Effects of spectral dispersion on clouds and precipitation in mesoscale convective systems

Abstract

[1] The effects of spectral dispersion on clouds and precipitation in mesoscale convective systems have been studied by conducting 10 numerical simulations with different values of spectral dispersion (" = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) in the clean, semipolluted, and polluted backgrounds. The simulation results show that spectral dispersion affects cloud microphysical properties markedly in each aerosol regime. With an increase in spectral dispersion, both the raindrop concentration and the rainwater content increase, while the mean radius of the raindrops diminishes substantially. Moreover, it is found that the effects of spectral dispersion on simulated precipitation differ in these three aerosol backgrounds and relative humidity. In the clean background and at relatively lower humidity, the average accumulated precipitation is reduced significantly with an increase in spectral dispersion. Precipitation varies nonmonotonically in the semipolluted background, increasing with spectral dispersion at smaller values, while decreasing at larger values. In the mean time, precipitation is continuously enhanced with increasing spectral dispersion in the polluted background. Furthermore, sensitivity tests demonstrate that the possible impacts of spectral dispersion on precipitation varies depending on the relative humidity. For instance, at high relative humidity, an increase in spectral dispersion even in a clean atmosphere leads to more precipitation. Our results could shed light on understanding the influences of aerosols on clouds and precipitation, especially the second aerosol indirect effect.Citation: Xie, X., and X. Liu (2011), Effects of spectral dispersion on clouds and precipitation in mesoscale convective systems,

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

“…It is suggested that, compared with the autoconversion rate, there is relatively more efficient accretion growth with higher cloud water contents in the polluted backgrounds (see Table 3 for PRA), which eventually results in larger radius of rain drops. Aerosols can increase the rain drop size, which is in general good agreement with previous studies (Cheng et al, 2007;Altaratz et al, 2008;Berg et al, 2008;Li et al, 2008;Li et al, 2009;Lim and Hong, 2010;Storer et al, 2010;Lim et al, 2011;Xie and Liu, 2011).…”

“…x c can be written as a formula x c ¼ 9:7 Â 10 À17 N 3=2 c L À2 c . In a previous study (Xie and Liu, 2011), a similar autoconversion parameterisation was adopted where the cloud droplet spectral dispersion is a fixed constant (o 00.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0). However, in this article, o is parameterised as the function of the cloud droplet number concentration N c , described by the functions eq.…”

“…For simulations of the convective cloud system on 31 March 2005 in Beijing, the thermodynamic conditions used by Xie and Liu (2011) are chosen. Soundings used are presented in Fig.…”

“…Rotstayn and Liu (2005) showed that these autoconversion parameterisation schemes, including o (Liu and Daum, 2004), derived analytically, without employing the unrealistic assumption of constant collection efficiency of cloud water can also account for the spectral dispersion effects on cloud microphysical properties and surface precipitation. More recently, based on sensitivity studies of various spectral dispersion (o 0 0.1, 0.2, 0.3 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) in the mesoscale convective systems, Xie and Liu (2011) found that the spectral dispersion significantly affects both the cloud microphysical properties and the surface accumulated precipitation in different aerosol backgrounds. Hence, the cloud droplet spectral dispersion is a key parameter which combines aerosols effect on cloud microphysical properties with the surface precipitation.…”

On the robustness of aerosol effects on an idealized supercell storm simulated with a cloud system-resolving model

“…Changes in the magnitude of these impacts were also evident for the baseline model configuration with small perturbations to the vertical shear of the environmental u-component wind. These changes were of similar magnitude to changes in aerosol effects on deep convective storms with altered microphysics process formulations reported in previous studies (Ekman et al, 2011;Xie and Liu, 2011). Thus, the rapid growth of small perturbations and limits on the predictability of convective systems can potentially obscure sensitivity of aerosol effects to different model configurations and parameterizations, especially for studies of individual storms.…”