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Documentation - CT2011_oi
Biosphere Oceans Observations Fires Fossil Fuel TM5 Nested Model Assimilation
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Terrestrial Biosphere Module [goto top]
1.   Introduction
The biospheric component of the terrrestrial 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 0.5° x 0.5° 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 is 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.

The record of atmospheric CO2 calls for a deeper terrestrial biosphere sink than that generally simulated by forward models like CASA-GFED. This is manifested by a larger annual cycle of terrestrial biosphere fluxes, and in particular a deeper boreal summer uptake of carbon dioxide, in the posterior optimized fluxes compared to the prior models (See Box 1). We call upon the atmospheric CO2 observations to make this change, and in order to handle these prior model differences the ensemble Kalman filter's prior covariance model has been re-tuned. In short, this prior uncertainty needs to comfortably span differences among the terrestrial biosphere priors, the fossil fuel emissions priors, and adjustments to fluxes required to bring model predictions into agreement with observations. As a result, the land biosphere prior uncertainty has been doubled in CT2011 in comparison to previous releases. Details can be found on the assimilation scheme documentation page.

Box 1. Comparison of terrestrial biosphere flux priors

Time series of global-total terrestrial biosphere flux between the two priors and the CT2011 posterior. Global CO2 uptake by the land biosphere, expressed in PgC yr-1, excluding emissions by wildfire. Positive flux represents emission of CO2 to the atmosphere, and the negative fluxes indicate times when the land biosphere is a sink of CO2. While both priors manifest similar annual cycles of uptake in boreal summer balanced by emission in boreal winter, the GFED3 prior (tan) has an annual cycle that is about 10% smaller than that of GFED2 (green). Optimization against atmospheric CO2 data requires a larger land sink than in either prior, which effectively requires a deeper annual cycle. This is shown by the CT2011 posterior (black).

Differences in long-term mean terrestrial biosphere fluxes between the two priors. Red indicates areas where the GFED3 prior has less terrestrial uptake (or more outgassing to the atmosphere) than the GFED2 prior, and blue represents the opposite. Units are gC m-2 yr-1.

Unlike CT2010, CarbonTracker 2011 is a full reanalysis of the 2000-2010 period using new fossil fuel emissions, CASA-GFEDv3 fire emissions, and first-guess biosphere model fluxes derived from CASA-GFEDv2 for 4 of our inversions, and from CASA-GFEDv3 for the remaining 4 inversions.

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