Microwave Melt Research Articles

The Determination of the Snow Optical Grain Diameter and Snowmelt Area on the Greenland Ice Sheet Using Spaceborne Optical Observations

The optical diameter of the surface snow grains impacts the amount of energy absorbed by the surface and therefore the onset and magnitude of surface melt. Snow grains respond to surface heating through grain metamorphism and growth. During melt, liquid water between the grains markedly increases the optical grain size, as wet snow grain clusters are optically equivalent to large grains. We present daily surface snow grain optical diameters (dopt) retrieved from the Greenland ice sheet at 1 km resolution for 2017–2019 using observations from Ocean and Land Colour Instrument (OLCI) onboard Sentinel-3A. The retrieved dopt are evaluated against 3 years of in situ measurements in Northeast Greenland. We show that higher dopt are indicative of surface melt as calculated from meteorological measurements at four PROMICE automatic weather stations. We deduce a threshold value of 0.64 mm in dopt allowing categorization of the days either as melting or nonmelting. We apply this simple melt detection technique in Northeast Greenland and compare the derived melting areas with the conventional passive microwave MEaSUREs melt flag for June 2019. The two flags show generally consistent evolution of the melt extent although we highlight areas where large grain diameters are strong indicators of melt but are missed by the MEaSUREs melt flag. While spatial resolution of the optical grain diameter-based melt flag is higher than passive microwave, it is hampered by clouds. Our retrieval remains suitable to study melt at a local to regional scales and could be in the future combined with passive microwave melt flags for increased coverage.

Comparing Greenland Ice Sheet Melt Variability From Different Satellite Passive Microwave Remote Sensing Products Over a Common 5-year Record

Satellite microwave brightness temperature (Tb) observations over the Greenland Ice Sheet permit determination of melted/frozen snow conditions at spatial and temporal scales that are uniquely suited for climate model validation and metrics of ice sheet change. Strong microwave sensitivity to the presence of liquid water in the snowpack is clear. Yet, a host of unique microwave-derived melt products covering the ice sheet are available, each based on different methodology, and with unknown inter-product agreement. Here, we compared five different published microwave melt products over a common 5-year (2003–2007) record to establish compatibility between products and agreement with in situ observations from a network of on-ice weather stations (AWS) spanning the ice sheet. A sixth product, leveraging both Tb seasonal trends and diurnal variability, was also introduced and included in the comparison. We found variable agreement between products and observations, with melt estimates based on microwave emissions modeling and the newly presented Adaptive Threshold (ADT) algorithm showing the best performance for AWS sites with more than 1-day average annual melt period (e.g., 68.9% of ADT melt days consistent with AWS observations; 31.1% of ADT frozen days contrasting with AWS observed melt). Spatial patterns of melting also varied between products. The different products showed substantial spread in melt occurrence even for products with the best AWS agreement. Product differences were generally larger under higher melt conditions; whereby, the fraction of the ice sheet experiencing ≥25 days of melting each year ranged from 4 to 25% for different products. While long-term satellite records have consistently shown increasing decadal trends in melt extent, our results imply that the melt frequency at any given location, particularly in the ice sheet interior where melting is less prevalent, is still subject to significant uncertainty.

Evaluation of passive microwave melt detection methods on Antarctic Peninsula ice shelves using time series of Sentinel-1 SAR

Passive microwave datasets have been used to quantify the extent and duration of surface melt in Greenland and Antarctica from 1978 on with daily and near-daily intervals. These results have important implications for climate analysis and may help evaluate ice shelf stability. However, the accuracy of passive microwave methods used to detect melt is difficult to quantify, especially on the Antarctic Peninsula. Here four different melt detection methods are employed, including a new formulation of a statistical analysis of brightness temperature time series using a K-means clustering algorithm. Strikingly, two of the most widely used passive microwave melt detection methods are found to vary by 48% mean days of melt per year across six different locations on the Larsen C, Wilkins, and George VI Ice Shelves. In the absence of ground truth observations, time series of Sentinel-1 SAR observations from 2016 on provide a comparison dataset. In topographically flat regions where surface melt is spatially uniform, the passive microwave melt detection method based on a K-means analysis and the cross-polarization gradient ratio method demonstrate the highest agreement and correlation with active radar melt detection methods. One issue which has plagued passive microwave analysis is its coarse spatial resolution. High resolution SAR images are able to demonstrate and quantify the spatial variability of melt within individual passive microwave pixels. Melt is shown to be suitably uniform in space for passive microwave applications at the study sites on Antarctic Peninsula ice shelves, but not so in other regions of the Antarctic Peninsula. Spatial heterogeneity of surface melt on the sub-pixel scale is often related to varying surface topography.

Passive Microwave Melt Onset Retrieval Based on a Variable Threshold: Assessment in the Canadian Arctic Archipelago

The Canadian Arctic Archipelago (CAA) presents unique challenges to the determination of melt onset (MO) using remote sensing data. High spatial resolution data is required to discern melt onset among the islands and narrow waterways of the region. Current passive microwave retrievals use daily averaged 19 GHz and 37 GHz data from the multi-channel microwave radiometer (SMMR) and/or the special sensor microwave/imager (SSM/I). The development of a new passive microwave melt onset method capable of using higher resolution data is desirable. The new passive microwave melt onset method described here, named the Dynamic Threshold Variability Method (DTVM), uses higher resolution data from the 37 GHz vertically-polarized channel from the advanced microwave scanning radiometers (AMSR-E and AMSR-2). The DTVM MO detection methodology differs from previously presented passive microwave Arctic MO methods in that it does not use a fixed threshold of a brightness temperature parameter. Instead, the DTVM determines MO dates based on the distribution of dates corresponding to the exceedance of a range of brightness temperature variability thresholds. The method also uses swath data instead of daily averaged brightness temperatures, which is found to lead to improved melt detection. Two current passive microwave MO methods are compared and evaluated for applicability in the CAA alongside the DTVM. The DTVM provides MO dates at a higher spatial resolution than earlier methods in addition to higher correlation with MO dates from surface air temperature (SAT) reanalyses. It is found that, for some years, MO dates in the CAA exhibit a latitudinal dependence, while in other years the MO dates in the CAA are relatively uniform across the domain.

Evaluation of Calcium Fluoroaluminosilicate Based Glass Ionomer Luting Cements Processed Both by Conventional and Microwave Assisted Methods

Calcium fluoroaluminosilicate glasses (CAS) are used in the formulation of glass ionomer cements for dental applications. However, the cements obtained from CAS glasses were found to be radiolucent. In this study, the influence of substituting Zn, Sr and Mg for Ca of CAS glasses was investigated with respect to the structure and setting characteristics, mechanical properties, and radiopacity of cements designed for luting applications. Three glass compositions based on substitution of Zn, Sr and Mg for Ca at 1:1 molar ratio was synthesized. They were coded as the G 021 (Ca: Zn), G 022 (Ca: Sr), G 023 (Ca: Mg). G 021 and G 022 glasses were processed by conventional melt quench route, whereas G 023 was processed by microwave melt–quench route. Each glass was then mixed with Fuji Type I GIC liquid in order to evaluate the properties of novel cements at different powder/liquid ratios. X-ray diffraction and Fourier Transform-Infrared spectroscopy analysis confirmed the structure of the processed glasses. The average particle size of the processed glass powders was within specification limits for luting applications (<15 μm). The substitution of Zn, Sr and Mg for Ca at 1:1 molar ratio increased the reactivity of the respective glasses. This has been reflected in their respective setting characteristics and mechanical properties. The optimal combination of setting time, strength and radiopacity for the cements examined here was shown by G 022 cements. The microwave melting can be utilized for processing ionomer glasses as it did not alter the structure and properties of G 023 cement.

Evaluation of a novel inversion model for surface melt magnitude over the Greenland ice sheet during the 2002 ablation season

This work evaluates the performance of an empirical inversion model for surface liquid water fraction (LWF) or melt magnitude, using data from the Moderate Resolution Imaging Spectroradiometer (MODIS). Estimates of melt magnitude were evaluated through quantitative comparison to surface melt occurrence and duration derived from the cross-polarization gradient ratio (XPGR) and diurnal amplitude variation (DAV) passive microwave melt indices over the 2002 melt season. Longer melt durations derived from XPGR tend to correspond to higher LWF amounts at lower elevations, where temperatures are warmer than at higher elevations. DAV indicates melt occurrence at higher elevations, with lower durations in melt corresponding to small amounts of LWF. The LWF inversion model demonstrates that it is sensitive to both high melt duration events within the ablation zone, yet capable of capturing greater spatial variability in melt conditions broadly over the entire sheet.

Recent changes in pan‐Arctic melt onset from satellite passive microwave measurements

A new satellite passive microwave (PMW) melt onset retrieval algorithm based on temporal variations in the differences of the brightness temperature between 19 and 37 GHz is shown to be as effective as radar (e.g., QuikScat) measurements. The PMW technique shows improved melt estimates that are more closely linked to observed snow‐off dates than previous studies. An integrated pan‐Arctic (north of 50°N) melt onset date (MOD) dataset is produced by combining estimates on land and sea ice for the entire satellite PMW record. During the 1979–2011 period, significant trends of 2~3 days (decade)−1 to earlier MOD are mainly concentrated over the Eurasian land sector of the Arctic, consistent with changes in spring snow cover extent observed with visible satellite data. The variability and change in melt onset are largely driven by spring surface air temperature, with insignificant influence from low‐frequency modes of atmospheric circulation.

A comparison between SSM/I passive microwave melt onset dates and satellite-derived albedo melt onset dates in the Arctic

Arctic sea ice undergoes a very strong annual cycle. This study sets out to look at the transition when the Arctic sea ice starts to melt using satellite-obtained passive microwave brightness temperatures and satellite-derived albedo data for 13 points within the Arctic, including both first-year and multiyear ice locations, for 1995–2000. Special sensor microwave imager (SSM/I) brightness temperature differences are used to determine melt onset dates once surface temperatures approach freezing. Independently, satellite-derived albedo data are obtained and a melt onset date is derived. Generally, the two methods produce the same date for melt onset with optimum conditions. However, in most cases there are clouds present, which for the albedo data restrict observations and generate melt dates that are several days later than the passive microwave melt onset which is not affected by cloud cover. Melt onset dates, determined from the passive microwave brightness temperatures, are compared to those from the albedo observation to determine differences between the two methods. For first-year ice (FYI) locations, the average differences in melt onset dates for the study locations between the passive microwave and albedo-derived methods are +/−3 days. The average difference for multiyear ice (MYI) locations melt onset dates is around 8 days, slightly longer than the (FYI) locations, however, this is due to more cloudy conditions. The results indicate that the passive microwave-derived melt onset dates and albedo-derived dates are very close and either method could be used to determine melt. The advantage of using microwave data would be the independence of having to have cloud free conditions.

Microwave Growth of ZnO Bulk Crystals

AbstractA novel approach for fabricating ZnO crystals using self-encapsulated microwave melt growth technique is developed, which permits idealized temperature profile for the nucleation of the ZnO crystals and their growth in a contamination-free environment. This paper reports that both un-doped ZnO and p-type doped ZnO (by P or Li:N doping) single crystals were successfully fabricated by this technique. The as-grown ZnO crystals were of light brown to clear in color depending on processing conditions. Their transparency and colorless form were restored after post-growth annealing in oxygen.

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

The unidirectional solidification method was used to prepare well textured YBa 2Cu 3O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a– b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R>2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.

A New Inductive Furnace Based on Microwave Irradiation for Growing Long YBa2Cu3O7-δ Single-Domain Bars

Growing long YBa2Cu3O7–δ single domain bars requires new designs of inductive furnaces based on microwave irradiation. In this paper the authors present a new susceptor design using inductive heating to obtain a positive radial thermal gradient from the heating source. With this system textured YBa2Cu3O7–δ (Y123) ceramics are produced and compared with the conventional microwave melt textured growth process. No regulation system is needed and thermal radiation from the sample placed inside the tube is totally eliminated, which may normally cause parasitic nucleations at the periphery of the Y123 bars.

Effect of and doping on the microstructure and superconducting properties of melt textured zone

The microwave melt textured growth process (MMTG) is used to investigate the influence of and doping and their combination with two different Sn/Ce initial ratios on the (Y123) growth mechanism, microstructure and superconducting properties. It was shown that the combination of `' allows us to increase the Y123 crystal growth velocity compared with used alone. Furthermore, this combination reduces the particle coarsening observed when doping is added alone. Secondary phase SEM observations show that square submicrometric precipitates are homogeneously distributed in the matrix when `' are used. Thus, the identified phase appears to be a solid solution derived from the phase . In this article, the initial Sn/Ce ratio was found to be a crucial parameter that affects the matrix purity. In terms of superconducting properties, `' doped samples exhibit the highest magnetic critical current density in zero field , even under 1 Tesla , depending on the Sn/Ce ratio. The superconducting properties are discussed in relation with the microstructure and the EDS analysis results.

A microwave melt texture growth process of YBa 2Cu 3O 7- δ

Microwave heating in a single mode cavity can produce large thermal gradients ( G>150°C cm −1), so this process was used to grow large YBa 2Cu 3O 7- δ (Y123) textured domain bars by using the classical MTG process operating in a TE102 cavity. Thus, this paper is devoted to presenting this new technology involving a material-conducting microwave interaction and describe a device allowing microwave heating of Y123. In this paper, a model has been developed concerning microwave–matter interactions for conducting materials and a study of the relevant parameters which control the process has been performed. Thus, a temperature regulation algorithm has been presented. In terms of material processing, we applied our results to a Y 2BaCuO 5 rich Y123 composition doped with cerium dioxide, CeO 2, and the growth of a large quasi-single domain at a 2 mm h −1 growth rate in a microwave cavity was obtained. Owing to the particular heating mechanism, there is no thermal inertia leading to a faster process than the classical. Besides, the good superconducting properties obtained demonstrates that this original process can be competitive with the classical MTG process operating in a vertical furnace.

The crystal growth anisotropy of YBa 2Cu 3O 7− δ fabricated by the MTG method in a microwave cavity

The aim of this work is to optimise a microwave melt texture growth process in a vertical travelling solvent zone technique in a single mode microwave cavity to grow large single-domain Y123 samples. Indeed, the large observed thermal gradient (>180°C/cm) in combination with the possibility of translating the sample across the microwave cavity allows this technique to achieve conditions similar to those used in the classical MTG using a vertical furnace. However, the particular heating mechanism, in the microwave case, allows for a novel range of thermal conditions to be accessed, particularly a larger thermal gradient, to grow large Y123 single-domain in novel thermal conditions. In a first effort to optimise this microwave technique, a composition corresponding to a Y211-rich Y123 mixture, doped with cerium dioxide, was sought in order to improve the Y123 domain size. Secondly, the influence of the melt temperature and the bar diameter on the size and orientation of the Y123 quasi-crystals was investigated. The influence of each parameter is discussed with respect to the classical Y123 growth mechanism, and an explanation of the angle, which is observed to be near to 45°, between the ab-plane and the pulling direction is proposed. The Bean critical current density without applied magnetic field, for a cleaved sample at 77 K reaches 50,000 A/cm 2 showing the high quality of the microwave melt processed material.

Microwave heating: A new process for texturing the high Tc superconductor YBa 2Cu 3O 7−δ

Microwave heating appears as a very promising method that can be applied to the melt texture growth (MTG) process for the texturing of the high Tc superconductor “YBaCuO”. In the new MMTG (Microwave Melt Teture Growth) process, the conventional furnace is replaced by a single mode microwave cavity and the “123” sample is placed orthogonally to the electrical field. A high termal gradient acting as a texturing strength is produced, that allows well textured domains to be developed. One of the advantages of this technique will be the texture formation of long samples by a translation of the bars in the microwave cavity.

The Timing of Initial Spring Melt in the Arctic from Nimbus-7 SMMR Data (Abstract)

The ablation of sea ice is an important feature in the global climate system. During the melt season in the Arctic, rapid changes occur in sea-ice surface conditions and areal extent of ice. These changes alter the albedo and vary the energy budgets. Understanding the spatial and temporal variations of melt is critical in the polar regions. This study investigates the spring onset of melt in the seasonal sea-ice zone of the Arctic Basin through the use of a melt signature derived by Anderson and others from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. The signature is recognized in the “gradient ratio” of the 18 and 37 GHz vertical brightness temperatures used to distinguish multi-year ice. A spuriously high fraction of multi-year ice appears rapidly during the initial melt of sea ice, when the snow-pack on the ice surface has started to melt. The brightness-temperature changes are a result of either enlarged snow crystals or incipient puddles forming at the snow/ice interface.The timing of these melt events varies geographically and with time. Within the Arctic Basin, the melt signatures are observed first in the Chukchi and Kara/Barents Seas. As the melt progresses, the location of the melt signature moves westward from the Chukchi Sea and eastward from the Kara/Barents Seas to the Laptev Sea region. The timing of the melt signal also varies with year. For example, the melt signature occurred first in the Chukchi Sea in 1979, while in 1980 the signature was first observed in the Kara Sea.There are also differences in the timing of melt for specific geographic locations between years. The melt signature varied almost 25 days in the Chukchi Sea region between 1979 and 1980. The other areas had changes in the 7–10 day range.The occurrence of these melt signatures can be used as an indicator of climate variability in the seasonal sea-ice zones of the Arctic. The timing of the microwave melt signature has also been examined in relation to melt observed on short-wave imagery. The melt events derived from the SMMR data are also related to the large-scale climate conditions.

The Timing of Initial Spring Melt in the Arctic from Nimbus-7 SMMR Data (Abstract)

The ablation of sea ice is an important feature in the global climate system. During the melt season in the Arctic, rapid changes occur in sea-ice surface conditions and areal extent of ice. These changes alter the albedo and vary the energy budgets. Understanding the spatial and temporal variations of melt is critical in the polar regions. This study investigates the spring onset of melt in the seasonal sea-ice zone of the Arctic Basin through the use of a melt signature derived by Anderson and others from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. The signature is recognized in the “gradient ratio” of the 18 and 37 GHz vertical brightness temperatures used to distinguish multi-year ice. A spuriously high fraction of multi-year ice appears rapidly during the initial melt of sea ice, when the snow-pack on the ice surface has started to melt. The brightness-temperature changes are a result of either enlarged snow crystals or incipient puddles forming at the snow/ice interface. The timing of these melt events varies geographically and with time. Within the Arctic Basin, the melt signatures are observed first in the Chukchi and Kara/Barents Seas. As the melt progresses, the location of the melt signature moves westward from the Chukchi Sea and eastward from the Kara/Barents Seas to the Laptev Sea region. The timing of the melt signal also varies with year. For example, the melt signature occurred first in the Chukchi Sea in 1979, while in 1980 the signature was first observed in the Kara Sea. There are also differences in the timing of melt for specific geographic locations between years. The melt signature varied almost 25 days in the Chukchi Sea region between 1979 and 1980. The other areas had changes in the 7–10 day range. The occurrence of these melt signatures can be used as an indicator of climate variability in the seasonal sea-ice zones of the Arctic. The timing of the microwave melt signature has also been examined in relation to melt observed on short-wave imagery. The melt events derived from the SMMR data are also related to the large-scale climate conditions.