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Mitchell T. Black and David J. Karoly
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D. L. Spittlehouse and T. A. Black

Abstract

Rates of evapotranspiration from a 14 m high Douglas fir forest on the southwest coast of British Columbia were obtained using the energy balance/Bowen ratio method and an energy balance/eddy correlation method. In the former method, the Bowen ratio was measured using reversing diode psychrometers. In the latter, the sensible heat flux was obtained by eddy correlation analysis of data obtained from a fast response thermistor and Gill anemometers mounted horizontally and at 30° from the vertical. The generally low wind speed above the forest resulted in occasional stalling of the anemometers and made obtaining adequate eddy correlation data difficult. Spectral analysis of the eddy correlation data indicates that a significant fraction of the sensible heat flux was at low frequencies. The regression relationship between evapotranspiration rate obtained using the energy balance/eddy correlation method (Ee) and that obtained using the energy balance/Bowen ratio method (Eβ) was found to be
The experiment suggests that an eddy correlation system using mechanical anemometers is not suitable for extended water balance studies of forests where low wind speeds predominate.
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E. H. Hogg, D. T. Price, and T. A. Black

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A large portion of the western Canadian interior exhibits a distinctive seasonal pattern in long-term mean surface temperatures characterized by anomalously warmer conditions in spring and autumn than would be expected from a sinusoidal model. The anomaly is greatest over the southern boreal forest of western Canada, where trembling aspen (Populus tremuloides Michx.)—a deciduous, broad-leaved species—is an important component. In this region, mean temperatures are 2°–3°C warmer in April and October but nearly 2°C cooler in June and July, relative to a best-fitting sinusoidal function. Another feature of the climate in this region is that average precipitation is low (15–30 mm month−1) from October to April but increases sharply during the summer growing season (50–100 mm month−1 from June to August). Eddy correlation and sap flow measurements in a boreal aspen forest indicate profound seasonal changes in transpiration and energy partitioning associated with the deciduous nature of the forest canopy. Latent heat (water vapor) flux reaches a maximum during the summer period when leaves are present, while sensible heat flux is highest in early spring when the forest is leafless. Thus, it is postulated that feedbacks of leaf phenology of aspen forests, which occupy a large area of the western Canadian interior, may contribute significantly to the distinctive seasonal patterns of mean temperature and precipitation that occur in this region.

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D. Yap, T. A. Black, and T. R. Oke

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A yaw sphere-thermometer assembly, to measure sensible heat flux density by the eddy correlation method, was built following the design of Tanner and Thurtell. Wind tunnel experiments indicate that the sphere constant should be 1.57, which is significantly less than the theoretical value of 2.25. The effects of tilt indicate that heat fluxes may be in error by 5% per degree of tilt In unstable conditions and up to 11% per degree in stable conditions. Field comparisons of the heat fluxes measured by the yaw sphere-thermometer system and a Bowen ratio apparatus produced satisfactory agreement.

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John T. Allen, Alexandre B. Pezza, and Mitchell T. Black

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A global climatology for rapid cyclone intensification has been produced from the second NCEP reanalysis (NCEP2), the 25-yr Japanese Reanalysis (JRA-25), and the ECMWF reanalyses over the period 1979–2008. An improved (combined) criterion for identifying explosive cyclones has been developed based on preexisting definitions, offering a more balanced, normalized climatological distribution. The combined definition was found to significantly alter the population of explosive cyclones, with a reduction in “artificial” systems, which are found to compose 20% of the population determined by earlier definitions. Seasonally, winter was found to be the dominant formative period in both hemispheres, with a lower degree of interseasonal variability in the Southern Hemisphere (SH). Considered over the period 1979–2008, little change is observed in the frequency of systems outside of natural interannual variability in either hemisphere. Significant statistical differences have been found between reanalyses in the SH, while in contrast the Northern Hemisphere (NH) was characterized by strong positive correlations between reanalyses in almost all examined cases. Spatially, explosive cyclones are distributed into several distinct regions, with two regions in the northwest Pacific and the North Atlantic in the NH and three main regions in the SH. High-resolution and modern reanalysis data were also found to increase the climatology population of rapidly intensifying systems. This indicates that the reanalyses have apparently undergone increasing improvements in consistency over time, particularly in the SH.

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Sim D. Aberson, Michael T. Montgomery, Michael Bell, and Michael Black

An unprecedented dataset of category-5 Hurricane Isabel was collected on 12–14 September 2003. This two-part series focuses on novel dynamical and thermodynamical aspects of Isabel's innercore structure on 13 September. In Part I, using a composite of dropwindsonde and in situ aircraft data, the authors suggested that the axisymmetric structure of Isabel showed that the storm was superintense. Mesocyclones seen clearly in satellite imagery within the eye of Hurricane Isabel are hypothesized to mix high-entropy air at low levels in the eye into the eyewall, stimulating explosive convective development and a concomitant local horizontal wind acceleration.

Part II focuses on a unique set of observations into an extraordinary small- (miso) scale cyclonic feature inside of the inner edge of the eyewall of Hurricane Isabel. A dropwindsonde released into this feature measured the strongest known horizontal wind in a tropical cyclone. This particular observation is discussed in the context of concurrent observations from airborne Doppler radar and other airborne instruments. These observations show wind even stronger than the system-scale superintense wind suggested in Part I. Speculation on the frequency of occurrence of these “little whirls” and their potentially catastrophic impacts are presented.

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Mitchell T. Black, David J. Karoly, and Andrew D. King
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Jing M. Chen, T. Andrew Black, David T. Price, and Reid E. Carter

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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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Michael T. Montgomery, Michael M. Bell, Sim D. Aberson, and Michael L. Black

This study is an observational analysis of the inner-core structure, sea surface temperature, outflow layer, and atmospheric boundary layer of an intense tropical cyclone whose intensity and structure is consistent with recent numerical and theoretical predictions of superintense storms. The findings suggest new scientific challenges for the current understanding of hurricanes.

Unprecedented observations of the category-5 Hurricane Isabel (2003) were collected during 12–14 September. This two-part article reports novel dynamic and thermodynamic aspects of the inner-core structure of Isabel on 13 September that were made possible by analysis of these data. Here, a composite of the axisymmetric structure of the inner core and environment of Isabel is estimated using global positioning system dropwindsondes and in situ aircraft data. In Part II, an extreme wind speed observation on the same day is discussed in the context of this work.

The axisymmetric data composite suggests a reservoir of high-entropy air inside the low-level eye and significant penetration of inflowing near-surface air from outside. The analysis suggests that the low-level air penetrating the eye is enhanced thermodynamically by acquiring additional entropy through interaction with the ocean and replaces air mixed out of the eye. The results support the hypothesis that this high-entropy eye air “turboboosts” the hurricane engine upon its injection into the eyewall clouds. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound by 10–35 m s−1 for the given environmental conditions. Additional study with other datasets is encouraged to further test the superintensity hypothesis.

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Andrew D. King, Mitchell T. Black, David J. Karoly, and Markus G. Donat
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