John R. Worden

New global observations of the terrestrial carbon cycle from GOSAT: patterns of plant fluorescence with gross primary productivity

Our ability to close the Earths carbon budget and predict feedbacks in a warming climate depends critically on knowing where, when and how carbon dioxide is exchanged between the land and atmosphere. Terrestrial gross primary production (GPP) constitutes the largest flux component in the global carbon budget, however significant uncertainties remain in GPP estimates and its seasonality. Empirically, we show that global spaceborne observations of solar induced chlorophyll fluorescence – occurring during photosynthesis – exhibit a strong linear correlation with GPP. We found that the fluorescence emission even without any additional climatic or model information has the same or better predictive skill in estimating GPP as those derived from traditional remotely-sensed vegetation indices using ancillary data and model assumptions. In boreal summer the generally strong linear correlation between fluorescence and GPP models weakens, attributable to discrepancies in savannas/croplands (18–48% higher fluorescence-based GPP derived by simple linear scaling), and high-latitude needleleaf forests (28–32% lower fluorescence). Our results demonstrate that retrievals of chlorophyll fluorescence provide direct global observational constraints for GPP and open an entirely new viewpoint on the global carbon cycle. We anticipate that global fluorescence data in combination with consolidated plant physiological fluorescence models will be a step-change in carbon cycle research and enable an unprecedented robustness in the understanding of the current and future carbon cycle.

Importance of rain evaporation and continental convection in the tropical water cycle.

Atmospheric moisture cycling is an important aspect of the Earth’s climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

Improved solar Lyman α irradiance modeling from 1947 through 1999 based on UARS observations

The solar Lyman α radiation is the brightest solar vacuum ultraviolet (VUV: λ < 200 nm) emission, and this radiation is deposited in Earths atmosphere above 70 km. The Lyman α irradiance and its variability are therefore important for many studies of the Earths upper atmosphere. A long-term data set of the solar Lyman α irradiance from 1947 through 1999 is constructed using the measurements from the Atmospheric Explorer E (AE-E), the Solar Mesospheric Explorer (SME), and the Upper Atmosphere Research Satellite (UARS) along with predictions from proxy models to fill in data gaps and to extrapolate back to 1947. The UARS measurement is used as the reference, and the AE-E and SME measurements and the proxy models are adjusted to agree with the UARS values. The estimated 1-σ uncertainty for this long-term Lyman α time series is 10%. The average solar rotation (27-day) variability in Lyman α is 9% from this composite times series, and the solar rotation variability averaged over 2 years at solar minimum and maximum conditions is 5 and 11%, respectively. The average solar cycle (11-year) variability is a factor of 1.5 when the data are smoothed over 2 years, and the extreme Lyman α variability is a factor of 2.1. The Lyman α irradiances averaged over 2 years during solar minimum and maximum conditions are 3.7 and 5.6 × 1011 photons s−1 cm−2, respectively. The proxy models include three components to better fit the UARS measurements; nonetheless, there remain differences between the proxy models and the observed Lyman α irradiance, which are related to the source of the Lyman α radiation being different than that for the proxies. The available proxies are primarily chromospheric and coronal emissions, whereas the Lyman α variability is manifested more in the transition region. It is shown that emissions throughout the chromosphere, transition region, and corona vary differently mainly because their contrasts for active network and plage components are different. A transition region proxy is needed to improve the empirical proxy models of solar irradiance, and this composite Lyman α time series could serve as a proxy for other transition region emissions.

Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopologues: 1. Comparison between models and observations

The goal of this study is to determine how H2O and HDO measurements in water vapor can be used to detect and diagnose biases in the representation of processes controlling tropospheric humidity in atmospheric general circulation models (GCMs). We analyze a large number of isotopic data sets (four satellite, sixteen ground-based remote-sensing, five surface in situ and three aircraft data sets) that are sensitive to different altitudes throughout the free troposphere. Despite significant differences between data sets, we identify some observed HDO/H2O characteristics that are robust across data sets and that can be used to evaluate models. We evaluate the isotopic GCM LMDZ, accounting for the effects of spatiotemporal sampling and instrument sensitivity. We find that LMDZ reproduces the spatial patterns in the lower and mid troposphere remarkably well. However, it underestimates the amplitude of seasonal variations in isotopic composition at all levels in the subtropics and in midlatitudes, and this bias is consistent across all data sets. LMDZ also underestimates the observed meridional isotopic gradient and the contrast between dry and convective tropical regions compared to satellite data sets. Comparison with six other isotope-enabled GCMs from the SWING2 project shows that biases exhibited by LMDZ are common to all models. The SWING2 GCMs show a very large spread in isotopic behavior that is not obviously related to that of humidity, suggesting water vapor isotopic measurements could be used to expose model shortcomings. In a companion paper, the isotopic differences between models are interpreted in terms of biases in the representation of processes controlling humidity. Copyright

Forest productivity and water stress in Amazonia: observations from GOSAT chlorophyll fluorescence

It is unclear to what extent seasonal water stress impacts on plant productivity over Amazonia. Using new Greenhouse gases Observing SATellite (GOSAT) satellite measurements of sun-induced chlorophyll fluorescence, we show that midday fluorescence varies with water availability, both of which decrease in the dry season over Amazonian regions with substantial dry season length, suggesting a parallel decrease in gross primary production (GPP). Using additional SeaWinds Scatterometer onboard QuikSCAT satellite measurements of canopy water content, we found a concomitant decrease in daily storage of canopy water content within branches and leaves during the dry season, supporting our conclusion. A large part (r2 = 0.75) of the variance in observed monthly midday fluorescence from GOSAT is explained by water stress over moderately stressed evergreen forests over Amazonia, which is reproduced by model simulations that include a full physiological representation of photosynthesis and fluorescence. The strong relationship between GOSAT and model fluorescence (r2 = 0.79) was obtained using a fixed leaf area index, indicating that GPP changes are more related to environmental conditions than chlorophyll contents. When the dry season extended to drought in 2010 over Amazonia, midday basin-wide GPP was reduced by 15 per cent compared with 2009.

Implementation of cloud retrievals for Tropospheric Emission Spectrometer (TES) atmospheric retrievals: part 1. Description and characterization of errors on trace gas retrievals

terms of a set of frequency-dependent nonscattering optical depths and a cloud height. These cloud parameters are retrieved jointly with surface temperature, emissivity, atmospheric temperature, and trace gases such as ozone from spectral data. We demonstrate the application of this approach using data from the Tropospheric Emission Spectrometer (TES) and test data simulated with a scattering radiative transfer model. We show the value of this approach in that it results in accurate estimates of errors for trace gas retrievals, and the retrieved values improve over the initial guess for a wide range of cloud conditions. Comparisons are made between TES retrievals of ozone, temperature, and water to model fields from the Global Modeling and Assimilation Office (GMAO), temperature retrievals from the Atmospheric Infrared Sounder (AIRS), tropospheric ozone columns from the Goddard Earth Observing System (GEOS) GEOS-Chem, and ozone retrievals from the Total Ozone Mapping Spectrometer (TOMS). In each of these cases, this cloud retrieval approach does not introduce observable biases into TES retrievals.

Solar Extreme Ultraviolet Irradiance Measurements During Solar Cycle 22

The solar extreme ultraviolet (EUV) irradiance, the dominant global energy source for Earths atmosphere above 100 km, is not known accurately enough for many studies of the upper atmosphere. During the absence of direct solar EUV irradiance measurements from satellites, the solar EUV irradiance is often estimated at the 30–50% uncertainty level using both proxies of the solar irradiance and earlier solar EUV irradiance measurements, primarily from the Air Force Geophysics Laboratory (now Phillips Laboratory) rockets and Atmospheric Explorer (AE) instruments. Our sounding rocket measurements during solar cycle 22 include solar EUV irradiances below 120 nm with 0.2 nm spectral resolution, far ultraviolet (FUV) airglow spectra below 160 nm, and solar soft X-ray (XUV) images at 17.5 nm. Compared to the earlier observations, these rocket experiments provide a more accurate absolute measurement of the solar EUV irradiance, because these instruments are calibrated at the National Institute of Standards and Technology (NIST) with a radiometric uncertainty of about 8%. These more accurate sounding-rocket measurements suggest revisions of the previous reference AE–E spectra by as much as a factor of 2 at some wavelengths. Our sounding-rocket flights during the past several years (1988–1994) also provide information about solar EUV variability during solar cycle 22.

Understanding the Sahelian water budget through the isotopic composition of water vapor and precipitation

The goal of this paper is to investigate the added value of water isotopic measurements to estimate the relative influence of large-scale dynamics, convection, and land surface recycling on the Sahelian water budget. To this aim, we use isotope data in the lower tropospheric water vapor measured by the SCIAMACHY and TES satellite instruments and in situ precipitation data from the Global Network for Isotopes in Precipitation and collected during the African Monsoon Multidisciplinary Analysis field campaign, together with water-tagging experiments with the Laboratoire de Meteorologie Dynamique general circulation model (LMDZ) fitted with isotopes. We show that some isotopic biases in LMDZ reveal the misrepresentation of dehydrating processes that would be undetected without isotopic measurements. In dry regions, the vapor isotopic composition is primarily controlled by the intensity of the air dehydration. In addition, it may also keep some memory of dehydration pathways that is erased in the humidity distribution, namely the relative contribution of dehydration in the tropical upper troposphere versus midlatitudes. In wet regions, vapor and rain isotope compositions are primarily controlled by changes in convection, through rain reevaporation and through the progressive depletion of the vapor by convective mixing along air mass trajectories. Gradients in vapor isotope composition along air mass trajectories may help estimate continental recycling intensity, provided that we could quantify the effect of convection on the isotopic composition of water vapor.

Ozone-CO correlations determined by the TES satellite instrument in continental outflow regions

0.4– 1.0 mol mol � 1 and consistent with ICARTT data. The GEOS-Chem model reproduces the O3-CO enhancement ratios observed in continental outflow, but model correlations are stronger and more extensive. We show that the discrepancy can be explained by spectral measurement errors in the TES data. These errors will decrease in future data releases, which should enable TES to provide better information on O3-CO correlations. Citation: Zhang, L., et al. (2006), Ozone-CO correlations determined by the TES satellite instrument in continental outflow regions, Geophys. Res. Lett., 33, L18804, doi:10.1029/2006GL026399.

Evolution of Chromospheric Structures Derived from Ca II K Spectroheliograms: Implications for Solar Ultraviolet Irradiance Variability

Over 1400 National Solar Observatory Ca II K spectroheliograms are analyzed for solar structure evolution for the years 1980, 1985, 1987, 1988, 1989, and 1992 through 1996 July with about a 50% time coverage. This time period includes the maximum of solar cycle 21 and almost all phases of solar cycle 22. The spectral bandpass and spatial resolution of these images are 0.5 A and about 85, respectively. The plage, enhanced-network, active-network, and quiet-Sun features are identified on each image with an algorithm that uses criteria of intensity, size, filling factor, and continuity. This decomposition leads to time series of the plage and network areas, and their spatial distribution on the solar disk, which describe the solar cycle evolution of these structures. Thus, either the resultant structure masks or structure time series can be used for modeling the solar irradiance at other wavelengths. We cannot accurately identify all of the active network on the solar disk in Ca II K because the active-network intensities are not much greater than those of the quiet Sun. Therefore, we identify only the brighter active-network structures. However, the active network we identify on the Ca II K image can be used as a proxy for active-network evolution over the solar cycle. Many results that are useful for modeling solar irradiance variability are also presented. We find that the intensity contrasts (ratio of structure intensity to quiet Sun intensity) of the plage, enhanced network, and active network, averaged over the solar disk, are 1.95, 1.51, and 1.33, respectively. These contrasts remain essentially constant over the solar cycle. As expected, we find that the plage and the enhanced-network time series show a strong 27 day rotational modulation, but we also find that the active network can have a weak rotational modulation despite its wider longitudinal dispersion. The plage and enhanced network typically cover about 13% and 10%, respectively, of the solar disk during solar-maximum time periods. During solar moderate and minimum activity levels, the total plage and enhanced-network areas can reach zero, but the active network can still cover a large portion of the solar disk.