Global Distribution Of Par As Observed From Satellites

I. Laszlo, R. Pinker
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引用次数: 1

Abstract

Information on the Photosynthetically Active Radiation (PAR) is needed for estimating Net Primary Productivity (NPP) over land and in oceans. The diurnal cycle of PAR is also of interest because the response of most ecosystems to the intensity of PAR is non-linear. On local scales, information on PAR is obtained from measurements in the 0.4-0.7 pm spectral interval; in absence af such measurements, PAR is estimated from total solar irradiance using predetermined "conversion constants". We have developed a spectral model for inferring surface solar irradiance from satellite observations [i] and [2]. The estimation of the surface solar irradiance is obtained by couplin satellite observed visible radiances at the top of the atmosphere with a radiative transfer model which allows to account for the extinction processes in the atmosphere. The absorption by ozone, water vapor and the extinction by molecules, aerosols and clouds are parameterized. Values of ozone amount , surf ace pressure , temperature , precipitable water and aerosol characteristics can be obtained from observations or from climatology. The boundary conditions in the model are represented by a global model of surface albedo which is wavelength and solar zenith angle dependent. The spectral intervals of the model are such that it is possible to compute the PAR (Fig. 1). The solar model was implemented with global satellite data that are now becoming available under the International Satellite Cloud Climatology Project (ISCCP) , to obtain the global distribution of PAR and of the 'conversion constants'. The ISCCP data are based on observations taken every three hours. Therefore, it is possible to study the diurnal cycle of PAR. These results and implications for the estimation of primary productivity will be discussed.
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卫星观测到的Par的全球分布
估算陆地和海洋的净初级生产力需要光合有效辐射(PAR)的信息。PAR的日循环也很有趣,因为大多数生态系统对PAR强度的响应是非线性的。在局部尺度上,PAR的信息是从0.4-0.7 pm光谱区间的测量中获得的;在没有这种测量的情况下,PAR是使用预定的“转换常数”从太阳总辐照度估计出来的。我们已经开发了一个光谱模型,用于从卫星观测推断地表太阳辐照度[1]和[2]。地表太阳辐照度的估计是通过couplin卫星观测到的大气顶部的可见辐射,并采用考虑大气中消光过程的辐射传输模型得到的。对臭氧、水蒸气的吸收和分子、气溶胶和云的消光进行了参数化。臭氧量、表面压力、温度、可降水量和气溶胶特征的数值可从观测或气候学中获得。模型中的边界条件由一个与波长和太阳天顶角有关的全球表面反照率模型来表示。该模式的光谱间隔使得计算PAR成为可能(图1)。太阳模式是利用国际卫星云气候学项目(ISCCP)提供的全球卫星数据来实现的,以获得PAR的全球分布和“转换常数”。ISCCP的数据是基于每三小时进行一次的观测。因此,研究PAR的日循环是可能的。本文将讨论这些结果及其对初级生产力估计的影响。
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