Aerodynamic roughness length （Z0m） is a key factor in surface flux estimations with remote sensing algorithms and/or land surface models. This paper calculates ZOrn over several land surfaces, with 3 years of experi...Aerodynamic roughness length （Z0m） is a key factor in surface flux estimations with remote sensing algorithms and/or land surface models. This paper calculates ZOrn over several land surfaces, with 3 years of experimental data from Xiaotangshan. The results show that Z0m is direction-dependent, mainly due to the heterogeneity of the size and spatial distribution of the roughness elements inside the source area along different wind directions. Furthermore, a heuristic parameterization of the aerodynamic roughness length for heterogeneous surfaces is proposed. Individual Z0m over each surface component （patch） is calculated firstly with the characteristic parameters of the roughness elements （vegetation height, leaf area index, etc.）, then Z0m over the whole experimental field is aggregated, using the footprint weighting method.展开更多
Based on the Monin-Obulchov similarity theory, a scheme was developed to calculate surface roughness length. Surface roughness length over the eastern Qinghai-Tibetan Plateau during the winter season was then estimate...Based on the Monin-Obulchov similarity theory, a scheme was developed to calculate surface roughness length. Surface roughness length over the eastern Qinghai-Tibetan Plateau during the winter season was then estimated using the scheme and eddy covariance measurement data. Comparisons of estimated and measured wind speeds show that the scheme is feasible to calculate surface roughness length. The estimated roughness lengths at the measurement site during unfrozen, frozen and melted periods are 3.23x10(-3), 2.27x10(-3) and 1.92x10(-3) m, respectively. Surface roughness length demonstrates a deceasing trend with time during the winter season. Thereby, setting the roughness length to be a constant value in numerical models could lead to certain degree of simulation errors. The variation of surface roughness length may be caused by the change in land surface characteristic.展开更多
Based on preliminary theoretical analysis and numerical experiment, it is found that land surface heterogeneity plays an important role in the models turbulent flux calculation. In nearly neutral atmosphere conditions...Based on preliminary theoretical analysis and numerical experiment, it is found that land surface heterogeneity plays an important role in the models turbulent flux calculation. In nearly neutral atmosphere conditions, variation coefficient of sub-scale roughness length, cell-average roughness, and reference height are main factors affecting the calculation of grid turbulent fluxes. The first factor has a determinant role on calculation deviation. The relative error generated by roughness heterogeneity could be more than 40% in some cases in certain areas （e.g., in vegetation-climate transition belt）. Selecting a specific reference height may improve the calculation of turbulent flux. In stable or unstable atmosphere conditions, with sensible heat flux as an example, analysis shows that the discrepancy is correlated to the sub-grid distributions of mean wind velocity, potential temperature gradient between land surface and reference levels, and atmosphere stability near surface layer caused by the heterogeneity of land surface roughness. The calculation of turbulent flux is the most sensitive to stability in the above three factors. The above analysis shows that it is necessary to make a further consideration for the calculation deviation of the turbulent fluxes brought from land surface heterogeneity in the present numerical models.展开更多
Based on slow- and fast-response measurements under neutral stratification conditions from a 325-m meteorological tower located in a built-up area of north-central Beijing as well as a descriptive survey of surface ro...Based on slow- and fast-response measurements under neutral stratification conditions from a 325-m meteorological tower located in a built-up area of north-central Beijing as well as a descriptive survey of surface roughness elements (i.e., buildings and trees) around the tower site, urban roughness lengths, zo, with zero-plane displacement height are estimated using logarithmic wind profile and morphometric methods in eight 45° directional sectors. When comparing their results with each other, the slow-response method tends to give smaller zo values. At a given location, considerable directional variations in values are observed. The effect of surface roughness on urban turbulence characteristics in terms of non-dimensional standard deviations of three-component velocity, σi/u*1 (where i = u, v, w and u*1 is local friction velocity), is investigated.展开更多
According to the need of popular land surface process models, characteristics and rules of some key land surface process and soil parameters over Gobi in typical arid region of Northwest China are analyzed by using th...According to the need of popular land surface process models, characteristics and rules of some key land surface process and soil parameters over Gobi in typical arid region of Northwest China are analyzed by using the data observed during the intensive observation period of the Dunhuang Land–Surface Process Field Experiment (DLSPFE) (May–June 2000). Using the relative reflection as weighting factor, the weighted mean of the surface albedo over Dunhuang Gobi in typical arid region is calculated and its values are 0.255 ± 0.021. After canceling the interference of the buildings, the mean values of the roughness length averaged with logarithm is 0.0019 ± 0.00071 m. After removing the influence of the oasis, the soil wetness factor computed with data under condition of no precipitation is 0.0045. After removing the influence of the precipitation , the mean values of the soil heat capacity over Dunhuang Gobi in typical arid region is 1.12 × 10<SUP>6</SUP> J m<SUP>−3</SUP>K<SUP>−1</SUP>, a bit smaller than the values observed in HEIFE. But the soil heat diffusivity and conductivity are about one of those observed in HEIFE. The soil water content over Dunhuang Gobi in typical synoptic condition is very little and does not exceed 1% basically.展开更多
The parameterization of surface turbulent fluxes over the Gobi Desert in arid regions is studied by using rationally screened observational data. First, the characteristics of Monin-Obukhov similarity functions are an...The parameterization of surface turbulent fluxes over the Gobi Desert in arid regions is studied by using rationally screened observational data. First, the characteristics of Monin-Obukhov similarity functions are analyzed and their empirical formulae are fitted. The results show that fitted curves of changes of similarity functions of wind speed and temperature with stability parameter differ little from the typical empirical curves and are within the ranges of scatter of the empirical curves, but their values in the neutral condition arc different from the typical values to some extent. Furthermore, average values of momentum and scalar (sensible heat) roughness lengths as well as changes of scalar roughness length with friction velocity are determined by utilizing the data. It is found that the average values of scalar roughness length are about one order smaller than that of the momentum roughness length and decrease with increasing friction velocity, but they are evidently larger than their theoretically forecasted values.展开更多
Roughness length and zero-plane displacement over three typical surfaces were calculated iteratively by least-square method, which are Yucheng Experimental Station for agriculture surfaces, Qianyanzhou Experimental St...Roughness length and zero-plane displacement over three typical surfaces were calculated iteratively by least-square method, which are Yucheng Experimental Station for agriculture surfaces, Qianyanzhou Experimental Station for complex and undulant surfaces, and Changbai Mountains Experimental Station for forest surfaces. On the basis of roughness length dynamic, the effects of roughness length dynamic on fluxes were analyzed with SEBS model. The results indicate that, aerodynamic roughness length changes with vegetation conditions (such as vegetation height, LAI), wind speed, friction velocity and some other factors. In Yucheng and Changbai Mountains Experimental Station, aerodynamic roughness length over the fetch of flux tower changes with vegetation height and LAI obviously, that is, with the increase of LAI, roughness length increases to the peak value firstly, and then decreases. In Qianyanzhou Experimental Station, LAI changes slightly, so the relationship between roughness length and LAI is not obvious. The aerodynamic roughness length of Yucheng and Changbai Mountains Experimental Station changes slightly with wind direction, while aerodynamic roughness length of Qianyanzhou Experimental Station changes obviously with wind direction. The reason for that is the terrain in Yucheng and Changbai Mountains Experimental Station is relatively flat, while in Qianyanzhou Experimental Station the terrain is very undulant and heterogeneous. With the increase of wind speed, aerodynamic roughness length of Yucheng Experimental Station changes slightly, while it decreases obviously in Qianyanzhou Experimental Station and Changbai Mountains Experimental Station. Roughness length dynamic takes great effects on fluxes calculation, and the effects are analyzed by SEBS model. By comparing 1 day averaged roughness length in Yucheng Experimental Station and 5 day averaged roughness length of Qianyanzhou and Changbai Mountains Experimental Station with roughness length parameter chosen by the model, the effects of roughness length dynamic on flux calculation is analyzed. The maximum effect of roughness length dynamic on sensible heat flux is 2.726%, 33.802% and 18.105%, in Yucheng, Qianyanzhou, and Changbai Mountains experimental stations, respectively.展开更多
Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteor...Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteorological profile observation data were used to analyze characteristics of these parameters on the diurnal, seasonal, and annual scales. Significant seasonal and annual variations of the aerodynamic roughness length are observed over the two surfaces. A large variation of kB-1 is measured during the day. Both kB-1 and the bulk transfer coefficients exhibit significant seasonal and annual variations. During the growing season (May to October), average Cd and Ch are 3.1×10-3 and 2.5×10-3 over the degrade grassland surface, and 4.7×10-3 and 3.1×10-3 over the cropland surface respectively. During the non-growing season, average Cd and Ch are 2.3×10-3 and 2.0×10-3 over the degrade grassland surface, and 2.9×10-3 and 2.2×10-3 over the cropland surface respectively.展开更多
Aerodynamic roughness length is an important physical parameter in atmospheric numerical models and microme- teorological calculations, the accuracy of which can affect numerical model performance and the level of mic...Aerodynamic roughness length is an important physical parameter in atmospheric numerical models and microme- teorological calculations, the accuracy of which can affect numerical model performance and the level of micrometeorological computations. Many factors influence the aerodynamic roughness length, but formulas for its parameterization often only con- sider the action of a single factor. This limits their adaptive capacity and often introduces considerable errors in the estimation of land surface momentum flux （friction velocity）. In this study, based on research into the parameterization relations between aerodynamic roughness length and influencing factors such as windrow conditions, thermodynamic characteristics of the sur- face layer, natural rhythm of vegetation growth, ecological effects of interannual fluctuations of precipitation, and vegetation type, an aerodynamic roughness length parameterization scheme was established. This considers almost all the factors that af- fect aerodynamic roughness length on flat land surfaces with short vegetation. Furthermore, using many years＇ data recorded at the Semi-Arid Climate and Environment Observatory of Lanzhou University, a comparative analysis of the application of the proposed parameterization scheme and other experimental schemes was performed. It was found that the error in the friction velocity estimated by the proposed parameterization scheme was considerably less than that estimated using a constant aero- dynamic roughness length and by the other parameterization schemes. Compared with the friction velocity estimated using a constant aerodynamic roughness length, the correlation coefficient with the observed friction velocity increased from 0.752 to 0.937, and the standard deviation and deviation decreased by about 20% and 80%, respectively. Its mean value differed from the observed value by only 0.004 m s-l and the relative error was only about 1.6%, which indicates a significant decrease in the estimation error of surface-layer momentum flux. The test results show that the multifactorial universal parameterization scheme of aerodynamic roughness length for flat land surfaces with short vegetation can offer a more scientific parameteriza- tion scheme for numerical atmospheric models.展开更多
Surface aerodynamic roughness length is usually taken as a constant. In fact, it displays a remarkable dynamic change over underlying vegetation surfaces, because of the coupling of land surface roughness elements and...Surface aerodynamic roughness length is usually taken as a constant. In fact, it displays a remarkable dynamic change over underlying vegetation surfaces, because of the coupling of land surface roughness elements and windflow conditions. Current international research on this dynamic change and associated mechanisms is very limited. Using observations from different underlying surfaces (including forest, farmland and grassland) provided by a northern China coordinated observation test, the variation of aerodynamic roughness length, along with wind speed and friction velocity, is analyzed. We introduce two relationship fits, between aerodynamic roughness length and wind speed u, and dynamic variable u2/u*. Results show that aerodynamic roughness length has a clear dynamic change, and has complicated interactions with near-surface windflow. Further, the relationship fits between aerodynamic roughness length, u and u2/u*, are not only related to the roughness properties of the underlying vegetation surface (e.g. plant height), but also to plant dynamic response characteristics (e.g. flexibility). Aerodynamic roughness length decreases with increasing wind speed, because near-surface windflow conditions can change both plant roughness properties and airflow. However, the change of aerodynamic roughness length with friction velocity is complicated, and its sensitivities and transition points significantly depend on vegetation type. For underlying surfaces of forest and corn, with relatively substantial vegetative cover, roughness length correlates well with wind speed. For a surface with short vegetative cover, like natural lawn, the correlation is low. However, for all of the three vegetative surfaces, there is a close relation between roughness length and u2/u*, and their coefficients of fit from testing essentially represent the plant height and flexibility of different vegetation types. The test results also indicate that the parameterized relationships of roughness length over the underlying vegetation surface hold prospects for application.展开更多
It has previously been shown that aerodynamic roughness length changes significantly along with near- surface atmospheric thermodynamic state; however, at present, this phenomenon remains poorly understood, and very l...It has previously been shown that aerodynamic roughness length changes significantly along with near- surface atmospheric thermodynamic state; however, at present, this phenomenon remains poorly understood, and very little research concerning this topic has been conducted. In this paper, by using the data of different underlying surfaces provided by the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China, aerodynamic roughness length （z0） values in stable, neutral, and unstable atmospheric stratifications are compared with one another, and the relationship between z0 and atmospheric thermodynamic stability （（） is analyzed. It is found that z0 shows great differences among the stable, neutral, and unstable atmospheric thermodynamic states, with the difference in z0 values between the fully thermodynamic stable condition and the neutral condition reaching 60% of the mean z0. F^trthermore, for the wind speed range in which the wind data are less sensitive to z0, the surface z0 changes more significantly with （, and is highly correlated with both the Monin-Obukhov stability （4o） and the overall Richardson number （Rib）, with both of their correlation coefficients greater than 0.71 and 0.47 in the stable and unstable atmospheric stratification, respectively. The empirical relation fitted with the experimental observations is quite consistent with the Zilitinkevich theoretical relation in the stable atmosphere, but the two are quite distinct and even show opposite variation tendencies in the unstable atmosphere. In application, however, verification of the empirical fitted relations by using the experimental data finds that the fitted relation is slightly more applicable than the Zilitinkevich theoretical relation in stable atmospheric stratification, but it is much more suitable than the Zilitinkevich relation in unstable atmospheric stratification.展开更多
基金supported by the Hi-tech Research and Development Program of China(2007AA12Z175)the Special Research Foundation of the Public Benefit Industry (GYHY200706046)+1 种基金the Project funded by the National Natural Science Foundation of China (Grant No. 40671128)the National Basic Research Program of China (2007CB714401)
文摘Aerodynamic roughness length （Z0m） is a key factor in surface flux estimations with remote sensing algorithms and/or land surface models. This paper calculates ZOrn over several land surfaces, with 3 years of experimental data from Xiaotangshan. The results show that Z0m is direction-dependent, mainly due to the heterogeneity of the size and spatial distribution of the roughness elements inside the source area along different wind directions. Furthermore, a heuristic parameterization of the aerodynamic roughness length for heterogeneous surfaces is proposed. Individual Z0m over each surface component （patch） is calculated firstly with the characteristic parameters of the roughness elements （vegetation height, leaf area index, etc.）, then Z0m over the whole experimental field is aggregated, using the footprint weighting method.
基金supported by the National Natural Science Foundation of China (41275016, 41405016, 41205006, 41275014, 41375077, 91537104, and 91537106)
文摘Based on the Monin-Obulchov similarity theory, a scheme was developed to calculate surface roughness length. Surface roughness length over the eastern Qinghai-Tibetan Plateau during the winter season was then estimated using the scheme and eddy covariance measurement data. Comparisons of estimated and measured wind speeds show that the scheme is feasible to calculate surface roughness length. The estimated roughness lengths at the measurement site during unfrozen, frozen and melted periods are 3.23x10(-3), 2.27x10(-3) and 1.92x10(-3) m, respectively. Surface roughness length demonstrates a deceasing trend with time during the winter season. Thereby, setting the roughness length to be a constant value in numerical models could lead to certain degree of simulation errors. The variation of surface roughness length may be caused by the change in land surface characteristic.
基金Cooperative Project (2007DFB20210) funded by the Ministry of Science and Technology of the People's Republic of Chinathe Key Project of Basic Scientific Research +1 种基金Operation fund of Chinese Academy of Meteorological Sciences (2008Z006)the Independent Research Project of LaSW (2008LASWZI04,2009LASWZF02)
文摘Based on preliminary theoretical analysis and numerical experiment, it is found that land surface heterogeneity plays an important role in the models turbulent flux calculation. In nearly neutral atmosphere conditions, variation coefficient of sub-scale roughness length, cell-average roughness, and reference height are main factors affecting the calculation of grid turbulent fluxes. The first factor has a determinant role on calculation deviation. The relative error generated by roughness heterogeneity could be more than 40% in some cases in certain areas （e.g., in vegetation-climate transition belt）. Selecting a specific reference height may improve the calculation of turbulent flux. In stable or unstable atmosphere conditions, with sensible heat flux as an example, analysis shows that the discrepancy is correlated to the sub-grid distributions of mean wind velocity, potential temperature gradient between land surface and reference levels, and atmosphere stability near surface layer caused by the heterogeneity of land surface roughness. The calculation of turbulent flux is the most sensitive to stability in the above three factors. The above analysis shows that it is necessary to make a further consideration for the calculation deviation of the turbulent fluxes brought from land surface heterogeneity in the present numerical models.
文摘Based on slow- and fast-response measurements under neutral stratification conditions from a 325-m meteorological tower located in a built-up area of north-central Beijing as well as a descriptive survey of surface roughness elements (i.e., buildings and trees) around the tower site, urban roughness lengths, zo, with zero-plane displacement height are estimated using logarithmic wind profile and morphometric methods in eight 45° directional sectors. When comparing their results with each other, the slow-response method tends to give smaller zo values. At a given location, considerable directional variations in values are observed. The effect of surface roughness on urban turbulence characteristics in terms of non-dimensional standard deviations of three-component velocity, σi/u*1 (where i = u, v, w and u*1 is local friction velocity), is investigated.
基金This research was sponsored by the National Key Program for Developing Basic Sciences Research on the Formation Mechanism and Pr
文摘According to the need of popular land surface process models, characteristics and rules of some key land surface process and soil parameters over Gobi in typical arid region of Northwest China are analyzed by using the data observed during the intensive observation period of the Dunhuang Land–Surface Process Field Experiment (DLSPFE) (May–June 2000). Using the relative reflection as weighting factor, the weighted mean of the surface albedo over Dunhuang Gobi in typical arid region is calculated and its values are 0.255 ± 0.021. After canceling the interference of the buildings, the mean values of the roughness length averaged with logarithm is 0.0019 ± 0.00071 m. After removing the influence of the oasis, the soil wetness factor computed with data under condition of no precipitation is 0.0045. After removing the influence of the precipitation , the mean values of the soil heat capacity over Dunhuang Gobi in typical arid region is 1.12 × 10<SUP>6</SUP> J m<SUP>−3</SUP>K<SUP>−1</SUP>, a bit smaller than the values observed in HEIFE. But the soil heat diffusivity and conductivity are about one of those observed in HEIFE. The soil water content over Dunhuang Gobi in typical synoptic condition is very little and does not exceed 1% basically.
基金This work was supported by the National Natu-ral Science Foundation of China under Grant No.40175004 and the National Key Program for Developing Basic Sci-ences of China under Grant No.G1998040904-2.
文摘The parameterization of surface turbulent fluxes over the Gobi Desert in arid regions is studied by using rationally screened observational data. First, the characteristics of Monin-Obukhov similarity functions are analyzed and their empirical formulae are fitted. The results show that fitted curves of changes of similarity functions of wind speed and temperature with stability parameter differ little from the typical empirical curves and are within the ranges of scatter of the empirical curves, but their values in the neutral condition arc different from the typical values to some extent. Furthermore, average values of momentum and scalar (sensible heat) roughness lengths as well as changes of scalar roughness length with friction velocity are determined by utilizing the data. It is found that the average values of scalar roughness length are about one order smaller than that of the momentum roughness length and decrease with increasing friction velocity, but they are evidently larger than their theoretically forecasted values.
基金This work was supported the Key Project of the NSFC (Grant No. 40471099) the National Basic Research Project (Grant No.G2002CB412501) the Knowledge Innovation Project of Institute of Geo-graphic Sciences and Natural Resources Research, CAS (Grant No. KZCX1-SW-01-01A).
文摘Roughness length and zero-plane displacement over three typical surfaces were calculated iteratively by least-square method, which are Yucheng Experimental Station for agriculture surfaces, Qianyanzhou Experimental Station for complex and undulant surfaces, and Changbai Mountains Experimental Station for forest surfaces. On the basis of roughness length dynamic, the effects of roughness length dynamic on fluxes were analyzed with SEBS model. The results indicate that, aerodynamic roughness length changes with vegetation conditions (such as vegetation height, LAI), wind speed, friction velocity and some other factors. In Yucheng and Changbai Mountains Experimental Station, aerodynamic roughness length over the fetch of flux tower changes with vegetation height and LAI obviously, that is, with the increase of LAI, roughness length increases to the peak value firstly, and then decreases. In Qianyanzhou Experimental Station, LAI changes slightly, so the relationship between roughness length and LAI is not obvious. The aerodynamic roughness length of Yucheng and Changbai Mountains Experimental Station changes slightly with wind direction, while aerodynamic roughness length of Qianyanzhou Experimental Station changes obviously with wind direction. The reason for that is the terrain in Yucheng and Changbai Mountains Experimental Station is relatively flat, while in Qianyanzhou Experimental Station the terrain is very undulant and heterogeneous. With the increase of wind speed, aerodynamic roughness length of Yucheng Experimental Station changes slightly, while it decreases obviously in Qianyanzhou Experimental Station and Changbai Mountains Experimental Station. Roughness length dynamic takes great effects on fluxes calculation, and the effects are analyzed by SEBS model. By comparing 1 day averaged roughness length in Yucheng Experimental Station and 5 day averaged roughness length of Qianyanzhou and Changbai Mountains Experimental Station with roughness length parameter chosen by the model, the effects of roughness length dynamic on flux calculation is analyzed. The maximum effect of roughness length dynamic on sensible heat flux is 2.726%, 33.802% and 18.105%, in Yucheng, Qianyanzhou, and Changbai Mountains experimental stations, respectively.
基金supported by the National Basic Research Program of China (Grant Nos. 2010CB951801 and 2006CB400500)
文摘Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteorological profile observation data were used to analyze characteristics of these parameters on the diurnal, seasonal, and annual scales. Significant seasonal and annual variations of the aerodynamic roughness length are observed over the two surfaces. A large variation of kB-1 is measured during the day. Both kB-1 and the bulk transfer coefficients exhibit significant seasonal and annual variations. During the growing season (May to October), average Cd and Ch are 3.1×10-3 and 2.5×10-3 over the degrade grassland surface, and 4.7×10-3 and 3.1×10-3 over the cropland surface respectively. During the non-growing season, average Cd and Ch are 2.3×10-3 and 2.0×10-3 over the degrade grassland surface, and 2.9×10-3 and 2.2×10-3 over the cropland surface respectively.
基金supported by State Key Program of National Natural Science Foundation of China(Grant No.40830957)
文摘Aerodynamic roughness length is an important physical parameter in atmospheric numerical models and microme- teorological calculations, the accuracy of which can affect numerical model performance and the level of micrometeorological computations. Many factors influence the aerodynamic roughness length, but formulas for its parameterization often only con- sider the action of a single factor. This limits their adaptive capacity and often introduces considerable errors in the estimation of land surface momentum flux （friction velocity）. In this study, based on research into the parameterization relations between aerodynamic roughness length and influencing factors such as windrow conditions, thermodynamic characteristics of the sur- face layer, natural rhythm of vegetation growth, ecological effects of interannual fluctuations of precipitation, and vegetation type, an aerodynamic roughness length parameterization scheme was established. This considers almost all the factors that af- fect aerodynamic roughness length on flat land surfaces with short vegetation. Furthermore, using many years＇ data recorded at the Semi-Arid Climate and Environment Observatory of Lanzhou University, a comparative analysis of the application of the proposed parameterization scheme and other experimental schemes was performed. It was found that the error in the friction velocity estimated by the proposed parameterization scheme was considerably less than that estimated using a constant aero- dynamic roughness length and by the other parameterization schemes. Compared with the friction velocity estimated using a constant aerodynamic roughness length, the correlation coefficient with the observed friction velocity increased from 0.752 to 0.937, and the standard deviation and deviation decreased by about 20% and 80%, respectively. Its mean value differed from the observed value by only 0.004 m s-l and the relative error was only about 1.6%, which indicates a significant decrease in the estimation error of surface-layer momentum flux. The test results show that the multifactorial universal parameterization scheme of aerodynamic roughness length for flat land surfaces with short vegetation can offer a more scientific parameteriza- tion scheme for numerical atmospheric models.
基金supported by the Key Program of National Natural Science Foundation of China (40830957)the National Special Meteorological Scientific Research Fund in Public Welfare of China (GYHY200806021)
文摘Surface aerodynamic roughness length is usually taken as a constant. In fact, it displays a remarkable dynamic change over underlying vegetation surfaces, because of the coupling of land surface roughness elements and windflow conditions. Current international research on this dynamic change and associated mechanisms is very limited. Using observations from different underlying surfaces (including forest, farmland and grassland) provided by a northern China coordinated observation test, the variation of aerodynamic roughness length, along with wind speed and friction velocity, is analyzed. We introduce two relationship fits, between aerodynamic roughness length and wind speed u, and dynamic variable u2/u*. Results show that aerodynamic roughness length has a clear dynamic change, and has complicated interactions with near-surface windflow. Further, the relationship fits between aerodynamic roughness length, u and u2/u*, are not only related to the roughness properties of the underlying vegetation surface (e.g. plant height), but also to plant dynamic response characteristics (e.g. flexibility). Aerodynamic roughness length decreases with increasing wind speed, because near-surface windflow conditions can change both plant roughness properties and airflow. However, the change of aerodynamic roughness length with friction velocity is complicated, and its sensitivities and transition points significantly depend on vegetation type. For underlying surfaces of forest and corn, with relatively substantial vegetative cover, roughness length correlates well with wind speed. For a surface with short vegetative cover, like natural lawn, the correlation is low. However, for all of the three vegetative surfaces, there is a close relation between roughness length and u2/u*, and their coefficients of fit from testing essentially represent the plant height and flexibility of different vegetation types. The test results also indicate that the parameterized relationships of roughness length over the underlying vegetation surface hold prospects for application.
基金Supported by the National Natural Science Foundation of China (40830957, 41075008, and 41075009)National Basic Research and Development (973) Program of China (2013CB430206)China Postdoctoral Science Foundation (20110490854)
文摘It has previously been shown that aerodynamic roughness length changes significantly along with near- surface atmospheric thermodynamic state; however, at present, this phenomenon remains poorly understood, and very little research concerning this topic has been conducted. In this paper, by using the data of different underlying surfaces provided by the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China, aerodynamic roughness length （z0） values in stable, neutral, and unstable atmospheric stratifications are compared with one another, and the relationship between z0 and atmospheric thermodynamic stability （（） is analyzed. It is found that z0 shows great differences among the stable, neutral, and unstable atmospheric thermodynamic states, with the difference in z0 values between the fully thermodynamic stable condition and the neutral condition reaching 60% of the mean z0. F^trthermore, for the wind speed range in which the wind data are less sensitive to z0, the surface z0 changes more significantly with （, and is highly correlated with both the Monin-Obukhov stability （4o） and the overall Richardson number （Rib）, with both of their correlation coefficients greater than 0.71 and 0.47 in the stable and unstable atmospheric stratification, respectively. The empirical relation fitted with the experimental observations is quite consistent with the Zilitinkevich theoretical relation in the stable atmosphere, but the two are quite distinct and even show opposite variation tendencies in the unstable atmosphere. In application, however, verification of the empirical fitted relations by using the experimental data finds that the fitted relation is slightly more applicable than the Zilitinkevich theoretical relation in stable atmospheric stratification, but it is much more suitable than the Zilitinkevich relation in unstable atmospheric stratification.
