Wintertime orographic precipitation over western Tasmania

Abstract

The wintertime (April - October) precipitation across western Tasmania (west of 147°E) 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 pas-sage 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 Ĥ²(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 un-blocked 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.