Lidar observations of equatorial cirrus clouds at Mahé Seychelles

Abstract

[1] In February-March 1999, nocturnal observations with a ground-based backscatter/ depolarization lidar were conducted during the Airborne Platform for Earth ObservationThird European Stratospheric Experiment on Ozone (APE-THESEO) campaign at Mahé, Seychelles (4.4°S, 55.3°E). Upper tropospheric cirrus clouds were a common feature in the lidar echoes; they were detected for about 67 hours out of a total measurement time of 125 hours. Optical and geometrical characteristics of the clouds detected at altitudes above 9 km throughout the campaign are derived and discussed. Cirri above 14.5 km present a layered structure and appear more persistent and thinner than cirri at lower altitudes. The behavior of average optical and geometrical depths, integrated depolarization, backscatter-to-extinction ratio, integrated backscatter, and extinction coefficient is studied as a function of midcloud temperature and is compared with results from previous measurements and models. The estimated cloud backscatter-to-extinction ratio ranges from 0.01 to 0.2 sr À1 , with an average value of 0.051 ± 0.030 sr À1 . In the temperature range between 200 and 240 K the integrated depolarization ranges between 0.27 and 0.19 and appears lower than that for midlatitude cirri. In a single night, very high values of the integrated depolarization for a layered cirrus at 16 km could be related to outflow from an active convective cell. The two highest and coldest (temperature around 188 K) layers show values of optical and geometrical depth and integrated depolarization lower than for cirri at temperatures between 190 and 200 K. The optical characteristics of the clouds decrease for temperatures from 190 to 200 K, possibly indicating different evolution phases of the clouds.

Interdependence of tropical cirrus properties and their variability

“…8b, the standard deviations are significantly large. A similar relation between cloud optical depth and geometrical depth (physical depth) has been reported by various other investigators for tropical cirrus (Pace et al, 2003;Thomas et al, 2002) and mid-latitude cirrus (Sassen and Cho, 1992).…”

“…In this figure corresponding parameters are plotted with a time resolution of 250 s for 80 nights, in which continuous observations were available for more than 4 h. This plot shows that for low values of τ c (<0.01) the asymmetry is less than 2% and increases nonlinearly with an increase in τ c , indicating that most of the sub-visual cirrus are symmetric. Studies conducted at Mahe Island (4.4 • S, 55.3 • E) also showed (Pace et al, 2003) that in most of the cases the observed cirrus is symmetric. They attributed the low values of ξ mainly to ice precipitation, leading to the accumulation of ice mass near the cloud base.…”

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

“…Recent studies have focused on the western Pacific warm pool [ Platt et al , 1998, 2002; Heymsfield et al , 1998; Sassen et al , 2000; Comstock et al , 2002]. Other lidar statistics of cirrus cloud properties in the tropics are available only for the Seychelles [ Pace et al , 2003] and eastern India [ Sunilkumar and Parameswaran , 2005].…”

A comparison of light backscattering and particle size distribution measurements in tropical cirrus clouds

“…High tropical cirrus were first reported by Uthe and Russell (1976) who observed cirrus clouds between 12 and 18 km, from the ground lidar station at Kwajalein (8.7 • N, 167.7 • E). Since then, a molteplicity of lidar studies characterized cirrus morphology, often in conjunction with other remote sensing instruments as infrared radiometers or millimeter radars (Platt et al, 1998;Comstock and Sassen, 2001;Wang et al, 2001;Pace et al, 2003;Immler et al, 2007).…”