Cirrus (Ci) clouds have an important influence on Earth's radiation budget, and they remain one of the most significant uncertainties in predicting Earth's climate. In this study, we use ground-based radiometers along with a sky imager to monitor clouds and retrieve cloud properties (cloud fraction (CF), cloud optical depth (COD) and effective radii (Re)) over a rain shadow region in south peninsular India during September and October months, 2011. Lower part of Ci clouds are identified using the thresholds pertaining to COD and cloud base height (CBH). The optical and radiative properties of Ci clouds showed large variability on temporal and diurnal scales. The CF, COD and R_e varied from 7 to 100%, 0.76 to 9.99, and 2.76 to 37.92 μm, respectively. The CBH and Cloud Base Temperature (CBT) are found to vary from 7.24 to 8.99 km and − 31.99 to − 13.75 °C. The Shortwave Cloud Radiative forcing (SWCRF) exerted by the lower part of Ci clouds over the region is observed to vary from − 435 to 148.87 W m⁻² on a temporal scale with an average value of − 23.06 W m⁻². The relationship between SWCRF and COD revealed radiative cooling effect with increase in COD with a dependency rate of − 18.53 W m⁻²/τ. SWCRF is found to be more sensitive to COD as compared to other cloud characteristics (CF, CBH and CBT). The case studies depict that the observed lower part of Ci clouds are advected from the ocean indicating the influence of large scale systems. Lower part of Ci optical and radiative properties showed wide variability depending up on the source of formation and evolution. This study also suggests that the high temporal variability of optical and radiative properties of Ci clouds needs to be well considered in climate models to reduce the uncertainty of cirrus radiative effects.

卷云 (Ci) 对地球的辐射预算具有重要影响，并且它们仍然是预测地球气候的最重要的不确定因素之一。在这项研究中，我们使用地基辐射计和天空成像仪来监测南半岛雨影区域的云并检索云特性（云分数（CF）、云光学深度（COD）和有效半径（Re））印度，2011 年 9 月和 10 月。Ci 云的下部是使用与 COD 和云底高度 (CBH) 相关的阈值来识别的。 Ci 云的光学和辐射特性在时间和昼夜尺度上表现出很大的变化。 CF、COD 和 Re 的_变化范围分别为 7 至 100%、0.76 至 9.99 和 2.76 至 37.92 μm。 CBH 和云底温度 (CBT) 的变化范围为 7.24 至 8.99 km 和 − 31.99 至 − 13.75 °C。据观察，该区域上空 Ci 云下部施加的短波云辐射强迫（SWCRF）在时间尺度上变化范围为 - 435 至 148.87 W m ^-2，平均值为 - 23.06 W m ^-2。 SWCRF 和 COD 之间的关系揭示了辐射冷却效应随 COD 的增加而变化，依赖率为 − 18.53 W m ⁻² /τ。与其他云特征（CF、CBH 和 CBT）相比，SWCRF 对 COD 更敏感。案例研究描述了观测到的 Ci 云的下部是从海洋平流而来，表明了大尺度系统的影响。 Ci 光学和辐射特性的下部显示出广泛的变化，具体取决于形成和演化的来源。这项研究还表明，在气候模型中需要充分考虑 Ci 云光学和辐射特性的高时间变化，以减少卷云辐射效应的不确定性。