青藏高原典型湖泊冰面反照率的参数化及其对水热交换的影响研究

The highest and largest number of lakes on the earth are located in the Tibetan Plateau, which have a long freezing period. Ice albedo can profoundly affect the freezing and thawing process of lake ice, and further affects the climate effect of lakes. The preliminary study shows that the ice surface albedos over some lakes in the Tibetan Plateau are much lower than those derived from the other regional lakes and the lake models. However, it is difficult to conduct the further study due to a lack of observation data, which hampers the in-depth understanding of climate effect of the frozen lake. Based on the existing data, the ice albedo observation over the lake will be carried out in this project. Using remote sensing products and the ice surface observation data, we will analyze the characteristics of ice surface albedo and the heat- moisture exchange over the Tibetan Plateau lakes, and quantify the percentage of annual heat- moisture exchange quantity taken by the frozen lake. On the basis of analysis of the observation fact, the temperature-dependent albedo parameterization scheme of lake ice in the existing models will be improved, and an independent snow albedo scheme over the ice surface also will be introduced, making it more suitable for lake simulation in the Tibetan Plateau. Finally, the effects of the ice albedo and its parameterization improvement on heat- moisture exchange of frozen lake will be revealed by sensitivity tests using the lake model. This study promises to establish a more appropriate model for the lake freezing and thawing process in the Tibetan Plateau, and shed light on the interaction between atmosphere and high-altitude lakes at the annual scale.

青藏高原分布着地球上海拔最高、数量最多的湖泊群，并且具有较长的冻结期。湖泊冰面反照率深刻影响着湖冰的冻融过程，进而影响湖泊的气候效应。初步研究发现高原湖泊的冰面反照率明显低于模式预报值和其他地区湖泊，但由于观测匮乏而难以深入研究，制约着对冻结期湖泊气候效应的全面认识。本项目在已有研究基础上开展湖泊冰面反照率补充观测，利用遥感产品和冰面观测数据，分析冻结期青藏高原典型湖泊冰面反照率和水热交换特征，定量评价湖冰对年尺度湖面水热交换的影响；改进现有湖泊模式中的温度依赖型冰面反照率方案，引入独立的冰面积雪反照率方案，使之适合于青藏高原湖泊模拟；通过敏感性试验，揭示冰面反照率及其参数化改进对湖泊水热交换过程模拟的影响。本研究有助于建立适用于高原湖泊冻融过程的数值模式，能够加深对全年尺度高寒湖泊与大气相互作用的认识。

青藏高原分布着地球上海拔最高的高寒湖泊群，具有较长的冻结期。湖冰的存在改变了湖泊—大气的能量水分交换，冰/雪-反照率反馈机制使得大多数寒区湖泊对于气候变化的响应非常敏感，尤其是在太阳辐射十分剧烈的青藏高原。由于观测匮乏，青藏高原的湖冰反照率及其对水热交换的影响程度至今未被定量认识。本项目在黄河源区的鄂陵湖开展了多尺度的冰面能量与辐射平衡观测，结合遥感产品，揭示了高原典型湖泊的冰面反照率和冻结期水热交换特征；评估和优化了现有的冰面反照率参数化方案；基于数值模拟，量化了冰面反照率等湖表关键参数对湖泊温度和湖面水热交换的影响。主要结果如下：（1）观测发现，小于0.12和0.16的冰面反照率分别占观测样本量的72.21%和93.38%，洁净的鄂陵湖冰面(0.6m厚)正午时段反照率仅为0.075，明显低于以往报道的湖冰反照率；（2）MOD10A1/MYD10A1等逐日冰雪反照率产品能够较好的表征冰面反照率的快速变化；无积雪时，遥感资料得到的青藏高原典型湖泊的冰面反照率大多小于0.20，即使包含了有积雪时的整个冻结期，反照率大多也小于0.4，反照率与湖冰厚度的相关性较弱；（3）目前被广泛使用的冰面温度依赖型反照率参数化方案在高原湖泊并不适用，各类冰面反照率参数化方案大多严重高估了高原湖冰的反照率，偏高0.2–0.6左右，可导致模拟的湖泊全年蒸发量减少8%–11%；（4）采用观测的反照率能较好地模拟秋末冬初和春末夏初的湖水翻转混合现象，对春季湖水增温和分层的模拟也较好；冰面反照率对湖表温度的影响主要体现在冻结期至六月，不同年份有所差异；与观测相比，耦合模式模拟的湖泊冻结开始时间均偏早，动量粗糙度扩大后湍流通量交换增强，湖面温度下降更快，结冰期提前。本研究有助于建立适用于高原湖泊冻融过程的数值模式，能够加深对全年尺度高寒湖泊与大气相互作用的认识。