We used continuous measurements of atmospheric CO2 at two stations to investigate potential errors in inversions of temporal averages of satellite clear‐sky column retrievals. Compared to the ...complete data sets, the mid‐day CO2 on clear days was systematically lower with a larger winter difference. Net ecosystem exchange (NEE) of CO2 was enhanced on clear vs. all days, the summer boundary layer was deeper, and the CO concentration was systematically lower. During winter these differences cannot account for the CO2 bias, which must be caused by advection. Summertime errors reflect a tradeoff between deeper mixing and enhanced NEE on clear days. If these sites represent mid‐latitude forests and if the CO2 difference is confined to the bottom 15% column mass, then inversions of temporally‐averaged satellite column data products will incur a −0.2 to −0.4 ppm bias. CO2 concentrations must therefore be assimilated at the place and time observed.
Convective turbulence within the atmospheric boundary layer (ABL) and movement of the ABL over the surface results in a large spatial (104–105 km2) integration of surface 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.
Representing spatially varying precipitation for current grid length scales used in General Circulation Models (GCMs) is a continuing challenge. Furthermore, to fully capture the hydrologic effects ...of nonuniform precipitation, a representation of soil moisture heterogeneity and distribution of spatially varying precipitation must exist within the same framework. For this study, the explicit and sampling methods of Sellers et al. (2007) are tested off‐line using the Simple Biosphere Model (SiB3) in an arid, semiarid, and wet site, and are numerically compared to the bulk method, which is currently used in GCMs. To carry out the numerical experiments, an arbitrary grid area was defined by (1) a single instance of SiB3 (bulk method), (2) 100 instances of SiB3 (explicit method), and (3) less than 100 instances of SiB3 (sampling method). Precipitation was randomly distributed over fractions of the grid area for the explicit and sampling methods, while the standard SiB3 exponential distribution relating precipitation intensity to the grid area wet fraction was used in the bulk method. Comparing the sampling and bulk method to the explicit method indicates that 10 instances of SiB3 in the sampling method better captures the spatial variability in soil moisture and grid area flux calculations produced by the explicit method, and deals realistically with spatially varying precipitation at little additional computational cost to the bulk method.
Key Points
Spatially varying precipitation and soil moisture are simulated with SiB3
Bulk and sampled method simulations are compared to an explicit method
Sampled method simulations approach the explicit method
Ecosystem fluxes of energy, water, and CO2 result in spatial and temporal variations in atmospheric properties. In principle, these variations can be used to quantify the fluxes through inverse ...modelling of atmospheric transport, and can improve the understanding of processes and falsifiability of models. We investigated the influence of ecosystem fluxes on atmospheric CO2 in the vicinity of the WLEF‐TV tower in Wisconsin using an ecophysiological model (Simple Biosphere, SiB2) coupled to an atmospheric model (Regional Atmospheric Modelling System). Model parameters were specified from satellite imagery and soil texture data. In a companion paper, simulated fluxes in the immediate tower vicinity have been compared to eddy covariance fluxes measured at the tower, with meteorology specified from tower sensors. Results were encouraging with respect to the ability of the model to capture observed diurnal cycles of fluxes. Here, the effects of fluxes in the tower footprint were also investigated by coupling SiB2 to a high‐resolution atmospheric simulation, so that the model physiology could affect the meteorological environment. These experiments were successful in reproducing observed fluxes and concentration gradients during the day and at night, but revealed problems during transitions at sunrise and sunset that appear to be related to the canopy radiation parameterization in SiB2.
We evaluated how climate influences interannual variability in the terrestrial Net Ecosystem Exchange (NEE) of CO2 using the Simple Biosphere Model, Version 2 (SiB2) for 1983 to 1993 on a global, 1° ...by 1° latitude/longitude grid with a 10‐min time step. We quantified climate influences on NEE, explained regional differences, and related NEE variability to the Arctic Oscillation (AO) and the El Niño‐Southern Oscillation (ENSO). The simulated NEE reproduces the salient features and magnitude of the measured global CO2 growth rate. The Northern Hemisphere shows a pattern of alternating positive and negative NEE anomalies that cancel such that the tropics dominate the global simulated NEE interannual variability. Climate influences have strong regional differences with precipitation dominating in the tropics and temperature in the extratropics. In tropical regions with drier soils, precipitation control of photosynthesis (i.e., drought stress) dominates; in nearly saturated soils, precipitation control of respiration dominates. Because of cancellation and competing effects, no single climate variable controls global or regional NEE interannual variability. Globally, precipitation accounts for 44% of NEE variability; followed by Leaf Area Index (23%), soil carbon (12%), and temperature (16%). The influence of ENSO on NEE variability is consistent with that expected for shifting precipitation patterns in the tropics. Except in northern Europe, temperature advection by the AO does not significantly influence NEE variability. Neither the AO nor ENSO fully explain the temperature influence on respiration or the simulated NEE anomaly pattern in the Northern Hemisphere.
Abstract
Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global ...monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO
2
concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO
2
in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO
2
with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.
Key Points
Carbonyl sulfide can help falsify carbon cycle models
Carbonyl sulfide can aid separation of NPP into GPP and Resp
The oceanic COS source is probably much larger than currently thought
Inverse estimation of carbon dioxide (CO2) sources and sinks uses atmospheric CO2 observations, mostly made near the Earth's surface. However, transport models used in such studies lack perfect ...representation of atmospheric dynamics and thus often fail to produce unbiased forward simulations. The error is generally larger for observations over the land than those over the remote/marine locations. The range of this error is estimated by using multiple transport models (16 are used here). We have estimated the remaining differences in CO2 fluxes due to the use of ocean‐only versus all‐sites (i.e., over ocean and land) observations of CO2 in a time‐independent inverse modeling framework. The fluxes estimated using the ocean‐only networks are more robust compared to those obtained using all‐sites networks. This makes the global, hemispheric, and regional flux determination less dependent on the selection of transport model and observation network.
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