitudinally extended over northwestern Argentina. By the afternoon, the warm zone becomes wider and potential temperature values over 350 K were observed in the region of interest (Fig 9).  Moisture content at 850 hPa., increased during the day  reaching values as high as 14 g/kg in the afternoon.
At 500 hPa. a weak ridge axis was located over the MCS area. A short wave trough was observed to the south of 33 S moving to the east with the frontal zone. This trough was too far from the MCS area to have played an important role on its development (Fig 10).
At 200 hPa. a strong NW-SE oriented jet can be seen just behind the position of the frontal zone at low levels. Wind speeds were around 60 m/s in the jet streak located over Patagonia (Fig 11). A well developed Bolivian High could also be seen near its climatological position and moving slowly to the west. By the afternoon at beginning of the MCS's genesis stage, the jet streak progressed quickly to the southeast while a narrow zone of upper level divergence appears over central Argentina oriented nearly north to south (Fig 12).

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Genesis Stage (From 18 UTC 17 Jan 2003 to 06 UTC 18t Jan 2003):

By 18 UTC 17 January,  isolated convection could be observed over northern Argentina within the warm and moist air (Fig 13 )  far ahead of the frontal zone,  which is now located between north of Cordoba and Buenos Aires city (Fig 14). By the end of the genesis stage the frontal zone progressed towards the northeast (Fig 15).

By 00 UTC the convection associated with the cold front began to merge with the pre-frontal convection observed over northern Argentina. As a result,  a well developed MCS could be recognized starting around 00 UTC, January 18 (Fig 16 & 17). Low level circulation was from the east and northeast just ahead of the MCS and warm advection was weak. Behind the MCS a small but strong stream from the southeast at 850 hPa. developed with a wind speed maximum of 16 m/s (Fig 18 ). Weak cold advection was associated with this feature (fig. 19).  The observed vertical wind profile at Resistencia, shows southeastelies with speeds around 9 m/s  at 850 hPa. which are in good agreement with the analysis. The Pilot Balloon launched at Tostado at 6 UTC also showed strong winds (greater than 18 m/s) from the southeast at 1000 m, this measurement is also consistent whit the analyzed wind field. Figure 20 shows the vertical structure of the stream at 28 S.
The strongest values of moisture convergence took place within the end of the genesis stage and the beginning of the maturity stage, when two centers exceeding 80 g/kg per day could be seen. The most intense one, located to the west of the MCS (Figure 21). Equivalent Potential temperature values greater than 350 K could be found ahead of the frontal zone where the MCS was growing.
CAPE values from the analysis ranged between 2000 and 4000 J/Kg over the genesis region (Fig 22) and moisture contents continued above 14 g/kg at 850 hPa. (Fig. 23). 700 hPa. vertical velocity shows two centers of maximum vertical velocities between -2 and -2.5 Pa/s (Fig 24). These two centers were consistently superimposed  with the convergence maxima at 850 hPa.. Figure 25  better depicts the vertical structure of these features and the position of the convergence and divergence zones. 

At mean levels, very short wave troughs and ridges appeared over the genesis area as the MCS developed (Fig 26). The short wave trough which was evolving south of 33 S reached the Atlantic Ocean by the end of the genesis stage. Behind this system a short wave ridge crossed the Andes towards central and southern Argentina.

By the end of the genesis stage strong divergence could be found to the east of the developing MCS and also to the west (divergence values exceeding 9.1e-6/s). These two divergence maxima are linked to the upward movement and low level convergence maxima previously discussed (Fig 27). The Bolivian High remained stationary over central Bolivia during this stage.

In order to provide an idea of whether synoptic forcing could be playing a positive role on the system development, a quasigeostrophic analysis of Q vector at 700 hPa. has been done. From the begining of the genesis stage, there is a good correspondence between Q vector convergence and the upward movement. At 18 UTC- January 17, upward movements were located just ahead of the frontal zone over central Argentina (Fig 28). By the end of the genesis stage, when upward movements increased due to the strong convection, Q vector convergence was still present in the area where the MCS was growing (Fig 29) suggesting that a synoptic-scale driving mechanism is active during the system genesis, though its hard to decide how important for system development it is.

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Maturity Stage (From 06 UTC to 12 UTC 18 Jan 2003):

The surface frontal zone began to move slowly, reaching Paraguay and the south of Brazil by 12 UTC where a weak low level cyclonic circulation developed (Fig 30). The  area with  strongest convection moved to the northwest reaching south of Bolivia (Fig 31). During the maturity stage, strong convergence was still present ahead of the MCS, with moisture convergence values between 50 and 60 g/Kg per day being observed at 850 hPa. over northern Argentina (Fig 32). Moisture contents and equivalent potential temperature were high in the warm air ahead of the frontal zone.
Figure 33 shows the vertical structure of the v-wind component: winds at low levels were from the north but not strong enough to meet Bonner's criterion.
At 500 hPa. the short waves became more defined and small cyclonic and anticyclonic relative vorticity centers appear over the MCS area. These vorticity centers remained nearly stationary (Fig 34). As can be seen in this figure anticlonyc vorticity centers are stronger than the ciclonyc ones at 500 hPa. this is similar to the vertical structure of the 31st May 1985 MCS showed by Torres and Nicolini (1999), as showed in their figures 7 and 8, in that case the ciclonic vorticity was stronger at low levels were convergence had a maximun. At 200 hPa. strong divergence generates anticiclonyc vorticity and at 500 hPa. they found a weak ciclonyc vorticity center sourrounded by a stronger anticiclonyc vorticity ring. The wind and height fields at 500 hPa.  indicated geostrophic imbalence in close agreenment with Maddox (1983).
The 200 hPa. field shows strong divergence over north and northeastern Argentina during the maturity stage. On the other hand,  the strongest convection was located to the north-west between Argentina and Bolivia, as is inferred from the satellite image (Fig 35). The missplacement of upper level divergence in the anaylisis compared with the actual position of convection denoted by the satellite image for the same time may be due to an erroneous handling of data by the objective analysis and/or to the relative low resolution of the analyzed fields.
The jet streak located over Buenos Aires moved slowly to the northeast and the Bolivian High began to move slowly to the southwest.

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Dissipating Stage (From 12 UTC to 18 UTC 18 Jan 2003):

The frontal zone remained almost stationary over Paraguay and Southern Brazil. At 18 UTC January 18, only isolated convection could be found over Paraguay, Bolivia and northwestern Argentina (Fig 36 ). Low level circulation was from the south and southeast over eastern Argentina and divergence was still observed over the MCS area (Fig 37). Tropical air could be observed over Paraguay ahead of the frontal zone but lacking the amounts of moisture convergence that chacacterized the MCS's previous stages (Fig 38).
At 500 hPa. the short wave pattern remained almost at the same location while begining its decay phase (Fig 39).
Strong divergence persisted at upper levels to the east of the MCS region. The jet streak located to the southeast over Uruguay and southern Brazil weakened. The Bolivian High moved to the south reaching northwestern Argentina by 18 UTC 18th (Fig 40). On the following day, the Bolivian High began its retur towards its climatological position.

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More plots from the GDAS dataset:

Complete evolution of selected fields.
Date	17 January 2003				18 January 2003				19 January 2003
Hour / Field	00utc	06utc	12utc	18utc	00utc	06utc	12utc	18utc	00utc	06utc	12utc
1000 hPa.
850 hPa. Wind
850 hPa. Theta-E
Bonner Criterion
CAPE
700 hPa. Omega
500 hPa. Vorticity
200 hPa. Divergence
Vector Q (700 hPa.)

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