Flash Flood event over central Argentina: a case Flash Flood event over central Argentina: a case studystudy

Paola Salio(1), Yanina Garcia Skabar(2) and Matilde Nicolini(1)

(1) Departamento de Ciencias de la Atmósfera y los Océanos. Centro de Investigaciones del Mar y la Atmósfera. CONICET – UBA(2) Servicio Meteorológico Nacional. Cátedra de Climatología y Fenología Agrícolas. FA – UBA.A primary goal of the present work is:A primary goal of the present work is:

to describe the synoptic and mesoscale characteristics of the environment associated to a flash flood over the central region of Argentina, with a special emphasis in the relationship between the behaviour of convective precipitation and the evolution of the low level jet. In order to achieve this objective a numerical simulation is performed considering a version of the Brazilian Regional Atmospheric Modeling System (BRAMS), that include a microphysics scheme and explicit convection in the finest resolution grid

and estimations of precipitation are considered in order to detect convective and stratiform precipitation areas.

Methodology: Estimation of Convective and Stratiform Methodology: Estimation of Convective and Stratiform PrecipitationPrecipitation

- IR Brightness Temperature (235 K contour) to determine precipitation areas- hourly CMOPRH Precipitation Estimation- FORTRACC technique to following the Mesoscale Convective System

Stratiform precipitation < 7.5 mm h-1

Convective precipitation>= 7.5 mm h-1

Evolution of the environmental conditionsEvolution of the environmental conditions

March 30 12 Z March 30 15 Z March 30 18 Z March 30 21 Z March 30 09 Z

Upper pannel: Reflectivity (shaded), ageostrophic wind (barbs) and vertical velocity (contour). Lower Pannel: Equivalent Potential temperature (shaded), ageostrophic wind (barbs) and meridional ageostrophic wind (contour)

Reflectivity 850 hPaReflectivity 850 hPa

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Convective and Stratiform Precipitation estimated Convective and Stratiform Precipitation estimated by the combination of CMORPH and IR areaby the combination of CMORPH and IR area

CMORPHCMORPHBRAMS Grid 3BRAMS Grid 33 hr - Accumulated Precipitation and reflectivity3 hr - Accumulated Precipitation and reflectivity

Stratiform, convective and total precipitation estimated for all systems that affect the central and Stratiform, convective and total precipitation estimated for all systems that affect the central and northern region of Argentina between March 21 to April 1, 2007.northern region of Argentina between March 21 to April 1, 2007.

Large Scale ConditionsLarge Scale Conditions

200 hPa Streamlinesand wind intensity

500 hPa Geopotentialand 24-hour tendency

850 hPa Geopotentialand 24-hour tendency

March 27 12ZMarch 27 12Z

Vertically Integrated Moisture Flux

Large scale is dominated by:Large scale is dominated by:

a strong trough centered on 75°Wa strong trough centered on 75°W, that remains stationary over the whole studied period.

Associated to the trough a cut-offa cut-off low remain stationary over the coast of Chile generating large scale upper motion over central Argentina.

The availability of moisture is favoured by the presence of a lowlow over northwestern Argentina.over northwestern Argentina.

The moisture flux presents an anticiclonic gyremoisture flux presents an anticiclonic gyre and supply moisture from the Atlantic Ocean, this situation differs from the classical low level jet situation associated with convection.

Methodology: BRAMS simulation Methodology: BRAMS simulation

BRAMS 3.2 Brazilian Regional Atmospheric Modeling System version 3.2Simulation period : March 2007 from 25 to 31 at 12UTCTwo-way grid interactive nesting techniqueNumber of atmospheric levels: 30; vertical cordinate: shaved etaHorizontal Resolution: Grid 1 - 50 km Grid 2 - 12.5 km, Grid 3 - 3.125 kmGDAS analyses from NOAA/NCEP as initial and boundary conditions.Model includes topography data (1km resolution)terrain land use (1kmresolution), soil types (50km resolution)weekly sea surface temperaturesdaily soil moisture heterogeneous fields from USP/CPTECParameterizations:Shallow cumulus:Sousa y Silva; Deep convection: Grell; Explicit convection on Grid 2 and 3Radiative: Chen and Cotton; Horizontal diffusion:Smagorinsky; Vertical diffusion: Mellor-Yamada; Microphysics: 7 water species, bulk water scheme

Observed, Estimated and Modeled PrecipitationObserved, Estimated and Modeled Precipitation

Observed CMORPH estimations

Total acumulated precipitation Total acumulated precipitation between 26-3-2007 12Z to 31-3-2007 12Zbetween 26-3-2007 12Z to 31-3-2007 12Z

BRAMS Grid 2 BRAMS Grid 3

Observed precipitation over central Argentina (red), CMORPH (left) and models forecast by (right) different grids accumulated between March 26 to 31, 2007. Grid 1 (light blue) represent the precipitation forecast for 50 km

resolution, Grid 2 (yellow) for 12.5 km and Grid 3 (bordeaux) for 3.125 km respectively.

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Observations CMORPH

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Observed Grid 3 Grid 2 Grid 1

S A U C E V IE J O A E R O

P A R A N A IN T A

P A R A N A A E R O

M O N T E C A S E R O S A E R O

C O N C O R D IA A E R O

E L T R E B O L

R O S A R IO A E R O

G U A L E G U A Y C H U A E R O

J U N IN A E R O

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S A N M IG U E L

E L P A L O M A R A E R O

M O R O N A E R OC A S T E L A R IN T A

E Z E IZ A A E R O

A E R O P A R Q U E B U E N O S A IR E SB U E N O S A IR E S

-62 -60 -58 -56

-34

-32

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M onte C aseros

C oncord ia

G ualeguaychú

P araná

Sauce V ie jo

R osario

Junín

E l T rebo l

Ezeiza

Buenos A ires

CMORPH estimation denotes an excelent performance over the studied are. Values of CMORPH estimation denotes an excelent performance over the studied are. Values of precipitation are overestimated close to Buenos Aires.precipitation are overestimated close to Buenos Aires.

The accumulated precipitation simulated by BRAMS results in most locations very close The accumulated precipitation simulated by BRAMS results in most locations very close to the observed values, but in some locations as in Rosario simulated rain is significant to the observed values, but in some locations as in Rosario simulated rain is significant overestimated, reaching nearly twice the observed value on grid 2 and 3.overestimated, reaching nearly twice the observed value on grid 2 and 3.

Although the model evidences some spatial and temporal misplacements in maximum Although the model evidences some spatial and temporal misplacements in maximum precipitation respect to CMORPH estimation, accumulated precipitation during the whole precipitation respect to CMORPH estimation, accumulated precipitation during the whole period are properly represented by the model.period are properly represented by the model.

Convective and Stratiform Activity:Convective and Stratiform Activity:Mesoscale convective activity tend to generate during the beginning of the night and decay during the day. The maximum extension of the systems varies from small systems to the bigger one on March 29 at 8Z that cover all area, and shows also developments over northwestern Argentina.

Strong convective rates overpass by three times the total stratiform precipitation. Systems during the rest of the period present an equivalent total stratiform and convective precipitation but, in general, convective maximum precipitation occurs before the stratiform precipitation.

EnvironmentThe thermodynamic environment is characterized by strong CAPE, low CINE and the presence of a deep flow from the north that shows a LLJ profile.The relationship between the LLJ and the development of convection, during an extreme of convective precipitation shows: deep layer of unstable air toward the storm. an increase in the northerly flow at the moment of the developing of the storm.AKNOWLEDGMENTS: This research is supported by UBA grant X266, ANPCyT grant Nº PICT 07 – 14420 and CONICET PIP N° 5582. First author would like to thank to the Organization Committee of 15 ICCP for the support to participate in the conference