WatertankexperimentswerecarriedouttoinvestigatethethermalconvectionduetothebottomheatinginanasymmetricalvalleyunderneutralandstablystratifiedapproachflowswiththeParticleImageVelometry(PIV)visualizationtechnique.Intheneutralstratificationapproachflow,theascendingdraftinducedbybottomheatingismainlylocatedinthecenterofthevalleyincalmambientwind.However,withambientwindflow,thethermalconvectionisshiftedleeward,andthedescendingdraftislocatedontheleewardsideofthevalley,whiletheascendingdraftislocatedonthewindwardside.Thedescendingdraftisminorlyturbulentandorganized,whiletheascendingdraftishighlyturbulent.Withtheincreaseofthetowingspeed,thedescendingandascendingdraftsinducedbythemechanicalelevationbegintoplayamoredominantroleinthevalleyflow,whiletheroleofthethermalconvectioninthevalleyairflowbecomeslimited.Inthestablestratificationapproachflow,thethermalconvectionislimitedbythestablestratificationandnodistinctcirculationisformedincalmambientwind.Withambientwind,agravitywaveappearsintheupperlayerinthevalley.Withtheincreaseoftheambientwindspeed,agravitywaveplaysanimportantroleinthevalleyflow,andthelocationandintensityofthethermalconvectionarealsomodulatedbythegravityinternalwaves.Thethermalconvectionhasdifficultypenetratingtheupperstablelayer.Itsexchangeislimitedbetweentheairintheupperlayerandthatinthelowerlayerinthevalley,anditisadversetothediffusionofpollutantsinthevalley.
