Wet deposition

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This page describes the current wet deposition scheme used in GEOS-Chem.


Original algorithm

The Harvard Atmospheric Chemistry Modeling Group developed a wet deposition scheme (including scavenging of soluble tracer in convective updrafts, as well as rainout and washout of soluble tracers) for the GMI model. This scheme was then implemented into GEOS-Chem. Jacob et al [2000] describes the algorithm in full. This scheme is also described in Liu et al [2001].

Further updates

Add scavenging by snow

The following update to the original wet scavenging algorithm will be added to GEOS-Chem following the v8-03-02 release:

Qiaoqiao Wang wrote:

Here is the description of the change I made:
  1. Add snow scavenging. 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 is considered to be IN and then could be rained out. The below-cloud scavenging coefficients are also higher for snow than for rain droplets.

  2. The bulk below-cloud scavenging parameterization of Dana and Hales used in the standard GEOS-Chem model integrates scavenging efficiencies over typical aerosol size distributions. This overestimates scavenging as it does not account fro 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 integrated over accumulation mode for most aerosols and over coarse mode for coarse dust and sea salt.

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

--Bob Y. 09:58, 13 July 2010 (EDT)

Updates for MERRA met fields

We are implementing a new wet deposition scheme which uses the precipitation fields directly from the MERRA reanalysis product. More information to follow.

--Bob Y. 12:43, 20 September 2010 (EDT)

The wet deposition algorithms/changes documented in the PDF above are subject to change. We are posting the most recent versions on the wiki and working to keep these documents as up-to-date as possible.

-- Helen Amos 1:31, 24 September 2010 (EDT)


See Liu et al [2001].


  1. Dana, M.T., and J.M. Hales, Statistical aspects of the washout of polydisperse aerosols, Atmos. Environ, 10, 45-50, 1976.
  2. Domine, F., and E. Thibert, Mechanism of incorporation of trace gases in ice grown from the gas phase, Geophys. Res. Lett., 23, 3627-3630, 1996.
  3. Giorgi, F., and W.L. Chameides, Rainout lifetimes of highly soluble aerosols as inferred from simulations with a general circulation model, J. Geophys. Res., 91, 14,367-14,376, 1986.
  4. Jacob, D.J., Heterogeneous chemistry and tropospheric ozone, Atmos. Environ., 34, 2131-2159, 2000. PDF
  5. Jacob, D.J. H. Liu, C.Mari, and R.M. Yantosca, Harvard wet deposition scheme for GMI, Harvard University Atmospheric Chemistry Modeling Group, revised March 2000. PDF
  6. Levine, S.Z., and S.E. Schwartz, In-cloud and below-cloud scavenging of nitric acid vapor, Atmos. Environ., 16, 1725-1734, 1982.
  7. Liu, H., D.J. Jacob, I. Bey, and R.M. Yantosca, Constraints from 210Pb and 7Be on wet deposition and transport in a global three-dimensional chemical tracer model driven by assimilated meteorological fields, J. Geophys. Res., 106, 12,109-12,128, 2001. PDF
  8. Mari, C., D.J. Jacob, and P. Bechtold, Transport and scavenging of soluble gases in a deep convective cloud, J. Geophys. Res., 105, 22,255-22,267, 2000. PDF
  9. Selin, N.E. and D.J. Jacob, Seasonal and spatial patterns of mercury wet deposition in the United States: North American vs. intercontinental sources, Atmospheric Environment, 42, 5193-5204, 2008. PDF

Known issues

Negative tracer in routine WETDEP because of negative RH

See this post: GEOS-5 issues#Small negative RH value in 20060206.a6.2x25 file

Fixes are available at ftp://ftp.as.harvard.edu/pub/geos-chem/patches/v8-01-01.

--phs 16:31, 6 June 2008 (EDT)

Negative tracer in routine WETDEP

Dylan Millet wrote:

I'm having a run die consistently at the same time (October 1, 2005; first time step of the month) in large-scale wetdep, with an STT element < 0.
  • Platform: Linux cluster
  • Threads: 8
  • Version: v7-4-13 out of the box.
  • GEOS4, 4x5, 30L, full chemistry
  • IFORT 10.1
In Section 6 (No Downward Precip) of wetscav_mod.f, subroutine safety is getting called.
    WETDEP - STT < 0 at    1   1  29 for tracer    7 in area    6
(First of all it seems odd to do wetdep for L=29, this is 63 km up). Have you seen anything like this? I ran for the whole year starting Jan 1 successfully until this point.
... By the way, the problem persists when I turn off chemistry altogether.

Philippe Le Sager replied:

I used your restart file and the same input.geos (w/ chemistry on and off). My code went thru without problem. I tried both Sun Studio and Ifort 9 compilers, and the later on two different machines (altix and ceres). I used v7-04-13 and v8-01-01. I never reproduced your error.
We just got the new Ifort 10, and tried it too. I run v8-01-01 without an error. But when I tried v7-04-13, I finally reproduced your error, with the exact same negative values!
In other words: the bug happens with IFort 10 and v7-04-13 only.
Also, have a look at this recent development. This is not the reason for your bug (I tried v8 w/ ifort 10 and isorropia -like v7-04-13- and it did not crash), but using RPMARES instead of Isorropia may be a way to fix it.
... More about the Ifort 10 / v7-04-13 issue. When I wanted to debug with TotalView, I could not reproduce the bug anymore.... because I simply suppress any optimization. So, I did more test and found that if the default -O2 optimization is used, GEOS-Chem crashes. But it works fine with -O1. It is hard to tell what happens, since only the emissions step is done between reading the restart file and the crash.
Bob and I will further test Ifort 10 for optimization on our machines. Maybe we will find something... For the time being, you may have to switch to -O1, at least for the run that crashes. You will find the optimization flag at the beginning of the Makefile.ifort.

Long story short: This appears to be an optimization issue with IFORT 10 and v7-04-13. Upgrading to GEOS-Chem v8-01-01 should solve this problem.

--Bmy 10:38, 17 April 2008 (EDT)