Data from the National Oceanic and Atmospheric Administration (NOAA) satellites' Advanced Very High Resolution Radiometers (AVHRRs) represent the longest record (more than 25 years) of continuously available satellite-based thermal measurements, and have well-chosen spatial and spectral resolutions. As a consequence, these data are used extensively to develop cloud climatologies. However, for such applications, accurate calibration and intercalibration of both solar and thermal channels of the AVHRRs is necessary so as to homogenize the data obtained from the different AVHRR sensors. AVHRR thermal channels 4 and 5 are routinely used in threshold-based hierarchical decision-tree cloud detection and classification algorithms, and therefore an evaluation of the stability of these channels at low temperatures is important. In this letter, the AVHRR channel 4 and 5 brightness temperatures (BTs) are compared at five stations in Antarctica. The data for the period of June, July and August (the coldest months of every year and with minimal atmospheric influence) from 1982 to 2006 were used for the evaluations. The calibration and intercalibration of the thermal channels are found to be very robust. The root mean square errors (RMSEs) range from 2.2 to 3.4 K and the correlation coefficients from 0.84 to 0.95. No apparent artefacts or artificial jumps in the BTs are visible in the data series after changes of sensors. The BTs from the thermal channels of the AVHRRs can be used for preparing cloud climatologies, as their intercalibration is found to be consistent across different afternoon satellites.

Deriving accurate time‐series of cloud cover from satellite sensor data still remains a challenging task. The instruments onboard polar orbiting NOAA satellites offer the opportunity to prepare cloud climatology on a global scale; however, the orbital drifts of these satellites can introduce uncertainty when deriving such cloud climatology. The aim of this letter is to point out the importance and to estimate the impact of orbital drift on long‐term time‐series of the observation of convective cloud frequency of occurrence. The 20 years of daytime AVHRR data from over the Indian subcontinent for the summer monsoon season is used in this study. All four AVHRRs onboard NOAA‐7, ‐9, ‐11, and ‐14 satellites show positive correlation between increased cloud frequency and the delay in equator crossing‐times during their lifetime. This increase is significant over land, but over the ocean, there is no discernible effect. This effect should be considered to avoid spurious trends in cloud cover. Further in‐depth investigations are needed to make possible corrections.

The East Asian monsoon (EAM) and the El Nino Southern Oscillation (ENSO) determine climate variability over much of East Asia, affecting vulnerable grain markets and food security in China. In this study, we investigated the variability of climate and of agricultural production in China in association with the East Asian summer monsoon (EASM) and ENSO. Data from China showed that a strong EASM decreased fall temperature in Gansu and Sichuan Provinces in western China, as well as winter temperature in Heilongjiang Province in NE China and in Shandong and Anhui Provinces in eastern China. Summer rainfall in Hunan Province in southern China increased in weak EASM years. Summer temperature increased in Heilongjiang in NE China and Gansu Province in NW China during the La Nina phase. Summer rainfall decreased in Gansu Province in NW China during the El Nino phase. Among staple crops in China (rice, wheat, maize), maize production was very vulnerable to a strong EASM and El Nino phase. In Henan Province in central China, seasonal climate variability associated with EASM and ENSO resulted in about 14.4 and 15.6 %, respectively, of maize yield variability. Maize Yield at the national scale decreased significantly by 5.2 % during the El Nino phase. Cropland area affected and damaged by floods in Hunan Province in southern China increased significantly by 11.3 and 8.5 %, respectively, in weak monsoon years. During the La Nina phase, total crop planting area increased significantly in Shandong, Henan and Anhui Provinces in central China, and in Heilongjiang Province in NE China; however, it decreased significantly in Sichuan Province in SW China. The large variability in seasonal climate and agricultural production in association with EASM and ENSO warrant applying EASM and ENSO information to agricultural and food market management.

Fresh water figures prominently in the machinery of the Earth system and is key to understanding the full scope of global change. Greenhouse warming with a potentially accelerated hydrologic cycle is already a well‐articulated science issue, with strong policy implications. A broad array of other anthropogenic factors—widespread land cover change, engineering of river channels, irrigation and other consumptive losses, aquatic habitat disappearance, and pollution—also influences the water system in direct and important ways. A rich history of site‐specific research demonstrates the clear impact of such factors on local environments. Evidence now shows that humans are rapidly intervening in the basic character of the water cycle over much broader domains. The collective significance of these many transformations on both the Earth system and human society remains fundamentally unknown [Framing Committee of the GWSP, 2004].

The Alps—the youngest and most elevated mountain range in Europe—have inspired ideas about orogenic evolution for a long time. During the late 1980s, the western Alps were the site of intensive research using seismic profiling methods by Swiss, Italian, and French national programs [ Rome et al. , 1990; Pfiffner et al. , 1997] .These investigations, some of which formed part of the European Traverse [ Blundell et al. , 1992], provided a great wealth of new data relevant to the Alpine orogeny. This orogeny is generally viewed in the context of the collision of the European and the Adriatic/African continental plates after the closure and subduction of the Penninic Ocean since about 40–50 Ma.

The external forcing of the German Bight system is largely due to the atmosphere. Energy fluxes that drive mixing processes and biological productivity, as well as atmospheric nutrient inputs outside the Elbe estuary, are important factors for biomass production. This study is based on the KUSTOS experiments focusing on air-sea exchange with intensive observations of a) radiative fluxes at the surface of the drifting water body; b) atmospheric surface layer parameters determining the mixing conditions in the planetary boundary layer; c) the speciation of atmospheric nitrogen compounds; d) and changes in aerosol and gas composition during transport over sea. These episodic data were complemented by a) synoptic data analysis of water and air temperature, wind, pressure and water vapour pressure over the sea; b) corresponding oceanic data on heat advection and mixed layer depth from an oceanic model driven by observations in the atmosphere; c) computations of the highly variable heat and radiative fluxes with the mesoscale atmospheric model METRAS; d) long-term atmospheric deposition measurements of nutrients in the German Bight; e) investigations of the atmospheric processes responsible for the formation of coarse particulate nitrate by means of a new aerosol submodel in the METRAS transport model. We present detailed seasonal or annual budgets for fluxes of heat, momentum, nitrate, ammonium, persistent organic pollutants. The atmospheric fluxes of heat and chemical matter are compared with load and fluxes in the water column in order to identify when and where the atmospheric impact is relevant and detectable. Spatial and temporal variability is discussed for the fluxes of heat, momentum and nitrogen. From the budgets we identify categories of potential atmospheric impact. Apart from the category “no atmospheric impact≓ valid e.g. for Cr, As, Ni and phosphate, we identify 4 others: 1) “atmosphere driven≓: short term, local dominant impact for Heat and momentum; 2) “episodic atmospheric impact≓: long term, local and dominant impact with large fluxes involved for radiation, PCB, Pb; 3) “persistent atmospheric pollutant≓: long term dominant but regionally indifferent impact for α- and γHCH; 4) “steadily perturbing the marine ecosystem≓: long term, widespread impact superimposed on the dynamic system driven by marine biology for nitrate, ammonium.

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instruments on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of aerosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These platforms will be followed by other international platforms with unique aerosol sensing capability, some still in this century (e.g., ENVISAT in 1999). These international spaceborne multispectral, multiangular, and polarization measurements, combined for the first time with international automatic, routine monitoring of aerosol from the ground, are expected to form a quantum leap in our ability to observe the highly variable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol effect on remote sensing of the oceans and be used to generate first real-time atmospheric corrections over the land. This special issue summarizes the science behind this change in remote sensing, and the sensitivity studies and applications of the new algorithms to data from present satellite and aircraft instruments. Background information and a summary of a critical discussion that took place in a workshop devoted to this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultaneously from several space platforms with different observation strategies, together with continuous validations around the world, is expected to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, we have only partial understanding of the information content and accuracy of the radiative transfer inversion of aerosol information from the satellite data, due to lack of sufficient theoretical analysis and applications to proper field data. This limitation will make the anticipated new data even more interesting and challenging. A main concern is the present inadequate ability to sense aerosol absorption, from space or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are the effects of white caps and dust on the correction scheme. Future improvement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingredient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instruments on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of aerosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These platforms will be followed by other international platforms with unique aerosol sensing capability, some still in this century (e.g., ENVISAT in 1999). These international spaceborne multispectral, multiangular, and polarization measurements, combined for the first time with international automatic, routine monitoring of aerosol from the ground, are expected to form a quantum leap in our ability to observe the highly variable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol effect on remote sensing of the oceans and be used to generate first real-time atmospheric corrections over the land. This special issue summarizes the science behind this change in remote sensing, and the sensitivity studies and applications of the new algorithms to data from present satellite and aircraft instruments. Background information and a summary of a critical discussion that took place in a workshop devoted to this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultaneously from several space platforms with different observation strategies, together with continuous validations around the world, is expected to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, we have only partial understanding of the information content and accuracy of the radiative transfer inversion of aerosol information from the satellite data, due to lack of sufficient theoretical analysis and applications to proper field data. This limitation will make the anticipated new data even more interesting and challenging. A main concern is the present inadequate ability to sense aerosol absorption, from space or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are the effects of white caps and dust on the correction scheme. Future improvement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingredient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.

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Iterative methods for inverse referencing from mean orbital elements or osculating position and velocity, accounting for all necessary orbital perturbations with respect to given nadir pixel size, are described. [Inverse referencing means that the geodetic coordinates of a point on the surface are given and the corresponding image coordinates (scan line number and pixel number) are obtained from satellite orbital elements or coordinates.] The idea is to treat a pixel like a satellite tracking station on the ground. This permits the use of existing software for the computation of satellite ephemerides and orbit determination. The time of culmination of a satellite over the pixel and the off-nadir angle at that moment have been computed. Two variants for such a computation have been tested. Numerical results for the NOAA-N meteorological satellites and ERS-1 are presented. The present state of our software for inverse referencing should fulfil ordinary requirements posed by meteorologists. For NOAA-N satellites, the accuracy achieved roughly the nadir pixel size. The main obstacle to an increase in accuracy is the low quality of the mean orbital elements usually available. For ERS-1, the accuracy may achieve a level of 100 m. A software package, containing versions of the FORTRAN 77 programs PIXPO 3, PIXPO 4 and PIXPOSC, for various data types, including US-2 line or TBUS mean elements or a state vector, is available for scientific exchange.

Two models of relative spectral reflectances as a function of chlorophyll-like pigment concentrations in the upper layer of oceanic waters and thus absorption and scattering properties of phytoplankton are described. These models are developed on the basis of statistical relationships between measured spectral diffuse attenuation coefficients and chlorophyll density in various oceanic waters (from oligotrophic to eutrophic). The semi-empirical model includes new spectral irradiance measurements in different waters. The strongly nonlinear regression equation (reflectance v. chlorophyll concentration) is compared with direct measurements, giving a negligible systematic error and a standard deviation of only 15% and of 20%, if applied to other spectra. The theoretical model is based on the principle of invariance of the emergent radiation for a semi-infinite plane-parallel medium, the delta function approximation for the forward scattering peak, the approximate solution of the radiative transfer equation as well as Cox and Munk's statistics of the roughness of the sea surface. It recovers formulas derived by others with more complicated models. Both models convert reflectance or reflected radiances reliably into inherent optical phytoplankton properties for different open ocean waters, indicating no need for a regional algorithm.

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The microwave sensor SSM/I (Special Sensor Microwave/Imager) on board of the DMSP satellite can be used to develop a retrieval method for the water vapour content in the atmospheric boundary layer close to the sea surface, by means of radiative transfer calculations. It is found that the SSM/I measurements are sufficiently sensitive to the water vapour w(l) in the lowermost 500 m of the atmosphere to allow a retrieval of w(l) from the 19, 22, 37 GHz vertical and 19 GHz horizontal polarization measurements with an accuracy of 0.06 g cm(-2). The technique is validated with globally distributed radiosonde measurements located together with satellite soundings during the months July and August 1987. A linear relationship is established statistically to determine the nearsurface specific humidity from w(l) with an accuracy of 1.2 g Kg(-1).

A series of radiative transfer calculations were performed to study the possibility of determining cloud-top pressure (height) from backscattered solar radiances within the oxygen A-band absorption. For the development of a cloud-top pressure algorithm, we also looked into the impact of perturbing effects, such as varying cloud properties, sun elevation, and surface albedo. The most important quantities are total cloud optical thickness delta-C and the vertical profile of liquid-water content.The effects of cloud optical thickness-if delta-C > 1-are already taken into account by a cloud-top algorithm, which only considers two radiances inside and outside the oxygen absorption band. For one-layer clouds, the cloud-top heights may be derived to within an accuracy of 200 m. Multilayer clouds or varying liquid-water content profiles can only be matched with an inverse technique using radiances at up to 16 wavelengths, which, however, give cloud-top height estimates to within an error of only 50 m for all 900 cloud cases considered.

Thematic mapper and ship data has been used to study small scale features in coastal waters of the North Sea. Three independent pieces of information from all 7 TM channesl were found with factor analysis: suspended matter concentration, atmospheric scattering and sea surface temperature.