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In this paper we present a quantitative comparison between a large data base of medium-scale atmospheric gravity waves (AGWs) observed by radio interferometry of transionospheric radio sources and the results of a numerical simulation of the observed effects. The simulation includes: (i) the propagation and dissipation of AGWs up to ionospheric heights and (ii) the calculation of the subsequent slant TEC perturbations integrated along the path to the radio sources. We show that the observed azimuthal distribution of AGWs can be deeply biased. Predicted results are found to be consistent with previous extensive observations using radio beacons aboard geostationary satellites. These observations are rediscussed in view of the present predictions.
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A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLB and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone <i>et al</i>. (1995). Within the POLB, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLB and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.
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A three-dimensional meteorological model and a Lagrangian particle dispersion model are used to study the effects of a uniform large-scale wind on the dispersion of a non-reactive pollutant in a coastal region with complex terrain. Simulations are carried out both with and without a background wind. A comparison between model results and measured data (wind and pollutant concentrations) indicates that the coupled model system provides a useful mechanism for analyzing pollutant dispersion in coastal regions.
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Empirical studies have shown that warm El Nino/Southern Oscillation (ENSO) episodes are associated during northern summer with, first, a southward location of the intertropical convergence zone (ITCZ) over the tropical Atlantic, and, second, a weakened convection over West Africa where the ITCZ is near its mean latitude. A modelling experiment presented here is used to help explain this apparent contradiction. In simulated ENSO conditions, the ITCZ is located southwards over the tropical Atlantic. Over West Africa the intertropical front is also displaced southwards, but more slightly; the ITCZ is located at its climatological latitude and the vertical development of convective clouds over West and Central Africa is reduced due to dynamical subsidence in the upper levels.
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A new optical sensor, the airborne Polar Nephelometer, is described. The sensor is designed to measure the optical and microphysical parameters of clouds containing either water droplets or ice crystals or a mixture of these particles ranging in size from a few micrometers to about 500 µm diameter. The probe measures the scattering phase function of an ensemble of cloud particles intersecting a collimated laser beam near the focal point of a paraboloïdal mirror. The light scattered from polar angles from 3.49° to 169° is reflected onto a circular array of 33 photodiodes. The signal processing electronics and computer storage can provide one measurement of the scattering phase function every 100 ms or every 0.2 ms. The first part of the paper describes the theoretical design of a prototype version of the probe.
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Mesospheric ice particles, known as noctilucent clouds or polar mesospheric clouds, have long been observed by rocket instruments, satellites and ground-based remote sensing, while models have been used to simulate ice particle growth and cloud properties. However, the fact that different measurement techniques are sensitive to different parts of the ice particle distribution makes it difficult to compare retrieved parameters such as ice particle radius or ice concentration from different experiments. In this work we investigate the accuracy of satellite retrieval based on scattered light and how this affects derived cloud properties. We apply the retrieval algorithm on spectral signals calculated from modelled cloud distributions and compare the results to the properties of the original distributions. We find that ice mass density is accurately retrieved whereas mean radius is often overestimated and high ice concentrations are generally underestimated. The reason is partly that measurements based on scattered light are insensitive to the smaller particles and partly that the retrieval algorithm assumes a Gaussian size distribution. Once we know the limits of the satellite retrieval we proceed to compare the properties retrieved from the modelled cloud distributions to those observed by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument on the Odin satellite. We find that a model with a stationary atmosphere, as given by average atmospheric conditions, does not yield cloud properties that are in agreement with the observations, whereas a model with realistic temperature and vertical wind variations does. This indicates that average atmospheric conditions are insufficient to understand the process of noctilucent cloud growth and that a realistic atmospheric variability is crucial for cloud formation and growth. Further, the agreement between results from the model, when set up with a realistically variable atmosphere, and the observations suggests that our understanding of the growth process itself is reasonable.
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The effects of polluted aerosol on cloud are examined over the Yangtze River Delta (YRD) using three-month satellite data during wintertime from December 2013 to January 2014. The relationships between aerosol properties and cloud parameters are analyzed in detail to clarify the differences of cloud development under varying aerosol and meteorology conditions. Complex relationships between aerosol optical depth (AOD) and cloud droplet radius (CDR), liquid water path (LWP) and cloud optical thickness (COT) exists in four sub-regions. High aerosol loading (AOD) does not obviously affect the distributions of cloud LWP and COT. In fact, an inhibiting effect of aerosol occurs in coastal area for low- and medium-low clouds, more pronounced in low clouds (< 5km) than high clouds. Low aerosol loading (AOD) plays a positive role in promoting COTs of high- and low-clouds in areas dominated by marine aerosol. The most significant effect presents in valley and coal industry districts for clouds except high-cloud. The smallest values and variations of cloud parameters are observed in dry-polluted area, which suggests that dust aerosol makes little difference on clouds properties. Synoptic conditions also cast strong impacts on cloud distribution, particularly the unstable synoptic condition leads to cloud development at larger horizontal and vertical scales. The ground pollution enhances the amount of low-level cloud coverage even under stable condition. Aerosol plays an important role in cloud evolution for the low layers of troposphere (below 5 km) in case of the stable atmosphere in wintertime.
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The summer of 2015 was an extreme forest fire year in the Pacific Northwest. Our sample site at the Mt. Bachelor Observatory (MBO, 2.7 km a.s.l.) in central Oregon observed biomass burning (BB) events more than 50 % of the time during August. In this paper we characterize the aerosol physical and optical properties of 19 aged BB events during August 2015. Six of the 19 events were influenced by Siberian fires originating near Lake Baikal that were transported to MBO over 4–10 days. The remainder of the events resulted from wildfires in Northern California and Southwestern Oregon with transport times to MBO ranging from 3 to 35 h. Fine particulate matter (PM<sub>1</sub>), carbon monoxide (CO), aerosol light scattering coefficients (<i>σ</i><sub>scat</sub>), aerosol light absorption coefficients (<i>σ</i><sub>abs</sub>), and aerosol number size distributions were measured throughout the campaign. We found that the Siberian events had a significantly higher Δ<i>σ</i><sub>abs</sub>∕ΔCO enhancement ratio, higher mass absorption efficiency (MAE; Δ<i>σ</i><sub>abs</sub>∕ΔPM<sub>1</sub>), lower single scattering albedo (<i>ω</i>), and lower absorption Ångström exponent (AAE) when compared with the regional events. We suggest that the observed Siberian events represent that portion of the plume that has hotter flaming fire conditions and thus enabled strong pyroconvective lofting and long-range transport to MBO. The Siberian events observed at MBO therefore represent a selected portion of the original plume that would then have preferentially higher black carbon emissions and thus an enhancement in absorption. The lower AAE values in the Siberian events compared to regional events indicate a lack of brown carbon (BrC) production by the Siberian fires or a loss of BrC during transport. We found that mass scattering efficiencies (MSE) for the BB events ranged from 2.50 to 4.76 m<sup>2</sup> g<sup>−1</sup>. We measured aerosol size distributions with a scanning mobility particle sizer (SMPS). Number size distributions ranged from unimodal to bimodal and had geometric mean diameters (<i>D</i><sub>pm</sub>) ranging from 138 to 229 nm and geometric standard deviations (<i>σ</i><sub>g</sub>) ranging from 1.53 to 1.89. We found MSEs for BB events to be positively correlated with the geometric mean of the aerosol size distributions (<i>R</i><sup>2</sup> = 0.73), which agrees with Mie theory. We did not find any dependence on event size distribution to transport time or fire source location.
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We use satellite-borne measurements collected over the last decade (2006–2015) from the Aura Microwave Limb Sounder (MLS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to investigate the nitric acid distribution and the properties of polar stratospheric clouds (PSCs) in the early winter Antarctic vortex. Frequently, at the very start of the winter, we find that synoptic-scale depletion of HNO<sub>3</sub> can be detected in the inner vortex before the first lidar detection of geophysically associated PSCs. The generation of "sub-visible" PSCs can be explained as arising from the development of a solid particle population with low number densities and large particle sizes. Assumed to be composed of nitric acid trihydrate (NAT), the sub-visible PSCs form at ambient temperatures well above the ice frost point, but also above the temperature at which supercooled ternary solution (STS) grows out of the background supercooled binary solution (SBS) distribution. The temperature regime of their formation, inferred from the simultaneous uptake of ambient HNO<sub>3</sub> into NAT and their Lagrangian temperature histories, is at a depression of a few kelvin with respect to the NAT existence threshold, <i>T</i><sub>NAT</sub>. Therefore, their nucleation requires a considerable supersaturation of HNO<sub>3</sub> over NAT, and is consistent with a recently described heterogeneous nucleation process on solid foreign nuclei immersed in liquid aerosol. We make a detailed investigation of the comparative limits of detection of PSCs and the resulting sequestration of HNO<sub>3</sub> imposed by lidar, mid-infrared, and microwave techniques. We find that the temperature history of air parcels, in addition to the local ambient temperature, is an important factor in the relative frequency of formation of liquid/solid PSCs. We conclude that the initiation of NAT nucleation and the subsequent development of large NAT particles capable of sedimentation and denitrification in the early winter do not emanate from an ice-seeding process. Finally, we investigate the patterns of interannual variability and compare the relative formation frequency of liquid and solid PSCs in the Antarctic lower polar stratosphere using the results of a cluster analysis to synthesize the combined CALIOP and MLS measurements into a relatively small number of interrelated categories.
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Understanding the processes controlling terrestrial carbon fluxes is one of the grand challenges of climate science. Carbon cycle process controls are readily studied at local scales, but integrating local knowledge across extremely heterogeneous biota, landforms and climate space has proven to be extraordinarily challenging. Consequently, top-down or integral flux constraints at process-relevant scales are essential to reducing process uncertainty. Future satellite-based estimates of greenhouse gas fluxes – such as CO<sub>2</sub> and CH<sub>4</sub> – could potentially provide the constraints needed to resolve biogeochemical process controls at the required scales. Our analysis is focused on Amazon wetland CH<sub>4</sub> emissions, which amount to a scientifically crucial and methodologically challenging case study. We quantitatively derive the observing system (OS) requirements for testing wetland CH<sub>4</sub> emission hypotheses at a process-relevant scale. To distinguish between hypothesized hydrological and carbon controls on Amazon wetland CH<sub>4</sub> production, a satellite mission will need to resolve monthly CH<sub>4</sub> fluxes at a ∼ 333 km resolution and with a ≤ 10 mg CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> flux precision. We simulate a range of low-earth orbit (LEO) and geostationary orbit (GEO) CH<sub>4</sub> OS configurations to evaluate the ability of these approaches to meet the CH<sub>4</sub> flux requirements. Conventional LEO and GEO missions resolve monthly ∼ 333 km Amazon wetland fluxes at a 17.0 and 2.7 mg CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> median uncertainty level. Improving LEO CH<sub>4</sub> measurement precision by <msqrt>2</msqrt> would only reduce the median CH<sub>4</sub> flux uncertainty to 11.9 mg CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup>. A GEO mission with targeted observing capability could resolve fluxes at a 2.0–2.4 mg CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> median precision by increasing the observation density in high cloud-cover regions at the expense of other parts of the domain. We find that residual CH<sub>4</sub> concentration biases can potentially reduce the ∼ 5-fold flux CH<sub>4</sub> precision advantage of a GEO mission to a ∼ 2-fold advantage (relative to a LEO mission). For residual CH<sub>4</sub> bias correlation lengths of 100 km, the GEO can nonetheless meet the ≤ 10 mg CH<sub>4</sub> m<sup>−2</sup> day<sup>−1</sup> requirements for systematic biases ≤ 10 ppb. Our study demonstrates that process-driven greenhouse gas OS simulations can enhance conventional uncertainty reduction assessments by quantifying the OS characteristics required for testing biogeochemical process hypotheses.
