Showing papers by "Goddard Space Flight Center published in 2005"

The MODIS Aerosol Algorithm, Products and Validation

Abstract: The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both NASA’s Terra and Aqua satellites is making near-global daily observations of the earth in a wide spectral range (0.41–15 m). These measurements are used to derive spectral aerosol optical thickness and aerosol size parameters over both land and ocean. The aerosol products available over land include aerosol optical thickness at three visible wavelengths, a measure of the fraction of aerosol optical thickness attributed to the fine mode, and several derived parameters including reflected spectral solar flux at the top of the atmosphere. Over the ocean, the aerosol optical thickness is provided in seven wavelengths from 0.47 to 2.13 m. In addition, quantitative aerosol size information includes effective radius of the aerosol and quantitative fraction of optical thickness attributed to the fine mode. Spectral irradiance contributed by the aerosol, mass concentration, and number of cloud condensation nuclei round out the list of available aerosol products over the ocean. The spectral optical thickness and effective radius of the aerosol over the ocean are validated by comparison with two years of Aerosol Robotic Network (AERONET) data gleaned from 132 AERONET stations. Eight thousand MODIS aerosol retrievals collocated with AERONET measurements confirm that one standard deviation of MODIS optical thickness retrievals fall within the predicted uncertainty of 0.03 0.05 over ocean and 0.05 0.15 over land. Two hundred and seventy-one MODIS aerosol retrievals collocated with AERONET inversions at island and coastal sites suggest that one standard deviation of MODIS effective radius retrievals falls within reff 0.11 m. The accuracy of the MODIS retrievals suggests that the product can be used to help narrow the uncertainties associated with aerosol radiative forcing of global climate.

Using the satellite-derived NDVI to assess ecological responses to environmental change

Abstract: Assessing how environmental changes affect the distribution and dynamics of vegetation and animal populations is becoming increasingly important for terrestrial ecologists to enable better predictions of the effects of global warming, biodiversity reduction or habitat degradation. The ability to predict ecological responses has often been hampered by our rather limited understanding of trophic interactions. Indeed, it has proven difficult to discern direct and indirect effects of environmental change on animal populations owing to limited information about vegetation at large temporal and spatial scales. The rapidly increasing use of the Normalized Difference Vegetation Index (NDVI) in ecological studies has recently changed this situation. Here, we review the use of the NDVI in recent ecological studies and outline its possible key role in future research of environmental change in an ecosystem context.

The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission

Abstract: We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on 2003 April 28. GALEX is performing the first space UV sky survey, including imaging and grism surveys in two bands (1350-1750 and 1750-2750 ?). The surveys include an all-sky imaging survey (mAB 20.5), a medium imaging survey of 1000 deg2 (mAB 23), a deep imaging survey of 100 deg2 (mAB 25), and a nearby galaxy survey. Spectroscopic (slitless) grism surveys (R = 100-200) are underway with various depths and sky coverage. Many targets overlap existing or planned surveys in other bands. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the relationship of the UV and global star formation rate in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 < z < 2 and probe the physical drivers of star formation in galaxies. The GALEX mission includes a guest investigator program, supporting the wide variety of programs made possible by the first UV sky survey.

The Swift X-ray telescope

Abstract: he Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of gamma-ray bursts (GRBs) and GRB afterglows. The X-ray telescope (XRT) enables Swift to determine GRB positions with a few arcseconds accuracy within 100 s of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM-Newton/EPIC MOS CCD detector to provide a sensitive broad-band (0.2–10 keV) X-ray imager with effective area of > 120 cm2 at 1.5 keV, field of view of 23.6 × 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2×10−14 erg cm−2 s−1 in 104 s. The instrument is designed to provide automated source detection and position reporting within 5 s of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.

An extended AVHRR 8‐km NDVI dataset compatible with MODIS and SPOT vegetation NDVI data

Abstract: Daily daytime Advanced Very High Resolution Radiometer (AVHRR) 4‐km global area coverage data have been processed to produce a Normalized Difference Vegetation Index (NDVI) 8‐km equal‐area dataset from July 1981 through December 2004 for all continents except Antarctica. New features of this dataset include bimonthly composites, NOAA‐9 descending node data from August 1994 to January 1995, volcanic stratospheric aerosol correction for 1982–1984 and 1991–1993, NDVI normalization using empirical mode decomposition/reconstruction to minimize varying solar zenith angle effects introduced by orbital drift, inclusion of data from NOAA‐16 for 2000–2003 and NOAA‐17 for 2003–2004, and a similar dynamic range with the MODIS NDVI. Two NDVI compositing intervals have been produced: a bimonthly global dataset and a 10‐day Africa‐only dataset. Post‐processing review corrected the majority of dropped scan lines, navigation errors, data drop outs, edge‐of‐orbit composite discontinuities, and other artefacts in the compos...

The Swift Ultra-Violet/Optical Telescope

Abstract: The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (∼1 min) UV and optical photons from the afterglow of gamma-ray bursts in the 170–600 nm band as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey–Chretien design with micro-channel plate intensified charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.

Efficacy of climate forcings

Abstract: [1] We use a global climate model to compare the effectiveness of many climate forcing agents for producing climate change. We find a substantial range in the “efficacy” of different forcings, where the efficacy is the global temperature response per unit forcing relative to the response to CO2 forcing. Anthropogenic CH4 has efficacy ∼110%, which increases to ∼145% when its indirect effects on stratospheric H2O and tropospheric O3 are included, yielding an effective climate forcing of ∼0.8 W/m2 for the period 1750–2000 and making CH4 the largest anthropogenic climate forcing other than CO2. Black carbon (BC) aerosols from biomass burning have a calculated efficacy ∼58%, while fossil fuel BC has an efficacy ∼78%. Accounting for forcing efficacies and for indirect effects via snow albedo and cloud changes, we find that fossil fuel soot, defined as BC + OC (organic carbon), has a net positive forcing while biomass burning BC + OC has a negative forcing. We show that replacement of the traditional instantaneous and adjusted forcings, Fi and Fa, with an easily computed alternative, Fs, yields a better predictor of climate change, i.e., its efficacies are closer to unity. Fs is inferred from flux and temperature changes in a fixed-ocean model run. There is remarkable congruence in the spatial distribution of climate change, normalized to the same forcing Fs, for most climate forcing agents, suggesting that the global forcing has more relevance to regional climate change than may have been anticipated. Increasing greenhouse gases intensify the Hadley circulation in our model, increasing rainfall in the Intertropical Convergence Zone (ITCZ), Eastern United States, and East Asia, while intensifying dry conditions in the subtropics including the Southwest United States, the Mediterranean region, the Middle East, and an expanding Sahel. These features survive in model simulations that use all estimated forcings for the period 1880–2000. Responses to localized forcings, such as land use change and heavy regional concentrations of BC aerosols, include more specific regional characteristics. We suggest that anthropogenic tropospheric O3 and the BC snow albedo effect contribute substantially to rapid warming and sea ice loss in the Arctic. As a complement to a priori forcings, such as Fi, Fa, and Fs, we tabulate the a posteriori effective forcing, Fe, which is the product of the forcing and its efficacy. Fe requires calculation of the climate response and introduces greater model dependence, but once it is calculated for a given amount of a forcing agent it provides a good prediction of the response to other forcing amounts.

The burst alert telescope (BAT) on the Swift MIDEX mission

Abstract: he burst alert telescope (BAT) is one of three instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and localizes the burst direction to an accuracy of 1–4 arcmin within 20 s after the start of the event. The GRB trigger initiates an autonomous spacecraft slew to point the two narrow field-of-view (FOV) instruments at the burst location within 20–70 s so to make follow-up X-ray and optical observations. The BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4×4×2 mm), and the coded-aperture mask is composed of ∼52,000 pieces of lead (5×5×1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 15–150 keV energy range with ∼7 keV resolution, a sensitivity of ∼10−8 erg s−1 cm−2, and a 1.4 sr (half-coded) FOV. We expect to detect > 100 GRBs/year for a 2-year mission. The BAT also performs an all-sky hard X-ray survey with a sensitivity of ∼2 m Crab (systematic limit) and it serves as a hard X-ray transient monitor.

The photometric performance and calibration of the Hubble Space Telescope advanced camera for surveys

Abstract: We present the photometric calibration of the Advanced Camera for Surveys (ACS). The ACS was installed in the Hubble Space Telescope (HST) in 2002 March. It comprises three cameras: the Wide Field Channel (WFC), optimized for deep near‐IR survey imaging programs; the High Resolution Channel (HRC), a high‐resolution imager that fully samples the HST point‐spread function (PSF) in the visible; and the Solar Blind Channel (SBC), a far‐UV imager. A significant amount of data has been collected to characterize the on‐orbit performance of the three channels. We give here an overview of the performance and calibration of the two CCD cameras (WFC and HRC) and a description of the best techniques for reducing ACS CCD data. The overall performance is as expected from prelaunch testing of the camera. Surprises were a better‐than‐predicted sensitivity in the visible and near‐IR for both the WFC and HRC and an unpredicted dip in the HRC UV response at ∼3200 A. On‐orbit observations of spectrophotometric stand...

The Photometric Performance and Calibration of the HST Advanced Camera For Surveys

Abstract: We present the photometric calibration of the HST Advanced Camera for Surveys (ACS). We give here an overview of the performance and calibration of the 2 CCD cameras, the Wide Field Channel (WFC) and the High Resolution Channel (HRC), and a description of the best techniques for reducing ACS CCD data. On-orbit observations of spectrophotometric standard stars have been used to revise the pre-launch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zeropoints for all filters in 3 magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of long wavelength photons, the width of the PSF increases significantly in the near-IR and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide encircled energy curves and a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: 1 based on the observation of 2 star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and hot pixel population.

Analysis and quantification of the diversities of aerosol life cycles within AeroCom

Abstract: Simulation results of global aerosol models have been assembled in the framework of the AeroCom intercomparison exercise. In this paper, we analyze the life cycles of dust, sea salt, sulfate, black carbon and particulate organic matter as simulated by sixteen global aerosol models. The differences among the results (model diversities) for sources and sinks, burdens, particle sizes, water uptakes, and spatial dispersals have been established. These diversities have large consequences for the calculated radiative forcing and the aerosol concentrations at the surface. Processes and parameters are identified which deserve further research. The AeroCom all-models-average emissions are dominated by the mass of sea salt (SS), followed by dust (DU), sulfate (SO 4 ), particulate organic matter (POM), and finally black carbon (BC). Interactive parameterizations of the emissions and contrasting particles sizes of SS and DU lead generally to higher diversities of these species, and for total aerosol. The lower diversity of the emissions of the fine aerosols, BC, POM, and SO 4 , is due to the use of similar emission inventories, and does therefore not necessarily indicate a better understanding of their sources. The diversity of SO 4 -sources is mainly caused by the disagreement on depositional loss of precursor gases and on chemical production. The diversities of the emissions are passed on to the burdens, but the latter are also strongly affected by the model-specific treatments of transport and aerosol processes. The burdens of dry masses decrease from largest to smallest: DU, SS, SO 4 , POM, and BC. The all-models-average residence time is shortest for SS with about half a day, followed by SO 4 and DU with four days, and POM and BC with six and seven days, respectively. The wet deposition rate is controlled by the solubility and increases from DU, BC, POM to SO 4 and SS. It is the dominant sink for SO 4 , BC, and POM, and contributes about one third to the total removal of SS and DU species. For SS and DU we find high diversities for the removal rate coefficients and deposition pathways. Models do neither agree on the split between wet and dry deposition, nor on that between sedimentation and other dry deposition processes. We diagnose an extremely high diversity for the uptake of ambient water vapor that influences the particle size and thus the sink rate coefficients. Furthermore, we find little agreement among the model results for the partitioning of wet removal into scavenging by convective and stratiform rain. Large differences exist for aerosol dispersal both in the vertical and in the horizontal direction. In some models, a minimum of total aerosol concentration is simulated at the surface. Aerosol dispersal is most pronounced for SO 4 and BC and lowest for SS. Diversities are higher for meridional than for vertical dispersal, they are similar for the individual species and highest for SS and DU. For these two components we do not find a correlation between vertical and meridional aerosol dispersal. In addition the degree of dispersals of SS and DU is not related to their residence times. SO 4 , BC, and POM, however, show increased meridional dispersal in models with larger vertical dispersal, and dispersal is larger for longer simulated residence times.

Solar wind spatial scales in and comparisons of hourly Wind and ACE plasma and magnetic field data

Abstract: [1] Hourly averaged interplanetary magnetic field (IMF) and plasma data from the Advanced Composition Explorer (ACE) and Wind spacecraft, generated from 1 to 4 min resolution data time-shifted to Earth have been analyzed for systematic and random differences. ACE moments-based proton densities are larger than Wind/Solar Wind Experiment (SWE) fits-based densities by up to 18%, depending on solar wind speed. ACE temperatures are less than Wind/SWE temperatures by up to ∼25%. ACE densities and temperatures were normalized to equivalent Wind values in National Space Science Data Center's creation of the OMNI 2 data set that contains 1963–2004 solar wind field and plasma data and other data. For times of ACE-Wind transverse separations <60 RE, random differences between Wind values and normalized ACE values are ∼0.2 nT for ∣B∣, ∼0.45 nT for IMF Cartesian components, ∼5 km/s for flow speed, and ∼15 and ∼30% for proton densities and temperatures. These differences grow as a function of transverse separation more rapidly for IMF parameters than for plasma parameters. Autocorrelation analyses show that spatial scales become progressively shorter for the parameter sequence: flow speed, IMF magnitude, plasma density and temperature, IMF X and Y components, and IMF Z component. IMF variations have shorter scales at solar quiet times than at solar active times, while plasma variations show no equivalent solar cycle dependence.

Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms

Abstract: [1] This work builds on and extends our previous effort (Tsyganenko et al, 2003) to develop a dynamical model of the storm-time geomagnetic field in the inner magnetosphere, using space magnetometer data taken during 37 major events in 1996–2000 and concurrent observations of the solar wind and interplanetary magnetic field (IMF) The essence of the approach is to derive from the data the temporal variation of all major current systems contributing to the distant geomagnetic field during the entire storm cycle, using a simple model of their growth and decay Each principal source of the external magnetic field (magnetopause, cross-tail current sheet, axisymmetric and partial ring currents, and Birkeland current systems) is driven by a separate variable, calculated as a time integral of a combination of geoeffective parameters NλVβBsγ, where N, V, and Bs are the solar wind density, speed, and the magnitude of the southward component of the IMF, respectively In this approach we assume that each source has its individual relaxation timescale and residual quiet-time strength, and its partial contribution to the total field depends on the entire history of the external driving of the magnetosphere during a storm In addition, the magnitudes of the principal field sources were assumed to saturate during extremely large storms with abnormally strong external driving All the parameters of the model field sources, including their magnitudes, geometrical characteristics, solar wind/IMF driving functions, decay timescales, and saturation thresholds, were treated as free variables, and their values were derived from the data As an independent consistency test, we calculated the expected Dst variation on the basis of the model output at Earth's surface and compared it with the actual observed Dst A good agreement (cumulative correlation coefficient R = 092) was found, in spite of the fact that ∼90% of the spacecraft data used in the fitting were taken at synchronous orbit and beyond, while only 37% of those data came from distances 25 ≤ R ≤ 4 RE The obtained results demonstrate the possibility to develop a truly dynamical model of the magnetic field, based on magnetospheric and interplanetary data and allowing one to reproduce and forecast the entire process of a geomagnetic storm, as it unfolds in time and space

Carbon-based ocean productivity and phytoplankton physiology from space

Abstract: carbon(C)andchlorophyll(Chl)biomassandshowthatderivedChl:Cratioscloselyfollow anticipated physiological dependencies on light, nutrients, and temperature. With this new information, global estimates of phytoplankton growth rates (m) and carbon-based NPP are made for the first time. Compared to an earlier chlorophyll-based approach, our carbonbased values are considerably higher in tropical oceans, show greater seasonality at middle and high latitudes, and illustrate important differences in the formation and demise of regional algal blooms. This fusion of emerging concepts from the phycological and remote sensing disciplines has the potential to fundamentally change how we model and observe carbon cycling in the global oceans.

Recent trends in vegetation dynamics in the African Sahel and their relationship to climate

Abstract: Contrary to assertions of widespread irreversible desertification in the African Sahel, a recent increase in seasonal greenness over large areas of the Sahel has been observed, which has been interpreted as a recovery from the great Sahelian droughts. This research investigates temporal and spatial patterns of vegetation greenness and rainfall variability in the African Sahel and their interrelationships based on analyses of Normalized Difference Vegetation Index (NDVI) time series for the period 1982–2003 and gridded satellite rainfall estimates. While rainfall emerges as the dominant causative factor for the increase in vegetation greenness, there is evidence of another causative factor, hypothetically a human-induced change superimposed on the climate trend. r 2005 Elsevier Ltd. All rights reserved.

Atmospheric Chemistry Experiment (ACE): Mission overview.

Abstract: SCISAT-1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earth's atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750-4400 cm-1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285-1030 nm and spectral resolution of 1-2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results. Copyright 2005 by the American Geophysical Union.

A review of measurement-based assessments of the aerosol direct radiative effect and forcing

Abstract: Aerosols affect the Earth's energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Our goal is to assess current observational capabilities and identify uncertainties in the aerosol direct forcing through comparisons of different methods with independent sources of uncertainties. Here we assess the aerosol optical depth (τ), direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport model (CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical depth (τ) on a daily scale, with a high accuracy of ±0.03±0.05τ over ocean. The annual average τ is about 0.14 over global ocean, of which about 21%±7% is contributed by human activities, as estimated by MODIS fine-mode fraction. The multi-angle MISR derives an annual average AOD of 0.23 over global land with an uncertainty of ~20% or ±0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global the ocean. A number of measurement-based approaches estimate the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about -5.5±0.2 Wm -2 (median ± standard error from various methods) over the global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to -5.0 Wm -2 . Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite retrievals, surface measurements, and model simulations, and are less constrained. Over the oceans the surface DRE is estimated to be -8.8±0.7 Wm -2 . Over land, an integration of satellite retrievals and model simulations derives a DRE of -4.9±0.7 Wm -2 and -11.8±1.9 Wm -2 at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30-40%, even after accounting for thin cirrus and cloud contamination. A number of issues remain. Current estimates of the aerosol direct effect over land are poorly constrained. Uncertainties of DRE estimates are also larger on regional scales than on a global scale and large discrepancies exist between different approaches. The characterization of aerosol absorption and vertical distribution remains challenging. The aerosol direct effect in the thermal infrared range and in cloudy conditions remains relatively unexplored and quite uncertain, because of a lack of global systematic aerosol vertical profile measurements. A coordinated research strategy needs to be developed for integration and assimilation of satellite measurements into models to constrain model simulations. Enhanced measurement capabilities in the next few years and high-level scientific cooperation will further advance our knowledge.

Overview of the ICESat Mission

Abstract: [1] The Geoscience Laser Altimeter System (GLAS) on the NASA Ice, Cloud and land Elevation Satellite (ICESat) has provided a view of the Earth in three dimensions with unprecedented accuracy. Although the primary objectives focus on polar ice sheet mass balance, the GLAS measurements, distributed in 15 science data products, have interdisciplinary application to land topography, hydrology, vegetation canopy heights, cloud heights and atmospheric aerosol distributions. Early laser life issues have been mitigated with the adoption of 33-day operation periods, three times per year, designed to document intra- and inter-annual polar ice changes in accordance with mission requirements. A variety of calibration/validation experiments have been executed which show that the elevation products, when fully calibrated, have an accuracy that meets the science requirements. The series of papers in this special ICESat issue demonstrate the utility and quality of the ICESat data.

An AeroCom Initial Assessment - Optical Properties in Aerosol Component Modules of Global Models

Abstract: The AeroCom exercise diagnoses multi-component aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of particular concern are large model diversities for contributions by dust and carbonaceous aerosol, because they lead to significant uncertainty in aerosol absorption (aab). Since aot and aab, both, influence the aerosol impact on the radiative energy-balance, the aerosol (direct) forcing uncertainty in modeling is larger than differences in aot might suggest. New diagnostic approaches are proposed to trace model differences in terms of aerosol processing and transport: These include the prescription of common input (e.g. amount, size and injection of aerosol component emissions) and the use of observational capabilities from ground (e.g. measurements networks) or space (e.g. correlations between aerosol and clouds).

Infrared radiation from an extrasolar planet

Abstract: For the first time, light from a planet outside our Solar System has been detected on Earth. The planet is HD 209458b, previously identified by the wobble its gravity induces in its host star's orbit. It is slightly larger than Jupiter, but orbits its star at less than a twentieth of the distance between the Earth and the Sun, making it a so-called ‘hot Jupiter’ planet. As HD 209458b passes behind the star, the amount of infrared light coming from the area drops slightly: that drop represents the planet's light contribution. A class of extrasolar giant planets—the so-called ‘hot Jupiters’ (ref. 1)—orbit within 0.05 au of their primary stars (1 au is the Sun–Earth distance). These planets should be hot and so emit detectable infrared radiation2. The planet HD 209458b (refs 3, 4) is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter5), which may be the result of ongoing tidal dissipation6, but this explanation requires a non-zero orbital eccentricity (∼ 0.03; refs 6, 7), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 µm) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24-µm flux is 55 ± 10 µJy (1σ), with a brightness temperature of 1,130 ± 150 K, confirming the predicted heating by stellar irradiation2,8. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within ± 7 min, 1σ), which means that a dynamically significant orbital eccentricity is unlikely.

A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future

Abstract: Precipitation is a key link in the global water cycle and a proxy for changing climate; therefore, proper assessment of the urban envi- ronment's impact on precipitation (land use, aerosols, thermal properties) will be increasingly important in ongoing climate diagnostics and prediction, Glob- al Water and Energy Cycle (GWEC) analysis and modeling, weather forecast- ing, freshwater resource management, urban planning-design, and land- atmosphere-ocean interface processes. These facts are particularly critical if current projections for global urban growth are accurate. The goal of this paper is to provide a concise review of recent (1990-present) studies related to how the urban environment affects precipitation. In addition to providing a synopsis of current work, recent findings are placed in context with historical investigations such as Metropolitan Meteorological Experiment (METROMEX) studies. Both observational and modeling studies of urban- induced rainfall are discussed. Additionally, a discussion of the relative roles of urban dynamic and microphysical (e.g., aerosol) processes is presented. The paper closes with a set of recommendations for what observations and capa- bilities are needed in the future to advance our understanding of the processes.

Estimates of forest canopy height and aboveground biomass using ICESat

Abstract: Exchange of carbon between forests and the atmosphere is a vital component of the global carbon cycle. Satellite laser altimetry has a unique capability for estimating forest canopy height, which has a direct and increasingly well understood relationship to aboveground carbon storage. While the Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud and land Elevation Satellite (ICESat) has collected an unparalleled dataset of lidar waveforms over terrestrial targets, processing of ICESat data to estimate forest height is complicated by the pulse broadening associated with large-footprint, waveform-sampling lidar. We combined ICESat waveforms and ancillary topography from the Shuttle Radar Topography Mission to estimate maximum forest height in three ecosystems; tropical broadleaf forests in Brazil, temperate broadleaf forests in Tennessee, and temperate needleleaf forests in Oregon. Final models for each site explained between 59% and 68% of variance in field-measured forest canopy height (RMSE between 4.85 and 12.66 m). In addition, ICESat-derived heights for the Brazilian plots were correlated with field-estimates of aboveground biomass (r(2) = 73%, RMSE = 58.3 Mgha(-1)).

Retrieval of biomass combustion rates and totals from fire radiative power observations: FRP derivation and calibration relationships between biomass consumption and fire radiative energy release

Abstract: Estimates of wildfire aerosol and trace gas emissions are most commonly derived from assessments of biomass combusted. The radiative component of the energy liberated by burning fuel can be measured by remote sensing, and spaceborne fire radiative energy (FRE) measures can potentially provide detailed information on the amount and rate of biomass consumption over large areas. To implement the approach, spaceborne sensors must be able to derive fire radiative power (FRP) estimates from subpixel fires using observations in just one or two spectral channels, and calibration relationships between radiated energy and fuel consumption must be developed and validated. This paper presents results from a sensitivity analysis and from experimental fires conducted to investigate these issues. Within their methodological limits, the experimental work shows that FRP assessments made via independent hyperspectral and MIR radiance approaches in fact show good agreement, and fires are calculated to radiate 14 ± 3% [mean ± 1S.D.] of their theoretically available heat yield in a form capable of direct assessment by a nadir-viewing MIR imager. The relationship between FRE and fuel mass combusted is linear and highly significant (r2 = 0.98, n = 29, p < 0.0001), and FRP is well related to combustion rate (r2 = 0.90, n = 178, p < 0.0001), though radiation from the still-hot fuel bed can sometimes contribute significant FRP from areas where combustion has ceased. We conclude that FRE assessment offers a powerful tool for supplementing existing burned-area based fuel consumption measures, and thus shows significant promise for enhancing pyrogenic trace gas and aerosol emissions estimates

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

Abstract: Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30°S–20°S), smoke (20°S–5°N), mineral dust (5°N–25°N), and pollution aerosols (30°N– 60°N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2–0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is –11 ± 3 W/m2 for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Abstract: Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of Nature), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole. Gamma-ray bursts (GRBs) come in two classes1: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars2. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect3,4 if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

An exceptionally bright flare from SGR 1806–20 and the origins of short-duration γ-ray bursts

Abstract: Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806–20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic γ-ray burst. At least a significant fraction of the mysterious short-duration γ-ray bursts may therefore come from extragalactic magnetars. On 27 December last year, SGR1806–20, a soft γ-ray repeater in Sagittarius, released a giant flare that has been called the brightest explosion ever recorded. SGRs are X-ray stars that sporadically emit low-energy γ-ray bursts. They are thought to be magnetars: neutron stars with observable emissions powered by magnetic dissipation. Five papers in this issue report initial and follow-up observations of this event. The data are remarkable: for instance in a fifth of a second, the flare released as much energy as the Sun radiates in a quarter of a million years. Such power can be explained by catastrophic global crust failure and magnetic reconnection on a magnetar. Releasing a hundred times the energy of the only two previous SGR giant flares, this may have been a once-in-a-lifetime event for astronomers, and for the star itself.

Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean

Abstract: [1] Meteorological observations, in situ data, and satellite images of dust episodes were used already in the 1970s to estimate that 100 Tg of dust are transported from Africa over the Atlantic Ocean every year between June and August and are deposited in the Atlantic Ocean and the Americas. Desert dust is a main source of nutrients to oceanic biota and the Amazon forest, but it deteriorates air quality, as shown for Florida. Dust affects the Earth radiation budget, thus participating in climate change and feedback mechanisms. There is an urgent need for new tools for quantitative evaluation of the dust distribution, transport, and deposition. The Terra spacecraft, launched at the dawn of the last millennium, provides the first systematic well-calibrated multispectral measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument for daily global analysis of aerosol. MODIS data are used here to distinguish dust from smoke and maritime aerosols and to evaluate the African dust column concentration, transport, and deposition. We found that 240 ± 80 Tg of dust are transported annually from Africa to the Atlantic Ocean, 140 ± 40 Tg are deposited in the Atlantic Ocean, 50 Tg fertilize the Amazon Basin (four times as previous estimates, thus explaining a paradox regarding the source of nutrition to the Amazon forest), 50 Tg reach the Caribbean, and 20 Tg return to Africa and Europe. The results are compared favorably with dust transport models for maximum particle diameter between 6 and 12 μm. This study is a first example of quantitative use of MODIS aerosol for a geophysical research.