| Lakes,as an important part of the land surface,can significantly alter the surface energy and water balance through differences in moisture,heat,and friction between the lake and surrounding land areas,and thus influence regional weather and climate.Poyang Lake and Dongting Lake,as the two largest freshwater lakes over the middle and lower reaches of the Yangtze River basin in China,have an important impact on the regional weather and climate change.In recent years,with the continuous development of numerical models,many lake models have appeared and coupled to climate models to achieve accurate simulation of hydrodynamic process in lake and interaction between lake and atmosphere.However,the applicability of coupled lake model in the two largest freshwater lakes in China is still unknown.Besides,the long-term climate simulation generally drifts away from the driving fields due to errors of model and forcing field,which ultimately cause large inaccuracy in climate simulation and the simulated climate effect of lakes.In order to accurate simulate the climate effect of Poyang and Dongting Lake,this paper firstly evaluates the applicability of the WRF/Lake model in China's two major freshwater lakes,and calibrates the important parameters that characterize the hydrothermal process and the lake-atmosphere interaction in the WRF/Lake model to improve the ability of the WRF/Lake model in simulating lake-atmosphere interaction.Secondly,a piecewise-integration method is introduced in the simulation study of the lake climate effect.Here,a series of ideal sensitivity experiments are designed to assess the ability of the piecewise-integration method applied in a complex model to simulate the climate effect of Poyang and Dongting Lake.Thirdly,two four dimensional assimilation methods(grid nudging and spectral nudging)which will be used to assimilate the observations and analysis in the piecewise-integration method are evaluated.Fourthly,the NCEP/FNL,NCEP/R2 and ERA-Interim reanalysis are evaluated to investigate the ability of the WRF model in simulating the regional climate over China during the period of 2009-2011 and elect the optimal reanalysis as the lateral boundary conditions and initial conditions.Simultaneously,the optimal reliable domains of the elected reanalysis are investigated and verified.Lastly,the climate effects of Poyang and Dongting Lake are investigated using the calibrated WRF/Lake model and the piecewise-integration method accompanied with the elected optimal four dimensional assimilation method and reanalysis.The main conclusions are listed as follows:(1)Several cases with different weather characteristics are chosen and a series of experiments are conducted with WRF3.6.1/Lake model.The results show that the performance of the lake model is significantly affected by weather conditions.For the winter with precipitation cases,the performance of the default lake model even is worse than the no use of the lake model.Calibrations of water absorption,extinction coefficients and surface roughness length can reduce the biases of temperature and dew-point temperature at 2 m obviously for the winter with precipitation cases.Although the performance of the default and new calibrated model is intricate for other cases,the new calibrated model has prominent advantages for dew-point temperature at 2 m.Moreover,a long time simulation of five months also shows that the new calibrated coupled lake model performs better than the default one.These implies that the new calibrated coupled lake model may be more suitable to be used in lake-influenced weather and climate studies in Poyang and Dongting Lake regions.(2)The piecewise-integration method is defined as that the continuous long-term simulation is segmented into subintervals of sequential short-term simulations and updates the modeled states at the end of each subinterval,among which the reference state are updated with analysis data and the perturbed states are updated with the sum of analysis data and the difference between the perturbed and the reference states.The ability of the piecewise-integration method applied in a complex model to simulate the climate effect of Poyang and Dongting Lake is assessed though a series of ideal sensitivity experiments.The results show that the piecewise-integration method is significantly superior to the conventional continuous integration method.It can effectively reduce the accumulation of errors caused by long-term continuous integration and improve the simulation accuracy of the reference state and the perturbed state,thus consequently improve the simulation accuracy of the change field caused by Poyang and Dongting Lake.However,the advantages of the piecewise integration method are affected by the model error.When the model error is very small,the improvement of the change field is not obvious.Overall,the climate effect of Poyang and Dongting Lake studied with the piecewise-integration method will be more accurate than with the conventional continuous integration method.(3)When using the piecewise integration method,a Newton relaxation four-dimensional assimilation method(nudging)should be employed to assimilate the observation data or analysis field into the simulation process coordinately.At present,there are two nudging methods for assimilating the analysis field,including grid nudging and spectral nudging.A series of experiments are conduted here to compare which nudging method performs well in simulating climate over China.The results show that the biases of precipitation area simulated by the conventional continuous integrations is largest.Grid nudging can capture the precipitation area,but it under-estimate the magnitude of precipitation seriously,so its biases of precipitation is largest.Spectral nudging can capture the area and magnitude of precipitation well,and its biases of precipitation is least.Further analysis about a sudden turn from drought to flood(STDF)event occurred in the middle and lower reaches of the Yangtze River basin show that the conventional continuous integrations can't reproduce the STDF event,while nudging can capture it.Furthermore,the spectral nudging outperforms the conventional continuous simulation in reproducing precipitation features,but grid nudging induces the largest biases for precipitation though it has the smallest biases for other meteorological elements.Scale separation analysis shows that the large-scale features of the conventional continuous simulation have been drifted far away from real fields and force wrong small-scale features,whereas the nudging successfully prevent the model to drift away from the forcing fields at large-scale.And the different performance among spectral nudging and grid nudging would be attributed to that the former method generates its own small-scale information by the RCM,but grid nudging over-suppresses the small-scale information while retaining the large-scale features.The difference in small-scale features affects simulating different moisture flux and convergence,as well as cloud,and then results in diverse precipitation.These results illustrate that both the consistency with large-scale and the local variability from small-scale are robust factors for reproducing precipitation features during extreme events using RCMs.(4)The WRF model should be driven by the lateral boundary conditions and initial conditions,which usually taken from reanalysis,so the differences in driving data can cause uncertainties on simulated results.Besides,the piecewise integration method is more effectively when the analysis data error is smaller.Here,the NCEP/FNL,NCEP/R2 and ERA-Interim reanalysis as the initial and boundary field to drive the WRF model are evaluated.The results show that the regional climate over China simulated by three reanalysis datasets is different significantly and the spectral nudging can reduce the simulated biases of every reanalysis obviously.Relatively,the ERA-Interim reanalysis data has better performance both for precipitation and temperature in the middle and lower reaches of the Yangtze River basin.Scale separation analysis show that the reliable domains of the NCEP/FNL,NCEP/R2 and ERA-Interim reanalysis are 1500-2000 km.Further,the ERA-Interim reanalysis is used to drive the WRF model with two nesting domains to verify its reliable domains and investigate the effects of different truncated wave-number.The results show that the optimal truncated wave-number for spectral nudging in the two nesting domains in this paper is 4 and 1,respectively,of which the corresponding wavelengths are 1950 km and 1830 km respectively.(5)Poyang Lake and Dongting Lake,as the two largest freshwater lakes in China,are major lakes in the middle and lower reaches of the Yangtze River basin and will significantly affect the regional weather and climate.However,the regional climate effect of these two lakes is still unknown.Here,the climate effects of Poyang and Dongting Lake during year of 2009-2011 are investigated using the calibrated WRF/Lake model and the piecewise-integration method accompanied with the elected optimal four dimensional assimilation method and reanalysis.The results show that although there are still biases for temperature,precipitation and atmospheric circulation,the piecewise-integration method is significantly superior to the conventional continuous integration method.Thus,the piecewise-integration method is more suitable to be used in the study of climate effects of Poyang and Dongting Lake.Poyang Lake and Dongting Lake are heat sources in summer,autumn and winter,but cold source in spring.Besides,they increase wind speed,evaporation,air humidity,sensible heat flux and latent heat flux almost in whole year.The regional climate effect of these two lakes is notable in summer and autumn,but negligible in winter,which can be attributed to their effect on temperature in summer and autumn is large.The effect of Poyang and Dongting Lake on precipitation is various in different seasons and regions.In summer and autumn,the two lakes increase precipitation in the extensive area northern to the lakes,but reduce precipitation in the other parts.The mechanism of precipitation affecting by lakes is that the two lakes cause a cyclonic circulation anomaly and increase the convergence of water vapor flux over the region of increased precipitation,but cause an anticyclonic circulation anomaly or increase the divergence of water vapor flux over the region of decreased precipitation.Moreover,the impacts of these two lakes on the 2 m temperature,latent heat flux,downward shortwave radiation flux and upward longwave radiation flux are basically contrary with their impact on precipitation,but their effects on the 2 m water vapor mixture ratio and sensible heat flux are basically consistent with their impact on precipitation.Therefore,in addition to affecting the climate in the lake area,Poyang and Dongting Lake will also affect surrounding precipitation,temperature,humidity,and surface energy balance by affecting the atmospheric circulation and divergence of water vapor flux,thus affect the regional climate. |
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