scholarly journals A Robust C-Band Hydrometeor Identification Algorithm and Application to a Long-Term Polarimetric Radar Dataset

2013 ◽  
Vol 52 (9) ◽  
pp. 2162-2186 ◽  
Author(s):  
Brenda Dolan ◽  
Steven A. Rutledge ◽  
S. Lim ◽  
V. Chandrasekar ◽  
M. Thurai
Keyword(s):  
Electric Fields ◽  
Drop Diameter ◽  
Identification Algorithm ◽  
Liquid Drops ◽  
Warm Rain ◽  
Wet Snow ◽  
Vertically Aligned ◽  
Differential Reflectivity ◽  
Ice Water

AbstractA new 10-category, polarimetric-based hydrometeor identification algorithm (HID) for C band is developed from theoretical scattering simulations including wet snow, hail, and big drops/melting hail. The HID is applied to data from seven wet seasons in Darwin, Australia, using the polarimetric C-band (C-POL) radar, to investigate microphysical differences between monsoon and break periods. Scattering simulations reveal significant Mie effects with large hail (diameter > 1.5 cm), with reduced reflectivity and enhanced differential reflectivity Zdr and specific differential phase Kdp relative to those associated with S band. Wet snow is found to be associated with greatly depreciated correlation coefficient ρhv and moderate values of Zdr. It is noted that large oblate liquid drops can produce the same electromagnetic signatures at C band as melting hail falling quasi stably, resulting in some ambiguity in the HID retrievals. Application of the new HID to seven seasons of C-POL data reveals that hail and big drops/melting hail occur much more frequently during break periods than during monsoon periods. Break periods have a high frequency of vertically aligned ice above 12 km, suggesting the presence of strong electric fields. Reflectivity and mean drop diameter D0 statistics demonstrate that convective areas in both monsoon and break periods may have robust coalescence or melting precipitation ice processes, leading to enhanced reflectivity and broader distributions of D0. Conversely, for stratiform regions in both regimes, mean reflectivity decreases below the melting level, indicative of evaporative processes. Break periods also have larger ice water path fractions, indicating substantial mixed-phase precipitation generation as compared with monsoonal periods. In monsoon periods, a larger percentage of precipitation is produced through warm-rain processes.

Modeling and Interpretation of S-Band Ice Crystal Depolarization Signatures from Data Obtained by Simultaneously Transmitting Horizontally and Vertically Polarized Fields

2014 ◽  
Vol 53 (6) ◽  
pp. 1659-1677 ◽  
Author(s):  
J. C. Hubbert ◽  
S. M. Ellis ◽  
W.-Y. Chang ◽  
S. Rutledge ◽  
M. Dixon
Keyword(s):  
Electric Fields ◽  
Ice Crystals ◽  
Polarization Data ◽  
Convective Storms ◽  
Vertically Aligned ◽  
Linear Depolarization Ratio ◽  
Reflectivity Data ◽  
Convective Cells ◽  
Differential Reflectivity ◽  
Ice Phase

AbstractData collected by the National Center for Atmospheric Research S-band polarimetric radar (S-Pol) during the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX) in Taiwan are analyzed and used to infer storm microphysics in the ice phase of convective storms. Both simultaneous horizontal (H) and vertical (V) (SHV) transmit polarization data and fast-alternating H and V (FHV) transmit polarization data are used in the analysis. The SHV Zdr (differential reflectivity) data show radial stripes of biased data in the ice phase that are likely caused by aligned and canted ice crystals. Similar radial streaks in the linear depolarization ratio (LDR) are presented that are also biased by the same mechanism. Dual-Doppler synthesis and sounding data characterize the storm environment and support the inferences concerning the ice particle types. Small convective cells were observed to have both large positive and large negative Kdp (specific differential phase) values. Negative Kdp regions suggest that ice crystals are vertically aligned by electric fields. Since high |Kdp| values of 0.8° km−1 in both negative and positive Kdp regions in the ice phase are accompanied by Zdr values close to 0 dB, it is inferred that there are two types of ice crystals present: 1) smaller aligned ice crystals that cause the Kdp signatures and 2) larger aggregates or graupel that cause the Zdr signatures. The inferences are supported with simulated ice particle scattering calculations. A radar scattering model is used to explain the anomalous radial streaks in SHV and LDR.

Numerical study of electric field effects on the deformation of two-dimensional liquid drops in simple shear flow at arbitrary Reynolds number

2009 ◽  
Vol 626 ◽  
pp. 367-393 ◽  
Author(s):  
STEFAN MÄHLMANN ◽  
DEMETRIOS T. PAPAGEORGIOU
Keyword(s):  
Steady State ◽  
Shear Flow ◽  
Electric Field ◽  
Electric Fields ◽  
Simple Shear ◽  
Simple Shear Flow ◽  
Physical Parameters ◽  
Shear Stresses ◽  
Two Dimensional ◽  
Liquid Drops

The effect of an electric field on a periodic array of two-dimensional liquid drops suspended in simple shear flow is studied numerically. The shear is produced by moving the parallel walls of the channel containing the fluids at equal speeds but in opposite directions and an electric field is generated by imposing a constant voltage difference across the channel walls. The level set method is adapted to electrohydrodynamics problems that include a background flow in order to compute the effects of permittivity and conductivity differences between the two phases on the dynamics and drop configurations. The electric field introduces additional interfacial stresses at the drop interface and we perform extensive computations to assess the combined effects of electric fields, surface tension and inertia. Our computations for perfect dielectric systems indicate that the electric field increases the drop deformation to generate elongated drops at steady state, and at the same time alters the drop orientation by increasing alignment with the vertical, which is the direction of the underlying electric field. These phenomena are observed for a range of values of Reynolds and capillary numbers. Computations using the leaky dielectric model also indicate that for certain combinations of electric properties the drop can undergo enhanced alignment with the vertical or the horizontal, as compared to perfect dielectric systems. For cases of enhanced elongation and alignment with the vertical, the flow positions the droplets closer to the channel walls where they cause larger wall shear stresses. We also establish that a sufficiently strong electric field can be used to destabilize the flow in the sense that steady-state droplets that can exist in its absence for a set of physical parameters, become increasingly and indefinitely elongated until additional mechanisms can lead to rupture. It is suggested that electric fields can be used to enhance such phenomena.

Electrohydrodynamic rotation of drops at large electric Reynolds numbers

2016 ◽  
Vol 788 ◽  
Author(s):  
Ehud Yariv ◽  
Itzchak Frankel
Keyword(s):  
Electric Fields ◽  
Numerical Solutions ◽  
Fluid Velocity ◽  
Reynolds Numbers ◽  
Mirror Image ◽  
Liquid Drops ◽  
Weakly Conducting ◽  
Strong Electric Fields ◽  
Quincke Rotation ◽  
Asymmetric Solution

When subject to sufficiently strong electric fields, particles and drops suspended in a weakly conducting liquid exhibit spontaneous rotary motion. This so-called Quincke rotation is a fascinating example of nonlinear symmetry-breaking phenomena. To illuminate the rotation of liquid drops we here analyse the asymptotic limit of large electric Reynolds numbers, $\mathit{Re}\gg 1$, within the framework of a two-dimensional Taylor–Melcher electrohydrodynamic model. A non-trivial dominant balance in this singular limit results in both the fluid velocity and surface-charge density scaling as $\mathit{Re}^{-1/2}$. The flow is governed by a self-contained nonlinear boundary-value problem that does not admit a continuous fore–aft symmetric solution, thus necessitating drop rotation. Furthermore, thermodynamic arguments reveal that a fore–aft asymmetric solution exists only when charge relaxation within the suspending liquid is faster than that in the drop. The flow problem possesses both mirror-image (with respect to the direction of the external field) and flow-reversal symmetries; it is transformed into a universal one, independent of the ratios of electric conductivities and dielectric permittivities in the respective drop phase and suspending liquid phase. The rescaled angular velocity is found to depend weakly upon the viscosity ratio. The corresponding numerical solutions of the exact equations indeed collapse at large $\mathit{Re}$ upon the asymptotically calculated universal solution.

scholarly journals Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions

2012 ◽  
Vol 12 (14) ◽  
pp. 6609-6628 ◽  
Author(s):  
M. W. Gallagher ◽  
P. J. Connolly ◽  
I. Crawford ◽  
A. Heymsfield ◽  
K. N. Bower ◽  
...  
Keyword(s):  
Electric Fields ◽  
Numerical Models ◽  
Average Particle Size ◽  
Average Particle ◽  
Ice Crystal ◽  
Crystal Aggregation ◽  
Ice Water Content ◽  
Differential Sedimentation ◽  
High Electric Fields ◽  
Ice Water

Abstract. Aircraft measurements of the microphysics of a tropical convective anvil (at temperatures ~−60 °C) forming above the Hector storm, over the Tiwi Islands, Northern Australia, have been conducted with a view to determining ice crystal aggregation efficiencies from in situ measurements. The observed microphysics have been compared to an explicit bin-microphysical model of the anvil region, which includes crystal growth by vapour diffusion and aggregation and the process of differential sedimentation. It has been found in flights made using straight and level runs perpendicular to the storm that the number of ice crystals initially decreased with distance from the storm as aggregation took place resulting in larger crystals, followed by their loss from the cloud layer due to sedimentation. The net result was that the mass (i.e. Ice Water Content) in the anvil Ci cloud decreased, but also that the average particle size (weighted by number) remained relatively constant along the length of the anvil outflow. Comparisons with the explicit microphysics model showed that the changes in the shapes of the ice crystal spectra as a function of distance from the storm could be explained by the model if the aggregation efficiency was set to values of Eagg~0.5 and higher. This result is supported by recent literature on aggregation efficiencies for complex ice particles and suggests that either the mechanism of particle interlocking is important to the aggregation process, or that other effects are occuring, such as enhancement of ice-aggregation by high electric fields that arise as a consequence of charge separation within the storm. It is noteworthy that this value of the ice crystal aggregation efficiency is much larger than values used in cloud resolving models at these temperatures, which typically use E~0.0016. These results are important to understanding how cold clouds evolve in time and for the treatment of the evolution of tropical Ci in numerical models.

scholarly journals The Impacts of Atmospheric and Surface Parameters on Long-Term Variations in the Planetary Albedo

2018 ◽  
Vol 31 (21) ◽  
pp. 8705-8718 ◽  
Author(s):  
Bida Jian ◽  
Jiming Li ◽  
Guoyin Wang ◽  
Yongli He ◽  
Ying Han ◽  
...  
Keyword(s):  
Liquid Water ◽  
Surface Albedo ◽  
Cloud Fraction ◽  
Liquid Water Path ◽  
Water Path ◽  
Ice Water Path ◽  
Planetary Albedo ◽  
Ice Water ◽  
Long Term Variations

Planetary albedo (PA; shortwave broadband albedo) and its long-term variations, which are controlled in a complex way by various atmospheric and surface properties, play a key role in controlling the global and regional energy budget. This study investigates the contributions of different atmospheric and surface properties to the long-term variations of PA based on 13 years (2003–15) of albedo, cloud, and ice coverage datasets from the Clouds and the Earth’s Radiant Energy System (CERES) Single Scanner Footprint edition 4A product, vegetation product from Moderate Resolution Imaging Spectroradiometer (MODIS), and surface albedo product from the Cloud, Albedo, and Radiation dataset, version 2 (CLARA-A2). According to the temporal correlation analysis, statistical results indicate that variations in PA are closely related to the variations of cloud properties (e.g., cloud fraction, ice water path, and liquid water path) and surface parameters (e.g., ice/snow percent coverage and normalized difference vegetation index), but their temporal relationships vary among the different regions. Generally, the stepwise multiple linear regression models can capture the observed PA anomalies for most regions. Based on the contribution calculation, cloud fraction dominates the variability of PA in the mid- and low latitudes while ice/snow percent coverage (or surface albedo) dominates the variability in the mid- and high latitudes. Changes in cloud liquid water path and ice water path are the secondary dominant factor over most regions, whereas change in vegetation cover is the least important factor over land. These results verify the effects of atmospheric and surface factors on planetary albedo changes and thus may be of benefit for improving the parameterization of the PA and determining the climate feedbacks.

scholarly journals Isotopic fractionation during grain coarsening of wet snow

1993 ◽  
Vol 18 ◽  
pp. 129-134 ◽  
Author(s):  
Masayoshi Nakawo ◽  
Shigeru Chiba ◽  
Hiroshi Satake ◽  
Shigeru Kinouchi
Keyword(s):  
Experimental Data ◽  
Simple Model ◽  
Rapid Change ◽  
Isotopic Fractionation ◽  
Water Interface ◽  
Grain Coarsening ◽  
Isotope Concentration ◽  
Large Particles ◽  
Wet Snow ◽  
Ice Water

Isotopic composition of solid and liquid portions of wet snow was investigated experimentally. The compositions changed with time, δ values of ice becoming heavier than those for water. A simple model was proposed to explain their temporal variation. It predicted, however, a more rapid change of δ values than the trend obtained in the experiments. This suggests the presence of a “diffusion layer” adjacent to growing snow particles, where isotope concentration has dropped at the ice-water interface because of the fractionation during grain coarsening. The slope in δD–δ18O diagram estimated by the model is compatible with the experimental data. It is considered, therefore, that the freezing fraction, the part of the liquid which refreezes to relatively large particles during grain coarsening, could be estimated by measuring the isotope concentration.

scholarly journals Ground penetrating radar detection of subsnow slush on ice-covered lakes in interior Alaska

2012 ◽  
Vol 6 (6) ◽  
pp. 1435-1443 ◽  
Author(s):  
A. Gusmeroli ◽  
G. Grosse
Keyword(s):  
Ground Penetrating Radar ◽  
Geophysical Methods ◽  
Arctic Region ◽  
Water Harvesting ◽  
Lake Ice ◽  
Wet Snow ◽  
Dry Conditions ◽  
Ground Penetrating ◽  
Ice Water

Abstract. Lakes are abundant throughout the pan-Arctic region. For many of these lakes ice cover lasts for up to two thirds of the year. The frozen cover allows human access to these lakes, which are therefore used for many subsistence and recreational activities, including water harvesting, fishing, and skiing. Safe traveling condition onto lakes may be compromised, however, when, after significant snowfall, the weight of the snow acts on the ice and causes liquid water to spill through weak spots and overflow at the snow-ice interface. Since visual detection of subsnow slush is almost impossible our understanding on overflow processes is still very limited and geophysical methods that allow water and slush detection are desirable. In this study we demonstrate that a commercially available, lightweight 1 GHz, ground penetrating radar system can detect and map extent and intensity of overflow. The strength of radar reflections from wet snow-ice interfaces are at least twice as much in strength than returns from dry snow-ice interface. The presence of overflow also affects the quality of radar returns from the base of the lake ice. During dry conditions we were able to profile ice thickness of up to 1 m, conversely, we did not retrieve any ice-water returns in areas affected by overflow.

A Comparative Study of Rainfall Retrievals Based on Specific Differential Phase Shifts at X- and S-Band Radar Frequencies

2006 ◽  
Vol 23 (7) ◽  
pp. 952-963 ◽  
Author(s):  
Sergey Y. Matrosov ◽  
Robert Cifelli ◽  
Patrick C. Kennedy ◽  
Steven W. Nesbitt ◽  
Steven A. Rutledge ◽  
...  
Keyword(s):  
Comparative Study ◽  
Radar Data ◽  
Drop Diameter ◽  
Horizontal Polarization ◽  
Differential Phase Shift ◽  
Phase Measurements ◽  
Differential Phase ◽  
X Band ◽  
Raindrop Size ◽  
Differential Reflectivity

Abstract A comparative study of the use of X- and S-band polarimetric radars for rainfall parameter retrievals is presented. The main advantage of X-band polarimetric measurements is the availability of reliable specific differential phase shift estimates, KDP, for lighter rainfalls when phase measurements at the S band are too noisy to produce usable KDP. Theoretical modeling with experimental raindrop size distributions indicates that due to some non-Rayleigh resonant effects, KDP values at a 3.2-cm wavelength (X band) are on average a factor of 3.7 greater than at 11 cm (S band), which is a somewhat larger difference than simple frequency scaling predicts. The non-Rayleigh effects also cause X-band horizontal polarization reflectivity, Zeh, and differential reflectivity, ZDR, to be larger than those at the S band. The differences between X- and S-band reflectivities can exceed measurement uncertainties for Zeh starting approximately at Zeh > 40 dBZ, and for ZDR when the mass-weighted drop diameter, Dm, exceeds about 2 mm. Simultaneous X- and S-band radar measurements of rainfall showed that consistent KDP estimates exceeding about 0.1° km−1 began to be possible at reflectivities greater than ∼26–30 dBZ while at the S band such estimates can generally be made if Zeh > ∼35–39 dBZ. Experimental radar data taken in light-to-moderate stratiform rainfalls with rain rates R in an interval from 2.5 to 15 mm h−1 showed availability of the KDP-based estimates of R for most of the data points at the X band while at the S band such estimates were available only for R greater than about 8–10 mm h−1. After correcting X-band differential reflectivity measurements for differential attenuation, ZDR measurements at both radar frequency bands were in good agreement with each other for Dm < 2 mm, which approximately corresponds to ZDR ≈ 1.6 dB. The ZDR-based retrievals of characteristic raindrop sizes also agreed well with in situ disdrometer measurements.

scholarly journals Ionization Gas Sensor Using Suspended Carbon Nanotube Beams

Sensors ◽  
2020 ◽  
Vol 20 (6) ◽  
pp. 1660
Author(s):  
Shivaram Arunachalam ◽  
Ricardo Izquierdo ◽  
Frederic Nabki
Keyword(s):  
Electric Fields ◽  
Sacrificial Layer ◽  
Gaseous Mixtures ◽  
Long Term Stability ◽  
Comparable Performance ◽  
Ionization Sensor ◽  
Gas Molecules ◽  
High Temperature Vapor ◽  
Temperature Surface

An ionization sensor based on suspended carbon nanotubes (CNTs) was presented. A suspended CNT beam was fabricated by a low-temperature surface micromachining process using SU8 photoresist as the sacrificial layer. Application of a bias to the CNT beam generated very high non-linear electric fields near the tips of individual CNTs sufficient to ionize target gas molecules and initiate a breakdown current. The sensing mechanism of the CNT ionization sensor was discussed. The sensor response was tested in air, nitrogen, argon, and helium ambients. Each gas demonstrated a unique breakdown signature. Further, the sensor was tested with gaseous mixtures. The sensor exhibited good long-term stability and had comparable performance to other reported CNT-based ionization sensors in literature, which use high-temperature vapor deposition methods to grow CNTs. The sensor notably allowed for lower ionization voltages due to its reduced ionization gap size.

scholarly journals Analysis and Modeling of Melt-Water Refreezing in Dry Snow

1990 ◽  
Vol 36 (123) ◽  
pp. 238-246 ◽  
Author(s):  
W. Tad Pfeffer ◽  
Tissa H. Illangasekare ◽  
Mark F. Meier
Keyword(s):  
Thermal Equilibrium ◽  
Numerical Models ◽  
Front Propagation ◽  
Wetting Front ◽  
Capillary Suction ◽  
Mass Fluxes ◽  
Melt Water ◽  
Wet Snow ◽  
Ice Water ◽  
Existing Data

AbstractA dynamic zone of thermal disequilibrium is described which separates wet snow at 0 ° C from dry, sub-freezing snow. The dynamic zone tends to be eliminated by thermal equilibrium through freezing, but is sustained and propagated into the sub-freezing snow by water flow from the wet snow. The width of the dynamic zone is controlled by the rate of water inflow, and by the rate of freezing of water on to sub-freezing ice grains, which is in turn controlled by the ice/water geometry. Two ice/water geometries are investigated: isolated ice spheres and capillary tubes of ice into which water is pulled by capillary suction. The rate of freezing of water is calculated for the two models for various initial dimensions and temperatures. Equilibrium times are short (typically about 0.5 s), but depend on the assumed geometry, which is poorly constrained by existing data. Equilibration times and freeze-on mass fluxes are calculated for a variety of general conditions. These results can be used in numerical models of wetting-front propagation into cold snow.

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