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Documentation (CT2009)
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 here. 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 Pg 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 1x1 degree 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.

The current CarbonTracker release was based on the CASA runs for the GFED2 project to estimate fire emissions. We found a significantly better match to observations when using this output compared to the fluxes from a neutral biosphere simulation. Due to the inclusion of fires, inter-annual variability in weather and NDVI, 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 of release, to 0.15 PgC/yr of uptake.

3.   Further Reading