Comment on “Measurements of erythemal irradiance near Davis Station, Antarctica: Effect of inhomogeneous surface albedo”
Comment on “Measurements of erythemal irradiance near Davis Station, Antarctica: Effect of inhomogeneous surface albedo”
复制标题
对“南极洲戴维斯站附近红斑辐照度的测量：不均匀表面反照率的影响”的评论
DOI：
10.1029/1999gl011171
复制
发表时间：
2000
影响因子：
5.2
通讯作者：
M. Degünther
中科院分区：
文献类型：
--
作者：
B. Mayer;M. Degünther
$molskaia等人[1999]（以下简称SNM 99）最近提出了关于非均匀表面散射对散射辐照度影响的实验结果。测量是在南极洲戴维斯站附近沿沿着垂直于冰/水边界的断面进行的。主要发现是在无云条件下，与海洋相比，在高度反射的雪表面上的太阳辐射最大增加了10%。沿着这些断面测得的剖面表明，在距离海岸约2公里以上的地方，太阳辐照度变化不大。考虑到水（0.05）和雪（>0.8）之间的对比度，这些测量结果与几项实验和理论研究相矛盾，这些研究通常发现增加幅度要大得多，并且随着距离海岸更远而发生变化。在这里，我们讨论了这种看似不一致的可能原因，并就如何提高这种实验研究的有用性提出了一些建议。对于0.8的典型降雪，例如Kylling et al. [1999]、McKenzie等人[1998]以及赫尔曼和McKenzie [1998]提出，与无雪条件相比，实测和模拟的太阳辐射度增加了约40%，即是SNM 99报告的增加量的四倍。此外，Degiinther等人[1998年]、Ricchiazzi等人[1998年]和Kylling等人[1999年]进行的模型研究已经解决了两种极端情况之间的过渡区域。Degfinther等人[1998年]表明，即使在距离冰/水边界20-30公里处，辐照度仍可能与其极限均匀值相差百分之几。这些数字当然取决于大气条件和辐射的波长。为了允许SNM 99的结果与建模数据的直接比较，我们提出了在戴维斯站的条件下的太阳辐照度的模拟结果。表1总结了1997年11月21日示例案例的输入数据。我们使用了由UVSPEC模型驱动的Monte Carlo辐射传递求解器[Kylling等人，2000; Mayer等人，199.虽然可能，但地形没有考虑在内，因为戴维斯站周围的地区没有高海拔或陡峭的斜坡。假定气溶胶含量低（水平能见度100公里）。冰和水的边界近似为一条直线。模型域为200 x 200 km 2，沿沿着垂直于冰/水边界的横断面每隔1 km计算辐照度。通过用CIE红斑作用光谱[McKinlay和Diffey，198]对光谱辐照度进行加权并在290-400 nm波长范围内积分来计算红斑辐照度。图1a显示了三种不同陆地覆盖率下，作为距冰/水边界距离的函数的垂直辐照度的绝对值（SNM 99报告的1997年11月21日的数值为0.84）。粗线是水面（0.05）和完全被雪覆盖的表面（0.84）的结果，由$tamnes等人[1988]的一维DISORT辐射传输解算器计算。距离海岸仅20多公里即可达到均匀极限值。当陆面温度为0.84时，均匀值（冰/水）之比为1.44，与SNM 99用同一UVSPEC模式计算的均匀值（冰/水）之比（1.42）非常接近。图lb和lc显示了与SNM 99图3a的实验数据相比的归一化结果。由于数据对测量日期没有明显的依赖性，因此所有数据点都在这里绘制，没有进一步区分。乍一看，所有的测量都很好地模拟了，假设陆地的平均速度为0.6。实测断面表明，较之模拟，实验数据更早达到水平。然而，为了正确解释这些数据，必须考虑指定的实验不确定性和少量数据点（见SNM 99图3a中的误差条）。造成这种差异的一个可能原因是海岸线偏离了直线。由于SNM 99图3a中的结果似乎与测量位置无关，因此与冰/水边界的确切形式无关，但这一点可能影响不大。也可以排除非朗伯雪反射作为差异的原因，因为Degiinther和MeerkStter [2000]表明，使用雪的真实双向反射函数只会对下降流irra引入小于1%的小校正。
$molskaia et al. [1999] (hereafter SNM99) recently presented experimental results concerning the effect of inhomogeneous surface albedo on erythemal irradiance. Measurements were taken near Davis Station, Antarctica, along transects perpendicular to the ice/water boundary. The main findings were a maximum increase of the erythemal irradiance of 10% over the highly reflecting snow surface compared to the ocean under cloudless conditions. The profiles measured along these transects suggested that the erythemal irradiance did not change significantly for distances of more than about 2 km from the coast. Taking into account the albedo contrast between water (0.05) and snow (>0.8) these measurements contradict several experimental and theoretical studies who generally found much larger increases, and changes over longer distances from the coast. Here we discuss possible causes for this seeming discrepancy and give some suggestions on how to improve the usefulness of such experimental studies. For typical snow albedos of 0.8, e.g. Kylling et aI. [1999], McKenzie et al. [1998], and Herman and McKenzie [1998] presented measured and simulated increases of erythemal irradiance of about 40% compared to snow-free conditions, that is, four times the increase reported by SNM99. Furthermore, model studies by Degiinther et al. [1998], Ricchiazzi et al. [1998], and Kylling et al. [1999] have addressed the transition region between the two extreme cases. Degfinther et al. [1998] showed that even at 20-30 km distance from the ice/water boundary, the irradiance might still differ from its limiting homogeneous value by a few percent. These numbers of course depend on the atmospheric conditions and on the wavelength of the radiation. To allow a direct comparison of the results of SNM99 with modeled data, we present the results of a simulation of erythemal irradiance for the conditions at Davis Station. The input data for the example case, 21 November 1997, are summarized in Table 1. We used a Monte Carlo radiative transfer solver, driven by the UVSPEC model [Kylling et al., 2000; Mayer et al., 199•. Although possible, topography was not taken into account because the region around Davis Station does not have high elevations or steep slopes. Low aerosol was assumed (horizontal visibility 100 km). The boundary between ice and water was approximated by a straight line. The model domain was 200 x 200 km 2 and the irradiance was calculated every 1 km along a transect perpendicular to the ice/water boundary. The erythemal irradiance was calculated by weighting the spectral irradiance with the CIE erythema action spectrum [McKinlay and Diffey, 198• and integrating over the wavelength range 290-400 nm. Absolute values of the erythemal irradiance as a function of the distance from the ice/water boundary are shown in Figure la, for three different land albedos (a value of 0.84 is reported by SNM99 for November 21, 1997). The thick lines are the results for a water surface (albedo 0.05) and a completely snow covered surface (albedo 0.84), calculated by the one-dimensional DISORT radiative transfer solver by $tamnes et al. [1988]. The limiting homogeneous value is reached only more than 20 km away from the coast. The ratio of the homogeneous values (ice/water) of 1.44 for a land albedo of 0.84 is very close to that calculated by SNM99 (1.42) using the same UVSPEC model. Figures lb and lc show the normalized results in comparison with the experimental data from Figure 3a of SNM99. As there is no obvious dependence of the data on the measurement date, all data points are plotted here without further distinction. At first glance, all measurements are quite well modeled, assuming a land albedo of 0.6. The measured transects suggest an earlier leveling of the experimental data compared to the simulations. To correctly interpret these data, however, the specified experimental uncertainties and the small number of data points has to be taken into account (see error bars in Figure 3a of SNM99). A possible reason for the discrepancy is the deviation of the coast from a straight line. As the results in Figure 3a of SNM99 seem to be independent of the measurement location and thus of the exact form of the ice/water boundary, this point, however, is probably of little influence. A non-Lambertian snow albedo can also be ruled out as a cause for the differences as Degiinther and MeerkStter [2000] showed that the use of realistic bidirectional reflectance functions of snow would only introduce a small correction of less than 1% for the down-welling irra-