Input convective precipitation flux is zero in GEOS-FP (6/2020+) and GEOS-IT

[lizziel]

Your name

Lizzie Lundgren

Your affiliation

Harvard University

What happened? What did you expect to happen?

Viral Shah reports:

There was an error in archiving the convective precipitation fields (PFLCU and PFLCI) in our GEOS-IT and GEOS-FP data after the switch to Grell-Freitas (the fields are just set to zero). These fields are used in GEOS-Chem to calculate convective washout and re-evaporation

This bug impacts all GEOS-FP fields starting in Jun 2020 and the entire GEOS-IT inventory.

[yuanjianz]

Glad to find this after I have just reported this to Randall (What a coincidence! :anguished: ). So it is confirmed to be a bug instead of a feature. It seems GEOS-IT has a much large convective rainfall re-evaporation than MERRA-2 here. Does it mean that we might be missing a lot of downward transport through convective precipitation and re-evaporation?

plot above is from 1 year MERRA-2 v.s. GEOS-IT transport tracer simulation for 2019

I am also curious if a complete re-run of GEOS is needed to regenerate these fields. If so, how long would it be estimated to take? @lizziel @viral211

[viral211]

Hi @yuanjianz. Thanks for the plots. We cannot redo the entire GEOS-IT run, but we might be able to implement a fix moving forward, at least for GEOS-FP. We could also try to calculate the missing fields from DQRCU and REEVAPCN. I am trying to find out if that would work.

[yuanjianz]

According to #1454, MERRA-2 precipitation flux reconstructed from DQR* - REEVAP* is lower than PFI* + PFL* by about 30-40%. In GEOS-IT, my tests shows that at least LSAN is lower when reconstructing from DQRLSAN and REEVAPLS. Maybe fixed GEOS-FP runs can give us more detailed hints, @viral211.

* from 1 year GEOS-IT transport tracer simulation for 2019 ** reconstruction is by DRQLSAN - REEVAPLS and cumulative summation from lev=0 to lev=72 (caveat: a center diagnostic while PF* is level diganostic)

I also notice that PRECCON is preserved in GEOS-IT archive. I wonder if it is possible to utlize this information. For example, scale grid-wise reconstructed precipitation flux with PRECCON(surface flux)/reconstructed surface flux.

[viral211]

In GF, DQRCU is the net precipitation source, and we can calculate the precipitation fluxes and the re-evaporated fraction from it alone - with just unit conversion. DQR is in units of kg(precip) / kg (air) / s, and precip fluxes in kg(precip) / m2 / s. REEVAPCN is the cumulative (over layers) re-evaporated flux, which is different from the previous definition, but we don’t need to use that field now anyway.

Below are some plots comparing the different fields and showing minor differences (probably because I am using dry air density instead of total for the unit conversion).

Surface convective precip flux from a 1-hour GEOS (1 degree) run:

Convective precip flux profile from the GEOS run (GEOS levels are flipped compared to GEOS-Chem):

Monthly convective precip flux from a GEOS-Chem run with GEOS-IT fields (from Lizzie’s transport tracer run at 2x2.5)

[viral211]

@yuanjianz

According to https://github.com/geoschem/geos-chem/issues/1454, MERRA-2 precipitation flux reconstructed from DQR - REEVAP is lower than PFI + PFL by about 30-40%. In GEOS-IT, my tests shows that at least LSAN is lower when reconstructing from DQRLSAN and REEVAPLS. Maybe fixed GEOS-FP runs can give us more detailed hints,

This is expected because DQRLSAN does not include all sources of precipitation (eg. accretion of cloud drops by rain). We don't have a separate 3D field with these precip sources, but they can be calculated from PFL+PFILSAN, and REEVAP if needed.

[lizziel]

@lizziel:

Thank you Viral. Would this approach work with RAS-generated fields as well? If yes, we could retire PFLC* met arrays and avoid awkward switches between using them or not based on meteorology and simulation date.

@viral211:

Unfortunately, this doesn’t work for RAS. DQRCU is defined differently there. Yes, it would be awkward to have a switch based on simulation time or met product. I suggest a switch based on a comparison of PFLCN+PFICN at level 1 with PRECCON. For RAS, they should be equal. If PRECCON>0, but PFLCN+PFICN==0 (or say < 0.1 * PRECCON), then calculate PFLCN+PFICN from DQR.

This is a great suggestion. Let's follow up after the GCSC meeting on next steps. I can put together a PR and test, or you can if you prefer. This fix will go into 14.5.1.

[lizziel]

The plan moving forwards is I will implement a fix in the code. Since we will use existing fields in the logic paths the data processing will not need to change. Tagging @yidant.