Index Geophysics

Items

• Prediction of geomagnetic storms from solar wind data with the use of a neural network

  An artificial feed-forward neural network with one hidden layer and error back-propagation learning is used to predict the geomagnetic activity index (<i>D_st</i>) one hour in advance. The <i>B_z</i>-component and <i>&#x03A3;_Bz</i>, the density, and the velocity of the solar wind are used as input to the network. The network is trained on data covering a total of 8700 h, extracted from the 25-year period from 1963 to 1987, taken from the NSSDC data base. The performance of the network is examined with test data, not included in the training set, which covers 386 h and includes four different storms. Whilst the network predicts the initial and main phase well, the recovery phase is not modelled correctly, implying that a single hidden layer error back-propagation network is not enough, if the measured <i>D_st</i> is not available instantaneously. The performance of the network is independent of whether the raw parameters are used, or the electric field and square root of the dynamical pressure.

• Acoustic gravity wave growth and damping in convecting plasma

  The propagation of acoustic gravity waves through steadily convecting plasma in the thermosphere has been analysed theoretically. The growth and damping rates of internal gravity waves due to the feedback effects of wave-modulated Joule heating and Laplace forcing have been calculated. It is found that large convection flow velocities lead to the growth of large-scale internal gravity waves, whilst small- and medium-scale waves are heavily damped, under similar conditions. It has also been shown that wave growth is favoured for waves travelling against the plasma flow direction. The effects of critical coupling when wave phase speeds match the plasma flow speed have also been investigated. The results of these calculations are discussed in the context of the atmospheric energy budget and thermosphere-ionosphere coupling.

• Neutral gas composition changes and E×B vertical plasma drift contribution to the daytime equatorial F2-region storm effects

  Theoretical model calculations along with ground-based observations from Huancayo ionosonde station and ESRO-4 gas analyzer data, were used to estimate the contribution of neutral gas composition changes and <strong>E</strong>×<strong>B</strong> vertical plasma drift to the observed F2-layer storm effects at the geomagnetic equator. Atomic oxygen concentration increase may give the main contribution to the positive NmF2 effect when drift velocity changes are small, but negative storm effects, on the other hand, are related mostly to vertical drift variations.

• Experimental study of long nonlinear internal waves in rotating fluid

  Experiments were performed on the rotating platform 14 m in diameter equipped with a simple internal wave generator. Internal waves were generated for a wide range of Coriolis parameters. When the rotation is very weak, i.e., when the internal Rossby radius of deformation is much larger than the wavelength, then the stable nonlinear waves generated are solitary waves. These have a horizontal crest, as in the nonrotating case. When the rotation is strong, i.e., when the internal Rossby radius is at most comparable with the wavelength, then Sverdrup-like periodic waves can be generated, but no solitary wave can then propagate. For the intermediate case, Ostrovsky waves are generated. Their phase speed increases with increasing amplitude. Then, there are two characteristic wave lengths: one which varies with the inverse square root of the amplitude, as for the KdV wave, and the other, linked with the rotation, which varies as the square root of the amplitude. The experimental results are thus in agreement with most of the conclusions in recent analytical developments.

• The impact of cloud inhomogeneities on the Earth radiation budget: the 14 October 1989 I.C.E. convective cloud case study

  Through their multiple interactions with radiation, clouds have an important impact on the climate. Nonetheless, the simulation of clouds in climate models is still coarse. The present evolution of modeling tends to a more realistic representation of the liquid water content; thus the problem of its subgrid scale distribution is crucial. For a convective cloud field observed during ICE 89, Landsat TM data (resolution: 30m) have been analyzed in order to quantify the respective influences of both the horizontal distribution of liquid water content and cloud shape on the Earth radiation budget. The cloud field was found to be rather well-represented by a stochastic distribution of hemi-ellipsoidal clouds whose horizontal aspect ratio is close to 2 and whose vertical aspect ratio decreases as the cloud cell area increases. For that particular cloud field, neglecting the influence of the cloud shape leads to an over-estimate of the outgoing longwave flux; in the shortwave, it leads to an over-estimate of the reflected flux for high solar elevations but strongly depends on cloud cell orientations for low elevations. On the other hand, neglecting the influence of cloud size distribution leads to systematic over-estimate of their impact on the shortwave radiation whereas the effect is close to zero in the thermal range. The overall effect of the heterogeneities is estimated to be of the order of 10 W m^-2 for the conditions of that Landsat picture (solar zenith angle 65°, cloud cover 70%); it might reach 40 W m^-2 for an overhead sun and overcast cloud conditions.

• Observations and implications of liquid–liquid phase separation at high relative humidities in secondary organic material produced by α-pinene ozonolysis without inorganic salts

  Particles consisting of secondary organic material (SOM) are abundant in the atmosphere. To predict the role of these particles in climate, visibility and atmospheric chemistry, information on particle phase state (i.e., single liquid, two liquids and solid) is needed. This paper focuses on the phase state of SOM particles free of inorganic salts produced by the ozonolysis of <i>α</i>-pinene. Phase transitions were investigated in the laboratory using optical microscopy and theoretically using a thermodynamic model at 290 K and for relative humidities ranging from  &lt;  0.5 to 100 %. In the laboratory studies, a single phase was observed from 0 to 95 % relative humidity (RH) while two liquid phases were observed above 95 % RH. For increasing RH, the mechanism of liquid–liquid phase separation (LLPS) was spinodal decomposition. The RH range over which two liquid phases were observed did not depend on the direction of RH change. In the modeling studies, the SOM took up very little water and was a single organic-rich phase at low RH values. At high RH, the SOM underwent LLPS to form an organic-rich phase and a water-rich phase, consistent with the laboratory studies. The presence of LLPS at high RH values can have consequences for the cloud condensation nuclei (CCN) activity of SOM particles. In the simulated Köhler curves for SOM particles, two local maxima were observed. Depending on the composition of the SOM, the first or second maximum can determine the critical supersaturation for activation. Recently researchers have observed inconsistencies between measured CCN properties of SOM particles and hygroscopic growth measured below water saturation (i.e., hygroscopic parameters measured below water saturation were inconsistent with hygroscopic parameters measured above water saturation). The work presented here illustrates that such inconsistencies are expected for systems with LLPS when the water uptake at subsaturated conditions represents the hygroscopicity of an organic-rich phase while the barrier for CCN activation can be determined by the second maximum in the Köhler curve when the particles are water rich.

• Evidence for ambient dark aqueous SOA formation in the Po Valley, Italy

  Laboratory experiments suggest that water-soluble products from the gas-phase oxidation of volatile organic compounds can partition into atmospheric waters where they are further oxidized to form low volatility products, providing an alternative route for oxidation in addition to further oxidation in the gas phase. These products can remain in the particle phase after water evaporation, forming what is termed as aqueous secondary organic aerosol (aqSOA). However, few studies have attempted to observe ambient aqSOA. Therefore, a suite of measurements, including near-real-time WSOC (water-soluble organic carbon), inorganic anions/cations, organic acids, and gas-phase glyoxal, were made during the PEGASOS (Pan-European Gas-AeroSOls-climate interaction Study) 2012 campaign in the Po Valley, Italy, to search for evidence of aqSOA. Our analysis focused on four periods: Period A on 19–21 June, Period B on 30 June and 1–2 July, Period C on 3–5 July, and Period D on 6–7 July to represent the first (Period A) and second (Periods B, C, and D) halves of the study. These periods were picked to cover varying levels of WSOC and aerosol liquid water. In addition, back trajectory analysis suggested all sites sampled similar air masses on a given day. The data collected during both periods were divided into times of increasing relative humidity (RH) and decreasing RH, with the aim of diminishing the influence of dilution and mixing on SOA concentrations and other measured variables. Evidence for local aqSOA formation was only observed during Period A. When this occurred, there was a correlation of WSOC with organic aerosol (<i>R</i>² = 0.84), aerosol liquid water (<i>R</i>² = 0.65), RH (<i>R</i>² = 0.39), and aerosol nitrate (<i>R</i>² = 0.66). Additionally, this was only observed during times of increasing RH, which coincided with dark conditions. Comparisons of WSOC with oxygenated organic aerosol (OOA) factors, determined from application of positive matrix factorization analysis on the aerosol mass spectrometer observations of the submicron non-refractory organic particle composition, suggested that the WSOC differed in the two halves of the study (Period A WSOC vs. OOA-2 <i>R</i>² = 0.83 and OOA-4 <i>R</i>² = 0.04, whereas Period C WSOC vs. OOA-2 <i>R</i>² = 0.03 and OOA-4 <i>R</i>² = 0.64). OOA-2 had a high O ∕ C (oxygen ∕ carbon) ratio of 0.77, providing evidence that aqueous processing was occurring during Period A. Key factors of local aqSOA production during Period A appear to include air mass stagnation, which allows aqSOA precursors to accumulate in the region; the formation of substantial local particulate nitrate during the overnight hours, which enhances water uptake by the aerosol; and the presence of significant amounts of ammonia, which may contribute to ammonium nitrate formation and subsequent water uptake and/or play a more direct role in the aqSOA chemistry.

• Heterogeneous reactions of NO2 with CaCO3–(NH4)2SO4 mixtures at different relative humidities

  In this work, the heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures with a series of weight percentage (wt %) of (NH₄)₂SO₄ were investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different relative humidity (RH) values. For comparison, the heterogeneous reactions of NO₂ with pure CaCO₃ particles and pure (NH₄)₂SO₄ particles, as well as the reaction of CaCO₃ with (NH₄)₂SO₄ particles, were also studied. The results indicated that NO₂ did not show any significant uptake on (NH₄)₂SO₄ particles, and it reacted with CaCO₃ particles to form calcium nitrate under both dry and wet conditions. The heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures were markedly dependent on RH. Calcium nitrate was formed from the heterogeneous reactions at all the RHs investigated, whereas CaSO₄  ⋅  0.5H₂O (bassanite), CaSO₄  ⋅  2H₂O (gypsum), and (NH₄)₂Ca(SO₄)₂  ⋅  H₂O (koktaite) were produced depending on RH. Under the dry condition, the heterogeneous uptake of NO₂ on the mixtures was similar to that on CaCO₃ particles with neglectable effects from (NH₄)₂SO₄; the duration of initial stages and the NO₃⁻ mass concentrations had a negative linear relation with the mass fraction of (NH₄)₂SO₄ in the mixtures. Under wet conditions, the chemical interaction of (NH₄)₂SO₄ with Ca(NO₃)₂ enhances the nitrate formation, especially at medium RHs, while the coagulation of (NH₄)₂SO₄ with CaCO₃ exhibits an increasing inhibiting effects with increasing RH at the same time. In addition, the heterogeneous uptake of NO₂ on the mixtures of CaCO₃ and (NH₄)₂SO₄ was found to favor the formation of bassanite and gypsum due to the decomposition of CaCO₃ and the coagulation of Ca²⁺ and SO₄²⁻. A possible reaction mechanism was proposed and the atmospheric implications were discussed.

• Global fine-mode aerosol radiative effect, as constrained by comprehensive observations

  Aerosols directly affect the radiative balance of the Earth through the absorption and scattering of solar radiation. Although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify, the consensus is that anthropogenic aerosols cool the climate, partially offsetting the warming by rising greenhouse gas concentrations. Recent estimates of global direct anthropogenic aerosol radiative forcing (i.e., global radiative forcing due to aerosol–radiation interactions) are −0.35 ± 0.5 W m⁻², and these estimates depend heavily on aerosol simulation. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total aerosol optical depth (AOD), its fine-mode fraction, the vertical distribution of aerosols and clouds, and the collocation of clouds and overlying aerosols. We find that the direct fine-mode aerosol radiative effect is −0.46 W m⁻² (−0.54 to −0.39 W m⁻²). Fine-mode aerosols include sea salt and dust aerosols, and we find that these natural aerosols result in a very large cooling (−0.44 to −0.26 W m⁻²) when constrained by observations. When the contribution of these natural aerosols is subtracted from the fine-mode radiative effect, the net becomes −0.11 (−0.28 to +0.05) W m⁻². This net arises from total (natural + anthropogenic) carbonaceous, sulfate and nitrate aerosols, which suggests that global direct anthropogenic aerosol radiative forcing is less negative than −0.35 W m⁻².

• Optical and microphysical characterization of aerosol layers over South Africa by means of multi-wavelength depolarization and Raman lidar measurements

  Optical and microphysical properties of different aerosol types over South Africa measured with a multi-wavelength polarization Raman lidar are presented. This study could assist in bridging existing gaps relating to aerosol properties over South Africa, since limited long-term data of this type are available for this region. The observations were performed under the framework of the EUCAARI campaign in Elandsfontein. The multi-wavelength Polly^XT Raman lidar system was used to determine vertical profiles of the aerosol optical properties, i.e. extinction and backscatter coefficients, Ångström exponents, lidar ratio and depolarization ratio. The mean microphysical aerosol properties, i.e. effective radius and single-scattering albedo, were retrieved with an advanced inversion algorithm. Clear differences were observed for the intensive optical properties of atmospheric layers of biomass burning and urban/industrial aerosols. Our results reveal a wide range of optical and microphysical parameters for biomass burning aerosols. This indicates probable mixing of biomass burning aerosols with desert dust particles, as well as the possible continuous influence of urban/industrial aerosol load in the region. The lidar ratio at 355 nm, the lidar ratio at 532 nm, the linear particle depolarization ratio at 355 nm and the extinction-related Ångström exponent from 355 to 532 nm were 52 ± 7 sr, 41 ± 13 sr, 0.9 ± 0.4 % and 2.3 ± 0.5, respectively, for urban/industrial aerosols, while these values were 92 ± 10 sr, 75 ± 14 sr, 3.2 ± 1.3 % and 1.7 ± 0.3, respectively, for biomass burning aerosol layers. Biomass burning particles are larger and slightly less absorbing compared to urban/industrial aerosols. The particle effective radius were found to be 0.10 ± 0.03, 0.17 ± 0.04 and 0.13 ± 0.03 µm for urban/industrial, biomass burning, and mixed aerosols, respectively, while the single-scattering albedo at 532 nm was 0.87 ± 0.06, 0.90 ± 0.06, and 0.88 ± 0.07 (at 532 nm), respectively, for these three types of aerosols. Our results were within the same range of previously reported values.