Long-term Trend Of Planetary Boundary Layer Height Derived From Multiple Dataset In China And Its Underlying Mechanism

The planetary boundary layer(PBL),the lower part of the Earth's atmosphere that is in direct contact with surface,controls the vertical exchange of heat,water vapor,momentum and air pollutants between the ground and the free atmosphere,which plays a significant role in the researches related to weather,climate and environmental protection.At present,the long-term spatiotemporal evolutional characteristics of the PBL height in China and its driving factors remain still unclear,especially the differences arisen from different data sources.Based on sounding data,reanalysis data,ground-based observation data and other ancillary data,this thesis analyzes the spatial-temporal distribution characteristics of PBL height in China and its relationship with relevant meteorological parameters.It not only evaluates the applicability of different reanalysis data in revealing PBL height throughout China,but also deepens our understanding of the long-term variation of PBL height in China and their related causes.In particular,three types of reanalysis data from 2011 to 2018 at 0800 BJT and 2000 BJT were compared with sounding data.The reanalysis data for comparison include ERA-Interim,MERRA2 and JRA-55.The temperature profiles from ERA-Interim are on average closest to those from radiosonde measurements.The differences between MERRA2 and sounding are similar to those between JRA-55 and sounding.By comparison,the differences of specific humidity profiles between JRA-55 and sounding are negligible.Furthermore,the wind speed profiles are highly uncertaint due to the largest standard error.Overall,the PBL heights calculated from soundings are generally higher than those from reanalysis,irrespective of the reanalysis used.Interestingly,the PBL heights calculated from both ERA-Interim and JRA-55 are somewhat similar to each other,and the PBL heights calculated by these two reanalysis data are found to deviate much less from radiosonde-derived PBL height,compared with those from MERRA2,even though the PBL height calculated by MERRA2 is the lowest.Comparing the spatial discrepancies in atmospheric profiles and PBL height from three reanalysis and radiosonde measurements,the results show the highest consistence and agreement in eastern China,most likely due to high-density radiosonde observational sites.Moreover,the terrain,interpolation methods applied to atmospheric variable profiles are other important factors affecting the data quality of reanalysis data.To elucidate the long-term trend in PBL height from radiosonde,a series of quality control procedures have been conducted on the original soundings,such as homogenization test.Afterwards,the spatial-temporal variation of PBL height has been determined in China from 1979 to 2016.In terms of geographical distribution,the PBL height exhibits obvious pattern of “northwest high and southeast low”,with an apparent decreasing gradient from western China to eastern China.Regarding the seasonality of PBL height,it reached the peak in spring due mainly to the highest wind speed,followed by summer and autumn,and the minima are observed in winter.A robust and abrupt change of PBL height is found to occur in 2004,after a shift regime detection is performed on the time series of PBL height from 1979 to 2016.To be specific,over the former period(from 1979 to 2003)a spatially uniform increase was found in the PBL height,while over the latter period the PBL height decreased in a spatially nonuniform way.Further causality analysis for the decreasing PBL height after 2004 shows that it could be to the simultaneous increases in soil moisture and vegetation coverage,leading to increased low-tropospheric stability,but decreased sensible heat flux,which all together results in a suppressed PBL.By contrast,global warming largely accounts for the increasing trend in PBL height before 2004.This thesis profoundly reveals the spatiotemporal characteristics of PBL height in China in the context of global warming and high anthropogenic emissions.The trend reversal of PBL height highlights the important role of land surface features such as increasing soil moisture and vegetation coverage in modulating the PBL development.Overall,these findings reported here not only provide fundamental long-term PBL height dataset but also propose a novel mechanism by which the atmospheric pollution in China changes over time.