Reassignment of `ash_mass` in `convert_emiss.F` discards previous value

[hcbelgingur]

Describe the bug While working with the chem/convert_emiss.F code, I noticed in lines 746-747 that ash_mass is reassigned with the value of eh, which uses the value of ash_height.

          eh=2600.*(ash_height(i,j)*.0005)**4.1494
          ash_mass(i,j)=eh*1.e9/area

The previous value of ash_mass is never referred to after the initial read, so in effect, this previous value of ash_mass gets lost in line 747.

I suspect line 746 should be eh=2600.*(ash_mass(i,j)*.0005)**4.1494, replacing height with mass, or that the constant 2600 should be ash_mass.

I am not a weather scientist (I work as a developer) so I don't have a grasp of the science behind these numbers to decide what the right formula is. But the current code looks a bit odd to me.

Expected behavior I believe the newer ash_mass value should depend on its previous value.

[hcbelgingur]

Found an article that describes this behavior, it is intended to replace the ash mass value defined by the user.

Inclusion of Ash and SO2 emissions from volcanic eruptions in WRF-CHEM: development and some applications M. Stuefer , S. R. Freitas , G. Grell , P. Webley , S. Peckham , and S. A. McKeen

While the emissions preprocessor provides total mass and injection height, the latter will most often be overwritten by the user in the namelist, assuming that observations are available that are much closer to the truth.

For historic cases with known injection heights h and duration d of an eruption, the default initialization parameters are then replaced by the total erupted mass m (kg), which is empirically derived from h (m) and d (s) according to Mastinet et al. (2009):

m = ρd (0.0005 h) ^ 4.1494

The variable ρ denotes the assumed magma density of 2600 kg/m^3.

m increases significantly with injection height. It is evident that the total mass strongly depends on accurate injection heights.

[weiwangncar]

@hcbelgingur Thanks for the finding!