Long-term trend analysis and climatology of tropical cirrus clouds using 16 years of lidar data set over Southern India

A K Pandit,K Raghunath,S V B Rao,H S Gadhavi,A Jayaraman,M Venkat Ratnam

doi:10.5194/acp-15-13833-2015

Abstract

Abstract. Sixteen-year (1998–2013) climatology of cirrus clouds and their macrophysical (base height, top height and geometrical thickness) and optical properties (cloud optical thickness) observed using a ground-based lidar over Gadanki (13.5° N, 79.2° E), India, is presented. The climatology obtained from the ground-based lidar is compared with the climatology obtained from 7 and a half years (June 2006–December 2013) of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. A very good agreement is found between the two climatologies in spite of their opposite viewing geometries and the differences in sampling frequencies. Nearly 50–55 % of cirrus clouds were found to possess geometrical thickness less than 2 km. Ground-based lidar is found to detect a higher number of sub-visible clouds than CALIOP which has implications for global warming studies as sub-visible cirrus clouds have significant positive radiative forcing. Cirrus clouds with mid-cloud temperatures between −50 to −70 °C have a mean geometrical thickness greater than 2 km in contrast to the earlier reported value of 1.7 km. Trend analyses reveal a statistically significant increase in the altitude of sub-visible cirrus clouds which is consistent with the recent climate model simulations. The mid-cloud altitude of sub-visible cirrus clouds is found to be increasing at the rate of 41 ± 21 m year−1. Statistically significant decrease in optical thickness of sub-visible and thick cirrus clouds is observed. Also, the fraction of sub-visible cirrus cloud is found to have increased by 9 % in the last 16 years (1998 to 2013). This increase is mainly compensated by a 7 % decrease in thin cirrus cloud fraction. This has implications for the temperature and water vapour budget in the tropical tropopause layer.

Highlights

Cirrus clouds are ubiquitous, high altitude, thin and wispy cold clouds predominantly consisting of non-spherical ice crystals
The altitude of peak percentage occurrence (PO) based on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data is in good agreement with National Atmospheric Research Laboratory (NARL) lidar; magnitude of peak PO differs significantly with CALIOP having higher values
To investigate whether the difference in time range (16 years vs. 7.5 years) or time of observation is responsible for differences in PO based on NARL lidar and PO based on CALIOP, a subset of entire NARL lidar data set for the period 2006–2013 is made

Summary

Introduction

High altitude, thin and wispy cold clouds predominantly consisting of non-spherical ice crystals. They exhibit a very high degree of spatio-temporal variability in their macrophysical, microphysical and optical properties (Liou, 1986; Lynch et al, 2002) These clouds affect the earth’s radiation budget through two competing radiative effects viz., albedo effect (by reflecting back the incoming shortwave solar radiation) and green-house effect (by trapping the outgoing long-wave terrestrial radiation) (Liou, 2005). Cirrus clouds are found to have net positive radiative forcing (Chen et al, 2000; Hartmann et al, 1992) at the top of the atmosphere (TOA) and they warm the climate system These estimates are based on the International Satellite Cloud Climatology Project (ISCCP) cloud data obtained from passive satellites that do not consider the overlap effect of multi-layered clouds. This difficulty can be overcome only by using ground and space-based lidars that provide vertical distribution of clouds with opposite viewing geometry

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Conclusion