The simulation of aerosols and its radiative forcing using the new coupled system of aerosol HAM model with the Weather Research and Forecasting (WRF) model

A new coupled system of the aerosol HAM model and the Weather Research and Forecasting (WRF) model is introduced and tested in this paper. Unlike the two other aerosol schemes currently coupled in the WRF model (MADE/Sorgam and MOSAIC), the HAM applies a new "pseudomodal" approach for the representation of the aerosol particles. The aerosol population in this model is represented by the superposition of seven modes based on size and solubility. The particle size is assumed to be distributed log-normally. The seven modes are categorized into four geometrical size classes, ranging from the nucleation, Aitken and accumulation modes to coarse modes. The aerosols are also divided into two types of internally mixed and water soluble particles (four modes), and externally mixed and insoluble particles (three modes). This classification makes possible the prediction of the hygroscopic properties of initially insoluble aerosol compounds which controls their atmospheric lifetimes and also their interactions with clouds. The WRF-HAM model includes the various microphysical processes of condensation of sulfuric acid, nucleation and new particle formation, coagulation of aerosol particles, and the thermodynamical equilibrium of aerosols with the water vapor. The main removal processes for the aerosol particles in the coupled WRF-HAM model are gravitational sedimentation and dry deposition. The model also considers the in-cloud scavenging of aerosol particles by precipitation within the convective cumulus clouds. The main global aerosol compounds including sulfate, black carbon, particulate organic carbon (POM), sea salt and mineral dust have been considered in this study. The emission fluxes of different aerosol compounds are based on the prescribed Emission Inventory for the Aerosol Model Inter-comparison Experiment B, AEROCOM representative for the year 2000. The simulations are carried out for a 6-day simulation period from 6 to 12 May 2006 over a domain with 30-km grid spacing, covering south-western Asia, North Africa and some parts of Europe. The diurnal variation of the simulated hourly PM10 mass concentration at Tehran is qualitatively close to the hourly observations made by the Air Quality Control Company (AQCC) of the Municipality of Tehran. The model captures diurnal cycle and the magnitude of the observed PM10 concentration during most of the simulation period. Coupling WRF model with HAM aerosol scheme improves the hourly PM10 mass concentration compared to the case where WRF was coupled with MADE scheme. A negative radiative forcing and cooling of the atmosphere are found mainly over the regions of high emission of mineral dust. The absorption of shortwave radiation by black carbon causes warming effects in some regions with positive radiative forcing. The inclusion of aerosol feedback in the shortwave radiation scheme improves the simulated daily mean shortwave radiation fluxes in Tehran. The difference between the simulated and observed mean daily downward shortwave radiation, temperature and surface pressure by the HAM model is smaller than by the aerosol MADE scheme. Compared to WRF-MADE, using the coupled WRF-HAM model improves the simulation of downward shortwave radiation by up to 40 Wm. The spatial variation in the simulated mean optical depth of aerosols at 500 nm wavelength by the WRF-HAM model is qualitatively close to the measurements by MODIS instrument. Furthermore, the comparison of the simulated aerosol optical depth by HAM and MADE aerosol schemes and the observations in Solar Village site of the global AERONET Network shows that the HAM model highly outperforms MADE.