The Impacts Of Boundary Layer Height On Dry And Wet Changes In East Asian Arid And Semiarid Regions

The atmospheric boundary layer serves as an important bridge for the exchange of matter and energy between the atmosphere and the surface,plays a key role in the atmospheric adjustment at the weather and climate scales,and also plays an important role in the extreme weather and climate events.Therefore,it is of great importance to understand dry and wet changes from the perspective of atmospheric boundary layer.Using the existing observation data,CRU grid data,ERA-Interim and ERA-20C reanalysis data,the paper analyzed the applicability of ERA-20C reanalysis data in arid and semiarid regions,synthesized the characteristics of the boundary layer height and dry-wet changes characteristics in the East Asian arid and semiarid regions and its subtype climate regions as well as its four seasons,investigated the main factors affecting the boundary layer height changes in East Asian arid and semiarid regions and its subtype climate regions as well as its four seasons,and revealed the impacts of the boundary layer height changes on the dry and wet changes in the arid and semiarid regions.Through the study,hope to gain a deep understanding of the dry and wet changes,provide more information for their predictions,and provide a scientific basis for better understanding of drought issues and improving the fragile ecological environment in arid and semiarid regions.The main results are as follows:The boundary layer height BLH,temperature T and precipitation P data of the ERA-20C and the BLH,T and P data of the actual observed data,ERA-Interim and CRU obtained in the same period,were compared,and found that their characteristics of the spatial distributions and temporal changes were consistent.The correlation coefficients of BLH of ERA-20C with ERA-Interim,the P of the ERA-20C and CRU,and the T of the ERA-20C and CRU were 0.9695,0.9972 and 0.9570,respectively.The mean absolute errors were 86.8142 m,0.8593?and 3.5774mm,respectively.The root mean square errors were 122.3089 m,1.0622?and 5.0457 mm,respectively.These showed small differences and indicated that ERA-20C reanalysis data have good applicability in arid and semiarid regions,and can be used for related research in arid and semiarid regions.The spatial distributions of the BLH in East Asian arid and semiarid regions took the west extreme arid area as the highest center and decreased to the surroundings,showing the longitude flat characteristics.In 1900²010,the BLH showed an upward trend.From the perspective of the subtype climate regions,the increase of the BLH was the result of the joint effect of all the subtype climate regions.And the arid-semiarid transitional regions,semiarid regions and semiarid-semihumid transitional regions had the greatest contributions,which were 22.74%,23.74%and 22.05%,respectively.From the perspective of the seasons,the increase of the BLH and PET was the result of the joint effect of all the four seasons.And the main contribution was in summer with the contribution rate of 58.50%.The spatial distributions of the AI,P and PET in East Asian arid and semiarid regions took the west extreme arid area as the lowest,lowest,highest center and increased,increased,decreased to the surroundings,showing the longitude flat characteristics.In 1900²010,the AI showed a decreasing trend,the P showed a decreasing trend and PET showed an increasing trend.From the perspective of the subtype climate regions,the decrease of the AI as the result of the joint effect of arid regions,arid-semiarid transitional regions,semiarid regions and semiarid-semihumid transitional regions.And semiarid regions and semiarid-semihumid transitional regions had the greatest contributions,which were 34.14%and39.15%,respectively.From the perspective of the seasons,the decrease of the P was caused by summer and autumn,and the increase of PET was the result of the joint effect of all the four seasons.The main contributions of the two were both in summer,with the contribution rate of76.56%and 51.70%,respectively.In the arid and semiarid regions,there are good correlations between the BLH and the thermal factors.Among them,the BLH is negatively correlated with the latent heat flux Le and is positively related to other thermodynamic factors?temperature Ta,surface temperature Ts,temperature difference between the surface and air TsTa,net radiation rn,sensible heat flux Hs?.There are certain differences in the correlation between the BLH and the dynamic factors.Among them,the BLH and the roughness Rou,the ground wind speed w and the U component of the wind?the u component of the 10 m wind u10,the u component of the 100 m wind u100 and the wind shear of the u component uQB?have good positive correlation,while the correlation between the BLH and the v component of the wind?the v component of the 10 m wind v10,the U component of the 100 m wind v100 and the wind shear of the v component vQB?is poor.The main thermal and dynamic factors that affect the BLH are different in the subtype climate regions and seasons.In the extreme arid regions,arid regions,arid-semiarid transitional regions,semiarid regions and semiarid-semihumid transitional regions,the main thermal influence factors of BLH are Le,Le,Hs,Hs and Hs,respectively,with the correlation coefficients of-0.3859,-0.8078,0.8432,0.8902and 0.9119,respectively,and the main dynamic factors of BLH are w,Rou,Rou,w and w,with the correlation coefficients of-0.3391,0.4230,0.5298,0.6661 and 0.6738.In spring,summer,autumn and winter,the main thermal influence factors of BLH are Hs,Hs,Hs and rn,respectively,with the correlation coefficients of 0.8194,0.9363,0.5967 and 0.5704,respectively,and the main dynamic factors of BLH are Rou,uQB,u10 and Rou,respectively,with the correlation coefficients of 0.5389,0.5452,0.3026 and 0.4800,respectively.On the whole,the relationship between BLH and thermal factors is more closely related to dynamic factors in arid and semiarid regions.There is a close relationship between the BLH and dry-wet change.On spatial distribution,the BLH had a good correspondence with the dry-wet conditions.The regions of higher?lower?BLH corresponded to the regions of the smaller?larger?AI,the smaller?larger?P,and the larger?smaller?PET,which were the more severely?more gently?drought regions.On temporal variation,the BLH was in good agreement with the dry-wet physical quantity in the change process.The increase?decrease?of BLH was usually accompanied by the decrease?increase?of the AI,the decrease?increase?of the P,the increase?decrease?of the PET,the outward expansion?inward contraction?of the climate distinction lines,and the expansion?reduction?of the arid and semiarid regions.The correlation analysis showed that BLH had negative correlations with AI and P,and a positive correlation with PET.In the overall East Asian arid and semiarid regions,some subtype climate regions?arid-semiarid transitional regions,semiarid regions,and semiarid-semihumid transitional regions?,and all the four seasons,the BLH had the greatest correlation with PET,and the PET change periods were exactly the same as BLH.The changes of BLH have impacts on the dry and wet changes.The changes of BLH affect the PET by changing the capacity of the boundary layer holding water vapor.That is,when the BLH rises,the capacity of the atmospheric boundary layer holding water vapor is larger,on the original basis,the air density in the atmosphere decreases,thus making the evaporation capacity of the atmosphere to the surface increasing and the PET increasing;when the BLH decreases,the capacity of the atmospheric boundary layer holding water vapor is smaller,on the original basis,the air density in the atmosphere increases,thus making the evaporation capacity of the atmosphere to the surface reducing and the PET decreasing.Preliminary quantitative estimation showed that in the extreme arid regions,arid regions,arid-semiarid transitional regions,semiarid regions and semiarid-semihumid transitional regions,when the average annual BLH increases?decreases?1m,the PET will increase?decrease?by 0.9181 mm,1.7518 mm,1.6958 mm,1.2568mm,and 1.3690 mm,respectively,and the AI will decrease?increase?1.4738*10^-4,6.4611*10^-4,0.0011,0.0015 and 0.0020,respectively.For the overall East Asian arid and semiarid regions,when the average annual BLH increases?decreases?1m,the PET will increase?decrease?1.6600mm,the AI will decrease?increase?0.0012,and the area of arid and semiarid regions will increase?decrease?2.2076*10⁴ km²,correspondingly.