A water vapour flux tool for precipitation forecasting

The skill of quantitative precipitation forecasts is poor, especially for extreme events. This paper describes a new tool that combines wind observations aloft (from wind profiling radars) with vertically integrated water vapour (IWV) measurements derived from global positioning system (GPS) receivers to estimate the bulk transport of water vapour. This transport strongly influences precipitation enhancement by mountains. Based on earlier research, a controlling wind layer is defined, which has maximum correlation between the horizontal component of the wind directed upslope at the coast and the rainfall measured downwind in the mountains. The altitude of the maximum correlation (∼ 1 km above sea level) often corresponds to the altitude of the low-level jet that typically resides in the region of enhanced water vapour transport ahead of an approaching cyclone's cold front (i.e. in the atmospheric river portion of the storm). The wind at this level usually differs from the wind at the surface, pointing to the need for wind measurements aloft. The upslope wind in the controlling layer is then combined with the IWV measurement to calculate hourly, layer-mean, bulk water vapour transport. Case studies and four winters of data, including rain-gauge networks, demonstrate the close relationship between bulk water vapour transport and mountain precipitation. For example, heavy rainfall capable of generating flooding (i.e. ≥10 mm/h) occurs at the mountain site almost exclusively when coastal observations of IWV and upslope flow exceed 2 cm and 12·5 m/s, respectively (i.e. the bulk water vapour transport surpasses 25 m/s cm). These results are integrated into a prototype real-time diagnostic tool that has the potential to improve short-term quantitative precipitation forecasts in coastal mountains.