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Robin J. Hogan, Lin Tian, Philip R. A. Brown, Christopher D. Westbrook, Andrew J. Heymsfield, and Jon D. Eastment

Abstract

The assumed relationship between ice particle mass and size is profoundly important in radar retrievals of ice clouds, but, for millimeter-wave radars, shape and preferred orientation are important as well. In this paper the authors first examine the consequences of the fact that the widely used “Brown and Francis” mass–size relationship has often been applied to maximum particle dimension observed by aircraft D max rather than to the mean of the particle dimensions in two orthogonal directions D mean, which was originally used by Brown and Francis. Analysis of particle images reveals that D max ≃ 1.25D mean, and therefore, for clouds for which this mass–size relationship holds, the consequences are overestimates of ice water content by around 53% and of Rayleigh-scattering radar reflectivity factor by 3.7 dB. Simultaneous radar and aircraft measurements demonstrate that much better agreement in reflectivity factor is provided by using this mass–size relationship with D mean. The authors then examine the importance of particle shape and fall orientation for millimeter-wave radars. Simultaneous radar measurements and aircraft calculations of differential reflectivity and dual-wavelength ratio are presented to demonstrate that ice particles may usually be treated as horizontally aligned oblate spheroids with an axial ratio of 0.6, consistent with them being aggregates. An accurate formula is presented for the backscatter cross section apparent to a vertically pointing millimeter-wave radar on the basis of a modified version of Rayleigh–Gans theory. It is then shown that the consequence of treating ice particles as Mie-scattering spheres is to substantially underestimate millimeter-wave reflectivity factor when millimeter-sized particles are present, which can lead to retrieved ice water content being overestimated by a factor of 4.

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K. P. Gallo, A. L. McNab, T. R. Karl, J. F. Brown, J. J. Hood, and J. D. Tarpley

Abstract

A vegetation index and a radiative surface temperature were derived from satellite data acquired at approximately 1330 LST for each of 37 cities and for their respective nearby rural regions from 28 June through 8 August 1991. Urban–rural differences for the vegetation index and the surface temperatures were computed and then compared to observed urban–rural differences in minimum air temperatures. The purpose of these comparisons was to evaluate the use of satellite data to assess the influence of the urban environment on observed minimum air temperatures (the urban heat island effect). The temporal consistency of the data, from daily data to weekly, biweekly, and monthly intervals, was also evaluated. The satellite-derived normalized difference (ND) vegetation-index data, sampled over urban and rural regions composed of a variety of land surface environments, were linearly related to the difference in observed urban and rural minimum temperatures. The relationship between the ND index and observed differences in minimum temperature was improved when analyses were restricted by elevation differences between the sample locations and when biweekly or monthly intervals were utilized. The difference in the ND index between urban and rural regions appears to be an indicator of the difference in surface properties (evaporation and heat storage capacity) between the two environments that are responsible for differences in urban and rural minimum temperatures. The urban and rural differences in the ND index explain a greater amount of the variation observed in minimum temperature differences than past analyses that utilized urban population data. The use of satellite data may contribute to a globally consistent method for analysis of urban heat island bias.

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K.A. Browning, J.C. Frankhauser, J.-P. Chalon, P.J. Eccles, R.G. Strauch, F.H. Merrem, D.J. Musil, E.L. May, and W.R. Sand

Abstract

A model of an evolving hailstorm is synthesized from data presented in four related papers in this issue. The storm model, which is applicable to a class of ordinary multicell hailstorms and similar to earlier models derived by workers in South Dakota and Alberta, is discussed in terms of the growth of hail and its implications for hail suppression. Hail is grown in time–evolving updrafts that begin as discrete new clouds on the flank of the storm. Low concentrations of embryos develop rapidly within each of these clouds. The embryos subsequently grow into small hailstones while suspended near or above, the −20°C level as each new cloud grows and becomes the main updraft. Recycling is not a feature of this model as it is in supercell models. To improve the chance of silver iodide seeding being effective in suppressing the growth of hall in multicell storms, it is proposed that the seeding should be carried out not in the main updraft as is often the practice, but, rather, in the regions of weaker updraft associated with the early stages of developing clouds an the flank of the storm.

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John L. Beven II, Lixion A. Avila, Eric S. Blake, Daniel P. Brown, James L. Franklin, Richard D. Knabb, Richard J. Pasch, Jamie R. Rhome, and Stacy R. Stewart

Abstract

The 2005 Atlantic hurricane season was the most active of record. Twenty-eight storms occurred, including 27 tropical storms and one subtropical storm. Fifteen of the storms became hurricanes, and seven of these became major hurricanes. Additionally, there were two tropical depressions and one subtropical depression. Numerous records for single-season activity were set, including most storms, most hurricanes, and highest accumulated cyclone energy index. Five hurricanes and two tropical storms made landfall in the United States, including four major hurricanes. Eight other cyclones made landfall elsewhere in the basin, and five systems that did not make landfall nonetheless impacted land areas. The 2005 storms directly caused nearly 1700 deaths. This includes approximately 1500 in the United States from Hurricane Katrina—the deadliest U.S. hurricane since 1928. The storms also caused well over $100 billion in damages in the United States alone, making 2005 the costliest hurricane season of record.

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P. R. Field, R. P. Lawson, P. R. A. Brown, G. Lloyd, C. Westbrook, D. Moisseev, A. Miltenberger, A. Nenes, A. Blyth, T. Choularton, P. Connolly, J. Buehl, J. Crosier, Z. Cui, C. Dearden, P. DeMott, A. Flossmann, A. Heymsfield, Y. Huang, H. Kalesse, Z. A. Kanji, A. Korolev, A. Kirchgaessner, S. Lasher-Trapp, T. Leisner, G. McFarquhar, V. Phillips, J. Stith, and S. Sullivan

Abstract

Measured ice crystal concentrations in natural clouds at modest supercooling (temperature ~>−10°C) are often orders of magnitude greater than the number concentration of primary ice nucleating particles. Therefore, it has long been proposed that a secondary ice production process must exist that is able to rapidly enhance the number concentration of the ice population following initial primary ice nucleation events. Secondary ice production is important for the prediction of ice crystal concentration and the subsequent evolution of some types of clouds, but the physical basis of the process is not understood and the production rates are not well constrained. In November 2015 an international workshop was held to discuss the current state of the science and future work to constrain and improve our understanding of secondary ice production processes. Examples and recommendations for in situ observations, remote sensing, laboratory investigations, and modeling approaches are presented.

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P. A. Durkee, R. E. Chartier, A. Brown, E. J. Trehubenko, S. D. Rogerson, C. Skupniewicz, K. E. Nielsen, S. Platnick, and M. D. King

Abstract

The physical and radiative properties of a composite ship track are described from the analysis of 131 ship–ship track correlation pairs collected during the Monterey Area Ship Track experiment. The significant variability of ship tracks around their average characteristics is also described. The nominal environmental conditions for the ship track set are also described. The composite ship track is 296 ± 233 km long, 7.3 ± 6 h old, and averages 9 ± 5 km wide. The ship is, on the average, 16 ± 8 km from of the head of the ship track along the relative wind vector and corresponds to a time of 25 ± 15 min. The set of ship tracks examined in this study formed in marine boundary layers that were between 300 and 750 m deep, and no tracks formed in boundary layers above 800 m. The tracks form in regions of high relative humidity, small air–sea temperature differences, and moderate winds (average of 7.7 ± 3.1 m s−1). The ambient cloud reflectance in advanced very high resolution radiometer channel 3 (3.7-μm wavelength) is 11 ± 4%, while the composite ship track value is 14 ± 5%. The relative track brightness is 7 ± 26% and 37 ± 34% for 0.63- and 3.7-μm wavelengths, respectively.

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Steven J. Abel, Ian A. Boutle, Kirk Waite, Stuart Fox, Philip R. A. Brown, Richard Cotton, Gary Lloyd, Tom W. Choularton, and Keith N. Bower

Abstract

Aircraft observations in a cold-air outbreak to the north of the United Kingdom are used to examine the boundary layer and cloud properties in an overcast mixed-phase stratocumulus cloud layer and across the transition to more broken open-cellular convection. The stratocumulus cloud is primarily composed of liquid drops with small concentrations of ice particles and there is a switch to more glaciated conditions in the shallow cumulus clouds downwind. The rapid change in cloud morphology is accompanied by enhanced precipitation with secondary ice processes becoming active and greater thermodynamic gradients in the subcloud layer. The measurements also show a removal of boundary layer accumulation mode aerosols via precipitation processes across the transition that are similar to those observed in the subtropics in pockets of open cells. Simulations using a convection-permitting (1.5-km grid spacing) regional version of the Met Office Unified Model were able to reproduce many of the salient features of the cloud field although the liquid water path in the stratiform region was too low. Sensitivity studies showed that ice was too active at removing supercooled liquid water from the cloud layer and that improvements could be made by limiting the overlap between the liquid water and ice phases. Precipitation appears to be the key mechanism responsible for initiating the transition from closed- to open-cellular convection by decoupling the boundary layer and depleting liquid water from the stratiform cloud.

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J. D. Price, S. Vosper, A. Brown, A. Ross, P. Clark, F. Davies, V. Horlacher, B. Claxton, J. R. McGregor, J. S. Hoare, B. Jemmett-Smith, and P. Sheridan

During stable nighttime periods, large variations in temperature and visibility often occur over short distances in regions of only moderate topography. These are of great practical significance and yet pose major forecasting challenges because of a lack of detailed understanding of the processes involved and because crucial topographic variations are often not resolved in current forecast models. This paper describes a field and numerical modeling campaign, Cold-Air Pooling Experiment (COLPEX), which addresses many of the issues.

The observational campaign was run for 15 months in Shropshire, United Kingdom, in a region of small hills and valleys with typical ridge–valley heights of 75–150 m and valley widths of 1–3 km. The instrumentation consisted of three sites with instrumented flux towers, a Doppler lidar, and a network of 30 simpler meteorological stations. Further instrumentation was deployed during intensive observation periods including radiosonde launches from two sites, a cloud droplet probe, aerosol monitoring equipment, and an instrumented car. Some initial results from the observations are presented illustrating the range of conditions encountered.

The modeling phase of COLPEX includes use of the Met Office Unified Model at 100-m resolution, and some brief results for a simulation of an intensive observation period are presented showing the model capturing a cold-pool event. As well as aiding interpretation of the observations, results from this study are expected to inform the design of future generations of operational forecasting systems

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L. Magnusson, J.-R. Bidlot, M. Bonavita, A. R. Brown, P. A. Browne, G. De Chiara, M. Dahoui, S. T. K. Lang, T. McNally, K. S. Mogensen, F. Pappenberger, F. Prates, F. Rabier, D. S. Richardson, F. Vitart, and S. Malardel

Abstract

Tropical cyclones are some of the most devastating natural hazards and the “three beasts”—Harvey, Irma, and Maria—during the Atlantic hurricane season 2017 are recent examples. The European Centre for Medium-Range Weather Forecasts (ECMWF) is working on fulfilling its 2016–25 strategy in which early warnings for extreme events will be made possible by a high-resolution Earth system ensemble forecasting system. Several verification reports acknowledge deterministic and probabilistic tropical cyclone tracks from ECMWF as world leading. However, producing reliable intensity forecasts is still a difficult task for the ECMWF global forecasting model, especially regarding maximum wind speed. This article will put the ECMWF strategy into a tropical cyclone perspective and highlight some key research activities, using Harvey, Irma, and Maria as examples. We describe the observation usage around tropical cyclones in data assimilation and give examples of their impact. From a model perspective, we show the impact of running at 5-km resolution and also the impact of applying ocean coupling. Finally, we discuss the future challenges to tackle the errors in intensity forecasts for tropical cyclones.

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Ben C. Bernstein, Frank McDonough, Marcia K. Politovich, Barbara G. Brown, Thomas P. Ratvasky, Dean R. Miller, Cory A. Wolff, and Gary Cunning

Abstract

The “current icing potential” (CIP) algorithm combines satellite, radar, surface, lightning, and pilot-report observations with model output to create a detailed three-dimensional hourly diagnosis of the potential for the existence of icing and supercooled large droplets. It uses a physically based situational approach that is derived from basic and applied cloud physics, combined with forecaster and onboard flight experience from field programs. Both fuzzy logic and decision-tree logic are applied in this context. CIP determines the locations of clouds and precipitation and then estimates the potential for the presence of supercooled liquid water and supercooled large droplets within a given airspace. First developed in the winter of 1997/98, CIP became an operational National Weather Service and Federal Aviation Administration product in 2002, providing real-time diagnoses that allow users to make route-specific decisions to avoid potentially hazardous icing. The CIP algorithm, its individual components, and the logic behind them are described.

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