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The Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) officially became the 33rd European Research Infrastructure Consortium (ERIC) on 25 April 2023 with the support of 17 founding member and observer countries. As a pan-European legal organization, ACTRIS ERIC will coordinate the provision of data and data products on short-lived atmospheric constituents and clouds relevant to climate and air pollution over the next 15-20 years. ACTRIS was designed more than a decade ago, and its development was funded at national and European levels. It was included in the European Strategy Forum on Research Infrastructures (ESFRI) roadmap in 2016 and, subsequently, in the national infrastructure roadmaps of European countries. It became a landmark of the ESFRI roadmap in 2021. The purpose of this paper is to describe the mission of ACTRIS, its added value to the community of atmospheric scientists, providing services to academia as well as the public and private sectors, and to summarize its main achievements. The present publication serves as a reference document for ACTRIS, its users, and the scientific community as a whole. It provides the reader with relevant information and an overview on ACTRIS governance and services, as well as a summary of the main scientific achievements of the last 20 years. The paper concludes with an outlook on the upcoming challenges for ACTRIS and the strategy for its future evolution. SIGNIFICANCE STATEMENT: The Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) is the new European Research Infrastructure Consortium for short-lived atmospheric constituents and clouds, supporting fundamental research and excellence in Earth system observation. The primary objective of ACTRIS is to produce high-quality integrated datasets in the field of atmospheric sciences and to provide services, including access to instrumented platforms, tailored for scientific and technological use. Established with a long-term perspective, with financial commitments from 17 European countries and the European Commission, ACTRIS services are open to the global community of scientists involved in atmospheric research and beyond. It is expected that ACTRIS will have a strong impact enhancing excellence in Earth system observation and research, providing information and knowledge for the development of sustainable solutions to societal needs. KEYWORDS: Atmosphere; Europe; Aerosols/particulates; Clouds; Trace gases, 1. Introduction Short-lived atmospheric constituents (SLACs) and clouds play a key role in regulating Earth's climate and impacting the quality of air and its effects on human and ecosystem health. [...]

Earth has had oceans for nearly four billion years.sup.1 and Mars had lakes and rivers 3.5-3.8 billion years ago.sup.2. However, it is still unknown whether water has ever condensed on the surface of Venus.sup.3,4 because the planet--now completely dry.sup.5--has undergone global resurfacing events that obscure most of its history.sup.6,7. The conditions required for water to have initially condensed on the surface of Solar System terrestrial planets are highly uncertain, as they have so far only been studied with one-dimensional numerical climate models.sup.3 that cannot account for the effects of atmospheric circulation and clouds, which are key climate stabilizers. Here we show using three-dimensional global climate model simulations of early Venus and Earth that water clouds--which preferentially form on the nightside, owing to the strong subsolar water vapour absorption--have a strong net warming effect that inhibits surface water condensation even at modest insolations (down to 325 watts per square metre, that is, 0.95 times the Earth solar constant). This shows that water never condensed and that, consequently, oceans never formed on the surface of Venus. Furthermore, this shows that the formation of Earth's oceans required much lower insolation than today, which was made possible by the faint young Sun. This also implies the existence of another stability state for present-day Earth: the 'steam Earth', with all the water from the oceans evaporated into the atmosphere. Global climate model simulations of early Venus and Earth show that differences in the cloud regimes prevented ocean formation on Venus but not on Earth., Author(s): Martin Turbet [sup.1] , Emeline Bolmont [sup.1] , Guillaume Chaverot [sup.1] , David Ehrenreich [sup.1] , Jérémy Leconte [sup.2] , Emmanuel Marcq [sup.3] Author Affiliations: (1) Observatoire astronomique de [...]

The interactions and feedbacks among clouds, aerosols, pollutants, and the thermodynamic and kinematic environment remains an area of active research with important implications for our understanding of climate, weather, and [...]

Australian scientists are rushing to develop new technologies -- such as ways to block sunlight -- to help preserve corals in the face of climate change. Australian scientists are rushing to develop new technologies -- such as ways to block sunlight -- to help preserve corals in the face of climate change., Author(s): Jeff Tollefson Author Affiliations: During a field trial, a turbine generates plumes of seawater droplets that rise into the sky. Credit: Brendan Kelaher/SCU A boat using the sea-spraying cloud [...]

Known sources of formic acid could not explain the observed atmospheric concentrations of this compound. The discovery of a previously unknown pathway that generates formic acid in the atmosphere resolves this discrepancy. Pathway to atmospheric formic acid from formaldehyde in cloud droplets., Author(s): Joost de Gouw, Delphine Farmer Author Affiliations: Cloud droplets aid the production of formic acid in the atmosphere Formic acid is one of the simplest and most abundant organic [...]

The Convective Precipitation Experiment (COPE) was a joint U.K.-U.S. field campaign held during the summer of 2013 in the southwest peninsula of England, designed to study convective clouds that produce [...]

--JACOB SEELEY, describing what happens to Earth's atmosphere during a hothouse climate. Seely and Robin Wordsworth, both of Harvard University, modeled the atmospheric response when sea surface temperatures reached 130[degrees]F, [...]

Wildfires may hamper clouds' ability to form precipitation, which leads to dry conditions that are more likely to trigger wildfires, thus creating a feedback loop. Atmospheric scientists studied cumulus clouds [...]

Planting forests outside the tropics can sequester carbon and counteract global warming. Planting forests outside the tropics can sequester carbon and counteract global warming., Author Affiliations: Clouds above temperate forests can help to reduce local temperatures. Credit: Johner Images/Alamy Pine trees silhouetted against an evening sky filled with white fluffy clouds. Clouds plus trees [...]

Melting of the world's major ice sheets can affect human and environmental conditions by contributing to sea-level rise. In July 2012, an historically rare period of extended surface melting was [...]

Mammatus clouds are the pouchlike lobes seen hanging from mid- to high-level clouds. They can look quite dramatic, but they are also interesting because they provide clues to what controls anvil cirrus dynamic evolution. Thus far, the most commonly accepted explanation for observed subsidence of mammatus lobes is that they are driven by evaporative cooling of precipitation, accelerated by mixing with dry subcloud air. Here, an alternative explanation is proposed: radiative temperature contrasts between cloud base and the lower troposphere destabilize cloudy air to create a rapidly deepening mixed layer, which creates positively buoyant intrusions of dry air into the cloud interior; mammatus lobes are just the descending branch of the resulting circulations. In this regard, mammatus cloud fields might be considered an upside-down analog to the radiatively driven formation of 'cloud holes' seen at the tops of stratocumulus layers. DOI: 10.1175/2010JAS3513.1

During the Ice in Clouds Experiment-Layer Clouds (ICE-L), aged biomass-burning particles were identified within two orographic wave cloud regions over Wyoming using single-particle mass spectrometry and electron microscopy. Using a suite of instrumentation, particle chemistry was characterized in tandem with cloud microphysics. The aged biomass-burning particles comprised -30%-40% by number of the 0.1-1.0-[micro]m clear-air particles and were composed of potassium, organic carbon, elemental carbon, and sulfate. Aerosol mass spectrometry measurements suggested these cloud-processed particles were predominantly sulfate by mass. The first cloud region sampled was characterized by primarily homogeneously nucleated ice particles formed at temperatures near -40[degrees]C. The second cloud period was characterized by high cloud droplet concentrations (~150-300 [cm.sup.-3]) and lower heterogeneously nucleated ice concentrations (7-18 [L.sup.-1]) at cloud temperatures of -24[degrees] to -25[degrees]C. As expected for the observed particle chemistry and dynamics of the observed wave clouds, few significant differences were observed between the clear-air particles and cloud residues. However, suggestive of a possible heterogeneous nucleation mechanism within the first cloud region, ice residues showed enrichments in the number fractions of soot and mass fractions of black carbon, measured by a single-particle mass spectrometer and a single-particle soot photometer, respectively. In addition, enrichment of biomass-burning particles internally mixed with oxalic acid in both the homogeneously nucleated ice and cloud droplets compared to clear air suggests either preferential activation as cloud condensation nuclei or aqueous phase cloud processing. DOI: 10.1175/2010JAS3330.1

The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 [micro]m. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case. DOI: 10.1175/2010JAS3266.1

Ice concentrations in orographic wave clouds at temperatures between -24[degrees] and -29[degrees]C were shown to be related to aerosol characteristics in nearby clear air during five research flights over the Rocky Mountains. When clouds with influence from colder temperatures were excluded from the dataset, mean ice nuclei and cloud ice number concentrations were very low, on the order of 1-5 [L.sup.-1]. In this environment, ice number concentrations were found to be significantly correlated with the number concentration of larger particles, those larger than both 0.1- and 0.5-[micro]m diameter. A variety of complementary techniques was used to measure aerosol size distributions and chemical composition. Strong correlations were also observed between ice concentrations and the number concentrations of soot and biomass-burning aerosols. Ice nuclei concentrations directly measured in biomass-burning plumes were the highest detected during the project. Taken together, this evidence indicates a potential role for biomass-burning aerosols in ice formation, particularly in regions with relatively low concentrations of other ice nucleating aerosols. DOI: 10.1175/2010JAS3310.1

An improvement was developed and tested for surface longwave flux algorithms used in the Clouds and the Earth's Radiant Energy System processing based on lessons learned during the validation of global results of those algorithms. The algorithms involved showed significant overestimation of downward longwave flux for certain regions, especially dry--arid regions during hot times of the day. The primary cause of this overestimation was identified and the algorithms were modified to (i) detect meteorological conditions that would produce an overestimation, and (ii) apply a correction when the overestimation occurred. The application of this correction largely eliminated the positive bias that was observed in earlier validation studies. Comparisons of validation results before and after the application of correction are presented. DOI: 10.1175/2010JAMC2463.1

The multiscale structure of a super cloud cluster (SCC) over the equatorial Indian Ocean, observed in November and December 2006, was investigated using data from satellite microwave sensors and surface-based radars. The smaller-scale structure of this SCC was marked by a complicated relationship between rainfall systems and upper-tropospheric cloud shields, which moved eastward and westward, respectively, with a cycle of 2-4 days. In the analyses, attention was given to the structure of slow eastward-propagating (5-11 m [s.sup.-1]) precipitating systems and related synoptic-scale (~2000 km) disturbances. A case study of one of the systems revealed that it consisted of several lines of convective cells with a depth that was usually shallower than 10 km unless the cells encountered the westward-moving cloud shields. The environment of the convective lines was characterized by persistent unstable conditions with an increase of the westerly flow in the lower troposphere, suggesting the existence of a synoptic-scale upward motion. Composite analyses revealed that each rainfall system formed in a region of zonal flow convergence near the surface and divergence near 300 hPa. The vertical temperature structure tilted westward with height below this pressure level and eastward aloft, similar to that of a convectively coupled Kelvin wave. These results suggest that a SCC involves a group of synoptic-scale shallow waves propagating eastward. An additional analysis over the western Pacific also showed the predominance of eastward propagation in a SCC, demonstrating the advantage of satellite microwave sensors over infrared ones in monitoring the multiscale structure of tropical convection. DOI: 10.1175/2009JAS3151.1

A groundbreaking new-concept multiwavelength dual-polarized Advanced Microwave Radiometer for Rain identification (ADMIRARI) has been built and continuously operated in two field campaigns: the Convective and Orographically Induced Precipitation Study (COPS) and the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). The radiometer has 6 channels working in horizontal and vertical polarization at 10.65, 21.0, and 36.5 GHz, and it is completely steerable both in azimuth and in elevation. The instrument is suited to be operated in rainy conditions and is intended for retrieving simultaneously water vapor, rain, and cloud liquid water paths. To this goal the authors implemented a Bayesian retrieval scheme based on many state realizations simulated by the Goddard Cumulus Ensemble model that build up a prior probability density function of rainfall profiles. Detailed three-dimensional radiative transfer calculations, which account for the presence of nonspherical particles in preferential orientation, simulate the downwelling brightness temperatures and establish the similarity of radiative signatures and thus the probability that a given profile is actually observed. Particular attention is devoted to the sensitivity of the ADMIRARI signal to 3D effects, raindrop size distribution, and axial ratio parameterizations. The polarization and multifrequency signals represent key information to separate the effects introduced by non-Rayleigh scatterers and to separate rainwater (r-LWP) from the cloud water component (c-LWP). Long-term observations demonstrate that observed brightness temperatures and polarization differences can be well interpreted and reproduced by the simulated ones for all three channels simultaneously. Rough estimates of r-LWP derived from collocated observations with a micro rain radar confirm the rain/no rain separation and the variability trend of r-LWP provided by the radiometer-based retrieval algorithm. With this work the authors demonstrate the potential of ADMIRARI to retrieve information about the rain/cloud partitioning for midlatitude precipitation systems; future studies with this instrument will provide crucial information on rain efficiency of clouds for cloud modelers that might lead toward a better characterization of rain processes. DOI: 10.1175/2009JAMC2340.1

The indirect aerosol effect (Twomey effect) is studied during a Saharan dust-transport event that presented an unusually favorable combination of a dust-loading gradient across clouds with warm cloud-top temperatures. Standard retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), and the Clouds and the Earth's Radiant Energy System (CERES) provide cloud-top temperature (a surrogate for height), liquid water path (LWP), classification of precipitation regime, and radiation flux. The authors correlate a retrieved mean effective droplet radius ([r.sub.e]) versus the number concentration of cloud condensation nuclei ([N.sub.CCN]), using the regressed slope d [1nr.sub.e]/d ln[N.sub.CCN] as the estimator of the aerosol indirect effect (AIE). Results demonstrate statistically significant AIE for only some of the segregated cloud classes. For nonprecipitating clouds (the most direct test of Twomey effect), the estimated AIE is effectively -0.07 over all wider temperature bands and is statistically significant from 1.1 to 1.9 [sigma]. Further classification by LWP strengthens both the AIE (for all LWP > 150 g [m.sup.-2]) to approximately -0.16, and substantially increases the statistical significance, to better than 5[sigma]. Shortwave radiation forcing of dust aerosols is also estimated directly from satellite measurements. The direct shortwave (SW) radiation effect of Saharan dusts at solar zenith angle 21.6[degrees] is 53.48 [+ or -] 8.56 W [m.sup.-2] per unit aerosol optical depth, with a correlation coefficient of 0.92. The indirect SW forcing of Saharan dust is 29.88 [+ or -] 2.42 W [m.sup.-2] per unit AOD for clouds with LWP of 100 g [m.sup.-2]. DOI: 10.1175/2009JAS3235.1

The authors examined the possible use of deep convective clouds (DCCs), defined as clouds that overshoot the tropical tropopause layer (TTL), for the calibration of satellite measurements at solar channels. DCCs are identified in terms of the Moderate Resolution Imaging Spectroradiometer (MODIS) 10.8-[micro]m brightness temperature ([TB.sub.11]) on the basis of a criterion specified by [TB.sub.11] [less than or equal to] 190 K. To determine the characteristics of these clouds, the MODIS-based cloud optical thickness (COT) and effective radius ([r.sub.e]) for a number of identified DCCs are analyzed. It is found that COT values for most of the 4249 DCC pixels observed in January 2006 are close to 100. Based on the MODIS quality-assurance information, 90% and 70.2% of the 4249 pixels have COT larger than 100 and 150, respectively. On the other hand, the [r.sub.e] values distributed between 15 and 25 [micro]m show a sharp peak centered approximately at 20 [micro]m. Radiances are simulated at the MODIS 0.646-[micro]m channel by using a radiative transfer model under homogeneous overcast ice cloudy conditions for COT = 200 and [r.sub.e] = 20 [micro]m. These COT and [r.sub.e] values are assumed to be typical for DCCs. A comparison between the simulated radiances and the corresponding Terra/Aqua MODIS measurements for 6 months in 2006 demonstrates that, on a daily basis, visible-channel measurements can be calibrated within an uncertainty range of [+ or -] 5 %. Because DCCs are abundant over the tropics and can be identified from infrared measurements, the present method can be applied to the calibration of a visible-channel sensor aboard a geostationary or low-orbiting satellite platform. DOI: 10.1175/2009JAMC2197.1

This study examines the seasonal and diurnal dynamics of convective cloud entities--small cells and a mesoscale convective complex-like pattern--in the foothills of the tropical eastern Andes. The investigation is based on Geostationary Operational Environmental Satellite-East (GOES-E) satellite imagery (2005-07), images of a scanning X-band rain radar, and data from regular meteorological stations. The work was conducted in the framework of a major ecological research program, the Research Unit 816, in which meteorological instruments are installed in the Rio San Francisco valley, breaching the eastern Andes of south Ecuador. GOES image segmentation to discriminate convective cells and other clouds is performed for a 600 x 600 [km.sup.2] target area, using the concept of connected component labeling by applying the 8-connectivity scheme as well as thresholds for minimum blackbody temperature, spatial extent, and eccentricity of the extracted components. The results show that the formation of convective clouds in the lowland part of the target area mainly occurs in austral summer during late afternoon. Nocturnal enhancement of cell formation could be observed from October to April (particularly February--April) between 0100 and 0400 LST (LST = UTC - 5 h) in the Andean foothill region of the target area, which is the relatively dry season of the adjacent eastern Andean slopes. Nocturnal cell formation is especially marked southeast of the Rio San Francisco valley in the southeast Andes of Ecuador, where a confluence area of major katabatic outflow systems coincide with a quasi-concave shape of the Andean terrain line. The confluent cold-air drainage flow leads to low-level instability and cellular convection in the warm, moist Amazon air mass. The novel result of the current study is to provide statistical evidence that, under these special topographic situations, katabatic outflow is strong enough to generate mainly mesoscale convective complexes (MCCs) with a great spatial extent. The MCC-like systems often increase in expanse during their mature phase and propagate toward the Andes because of the prevailing upper-air easterlies, causing early morning peaks of rainfall in the valley of the Rio San Francisco. it is striking that MCC formation in the foothill area is clearly reduced during the main rainy season [June--August (JJA)] of the higher eastern Andean slopes. At a first glance, this contradiction can be explained by rainfall persistence in the Rio San Francisco valley, which is clearly lower during the time of convective activity (December--April) in comparison with JJA, during which low-intensity rainfall is released by predominantly advective clouds with greater temporal endurance.

Toward improving parameterization of cloud droplet activation in cloud and climate models, the integro-differential equation for supersaturation is solved analytically for the algebraic size spectrum of the cloud condensation nuclei (CCN) that is equivalent to the lognormal spectrum. The analytical solutions are obtained for four limiting cases that are combinations of two different values of the updraft vertical velocity (small and large) and two different values of the condensation coefficient that correspond to pure and polluted cloud drops. The characteristics of the CCN can vary within each limit. Thus, these four limits and interpolation among them cover the vast majority of cloudy conditions. Analytical expressions are obtained for the time of CCN activation, maximum supersaturation, and the concentration of activated droplets. For small updraft vertical velocities, these quantities are the products of the power laws by six variables: CCN concentration, mean radius, soluble fraction, vertical velocities, surface tension, and condensation coefficient. At large updraft vertical velocities, the activation time and maximum supersaturation are the products of the power laws of only two variables--CCN concentration and vertical velocity--and are independent of the CCN physicochemical properties. The first limit is a generalization of the Twomey power laws, with Twomey's coefficient [C.sub.T] and index k expressed via CCN physicochemical properties; the other three limits are new. The accuracy and regions of validity of these limits are determined by comparison with the exact numerical solution to the supersaturation equation. These solutions can be used for parameterization of drop activation in cloud and climate models and for control of numerical solutions. An advantage of this method is that it does not require running parcel models, and the drop concentrations can be obtained from lookup tables or as simple interpolation among the limiting solutions for the instantaneous model parameters.

Analyses of tropical cyclones created in an idealized environment reveal how and why cloud microphysical assumptions can influence storm motion, including speed and direction. It is well known that in the absence of a mean flow, a leading factor in storm propagation is the establishment of 'beta gyres' owing to planetary vorticity advection by the storm's circulation. Previous research demonstrated that tangential winds well beyond the core influence storm motion by helping to determine the gyres' orientation and intensity. Microphysical assumptions, especially involving average particle fall speeds, can strongly influence the winds at outer radius. More specifically, microphysics modulates the radial distribution of column-average virtual temperature, which largely determines the radial surface pressure gradient and therefore the winds because they tend to be in gradient balance beyond the core. Microphysics schemes can differ markedly with respect to average fall speed, depending on the complexity of the scheme and how interactions among condensation types are handled. Average fall speed controls the outward movement of particles produced in the eyewall into the anvil, where they can influence the environment through cloud-radiative interactions and phase changes. With the assistance of some special sensitivity tests, the influence of microphysics and fall speed on radial temperature gradients, leading to different outer wind strengths and tracks, is shown. Among other things. this work demonstrates that the treatment of outer rainbands in operational models can potentially influence how simulated storms move, thus affecting position forecasts.

The scattering of stellar orbits by galactic molecular clouds is studied in hope of explaining isotopic peculiarities of presolar grains from Asymptotic Giant Branch (AGB) stars. Silicon isotopic anomalies found in the mainstream Silicon Carbide (SiC) grains are observed to have heavy isotopes enriched. To explain the isotopic heaviness I propose that AGB stars that formed in the interior region of the galaxy, with metallicity higher than solar, change their orbits during close encounters with molecular clouds. The AGB stars give up their SiC grains in winds deposited in the solar neighborhood prior to the birth of the Sun. I model the dynamics by superposing on a standard axisymmetric galactic potential the discrete potentials of thousands of molecular clouds moving in constant circular orbits. Gravity assist occurs when a star scattered into a crossing orbit by one cloud experiences forward scattering from a second cloud. Key words: Silicon carbide, AGB stars, gravity assist, INTRODUCTION Many studies (e.g., 5 and 14) have shown that the mainstream presolar Silicon Carbide (SiC) grains extracted from meteorites formed in asymptotic giant branch stars. Ion probe studies show [...]

Previous work has shown that a convective cloud feedback can greatly increase high-latitude surface temperature upon the removal of sea ice and can keep sea ice from forming throughout polar night. This feedback activates at increased greenhouse gas concentrations. It may help to explain the warm 'equable climates' of the late Cretaceous and early Paleogene eras ~100 to ~35 million years ago) and may be relevant for future climate under global warming. Here, the factors that determine the critical threshold C[O.sub.2] concentration at which this feedback is active and the magnitude of the warming caused by the feedback are analyzed using both a highly idealized model and NCAR's single-column atmospheric model (SCAM) run under Arctic-like conditions. The critical C[O.sub.2] is particularly important because it helps to establish the relevance of the feedback for past and future climates. Both models agree that increased heat flux into the high latitudes at low altitudes generally decreases the critical C[O.sub.2]. Increases in oceanic heat transport and in solar radiation absorbed during the summer should cause a sharp decrease in the critical C[O.sub.2], but the effect of increases in atmospheric heat transport depends on its vertical distribution. It is furthermore found (i) that if the onset of convection produces more clouds and moisture, the critical C[O.sub.2] should decrease, and the maximum temperature increase caused by the convective cloud feedback should increase and (ii) that reducing the depth of convection reduces the critical C[O.sub.2] but has little effect on the maximum temperature increase caused by the convective cloud feedback. These results should help with interpretation of the strength and onset of the convective cloud feedback as found, for example, in Intergovernmental Panel on Climate Change (IPCC) coupled ocean-atmosphere models with different cloud and convection schemes.

Idealized numerical simulations of moist strongly stratified flow over topography are used to study the processes that control orographic clouds in terrain-blocked flows as a joint function of the nondimensional flow parameter Nh/U, the horizontal topographic aspect ratio [beta], and the Rossby radius of deformation Nh/f. The simulations show the competition between enhanced upstream condensation in the secondary vertically propagating gravity wave and the reduction of condensation owing to enhanced low-level flow deflection. As Nh/U increases above about 1.5. the tendency for flow to be deflected around the barrier reduces cloud formation in the primary gravity wave over the ridge, while increasing [beta] expands low-level clouds over a broader upstream area. Ice clouds may form aloft in the secondary vertically propagating gravity wave and extend upstream for several hundred kilometers. In terrain-blocked flows, more than half of the condensate mass develops upstream of the barrier in the secondary gravity wave, while in unblocked flows most of the condensate is downstream of the barrier in the primary lee wave. In 2D, none of the flow can be diverted around the barrier, and it therefore produces a much more vigorous hydrologic cycle than over long ([beta] = 8) 3D ridges, increasing upstream lifting and cloud water content by at least a factor of 2, and generating primary wave clouds that are not produced in the 3D case. Rotation reduces the upstream extent of condensation in blocked flows to a region on the order of the radius of deformation and in 3D induces a marked asymmetry in the lifting and condensation upstream of the terrain.

The significant developments that took place in the study of cold dark clouds are presented. Findings reveal that by clarifying the combined structure of physical properties and complex chemical, the condition for star formation can be understood.

The potential of ground-based microwave weather radar systems for volcanic ash cloud detection and quantitative retrieval is evaluated. The relationship between radar reflectivity factor, ash concentration, and fall rate is statistically derived for various eruption regimes and ash sizes by applying a radar-reflectivity microphysical model. To quantitatively evaluate the ash detectability by weather radars, a sensitivity analysis is carried out by simulating synthetic ash clouds and varying ash concentration and size as a function of the range. Radar specifications are taken from typical radar systems at S-, C-, and X-band. A prototype algorithm for volcanic ash radar retrieval (VARR) is discussed. Starting from measured single-polarization reflectivity, the statistical inversion technique to retrieve ash concentration and fall rate is based on two cascade steps, namely: 1) classification of eruption regime and volcanic ash category and 2) estimation of ash concentration and fall rate. Expected accuracy of the VARR algorithm estimates is evaluated using a synthetic data set. An application of the VARR technique is finally shown, taking into consideration the eruption of the Grimsvotn volcano in Iceland on November 2004. Volume scan data from a Doppler C-band radar, which is located at 260 km from the volcano vent, are processed by means of the VARR algorithm. Examples of the achievable VARR products are presented and discussed. Index Terms--Ash retrieval, inversion methods, microwave radars, volcanic eruption clouds.

The Naval Research Laboratory W-band Advanced Radar for Low Observable Control (WARLOC) is a high-power 94-GHz radar, with 3-10-kW average and 80-kW peak power, now set up on the western shore of the Chesapeake Bay. It has three orders of magnitude more power and sensitivity than other W-band radar systems. This enables cloud reflectivity to be measured with high signal-to-noise ratios and a resolution of about 15 m over a two (or three)-dimensional region, which can be as large as tens of kilometers on a side. This allows imaging of the internal structure of clouds over great distances in very great detail. At the shortest scale lengths, the structure has a speckle pattern indicating that it is governed at least in part by stochastic processes. The WARLOC data allow the reflectivity correlation function and its Fourier transform, which is the power spectrum, to be computed for scale lengths ranging from 30 m to 10 km. For measurements taken of cirrus clouds on several occasions as well as one case of a precipitating cloud, it was found that for small correlation distance r, the correlation function usually decreases as [r.sup.2/3], or, equivalently, the wavenumber spectrum scales as [k.sup.-5/3], where k is the wavenumber. This suggests that fluid turbulence in the inertial range is playing a role; however, unlike classical fluid turbulence, the results suggest that the turbulence here is generally quite anisotropic. Furthermore, for longer scale lengths (the outer range according to the Kolmogorov theory), the reflectivity fluctuations usually show a wavelike behavior in the vertical direction but only occasionally in the horizontal direction. Index Terms--Clouds, gyrotrons, millimeter-wave radar.

Observations show that convective perturbations of the tropical atmosphere are associated with substantial variations of clouds and water vapor. Recent studies suggest that these variations may play an active role in the large-scale organization of the tropical atmosphere. The present study investigates that possibility by using a two-dimensional, nonrotating model that includes a set of physical parameterizations carefully evaluated against tropical data. In the absence of cloud--radiation interactions, the model spontaneously generates fast upwind (eastward) moving planetary-scale oscillations through the wind-induced surface heat exchange mechanism. In the presence of cloud--radiative effects, the model generates slower upwind (eastward) propagating modes in addition to small-scale disturbances advected downwind (westward) by the mean flow. Enhanced cloud--radiative effects further slow down upwind propagating waves and make them more prominent in the spectrum. On the other hand, the model suggests that interactions between moisture and convection favor the prominence of moist Kelvin-like waves in tropical variability at the expense of small-scale advective disturbances. These numerical results, consistent with theoretical predictions, suggest that the interaction of water vapor and cloud variations with convection and radiation plays an active role in the large-scale organization of the tropical atmosphere.

The frequent presence of clouds in passive remotely sensed imagery severely limits its regular exploitation in various application fields. Thus, the removal of cloud cover from this imagery represents an important preprocessing task consisting in the reconstruction of cloud-contaminated data. The intent of this study is to propose two novel general methods for the reconstruction of areas obscured by clouds in a sequence of multitemporal multispectral images. Given a cloud-contaminated image of the sequence, each area of missing measurements is reconstructed through an unsupervised contextual prediction process that reproduces the local spectro-temporal relationships between the considered image and an opportunely selected subset of the remaining temporal images. In the first method, the contextual prediction process is implemented by means of an ensemble of linear predictors, each trained over a local multitemporal region that is spectrally homogeneous in each temporal image of the selected subset. In order to obtain such regions, each temporal image is locally classified by an unsupervised classifier based on the expectation-maximization (EM) algorithm. In the second method, the local spectro-temporal relationships are reproduced by a single nonlinear predictor based on the support vector machines (SVM) approach. To illustrate the performance of the two proposed methods, an experimental analysis on a sequence of three temporal images acquired by the Landsat-7 Enhanced Thematic Mapper Plus sensor over a total period of four months is reported and discussed. It includes a detailed simulation study that aims at assessing with different reconstruction quality criteria the accuracy of the methods in different qualitative and quantitative cloud contamination conditions. Compared with two techniques based on compositing algorithms for cloud removal, the proposed methods show a clear superiority, which makes them a promising and useful tool in solving the considered problem, whose great complexity is commensurate with its practical importance. Index Terms--Cloud removal, expectation-maximization (EM) algorithm, image reconstruction, linear prediction, multitemporal multispectral images, spatio-temporal context, support vector machines (SVM).