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R. Damiani, J. Zehnder, B. Geerts, J. Demko, S. Haimov, J. Petti, G. S. Poulos, A. Razdan, J. Hu, M. Leuthold, and J. French

The finescale structure and dynamics of cumulus, evolving from shallow to deep convection, and the accompanying changes in the environment and boundary layer over mountainous terrain were the subjects of a field campaign in July–August 2006. Few measurements exist of the transport of boundary layer air into the deep troposphere by the orographic toroidal circulation and orographic convection. The campaign was conducted over the Santa Catalina Mountains in southern Arizona, a natural laboratory to study convection, given the spatially and temporally regular development of cumulus driven by elevated heating and convergent boundary layer flow. Cumuli and their environment were sampled via coordinated observations from the surface, radiosonde balloons, and aircraft, along with airborne radar data and stereophotogrammetry from two angles.

The collected dataset is expected to yield new insights in the boundary layer processes leading to orographic convection, in the cumulus-induced transport of boundary layer air into the troposphere, and in fundamental cumulus dynamics. This article summarizes the motivations, objectives, experimental strategies, preliminary findings, and the potential research paths stirred by the project.

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Peter G. Black, Eric A. D'Asaro, William M. Drennan, Jeffrey R. French, Pearn P. Niiler, Thomas B. Sanford, Eric J. Terrill, Edward J. Walsh, and Jun A. Zhang

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (C D) values derived from CBLAST momentum flux measurements show C D becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements.

The Dalton number estimates (C E) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a C E/C D ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

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Ronald B. Smith, Justin R. Minder, Alison D. Nugent, Trude Storelvmo, Daniel J. Kirshbaum, Robert Warren, Neil Lareau, Philippe Palany, Arlington James, and Jeffrey French

The Dominica Experiment (DOMEX) took place in the eastern Caribbean from 4 April to 10 May 2011 with 21 research flights of the Wyoming King Air and several other observing systems. The goal was an improved understanding of the physics of convective orographic precipitation in the tropics. Two types of convection were found. During a period of weak trade winds, diurnal thermal convection was seen over Dominica. This convection caused little precipitation but carried aloft air with island-derived aerosol and depleted CO2. During periods of strong trades, mechanically forced convection over the windward slopes brought heavy rain to the high terrain. This convection was “seeded” by trade-wind cumuli or neutrally buoyant cool wet patches of air. In this mechanically forced convection, air parcels did not touch the island surface to gain buoyancy so no island-derived tracers were lofted. With fewer aerosols, the mean cloud droplet diameter increased from 15 to 25 μm. Plunging airflow and a wake were found in the lee of Dominica. The DOMEX dataset will advance our understanding and test our theories of cumulus triggering and aerosol influence on precipitation.

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Sarah A. Tessendorf, Jeffrey R. French, Katja Friedrich, Bart Geerts, Robert M. Rauber, Roy M. Rasmussen, Lulin Xue, Kyoko Ikeda, Derek R. Blestrud, Melvin L. Kunkel, Shaun Parkinson, Jefferson R. Snider, Joshua Aikins, Spencer Faber, Adam Majewski, Coltin Grasmick, Philip T. Bergmaier, Andrew Janiszeski, Adam Springer, Courtney Weeks, David J. Serke, and Roelof Bruintjes


The Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) project aims to study the impacts of cloud seeding on winter orographic clouds. The field campaign took place in Idaho between 7 January and 17 March 2017 and employed a comprehensive suite of instrumentation, including ground-based radars and airborne sensors, to collect in situ and remotely sensed data in and around clouds containing supercooled liquid water before and after seeding with silver iodide aerosol particles. The seeding material was released primarily by an aircraft. It was hypothesized that the dispersal of the seeding material from aircraft would produce zigzag lines of silver iodide as it dispersed downwind. In several cases, unambiguous zigzag lines of reflectivity were detected by radar, and in situ measurements within these lines have been examined to determine the microphysical response of the cloud to seeding. The measurements from SNOWIE aim to address long-standing questions about the efficacy of cloud seeding, starting with documenting the physical chain of events following seeding. The data will also be used to evaluate and improve computer modeling parameterizations, including a new cloud-seeding parameterization designed to further evaluate and quantify the impacts of cloud seeding.

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David C. Leon, Jeffrey R. French, Sonia Lasher-Trapp, Alan M. Blyth, Steven J. Abel, Susan Ballard, Andrew Barrett, Lindsay J. Bennett, Keith Bower, Barbara Brooks, Phil Brown, Cristina Charlton-Perez, Thomas Choularton, Peter Clark, Chris Collier, Jonathan Crosier, Zhiqiang Cui, Seonaid Dey, David Dufton, Chloe Eagle, Michael J. Flynn, Martin Gallagher, Carol Halliwell, Kirsty Hanley, Lee Hawkness-Smith, Yahui Huang, Graeme Kelly, Malcolm Kitchen, Alexei Korolev, Humphrey Lean, Zixia Liu, John Marsham, Daniel Moser, John Nicol, Emily G. Norton, David Plummer, Jeremy Price, Hugo Ricketts, Nigel Roberts, Phil D. Rosenberg, David Simonin, Jonathan W. Taylor, Robert Warren, Paul I. Williams, and Gillian Young


The Convective Precipitation Experiment (COPE) was a joint U.K.–U.S. field campaign held during the summer of 2013 in the southwest peninsula of England, designed to study convective clouds that produce heavy rain leading to flash floods. The clouds form along convergence lines that develop regularly as a result of the topography. Major flash floods have occurred in the past, most famously at Boscastle in 2004. It has been suggested that much of the rain was produced by warm rain processes, similar to some flash floods that have occurred in the United States. The overarching goal of COPE is to improve quantitative convective precipitation forecasting by understanding the interactions of the cloud microphysics and dynamics and thereby to improve numerical weather prediction (NWP) model skill for forecasts of flash floods. Two research aircraft, the University of Wyoming King Air and the U.K. BAe 146, obtained detailed in situ and remote sensing measurements in, around, and below storms on several days. A new fast-scanning X-band dual-polarization Doppler radar made 360° volume scans over 10 elevation angles approximately every 5 min and was augmented by two Met Office C-band radars and the Chilbolton S-band radar. Detailed aerosol measurements were made on the aircraft and on the ground. This paper i) provides an overview of the COPE field campaign and the resulting dataset, ii) presents examples of heavy convective rainfall in clouds containing ice and also in relatively shallow clouds through the warm rain process alone, and iii) explains how COPE data will be used to improve high-resolution NWP models for operational use.

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