Aerosol Indirect Effects on Tropical Convection Characteristics under Conditions of Radiative–Convective Equilibrium

Abstract

The impacts of enhanced aerosol concentrations such as those associated with dust intrusions on the trimodal distribution of tropical convection have been investigated through the use of large-domain (10 000 grid points), fine-resolution (1 km), long-duration (100 days), two-dimensional idealized cloud-resolving model simulations conducted under conditions of radiative-convective equilibrium (RCE). The focus of this research is on those aerosols that serve primarily as cloud condensation nuclei (CCN). The results demonstrate that the large-scale organization of convection, the domain-averaged precipitation, and the total cloud fraction show only show a weak response to enhanced aerosol concentrations. However, while the domainwide responses to enhanced aerosol concentrations are weak, aerosol indirect effects on the three tropical cloud modes are found to be quite significant and often opposite in sign, a fact that appears to contribute to the weaker domain response. The results suggest that aerosol indirect effects associated with shallow clouds may offset or compensate for the aerosol indirect effects associated with congestus and deep convection systems and vice versa, thus producing a more moderate domainwide response to aerosol indirect forcing. Finally, when assessing the impacts of aerosol indirect forcing associated with CCN on the characteristics of tropical convection, several aspects need to be considered, including which cloud mode or type is being investigated, the field of interest, and whether localized or systemwide responses are being examined.

Cloud-resolving modelling of aerosol indirect effects in idealised radiative-convective equilibrium with interactive and fixed sea surface temperature

“…Despite constant precipitable water in the fSST cases, the precipitation rate tends to increase with increasing CCN count, with estimated relative susceptibility of 0.06 mm day −1 or about 2 % relative to the control case. Such a relatively minor change in precipitation rate is consistent with other studies (e.g., Rotstayn and Penner, 2001;van den Heever et al, 2011;Grabowski and Morrison, 2011;Morrison and Grabowski, 2011). The increase is the response to the modest increase of radiative cooling as indicated by the increase of OLR (see Fig.…”

“…The GCMs generally do not resolve individual clouds; therefore, virtually all the complexities of aerosol-cloud-radiation interactions have to be parameterised (e.g., Abdul-Razzak and Ghan, 2002;Nenes and Seinfeld, 2003;Liu and Penner, 2005;Hoose et al, 2010). On the other hand, many details of these interactions including convection, large-scale forcing, aerosol, cloud microphysics and radiation can be explicitly represented by CRMs (e.g., Lu and Seinfeld, 2005;Grabowski, 2006;Tao et al, 2007;van den Heever et al, 2011;Morrison and Grabowski, 2011). Recently, a GCM that uses a CRM as a super-parameterisation of clouds has been developed to link the explicitly simulated clouds and aerosol processes on global scale (Wang et al, 2011).…”

“…The RCE has been rather extensively used in the past to study processes driving the hydrological cycle in the Tropics (e.g., Tompkins and Craig, 1998;Xu and Randall, 1999;Grabowski, 2006;Stephens et al, 2008;Romps, 2011). Previous CRM studies of AIEs have examined the variations of tropical convection over the ocean with different sea-surface temperature (SST), which was prescribed from observations or simply fixed at some value (e.g., Grabowski, 2006;van den Heever et al, 2011;Morrison and Grabowski, 2011). The AIEs in the case of the fixed SST can be viewed as the fast response of a cloudy atmosphere to aerosol forcing on relatively short time scales when the SST does not have enough time to respond due to the ocean's large thermal inertia.…”

“…By convention, the aerosol indirect effects (AIEs) are subdivided into subcategories depending on their effects on cloud properties. For example, the effect of aerosols on cloud albedo is called the first indirect or Twomey effect (Twomey, 1974), the effect on precipitation efficiency and cloud water path (Albrecht, 1989) and cloud lifetime (Pincus and Baker, 1994) is generally referred to as the second indirect effect. The current consensus reflected in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2007) is that aerosols have a predominantly cooling effect on climate with the magnitude of the forcing similar to the net radiative…”

Coupling spectral‐bin cloud microphysics with the MOSAIC aerosol model in WRF‐Chem: Methodology and results for marine stratocumulus clouds

“…Aerosolenhanced cloud fraction and cloud top height for DCCs have been ubiquitously observed [Koren et al, 2010;Li et al, 2011;Storer and van den Heever, 2013], and microphysical invigoration has been proposed as a dominant mechanism based on model simulations with a bin cloud microphysics . In spite of substantial efforts made to identify the governing factors and mechanisms in aerosol-cloud interactions (ACI) in recent studies [Feingold and Chuang, 2002;Yin et al, 2005;van den Heever et al, 2006van den Heever et al, , 2011Rosenfeld et al, 2008;Lee et al, 2009;Saleeby et al, 2010;Fan et al, 2009aFan et al, , 2013, aerosol impacts on the evolution of various types of clouds and precipitation remain poorly known. Quantification of AIE is still challenging and represents one of the largest uncertainties in climate predictions.…”