Cosmic Rays,Clouds, and Climate

A correlation between a global average of low cloud cover and the flux of cosmic rays incident in the atmosphere has been observed during the last solar cycle. The ionising potential of Earth bound cosmic rays are modulated by the state of the heliosphere, while clouds play an important role in the Earth's radiation budget through trapping outgoing radiation and reflecting incoming radiation. If a physical link between these two features can be established, it would provide a mechanism linking solar activity and Earth's climate. Recent satellite observations have further revealed a correlation between cosmic ray flux and low cloud top temperature. The temperature of a cloud depends on the radiation properties determined by its droplet distribution. Low clouds are warm (>273K) and therefore consist of liquid water droplets. At typical atmospheric supersaturations (1%) a liquid cloud drop will only form in the presence of an aerosol, which acts as a condensation site. The droplet distribution of a cloud will then depend on the number of aerosols activated as cloud condensation nuclei (CCN) and the level of super saturation. Based on observational evidence it is argued that a mechanism to explain the cosmic ray-cloud link might be found through the role of atmospheric ionisation in aerosol production and/or growth. Observations of local aerosol increases in low cloud due to ship exhaust indicate that a small perturbation in atmospheric aerosol can have a major impact on low cloud radiative properties. Thus, a moderate influence on atmospheric aerosol distributions from cosmic ray ionisation would have a strong influence on the Earth's radiation budget. Historical evidence over the past 1000 years indicates that changes in climate have occurred in accord with variability in cosmic ray intensities. Such changes are in agreement with the sign of cloud radiative forcing associated with cosmic ray variability as estimated from satellite observations.     

Aerosol-Cloud Interactions Control of Earth Radiation and Latent Heat Release Budgets

Aircraft observations and model simulations show that cloud development is strongly modulated by the impact of cloud-aerosol interactions on precipitation forming processes. New insights into the mechanisms by which aerosols dominate the cloud cover of marine shallow clouds suggest that feedbacks between the cloud microstructure and cloud dynamics through precipitation processes play a major role in determining when a solid cloud cover will break up into a field of trade wind cumulus. Cloud-aerosol interactions dominate not only the dynamics of marine shallow clouds, but also the lifetime and the vertical disposition of latent heat of deep convective clouds over ocean and even more strongly over land. Recent coincident satellite measurements of aerosols and cloud properties quantify the aerosol effects on cloud cover and radiative forcing on regional and global scales. The shapes of the satellite retrieved relations between aerosols and cloud properties are consistent with the suggested ways by which aerosols affect clouds via precipitation processes, particularly by affecting the intensity of the cloud vertical air motions and its vertical development.     

Atmospheric Aerosol and Cloud Condensation Nuclei Formation: A Possible Influence of Cosmic Rays?

A physical mechanism which may have a potential to connect climate with cosmic rays (CR) involves aerosol particle formation by CR generated atmospheric ions followed by new particle growth. Only grown particles can scatter sunlight efficiently and can eventually act as cloud condensation nuclei (CCN) and thereby may influence climate. Moreover grown particles live longer as they are less rapidly scavenged by pre-existing larger particles. The present paper discusses aerosol particle formation and growth in the light of new measurements recently made by our MPIK Heidelberg group. Emphasis is placed upon the upper troposphere where very low temperatures tend to facilitate new particle formation by nucleation. The new measurements include: laboratory measurements of cluster ions, aircraft measurements of ambient atmospheric ions, and atmospheric measurements of the powerful nucleating gas H₂SO₄ and its precursor SO₂. The discussion also addresses model simulations of aerosol formation and growth. It is concluded that in the upper troposphere new aerosol formation by CR generated ions is a frequent process with relatively large rates. However new particle formation by homogeneous nucleation (HONU) which is not related to CR also seems to be efficient. The bottleneck in the formation of upper troposphere aerosol particles with sizes sufficiently large to be climate relevant is not nucleation but growth of small particles. Our recent upper troposphere SO₂ measurements suggest that particle growth by gaseous sulphuric acid condensation is at least occasionally efficient. If so CR mediated formation of CCN sized particles should at least occasionally be operative in the upper troposphere.     

Radiative Forcing of Climate Change

Our current understanding of mechanisms that are, or may be, acting to cause climate change over the past century is briefly reviewed, with an emphasis on those due to human activity. The paper discusses the general level of confidence in these estimates and areas of remaining uncertainty. The effects of increases in the so-called well-mixed greenhouse gases, and in particular carbon dioxide, appear to be the dominant mechanism. However, there are considerable uncertainties in our estimates of many other forcing mechanisms; those associated with the so-called indirect aerosol forcing (whereby changes in aerosols can impact on cloud properties) may be the most serious, as its climatic effect may be of a similar size as, but opposite sign to, that due to carbon dioxide. The possible role of volcanic eruptions as a natural climate change mechanism is also highlighted.     

Nucleation processes and aerosol chemistry

As the result of many observable optical phenomena, the occasional existence of upper atmospheric aerosols has been known since the beginning of the Century. Nevertheless, it is only during the last two decades that their persistent nature and extent of global distribution have become recognized. This review is addressed to the chemistry of upper atmospheric aerosols with particular attention to the chemical reactions and nucleation mechanisms responsible for their formation.Considering the stratosphere, it is now generally agreed that sulfate, mainly in the form of sulfuric acid, represents the single most abundant constituent of stratospheric aerosols. There is now strong evidence that these aerosols are formed in situ from sulfur bearing gases oxidized by free radicals. The various nucleation processes of atmospheric importance are reviewed, and it is shown that heteromolecular nucleation processes are almost certainly responsible for new particle formation in the stratosphere.The nature, and even existence, of aerosol layers above the stratosphere is less certain. The influx of extraterrestrial materials into the Earth atmosphere is estimated to be in the order of 50 to 500 tons per day. Considerations are given to the possible mechanisms which convert these materials to aerosol size particles. Although these particles provide one possible explanation for the appeareance of noctilucent clouds, nucleation about ions, most probably H₃O⁺ hydrates, is a strong contender as a more plausible formation mechanism. Considerable attention is given to this important atmospheric process, where it is shown that chemical forces contribute to the stability of the prenucleation clusters and the overall height of the energy barrier to nucleation. Finally, the role of aerosols in the sodium and other cycles of trace upper atmospheric constituents, is discussed.This research was performed under the auspices of the U.S. Atomic Energy Commission and partially financed by the Department of Transportation and the National Science Foundation under interagency agreements.     

Profiles of Ion and Aerosol Interactions in Planetary Atmospheres

In planetary atmospheres the nature of the aerosols varies, as does the relative importance of different sources of ion production. The nature of the aerosol and ion production is briefly reviewed here for the atmospheres of Venus, Mars, Jupiter and Titan using the concepts established for the terrestrial atmosphere. Interactions between the ions formed and aerosols present cause (1) charge exchange, which can lead to substantial aerosol charge and (2) ion removal. Consequences of (1) are that (a) charged aerosol are more effectively removed by conducting liquid droplets than uncharged aerosol and (b) particle–particle coagulation rates are modified, influencing particle residence times in the relevant atmosphere. Consequences of (2) are that ions are removed in regions with abundant aerosol, which may preclude charge flow in an atmosphere, such as that associated with an atmospheric electrical circuit. In general, charge should be included in microphysical modeling of the properties of planetary aerosols.     

Solar Variability and the Search for Corresponding Climate Signals

Instrumental and paleodata from the last centuries are investigated to get circumstantial evidence for external influences on the Earth's climate machine. Such influences could be of extraterrestrial and/or anthropogenic origin. Anthropogenic influences are separated from solar on superdecadal time scales and on a hemispheric level using a non-linear regression model. The function to be explained is the northern hemispheric temperature. The model contains two forcing components explicitly: A parameterized anthropogenic component, which describes the aggregated effect of greenhouse gases, aerosols and other anthropogenic climate impacts. A solar component, which describes the solar variability history. The solution of the regression model allows, under certain assumptions, a functional separation of the variability components and provides an estimation of their relative contributions to global warming during the last 140 years.     

Aerosol Effects on Clouds and Climate

Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei, they may inhibit freezing and they could have an influence on the hydrological cycle. While the cloud albedo enhancement (Twomey effect) of warm clouds received most attention so far and traditionally is the only indirect aerosol forcing considered in transient climate simulations, here I discuss the multitude of effects.     

In Situ Collection of Refractory Dust in the Upper Stratosphere: The DUSTER Facility

DUSTER (Dust from the Upper Stratosphere Tracking Experiment and Retrieval) is an instrument designed to collect nanometer to micrometer scale solid aerosol particles in the upper stratosphere on board balloons. With three DUSTER flights we have demonstrated that: (1) the instrument’s performance was within the design parameters of environmental specifications (?80 ^°C; 3–10?mbar); (2) inertial impact collection of aerosols ～500?nm to 24 microns on holey-carbon thin films mounted on Transmission Electron Microscope mesh grids was achieved; (3) the design of an active collector exposed to the air flux and an identical collector “blank”, not exposed to the air flux, to monitor possible contamination permits unambiguous identification of collected particles; (4) save storage of collected samples and subsequent retrieval in the laboratory was achieved with no measurable contamination; (5)?reduced sample manipulation allowed the chemical and structural characterization of collected dust particles by Field-emission scanning electron microscopy and energy dispersive X-Ray analyses, and infrared and Raman micro-spectroscopy. The main and most ambitious goal is the collection and characterization of solid aerosol particles smaller than 3 microns of solar system debris that are currently not sampled on a routine basis by other instruments in the upper stratosphere. DUSTER will provide a record of the amount of solid aerosols, their size, shapes and chemical properties in the upper stratosphere, including particles less than 3 microns in size. The DUSTER program identified 25 particles as collected during the 2008 flight with sizes in the range of 0.4 to 24 microns.     

Effectiveness of MMW aerosols in defeating a battlefieldsurveillance radar: field demonstration preliminary results

The US Army ERDEC is developing advanced aerosol systems to combat threat surveillance, fire control, and seeker systems operating in the visible, infrared, and millimeter wave portions of the electromagnetic spectrum. One such system is the M56 multispectral smoke generator, which presently operates in the visible and infrared portions of the spectrum; a millimeter wave (MMW) Module is in development to extend the M56 spectral range. This paper documents preliminary results of a field demonstration test of the M56 MMW Module. An MMW instrumentation radar was modified to simulate the scan pattern and radar parameters of a tactical battlefield surveillance radar system. A test grid was populated with both stationary and moving tactical targets, and the radar scanned the grid to simulate a surveillance radar in operation. Once a realistic tactical engagement scenario was developed, MMW aerosols were deployed to demonstrate the impact such aerosols could have on radar detection and classification performance     

Ions in the Terrestrial Atmosphere and Other Solar System Atmospheres

Charged molecular clusters, traditionally called small ions, carry electric currents in atmospheres. Charged airborne particles, or aerosol ions, play an important role in generation and evolution of atmospheric aerosols. Growth of ions depends on the trace gas content, which is highly variable in the time and space. Even at sub-ppb concentrations, electrically active organic compounds (e.g. pyridine derivatives) can affect the ion composition and size. The size and mobility are closely related, although the form of the relationship varies depending on the critical diameter, which, at 273 K, is about 1.6 nm. For ions smaller than this the separation of quantum levels exceeds the average thermal energy, allowing use of a molecular aggregate model for the size-mobility relation. For larger ions the size-mobility relation approaches the Stokes-Cunningham-Millikan law. The lifetime of a cluster ion in the terrestrial lower atmosphere is about one minute, determined by the balance between ion production rate, ion-ion recombination, and ion-aerosol attachment.     

The Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan

The payload of the Huygens Probe into the atmosphere of Titan includes the Descent Imager/Spectral Radiometer (DISR). This instrument includes an integrated package of several optical instruments built around a silicon charge coupled device (CCD) detector, a pair of linear InGaAs array detectors, and several individual silicon detectors. Fiber optics are used extensively to feed these detectors with light collected from three frame imagers, an upward and downward-looking visible spectrometer, an upward and downward looking near-infrared spectrometer, upward and downward looking violet phtotometers, a four-channel solar aerole camera, and a sun sensor that determines the azimuth and zenith angle of the sun and measures the flux in the direct solar beam at 940 nm. An onboard optical calibration system uses a small lamp and fiber optics to track the relative sensitivity of the different optical instruments relative to each other during the seven year cruise to Titan. A 20 watt lamp and collimator are used to provide spectrally continuous illumination of the surface during the last 100 m of the descent for measurements of the reflection spectrum of the surface. The instrument contains software and hardware data compressors to permit measurements of upward and downward direct and diffuse solar flux between 350 and 1700 nm in some 330 spectral bands at approximately 2 km vertical resolution from an alititude of 160 km to the surface. The solar aureole camera measures the brightness of a 6° wide strip of the sky from 25 to 75° zenith angle near and opposite the azimuth of the sun in two passbands near 500 and 935 nm using vertical and horizontal polarizers in each spectral channel at a similar vertical resolution. The downward-looking spectrometers provide the reflection spectrum of the surface at a total of some 600 locations between 850 and 1700 nm and at more than 3000 locations between 480 and 960 nm. Some 500 individual images of the surface are expected which can be assembled into about a dozen panoramic mosaics covering nadir angles from 6° to 96° at all azimuths. The spatial resolution of the images varies from 300 m at 160 km altitude to some 20 cm in the last frames. The scientific objectives of the experiment fall into four areas including (1) measurement of the solar heating profile for studies of the thermal balance of Titan; (2) imaging and spectral reflection measurements of the surface for studies of the composition, topography, and physical processes which form the surface as well as for direct measurements of the wind profile during the descent; (3) measurements of the brightness and degree of linear polarization of scattered sunlight including the solar aureole together with measurements of the extinction optical depth of the aerosols as a function of wavelength and altitude to study the size, shape, vertical distribution, optical properties, sources and sinks of aerosols in Titan's atmosphere; and (4) measurements of the spectrum of downward solar flux to study the composition of the atmosphere, especially the mixing ratio profile of methane throughout the descent. We briefly outline the methods by which the flight instrument was calibrated for absolute response, relative spectral response, and field of view over a very wide temperature range. We also give several examples of data collected in the Earth's atmosphere using a spare instrument including images obtained from a helicopter flight program, reflection spectra of various types of terrain, solar aureole measurements including the determination of aerosol size, and measurements of the downward flux of violet, visible, and near infrared sunlight. The extinction optical depths measured as a function of wavelength are compared to models of the Earth's atmosphere and are divided into contributions from molecular scattering, aerosol extinction, and molecular absorption. The test observations during simulated descents with mountain and rooftop venues in the Earth's atmosphere are very important for driving out problems in the calibration and interpretion of the observations to permit rapid analysis of the observations after Titan entry. This revised version was published online in August 2006 with corrections to the Cover Date.     

Atmospheric Ions and Aerosol Formation

This paper discusses atmospheric ions and their role in aerosol formation. Emphasis is placed upon the upper troposphere where very low temperatures tend to facilitate new particle formation by nucleation. New measurements addressed include: Laboratory measurements of cluster ions, aircraft measurements of ambient atmospheric ions, atmospheric measurements of the powerful nucleating gas H₂SO₄ and its gaseous precursor SO₂. The paper also discusses model simulations of aerosol formation and growth. It is concluded that in the upper troposphere new aerosol formation via ions is a frequent process with relatively large rates. However new particle formation by homogeneous nucleation which does not involve ions also seems to be efficient. The bottleneck in the formation of upper troposphere aerosol particles with sizes sufficiently large to be climate relevant is mostly not nucleation but sufficient growth of new and still very small particles. Our recent upper troposphere SO₂ measurements suggest that particle growth by gaseous sulphuric acid condensation can be efficient in certain circumstances. If so, cosmic ray mediated formation of CCN sized particles should at least occasionally be operative in the upper troposphere.     

Galileo Photopolarimeter/Radiometer experiment

The Photopolarimeter/Radiometer (PPR) is a remote sensing instrument on the Galileo Orbiter designed to measure the degree of linear polarization and the intensity of reflected sunlight in ten spectral channels between 410 and 945 nm to determine the physical properties of Jovian clouds and aerosols, and to characterize the texture and microstructure of satellite surfaces. The PPR also measures thermal radiation in five spectral bands between 15 and 100 m to sense the upper tropospheric temperature structure. Two additional channels which measure spectrally integrated solar and solar plus thermal radiation are used to determine the planetary radiation budget components. The PPR photopolarimetric measurements utilize previously flown technology for high-precision polarimetry using a calcite Wollaston prism and two silicon photodiodes to enable simultaneous detection of the two orthogonal polarization components. The PPR radiometry measurements are made with a lithium tantalate pyroelectric detector utilizing a unique arrangement of radiometric stops and a scene/space chopper blade to enable a warm instrument to sense accurately the much colder scene temperatures.     

Electrical Characteristics of Silicon Solar Cells at Low Temperatures

Experimentally determined values of open-circuit voltage, short-circuit current, and maximum power for p on n and n on p silicon solar cells are presented for temperatures ranging from -196°C to + 50°C under equivalent space sunlight intensities of 58 mW/cm2 and 268 mW/cm2. An anomalous behavior is observed in the n on p cells at low temperatures; namely, the open-circuit voltage becomes nearly independent of temperature below a transition temperature Tt that depends on the sunlight intensity.     

Anthropogenic and natural causes of twentieth century temperature change

We analyse spatio-temporal patterns of near-surface temperature change to provide an attribution of twentieth century climate change. We apply an ``optimal detection' methodology to seasonal and annual data averaged over a range of spatial and temporal scales. We find that solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects.     

Solar Influence on Earth's Climate

An increasing number of studies indicate that variations in solar activity have had a significant influence on Earth's climate. However, the mechanisms responsible for a solar influence are still not known. One possibility is that atmospheric transparency is influenced by changing cloud properties via cosmic ray ionisation (the latter being modulated by solar activity). Support for this idea is found from satellite observations of cloud cover. Such data have revealed a striking correlation between the intensity of galactic cosmic rays (GCR) and low liquid clouds (<3.2 km). GCR are responsible for nearly all ionisation in the atmosphere below 35 km. One mechanism could involve ion-induced formation of aerosol particles (diameter range, 0.001–1.0 μm) that can act as cloud condensation nuclei (CCN). A systematic variation in the properties of CCN will affect the cloud droplet distribution and thereby influence the radiative properties of clouds. If the GCR-Cloud link is confirmed variations in galactic cosmic ray flux, caused by changes in solar activity and the space environment, could influence Earth's radiation budget. This revised version was published online in August 2006 with corrections to the Cover Date.     

Global aerodynamic design optimization based on data dimensionality reduction

In aerodynamic optimization, global optimization methods such as genetic algorithms are preferred in many cases because of their advantage on reaching global optimum. However, for complex problems in which large number of design variables are needed, the computational cost becomes prohibitive, and thus original global optimization strategies are required. To address this need, data dimensionality reduction method is combined with global optimization methods, thus forming a new global optimization system, aiming to improve the efficiency of conventional global optimization. The new optimization system involves applying Proper Orthogonal Decomposition (POD) in dimensionality reduction of design space while maintaining the generality of original design space. Besides, an acceleration approach for samples calculation in surrogate modeling is applied to reduce the computational time while providing sufficient accuracy. The optimizations of a transonic airfoil RAE2822 and the transonic wing ONERA M6 are performed to demonstrate the effectiveness of the proposed new optimization system. In both cases, we manage to reduce the number of design variables from 20 to 10 and from 42 to 20 respectively. The new design optimization system converges faster and it takes 1/3 of the total time of traditional optimization to converge to a better design, thus significantly reducing the overall optimization time and improving the efficiency of conventional global design optimization method.     

Oxygen Isotopes and Sampling of the Solar System

The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as ‘zones.’ Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as ‘belts.’ On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn’s albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of “stacked circulation cells,” with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.

     

Resonance in a coupled solar-climate model

The 11–year solar activity cycle is magnetic in origin and is responsible for small changes in solar luminosity and the modulation of the solar wind. The terrestrial climate exhibits much internal variability supporting oscillations with many frequencies. The direct effect of changing solar irradiance in driving climatic change is believed to be small, and amplification mechanisms are needed to enhance the role of solar variability. In this paper we demonstrate that resonance may play a crucial role in the dynamics of the climate system, by using the output from a nonlinear solar dynamo model as a weak input to a simplified climate model. The climate is modelled as oscillating about two fixed points (corresponding to a warm and cold state) with the weak chaotically modulated solar forcing on average pushing the solution towards the warm state. When a typical frequency of the input is similar to that of the chaotic climate system then a dramatic increase in the role of the solar forcing is apparent and complicated intermittent behaviour is observed. The nonlinear effects are subtle however, and forcing that on average pushes the solution towards the warm state may lead to increased intervals of oscillation about either state. Owing to the intermittent nature of the timeseries, analysis of the relevant timeseries is shown to be non-trivial.