Mengistu Wolde

Polarimetric Signatures from Ice Crystals Observed at 95 GHz in Winter Clouds. Part I: Dependence on Crystal Form

Abstract Based on observations made with an airborne 95-GHz polarimetric cloud radar and in situ microphysical probes, the dependence of ZDR and linear depolarization ratio (LDR) values on ice crystal type and radar beam orientation was examined. Distinct ranges of ZDR and LDR values at various radar beam orientations were identified for simple planar and columnar crystals and for melting particles. The results also show that, based on ZDR and LDR values for different beam orientations, dendritic crystals can be distinguished from simpler hexagonal and branched crystals. Polarimetric signatures are almost exclusively associated with unrimed or slightly rimed crystals, therefore the presence of such signatures can help to identify cloud regions where such crystals dominate. The data generally agrees with previously reported results, though some differences are also noted. The observed ZDR and LDR values for simple crystal types are in reasonable agreement with theoretical predictions.

Extratropical Transition of Hurricane Michael: An Aircraft Investigation

In order to better understand the behavior and impacts of tropical cyclones undergoing extratropical transition (ET), the Meteorological Service of Canada (MSC) conducted a test flight into Hurricane Michael. Between 16 and 19 October 2000 the transition of Hurricane Michael from a hurricane to an intense extratropical storm was investigated using a Canadian research aircraft instrumented for storm research. This paper presents the various data collected from the flight with a detailed description of the storm structure at the time when Michael was in the midst of ET. Hurricane Michael was moving rapidly to the northeast, approximately 300 km southeast of Nova Scotia, Canada, during the time of the aircraft mission. A period of rapid intensification had also occurred during this time as the system moved north of the warm Gulf Stream waters and merged with a baroclinic low pressure system moving offshore of Nova Scotia. Consequently, the hurricane was sampled near the period of its lowest surface pressure ...

Global Precipitation Measurement Cold Season Precipitation Experiment (GCPEx): For Measurement Sake Let it Snow

AbstractAs a component of Earth’s hydrologic cycle, and especially at higher latitudes, falling snow creates snowpack accumulation that in turn provides a large proportion of the freshwater resources required by many communities throughout the world. To assess the relationships between remotely sensed snow measurements with in situ measurements, a winter field project, termed the Global Precipitation Measurement (GPM) Cold Season Precipitation Experiment (GCPEx), was carried out in the winter of 2011/12 in Ontario, Canada. Its goal was to provide information on the precipitation microphysics and processes associated with cold season precipitation to support GPM snowfall retrieval algorithms that make use of a dual-frequency precipitation radar and a passive microwave imager on board the GPM core satellite and radiometers on constellation member satellites. Multiparameter methods are required to be able to relate changes in the microphysical character of the snow to measureable parameters from which precip...

Polarimetric Signatures from Ice Crystals Observed at 95 GHz in Winter Clouds. Part II: Frequencies of Occurrence

Abstract Data are presented, from a large collection of observations in wintertime clouds in Wyoming, which show that the fraction of cloud volumes from which significant radar polarimetric information can be obtained is small. For example, when averaged over all available samples, signals exceeding the chosen limits of 3 dB for ZDR and −18 dB for linear depolarization ratio were found in just a few percent of the observations for radar beam incidence angles of less than 45°. In general, the polarimetric signatures are interpreted as indicators of the prevalence of pristine and lightly rimed crystals, as opposed to more densely rimed crystals, graupel, or aggregates. However, specific cases are presented to illustrate exceptions to this interpretation. The polarimetric signatures provide information regarding ice crystal types from larger cloud volumes than can be observed with in situ probes, and thus may aid in understanding the evolution and possible origin of hydrometeors in the clouds. They may also ...

STORM STUDIES IN THE ARCTIC (STAR)

The Storm Studies in the Arctic (STAR) network (2007–2010) conducted a major meteorological field project from 10 October–30 November 2007 and in February 2008, focused on southern Baffin Island, Nunavut, Canada—a region that experiences intense autumn and winter storms. The STAR research program is concerned with the documentation, better understanding, and improved prediction of meteorological and related hazards in the Arctic, including their modification by local topography and land–sea ice–ocean transitions, and their effect on local communities. To optimize the applicability of STAR network science, we are also communicating with the user community (northern communities and government sectors). STAR has obtained a variety of surface-based and unique research aircraft field measurements, high-resolution modeling products, and remote sensing measurements (including Cloudsat) as part of its science strategy and has the first arctic Cloudsat validation dataset. In total, 14 research flights were flown b...

Measurements of Differential Reflectivity in Snowstorms and Warm Season Stratiform Systems

AbstractThe organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilut...

Differences between measured and reported volatile organic compound emissions from oil sands facilities in Alberta, Canada.

Significance Validation of volatile organic compound (VOC) emission reports, especially from large industrial facilities, is rarely attempted. Given uncertainties in emission reports, their evaluation and validation will build confidence in emission inventories. It is shown that a top-down approach can provide measurement-based emission rates for such emission validation. Comparisons with emission reports from Alberta oil sands surface mining facilities revealed significant differences in VOC emissions between top-down emissions rates and reports. Comparison with VOC species emission reports using currently accepted estimation methods indicates that emissions were underestimated in the reports for most species. This exercise shows that improvements in the accuracy and completeness of emissions estimates from complex facilities would enhance their application to assessing the impacts of such emissions. Large-scale oil production from oil sands deposits in Alberta, Canada has raised concerns about environmental impacts, such as the magnitude of air pollution emissions. This paper reports compound emission rates (E) for 69–89 nonbiogenic volatile organic compounds (VOCs) for each of four surface mining facilities, determined with a top-down approach using aircraft measurements in the summer of 2013. The aggregate emission rate (aE) of the nonbiogenic VOCs ranged from 50 ± 14 to 70 ± 22 t/d depending on the facility. In comparison, equivalent VOC emission rates reported to the Canadian National Pollutant Release Inventory (NPRI) using accepted estimation methods were lower than the aE values by factors of 2.0 ± 0.6, 3.1 ± 1.1, 4.5 ± 1.5, and 4.1 ± 1.6 for the four facilities, indicating underestimation in the reported VOC emissions. For 11 of the combined 93 VOC species reported by all four facilities, the reported emission rate and E were similar; but for the other 82 species, the reported emission rate was lower than E. The median ratio of E to that reported for all species by a facility ranged from 4.5 to 375 depending on the facility. Moreover, between 9 and 53 VOCs, for which there are existing reporting requirements to the NPRI, were not included in the facility emission reports. The comparisons between the emission reports and measurement-based emission rates indicate that improvements to VOC emission estimation methods would enhance the accuracy and completeness of emission estimates and their applicability to environmental impact assessments of oil sands developments.

Cloud structure and crystal growth in nimbostratus

Abstract Cloud structure and crystal growth in two nimbostratus were examined using in situ and airborne radar observations. In both cases, structure throughout the cloud depth was modulated by generating cells at about 8-km altitude. Large-scale horizontal homogeneity at altitudes below the generating cells was due to the rapid movement of the generating cells relative to the main cloud mass. In addition, significant horizontal variability was evident on the scale of few hundred meters, principally in the radar reflectivity data but also readily detectable in the particle data. The melting layer was clearly defined in the radar images. Thin dry layers just above the melting layer were also observed in both cases. In agreement with earlier studies, particle spectra in these clouds are adequately described by exponential relationships. There is a strong correlation between the slope (λ) and intercept (No) parameters of an exponential size distribution; this relationship is well characterized by a power law with constants varying from case to case. Radar reflectivity is negatively correlated with the slope parameter λ. It is concluded from the observations that the vapor supply made available by large-scale lifting was taken up by depositional growth of the ice crystals. Aggregation of the crystals led to the final shaping of the precipitation size spectra.

On the role of ice-nucleating aerosol in the formation of ice particles in tropical mesoscale convective systems

Over decades, the cloud physics community has debated the nature and role of aerosol particles in ice initiation. The present study shows that the measured concentration of ice crystals in tropical mesoscale convective systems exceeds the concentration of ice nucleating particles (INPs) by several orders of magnitude. The concentration of INPs was assessed from the measured aerosol particles concentration in the size range of 0.5 to 1 µm. The observations from this study suggest that primary ice crystals formed on INPs make only a minor contribution to the total concentration of ice crystals in tropical mesoscale convective systems. This is found by comparing the predicted INP number concentrations with in-situ ice particle number concentrations. The obtained measurements suggest that ice multiplication is the likely explanation for the observed high concentrations of ice crystals in this type of convective system.

Aircraft Observations of Orographic Cloud and Precipitation Features over Southern Baffin Island, Nunavut, Canada

Abstract This study evaluates cloud and precipitation features over the orography of southern Baffin Island in the southeast Canadian Arctic during the Storm Studies in the Arctic (STAR) field project in autumn 2007. Three case studies provide the basis for a comparative analysis of how cloud and precipitation features from upstream ocean regions are modified by the orography, in addition to the variability of these features over diverse synoptic and sea-ice conditions. Using data collected by a research aircraft with an onboard W-band Doppler radar and microphysical instrumentation, multiple factors were found to play roles in enhancing and/or reducing cloud and precipitation over the orography of the region. Gravity waves, terrain shape, atmospheric stability, and atmosphere–ocean exchanges were all associated with precipitation enhancement. In addition, several factors that reduce precipitation were identified, including sublimation, high sea-ice extent, and low-level blocking in the upstream environment. Accretion and aggregation were identified as important particle growth mechanisms over the orography. By increasing particle density and/or mass, the probability of ice particles precipitating to the surface increased. These results indicate that the complexity of these critical features over terrain in high-latitude regions poses considerable challenges for modelling.