Short-term quantitative precipitation forecast experiments based on blending of nowcasting with numerical weather prediction

In order to overcome the deficiency of the quantitative precipitation forecast (QPF) by a mesoscale numerical weather prediction (NWP) model for the very short range at connective scales and the vanishingly low skill of radar-based rainfall now castings beyond the first couple of hours, a QPF scheme to blend these two sources has been developed.It consists of the following three major steps.Firstly, a phase correction technique is applied to the NWP model QPF to correct their systematic timing or location errors by comparing to the corresponding quantitative precipitation estimation (QPE) derived from the radar reflectivity and automatic raingauge data.Secondly, an intensity calibration technique is applied to the phase-corrected model QPF to adjust its intensity so that the calibrated intensity distribution can match the actual distribution calculated from QPE.Thirdly, the blended QPF is obtained as a weighted average between the radar-based nowcast and the phase-corrected intensity-adjusted model QPF with the weights at the different lead times assigned according to a parameterized hyperbolic tangent funetion.Such parameters can vary dynamically according to an index that could quantify the skill of extrapolative nowcast relative to the model QPF and,such an index is found to be different for the different precipitation types.In general,higher weights are given to nowcast in the couple of hours.At about 3 hours and beyond,higher weights are given to model QPF as the now-cast skill drops.This blending scheme was applied to five typical heavy rain cases in 2011 and two in 2012 over the Bcijing-Tianjin-Hebei region with about eighty tests done.The QPF verification results indicated that the performance of the blended QPF was significantly improved in the 0-6 forecast range with the overall forecast skill generally higher than either radar-based nowcast or model QPF taken individually.Given such encouraging results, the present blending scheme is expected to be promising as an effective tool when deployed for operation.