Improvement Of A Lake Model And Numerical Simulations And Future Predictions Of Lake Thermal Processes On The Tibetan Plateau

Under global warming,the Tibetan Plateau(TP)has experienced dramatic climate changes.Lakes play a key role in the regional climate and ecology systems in the TP.Accurately and scientifically assessing historical lake thermal process changes and predicting future lake thermal processes can provide the theoretical basis for climate change and environment construction management for the TP.There exists insufficient deficiency on the historical and future studies of TP lake thermal processes for current research.The lake model embedded in the Community Land Model(CLM)was used as the modeling tool and theoretical foundation.The long time meteorological data were mainly input into the lake model as the model forcing.Results from the CLM lake model were evaluated mainly using the remote sensing data.The key processes and factor were figured out.Then improvements were made for this model.The effects of water clarity on lake thermal processes were explored with the improved model.The spatiotemporal lake thermal process trends were investigated during 1985?2015.Finally,future lake thermal processes were analyzed during 2070?2100.The main conclusions were drawn as follows:(1)The CLM lake model generated biases in lake water surface temperature(LWST)simulations for TP lakes.LWST Simulations generally reflected observed temperature distributions in the TP when compared to Moderate-resolution Imaging Spectroradiometer(MODIS).However,the CLM lake model could not accurately simulate the seasonal variations of LWST.Lake Qinghai,Lake Zhaling,and Namco were chose to further examine the simulation errors.LWST simulations for Namco were worst.Lake temperature profile simulations also had large biases.Through data correction and sensitivity tests,results indicated that radiation penetrating(extinction parameter)and water mixing(water diffusivity parameter)processes were main processes(factors)influencing lake temperature modeling.Correcting the input extinction parameter and enlarging the diffusivity parameter improved LWST and temperature profiles.Whereas,simply enhancing water mixing cannot reasonably reflect water mixing,and cannot meet the demand for the vast TP lakes.(2)K profile parameterization(KPP)in ocean modeling was coupled and adopted into the CLM lake model(CLM-KPP).In this new model,water mixing was estimated separately for the water boundary layer and its below.Several factors affecting water mixing includes thermal and dynamic forcing in the boundary layer and shear instability and internal waves below the layer.CLM-KPP was able to reflect the observed lake water mixing processes and temperature profiles when compared to observations.(3)Lake Qinghai was found with obvious water clarity changes,and undergone the decreased water clarity after 2005 with the extinction parameter increasing.Two simulations without and with water extinction changes after 2005 were carried out.Results showed that the reduced water clarity leaded to more radiation absorbed in the upper and less in the lower.This increased temperature in the upper and decreased in the ice-free period from April to August,resulting in strong lake-air interactions.During ice-on seasons,LWST decreased,turbulent fluxes decreased,and lake ice thickness increased.(4)The spatiotemporal trends of lake thermal processes for TP lakes were analyzed.Lake temperatures increased in 1985?2015 and 1995?2015,while decreased in 2005?2015in the northwest of TP.The warm extreme events were enhanced,and the cold events were weakened.The cold events were strengthened in 2005?2015.In 1985?2015,the trends of lake thermal processes were in high correlations with downward longwave radiation,air temperature,air pressure,and specific humidity.In 1995?2015,the correlations reduced.In2005-2015,air temperature,air pressure,and wind speed were key factors.(5)The lake thermal processes in the TP were projected in the future 31 years2070?2100 using the General circulation models(GCMs)with the Representative concentration Pathways 8.5(RCP8.5)during 2070?2100.The future climate made TP lakes undergo dramatic thermal changes.Lake temperature increased,lake ice thickness decreased,and lake ice duration decreased significantly.In this study,CLM lake model was used to evaluate simulations for TP lakes.Key physical processes and parameters were investigated.Results showed water mixing was the main reason to cause larger biases in lake models.Thus,a vertical mixing scheme from ocean models,KPP,was improved and coupled into the lake model.This new model reproduces the observed lake mixing processes and greatly improved lake temperature simulations.Then,this new model was used to study the effects of lake clarity on lake thermal processes for Lake Qinghai.In addition,lakes in the TP were simulated to explore temporal and spatial variation during 1985?2015.Future predictions of lake thermal processes were performed during 2070?2100.These results in this study can provide scientific basis for the sustainable ecological environment construction and theoretical support for the rational management of regional water resource and climate change in the TP.