Stage-Dependent Predictability of a Heavy Rainfall Event in South China: An Ensemble-Based Analysis

Persistent heavy rainfall during warm-season sometimes presents a stage-dependent predictability, which makes it a big challenge for operational forecasting. Based on ensemble forecast, this study investigated the key influencing factors of two different rainfall stages with different predictability in a persistent heavy rainfall in South China, Guangdong Province during 29-31 March 2014, aiming at explaining the lower predictability in the earlier stage of this rainfall event.

By applying ensemble-based sensitivity analysis (ESA) on operational global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), different key factors of the two rainfall stages were diagnosed by correlating accumulated precipitation of each stage to atmospheric state variables at respective times. The precipitation in both stages was found to be strongly correlated with the low-level vortex and the low-level jet on the southeast flank of the vortex. The key influencing factor in earlier stage S1 was the location of the low-level jet. A more to the southeast low-level vortex resulted in a more to the southeast low-level jet, leading to more precipitation in Guangdong. In later stage S2, with the relatively accurate low-level jet location, the width of the jet became more important. A narrower low-level jet associated with a weaker low-level vortex was found to be favorable for heavy rainfall to occur because the moisture from the southwest can be concentrated over Guangdong rather than transported more to the north in this situation.

The stage-dependent precipitation predictability in this case can be attributed to the different predictability of low-level jet over stages, which might be relevant to the juxtapositions between orientation of the low level jet and the direction of low-level vortex movement in the target area. The locations of low-level jet axis in S1 were very diverse among ensemble members, while in S2 were more concentrated and better captured by model. The lower predictability of precipitation in S1 was strongly associated with the less accurate low-level jet location in ensemble forecast. In this stage, the low-level vortex moved southeast perpendicular to the low level jet, and thus the uncertainty of the geopotential height in the target area was large within the ensemble, resulting in wide ensemble spread of the jet location. In S2, however, the low-level vortex moved northeastward parallel to the low level jet, leading to small height uncertainty in the target area and thus the jet locations were more concentrated and well predicted.