Quantifying the impact of sub-grid surface wind variability on sea salt and dust emissions in CAM5

Abstract

Abstract. This paper evaluates the impact of sub-grid variability of surface wind on sea salt and dust emissions in the Community Atmosphere Model version 5 (CAM5). The basic strategy is to calculate emission fluxes multiple times, using different wind speed samples of a Weibull probability distribution derived from model-predicted grid-box mean quantities.In order to derive the Weibull distribution, the sub-grid standard deviation of surface wind speed is estimated by taking into account four mechanisms: turbulence under neutral and stable conditions, dry convective eddies, moist convective eddies over the ocean, and air motions induced by mesoscale systems and fine-scale topography over land. The contributions of turbulence and dry convective eddy are parameterized using schemes from the literature. Wind variabilities caused by moist convective eddies and fine-scale topography are estimated using empirical relationships derived from an operational weather analysis data set at 15 km resolution. The estimated sub-grid standard deviations of surface wind speed agree well with reference results derived from 1 year of global weather analysis at 15 km resolution and from two regional model simulations with 3 km grid spacing.The wind-distribution-based emission calculations are implemented in CAM5. In terms of computational cost, the increase in total simulation time turns out to be less than 3 %. Simulations at 2 • resolution indicate that sub-grid wind variability has relatively small impacts (about 7 % increase) on the global annual mean emission of sea salt aerosols, but considerable influence on the emission of dust. Among the considered mechanisms, dry convective eddies and mesoscale flows associated with topography are major causes of dust emission enhancement. With all the four mechanisms included and without additional adjustment of uncertain parameters in the model, the simulated global and annual mean dust emission increase by about 50 % compared to the default model. By tuning the globally constant dust emission scale factor, the global annual mean dust emission, aerosol optical depth, and top-of-atmosphere radiative fluxes can be adjusted to the level of the default model, but the frequency distribution of dust emission changes, with more contribution from weaker wind events and less contribution from stronger wind events. In Africa and Asia, the overall frequencies of occurrence of dust emissions increase, and the seasonal variations are enhanced, while the geographical patterns of the emission frequency show little change.

Quantification of marine aerosol subgrid variability and its correlation with clouds based on high‐resolution regional modeling

“…Gustafson et al [] further showed that the neglect of subgrid processes over Mexico could lead to an average daytime mean bias of over 30% in shortwave aerosol radiative forcing obtained from WRF‐Chem simulations. In addition, Zhang et al [] investigated the effect of subgrid wind variability on sea‐salt and dust emission in the community atmospheric model (CAM5). They found that the simulated global dust emissions could increase by 50% when considering the subgrid wind variability, while the subgrid wind variability has relatively small impacts (about 7% increase) on global averaged sea‐salt emissions.…”

Modeling Dust in East Asia by CESM and Sources of Biases

“…To better understand the model biases in East Asia, dust emission schemes of Kok, Mahowald, et al (), Kok, Albani, et al () (hereafter K14) and Ginoux et al () (hereafter G01) were implemented in the Community Land Model version 4 (CLM4.0) of CESM. Treatment of subgrid surface wind variability in calculating dust emission by Zhang et al () was also applied in CLM4.0. The goal of this study is to evaluate the performance of CESM over East Asia in the simulations of (1) dust extinction profiles and DOD, including their spatial distributions and temporal variations; (2) dust mass budgets in the Taklamakan desert and Tibetan Plateau (see Figure ); and (3) dust surface concentrations and 10‐m wind speed with a special focus on their PDFs.…”

An Evaluation of the CHIMERE Chemistry Transport Model to Simulate Dust Outbreaks across the Northern Hemisphere in March 2014

“…Moreover, a recent study describing a conceptual model of dust dynamics showed that vertical transport can efficiently counteracts and limits the gravitational settling of coarse particles [103], according to Lidar observations in the frame of the SALTRACE experiment. The evolution of the particle sizes is also shown in Figure 7 [35][36][37][38][39][40] • N resulting mainly of the trans-Pacific transport of Asian dust emissions mixed with diluted Saharan dust transported by southwesterly fluxes. In Figure 7, the deep convection updraft flux integrated over the column is displayed as an indicator of the convective activity.…”

“…A major West African dust storm was simulated at 5 km resolution with the French modeling system AROME coupled with the ORILAM aerosol model [37]. A high-resolution modeling of dust phenomena is computationally demanding and requires high-resolution input fields [38], however, high resolution simulations ensure a better quantification of dust source regions, meteorological mechanisms that control dust emission fluxes [39,40], transport pathways [41,42], dust radiative direct and indirect effects, complex atmospheric chemistry, and deposition processes. For anthropogenic pollution, a statistical evaluation performed by [43] showed that the increased resolution better reproduces the spatial gradients in pollution regimes, but does not help to improve significantly the model performance for reproducing observed temporal variability.…”