Criteria for the onset and breakup of summer lake stratification based on routine temperature measurements
Criteria for the onset and breakup of summer lake stratification based on routine temperature measurements
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DOI：
10.1127/1863-9135/2014/0582
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发表时间：
2014-06
影响因子：
1
通讯作者：
C. Engelhardt;G. Kirillin
中科院分区：
文献类型：
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作者：
C. Engelhardt;G. Kirillin
温带湖泊夏季分层的强度和持续时间的增加是许多湖泊研究中报告的气候变化的一个良好描述的哨兵。早春热层结的开始和秋季翻转过程中热层结的破裂的定义通常是主观选择的二元阈值。然而，混合和停滞之间的过渡阶段（夏季层化的开始）和停滞和混合之间的过渡阶段（夏季层化的结束）是连续的过程。因此，分层期的时间限制取决于定义，这使得不同研究结果的比较复杂化。夏季层化持续时间的实验测定需要统一的标准来决定层化的建立和终止。我们表明，分层期（秋季混合开始）的终止可以检测到足够的精度从近底部温度记录在湖的最深点。然而，在这一时刻之后，整个水柱完全混合需要几天时间，分层期结束需要一个稳定阈值。为了定义分层期的开始，之前建议了不同稳定性指数的阈值。虽然阈值的选择影响每年的估计持续时间的分层，它似乎没有显着影响分层的长期趋势。因此，比选择稳定性阈值更重要的是用于分析的观测数据的质量。使用测量数据，从两个二分湖在北方德国，我们表明，分层长度的估计强烈依赖于时间分辨率的测量。对于旨在通过实验验证未来温带湖泊分层变化的比较研究，我们建议（i）使用高分辨率水温探测器的数据（完全丢弃每周、每两周或更稀疏采样周期的数据集），（ii）使用一致的稳定性阈值（例如施密特稳定性的固定值）来均匀地导出分层周期的持续时间，以及（iii）在大型相互比较研究中应用标准化算法来确定分层指数。
The increase in strength and duration of summer stratification in temperate lakes is a well-described sentinel of climatic change reported in numerous lake studies. The definitions of the onset of thermal stratification in early spring and its breakup during the autumn overturn are usually binary thresholds chosen on a subjective basis. However, the transition phases between mixing and stagnation (start of summer stratification) and between stagnation and mixing (end of summer stratification) are continuous processes. Therefore, the time limits of the stratified period are definition-dependent, which complicates comparison of results from different studies. An experimental determination of the duration of summer stratification needs unified criteria to decide when stratification is established and when it terminates. We demonstrate that the termination of the stratified period (start of autumn mixing) can be detected with sufficient accuracy from near bottom temperature records at the deepest point of the lake. Nevertheless, it takes days after this moment until the whole water column is mixed completely and a stability threshold is required for the end of the stratified period. To define the beginning of the stratified period, thresholds for different stability indices were suggested previously. Although the threshold choice influences the estimated duration of stratification each year, it appears not to significantly affect the long-term trends of stratification. More important than the selection of stability thresholds is therefore the quality of observational data used for analysis. Using measured data from two dimictic Lakes in Northern Germany we show that the estimation of the stratification length depends strongly on the temporal resolution of the measurements. For comparative studies which aim to verify a shift in future temperate lake stratification experimentally, we suggest (i) to use data from high-resolution water temperature probes (discarding completely data sets with weekly, biweekly or sparser sampling period), (ii) to use a consistent stability threshold (e.g. a fixed value of Schmidt stability) to derive the duration of the stratified period uniformly, and (iii) to apply standardized algorithms in large intercomparison studies to determine indices of stratification.