Variations In Liquid Water Content Research Articles

Is a more physical representation of aerosol chemistry needed for fog forecasting?

AbstractWith the changing climate, the study of fog formation is essential due to the impact of the complexity of natural and anthropogenic aerosols. The evolution of the droplet size distribution in the presence of different aerosol species remains poorly understood. To make progress towards reducing the uncertainty of fog forecasts, the Eulerian–Lagrangian particle‐based small‐scale model for the diffusional growth of droplets is used to better understand the droplet activation and growth. The small‐scale model simulations are performed using observed data from the Winter Fog Experiment study over Indira Gandhi International Airport, New Delhi. The microphysical properties, such as droplet number concentrations (Nd) and liquid water content (LWC), important for fog simulation, are evaluated to gain more insights. The small‐scale simulations have shown the droplet microphysical properties at different evolutionary stages. The Nd and effective radius change with variations in LWC for different aerosol chemistries (i.e., organics, mix, and inorganic). The calculated visibility at small scale is also shown with the variation of Nd and LWC. This study compared visibility from an existing parametrization with parcel–direct numerical simulation calculation. The hygroscopicity , which is highly related to the activation of aerosols to condensation nuclei, is taken into account to demonstrate the contribution of aerosol chemistry to fog droplet formation. The results highlight that hygroscopicity is essential in the numerical model for fog and visibility prediction as the microphysical properties of fog are regulated by aerosol species.

Dynamic Fluid Ingress Detection in Geomaterials Using K-Band Frequency Modulated Continuous Wave Radar

Frequency modulated continuous wave (FMCW) radar in the K-band has been shown to be an effective detector of geomaterial physical properties being highly sensitive to rock characteristics, particularly mineral composition and, for porous rock, variations in liquid water content. This research demonstrates that contrasts in FMCW return signals with time correlate with changes in geomaterial water content. FMCW signal returns were acquired for porous sandstone samples subjected to controlled water injection while also in a neutron beam, taking advantage of the well-known, and well-calibrated, attenuation of neutrons by hydrogen atoms for the water-containing porous sandstone samples. The sequential neutron tomographic images clearly show water moving up the sample with time while the FMCW observations show increases in radar reflection coefficient as a function of water position in the field of view. The observed FMCW detection of flood-front position is corroborated by the synchronous neutron tomographic images. We also observe repeatable variations in the radar reflection coefficient as a function of sample orientation during fluid injection, verifying that FMCW sensing offers real-time insight into the interactions between fluid movement and sample heterogeneity, via non-contact and non-invasive flood-front tracking. This research demonstrates that FMCW has potential to be a more accessible and easily deployable sensing modality than neutron tomography, enabling dynamic geomaterial testing to be conducted outwith the confines of the highly controlled laboratory environment required for neutron investigation.

On Which Microphysical Time Scales to Use in Studies of Entrainment‐Mixing Mechanisms in Clouds

AbstractThe commonly used time scales in entrainment‐mixing studies are examined to seek the most appropriate one, based on aircraft observations of cumulus clouds from the RACORO campaign and numerical simulations with the Explicit Mixing Parcel Model. The time scales include the following: τevap, the time for droplet complete evaporation; τphase, the time for saturation ratio deficit (S) to reach 1/e of its initial value; τsatu, the time for S to reach −0.5%; and τreact, the time for complete droplet evaporation or S to reach −0.5%. It is found that the proper time scale to use depends on the specific objectives of entrainment‐mixing studies. First, if the focus is on the variations of liquid water content (LWC) and S, then τreact for saturation, τsatu and τphase are almost equivalently appropriate, because they all represent the rate of dry air reaching saturation or of LWC decrease. Second, if one focuses on the variations of droplet size and number concentration, τreact for complete evaporation and τevap are proper because they characterize how fast droplets evaporate and whether number concentration decreases. Moreover, τreact for complete evaporation and τevap are always positively correlated with homogeneous mixing degree (ψ); thus, the two time scales, especially τevap, are recommended for developing parameterizations. However, ψ and the other time scales can be negatively, positively, or not correlated, depending on the dominant factors of the entrained air (i.e., relative humidity or aerosols). Third, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.

Simplified Normalization of C-Band Synthetic Aperture Radar Data for Terrestrial Applications in High Latitude Environments

Synthetic aperture radar (SAR) applications often require normalization to a common incidence angle. Angular signatures of radar backscatter depend on surface roughness and vegetation cover, and thus differ, from location to location. Comprehensive reference datasets are therefore required in heterogeneous landscapes. Multiple acquisitions from overlapping orbits with sufficient incidence angle range are processed in order to obtain parameters of the location specific normalization function. We propose a simpler method for C-band data, using single scenes only. It requires stable dielectric properties (no variations of liquid water content). This method is therefore applicable for frozen conditions. Winter C-band data have been shown of high value for a number of applications in high latitudes before. In this paper we explore the relationship of incidence angle and Sentinel-1 backscatter across the tundra to boreal transition zone. A linear relationship (coefficient of determination R 2 = 0.64) can be found between backscatter and incidence angle dependence (slope of normalization function) as determined by multiple acquisitions on a pixel by pixel basis for typical land cover classes in these regions. This allows a simplified normalization and thus reduced processing effort for applications over larger areas.

Water and Salt Migration with Phase Change in Saline Soil during Freezing and Thawing Processes.

Water and salt transfer coupled with phase change may cause serious damage to engineering structures in saline soil regions. In this study, the migration of water and salt in silty clay collected from the Qinghai-Tibet Plateau is explored experimentally and numerically during freezing and thawing processes. The results revealed that there are significant differences in the variations of liquid water content and solution concentration for different initial salt contents, due to salt crystallization and dissolution. The temperature-induced water migration is determined by the soil properties, which can be well explained by the thermodynamics of mass transfer. The amount of salt migrated upward during cooling is slightly larger than that transported downward in the warming period, implying that salt may be accumulated in the surface soil after a large number of circulations and finally result in soil salinization.

Sensitivity of Aircraft Performance to Icing Parameter Variations

A IRCRAFT icing is widely recognized as a significant hazard to aircraft operations. For this reason, aircraft and ice protection systems must be certified for flight into icing conditions. The aircraft certification and icing research communities rely on icing wind tunnels as an efficient way to produce ice accretions in a controlled environment. The aerodynamic performance of aircraft with these ice accretions contributes to the certification of aircraft. It is therefore critical to ensure that the ice accretions are simulated within a known aerodynamic uncertainty. The aerodynamic performance of an airfoil containing an ice accretion is highly dependent on the geometry of the ice accretion, which is in turn dependent on icing conditions such as temperature, liquid water content (LWC), and median volume diameter (MVD). Icing wind tunnels have the capability to vary LWC and MVD, however, the accuracy in LWC and MVD required to create an aerodynamically representative ice accretion is not known. This research addressed the problem of “how good is good enough” by determining the relationship and sensitivity of iced-airfoil performance to these icing cloud parameters. In addition, these data were placed in perspective by relating measurable or significant aircraft performance changes to the underlying changes in airfoil aerodynamic performance. Recent NASA studies [1,2] in the Icing Research Tunnel (IRT) measured the effect of icing parameter variations on ice-accretion geometry. These studies showed that small variations in LWC and MVD corresponded to distinct changes in ice-accretion geometry. In addition, the effect of ice-accretion geometry on aerodynamic performance has been recently investigated [3–6]. Papadakis et al. [3,4] used spoilers to simulate horn ice and showed that Clmax degradation was related to the horn height (k=c). Kim and Bragg [5], and Broeren et al. [6] showed that k=c and surface location (s=c) had the biggest impact on airfoil performance degradation. This study used ice tracings from theNASA studies [1,2] as a basis to examine the sensitivity of aerodynamic performance to icing parameter variations. Eleven ice-accretion tracings were selected from the 39measured byMiller et al. [2] to reasonably span the range of LWC andMVD tested. The selected ice tracings were modeled as two-dimensional smooth simulated ice shapes for wind-tunnel testing. The experiments for this research were performed in the Illinois subsonic, low-turbulence, open-return wind tunnel. The airfoil model was an aluminum NACA 0012 airfoil with an 18 in. chord, 33.6 in. span, and a removable leading edge to facilitate installation of the ice simulations. Testing was performed at a Reynolds number of 1.8 million, and a Mach number of 0.18. The results of the aerodynamic testing were related to the corresponding icing parameters in the form of two sensitivities: airfoil performance to icing parameter variations, and derived aircraft performance to icing parameter variations. More details can be found in Campbell et al. [7,8].

Cloud physics and cloud water sampler comparison during FEBUKO

Optical methods for counting and sizing cloud droplets and a wide range of cloud water sampling methods were used to characterize the atmospheric liquid phase during the FEBUKO cloud experiments. Results near cloud base as well as more than 300 m inside the hill cap clouds are presented, reflecting their inhomogeneous nature. The cloud droplet number varies from 50 to 1000 cm � 3 and drop sizes between 1 and 20mm diameter are most frequent. Variations in the liquid water content (LWC) and in the total ion content (TIC) are much smaller when the measurement position is deeper in the cloud. Near cloud base variability in updraft strength and, near cloud top, entrainment processes (droplet evaporation by mixing with drier air, aerosol and gas scavenging) disturb the adiabatic conditions and produce large variations in LWC and chemical composition. Six different active cloud water collectors and impactors were running side by side; they differ in the principle of sampling, in the throughput of cloudy air per unit time and in the calculated 50% cutoff diameter, which influence also their sampling efficiency. Two of them are designed to collect cloud water in two droplet size fractions. Three cloud events were selected by the FEBUKO team for detailed cloud physical and chemical analyses because they serve best the modelling demands concerning connected flow between the upwind, summit and downwind sites for process studies. Frequency distributions of the LWC and, also of the cloud base height are given as statistical parameters for both FEBUKO experiments. r 2005 Elsevier Ltd. All rights reserved.

Coherent Scattering of Microwaves by Particles: Evidence from Clouds and Smoke

Many radar measurements of the atmosphere can be explained in terms of two scattering mechanisms: incoherent scattering from particles, and coherent scattering from variations in the refractive index of the air, commonly called clear-air or Bragg scattering. Spatial variations in the liquid water content of clouds may also give a coherent contribution to the radar return, but it is commonly believed that this coherent scattering from the droplets is insignificant because variations in humidity have a much larger influence on the refractive index than equal variations in liquid water content. It is argued that the fluctuations in water vapor mixing ratio in clouds can be much smaller than those in liquid water mixing ratio. In this article an expression for the strength of the coherent scattering from particles will be derived for fluctuations caused by turbulent mixing with clean (i.e., particle-free) air, where it will be assumed that the particles follow the flow, that is, their inertia is neglected. It will be shown that the coherent contribution adds to the incoherent contribution, the latter always being present. The coherent particle scattering can be stronger than the incoherent scattering, especially at longer wavelengths and high particle concentrations. Recently published dual-frequency measurements of developing cumulus clouds and smoke show a correlation for which no explanation has been found in terms of incoherent particle scattering and coherent air scattering. Scatterplots of the reflectivity factors at both frequencies show a clustering of points in between the values that correspond to pure clear-air and pure incoherent scattering. Those differences in the radar reflectivity factors could be due to a mixture of Bragg scattering and incoherent particle scattering, but then no correlation is expected, because the origin of the scattering mechanism that dominates at each wavelength is different. However, coherent scattering from the particles can cause the radar reflectivities of dual-wavelength radar measurements to become correlated with each other. It may explain the slopes and the differences seen in the scatterplots of the radar reflectivities of cloud and smoke measurements, with reasonable values of the parameters involved. However, the correlation between the radar reflectivities is very tight near the cloud top and seems to be present in adiabatic cores as well. This is an indication that, apart from mixing with environmental air, the inertia of the droplets could also be important for the creation of small-scale fluctuations in droplet concentration.