Characteristics of Pre-summer Daytime Cloud Regimes over Coastal South China from the Himawari-8 Satellite

Abstract

Using the high spatiotemporal resolution (2 km-and-10 min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study documents the fine-scale characteristics of daytime cloud regimes (CRs) over coastal South China during the pre-summer rainy season (April–June). Six CRs (CR1–CR6) are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness, namely, the optically thin-to-moderate cloud mixture, optically thin warm clouds with cirrus, optically thick warm clouds, weak convective cloud mixture, strong convective clouds, and extreme, deep convective clouds. The optically thick warm clouds are the major CR during April and May, with higher frequencies over land, especially along the urban agglomeration, rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities. The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land, while the two CRs with optically thinner clouds occur mainly offshore.

Synoptic flow patterns (SPs) are objectively identified and examined focusing on those favoring the two major rain-producing CRs (CR5 and CR6) and the highly reflective CR with optically thick warm clouds (CR3). The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset, and a northwest high-southeast low pattern with strong dynamic convergence along the coastline, respectively. The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward, leading to a moderate moisture supply and a wide range of convective available potential energy, but also, large convective inhibition.

摘要

本研究基于高时空分辨率的葵花 8 号卫星数据分析了华南沿海 4–6 月日间云系特征. 利用云顶亮温和云光学厚度二维频数分布, 本文得到六种云系 (CR1-CR6)——薄到中等光学厚度的混合云系, 光学厚度薄的暖云和卷云, 光学厚度厚的暖云, 弱对流混合云系, 强对流云系和极端深厚对流云系. 4–5 月华南沿海前汛期日间的主要云系是光学厚度厚的暖云. 相对于近海区域来说, 它更多发生在陆地上, 尤其是城市群地区, 这可能指示了城市群地区气溶胶的作用. 弱对流和强对流云系在陆地频发, 而光学厚度较薄的两种云系更多发生在近海区域.

利用客观分型方法找出了利于主要降水云系(CR5-CR6)和强辐射反射作用的云系(CR3)发生的天气背景. 分析表明, 有利于强对流和极端深厚对流云系发展的天气背景分别是季风爆发后低空急流输送水汽型和西北高压-东南低压对峙在沿海地区增强动力辐合型; 强辐射反射作用的云系则与西伸的副热带高压型更为相关, 这种天气形势导致了华南沿海水汽输送的强度中等、 对流有效位能的值跨度较大, 但对流抑制能也较大.

Electronic Supplementary Material to

Characteristics of Pre-summer Daytime Cloud Regimes over Coastal South China from the Himawari-8 Satellite