A Tropical 'NAT-like' belt observed from space

A tropical «  NAT-like  » belt observed from space  Chepfer, H.1 and Noel, V.2 1 Laboratoire de Meteorologie Dynamique / IPSL, UPMC. 2 Laboratoire de Meterologie Dynamique / IPSL, CNRS. Proposed for publication in Geophysical Research Letters Corresponding author:! H. Chepfer, ! ! ! ! Laboratoire de Meterologie Dynamique ! ! ! ! Ecole Polytechnique ! ! ! ! 91128 Palaiseau ! ! ! ! France ! ! ! ! helene.chepfer@lmd.polytechnique.fr 1 Abstract. The optical properties of cold tropical tropopause clouds are examined on a global scale, using two years of space-borne lidar observations from CALIPSO (June 2006 - May 2008). The linear depolarization ratio, color ratio and backscatter signal are analyzed in tropical clouds colder than 200 K in a way similar to recent studies of Polar Stratospheric Clouds (PSCs). It is found that the three categories of particles encountered in PSC (Ice, Sulfate Ternary Solutions or STS, and Nitric Acid Trihydrate or NAT) do also occur in tropical cold cloud layers. Particles with optical properties similar to NAT are few, but they cover the tropical belt and represent about 20% of cold cloud tropical particles all year long. The optical behavior of these particles requires them to be very small, non-spherical, optically thin, and persistent in the TTL at temperatures colder than 200 K; NAT particles and very small ice crystals meet these criteria and are right now the best candidates to explain the presented observations. 2 1. Introduction As NAT particles play an important role in the vertical nitric acid redistribution (Krämer et al. 2008) and in the regulation of nitric acid and water vapour (Krämer et al. 2006), with a possible impact on the radiative budget at the top of the troposphere (Gao et al. 2004), it is a key step to get a global quantitative view of NAT-containing particles in the tropics. The possibility of observing NAT particles in the Tropical Tropopause Layer (TTL) with space-borne lidar was first proposed by Omar and Gardner (2001) and Hervig and McHugh (2002), based on the analysis of a limited number of lidar profiles without polarization capability. Their interpretation of this dataset was controversial (Jensen and Drdla, 2002) because the amount of NAT that can form in TTL conditions is so small that it seemed hardly detectable from space. Since then, several in situ observations have confirmed the existence of NAT in very small amount in the TTL (Popp et al. 2004, Popp et al. 2007, Voigt et al. 2008) as predicted theoretically by Hamill and Fiocco (1988). TTL NAT particles were also observed from a ground-based polarization lidar (Immler et al. 2007). For NAT detection purposes, the new CALIOP / CALIPSO lidar (Winker et al. 2007) presents two advances compared to previous lidars in space: it measures the linear depolarization ratio and has collected a statistically significant dataset (more than 2 years) in the tropics and polar regions where NAT are known to occur during winter in PSCs (Voigt et al. 2000, Höpfner et al. 2006). Here we analyze CALIPSO observations in a similar way in Austral PSCs and cold TTL clouds (T < 200 K) to evaluate how frequently NAT-like particles occur therein on a global scale. The detection of NAT-like particles uses two constraints (lidar backscatter intensity and linear depolarization ratio) following a classification method that has proven to be 3 effective to discriminate ice, NAT and STS particles in PSC from ground-based (e.g. Stein et al. 1999, Massoli et al. 2006), airborne (Browell et al. 1990) and space-borne lidar observations (e.g. von Savigny et al. 2005, Pitts et al. 2007). We first detect optically thin clouds colder than 200 K (Sect.2), then we study their optical properties in tropical and polar regions (Sect.3), before focusing on the NAT-like particles spatial distribution in the tropics (Sect. 4). Results are discussed and conclusions drawn in Sect. 5. 2.TTL clouds and PSCs colder than 200 K TTL clouds and PSCs were detected for all CALIPSO night-time observations collected between June 2006 and May 2008. CALIPSO follows a nearly sun-synchronous orbit which crosses the equator at 01:30 Local Solar Time (night time orbits), two successive orbits are closer in space at polar latitudes than in the tropics. The altitudes of each cloud layer top (ztop) and base (zbase) are derived first, then clouds are selected based on mid- layer temperatures. 2.1. Cloud detection CALIOP level 1 data are a succession of vertical profiles of altitude-dependent backscattered signal, each a vertical stack of individual lidar data points. Each CALIOP point describes the optical properties of a slice of the atmosphere at a given altitude; near the tropopause, such a slice is 60 meters thick. For TTL clouds from 30°S to 30°N, ztop and zbase were derived from the level 1B product Attenuated Total Backscatter at 532 nm (ATB532). ATB532 profiles were averaged horizontally over 5 km to improve the s

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