Talk:GEOS-Chem v8-02-04

Discussion about Mean OH in v8-02-04 benchmarks

Jennifer Logan wrote:

Hello all.

Can anyone explain what is going on with global mean OH? It has become incredibly version dependent.

Starting with the reprocessed met fields in mid-2009:

   v8-01-04 Run2   11.10
   v8-02-01 Run0   11.82  (updated J values and mechanism)
   v8-02-04 Run0   10.45  (new hybrid MEGAN)
   v8-02-04 Run1   11.05   (as above with new LAI)
   v8-02-04 Run2   11.07   (as above, with LINOZ)

So we are back where we started. Claire's link about HNO4 doesn't help me. Was there an error in the new chemistry release, and was the result in v8-02-01 just plain wrong? Did the new MEGAN affect global mean OH in going to v8-02-04 Run0? How could we tell, if there was a mistake in v8-02-01 Run0.

If I am confused, imagine the outside user. Once this is sorted out, there needs to be some clarification on the wiki.

Jennifer

Jingqiu Mao wrote:

I believe that there was a typo in jv_spec.dat of v8-02-01 for HCHO photolysis. And it was fixed in v8-02-04.

So the number in v8-02-01 is incorrect. But I am not sure how the new MEGAN affect OH.

Jingqiu

Claire Carouge wrote:

I had a look at the Isoprene emissions from MEGAN in both Run0 (old LAI) and Run1 (MODIS LAI). The total emissions are [below].

   ISOP emissions in v8-02-04-Run0 (with AVHRR LAI)
   ------------------------------------------------
   200501 Total =      45.6478 Tg C
   200502 Total =      41.4675 Tg C
   200503 Total =      43.6536 Tg C
   200504 Total =      42.3698 Tg C
   200505 Total =      40.7229 Tg C
   200506 Total =      44.8216 Tg C
   200507 Total =      50.4102 Tg C
   200508 Total =      51.3827 Tg C
   200509 Total =      51.2387 Tg C
   200510 Total =      53.3078 Tg C
   200511 Total =      49.7893 Tg C
   200512 Total =      43.1464 Tg C
                      -------------
   Annual emissions:  557.9583 Tg C 

   ISOP emissions in v8-02-04-Run1 (with MODIS LAI)
   ------------------------------------------------
   200501 Total =      39.2219 Tg C
   200502 Total =      37.0537 Tg C
   200503 Total =      39.6395 Tg C
   200504 Total =      38.7458 Tg C
   200505 Total =      37.3399 Tg C
   200506 Total =      39.9473 Tg C
   200507 Total =      43.8768 Tg C
   200508 Total =      45.0809 Tg C
   200509 Total =      44.6310 Tg C
   200510 Total =      44.2945 Tg C
   200511 Total =      40.0349 Tg C
   200512 Total =      35.9431 Tg C
                     --------------
   Annual emissions:  485.8093 Tg C 

There is quite a big drop in Isoprene emissions which may be can explain the increase in OH.

I'm not sure what is different in the LAI that would cause such a decrease in Isoprene emissions.

Claire

Daniel Jacob wrote:

Is the new "hybrid" MEGAN mainly defined by MODIS LAI? I thought there were other changes. Did Michael Barkley approve of the change in isoprene emissions? If not we should submit those differences to him (and to Dylan M.) to make sure they're understood. Daniel

Bob Yantosca wrote:

Hi all,

As you know, we have been benchmarking the v8-02-04 release w/ some 1-yr simulations. The mean OH seems to drop quite a bit in v8-02-04-Run0, which uses the new hybrid MEGAN scheme (see email trail below). We have been trying to understand this.

Here is what I've found out:

1. The implementation of MEGAN in v8-02-03 and prior versions uses the AVHRR LAI data. However, all other parts of the code that needs to use LAI data uses the "old" LAI data that goes back to Yuhang Wang.
2. The benchmark v8-02-04-Run0 uses the new MEGAN hybrid model, but still the AVHRR LAI...so this should be "close" to the MEGAN in prior versions.
3. The benchmark v8-02-04-Run1 uses the MODIS LAI. Also all other parts of the code that need LAI also use MODIS LAI.

It's apparent that the switch from AVHRR LAI to MODIS LAI causes a decrease in isoprene emissions totals:

1. ISOP emissions in v8-02-04-Run0 (with AVHRR LAI): 557.9583 Tg C
2. ISOP emissions in v8-02-04-Run1 (with MODIS LAI): 485.8093 Tg C

I have made a couple of PDF files to plot out the monthly differences for LAI and ISOP emissions from the 2 benchmark runs. The files are:

1. MODIS_LAI_vs_AVHRR_LAI.pdf
2. Isoprene_Run0_vs_Run1.pdf

You can see that the MODIS LAI are much lower than the AVHRR LAI in most places on the globe, particularly over Amazonia and North America. I believe that this is what is driving the lower isoprene emissions.

Also note: the AVHRR LAI data are for the year 2000 only, whereas the MODIS LAI are for 2005 (the same year as the benchmark simulation). So this should be a robust representation of the difference between the two benchmark simulations.

Mike B, Dylan M, May: Should we expect such differences in ISOP emissions? Do you see any problems w/ the MODIS LAI data? Comments appreciated...

Thanks,

Bob Y.

Jennifer Logan wrote:

FYI: The drop in mean OH from

   v8-02-01 Run0   11.82  (updated J values and mechanism)

to

   v8-02-04 Run0   10.45  (new hybrid MEGAN)

was not just because of MEGAN. There was an error in v8-02-01 Run0.

To quote Jingqiu:

"I believe that there was a typo in jv_spec.dat of v8-02-01 for HCHO photolysis. And it was fixed in v8-02-04. So the number in v8-02-01 is incorrect. But I am not sure how the new MEGAN affects OH."

Jennifer

Jingqiu Mao wrote:

One more thing, this error in v8-02-01 Run0 may only explain 0.8e6 (or less) of global mean OH, considering the change from v8-01-04 to v8-02-01. So there has to be other reasons for this drop.

Jingqiu

Daniel Jacob wrote:

OK, and now we see from Bob's email that the global drop in isoprene emissions from the updated MODIS LAI is only 15%, which doesn't seem worth worrying about. So do we still have a problem with OH trends? Daniel

Dylan Millet wrote:

Hi everyone -- The emission totals Bob lists are in line with what others (Mike, Claire, Eloise) have reported for MEGANv2.1 and MODIS LAI (~460 TgC using GEOS-4). I don't see a problem there.

The 5% increase in OH (10.45 to 11.05) from v8-2-4 Run0 to v8-2-4 Run1 seems consistent with the 15% drop in isoprene emissions between the two runs (due to changing LAI). I don't know how to interpret the change from v8-2-1 given the error Jingqiu mentions in v8-2-1. And v8-1-4 was before the big chemistry update.

The pre-v8-2-4 implementation of MEGAN gives ~400TgC isoprene emission with GEOS-4. 560TgC is high; in my opinion we should go ahead with the MEGAN update, but not advise its use with the old AVHRR LAI .

Dylan

Mike Barkley wrote:

Hi all,

Both the hybrid & PCEEA models accurately follow the latest MEGAN algorithms. Additionally, AVHRR LAI was thought to be biased too high (sorry can't presently locate the relevant paper) & that MODIS C5 collection was an improvement. On this basis, the emissions should be more accurate than in previous GEOS-Chem versions, so irrespective of the impact on OH (5% is reasonable anyway), it would seem logical to press ahead with their implementation. If the isoprene emissions are > 500 Tg C then then subsequent chemistry is going to be badly affected. Personally, I would not use the AVHRR LAI data.

Mike

Bob Yantosca wrote:

Hi Mike (and all),

Thanks for clarifying about the isoprene.

We chose the benchmark runs such that Run0 would kind of mimic (as much as possible) the prior benchmark simulation v8-02-01. So we used MEGAN + AVHRR LAI, since that what was used before. Then we set up Run1 to see the impact of turning on the MODIS LAI had on the biogenic emissions. Now we have our answer.

Maybe it might not be a bad idea to force using MODIS LAI w/ the new MEGAN (i.e. if the user sets the switch to use AVHRR LAI in input.geos, then override it manually and print out a message to the log file). That would prevent any errors caused by someone forgetting to set the right switch in input.geos. We have already frozen v8-02-04 but could easily add this to v8-02-05.

Thanks again for your feedback! Best,

Bob Y.

Mike Barkley wrote:

One thing about the MODIS LAI, at high northern latitudes (typically > 60ish degrees) during the winter months (DJF) there are missing pixels that haven't been processed owing to cloud cover, high surface reflectance, etc. In such cases here I've assigned a default LAI value of zero. The true LAI could be > 0 but is most likely to be small <1.0. The cold temperatures, low light levels & snow cover would make the bigoenic VOC emissions negligible anyway, so using LAI=0 should be ok in this respect.

--Bob Y. 11:10, 17 February 2010 (EST)