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Mengistu Wolde and Gabor Vali

Abstract

Based on observations made with an airborne 95-GHz polarimetric cloud radar and in situ microphysical probes, the dependence of Z DR and linear depolarization ratio (LDR) values on ice crystal type and radar beam orientation was examined. Distinct ranges of Z DR 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 Z DR 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 Z DR and LDR values for simple crystal types are in reasonable agreement with theoretical predictions.

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Mengistu Wolde and Gabor Vali

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 Z DR 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 help to refine assumptions made in the modeling of radiative transfer through clouds.

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James Abraham, J. Walter Strapp, Christopher Fogarty, and Mengistu Wolde

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 and maximum surface winds. It is estimated that the aircraft passed approximately 10 km south of the estimated 42.7°N, 59.7°W position of the surface low pressure center at about 1645 UTC 19 October. Sixteen dropsondes were deployed in a single traverse from northwest to east of the storm center, and then back westbound south of the center. Winds were found to be highest on the southeast side of the hurricane where the storm movement adds to the hurricane rotational flow. A southwesterly jet with winds exceeding 70 m s−1 was observed between 500 and 2000 m approximately 85 km southeast of the center. This low-level jet was much deeper than the usual lowlevel maximum winds found in hurricanes. Michael was observed to have an elevated warm core similar to purely tropical systems, but low-altitude humidity appeared to be eroded by entrainment of dry midlatitude air surrounding the storm, which is typically observed during the ET process.

A cloud-profiling 35-GHz radar provided data on the distribution of precipitation across the system, and cloud microphysical probes measured cloud water contents, particle phases, and spectra. Although a wide variety of liquid, mixed phase, and deep glaciated clouds were observed, the glaciated cloud encountered on the northwest side of the center, associated with the most significant precipitation area, was relatively stratiform in nature, with a broad area of high ice water content reaching 1.5 g m−3, and very high concentrations of small ice particles.

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William P. Kustas, John H. Prueger, J. Ian MacPherson, Mengistu Wolde, and Fuqin Li

Abstract

Eddy covariance measurements of wind speed u and shear velocity u * from tower- and aircraft-based systems collected over rapidly developing corn- (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields were used in determining the local and regional (effective) surface roughness length zo and 〈zo〉, respectively. For corn, canopy height increased from ∼1 to 2 m and the leaf area index changed from ∼1 to 4 during the study period, while for soybean, canopy height increased from ∼0.1 to 0.5 m and the leaf area index increased from ∼0.5 to 2. A procedure for the aggregation of local roughness values from the different land cover types based on blending-height concepts yielded effective surface roughness values that were from ∼1/2 to 1/4 of the magnitude estimated with the aircraft data. This indicated additional kinematic stress caused by form drag from isolated obstacles (i.e., trees, houses, and farm buildings), and the interaction of adjacent corn- and soybean fields were probably important factors influencing the effective surface roughness length for this landscape. The comparison of u * measurements from the towers versus the aircraft indicated that u * from aircraft was 20%–30% higher, on average, and that u * over corn was 10%–30% higher than over soybean, depending on stability. These results provide further evidence for the likely sources of additional kinematic stress. Although there was an increase in zo and 〈zo〉 over time as the crops rapidly developed, particularly for corn, there was a more significant trend of increasing roughness length with decreasing wind speed at wind speed thresholds of around 5 m s−1 for the aircraft and 3 m s−1 for the tower measurements. Other studies have recently reported such a trend. The impact on computed sensible heat flux H using 〈zo〉 derived from the aggregation of zo from the different land cover types, using the blending-height scheme, and that estimated from the aircraft observations, was evaluated using a calibrated single-source/bulk resistance approach with surface–air temperature differences from the aircraft observations. An underestimate of 〈zo〉 by 50% and 75% resulted in a bias in the H estimates of approximately 10% and 15%, respectively. This is a relatively minor error when considering that the root-mean-square error (rmse) value between single-source estimates and the aircraft observations of H was 15 W m−2 using the aircraft-derived 〈zo〉, and only increased to approximately 20 and 25 W m−2 using the 1/2 and 1/4 〈zo〉 values, as estimated from the blending-height scheme. The magnitude of the excess resistance relative to the aerodynamic resistance to heat transfer was a major contributing factor in minimizing the error in heat flux calculations resulting from these underestimations of 〈zo〉.

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Earle R. Williams, David J. Smalley, Michael F. Donovan, Robert G. Hallowell, Kenta T. Hood, Betty J. Bennett, Raquel Evaristo, Adam Stepanek, Teresa Bals-Elsholz, Jacob Cobb, Jaclyn Ritzman, Alexei Korolev, and Mengistu Wolde

Abstract

The 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 dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.

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John Hanesiak, Ronald Stewart, Peter Taylor, Kent Moore, David Barber, Gordon McBean, Walter Strapp, Mengistu Wolde, Ron Goodson, Edward Hudson, David Hudak, John Scott, George Liu, Justin Gilligan, Sumita Biswas, Danielle Desjardins, Robyn Dyck, Shannon Fargey, Robert Field, Gabrielle Gascon, Mark Gordon, Heather Greene, Carling Hay, William Henson, Klaus Hochheim, Alex Laplante, Rebekah Martin, Marna Albarran Melzer, and Shunli Zhang

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 between 5 and 30 November 2007, with eight coinciding with Cloudsat passes. The aircraft was outfitted with many instruments that measure cloud microphysical parameters and three unique Doppler-polarized airborne radars operating in Ka, X and W bands. The project area, instrumentation platforms, real-time forecasts, storm cases, and results thus far are discussed in this article. A number of synoptic and mesoscale features were sampled—such as fronts, upslope/terrain-enhanced precipitation, convective precipitation, and boundary layer clouds/precipitation—as well as targeted Cloudsat missions. One significant and unique event included a research flight into an intense high-latitude storm leftover from Hurricane Noel—an intense tropical and extratropical disturbance that caused many fatalities in the tropics and extensive damage on the eastern North American seaboard. These synoptic and mesoscale features and high-latitude storms will be studied in detail over the next several years. It is anticipated that scientific progress in better understanding the nature of these arctic storms and their hazards will lead to improved conceptual models and improved prediction of such events.

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Gail Skofronick-Jackson, David Hudak, Walter Petersen, Stephen W. Nesbitt, V. Chandrasekar, Stephen Durden, Kirstin J. Gleicher, Gwo-Jong Huang, Paul Joe, Pavlos Kollias, Kimberly A. Reed, Mathew R. Schwaller, Ronald Stewart, Simone Tanelli, Ali Tokay, James R. Wang, and Mengistu Wolde

Abstract

As 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 precipitation detection and estimation can be based. The data collection strategy was coordinated, stacked, high-altitude, and in situ cloud aircraft missions with three research aircraft sampling within a broader surface network of five ground sites that in turn were taking in situ and volumetric observations. During the field campaign 25 events were identified and classified according to their varied precipitation type, synoptic context, and precipitation amount. Herein, the GCPEx field campaign is described and three illustrative cases detailed.

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Indirect and Semi-direct Aerosol Campaign

The Impact of Arctic Aerosols on Clouds

Greg M. McFarquhar, Steven Ghan, Johannes Verlinde, Alexei Korolev, J. Walter Strapp, Beat Schmid, Jason M. Tomlinson, Mengistu Wolde, Sarah D. Brooks, Dan Cziczo, Manvendra K. Dubey, Jiwen Fan, Connor Flynn, Ismail Gultepe, John Hubbe, Mary K. Gilles, Alexander Laskin, Paul Lawson, W. Richard Leaitch, Peter Liu, Xiaohong Liu, Dan Lubin, Claudio Mazzoleni, Ann-Marie Macdonald, Ryan C. Moffet, Hugh Morrison, Mikhail Ovchinnikov, Matthew D. Shupe, David D. Turner, Shaocheng Xie, Alla Zelenyuk, Kenny Bae, Matt Freer, and Andrew Glen

Abstract

A comprehensive dataset of microphysical and radiative properties of aerosols and clouds in the boundary layer in the vicinity of Barrow, Alaska, was collected in April 2008 during the Indirect and Semi-Direct Aerosol Campaign (ISDAC). ISDAC's primary aim was to examine the effects of aerosols, including those generated by Asian wildfires, on clouds that contain both liquid and ice. ISDAC utilized the Atmospheric Radiation Measurement Pro- gram's permanent observational facilities at Barrow and specially deployed instruments measuring aerosol, ice fog, precipitation, and radiation. The National Research Council of Canada Convair-580 flew 27 sorties and collected data using an unprecedented 41 stateof- the-art cloud and aerosol instruments for more than 100 h on 12 different days. Aerosol compositions, including fresh and processed sea salt, biomassburning particles, organics, and sulfates mixed with organics, varied between flights. Observations in a dense arctic haze on 19 April and above, within, and below the single-layer stratocumulus on 8 and 26 April are enabling a process-oriented understanding of how aerosols affect arctic clouds. Inhomogeneities in reflectivity, a close coupling of upward and downward Doppler motion, and a nearly constant ice profile in the single-layer stratocumulus suggests that vertical mixing is responsible for its longevity observed during ISDAC. Data acquired in cirrus on flights between Barrow and Fairbanks, Alaska, are improving the understanding of the performance of cloud probes in ice. Ultimately, ISDAC data will improve the representation of cloud and aerosol processes in models covering a variety of spatial and temporal scales, and determine the extent to which surface measurements can provide retrievals of aerosols, clouds, precipitation, and radiative heating.

A supplement to this article is available online:

DOI: 10.1175/2010BAMS2935.2

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