Validation of objective forecast for categories of precipitation in flood season in Ningxia

Using T639 model products and the precipitation observations from 25 national automatic weather stations in Ningxia between May and September from 2010 to 2013, we have established the precipitation probability forecast equations of light rain, moderate rain and heavy rain within forecast time length of 168 h between May and September in Ningxia based on probability regression and cross-validation method, and compared and validated the forecast skills of objective T639-MOS products, T639 simulations and NMC forecasts on precipitation in Ningxia between May and September in 2014. Results show that comparing to T639 and NMC, T639-MOS presents much lower false alarm ratio, a little higher miss ratio and much higher forecast accuracy, and its forecast skill increases by an average of about 10%. T639-MOS has a good forecast result to large-scale precipitation and severe precipitation, especially with the approach of forecast time and the function adjustment of numerical forecast model. Representativeness and physical meaning of predictors selected are definite based on the objective forecast method. Water vapor, dynamic and thermodynamic factors are important for precipitation forecast in Ningxia. A key forecast factor of light rain is water vapor flux in the mid- and low-level, and those for moderate rain and heavy rain are moist potential vorticity and meridional wind. Additionally, the configuration of critical dynamic factors in the upper- and low-level is considered for heavy rain. Samples of precipitation in Ningxia are relatively low, and most of them are light rain; precipitation samples for moderate rain are only a few. So, the higher the precipitation intensity is, the higher the miss ratio is, and the lower the forecast accuracy is. Nevertheless, the T639-MOS has the highest forecast accuracy to the different levels of precipitation.