Hourly-Scale Evaluation Of Precipitation Simulation In China

Due to the uniqueness and complexity of the East Asian precipitation systems,the precipitation simulation biases over East Asia generally exist in various current state-of-the-art numerical models.It has been a difficult problem for numerical simulation to reproduce the precipitation characteristics over East Asia reasonably and accurately.Based on the characteristics of hourly precipitation and event-based assessment methods,a comprehensive and detailed evaluation of the biases of precipitation simulations over East Asia is performed.This study aims to promote the in-depth understanding of the key factors affecting the precipitation simulation and provide more information for model improvements.The main conclusions are summarized as follows:1.Overall evaluation of precipitation simulation over East Asia.The biases of summer precipitation over East Asia simulated by high-resolution climate system model are closely related to topography.Over the Tibetan Plateau,the simulated precipitation amount is significantly larger than that of the observation,and this amount bias is mostly contributed by the overestimated precipitation frequency.On the contrary,over the third terrain step of China(the area to the east of 500 m contour line in the east of China),the simulation presents an underestimation of precipitation amount,frequency and intensity.For summer precipitation simulation,the model presents six regions with large discrepancies.Four regions present large positive biases in both amount and frequency,which are located at the southern edge of the Tibetan Plateau(STP),the northeastern edge of the Tibetan Plateau(NETP),the eastern periphery of the Tibetan Plateau(EPTP),and the mountainous area of North China(NCM).The other two regions with negative biases include the lower reaches of the Yangtze River(LYR)and the South China coast(SCC)over the third terrain step.For the four regions with positive biases(two regions with negative biases),the overestimation(underestimation)of precipitation frequency is mainly due to the positive(negative)frequency biases of weak(moderate)precipitation.Further evaluation of the precipitation diurnal variation shows that the biases over different regions are caused by different types of precipitation events.Compared to the observation,the smaller diurnal amplitude over the regions around TP(STP,NETP and EPTP)are mainly affected by the long-duration precipitation events.Affected by the long-duration precipitation events,the diurnal amplitudes over the regions around TP(STP,NETP and EPTP)are smaller than the observation.Affected by long-duration early moring precipitation events and short-duration afternoon precipitation events,the peak time of precipitation diurnal variation over the NCM is earlier than the observation,and the diurnal amplitude is smaller than the observation.The precipitation over the LYR and SCC peaks earlier than the observation is mainly due to the short-duration afternoon precipitation events.2.The evaluation of precipitation simulation over the east periphery of TP.Considering the significant biases of the simulation of frequency-intensity structure over this region,the heavy and weak precipitation events were distinguished and evaluated respectively.Focusing on the temporal and spatial evolution characteristics of heavy precipitation processes over the EPTP,the climate model reproduce the heavy precipitation systems moving from the northwest to the southeast of the key region.However,the model shows that the precipitation events start earlier than that of the observed events,and this bias may be closely related to the state of the low-level divergence field in the model.In view of the problem of excessive weak precipitation over the EPTP,the climate model is integrated as a weather prediction model using the Transpose-AMIP(TAMIP)-type intergration.By comparing with the actual precipitation processes,the timesteps with artificial weak precipitation in the TAMIP experiment are selected and the corresponding circulation biases are investigated.It is found that the artificial weak precipitation in the model may be related to the abnormal convergence of the lower troposphere and to the colder and wetter atmosphere near the surface.Furthermore,the sensitivity test was carried out to add an additional moisture divergence term in the moisture prognostic equation.It can significantly reduce the excessive precipitation over the regions around the steep terrain.This improvement is mainly due to the reduction of overestimation of the weak precipitation frequency by sensitivity test,which confirms the effect of the biases of simulated moisture divergence on the artifical weak precipitation.3.The evaluation of precipitation simulation over North China.Focusing on the steep terrain region of North China,the climate model can not reproduce the spatial distribution pattern of precipitation decreasing with the increase of terrain height.The precipitation amount simulation presents positive(negative)biases over the high-altitude(low-altitude)region.In addition,the climate model can not reproduce the regional differences in the diurnal peak of precipitation amount over North China.Under the control of relatively real circulation,the TAMIP run can simulate the pattern of the peak time of precipitation diurnal cycle over North China,but the simulaton biases of the spatial distribution of precipitation amount has not been significantly improved.Futhermore,the ECMWF forecast products with real initial field and higher resolution are evaluated.It is found that the model can not only reproduce the distribution of precipitation amount with terrain height,but also reasonably simulate the regional differences of the peak phase of precipitation diurnal variation.It indicates that the relation between precipitation and topography is influenced by the horizontal resolution of the model,while the diurnal cycle is mainly controlled by the circulation field.It is worth noting that the diurnal varitation of precipitation amount and the diurnal varitation of start,peak and end time of events all present lagging features from the high-altitude region to the downstream plain region,while the TAMIP and ECMWF show poor ability to simulate the precipitation evolution.The lagging feature of diurnal variation of precipitation simulated by TAMIP and ECMWF is discontinuous along the terrain,indicating that there is still a deviation in the simulation of the downhill process of the precipitation systems.4.The evaluation of precipitation simulation over the coastal area along South China.Affected by the land and sea distribution and by the small-scale mountains in the interior of South China,the spatial and temporal distribution of precipitation over the coastal area of South China is more complicated.The precipitation amount and frequency along the coastline present tripole patterns.Correspondingly,the diurnal peaks in precipitation amount along the coast present a “morning-afternoon-morning” pattern.That is,there are two precipitation amount centres with high precipitation frequencies located to the west and east of the Leizhou Peninsula,and the diurnal peaks in precipitation in that location occur in the morning.The Leizhou Peninsula is a relatively dry region with a low precipitation frequency,and the diurnal precipitation amount presents an afternoon peak.The the climate model presents a systematic underestimation of the precipitation amount over the coastal area,and the high-resolution climate model cannot reasonably reproduce the tripole patterns of precipitation amount along the coastline.Compared with the climate model,ECMWF forecast products reasonably reproduce the tripole patterns of precipitation amount.However,the ECMWF forecast product has no obvious advantage for the simulation of the precipitation diurnal cycle.The high-resolution climate model can reasonably reproduce the changing characteristics of the diurnal peak phase along the coastline.In particular,the climate model can well simulate the the single early-morning peak of precipitation diurnal variation to the west of Leizhou Peninsula.ECMWF overestimated the afternoon precipitation to the west of Leizhou Peninsula,which makes the diurnal variation of precipitation over this region presents double-peak(one in the afternoon and the other in the after midnight).