Dynamical Processes In Atmosphere Research Articles

Technical note: Characterization of a single-beam gradient force aerosol optical tweezer for droplet trapping, phase transition monitoring, and morphology studies

Abstract. Single particle analysis is essential for a better understanding of the particle transformation process and to predict its environmental impact. In this study, we developed an aerosol optical tweezer (AOT) Raman spectroscopy system to investigate the phase state and morphology of suspended aerosol droplets in real time. The system comprises four modules: optical trapping, reaction, illumination and imaging, and detection. The optical trapping module utilizes a 532 nm laser and a 100 × oil immersion objective to stably trap aerosol droplets within 30 s. The reaction module allows us to adjust relative humidity (RH) and introduce reaction gases into the droplet levitation chamber, facilitating experiments to study liquid–liquid phase transitions. The illumination and imaging module employs a high-speed camera to monitor the trapped droplets, while the detector module records Raman scattering light. We trapped sodium chloride (NaCl) and 3-methyl glutaric acid (3-MGA) mixed droplets to examine RH-dependent morphology changes. Liquid–liquid phase separation (LLPS) occurred when RH was decreased. Additionally, we introduced ozone and limonene/pinene to generate secondary organic aerosol (SOA) particles in situ, which collided with the trapped droplet and dissolved in it. To determine the trapped droplet's characteristics, we utilized an open-source program based on Mie theory to retrieve diameter and refractive index from the observed whispering gallery modes (WGMs) in Raman spectra. It is found that mixed droplets formed core–shell morphology when RH was decreased, and the RH dependence of the droplets' phase transitions generated by different SOA precursors varied. Our AOT system serves as an essential experimental platform for in situ assessment of morphology and phase state during dynamic atmospheric processes.

Aerosol particle properties at a remote tropical rainforest in Borneo

This paper aims to investigate aerosol particle properties and their seasonal variabilities at a remote tropical forest site. It also attempts to study the relationship between aerosol particles and dynamic atmospheric processes that modulate aerosol variability. Mass concentrations for particles smaller than 10 μm aerodynamic diameter (PM10) and optical aerosol particle properties were measured at Danum Valley tropical rainforest in Borneo Island. The particle mass concentrations were measured using a Tapered Element Oscillating Microbalance (TEOM). Measurements of aerosol particle optical properties, i.e., the particle light absorption and scattering coefficients, were conducted using a Multi-Angle Absorption Photometer (MAAP) and nephelometer, respectively. The single scattering albedo (SSA) and scattering Angstrom exponent (SAE) were calculated from the aerosol scattering and absorption coefficients. The results showed that the daily average of PM10 mass concentrations was 12.3 μgm−3, while the particle light absorption coefficients at wavelength 637 nm and scattering coefficients at wavelength 525 nm were reported as 2.7 Mm−1 and 21.7 Mm−1, respectively. The daily average SSA and SAE recorded were 0.89 and 1.44, respectively. The SSA showed no seasonal variations while higher SAE was observed during the dry season due to the influence of biomass burning originating from the southern region of Borneo. The wavelet analysis showed strong intra-seasonal cycles (12–20 and 32–64 days) which suggest the influence of Quasi-BiWeekly (QBW) oscillation and Madden-Julian Oscillation (MJO) on the aerosol variability over the tropics. The analysis also showed seasonal and annual variability bands, generally associated with the dry and wet seasons over the region.

Квазидвухлетние осцилляции зонального ветра в экваториальной стратосфере и их влияние на межгодовые изменения мощности озоновой дыры в Антарктике

The Antarctic ozone hole is observed annually in spring due to the complex influence of photochemical and dynamical processes. The increased concentration of ozone-depleting substances in the atmosphere causes a long-term negative trend in total ozone (TO). Intense interannual fluctuations in TO against a background of the long-term trend associated with dynamic atmospheric processes do not allow assessing definitely the direction of the trend (growth/decline) in the recent years. Studying the dependence of interannual fluctuations in the ozone hole intensity on the equatorial quasi-biennial oscillation (QBO) allows identifying natural causes of variations and assessing the trend due to anthropogenic factors. The long-term QBO forecast allows predicting different phenomena that depend on the QBO.

Helicity and Turbulence in the Atmospheric Boundary Layer

Helicity is inherent in many circulating motions and structures in the atmospheric boundary layer (ABL), where it is continuously reproduced due to the combined action of the Earth’s rotation and friction, which also is closely related to the phenomena of the inverse cascade and large-scale restructuring of flows. The helicity factor should be correctly considered during the construction of atmosphere models, and, accordingly, the knowledge of the helicity distribution in the ABL and its relation to dynamic atmospheric processes is required. In this study, the helicity of the circulation structures of various spatial and temporal scales in the ABL is determined from an analysis of field measurement data. A qualitative and quantitative comparison with the observed values is carried out on the basis of results of numerical simulations using the quasi-two-dimensional model and a WRF-ARW mesoscale atmospheric nonhydrostatic model, in particular, WRF-LES. A good agreement with the observed spatial distributions of circulating motions is obtained. A link between the turbulent characteristics and helicity of the structures under study is shown. The helicity estimates for circulation structures of various scales in the ABL and in the free atmosphere are compared.

Warm bias of sea surface temperature in Eastern boundary current regions—a study of effects of horizontal resolution in CESM

Warm bias of modeled sea surface temperature (SST) in the eastern boundary upwelling systems (EBUS) is a ubiquitous feature in coupled climate models. This paper investigates the causes underlying this warm bias, with a focus on the effect of horizontal resolution in the atmospheric component of coupled models, by using Community Earth System Model (CESM) as an example. By breaking down the energy budget of the upper ocean, we conclude that surface net heat flux and Ekman upwelling process exert a considerable influence (over 80%) on upper ocean temperature of EBUS in CESM. Besides, the problem of underestimation of stratocumulus cloud is not present near the coast, and hence not responsible for this warm bias in CESM. On the contrary, downward shortwave radiation bias is overcompensated by longwave radiation and latent flux bias on the open ocean. Therefore, the insufficient ocean dynamic upwelling is the dominantly cause for SST warm bias. Finer horizontal resolution atmosphere component of CESM enables better representation of low-level coastal jet structure, with stronger and closer alongshore wind stress and curl leading to realistic representation of upwelling process and horizontal water mass transportation. Furthermore, low-level coastal jet is shown to be sensitive to coastal mountain topography, especially in South East Pacific region, through both thermodynamic and dynamic atmospheric processes and oceanic response. This article provides further proof of improving coupled climate models in reducing the SST biases in EBUS regions.

Helicity in dynamic atmospheric processes

An overview on the helicity of the velocity field and the role played by this concept in modern research in the field of geophysical fluid dynamics and dynamic meteorology is given. Different (both previously known in the literature and first presented) formulations of the equation of helicity balance in atmospheric motions (including those with allowance for effects of air compressibility and Earth’s rotation) are brought together. Equations and relationships are given which are valid in different approximations accepted in dynamic meteorology: Boussinesq approximation, quasi-static approximation, and quasi-geostrophic approximation. Emphasis is placed on the analysis of helicity budget in large-scale quasi-geostrophic systems of motion; a formula for the helicity flux across the upper boundary of the nonlinear Ekman boundary layer is given, and this flux is shown to be exactly compensated for by the helicity destruction inside the Ekman boundary layer.

Polarisation of very-low-mass stars and brown dwarfs

Ultra-cool dwarfs of the L spectral type (Teff=1400-2200K) are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering. We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and the grain-size effects on the polarisation degree in a sample of nine late M, L and early T dwarfs. We obtain linear polarimetric imaging measurements using FORS1 at the Very Large Telescope, in the Bessel I filter, and for a subset in the Bessel R and the Gunn z filters. We measure a polarisation degree of (0.31+/-0.06)% for LHS102BC. We fail to detect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree of each object of 0.09% to 0.76% (95% CL). For those targets we do not find evidence of large-scale cloud horizontal structure in our data. Together with previous surveys, our results set the fraction of ultra-cool dwarfs with detected linear polarisation to (30+10-6)% (1-sigma). For three brown dwarfs, our observations indicate polarisation degrees different (at the 3-sigma level) than previously reported, giving hints of possible variations. Our results fail to correlate with the current model predictions for ultra-cool dwarf polarisation for a flattening-induced polarisation, or with the variability studies for a polarisation induced by an hetereneous cloud cover. This stresses the intricacy of each of those tasks, but may as well proceed from complex and dynamic atmospheric processes.

Some remarks on light pressure in atmosphere

Attention is paid to the effect of light pressure, completely ignored in the present explanations on the energetics of dynamical processes in atmosphere. It seems even to be of great importance and play a great role, especially in the upper atmosphere. It seems there are three phenomena produced probably by this process: 1) Appearing of the earth atmosphere tide on the night side. 2) Presence of strong west winds on the great altitude. 3) Vertical oscillations of the upper atmosphere density.