Wintertime orographic precipitation over western Tasmania

Abstract

The wintertime (April - October) precipitation across western Tasmania (west of 147E) has been studied for two years (2014 and 2015). Using the AWAP precipitation analysis, the average daily rainfall across western Tasmania was found to be 4.49 mm day-1 for all winter days and 6.99 mm day-1 for rain days (average precipitation greater than 1 mm day-1). Rain days were observed for ~63% of all days during the winter months. Rain days were frequently recorded after the passage of a cold front, when winds are typically from the west and southwest, off the open Southern Ocean. The daily precipitation was found to be highly correlated (r = 0.55) with the 12 UTC ERA-Interim 1000 m wind speed at a point upwind of Tasmania, roughly 100 km off the west coast. Given the highly variable meteorology of the Southern Ocean storm track and the complex topography, western Tasmania is a natural testbed for studying orographic precipitation. Both locally blocked and unblocked flows, caused by changes in the low-level thermodynamic stability, occur frequently over the course of a winter with a stable environment having a lower average precipitation rate (3.66 mm day-1) than an unstable environment (8.40 mm day-1), although only a weak correlation (r = -0.07) was found between precipitation and 2 (the square of non-dimensional mountain height). Simulated precipitation from the Australian Bureau of Meteorology's ACCESS-VT model was found to underestimate the AWAP precipitation by ~20%. The greatest negative relative errors between the AWAP and ACCESS-VT precipitation in unblocked flow were in the lee of the mountains, over central and south-central Tasmania. For days when the flow was blocked, this region had large positive relative errors in precipitation. Over the upwind side of western Tasmania, ACCESS-VT underestimated precipitation in comparison to AWAP in both unblocked and blocked flows. However, the network of surface sites is quite sparse over this region, which limits our confidence in both the ACCESS-VT and the AWAP precipitation products. A more detailed investigation is necessary to better appreciate limitations in the ACCESS-VT forecasts in this region.