Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?

Abstract

Abstract. Three configurations of a bulk microphysics scheme in conjunction with a detailed bin scheme are implemented in the Weather Research and Forecasting (WRF) model to specifically address the role of the saturation adjustment assumption (i.e., condensing/evaporating the surplus/deficit water vapor relative to saturation in one time step) on aerosol-induced invigoration of deep convective clouds. The bulk model configurations are designed to treat cloud droplet condensation/evaporation using either saturation adjustment, as employed in most bulk models, or an explicit representation of supersaturation over a time step, as used in bin models. Results demonstrate that the use of saturation adjustment artificially enhances condensation and latent heating at low levels and limits the potential for an increase in aerosol concentration to increase buoyancy at mid to upper levels. This leads to a small weakening of the time-and domain-averaged convective mass flux (∼ −3 %) in polluted compared to clean conditions. In contrast, the bin model and bulk scheme with explicit prediction of supersaturation simulate an increase in latent heating aloft and the convective updraft mass flux is weakly invigorated (∼5 %). The bin model also produces a large increase in domainmean cumulative surface precipitation in polluted conditions (∼18 %), while all of the bulk model configurations simulate little change in precipitation. Finally, it is shown that the cold pool weakens substantially with increased aerosol loading when saturation adjustment is applied, which acts to reduce the low-level convergence and weaken the convective dynamics. With an explicit treatment of supersaturation in the bulk and bin models there is little change in cold pool strength, so that the convective response to polluted conditions is influenced more by changes in latent heating aloft. It is concluded that the use of saturation adjustment can explain differences in the response of cold pool evolution and convective dynamics with aerosol loading simulated by the bulk and bin models, but cannot explain large differences in the response of surface precipitation between these models.

Uncertainty from the choice of microphysics scheme in convection-permitting models significantly exceeds aerosol effects

“…Morrison and Grabowski, 2011). However, while some argue (fairly) that this is at least in part due to the limitations of bulk schemes to fully represent aerosol-cloud interactions (such as saturation adjustment; Lebo et al, 2012), others argue that this is consistent with the concept of clouds as a buffered system hypothesised by Stevens and Feingold (2009). Month-long simulations approaching the climatological scale using bin microphysics performed by Fan et al (2013) also showed aerosol impacts on precipitation on the order of a few percent.…”

“…Lynn and Khain (2007) found that, while all schemes overestimated maximum rain rates in a simulated MCS, all bulk schemes tested overpredicted average and maximum rain rates by a factor of 2 to 3, while bin schemes overestimated maximum rain rates by about 20 %. In idealised supercell simulations, Khain and Lynn (2009) found that the Thompson et al (2004) double-moment bulk scheme produced 2 times more accumulated surface rain than a bin scheme, while Lebo et al (2012) found that the bulk scheme also produced twice as much surface rain as the same bin scheme used by Khain and Lynn (2009) in simulations of the same supercell. Investigations of the shape of the cloud droplet size distribution in large-eddy simulations of non-precipitating shallow cumulus clouds with a bin (Igel and van den Heever, 2017a) and bulk (Igel and van den Heever, 2017b) scheme showed the importance of the cloud droplet size distribution shape parameter.…”

“…Although we use two bulk microphysics schemes to show this, there is a body of literature which identifies signals of aerosol impact on cloud in bulk schemes (e.g. Morrison and Grabowski, 2011;Morrison, 2012;Lebo et al, 2012;Kalina et al, 2014), albeit not always convective invigoration (see especially Lebo et al, 2012), and in bin-emulating bulk schemes (e.g. van den Heever et al, 2006;Lee andFeingold, 2010, 2013).…”

“…Bryan and Morrison (2012) found that even at very high resolution, convective cores in an idealised squall line simulation remained undiluted due to the saturation adjustment used in the bulk microphysics scheme. However, saturation adjustment alone is insufficient to explain all differences between bin and bulk schemes: in idealised supercell simulations using bulk microphysics both with saturation adjustment and without (in which the scheme was modified to include an explicit representation of supersaturation predicted over each time step), Lebo et al (2012) found that the use of saturation adjustment was able to explain differences between a bulk and bin scheme in the response of cold pool evolution and convective dynamics under polluted conditions, but was not sufficient to explain the large differences in the response of surface precipitation to aerosol loading.…”

“…Morrison and Grabowski, 2011) to 3-D simulations of idealised supercell storms (e.g. Khain and Lynn, 2009;Lebo and Seinfeld, 2011;Morrison, 2012;Lebo et al, 2012). With such a wide range of model configurations, convective and largescale environments, microphysics parameterisations (bin and bulk models are both frequently used in idealised studies of aerosol-convection interactions) and proxy variables used to represent aerosol processes, it is perhaps not surprising that a consistent response of idealised convection to aerosol has not been seen; indeed, due to environment and regime dependence, it may not exist.…”