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Documentation - CT2010
Biosphere Oceans Observations Fires Fossil Fuel TM5 Nested Model Assimilation
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Biosphere Module [goto top]
1.   Introduction
The biospheric component of the carbon cycle consists of all the carbon stored in 'biomass' around us. This includes trees, shrubs, grasses, carbon within soils, dead wood, and leaf litter. Such reservoirs of carbon can exchange CO2 with the atmosphere. Exchange starts when plants take up CO2 during their growing season through the process called photosynthesis (uptake). Most of this carbon is released back to the atmosphere throughout the year through a process called respiration (release). This includes both the decay of dead wood and litter and the metabolic respiration of living plants. Of course, plants can also return carbon to the atmosphere when they burn, as described our fire emissions module documentation. Even though the yearly sum of uptake and release of carbon amounts to a relatively small number (a few petagrams (one Pg=1015 g)) of carbon per year, the flow of carbon each way is as large as 120 PgC each year. This is why the net result of these flows needs to be monitored in a system such as ours. It is also the reason we need a good physical description (model) of these flows of carbon. After all, from the atmospheric measurements we can only see the small net sum of the large two-way streams (gross fluxes). Information on what the biospheric fluxes are doing in each season, and in every location on Earth is derived from a specialized biosphere model, and fed into our system as a first guess, to be refined by our assimilation procedure.

2.   Detailed Description
The biosphere model currently used in CarbonTracker is the Carnegie-Ames Stanford Approach (CASA) biogeochemical model. This model calculates global carbon fluxes using input from weather models to drive biophysical processes, as well as satellite observed Normalized Difference Vegetation Index (NDVI) to track plant phenology. The version of CASA model output used so far was driven by year specific weather and satellite observations, and including the effects of fires on photosynthesis and respiration (see van der Werf et al., [2006] and Giglio et al., [2006]). This simulation gives 1° x 1° global fluxes on a monthly time resolution.

Net Ecosystem Exchange (NEE) is re-created from the monthly mean CASA Net Primary Production (NPP) and ecosystem respiration (RE). Higher frequency variations (diurnal, synoptic) are added to Gross Primary Production (GPP=2*NPP) and RE(=NEE-GPP) fluxes every 3 hours using a simple temperature Q10 relationship assuming a global Q10 value of 1.5 for respiration, and a linear scaling of photosynthesis with solar radiation. The procedure is very similar, but NOT identical to the procedure in Olsen and Randerson [2004] and based on ECMWF analyzed meteorology. Note that the introduction of 3-hourly variability conserves the monthly mean NEE from the CASA model. Instantaneous NEE for each 3-hour interval is thus created as:

NEE(t) = GPP(I, t) + RE(T, t)

GPP(t) = I(t) * (∑(GPP) / ∑(I))

RE(t) = Q10(t) * (∑(RE) / ∑(Q10))

Q10(t) = 1.5((T2m-T0) / 10.0)

where T=2 meter temperature, I=incoming solar radiation, t=time, and summations are done over one month in time, per gridbox. The instantaneous fluxes yielded realistic diurnal cycles when used in the TransCom Continuous experiment.

Fig 1. Map of optimized global biosphere fluxes. The pattern of net ecosystem exchange (NEE) of CO2 of the land biosphere averaged over the time period indicated, as estimated by CarbonTracker. This NEE represents land-to-atmosphere carbon exchange from photosynthesis and respiration in terrestrial ecosystems, and a contribution from fires. It does not include fossil fuel emissions. Negative fluxes (blue colors) represent CO2 uptake by the land biosphere, whereas positive fluxes (red colors) indicate regions in which the land biosphere is a net source of CO2 to the atmosphere. Units are gC m-2 yr-1.

CarbonTracker uses fluxes from CASA runs for the GFED project as its first guess for terrestrial biosphere fluxes. We have found a significantly better match to observations when using this output compared to the fluxes from a neutral biosphere simulation. Prior to CT2010, we used version 2 of the CASA-GFED model, which was driven by AVHRR NDVI, scaled to represent MODIS fPAR. Recently the GFED team has transitioned to version 3.1 of their model, driven directly by MODIS fPAR. We have found that the newer CASA-GFEDv3 product has a smaller seasonal cycle than the older CASA-GFEDv2. While we would eventually like to use the CASA-GFEDv3 results for our first-guess terrestrial fluxes, this will require a re-tuning of the CarbonTracker optimization system, which is a research problem currently under investigation.

CarbonTracker 2010 is a reanalysis of the 2007-2009 period using new fossil fuel emissions, CASA-GFEDv3 fire emissions, and first-guess biosphere model fluxes derived from CASA-GFEDv2. Prior to 2007, the results are identical to CT2009. For the reanalyzed period (2007-2009), we use fire emissions from CASA-GFEDv3 but NPP and Re from CASA-GFEDv2. This hybrid terrestrial biosphere model approach allows us to use CASA-GFEDv2 NPP and Re for which CarbonTracker is currently tuned, while also imposing the fire emissions from the most up-to-date CASA-GFEDv3 model. Note that NPP and Re are driven by real NDVI data in 2007 and 2008, while 2009 NPP and Re fluxes are composed of the climatological prior for 2001-2008.

Use of a climatological prior is not unprecedented. For several previous releases, we have used climatological first-guess fluxes for the final year in the analysis cycle.

Due to the inclusion of fires, inter-annual variability in weather and NDVI (or fPAR), the fluxes for North America start with a small net flux even when no assimilation is done. This flux ranges from 0.05 PgC yr-1 of release, to 0.15 PgC yr-1 of uptake.

3.   Further Reading