Estimation of clear-sky longwave downward radiation from HJ-1B thermal data

Estimation of clear-sky longwave downward radiation from HJ-1B thermal data

Science China Earth Sciences

To satisfy the requirement of surface energy budget research on the meso- and micro-scale, a parameterization is developed to calculate high spatial resolution, clear-sky downward longwave radiation (DLR) from HJ-1B thermal data. The DLR algorithm is established based on extensive radiative transfer simulation and statistical analysis. To address the problem that HJ-1B has a single thermal channel and lacks atmospheric information, the brightness temperature of HJ-1B and water vapor content are used in the algorithm. An accuracy evaluation and error analysis for the algorithm is conducted using a simulated radiation dataset. The result shows that the algorithm performs well in most circumstances, but there is obvious underestimation when water vapor content is greater than 4 g/cm2. Error analysis indicates the accuracy of estimated DLRs is affected by uncertainties in input parameters, including water vapor content and top-of-atmosphere radiance. It is also affected by the difference between ground and near-surface air temperature. The algorithm is applied to actual HJ-1B data, and validated by ground data from six stations in the Heihe River and Haihe River basins. The estimated DLRs have good consistency with measured data except at Huazhaizi, and root mean square errors at most sites are around 20 W/m2, which is slightly better than the result of MODIS. There is significant overestimation of DLR at Huazhaizi during summer, which is mainly produced by the large ground-air temperature difference. A correction process based on temperature difference is proposed and applied at Huazhaizi. The result shows that the positive bias is largely diminished after correction.