Bidirectional Turbulent Fluxes of Fog at a Subtropical Montane Cloud Forest Covering a Wide Size Range of Droplets

Size-resolved turbulent fluxes of fog droplets are investigated above a subtropical montane cloud forest in Taiwan. By integrating an aerosol spectrometer into an eddy-covariance set-up, we measure droplet number fluxes and liquid water fluxes in a size range of aerosol particles and droplets with diameters ranging from 0.25 $${\upmu }\!\mathrm{m}$$ μ m to 17.3 $${\upmu }\!\mathrm{m}$$ μ m . We find two flux-direction changes within this size range: a downward flux occurs for accumulation-mode aerosols of diameters between 0.25 $${\upmu }\!\mathrm{m}$$ μ m and 0.83 $${\upmu }\!\mathrm{m}$$ μ m , an upward flux occurs for hydrated aerosols with diameters between 1.1 $${\upmu }\!\mathrm{m}$$ μ m and 2.4 $${\upmu }\!\mathrm{m}$$ μ m , and a downward flux occurs again for activated fog droplets between diameters of 3 $${\upmu }\!\mathrm{m}$$ μ m and 17.3 $${\upmu }\!\mathrm{m}$$ μ m . The droplet size distributions can be modelled by a trimodal log-normal distribution, and the modes correlate with the different flux directions. The formation of the three modes and the establishment of the respective flux directions can be explained by combining the Köhler theory on the basis of measured ion concentrations in fog with the turbulent transport of droplets. Finally, from the combined analysis of droplet fluxes and size distributions, we infer relevant processes of droplet development and dissolving during various phases of the life cycles of the fog events.

Downward particle flux in the open ocean: Future global opportunities and insights from time-series measurements at the PAP Sustained Observatory in the Northeast Atlantic.

<p>The Joint Global Ocean Flux Study (JGOFS) started in 1987 and stimulated massive integration of efforts to measure and understand the processes of downward flux in the ocean. A small number of sustained observatories have persisted from this time in the belief that a key to gaining this understanding is by prolonged time-series observations. The sustained observatory over the Porcupine Abyssal Plain (PAP-SO) in the Northeast Atlantic is one such site which has provided nearly continuous measurements of downward flux at a depth of 3000m since 1989. This was using sediment traps but, in order to understand the data, we have exploited a wide range of other approaches such as optical methods, drifting sediment traps in the upper 1000m, chemical variables near the surface and computational modelling. Insights are also gained from the Continuous Plankton Recorder (CPR), satellite observations and various climatic indices.</p><p>This presentation draws together these measurements to quantify and understand flux at 3000m. Seasonal and interannual variability is large but after 30 years of observation, explanations are now possible. In addition, some of the conclusions identify major and surprising features about the ways ecosystems in one year may be influenced by their structure and function in previous years.</p><p>At the same time as these time-series observations have been in progress, major new developments have taken place globally and at PAP-SO to provide additional and alternative means to asses flux. The sediment trap has significant advantages as well as uncertainties which have been described previously. The new era of approaches using, for instance, BGC Argo, the Carbon Flux Explorer and a variety of other optical techniques offer what may be a quantum leap in our understanding of downward particle flux now and how it is changing in response to changes in the global climate. This presentation will give a personal and optimistic view of the opportunities which are now developing to quantify and understand this crucially important process.</p>

Longitudinal peculiarities of planetary waves-zonal flow interaction and its role in stratosphere-troposphere dynamical coupling

The three-dimensional (3D) planetary wave analysis provides more regionalized information on stratospheric-tropospheric dynamic interactions. The upward wave flux from the troposphere to the stratosphere is maximized above north-eastern Eurasia, while the downward flux is mainly over the North America and North Atlantic (NANA) region, which is much stronger in mid and late winter. This distribution is determined by the wave-wave interaction between the different wavenumbers of planetary waves, especially between wavenumber 1 and wavenumber 2. The upward wave flux anomalies in early winter are negatively correlated with the strength of the stratospheric polar vortex (SPV). In the mid and late winter months, the strength of the SPV is positively correlated with the first mode of 3D wave flux and has a leading relationship of approximately one month. The stronger SPV corresponds to a stronger upward wave flux above northern Eurasia and stronger downward flux over the NANA region. The interannual variation in wave flux in early winter is closely associated with the Scandinavian wave train pattern. In contrast, the wave flux variation is related to the circulation anomaly corresponding to Arctic Oscillation in mid and late winter, which causes climate anomalies across the Northern Hemisphere, especially coherent temperature changes in northern Europe, eastern United States and northeastern China.

Intra-Annual Patterns in the Benthic-Pelagic Fluxes of Dissolved and Particulate Matter

In coastal temperate environments, many processes known to affect the exchange of particulate and dissolved matter between the seafloor and the water column follow cyclical patterns of intra-annual variation. This study assesses the extent to which these individual short term temporal variations affect specific direct drivers of seafloor-water exchanges, how they interact with one another throughout the year, and what the resulting seasonal variation in the direction and magnitude of benthic-pelagic exchange is. Existing data from a multidisciplinary long-term time-series from the Western Channel Observatory, United Kingdom, were combined with new experimental and in situ data collected throughout a full seasonal cycle. These data, in combination with and contextualized by time-series data, were used to define an average year, split into five ‘periods’ (winter, pre-bloom, bloom, post-bloom, and autumn) based around the known importance of pelagic primary production and hydrodynamically active phases of the year. Multivariate analyses were used to identify specific sub-sets of parameters that described the various direct drivers of seafloor-water exchanges. Both dissolved and particulate exchange showed three distinct periods of significant flux during the year, although the specific timings of these periods and the cause-effect relationships to the direct and indirect drivers differed between the two types of flux. Dissolved matter exchange was dominated by an upward flux in the pre-bloom period driven by diffusion, then a biologically induced upward flux during the bloom and an autumn downward flux. The latter was attributable to the interactions of hydrodynamic and biological activity on the seafloor. Particulate matter exchanges exhibited a strongly hydrologically influenced upward flux during the winter, followed by a biologically induced downward flux during the bloom and a second period of downward flux throughout post-bloom and autumn periods. This was driven primarily through interactions between biological activity, and physical and meteorological drivers. The integrated, holistic and quantitative data-based analysis of intra-annual variability in benthic/pelagic fluxes presented in this study in a representative temperate coastal environment, demonstrates not only the various process’ inter-connectivity, but also their relative importance to each other. Future investigations or modeling efforts of similar systems will benefit greatly from the relationships and baseline rules established in this study.

Major role of particle fragmentation in regulating biological sequestration of CO2 by the oceans

A critical driver of the ocean carbon cycle is the downward flux of sinking organic particles, which acts to lower the atmospheric carbon dioxide concentration. This downward flux is reduced by more than 70% in the mesopelagic zone (100 to 1000 meters of depth), but this loss cannot be fully accounted for by current measurements. For decades, it has been hypothesized that the missing loss could be explained by the fragmentation of large aggregates into small particles, although data to test this hypothesis have been lacking. In this work, using robotic observations, we quantified total mesopelagic fragmentation during 34 high-flux events across multiple ocean regions and found that fragmentation accounted for 49 ± 22% of the observed flux loss. Therefore, fragmentation may be the primary process controlling the sequestration of sinking organic carbon.

Characterization of total ecosystem-scale biogenic VOC exchange at a Mediterranean oak–hornbeam forest

Recently, the number and amount of biogenically emitted volatile organic compounds (VOCs) has been discussed in great detail. Depending on the ecosystem, the published number varies between a dozen and several hundred compounds. We present ecosystem exchange fluxes from a mixed oak–hornbeam forest in the Po Valley, Italy. The fluxes were measured by a proton transfer reaction-time-of-flight (PTR-ToF) mass spectrometer and calculated using the eddy covariance (EC) method. Detectable fluxes were observed for up to 29 compounds, dominated by isoprene, which comprised over 60 % of the total upward flux (on a molar basis). The daily average of the total VOC upward flux was 10.4 nmol m−2 s−1. Methanol had the highest concentration and accounted for the largest downward flux. Methanol seemed to be deposited to dew, as the downward flux happened in the early morning, right after the calculated surface temperature came closest to the calculated dew point temperature.We estimated that up to 30 % of the upward flux of methyl vinyl ketone (MVK) and methacrolein (MACR) originated from atmospheric oxidation of isoprene. A comparison between two methods for the flux detection (manual and automated) was made. Their respective advantages and disadvantages were discussed and the differences in their results shown. Both provide comparable results.

Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions

A high-resolution, two-dimensional numerical model is used to study the moisture redistribution following homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions and three levels of complexity for the representation of cloud processes: complete microphysics and cloud radiative effects, likewise but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation. Cloud evolution and moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated). Only a fraction (~25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated and hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level. The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations.