Cirrus clouds are high-altitude thin, wispy clouds that cover almost 20–50% of the Earth's atmosphere (Veerabudhiran,2004; Liou,1986; Lynch,1999; Stubenrauch, et al., 2013). Owing to their interaction with the incoming solar radiation, they have the potential to modulate radiation budget ( Choi and Ho, 2006; Podglajen et al., 2015; Hansen, et al., 1997; Chen, et al., 2000; Hartmann, et al., 1992; Burkhardt and Kärcher, 2011; Takano and Liou, 1995; Zeng et al., 2021; Campbell et. al., 2021) of the Earth and can influence the weather and climate (Baran et al., 2012; Muri et al., 2014; Yang et al. 2015; Kärcher, B. 2018; Lawson et al., 2019). The macro and microphysical properties of cirrus decide the cooling or warming effect (Noel et al., 2006, Stubenrauch, et al., 2010). The cirrus clouds mainly composed of ice crystal. However, occasionally they are enriched with super-cooled water droplets (Kramer et al., 2009; Jeffery et al.,1997). Cirrus cloud formation occurs at typical temperatures of less than − 20oC (Liou et al., 1986; Jensen et al., 2001; Korolev and Isaac, 2006; Koop et al., 2000) and shape and size of ice crystal within these clouds depend on the ambient temperature and relative humidity (Liou, 2002). In general, the genesis of cirrus cloud can be explained either via deep convective activity or due to nucleation of the ice crystals near the tropopause(Veerabudhiran, 2004). The typical lifetime of cirrus cloud varies from a few hours to days (Taylor et al., 2011; Meenuet al., .2011). The interaction with incoming solar radiation, which includes reflection, absorption, and transmission, depends on the coverage, position, thickness, and size distribution of ice crystals (Liou et al., 1986; Baran et al., 2009; Yang et al., 2015), which eventually determine their role in impacting the climate. In the upper altitude the optical thin cirrus clouds having a low albedo which causes positive radiative forcing and the greenhouse effect, which will warm the atmosphere. The optically thick cirrus clouds have high albedo, which produces negative radiative forcing, and the albedo effect cools the atmosphere (Jensen et al. 1996; McFarquhar et al. 2000, Fu and Liou, 1993; Fahey and Schumann, 1999). The degree and extent of greenhouse or albedo effects will lead to a significant atmospheric differential heating and cooling in vertical as well as horizontal scales and appear to have a profound influence on synoptic and regional weather systems (Liou, 1986; Forster et al., 2007; Campra et al., 2008). For well-mixed greenhouse gases, a positive radiative forcing implies global warming (Ramanathan and Collins, 1991;Hansen et al., 1997). However, cirrus induces a radiative heat source profile which tends to warm the upper troposphere but may cool the surface (Liou, 1986; Fu and Liou, 1993; Fahey and Schumann, 1999). Choi and Ho (2006), found that the longwave radiative forcing of cirrus often exceeds corresponding shortwave radiative forcing due to their low optical depth and cool ambient temperature that give rise to net surface warming of the earth's surface.
Lidar (Light Detection and Ranging)is a robust tool widely used for studies of cirrus cloud which include their identification and quantitative assessment (Saseen and Cambell, 2001;Ramaswamy, 2006; Nair et al., 2012, Das et al., 2009; Manoj et al., 2020; Lolli et al., 2020, Saha et al., 2020; 2021). The lidar can locate cirrus cloud base (from below) and cloud top with a precision which cannot be retrieved by any other technique (Veerabudhiran, 2004). It provides retrieval parameters such as normalized relative backscatter, extinction coefficient, backscatter and depolarization ratio, which are used for characterizing the cirrus cloud (Das et al., 2009; Ramaswamy, 2006; Nair et al., 2012; Manoj kumar and Venkatratnam, 2020; Manoj kumar et al., 2021). Majority of cirrus in the tropical region are sub-visual and thin (Immleretal., 2007; Seifert et al., 2007) and cover up to 60% which is mainly as a result of anvils produced directly by deep convection in the Inter-Tropical Convergence Zone (ITCZ) and due to monsoonal circulations (Wylie et al., 1994; Chenand Liu, 2005; Fortuin et al., 2007; Kox et al., 2014). Observational studies have indicated that during summer, the ITCZ continuously pumps moisture and aerosol from the lower troposphere into the upper troposphere resulting in deep convection that often leads to the formation of cirrus clouds (Veerabuthiran, 2004; Meenuet al., 2011). Das et al. (2011) have reported the formation of tropical cirrus cloud coupled with the tropical easterly jet (TEJ) during summer monsoon using the CALIOP data over the Asian Summer Monsoon region. The largest frequency of occurrence of cirrus clouds are over the tropical Indian Ocean (Sunil kumar and Parameswaran 2005; Seifert et al., 2007) as well as over the western Pacific (Heymsfield et al., 1998; Sassen et al., 2019) and Southeast Asia (He et al., 2013; Wang et al., 2020), a significant fraction of them being optically and physically thin (Meenu et al., 2011). The cirrus clouds have a broad range of optical depth, positively and negatively impacting the radiation budget. Usually, thin cirrus clouds may produce positive radiative forcing at the top of the atmosphere, whereas there can be cooling due to thick cirrus clouds (Stephens and Webster 1981; Fu and Liou 1993; Fahey and Schumann 1999).
Cirrus clouds radiative transfer is one of the important components in the Earth heat budget equations. In the event of these high cirrus clouds descending to lower altitudes, it could modify regional weather patterns( Prabhakara et al., 1997; Schumann et al., 2021). Cirrus descending with time is attributed to either gravitational settling of ice crystals within the cirrus or the downward movement of ice due to subsidence of upper tropospheric air mass (Heymsfield and Iaquinta, 2000; Mitchell et al., 2008). The triggering mechanism for the vertical movement could be due to the atmospheric waves or the general circulation. A descending pattern of cirrus is reported by Nair et al.(2012) in Trivandrum, throughout the year, where they observed the maximum descending (1.5–2.5 Km) in March-May and June-September months. The descent of cirrus can significantly modulate the moisture content and the radiation budget of the upper troposphere (Jakob, 2002; Jensen et al., 2011). The ice particle content in cirrus undergoes sedimentation during its descent, resulting in the sublimation of these particles, increasing water vapour content at higher altitudes, and impacting radiative forcing significantly (Jakob 2002; Jensen et al., 2011) subsequent to cloud formation.
The above studies have highlighted the role of cirrus clouds in the radiative balance of the earth’s atmospheric system, which usually depends on cirrus optical properties, vertical and horizontal coverage (Fu and Liou 1993; Zerefos et al., 2003; Seifert et al., 2007; Das et al., 2009). Due to limited knowledge of cirrus geometrical and optical properties, significant uncertainties still remain with respect to the radiative and climate effects of cirrus clouds ( Solomon et al., 2007; Stocker et al., 2013). Thus, observational studies can improve our present understanding of cirrus characteristics and their associated process. In this context, we monitored these clouds from July 2013 to June 2015 using a Micro-Pulse Lidar over a coastal station (Goa) located on the west coast of India. In this paper, we report their characteristics, including occurrence, descending pattern, and geometrical thickness during the study period. In addition, we will also discuss two cases of abnormal descending patterns during the summer monsoon and examine the possible mechanism responsible for such rapid descending of cirrus clouds. These results are further complemented by the radiosonde as well as CALIPSO observations at the study site.The paper is organized as follows. Section 2 provides a brief description of the study site. Section 3 describes the lidar system, other datasets used and the data analysis methods. Section 4 presents the major results and discussion followed by comclusion in section 5.