Cirrus optical properties observed with lidar, radiosonde, and satellite over the tropical Indian Ocean during the aerosol‐polluted northeast and clean maritime southwest monsoon

Abstract

[1] Cirrus formation and geometrical and optical properties of tropical cirrus as a function of height and temperature are studied on the basis of INDOEX (Indian Ocean Experiment) lidar and radiosonde measurements and satellite observations of deep convection causing the generation of anvil cirrus. Lidar and radiosonde measurements were conducted at Hulule (4.1°N, 73.3°E), Maldives, during four field campaigns carried out in FebruaryMarch 1999 and March 2000 (northeast (NE) monsoon season, characterized by increased concentrations of anthropogenic aerosols over the Indian Ocean) and in July and October 1999 (southwest (SW) monsoon season, characterized by clean maritime conditions). As a result of a stronger impact of deep convection on cirrus formation during the SW monsoon season, cirrus clouds covered the sky over the lidar site in only 35% (NE), but 64% (SW) of the measurement time. Subvisible cirrus (optical depth 0.03), thin (optical depth from 0.03 to 0.3), and opaque cirrus (optical depth !0.3) were observed in 18%, 48%, and 34% (NE) and in 8%, 52%, and 40% (SW) out of all cirrus cases, respectively. Mean midcloud heights were rather similar with values of 12.9 ± 1.5 km (NE) and 12.7 ± 1.3 km (SW). In 25% of the cases the cirrus top height was found close to the tropopause. Mean values of the multiple-scattering-corrected cirrus optical depth, cirrus layer mean extinction coefficient, and extinction-to-backscatter ratio were 0.25 ± 0.26 (NE) and 0.34 ± 0.29 (SW), 0.12 ± 0.09 km À1 (NE) and 0.12 ± 0.10 km À1 (SW), and 33 ± 9 sr (NE) and 29 ± 11 sr (SW), respectively. A functional dependency of the extinction coefficient of the tropical cirrus on temperature is presented. All findings are compared with several other cirrus lidar observations in the tropics, subtropics, and at midlatitudes. By contrasting the cirrus optical properties of the different seasons, a potential impact of anthropogenic particles on anvil cirrus optical properties was examined. Differences in the cirrus extinction-to-backscatter ratio suggest that NE monsoon anvil cirrus originating from deep-convection cumulus clouds had more irregularly shaped and thus slightly larger ice crystals than respective SW monsoon anvil cirrus. Because the meteorological conditions were found to vary significantly between the seasons, an unambiguous identification of the influence of Asian haze on cirrus optical properties is not possible.

Optical and geometrical characteristics of cirrus clouds over a Southern European lidar station

“…This model has been compared to Eloranta's model (Eloranta, 1998) for a wide range of cloud profiles and lidar parameters and found that reproduces the high-order Eloranta calculation when the latter takes into account 5th or 6th order scattering (Hogan, 2006). This model was also used by Seifert et al (2007) to estimate the contribution of multiple scattering effects to the effective optical properties of cirrus clouds during Indian Ocean Experiment (INDOEX). A laser beam of 0.5 mrad full-angle divergence with a wavelength of 355 nm, and a receiver field of view of 1 mrad are used in the model simulation.…”

“…Cirrus climatological results from Observatoire de haute Provence in France are presented by Goldfarb et al (2001) and suggest as more prevalent optical depth in the region from 0.03 to 0.1. In addition, Seifert et al (2007) have studied the optical properties of cirrus clouds over the tropical Indian ocean during Northeast (NE) and Southwest (SW) monsoon seasons. They found a multiple-scattering corrected optical depth of 0.25±0.26 (NE) and 0.34±0.29 (SW), while the lidar ratio values were found to be 33±9 sr (NE) and 29±11 sr (SW).…”

“…Sassen and Campbell (2001) have presented a mid-latitude climatology showing that optical depth of cirrus clouds is greater than 0.3 for ∼50% of detected cases. Seifert et al (2007) have studied optical properties of tropical cirrus clouds and found 13%, 50% and 37% for subvisual, thin and opaque cirrus clouds, respectively. Many other studies have been performed and mainly focused on tropical cirrus clouds because of the high frequency of occurrence of sub-visual cirrus clouds and their significant influence on the Earth's atmospheric system (Comstock and Ackerman, 2002), as observed by SAGE (Stratospheric Aerosol and Gas Experiment) II (Wang et al, 1996;Wang et al, 1998).…”

Optical and Geometrical Properties of Cirrus Clouds over the Tibetan Plateau Measured by Lidar and Radiosonde Sounding at the Summertime in 2014

“…With the 15 increase of the height, the temperatures gradually decrease. According to the previous work (Chen et al, 2002, Sunilkumar and Parameswaran, 2005, Seifert et al, 2007, He et al, 2013, temperature has great impact on linear particle depolarization ratio and extinction coefficient by dominating the crystal shape, number concentration and other microphysics properties of cirrus. Furthermore, the cloud base and top height of the first cirrus layer are 6.02  0.60 km and 8.56  0.69 km, respectively.…”

“…In this study, for accurately correcting the MS influence, we perform a full treatment of multiple scattering following the model of Hogan (2008). The model is also introduced and applied by Seifert et al (2007), Kienast-Sjögren et al (2016) and Gouveia et al (2017). In this model, the typically operated FOV of 1.3 mrad and the full divergence angle of 0.5 mrad in WACAL is input.…”