Hydrometeorological behaviour of pine and larch forests in eastern Siberia

Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (R_n ? G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded R_n ? G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day^?1. The contributions of understory evapotranspiration (E_u) and overstory transpiration (E_o) to the evapotranspiration of the entire ecosystem (E_t) were both from 25 to 50% throughout the period analysed. These results suggest that E_u plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ?26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.     

Review of the Researches on Changma and Future Observational Study(KORMEX)

ＲｅｖｉｅｗｏｆｔｈｅＲｅｓｅａｒｃｈｅｓｏｎＣｈａｎｇｍａａｎｄＦｕｔｕｒｅＯｂｓｅｒｖａｔｉｏｎａｌＳｔｕｄｙ（ＫＯＲＭＥＸ）Ｊａｉ－ＨｏＯｈ，Ｗｏｎ－ＴａｅＫｗｏｎａｎｄＳａｎｇ－ＢｏｍＲｙｏＭＥＴＲＩ，ＫｏｒｅａＭｅｔｅｏｒｏｌｏｇｉｃａｌＡ...     

青藏高原复杂地表能量通量研究

“全球能量水循环之亚洲季风青藏高原试验研究”（GAME/Tibet）和“全球协调加强观测计划（CEOP）亚澳季风之青藏高原试验研究”（CAMP/Tibet）的加强期观测和长期观测已经进行了9年多,并且已取得了大量的珍贵资料。首先介绍了GAME/Tibet 和CAMP/Tibet 试验的情况,并利用观测资料给出了局地能量分布（日变化和月际变化）特征。复杂地表区域能量通量研究是青藏高原地气相互作用研究中的重中之重。卫星遥感的应用成为解决这一问题,即实现GAME/Tibet和CAMP/Tibet试验主要初衷的必不可少的手段。利用卫星遥感观测（Landsat 7 ETM）资料结合地面观测的方法,计算得到了相关地区非均匀地表区域上的地表温度、地表反射率、标准化差值植被指数(NDVI)、校准的调整土壤植被指数（MSAVI）、植被覆盖度和叶面指数(LAI)及能量平衡各分量（净辐射通量、土壤热通量、感热和潜热通量）的分布图像,所得结果基本可信。为了得到整个青藏高原复杂地表的热通量分布,中国科学院青藏高原研究所正在与其他研究单位一起建立青藏高原地表和大气过程监测系统（MORP）。最后介绍了该监测计划和已建立的3个综合观测研究站及如何利用建立的台站把站点观测的热通量推广到整个青藏高原的途径。     

考虑冻土的陆面过程模型及其在青藏高原GAME/Tibet试验中的应用

陆面过程的研究对于更好地认识气候和天气系统的演变规律、陆地-大气水热交换过程、人类活动对气候和环境的影响等具有重要意义. 建立了综合考虑土壤冻融、土壤水汽通量、植被覆盖和陆面-大气近地层水热交换的一维冻土-植被-大气连续体模型, 模拟了固液相变、汽态水迁移、土壤水、汽、热耦合迁移等过程, 反映了液态水从未冻区向冻结区迁移、冻结及其引起的潜热迁移的冻土物理本质, 也反映了汽态水分从高温区向低温区迁移所引起的温度及水分场的变化, 并对模型进行了检验. 水分运动方程采用混合Richards方程, 可适应各种边界条件. 土壤水热传输模型求解引入了修正的Picard迭代法, 不仅使计算迭代收敛更快, 而且能更好地保证数值计算过程中的水量平衡. 结合GAME/Tibet实验1998年5月份、7月份的观测数据, 应用该模型对青藏高原安多观测点的水热交换过程进行了模拟分析. 模拟结果表明: 土壤的冻融过程对地温变化会产生负反馈作用; 若净辐射相同, 土壤表层含水量较高的情况下考虑冻结时其地热通量在冰融化时明显增加, 显热通量减少, 而潜热通量变化不大, 但是冻结时各通量的变化不明显; 而土壤发生融化时, 尽管地热通量增加, 但是地表温度仍然减小; 土壤发生冻结时, 尽管土壤负温要比不考虑冻结时高, 但整体上热通量变化不大.     

A land surface model incorporated with soil freeze/thaw and its application in GAME/Tibet

Land surface process is of great importance in global climate change, moisture and heat exchange in the interface of the earth and atmosphere, human impacts on the environment and eco- system, etc. Soil freeze/thaw plays an important role in cold land surface processes. In this work the diurnal freeze/thaw effects on energy partition in the context of GAME/Tibet are studied. A sophisti- cated land surface model is developed, the particular aspect of which is its physical consideration of soil freeze/thaw and vapor flux. The simultaneous water and heat transfer soil sub-model not only reflects the water flow from unfrozen zone to frozen fringe in freezing/thawing soil, but also demon- strates the change of moisture and temperature field induced by vapor flux from high temperature zone to low temperature zone, which makes the model applicable for various circumstances. The modified Picard numerical method is employed to help with the water balance and convergence of the numerical scheme. Finally, the model is applied to analyze the diurnal energy and water cycle char- acteristics over the Tibetan Plateau using the Game/Tibet datasets observed in May and July of 1998. Heat and energy transfer simulation shows that: (i) There exists a negative feedback mechanism between soil freeze/thaw and soil temperature/ground heat flux; (ii) during freezing period all three heat fluxes do not vary apparently, in spite of the fact that the negative soil temperature is higher than that not considering soil freeze; (iii) during thawing period, ground heat flux increases, and sensible heat flux decreases, but latent heat flux does not change much; and (iv) during freezing period, soil temperature decreases, though ground heat flux increases.