TY - JOUR

T1 - Cirrus parameterisation and the role of ice nuclei.

AU - MacKenzie, Rob

AU - Ren, C.

N1 - It refines and extends a method for parameterisation of cirrus cloud. The parameterisation is designed for use in global climate-and-chemistry models (i.e., CCMs, GCMs and CTMs) but has also been used in a Lagrangian framework to provide forecasts for the TroCCiNOx, SCOUT-O3, and AMMA measurement campaigns RAE_import_type : Journal article RAE_uoa_type : Earth Systems and Environmental Sciences

PY - 2005/4/1

Y1 - 2005/4/1

N2 - A parametrization of cirrus clouds formed by homogeneous nucleation is improved so that it can be used more easily in general-circulation models (GCMs) and climate models. The improved parametrization is completely analytical and requires no fitting of parameters to models or measurements; it compares well with full microphysical model results even when monodisperse aerosol particles are used in the parametrization to determine cirrus ice-crystal number densities. However, the presence of ice nuclei in the atmosphere can modify the formation of cirrus clouds. If sufficient ice particles have been generated by heterogeneous nucleation, the saturation ratio of the air parcel will never reach that required for homogeneous nucleation. We calculate the critical number density of ice nuclei, above which homogeneous nucleation will be suppressed. The critical number density depends on the temperature, the updraught velocity, and the supersaturation at which ice nuclei activate. The theory points to key uncertainties in our observations of ice nuclei in the upper troposphere; for ice nuclei that activate at relatively low supersaturations, number density is more important than a precise knowledge of the activation supersaturation. Overall, the theory provides a general framework within which to interpret observations and the results of full microphysical cloud models. The theory can provide analytical test cases as benchmarks for the testing of models in development, and can be implemented itself into larger-scale atmospheric models, such as GCMs. Copyright © 2005 Royal Meteorological Society

AB - A parametrization of cirrus clouds formed by homogeneous nucleation is improved so that it can be used more easily in general-circulation models (GCMs) and climate models. The improved parametrization is completely analytical and requires no fitting of parameters to models or measurements; it compares well with full microphysical model results even when monodisperse aerosol particles are used in the parametrization to determine cirrus ice-crystal number densities. However, the presence of ice nuclei in the atmosphere can modify the formation of cirrus clouds. If sufficient ice particles have been generated by heterogeneous nucleation, the saturation ratio of the air parcel will never reach that required for homogeneous nucleation. We calculate the critical number density of ice nuclei, above which homogeneous nucleation will be suppressed. The critical number density depends on the temperature, the updraught velocity, and the supersaturation at which ice nuclei activate. The theory points to key uncertainties in our observations of ice nuclei in the upper troposphere; for ice nuclei that activate at relatively low supersaturations, number density is more important than a precise knowledge of the activation supersaturation. Overall, the theory provides a general framework within which to interpret observations and the results of full microphysical cloud models. The theory can provide analytical test cases as benchmarks for the testing of models in development, and can be implemented itself into larger-scale atmospheric models, such as GCMs. Copyright © 2005 Royal Meteorological Society

KW - Aerosols • Heterogeneous nucleation • Homogeneous nucleation • Modelling

U2 - 10.1256/qj.04.126

DO - 10.1256/qj.04.126

M3 - Journal article

VL - 131

SP - 1585

EP - 1605

JO - Quarterly Journal of the Royal Meteorological Society

JF - Quarterly Journal of the Royal Meteorological Society

SN - 0035-9009

IS - 608

ER -