Numerical simulation of clouds and precipitation depending on different relationships between aerosol and cloud droplet spectral dispersion

Abstract

The aerosol effects on clouds and precipitation in deep convective cloud systems are investigated using the Weather Research and Forecast (WRF) model with the Morrison two-moment bulk microphysics scheme. Considering positive or negative relationships between the cloud droplet number concentration (Nc) and spectral dispersion (ε), a suite of sensitivity experiments are performed using an initial sounding data of the deep convective cloud system on 31 March 2005 in Beijing under either a maritime (‘clean’) or continental (‘polluted’) background. Numerical experiments in this study indicate that the sign of the surface precipitation response induced by aerosols is dependent on the ε−Nc relationships, which can influence the autoconversion processes from cloud droplets to rain drops. When the spectral dispersion ε is an increasing function of Nc, the domain-average cumulative precipitation increases with aerosol concentrations from maritime to continental background. That may be because the existence of large-sized rain drops can increase precipitation at high aerosol concentration. However, the surface precipitation is reduced with increasing concentrations of aerosol particles when ε is a decreasing function of Nc. For the ε−Nc negative relationships, smaller spectral dispersion suppresses the autoconversion processes, reduces the rain water content and eventually decreases the surface precipitation under polluted conditions. Although differences in the surface precipitation between polluted and clean backgrounds are small for all the ε−Nc relationships, additional simulations show that our findings are robust to small perturbations in the initial thermal conditions

Sensitivity study of cloud parameterizations with relative dispersion in CAM5.1: impacts on aerosol indirect effects

“…Table 6 shows the global, Northern Hemisphere, and Southern Hemisphere annual mean changes of liquid water path ( LWP), cloud-top effective radius ( REL), shortwave cloud radiative forcing ( SWCF), longwave cloud radiative forcing ( LWCF), and total cloud radiative forcing (AIF) induced by aerosols in News. With an increase in anthropogenic aerosols, the LWP can be increased by the decreased autoconversion rate to form raindrops, and additionally the REL can be reduced significantly due to the enhanced activation of aerosols to cloud droplets (Xie et al, 2013). Due to the increased LWP and the decreased REL, the SWCF and LWCF can be increased by anthropogenic aerosols, and the total cloud radiative forcing (SWCF+LWCF) can also be increased, where the aerosol-induced SWCF is dominated for changes in the total cloud radiative forcing.…”

“…Several empirical expressions have been proposed to represent ε in terms of the droplet number concentration (reviewed by Xie et al, 2013). Here, three commonly used expressions are used to investigate the dispersion effect.…”

The relative dispersion of cloud droplets: its robustness with respect to key cloud properties

“…They concluded that the N c -ε relationship (positive or negative change of ε with N c ) influences the autoconversion process (i.e conversion of cloud droplets to raindrops), and therefore affects the response of ground precipitation to a change in aerosols concentration. Xie et al (2013) suggested that for a positive N c -ε relationship, the large-sized rain drops at high aerosol concentrations enhance the efficiency of the surface precipitation. The diversity of schemes for the N c -ε relationship (as shown in Fig.…”

“…Therefore the impact of aerosol loading on ε should be well understood for enabling suitable use of climate models in quantifying the impact of aerosol loading on the droplet size distribution, effective radius, and clouds' reflectivity. Xie et al (2013) studied four types of parameterizations for treating the relationship between N c and ε. They implemented these schemes into the Weather Research and Forecasting (WRF) model, aimed at studying the effects of aerosol on cloud microphysics and ground precipitation.…”

Skewness of cloud droplet spectrum and an improved estimation for its relative dispersion

“…The effective cloud droplet radius is related to the volume radius (r 3 ) by a ratio (b, r e = br 3 ), which in turn is a function of the relative dispersion of the cloud droplet spectrum (e) (Pontikis and Hicks 1992;Martin et al 1994;Daum 2000, 2002).This indicates that the relative dispersion therefore plays a key role in determining the cloud optical properties because of its intrinsic relationship with effective cloud droplet radius. Moreover, e also affects the automatic conversion rate from cloud water into rain water (Liu et al 2006a;Xie et al 2013), and consequently alters the lifetime of clouds. It is thus evident that the relative dispersion of cloud droplet spectrum (e) is a highly important cloud microphysical parameter.…”