Daniel M. Ricciuto

Evaluation of remote sensing based terrestrial productivity from MODIS using regional tower eddy flux network observations

The Moderate Resolution Spectroradiometer (MODIS) sensor has provided near real-time estimates of gross primary production (GPP) since March 2000. We compare four years (2000 to 2003) of satellite-based calculations of GPP with tower eddy CO2 flux-based estimates across diverse land cover types and climate regimes. We examine the potential error contributions from meteorology, leaf area index (LAI)/fPAR, and land cover. The error between annual GPP computed from NASAs Data Assimilation Offices (DAO) and tower-based meteorology is 28%, indicating that NASAs DAO global meteorology plays an important role in the accuracy of the GPP algorithm. Approximately 62% of MOD15-based estimates of LAI were within the estimates based on field optical measurements, although remaining values overestimated site values. Land cover presented the fewest errors, with most errors within the forest classes, reducing potential error. Tower-based and MODIS estimates of annual GPP compare favorably for most biomes, although MODIS GPP overestimates tower-based calculations by 20%-30%. Seasonally, summer estimates of MODIS GPP are closest to tower data, and spring estimates are the worst, most likely the result of the relatively rapid onset of leaf-out. The results of this study indicate, however, that the current MODIS GPP algorithm shows reasonable spatial patterns and temporal variability across a diverse range of biomes and climate regimes. So, while continued efforts are needed to isolate particular problems in specific biomes, we are optimistic about the general quality of these data, and continuation of the MOD17 GPP product will likely provide a key component of global terrestrial ecosystem analysis, providing continuous weekly measurements of global vegetation production

Estimates of net CO2 flux by application of equilibrium boundary layer concepts to CO2 and water vapor measurements from a tall tower

fluxes that affects the CO2 and water vapor mixing ratios. We apply quasi-equilibrium concepts for the terrestrial ABL to measurements of CO2 and water vapor made within the ABL from a tall tower (396 m) in Wisconsin. We suppose that CO2 and water vapor mixing ratios in the ABL approach an equilibrium on timescales longer than a day: a balance between the surface fluxes and the exchange with the free troposphere above. By using monthly averaged ABL-to-free-tropospheric water vapor differences and surface water vapor flux, realistic estimates of vertical velocity exchange with the free troposphere can be obtained. We then estimated the net surface flux of CO2 on a monthly basis for the year of 2000, using ABL-to-free-tropospheric CO2 differences, and our flux difference estimate of the vertical exchange. These ABL-scale estimates of net CO2 flux gave close agreement with eddy covariance measurements. Considering the large surface area which affects scalars in the ABL over synoptic timescales, the flux difference approach presented here could potentially provide regional-scale estimates of net CO2 flux. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1818 Hydrology: Evapotranspiration; 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: boundary layer, CO2 exchange, evapotranspiration

Decomposing CO2 fluxes measured over a mixed ecosystem at a tall tower and extending to a region: A case study

[1]xa0CO2 fluxes for six stand types are inferred by decomposing eddy-covariance (EC) fluxes measured at a 447-m tower using footprint models and ecosystem models in a case study. The functional parameters in the ecosystem models are estimated for each stand type utilizing temporal EC flux series. The results show differences in terms of the functional parameters and fluxes among the different stand types that are consistent with general expectations for the respective stand types. The fluxes, in addition to measurements at two nearby short towers, are used for flux aggregation in the region. Comparisons suggest that it is critical for flux aggregation to distinguish the wetland from the upland. A distinction among three upland forests and between forested and lowland wetlands could be important, too. The difference in aggregated values of net ecosystem-atmospheric exchange of CO2 with the watershed function classification scheme and with the stand-type level classification scheme can reach about 250 gC m−2 season−1 over the entire growing season. Analyses suggest that the six-stand classification scheme still does not capture all the variability in stand characteristics relevant to CO2 exchange. In addition, the varying fluxes for the same stand type with location in the region challenge the widely used land-cover-based ecosystem classification scheme. It is improper to use EC measurements at any single tower to approximate CO2 fluxes in the region. Implications may help identify key ecosystem types and design more measurements in the region. Limitations and future efforts are discussed.

A nonparametric method for separating photosynthesis and respiration components in CO2 flux measurements

[1] Future climate change is expected to affect ecosystematmosphere CO2 exchange, particularly through the influence of temperature. To date, however, few studies have shown that differences in the response of net ecosystem CO2 exchange (NEE) to temperature among ecosystems can be explained by differences in the photosynthetic and respiratory processes that compose NEE. Using a new nonparametric statistical model, we analyzed data from four forest ecosystems. We observed that differences among forests in their ability to assimilate CO2 as a function of temperature were attributable to consistent differences in the temperature dependence of photosynthesis and respiration. This observation provides empirical validation of efforts to develop models of NEE from the first-principle relationships between photosynthetic and respiratory processes and climate. Our results also showed that models of seasonal dynamics in NEE that lack specific consideration of the temperature dependence of respiration and photosynthesis are likely to carry significant uncertainties. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 4806 Oceanography: Biological and Chemical: Carbon cycling. Citation: Yi, C., et al. (2004), A nonparametric method for separating photosynthesis and respiration components in CO2 flux measurements, Geophys. Res. Lett., 31, L17107, doi:10.1029/2004GL020490.

Estimating daytime CO2 fluxes over a mixed forest from tall tower mixing ratio measurements

[1]xa0Difficulties in estimating terrestrial ecosystem CO2 fluxes on regional scales have significantly limited our understanding of the global carbon cycle. This paper presents an effort to estimate daytime CO2 fluxes over a forested region on the scale of 50 km in northern Wisconsin, USA, using the tall-tower-based mixed layer (ML) budget method. Budget calculations were conducted for 2 years under fair-weather conditions as a case study. With long-term measurements of CO2 mixing ratio at a 447-m-tall tower, daytime regional CO2 fluxes were estimated on the seasonal scale, longer than in earlier studies. The flux derived from the budget method was intermediate among those derived from the eddy-covariance (EC) method at three towers in the region and overall closest to that derived from EC measurements at 396 m of the tall tower. The dormant season average daytime-integrated regional CO2 flux was about 0.35 ± 0.18 gC m−2. During the growing season, the monthly averaged daytime-integrated regional CO2 flux varied from −1.58 ± 0.19 to −4.15 ± 0.32 gC m−2, suggesting that the region was a net sink of CO2 in the daytime. We also discussed the effects on theses estimates of neglecting horizontal advection, selecting for fair-weather conditions, and using single-location measurements. Daytime regional CO2 flux estimates from the ML budget method were comparable to those from three aggregation experiments. Differences in results from the different methods, however, suggest that more constraints are needed to estimate regional fluxes with more confidence. Despite uncertainties, our analyses indicate that it is feasible to estimate daytime regional CO2 fluxes on long timescales using tall tower measurements.