Importance of Chemical Composition of Ice Nuclei o

Authors: Setigui Aboubacar Keita Eric Girard

Publish Date: 2016/04/23

Volume: 173, Issue: 9, Pages: 3141-3163

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Abstract

Ice clouds play an important role in the Arctic weather and climate system but interactions between aerosols clouds and radiation remain poorly understood Consequently it is essential to fully understand their properties and especially their formation process Extensive measurements from groundbased sites and satellite remote sensing reveal the existence of two Types of Ice Clouds TICs in the Arctic during the polar night and early spring TICs1 are composed by nonprecipitating small radarunseen ice crystals of less than 30 μm in diameter The second type TICs2 are detected by radar and are characterized by a low concentration of large precipitating ice crystals ice crystals 30 μm To explain these differences we hypothesized that TIC2 formation is linked to the acidification of aerosols which inhibits the ice nucleating properties of ice nuclei IN As a result the IN concentration is reduced in these regions resulting to a lower concentration of larger ice crystals Water vapor available for deposition being the same these crystals reach a larger size Current weather and climate models cannot simulate these different types of ice clouds This problem is partly due to the parameterizations implemented for ice nucleation Over the past 10 years several parameterizations of homogeneous and heterogeneous ice nucleation on IN of different chemical compositions have been developed These parameterizations are based on two approaches stochastic that is nucleation is a probabilistic process which is time dependent and singular that is nucleation occurs at fixed conditions of temperature and humidity and timeindependent The best approach remains unclear This research aims to better understand the formation process of Arctic TICs using recently developed ice nucleation parameterizations For this purpose we have implemented these ice nucleation parameterizations into the Limited Area version of the Global Multiscale Environmental Model GEMLAM and use them to simulate ice clouds observed during the Indirect and SemiDirect Aerosol Campaign ISDAC in Alaska Simulation results of the TICs2 observed on April 15th and 25th acidic cases and TICs1 observed on April 5th nonacidic cases are presented Our results show that the stochastic approach based on the classical nucleation theory with the appropriate contact angle is better Parameterizations of ice nucleation based on the singular approach tend to overestimate the ice crystal concentration in TICs1 and TICs2 The classical nucleation theory using the appropriate contact angle is the best approach to use to simulate the ice clouds investigated in this researchWe thank NETCARE Network on Climate and Aerosols Addressing Key Uncertainties in Remote Canadian Environments and NSERC Natural Sciences and Engineering Research Council of Canada for funding support and ARM Atmospheric Radiation Measurement Program for the data collected during ISDAC

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