Modeling The Impacts Of CP-El Ni(?)o On A Drought Event In Southwestern China With A Piecewise-integration Method

A drought event spanning from the autumn of 2009 to the spring of 2010 occurred in southwestern China(SWC).This drought was the most serious in the past 50 years and led to a huge economic losses.Model-based sensitivity experiment is a widely used method for studying climate change attributions.In traditional climate sensitivity studies,"climate drift" occurs because of the accumulation of model errors during long-term integrations,both in "known" and "unknown" states.The credibility of traditional sensitivity experiments is relatively low.A piecewise-integration method,dividing the long-term continuous simulation into a series of sequential short-term simulations,is used to reduce the model errors.The model fields are updated at the end of each subinterval with analysis data for the "known" state run,and with the sum of the analysis data and "perturbation field"(the difference between "unknown" and "known" state)for the "unknown" state run.This paper conducts sensitivity experiments with the piecewise-integration method to evaluate the impacts of the CP-El Ni?o(Central Pacific El Ni?o)on the severe drought in SWC.First,two groups of ideal sensitivity experiments based on Weather Research and Forecasting model(WRF)are conducted to evaluate the ability of piecewise-integration method in simulating the climate effects of sea surface temperature.The results of ideal experiments show that piecewiseintegration method can effectivity reduce the accumulation of model errors both in the simulation of "known" state and "unknown" state,and the simulation accuracy of directly updated variables(such as U,V,T,Q)and model diagnostic variables(such as precipitation and 2 m temperature)can obviously increase.The simulation effect of piecewise-integration for "known" state is better than "unknown" state.It is mainly because there is no accuracy observation data in the simulation of "unknown" state and more model errors will be introduced in the process of updating the model field.It can distinctly improve the simulation accuracy of "change field"(climate effects of sea surface temperature anomalies),but the improvement effect of ideal experiment with larger model errors(ideal experiment ?)is poor than ideal experiment with smaller model errors(ideal experiment ?).The major reason is that model errors are obviously greater than the climate effect of sea surface temperature anomalies in ideal experiment ?.In this situation,the simulation of continuousintegration is not credible,and the improvement of piecewise-integration is not outstanding.For different variables,the improvement degree of the piecewise-integration is also different,the improvement rate of the variables T and Q is higher,while the improvement rate of the variables U and V is lower in this study.In general,piecewise-integration method apply to study the climate effect of sea surface temperature anomalies.Based on the ideal experiments result,real case study using piecewise-integration method based on WRF model is designed to study the effect of CP-El Ni?o on the severe SWC drought in 2009/2010.The results show that model errors in piecewise-integration experiment are less than in continuous-integration experiment.The degree of drought over SWC in continuous-integration sensitivity experiment is too strong to be believed.However,the magnitude and the distribution of the drought simulated by piecewise-integration sensitivity experiment are consistent with the observation.In addition,the results of piecewise-integration show that CP-El Ni?o may lead to warmer temperature in SWC.Then,based on the piecewise-integration simulation,the mechanisms of severe drought caused by CP-El Ni?o are analyzed.The anomalous anticyclone circulation in the lower troposphere over the Philippines,which originated from the strong development of the CP-El Ni?o event,is shown to inhibit the water vapor transport from the South China Sea to SWC,leading to a lack of moisture in the study region.On the other hand,pronounced subsidence and warm temperatures in the lower troposphere is proved to be the dynamic factor and thermodynamic factor that emerged and maintained the drought.Through the synthesis of several El Ni?o events in recent 30 years,drought events with different degrees occurred over SWC in CP-El Ni?o years while the precipitation increased over SWC in EP-El Ni?o years.The latter is also verified through piecewiseintegration method in this study.