A Statistical Method for Determining Optical and Geometrical Characteristics of Cirrus Clouds

Manoj Kumar Nellore,Arul Lenus Roy Vellaisamy,Venkatramanan Kannan

doi:10.3390/cli7050072

Abstract

Altitude occurrences of cirrus clouds, their base, mid, and top heights are identified by using a powerful statistical approach called the Variance Centroid Method (VCM). This method is effective in determining the geometrical properties of cirrus clouds at a height range of 8 km to 20 km. This statistical method provides in-depth information on Generation Circulation Models (GCM’s) because of its significant role in the radiative balance of the earth’s atmosphere. The output related to geometrical and optical properties of cirrus clouds obtained from this statistical method for the years 2014 and 2015 are studied. Altitude distributions of base, mid, top, and thickness of tropical cirrus clouds are analyzed in terms of percentage occurrences. For the year 2014, it was found that 14.8%, 75.2%, and 9.9% of cirrus clouds were sub-visual, thin, and thick clouds and for the year 2015, 71.2% and 28.7% of cirrus clouds were found to be thin and thick clouds, and sub-visual types of clouds were not observed. The interdependence of optical depth with the depolarization ratio is discussed and it is observed that the correlation is negative (−0.0303) for the year 2014 and positive (0.1311) for the year 2015. High optical depths with values from 0.502 to 0.849 are observed in the height regions from 9 km to 15 km (for 2014) and for the year 2015, the observed value ranges from 0.514 to 0.822 for the height regions from 9 km to 11 km. Statistical variation of characteristics of tropical cirrus clouds is presented for the period of study. The characterization of these cirrus clouds is highly useful for climate modeling studies, and their impact plays a vital role in the Earth’s radiation budget at the top of the atmosphere.

Highlights

Cirrus clouds are thin and wispy clouds which are entirely made of ice [1]
Using Variance Centroid Method (VCM), it is possible to locate the base, middle, and top heights of cirrus clouds. The results of this method are presented and discussed by taking into account the complexities of the lidar signal returns that are generated by the cirrus clouds in a highly variable atmosphere and by considering the limitations of the instrument hardware
The base height of the first cirrus cloud signal when searching upwards is denoted as rb (CiBH), the mean height of the cirrus cloud signal is denoted as rp (CiMH), and the top height of the cirrus cloud signal is denoted as rt (CiTH)

Summary

Introduction

Cirrus clouds are thin and wispy clouds which are entirely made of ice [1]. They form in very cold air at high altitudes in the range from 7 km to 20 km. They are common high clouds which can be seen at any time during the year They are found generally in the Upper Troposphere/Lower Stratosphere (UTLS) region, and most of the regions of these clouds are composed of non-spherical ice crystals. The presence of these clouds increases the fraction of the solar radiation reflected back to space known as the solar albedo effect. These clouds absorb outgoing thermal radiation from the earth known as the infrared greenhouse effect [2] Both of these phenomena affect the Earth’s radiation and climate, the former leading to a cooling of the system and later to the warming of the system [3]. A clear understanding of cirrus cloud parameterization in terms of their occurrence characteristics, geometrical, and optical characteristics at distinctive geographical areas

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