Wet deposition

GEOS-Chem has two wet deposition schemes,

Jacob et al. [2000], the default scheme
Luo and Yu [2023], an optional scheme

Jacob et. al. [2000] wet deposition scheme

The default wet deposition scheme is described in Jacob et al. [2000]. This scheme was originally developed for the NASA GMI model. Its implementation into GEOS-Chem is described fully in Liu et al. [2001].

Several updates have been made to the original implementation. These are described in the sections below.

Allow both washout and rainout when precipitation forms

From Wang et al. [2011]:

When there is new formation of precipitation in lower layer \(k\), rainout will be applied to the whole precipitation area: \(max(F_k, F_{k+1})\), considering the contribution of precipitation formation overhead. This will overestimate rainout effect when \(F_{k+1}\) is much larger than \(F_k\). Therefore, we now apply rainout effect to precipitation area \(F_k\) and washout effect to the area: \(max(0, F_{k+1}-F_k\)) in the same grid box.

Modifications for aerosol scavenging efficiency

From Wang et al. [2011]:

The bulk below-cloud scavenging parameterization of Dana and Hales [1976] (\(k = 0.1P\), where \(P\) is the precipitation rate mm h^-1) used in the standard GEOS-Chem model integrates scavenging efficiencies over typical aerosol size distributions. This overestimates scavenging as it does not account for the preferential removal of the very fine and coarse particles over the course of the precipitation event, shifting the aerosol size distribution toward the more scavenging-resistant accumulation mode that accounts for most of aerosol mass. Now we use the below-cloud scavenging coefficients (\(k = a P^{b}\)) constructed by Feng (2007, 2009) integrated over accumulation mode for most aerosols and over coarse mode for coarse dust and sea salt.

Obtain precipitation fields directly from meteorology

In GEOS-Chem v9-01-01 and later versions, we use an improved wet deposition scheme which uses the precipitation fields directly from the meteorology. At the time, the MERRA meteorology product was used, but this same methodology extends to other met field products as well.

Add scavenging by snow

From Wang et al. [2011]:

For in-cloud scavenging by rain droplets, we assume 100% of water-soluble aerosols are rained out. But in the case of snow, only dust and hydrophobic BC are considered to be IN and then could be rained out. Note that HNO3 is also assumed to be rained out by snow as it forms a monolayer in ice crystal. The below-cloud scavenging coefficients are also higher for snow than for rain droplets.

Impaction scavenging for hydrophobic BC and homogeneous IN removal

From Wang et al. [2014]:

We modify the scheme by (1) scavenging hydrophobic aerosol (hydrophobic BC and OC) in convective updrafts, since this would take place by impaction (Ekman et al. [2004]) and (2) scavenging water-soluble aerosol from cold clouds by homogeneous freezing of solution droplets at T < 237 K (Friedman et al. [2011]). We conducted ²²²Rn - ²¹⁰Pb simulations to test the general model representation of aerosol deposition and found a lifetime of tropospheric ²¹⁰Pb aerosol against deposition of 8.6 days.

Luo and Yu [2003] wet deposition scheme

Luo and Yu [2023] have developed an optional wet deposition scheme that you may use instead of the default Jacob et al. [2000]. This work builds on their earlier research (see Luo et al. [2020]).

Note

To use the Luo and Yu [2023] scheme you must use the -DLUO_WETDEP=y option at configuration time.

$ cmake -DLUO_WETDEP=y ... etc. other Cmake options ...

From the abstract to Luo and Yu [2023]:

The impacts of cloud mixing and uptake on wet scavenging are not adequately resolved in global models which can lead to an overestimation of the removal of water-soluble gases and aerosols from the atmosphere. To address this issue, we develop and implement novel parameterizations to consider the impacts of these processes. Our analysis of vertical profiles of nitric acid, inorganic nitrate, ammonium, and sulfate concentrations during the Atmospheric Tomography Mission periods indicates that air refreshing limitation has a significant impact above 800 hPa, while cloud ice uptake limitation plays an important role above 500 hPa. Incorporating these two processes resulted in a reduction of wet depositions of these species across source regions and a slight increase in their downwind regions. Wet depositions of nitrate, ammonium, and sulfate were reduced in source regions by 22.7%, 8.4%, and 8.3%, respectively and increased in downwind regions by 10.1%, 7.0%, and 4.3%, respectively.