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R. J. Harding and J. W. Pomeroy

Abstract

During the winter of 1993/94 a study to quantify the winter energy balance of the main cover types of the boreal landscape took place. The study was based on the southern edge of boreal forest in Canada. Measurements were made over a mature jack pine stand and a frozen lake. Shortwave albedos of 12% to 14% over the jack pine and 20% to 88% on the frozen lake (both depending on snow cover) were measured. There were correspondingly large contrasts in the total radiation inputs and the turbulent heat fluxes. The mean net all-wave radiation input was large and positive into the forest and negative over the lake. The sensible heat fluxes were of the same sign as the radiative inputs with positive values over the forest peaking at +200 W m−2 and failing to − 100 W m−2 over the lake. The evaporation from the forest depended on whether the there was snow on the canopy. When the canopy was snow-free, the evaporation was low, about 50% of net radiation but, when there was snow on the canopy, the evaporation was large, 4 mm over a 36-hour period. The results of these experiments are being used to design much-improved descriptions of boreal forest within the next generation of climate, models.

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I. R. Calder, R. L. Hall, R. J. Harding, and I. R. Wright

Abstract

A portable, wet-surface lysimeter system, for measuring the in situ interception characteristics of short to medium height vegetation is described. The system, comprising an electronic balance and meteorological sensors linked to a microcomputer, has been used successfully at a number of U.K. sites of widely varying topography. In this paper, results are presented from two heather sites near Killin in Scotland. The aerodynamic resistance (rv) to the transport of water vapor from a canopy of wet heather was found, from solution of the Penman-Monteith equation, to be rv = (20.0 ± 2.0)ū −(0.60±0.06), where ū is the mean windspeed, and the canopy capacity of intercepted water was found to be (1.4 ± 0.2) mm. A review of methods of determining aerodynamic resistance is also given.

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R. M. Clancy, J. M. Harding, K. D. Pollak, and P. May

Abstract

Global-scale analyses of ocean thermal structure produced operationally at the U.S. Navy's Fleet Numerical Oceanography Center are verified, along with an ocean thermal climatology, against unassimilated bathythermograph (bathy), satellite multichannel sea surface temperature (MCSST), and ship sea surface temperature (SST) data. Verification statistics are calculated from the three types of data for February–April of 1988 and February–April of 1990 in nine verification areas covering most of the open ocean in the Northern Hemisphere. The analyzed thermal fields were produced by version 1.0 of the Optimum Thermal Interpolation System (OTIS 1.0) in 1988, but by an upgraded version of this model, referred to as OTIS 1.1, in 1990. OTIS 1.1 employs exactly the same analysis methodology as OTIS 1.0. The principal difference is that OTIS 1.1 has twice the spatial resolution of OTIS 1.0 and consequently uses smaller spatial decorrelation scales and noise-to-signal ratios. As a result, OTIS 1.1 is able to represent more horizontal detail in the ocean thermal fields than its predecessor.

Verification statistics for the SST fields derived from bathy and MCSST data are consistent with each other, showing similar trends and error levels. These data indicate that the analyzed SST fields are more accurate in 1990 than in 1988, and generally more accurate than climatology for both years. Verification statistics for the SST fields derived from ship data are inconsistent with those derived from the bathy and MCSST data, and show much higher error levels indicative of observational noise.

Verification of the subsurface thermal fields with bathy data clearly show improvements in the accuracy of the analyzed thermal fields between 1988 and 1990, even though the number of hathy observations available for assimilation into the analysis is less in 1990 than in 1988. The analyzed subsurface thermal structure is also generally more accurate than climatology, particularly in 1990, indicating that the OTIS model makes effective use of the bathy data. Errors are much larger in the western halves of the midiatitude ocean basins than in the eastern halves, primarily as a result of the strong and unresolved fronts and eddies associated with the western boundary currents. Prominent subsurface maxima in the error profiles for both the analysis and climatology, probably a result of unresolved thermocline variability, are present in all five tropical verification areas.

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Justin E. Bagley, Ankur R. Desai, Keith J. Harding, Peter K. Snyder, and Jonathan A. Foley

Abstract

Expansion of agricultural lands and inherent variability of climate can influence the water cycle in the Amazon basin, impacting numerous ecosystem services. However, these two influences do not work independently of each other. With two once-in-a-century-level droughts occurring in the Amazon in the past decade, it is vital to understand the feedbacks that contribute to altering the water cycle. The biogeophysical impacts of land cover change within the Amazon basin were examined under drought and pluvial conditions to investigate how land cover and drought jointly may have enhanced or diminished recent precipitation extremes by altering patterns and intensity. Using the Weather Research and Forecasting (WRF) Model coupled to the Noah land surface model, a series of April–September simulations representing drought, normal, and pluvial years were completed to assess how land cover change impacts precipitation and how these impacts change under varied rainfall regimes. Evaporative sources of water vapor that precipitate across the region were developed with a quasi-isentropic back-trajectory algorithm to delineate the extent and variability that terrestrial evaporation contributes to regional precipitation. A decrease in dry season latent heat flux and other impacts of deforestation on surface conditions were increased by drought conditions. Coupled with increases in dry season moisture recycling over the Amazon basin by ~7% during drought years, land cover change is capable of reducing precipitation and increasing the amplitude of droughts in the region.

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