Aerosol impacts on clouds and precipitation in eastern China: Results from bin and bulk microphysics

Abstract

Using the Weather Research and Forecasting model coupled with a spectral‐bin microphysics (“SBM”) and measurements from the Atmospheric Radiation Measurement Mobile Facility field campaign in China (AMF‐China), the authors examine aerosol indirect effects (AIE) in the typical cloud regimes of the warm and cold seasons in Southeast China: deep convective clouds (DCC) and stratus clouds (SC), respectively. Comparisons with a two‐moment bulk microphysics (“Bulk”) are performed to gain insights for improving bulk schemes in estimating AIE in weather and climate simulations. For the first time, measurements of aerosol and cloud properties acquired in China are used to evaluate model simulations to better understand aerosol impact on clouds in the southeast of China. It is found that changes in cloud condensation nuclei (CCN) concentration significantly change the timing of storms, the spatial and temporal distributions of precipitation, the frequency distribution of precipitation rate, as well as cloud base and top heights for the DCC, but not for the SC. Increasing CCN increases cloud droplet number (Nc) and mass concentrations, decreases raindrop number concentration, and delays the onset of precipitation. Compared with SBM, Bulk predicts much higher Ncand the opposite CCN effects on convection and heavy rain, stemming from the fixed CCN prescribed in Bulk. CCN have a significant effect on ice microphysical properties with SBM but not Bulk and different condensation/deposition freezing parameterizations employed could be the main reason. This study provided insights to further improve the bulk scheme to better account for aerosol‐cloud interactions in regional and global climate simulations, which will be the focus for a follow‐on paper.

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

“…Not only do bin and bulk schemes often produce different results in terms of cloud and precipitation, but Fan et al (2012a) found that the use of fixed CCN in a bulk scheme led to opposite CCN effects on convection and heavy rain compared to CCN effects when using a bin scheme. Similarly, Lebo and Seinfeld (2011) found an opposite response of accumulated surface rain to CCN in idealised supercell simulations using a bulk and bin scheme.…”

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

“…Generally, constant CCN source is set for the lateral boundary grid points to avoid advective depletion of CCN and to replenish CCN due to nucleation depletion of CCN. This setup often causes unrealistically high aerosol concentrations (and droplet nucleation) at the lateral boundaries, so different ways are used to mitigate the problem [e.g., Fan et al, 2012]. With the coupling with chemistry/aerosols that is done by this study, this unrealistic setup is no longer needed.…”

“…A bin microphysical scheme, the spectral-bin microphysics (SBM), has been coupled into the WRF model and extensively used or modified to simulate various cloud cases and study aerosol indirect effects Fan et al, 2012Fan et al, , 2013. Due to its use of simplified aerosol treatments, application of the SBM has been limited to idealized aerosol scenarios, which are often not consistent with real-world conditions.…”

“…The SBM scheme that we coupled to MOSAIC is a fast version of the Hebrew University Cloud Model (HUCM) described by Khain et al [2004], which is similar to Fan et al [2012]. The original SBM parameterization (i.e., the full version of HUCM) solves a system of prognostic equations for the size distribution functions of seven hydrometeor types (liquid drops, plate, columnar and branch ice crystal types, snow/aggregates, graupel, and hail/frozen drops), as well as CCN.…”

“…However, due to the expensive computation cost, bin cloud microphysics is often run with a prescribed aerosol spectrum with a fixed composition and a simple aerosol budget treatment without connecting with chemistry/aerosol calculations Chen et al, 2011;Fan et al, 2012;Xue and Feingold, 2006;Xue et al, 2010]. For instance, Xue and Feingold [2006] investigated the effects of aerosol on warm trade cumulus clouds using a large-eddy simulation with bin cloud microphysics, in which aerosol properties were prescribed to be constant over time and space.…”