Assessing the Regional Model ETA-15km and Wind Anomalies to Forecast Central Andes Winter Snowstorms

Although the high mountainous region of the Central Andes (30º/35º S) has a winter precipitation regime, the heavy winter snowstorms are not so frequent. When they do occur, there are severe socio-economical impacts especially on transportation and commerce between Argentina and the main Chilean port. During some heavy winter snowstorms, thousands of truck drivers and tourists were dangerously blocked on the principal international highway and in the sky resorts throughout long periods. In the context of improving the winter snowstorms forecast in the region, this study presents the performance of the recently implemented regional model ETA-15km to forecast the atmosphere vertical profile during one severe episode along the Central Andes. Moreover, a quantification of wind anomalies was examined as a possible important forecast signal for potentially significant snowstorm that could be used as an extra tool to assist forecasters. This study focuses on the low- and upper-level wind structure and its anomalies associated with winter storm, and also with strong downslope wind on leeward of the mountain range ( Zonda wind in this region).

The winds forecasted of one significant snowstorm event, that took place around 27 August 2005, are verified with radiosonde and aircraft data observations. The model ETA-15km used is a regional version of the model ETA-40km South American domain from the Centro de Previsao do Tempo e Estudos Climáticos (CPTEC-Brazil). In addition, the observed and forecasted low-level (850 and 700 hPa) and upper-level (300 and 250 hPa) westerly wind anomalies are compared with a 10-yr (1974-83) winter radiosonde climatology at windward station. Anomalous westerly winds are correlated with: enhanced upslope precipitation at windward, heavy snowstorms on high mountains and strong downslope wind at leeward. This phenomenon is common in other parts of the world with mountain ranges perpendicular to the westerlies flow in middle latitudes. The wind vertical profile observed reveals substantial west and north component anomalies at 850 and 700 hPa, respectively; and the westerly wind anomalies at upper-level departs significantly from normal. The model had an acceptable performance, thus the examination of its wind anomalies may assist forecasters in identifying significant winter storms in the short range (2-3 days).