科研之友
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摘要
Three kinds of "visible" cloud optical thickness tau-matching shortwave direct, global, and scattering solar irradiances (I-ds, I-gs and I-ss)-are defined, which are marked as tau(d), tau(g), and tau(s), respectively. It is found from radiation calculations that a ratio of I-ss to I-gs in the small-tau case has a unique characteristic: strong sensitivity to tau but weak sensitivity to the cloud scattering phase function. On the basis of this characteristic, a method to retrieve I-ss-equivalent tau(s) from the ratio is proposed. This method is validated by way of simulation and application tests, in which the Discrete Ordinate Radiative Transfer model (DISORT) is used to calculate irradiances. As shown in simulations with tau < 2, there may be unrealistically negative or grossly overestimated tau values from I-gs, owing to the difference between tau(s) and tau(d), while the new method can lead to a very good agreement of tau(s) retrieval with its input. Furthermore, this method is used to retrieve small tau from the pyrheliometer and pyranometer measurements in Lhasa during 2006. It is found that tau retrieved from I-gs was often negative because of cloud inhomogeneity, while the application of the new method resulted in stable yet reasonable tau(s) values. The I-ss calculations using 1293 sets of tau(s) retrievals fit well into the I-ss determinations from pyrheliometer and pyranometer measurements with an annual-mean deviation of 0.18%, but the deviation was raised to 46.4% when using tau(g) retrievals.
- 出版日期2014-4
- 单位中国科学院大气物理研究所
