Assessment of FY-4A and Himawari-8 Cloud Top Height Retrieval through Comparison with Ground-Based Millimeter Radar at Sites in Tibet and Beijing

Abstract

The accuracy of passive satellite cloud top height (CTH) retrieval shows regional dependence. This paper assesses the CTH derived from the FY-4A and Himawari-8 satellites through comparison with those from the ground-based millimeter radar at two sites: Yangbajing, Tibet, China (YBJ), and the Institute of Atmospheric Physics (IAP), Beijing, China. The comparison shows that Himawari-8 missed more CTHs at night than FY-4A, especially at YBJ. It is found that the CTH difference (CTHD; radar CTH minus satellite CTH) for FY-4A and Himawari-8 is 0.06 ± 1.90 km and −0.02 ± 2.40 km at YBJ respectively, and that is 0.93 ± 2.24 km and 0.99 ± 2.37 km at IAP respectively. The discrepancy between the satellites and radar at IAP is larger than that at YBJ. Both satellites show better performance for mid-level and low-level clouds than for high-level clouds at the two sites. The retrievals from FY-4A agree well with those from Himawari-8, with a mean difference of 0.08 km at YBJ and 0.06 km at IAP. It is found that the CTHD decreases as the cloud depth increases at both sites. However, the CTHD has no obvious dependence on cloud layers and fractions. Investigations show that aerosol concentration has little impact on the CTHD. For high and thin clouds, the CTHD increases gradually with the increase of the surface temperature, which might be a key factor causing the regional discrepancy between IAP and YBJ.

摘要

由于仪器性能、黑体云的理论假设、辐射传输模型计算的固有误差、数值模型计算的温度廓线与实际不一致以及下垫面的复杂性等原因,卫星红外遥感反演云顶高度(CTH)总是存在误差。而更不满足黑体云假设的高云和薄云所占的比例在不同地区有所不同;此外,影响地面和大气辐射的下垫面特征和大气特性在不同地区也有所不同,从而可能导致不同地区的卫星CTH反演误差不同。即卫星CTH反演的精度具有地区依赖性。本文通过与西藏羊八井(YBJ)和北京大气物理研究所(IAP)两个站点的地面毫米波雷达的比较,对FY-4A和Himawari-8两卫星的CTH反演进行了评估。结果表明,Himawari-8在夜间比FY-4A缺测了更多的CTH,尤其是在YBJ。FY-4A和Himawari-8的CTHD(CTHD:雷达CTH减去卫星CTH)在YBJ分别为0.06±1.9 km和-0.02±2.4 km,在IAP分别为0.93±2.24 km和0.99±2.37 km。IAP处卫星与雷达之间的差异比YBJ的大。这两颗卫星在两个站点对中云和低云的反演效果均好于对高云的反演。FY-4A和Himawari-8的反演结果吻合较好,两者之间的平均误差在YBJ和IAP分别为0.08 km和0.06 km。结果表明,两个站点的CTHD均随云厚度的增加而减小。研究时段内CTHD对云层数和云分数没有表现出明显的依赖性。研究表明,气溶胶浓度对CTHD的影响不大。对于高薄云,CTHD随着地表温度的升高而逐渐增大,这可能是造成IAP和YBJ地区差异的一个关键因素。