Analysis of an extreme precipitation episode over the central subtropical Andes using ETA/PRM regional model

The high mountains of central Andes (30º/36º S) and the central Chile region (windward of mountain range) have a winter precipitation regime. The extreme precipitation episodes(EPE)are not so frequent and when it does occur, there are severe socio-economical impacts specially on transportation and commerce. Snowfall over the high mountain of the Andes is the product of complex interactions between the synoptic scale flow and the mesoscale topog-raphy.

The aims of this study are to analyze the thermodynamics, synoptic, and mesoscale conditions for a heavy rain and snowstorm event occurred during August 26th - 28th 2005 over Central Andes of Argentina and Chile; and to evaluate the ETA-Programa Regional de Meteorología (PRM) regional model as a potential forecasting tool. It is mainly intended to determine the potential of the model to capture mesoscale-topography conditions related to extreme precipi-tation accumulated on the western slopes and high-mountain of central Andes and to the oro-graphic winds (Zonda /chinook/foehn wind) on the eastern slopes.

During August 2005 a monthly positive precipitation anomaly was registered over central Chile and on the high mountain. Five cold frontal systems passed across the region in this winter month and the last one (August 26th - 28th, EPE) had the heaviest precipitation. In par-ticular on high areas of the Santiago de Chile city surroundings, the total precipitation of the EPE reached 151 mm and the maximum precipitation accumulated in 24 hours was 65.7 mm on August 27th. Simultaneously over high Cordillera in Punta de Vacas meteorological station (some miles to the east of the international boundary) located at 32º49'S 62º54'W 2395 m.a.s.l., the snowfall reached 44 mm in 24 hours also overcoming the August monthly mean. These extreme precipitation conditions caused floods in Santiago city with many houses overflowed and hundreds of people damaged; the international roadway was closed during many hours; and strong Zonda wind gusts were observed over the plains of San Juan province in Argentina.

The general Southern Hemisphere circulation during August 2005 showed positive sea level pressure anomalies and geopotential heights in all pressure levels in a large band covering the South Pacific Ocean in middle latitudes. This band was extended from southeast Australia and New Zealand to the south border of South America, and negative anomalies were observed to the north of this band. This pattern caused a weak west flow on middle latitudes and an inten-sified flow over subtropical region.

The principal synoptic conditions during August 27th when the EPE's maximum intensity occurred were:

• A frontal zone and an intense zonal flow in middle and high levels. • Absence of a clear trough in middle and high levels (as typical cases) and absence of an intense surface extratropical cyclone. • Persistence of a wide low pressure belt (35ºS-50ºS) linked to a high pressure blocking area in higher latitudes (55ºS-65ºS).

The atmosphere vertical profile windward of Andes Cordillera showed a typical configuration with the subsidence inversion associated to the semi permanent subtropical South Pacific an-ticyclone, during August 25th. Since August 26th start an instability process reaching maxi-mum value in August 27th at 00 UTC and 12UTC (with saturated column from surface to 400hPa level).These coincide with maximum rainfall on central Chile and maximum snowfall on the Andes Cordillera, and with highest Zonda wind gusts on downslopes plains. At leeward of the mountains over Mendoza city (32º53xS 68º51xS, 750 m.a.s.l.) instability just start in August 27th at 00UTC with an increasing instability and wind velocity from high and middle atmospheric levels (500 hPa - 300 hPa); the maximum instability in the whole column can be seen from August 28th at 12 UTC. The absence of a well defined trough is confirmed because of southwest and west wind direc-tion observed by the radiosonde between 500 hPa and 250 hPa levels, during all the time, spe-cially at EPE's maximum intensity.

The ETA-PRM model is hydrostatic and uses the eta vertical coordinate. The model is inte-grated with horizontal resolution of 15 km and 38 vertical levels over the central Chile and central-western Argentina region from 40ºS to 29ºS and from 75ºW to 56ºW. The model inte-grations began at 1200UTC some hours (48h -72h) before the occurrence of maximum pre-cipitation intensity, in order to forecast the beginning and ending of the episode. The initial condition and the lateral boundaries (are provided at 6-h intervals) was taken from the ETA-Centro de Previsao de Tempo e Estudos Climáticos (CPTEC-Brazil) South American conti-nent version analysis.

The model was able to forecast several aspects of the analyzed EPE, such as heavy liquid and solid precipitation and starting and ending precipitation times. The ability to reproduce the zonal precipitation gradient across and on both slopes of the central Andes, with maximum precipitation accumulated on the west areas close to the Chile-Argentina border (on the windward of mountain range), provides successfully confidence in the model. In addition, the wind strength, temperature and humidity changes were able to forecast, associated with Zonda winds occurrence observed at some stations on the downslopes of central Andes.