A Tropical "NAT-like" belt observed from space

TL;DR

This study uses space-borne lidar data to identify and analyze the optical properties of cold tropical clouds, revealing the presence of NAT-like particles across the tropical belt and their potential role in cloud composition.

Contribution

It is the first global analysis of tropical cold clouds showing NAT-like particles using lidar data, expanding understanding of cloud particle diversity in the tropics.

Findings

NAT-like particles are present in tropical cold clouds.

These particles constitute about 20% of cold cloud particles in the tropics.

NAT particles are small, non-spherical, and persistent at temperatures below 200 K.

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. The optical behavior of these particles requires them to be 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.