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J. D. Price and M. R. Bush

Abstract

In this study comparisons are made between Met Office mesoscale model boundary layer profiles, and radiosonde data collected in the central United Kingdom during three intensive boundary layer cloud experiments. Significant differences between forecast and observed profiles were found. An assessment of whether these differences are dominated by sonde random error or model error is performed. Results suggest that sonde random errors are insignificant for stratocumulus fields, but not for cumulus ones.

Results show that model fields do not represent the fine vertical structure seen in the observations. The impact of this result for cloud prediction is discussed, and an estimate for the vertical resolution required to adequately parameterize the cloud field is provided.

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T. Rossby, J. Price, and D. Webb

Abstract

Up to forty neutrally buoyant floats (20 at 700 m, 20 at 1300 m) were used in the POLYMODE Local Dynamics Experiment (LDE) to provide a quasi-Lagrangian description of the structure and evolution of the mesoscale eddy field in a limited region characterized by higher kinetic energy levels than those obtained in MODE. This paper is an overview and summary of the data collected.

The temporal development of the two-level float array is presented in a sequence of maps, each spanning five days. In these one readily notices an “oscillation” of floats at 1300 m in a NE-SW direction before the cluster breaks apart. At 700 m the float cluster subdivides much more rapidly. The first setting of 700-m floats drifts to the west; the second group, launched two months later, goes far to the east.

Ensemble averages as a function of time of the floats at 1300 m reveal great sensitivity to the “oscillatory” velocity field while the array is “tight” or coherent the corresponding 700-in averages, although noisy, show clearly the westward and eastward motion of the first and second clusters respectively. Grand averages for μ, υ (cm s−1), and kinetic energy (ergs gm−1) are (−1.8, 0., 80) and (−1.6, 0., 34) for the shallow and deep floats.

The spatial correlation functions show 1arge scales of coherence. The zero-crossing of the transverse-velocity correlation function is about 100 km for both the 700- and 1300-m floats compared to 55 km for the 1500-m goats in MODE. The longitudinal correlation scale is also much larger.

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P. L. Richardson, J. F. Price, D. Walsh, L. Armi, and M. Schröder

Abstract

Three Meddies were tracked for up to two years in the Canary Basin using neutrally buoyant SOFAR floats. These Meddies have cores of warm, salty Mediterranean Water and are approximately 100 km in diameter, 800 m thick, and are centered at a depth of 1100 m. Meddy 1 was tracked for two years (1984–86) with five floats as it drifted 1090 km southward with a mean velocity of 1.8 cm s−1. Four shipboard surveys made during these two years revealed the nearly total decay of Meddy 1 by gradual mixing processes. Meddy 2 drifted 530 km southwestward over 8.5 months with a mean velocity of 2.3 cm s−1 until it collided with Hyères Seamount near 31°N, 29°W. The floats trapped in this Meddy then stopped looping abruptly, implying a major disruption of this Meddy. Meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1.1 cm s−1. A comparison of the velocity of Meddies to the velocity of nearby floats at 1100 m depth outside of the Meddies shows clearly that all three Meddies moved southwestward through the surrounding water at a speed of about 1.3 cm s−1.

The floats inside the Meddies looped anticyclonically in a nearby solid-body rotation with a period of 6 days for Meddy 1, 4 days for Meddy 2, and 5 days for Meddy 3. The rotation period of Meddy 1 appeared to remain constant over nearly two years despite a large decrease in the Meddy's thickness and diameter due to mixing. Rotation velocities in the Meddies were as great as 34 cm s−1 (Meddy 2), much faster than speed of nearby floats outside of the Meddies.

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J. D. Price, S. Lane, I. A. Boutle, D. K. E. Smith, T. Bergot, C. Lac, L. Duconge, J. McGregor, A. Kerr-Munslow, M. Pickering, and R. Clark

Abstract

Fog is a high-impact weather phenomenon affecting human activity, including aviation, transport, and health. Its prediction is a longstanding issue for weather forecast models. The success of a forecast depends on complex interactions among various meteorological and topographical parameters; even very small changes in some of these can determine the difference between thick fog and good visibility. This makes prediction of fog one of the most challenging goals for numerical weather prediction. The Local and Nonlocal Fog Experiment (LANFEX) is an attempt to improve our understanding of radiation fog formation through a combined field and numerical study. The 18-month field trial was deployed in the United Kingdom with an extensive range of equipment, including some novel measurements (e.g., dew measurement and thermal imaging). In a hilly area we instrumented flux towers in four adjacent valleys to observe the evolution of similar, but crucially different, meteorological conditions at the different sites. We correlated these with the formation and evolution of fog. The results indicate new quantitative insight into the subtle turbulent conditions required for the formation of radiation fog within a stable boundary layer. Modeling studies have also been conducted, concentrating on high-resolution forecast models and research models from 1.5-km to 100-m resolution. Early results show that models with a resolution of around 100 m are capable of reproducing the local-scale variability that can lead to the onset and development of radiation fog, and also have identified deficiencies in aerosol activation, turbulence, and cloud micro- and macrophysics, in model parameterizations.

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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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The JET2000 Project: Aircraft Observations of the African Easterly Jet and African Easterly Waves

Aircraft Observations of the African Easterly Jet and African Easterly Waves

C. D. Thorncroft, D. J. Parker, R. R. Burton, M. Diop, J. H. Ayers, H. Barjat, S. Devereau, A. Diongue, R. Dumelow, D. R. Kindred, N. M. Price, M. Saloum, C. M. Tayor, and A. M. Tompkins

Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight CI30 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ.

Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale system) are presented and discussed. Analysis of dropsonde data on 28 August indicates contrasting convective characteristics north and south of the AEJ with dry convection more dominant to the north and moist convection more dominant to the south. The consequences of this for the AEJ and the relationship with the boundary layer observations are briefly discussed.

Preliminary NWP results indicate little sensitivity to the inclusion of the dropsonde data on the AEJ winds in European Centre for Medium-Range Weather Forecasts (ECMWF) and Met Office analyses. It is proposed that this may be due to a good surface analysis and a realistic model response to this. Both models poorly predict the AEJ in the 5-day forecast indicating the need for more process studies in the region.

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Hyein Jeong, Xylar S. Asay-Davis, Adrian K. Turner, Darin S. Comeau, Stephen F. Price, Ryan P. Abernathey, Milena Veneziani, Mark R. Petersen, Matthew J. Hoffman, Matthew R. Mazloff, and Todd D. Ringler

Abstract

The Southern Ocean overturning circulation is driven by winds, heat fluxes, and freshwater sources. Among these sources of freshwater, Antarctic sea ice formation and melting play the dominant role. Even though ice-shelf melt is relatively small in magnitude, it is located close to regions of convection, where it may influence dense water formation. Here, we explore the impacts of ice-shelf melting on Southern Ocean water-mass transformation (WMT) using simulations from the Energy Exascale Earth System Model (E3SM) both with and without the explicit representation of melt fluxes from beneath Antarctic ice shelves. We find that ice-shelf melting enhances transformation of Upper Circumpolar Deep Water, converting it to lower density values. While the overall differences in Southern Ocean WMT between the two simulations are moderate, freshwater fluxes produced by ice-shelf melting have a further, indirect impact on the Southern Ocean overturning circulation through their interaction with sea ice formation and melting, which also cause considerable upwelling. We further find that surface freshening and cooling by ice-shelf melting cause increased Antarctic sea ice production and stronger density stratification near the Antarctic coast. In addition, ice-shelf melting causes decreasing air temperature, which may be directly related to sea ice expansion. The increased stratification reduces vertical heat transport from the deeper ocean. Although the addition of ice-shelf melting processes leads to no significant changes in Southern Ocean WMT, the simulations and analysis conducted here point to a relationship between increased Antarctic ice-shelf melting and the increased role of sea ice in Southern Ocean overturning.

Open access
G.-K. Plattner, R. Knutti, F. Joos, T. F. Stocker, W. von Bloh, V. Brovkin, D. Cameron, E. Driesschaert, S. Dutkiewicz, M. Eby, N. R. Edwards, T. Fichefet, J. C. Hargreaves, C. D. Jones, M. F. Loutre, H. D. Matthews, A. Mouchet, S. A. Müller, S. Nawrath, A. Price, A. Sokolov, K. M. Strassmann, and A. J. Weaver

Abstract

Eight earth system models of intermediate complexity (EMICs) are used to project climate change commitments for the recent Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report (AR4). Simulations are run until the year 3000 a.d. and extend substantially farther into the future than conceptually similar simulations with atmosphere–ocean general circulation models (AOGCMs) coupled to carbon cycle models. In this paper the following are investigated: 1) the climate change commitment in response to stabilized greenhouse gases and stabilized total radiative forcing, 2) the climate change commitment in response to earlier CO2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the twenty-first century compare reasonably well with results from AOGCMs, and the suite of EMICs proves well suited to complement more complex models. Substantial climate change commitments for sea level rise and global mean surface temperature increase after a stabilization of atmospheric greenhouse gases and radiative forcing in the year 2100 are identified. The additional warming by the year 3000 is 0.6–1.6 K for the low-CO2 IPCC Special Report on Emissions Scenarios (SRES) B1 scenario and 1.3–2.2 K for the high-CO2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3–1.1 m for SRES B1 and 0.5–2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO2 stabilization. In contrast, surface temperature changes slow down after a century. The meridional overturning circulation is weakened in all EMICs, but recovers to nearly initial values in all but one of the models after centuries for the scenarios considered. Emissions during the twenty-first century continue to impact atmospheric CO2 and climate even at year 3000. All models find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49%–62%) in year 3000, and that a substantial fraction (15%–28%) is still airborne even 900 yr after carbon emissions have ceased. Future stabilization of atmospheric CO2 and climate change requires a substantial reduction of CO2 emissions below present levels in all EMICs. This reduction needs to be substantially larger if carbon cycle–climate feedbacks are accounted for or if terrestrial CO2 fertilization is not operating. Large differences among EMICs are identified in both the response to increasing atmospheric CO2 and the response to climate change. This highlights the need for improved representations of carbon cycle processes in these models apart from the sensitivity to climate change. Sensitivity simulations with one single EMIC indicate that both carbon cycle and climate sensitivity related uncertainties on projected allowable emissions are substantial.

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H. J. S. Fernando, E. R. Pardyjak, S. Di Sabatino, F. K. Chow, S. F. J. De Wekker, S. W. Hoch, J. Hacker, J. C. Pace, T. Pratt, Z. Pu, W. J. Steenburgh, C. D. Whiteman, Y. Wang, D. Zajic, B. Balsley, R. Dimitrova, G. D. Emmitt, C. W. Higgins, J. C. R. Hunt, J. C. Knievel, D. Lawrence, Y. Liu, D. F. Nadeau, E. Kit, B. W. Blomquist, P. Conry, R. S. Coppersmith, E. Creegan, M. Felton, A. Grachev, N. Gunawardena, C. Hang, C. M. Hocut, G. Huynh, M. E. Jeglum, D. Jensen, V. Kulandaivelu, M. Lehner, L. S. Leo, D. Liberzon, J. D. Massey, K. McEnerney, S. Pal, T. Price, M. Sghiatti, Z. Silver, M. Thompson, H. Zhang, and T. Zsedrovits

Abstract

Emerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multi-institutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms, has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent submesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.

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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

Abstract

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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