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Colin Price and David Rind

Abstract

Future climate change could have significant repercussions for lightning-caused wildfires. Two empirical fire models are presented relating the frequency of lightning fires and the area burned by these fires to the elective precipitation and the frequency of thunderstorm activity. One model deals with the seasonal variations in lightning fires, while the second model deals with the interannual variations of lightning fires. These fire models are then used with the Goddard Institute for Space Studies General Circulation Model to investigate possible changes in fire frequency and area burned in a 2 × CO2 climate. In the United States, the annual mean number of lightning fires increases by 44%, while the area burned increases by 78%. On a global scale, the largest increase in lightning fires can be expected in untouched tropical ecosystems where few natural fires occur today.

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Colin Price and David Rind

Abstract

A general circulation model (GCM) is used to model global lightning distributions and frequencies. Both total and cloud-to-ground lightning frequencies are modeled using parameterizations that relate the depth of convective clouds to lightning frequencies. The model's simulations of lightning distributions in time and space show good agreement with available observations. The model's annual mean climatology shows a global lightning frequency of 77 flashes per second, with cloud-to-ground lightning making up 25% of the total. The maximum lightning activity in the GCM occurs during the Northern Hemisphere summer, with approximately 91% of all lightning occurring over continental and coastal regions.

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Jing M. Chen, T. Andrew Black, David T. Price, and Reid E. Carter

Abstract

A model has been developed to calculate the spatial distribution of the photosynthetic photon flux density (PPFD) in elliptical forest openings of given slopes and orientations. The PPFD is separated into direct and diffuse components. The direct component is calculated according to the opening and radiation geometries, and pathlength of the solar beam through the forest canopy. The diffuse component is obtained from the sky, tree, and landscape view factors. In this model, the distribution of foliage area with height and the effect of foliage clumping on both direct and diffuse radiation transmission are considered.

The model has been verified using measurements for six quantum sensors (LI-COR Inc.) located at different positions in a small clear-cut (0.37 ha) in a 90-year-old western hemlock-Douglas fir forest.

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John H. Pedlar, Daniel W. McKenney, Kevin Lawrence, Pia Papadopol, Michael F. Hutchinson, and David Price

Abstract

This study produced annual spatial models (or grids) of 27 growing-season variables for Canada that span two centuries (1901–2100). Temporal gaps in the availability of daily climate data—the typical and preferred source for calculating growing-season variables—necessitated the use of two approaches for generating these growing-season grids. The first approach, used only for the 1950–2010 period, employed a computer script to directly calculate the suite of growing-season variables from existing daily climate grids. Since daily grids were not available for the remaining years, a second approach, which employed a machine-learning method called boosted regression trees (BRT), was used to generate statistical models that related each growing-season variable to a suite of climate and water-related predictors. These BRT models were used to generate grids of growing-season variables for each year of the study period, including the 1950–2010 period to allow comparison between the two approaches. Mean absolute errors associated with the BRT-based grids were approximately 30% higher than those associated with the daily-based grids. The two approaches were also compared by calculating trends in growing-season length over the 1950–2010 period. Significant increases in growing-season length were obtained for nearly all ecozones across Canada, and there were no significant differences in the trends obtained from the two approaches. Although the daily-based approach tended to have lower errors, the BRT approach produced comparable map products that should be valuable for periods and regions for which daily data are not available.

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James B. Girton, Lawrence J. Pratt, David A. Sutherland, and James F. Price

Abstract

The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control—primarily an asymmetry across the sill reminiscent of flow over a dam. However, this aspect has never been confirmed by any quantitative measure, nor is the position of the control section known. This paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the FBC. These include 1) the cross-stream variation in the local Froude number, including a modified form that accounts for stratification and vertical shear, 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section, and 3) direct computation of shallow water wave speeds from the observed overflow structure. Though differences exist, the three methods give similar answers, suggesting that the FBC is indeed controlled, with a critical section located 20–90 km downstream of the sill crest. Evidence of an upstream control with respect to a potential vorticity wave is also presented. The implications of these results for hydraulic predictions of overflow transport and variability are discussed.

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David S. Hosom, Gennaro H. Crescenti, Clifford L. Winget, Sumner Weisman, Donald P. Doucet, and James F. Price

Abstract

An intelligent, chilled mirror humidity instrument has been designed for use on buoys and ships. Our design goal is for the instrument to make high-quality dewpoint temperature measurements for a period of up to one year from an unattended platform, while consuming as little power as possible.

The instrument uses a General Eastern Dew-10 chilled mirror sensor and is controlled by an onboard digital processor that is programmable in BASIC. Communications to an external logger are provided by an RS-232 compatible interface. The housing is made of PVC and is approximately 76 cm long × 11.5 cm in diameter; the complete instrument weight is 5.8 kg. Energy consumption is typically 800 J per measurement; standby power consumption is 0.05 W.

A series of dockside tests have been carried out to evaluate the long-term accuracy and reliability of the D10IQ. As a standard, we used an EG&G 200M Dewtrak chilled mirror instrument that was cleaned manually at frequent intervals. We found that the mean difference between the D10IQ and EG&E 200M was roughly 0.9°C, which is within the expected error of our calibration procedure, and that the standard deviation of the difference was about 0.8°C. The variance is contributed equally by a fast time scale random fluctuation and a 5- to 10-day period variation. For our purpose it is most significant that there was very little calibration drift of the D10IQ so long as the mirror reflectance stayed above a well-defined and readily monitored threshold value. Thus it appears that the D10IQ can provide a fairly reliable and accurate means for measuring humidity from unattended platforms.

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Daniel W. McKenney, Michael F. Hutchinson, Pia Papadopol, Kevin Lawrence, John Pedlar, Kathy Campbell, Ewa Milewska, Ron F. Hopkinson, David Price, and Tim Owen

Over the past two decades, researchers at Natural Resources Canada's Canadian Forest Service, in collaboration with the Australian National University (ANU), Environment Canada (EC), and the National Oceanic and Atmospheric Administration (NOAA), have made a concerted effort to produce spatial climate products (i.e., spatial models and grids) covering both Canada and the United States for a wide variety of climate variables and time steps (from monthly to daily), and across a range of spatial resolutions. Here we outline the method used to generate the spatial models, detail the array of products available and how they may be accessed, briefly describe some of the usage and impact of the models, and discuss anticipated further developments. Our initial motivation in developing these models was to support forestry-related applications. They have since been utilized by a wider range of agencies and researchers. This article is intended to further raise awareness of the strengths and weaknesses of these climate models and to facilitate their wider application.

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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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