【作者】 李宏宇；

【导师】 张强；

【作者基本信息】 兰州大学 ， 干旱气象与灾害， 2012， 硕士

【摘要】 地表能量不平衡问题一直是陆面过程研究的一个重要科学难题,而陆面过程参数化对于陆面模式则有着重要的意义。利用兰州大学半干早气候与环境观测站(SACOL)2007-2009年的观测资料,在分析陇中黄土高原陆面辐射和热量收支特征的基础上,探讨了其地表能量不平衡特征及其影响机制,并计算和讨论了黄土高原自然植被下垫面陆面过程相关参数。主要结论如下：(1)通过研究不同典型天气条件对陆面过程微气象特征的影响,发现地表反照率在晴天会出现早晨偏大的不对称结构；晴天与多云的天气相比能量收支不平衡量较大,而阴天时的阵性降水会使局地能量收支出现不平衡。在半干旱区,云和降水对能量和辐射收支的影响不容忽视,可达到约25%的削弱程度,比极端干旱的敦煌荒漠区要大,进一步证明了半干旱区夏季的平均气候特征与云量较多的多云天气(5≦Mean total cloud amount<8)接近。另外,7月日平均波恩比最大是4.1,平均是1.95,比处于极端干旱区的敦煌波恩比小一个数量级,说明榆中所处的黄土高原半干旱区比极端干旱区在气候上要湿润很多。(2)计算了地表和5cm深处土壤热通量板之间的热储存,并对地表能量不平衡特征重新进行了评价。当把土壤热储存作为地表能量收支的一部分考虑后,不平衡差额绝对值平均降低了23Wm-2,能量不闭合度平均减小了0.104,利用最小二乘法(OLS)进行线性回归得到的不闭合度减小了0.085。从白天和晚上的能量不闭合度频率分布图上也能够看出,土壤热储存的考虑对地表能量平衡有很大改善。即使考虑了土壤热储存项,仍然有很明显的地表能量不闭合。(3)估算了空气热储存和光合作用储存的大小,并分别用水分守恒关系和两层土壤温度方法计算了浅层土壤水分垂直通量,考察了空气热储存、光合作用储存和水分垂直运动热量输送对地表能量平衡的影响。黄土高原区自然植被下垫面的空气热储存、光合作用储存和土壤水分垂直运动热量输送平均日变化峰值分别达到1.5、2.0和7.9Wm-2；在能量平衡方程中引入这三项后,地表能量闭合度由88.1%提高到89.6%.空气热储存、光合作用储存和水分垂直运动热量输送对于改善黄土高原地表能量不平衡状况有一定作用,研究区域的半干旱气候背景和植被状况是导致各热储存量与其他试验区存在差异的根本原因.(4)分析了总体输送系数的不同季节平均日变化和频率分布特征,考察了地面粗糙度的变化趋势以及降水对其的影响。降水正常年份的粗糙度是0.009m,偏干年份粗糙度是0.006m,月平均粗糙度变化与正常年份相比较为平缓。对总体输送系数与粗糙度以及总体理查孙数的关系分别进行了讨论,在中性层结下黄土高原区动力输送作用占主要地位,发现动量总体输送系数和奈曼流动沙丘下垫面很接近,而感热输送系数与戈壁下垫而接近。分析了反照率和太阳高度角以及土壤湿度的关系,并拟合得到以这两个物理量为因子的参数化公式。总体上,黄土高原自然植被下垫面的反照率比敦煌荒漠小,而大于长白山松林下垫面,与三个地区植被覆盖和土壤质地的不同有关。通过对参数化公式模拟效果的检验,发现低太阳高度角下的反照率对土壤湿度和太阳高度角以外的其他因素敏感,而对应高太阳高度角的反照率受土壤湿度和太阳高度角的控制较强。最后,计算了土壤热传导率和热扩散率等土壤热力参数,相同湿度的热传导率比敦煌荒漠要大并拟合得到热传导率以土壤湿度为自变量的参数化公式。更多还原

【Abstract】 The surface energy imbalance problem has been a challenge in the study of surface land process since it was found in the late1980s. Besides, physical parameters related to land-surface processes are significant to land surface process models. Based on the data collected at the Semi-Arid Climate and Environment Observatory of Lanzhou University （SACOL） from2007to2009, the paper analyzed characteristics of land surface radiation and energy budget in summer, investigated characteristics of the surface energy imbalance and mechanisms causing it and calculated physical parameters related to land-surface processes over the natural vegetation surface in the Loess Plateau of middle Gansu. Main results arc concluded as follows:（l）By studying the impacts of different weather conditions on micrometcorological characteristics, the asymmetric structure of diurnal variation of surface albedo appears on the morning of sunny days with larger albedo values. The surface energy budget on a sunny day has a larger amount of imbalance, compared with that on a cloudy day. Besides.the shower on an obscure day can also lead to large energy imbalance. The unclosure of energy balance obtained by using OLS（Ordinary Least Squares） linear regression method is19.6%.In the semi-arid area, the clouds and the precipitation cause about25%disturbances to each component of energy balance. Weakening impact of clouds and precipitation on surface energy budget is apparent and much stronger than Dunhuang Gobi. Furthermore, it shows that the mean climatic characteristics in summer relatively close to those of cloudy days with more clouds（5=Mean total cloud amount<8）. Moreover, the maximum daily average Bowen ratio in July reaches4.1and the monthly mean value is1.95. The Bowen ratio in Yuzhong is much smaller than that in Dunhuang Gobi, which reflects the semi-arid area is more humid than the extreme arid area.（2）The soil heat storage between the ground surface and the heat flux plate at5cm depth was calculated from some soil thermal parameters, and the surface energy imbalance condition was re-estimated. When the soil heat storage as an energy budget component was taken into the surface energy balance equation, the mean absolute value of surface energy balance residual decreases23W m^-2 and the mean unclosure of energy balance is reduced by0.104. Moreover, the unclosure of energy balance obtained by using OLS（Ordinary Least Squares） linear regression method decreases0.085.Overall,the storage term can improve the surface energy budget condition and makes a large contribution to the unclosurc of energy balance, which can also be seen from the frequency distributions of the surface energy balance unclosure during the day and night.However,even if we take the soil heat storage into account, the surface energy imbalance still exists and it remains apparent.（3）The paper estimates the heat storage associated with change of air temperature and humidity as well as the energy stored in plants due to the photosynthesis, determines the water vertical flux in the shadow soil layer both by water conservation principle and two-level soil temperature, and investigates impacts of air and plant photosynthesis energy storages and heat transferred by the soil water movement on the surface energy budget. It is found that the diurnal variation peaks of averaged energy storages of air and plant photosynthesis reach1.5and2.0W m^-2 respectively. Additionally, the diurnal variation peak of mean heat transferred by vertical water movement is close to8.0W^-2. The closure of energy balance is improved from88.1%to89.6%by adding the three additional energy terms to the energy balance equation. As a whole, the energy storage related to air and the plant photosynthesis, and the heat transferred by the soil water movement both promote the surface energy balance to some extent. Furthermore, the semi-arid climate and the vegetation condition of Loess Plateau essentially lead to significant differences of various energy storages among this area and other climatic districts.（4）The paper studied averaged diurnal variations and the frequency distributions of the bulk transfer coefficients in different seasons, and analyzed the trend of surface roughness as well as effects of precipitation on the roughness. Monthly averages of the roughness in a normal year for precipitation have more changes compared with those in a dryer year and increasing the precipitation tends to increase the roughness. The roughness in a normal year is9×10^-3m, while the roughness in a dryer year decreases to6×10^-3m. Relationships among the bulk transfer coefficients and the two factors including surface roughness and the Richardson number are discussed. The role of dynamical transfer in land-atmosphere energy exchange over the Loess Plateau is dominant. Additionally, the neutral bulk transfer coefficient for momentum is close to those over movable dune at Naiman in Inner Mongolia and the neutral bulk transfer coefficient for momentum is close to those over Gobi. We also analyzed effects of solar elevation angle and soil moisture on surface albedo and preliminarily a multiple factorial parameterization formulae of surface albedo are raised.Generally, the albedo over the Loess Plateau is smaller than that over the desert of Dunhuang and larger than that over pine forest in Changbai Mountain. Various vegetation covers and soil types in the three regions lead to differences of albedo.By testing the simulation results of the albedo formula, it is found that albedo with low solar elevation angles is sensitive to other factors except soil moisture and solar elevation angle, while soil moisture and solar elevation angle affect albedo with high solar elevation angles significantly.What’s more, soil thermal parameters including soil thermal conductivity and thermal diffusion are calculated. In the same soil moisture, the soil thermal conductivity larger than that in desert of Dunhuang In the end, a parameterization formula of soil thermal conductivity is gotten by fitting thermal conductivity and soil moisture.更多还原