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Xiusheng Yang
and
David R. Miller

Abstract

A spectral model was assembled and used to compute the potential solar irradiance in five broad bands, that is, ultraviolet-B (280–320 nm in wavelength), ultraviolet-A (320–400 nm), photosynthetically active (400–700 nm), near infrared (700–1500 nm), and far infrared (1500–4000 nm), above a green vegetation canopy defined by ground albedo. Starting from the spectral solar irradiance above the atmosphere, the spectral model calculates unweighted potential (clear-sky) direct, diffuse, and global irradiance over the total solar spectrum at the ground-level by considering molecular scattering, scattering and absorption by aerosols, and absorption by ozone, uniformly distributed gases, and water vapor.

Broadband irradiances and atmospheric transmittances were determined by spectral integration of the predictions from the spectral model over the five spectral regions. The effects of solar position, altitude, column ozone, column water vapor, and turbidity in the lower atmosphere on the broadband atmospheric transmittance were examined and quantified. Based on the analysis, a set of simple regression equations was developed for estimating the broadband values of the potential atmospheric transmittance in the five spectral regions. The regression model predicts global irradiance in any of the five broad bands at sea level for a fixed solar position, and then extends the predictions with multipliers of altitude and solar zenith angle. The major advantage of the regression model lies in the convenience of using easy-to-obtain, wavelength independent, parameters as input variables in predicting potential broadband irradiances for ecological studies in engineering units (W m−2) for any given location and time.

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Jesse O. Bash
and
David R. Miller

Abstract

A relaxed eddy accumulation (REA) system was designed to continuously measure total gaseous mercury (TGM) fluxes over a forest canopy. TGM concentration measurements were measured at 5-min intervals with a Tekran model 2537A mercury analyzer located above the forest canopy on a walk-up meteorological tower. Ten-minute averages for up- and downdraft mercury concentrations were used to calculate the flux. The multiresolution decomposition technique was used to determine day- and nighttime averaging periods for the turbulent statistics used in the REA technique. This paper documents the REA system for mercury flux measurements and its use over a forest canopy.

The REA system response to the averaging times for the turbulent statistics and corrections to up- and downdraft concentrations are major considerations when using the technique with the Tekran mercury analyzer over a forest canopy. TGM flux data collected from 18 August to 12 September 2005 are used here to demonstrate the capabilities of the REA system to measure both short- (1-h time periods) and long-term flux dynamics. During the demonstration period the TGM median flux was 21.9 ± 32.6 ng m−2 h−1 and the median atmospheric TGM concentrations were 1.34 ± 0.13 ng m−2 h−1. Maximum short-term TGM evasive fluxes occurred during the daylight hours with minimums during the nighttime. A consistent bimodal emission pattern was observed during the daytime emissions over the canopy. The first peak occurred immediately following the evaporation of the nighttime dew on the canopy and the second peak occurred in the late afternoon.

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Bernadette H. Connell
and
David R. Miller

Abstract

The authors review sources of error in radiosonde measurements in the atmospheric boundary layer and analyze errors of two radiosonde models manufactured by Atmospheric Instrumentation Research, Inc. The authors focus on temperature and humidity lag errors and wind errors. Errors in measurement of azimuth and elevation angles and pressure over short time intervals and at higher attitudes introduce wind vector errors greater than 5 m s−1. Mean temperature and humidity lag errors were small, and collectively, these tag errors had little effect on the calculation of the vertically integrated water vapor content. However, individual large lag errors occurred with dramatic changes in the environment, such as near the surface or at the top of the boundary layer. Dual-sonde flights showed mean instrument error comparable to lag error and had little effect on the calculation of the columnar water vapor content. A hypothetical consistent error of 5% in the measurement of relative humidity in a dry environment could introduce error in the calculation of columnar water vapor content up to 1 kg m−2.

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April L. Hiscox
,
Carmen J. Nappo
, and
David R. Miller

Abstract

In this note a methodology is presented for measuring dispersion parameters based on lidar images, which can be used as an efficient way to remotely monitor time variations of plume dispersion parameters. Lidar images of a smoke plume cross section over a forest canopy during nighttime conditions are analyzed to estimate vertical dispersion parameters and vertical meander of the plume centerline in the near field. Dispersion parameters 60 m downwind are found to have a median value of 2.31 m, with values ranging from a minimum of 0.56 m to a maximum of 5.45 m. Measurements of these parameters have not previously been made outside the restraints of a wind tunnel experiment.

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Neil F. Laird
,
L. Jay Miller
, and
David A. R. Kristovich

Abstract

This article presents a detailed examination of the kinematic structure and evolution of the 5 December 1997 winter mesoscale vortex in the vicinity of Lake Michigan using the synthetic dual-Doppler (SDD) technique. When such a mesoscale event propagates a distance large enough that the viewing angle from a single-Doppler radar changes by about 30° and the circulation is sufficiently steady during this time period, then the SDD method can reveal reliable details about the circulation. One such detail of the observed vortex was a pattern of convergence and divergence associated with radial bands, where heavier snowfall was located. Another was the steadiness and vertical coherence of the derived vorticity and convergence patterns within the cyclonic circulation.

On 5 December 1997, the observed reflectivity field remained remarkably steady for nearly 2.5 h as the vortex moved southeastward allowing for the application of the SDD technique. The reflectivity field exhibited a pronounced asymmetric convective structure with at least three well-defined, inward-spiraling radial snowbands, and a distinct weak-reflectivity region or “eye” near the center of cyclonic circulation. The SDD results showed the vortex circulation was composed of a combination of rotation on the meso-β scale and convergence on the meso-γ scale associated with the embedded radial snowbands. Vertical profiles of derived meso-β-scale, area-mean convergence and vorticity suggest that this winter vortex was likely a warm-core system, similar to both tropical cyclones and polar lows.

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Kirk M. Ducharme
,
David R. Miller
, and
Donald E. Aylor

Abstract

Cylindrical, platinum-coated, hot-film anemometers were struck with a series of individual drops of water while immersed in controlled airflows with velocities ranging from 0.3 to 5.0 m s−1. Subjecting the sensor to water drops caused slight, but permanent, changes in calibration. In an effort to overcome calibration changes following drop impacts, a Teflon-coated sensor was also tested. A filtering algorithm was devised to remove drop-caused spikes in the recorded time series. An average spike duration of 0.32 s per drop impact was found, and maximum record loss was estimated to be 1.7% for rainfall rates less than 30 mm h−1.

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Jesse O. Bash
,
Patricia Bresnahan
, and
David R. Miller

Abstract

This paper presents a review of recent natural surface mercury exchange research in the context of a new modeling framework. The literature indicates that the mercury biogeochemical flux is more dynamic than the current models predict, with interacting multimedia storage and processes. Although several natural mercury emissions models have been created and incorporated into air quality models (AQMs), none are coupled with air quality models on a mass balance basis, and all lack the capacity to explain processes that involve the transport of mercury across atmosphere–surface media concentration gradients. Existing natural mercury emission models treat the surface as both an infinite source and infinite sink for emissions and deposition, respectively, and estimate emissions through the following three pathways: soil, vegetation, and surface waters. The use of these three transport pathways, but with compartmentalized surface storage in a surface–vegetation–atmosphere transport (SVAT) resistance model, is suggested. Surface water fluxes will be modeled using a two-film diffusion model coupled to a surface water photochemical model. This updated framework will allow both the parameterization of the transport of mercury across atmosphere–surface media concentration gradients and the accumulation/depletion of mercury in the surface media. However, several key parameters need further experimental verification before the proposed modeling framework can be implemented in an AQM. These include soil organic mercury interactions, bioavailability, cuticular transport of mercury, atmospheric surface compensation points for different vegetation species, and enhanced soil diffusion resulting from pressure perturbations.

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Patrick Minnis
,
Louis Nguyen
,
David R. Doelling
,
David F. Young
,
Walter F. Miller
, and
David P. Kratz

Abstract

To establish a more reliable reference instrument for calibration normalization, this paper examines the differences between the various thermal infrared imager channels on a set of research and operational satellites. Mean brightness temperatures from the Visible Infrared Scanner (VIRS) on the Tropical Rainfall Measuring Mission (TRMM) satellite and the second Along-Track Scanning Radiometer (ATSR-2) on the second European Remote Sensing Satellite (ERS-2) are correlated with matched data from the eighth Geostationary Operational Environmental Satellite (GOES-8), the fifth Geostationary Meteorological Satellite (GMS-5), and with each other. VIRS data are also correlated with the Terra satellite's Moderate Resolution Imaging Spectroradiometer (MODIS) provisional data as a preliminary assessment of their relative calibrations. As an additional check on their long-term stability, the VIRS data are compared to the broadband longwave radiances of the Clouds and the Earth's Radiant Energy System (CERES) scanners on TRMM. No statistically significant trend in the calibration of any of the three (3.7, 10.8, and 12.0 μm) VIRS thermal channels could be detected from the comparisons with CERES data taken during 1998 and 2000 indicating that the VIRS channels can serve as a reliable reference for intercalibrating satellite imagers. However, a small day–night difference in the VIRS thermal channels detected at very low temperatures should be taken into account. In general, most of the channels agreed to within less than ±0.7 K over a temperature range between 200 and 300 K. Some of the smaller differences can be explained by spectral differences in the channel response functions. A few larger differences were found at 200 K for some of the channels suggesting some basic calibration differences for lower temperatures. A nearly 3-K bias in the ATSR-2 11-μm channel relative to VIRS and GOES-8 was found at the cold end of the temperature range. The intercalibrations described here are being continued on a routine basis.

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Patrick Minnis
,
Louis Nguyen
,
David R. Doelling
,
David F. Young
,
Walter F. Miller
, and
David P. Kratz

Abstract

Operational meteorological satellites generally lack reliable onboard calibration systems for solar-imaging channels. Current methods for calibrating these channels and for normalizing similar channels on contemporaneous satellite imagers typically rely on a poorly calibrated reference source. To establish a more reliable reference instrument for calibration normalization, this paper examines the use of research satellite imagers that maintain their solar-channel calibrations by using onboard diffuser systems that rely on the sun as an absolute reference. The Visible Infrared Scanner (VIRS) on the Tropical Rainfall Measuring Mission (TRMM) satellite and the second Along-Track Scanning Radiometer (ATSR-2) on the second European Remote Sensing Satellite (ERS-2) are correlated with matched data from the eighth Geostationary Operational Environmental Satellite (GOES-8), the fifth Geostationary Meteorological satellite (GMS-5), and with each other to examine trends in the solar channels. VIRS data are also correlated with the Terra satellite's Moderate Resolution Imaging Spectroradiometer (MODIS) provisional data as a preliminary assessment of their relative calibrations. As an additional check on their long-term stability, the VIRS data are compared to the relevant corresponding broadband shortwave radiances of the Clouds and the Earth's Radiant Energy System (CERES) scanners on TRMM. No statistically significant trend in the calibration of the VIRS 0.65- and 1.64-μm channels could be detected from the comparisons with CERES data taken during 1998 and 2000. The VIRS-to-GOES-8 correlations revealed an annual degradation rate for the GOES-8 visible (0.67 μm) channel of ∼7.5% and an initial drop of 16% in the gain from the prelaunch value. The slopes in the GOES-8 visible-channel gain trend lines derived from VIRS data taken after January 1998 and ATSR-2 data taken between October 1995 and December 1999 differed by only 1%–2% indicating that both reference instruments are highly stable. The mean difference of 3%–4.8% between the VIRS–GOES-8 and ATSR-2–GOES-8 gains is attributed to spectral differences between ATSR-2 and VIRS and to possible biases in the ATSR-2 channel-2 calibration. A degradation rate of 1.3% per year found for the GMS-5 visible channel was confirmed by comparisons with earlier calibrations. The MODIS and VIRS calibrations agreed to within −1% to 3%. Some of the differences between VIRS and the provisional MODIS radiances can be explained by spectral differences between the two instruments. The MODIS measures greater reflectance than VIRS for bright scenes. Although both VIRS and ATSR-2 provide temporally stable calibrations, it is recommended that, at least until MODIS calibrations are finalized, VIRS should be used as a reference source for normalizing operational meteorological satellite imagers because of its broader visible filter.

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Michael J. DeFlorio
,
David W. Pierce
,
Daniel R. Cayan
, and
Arthur J. Miller

Abstract

Water resources and management over the western United States are heavily impacted by both local climate variability and the teleconnected responses of precipitation to the El Niño–Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO). In this work, regional precipitation patterns over the western United States and linkages to ENSO and the PDO are analyzed using output from a Community Climate System Model version 4 (CCSM4) preindustrial control run and observations, with emphasis on extreme precipitation events. CCSM4 produces realistic zonal gradients in precipitation intensity and duration over the western United States, with higher values on the windward side of the Cascade Mountains and Sierra Nevada and lower values on the leeward. Compared to its predecessor CCSM3, CCSM4 shows an improved teleconnected signal of both ENSO and the PDO to large-scale circulation patterns over the Pacific–North America region and also to the spatial pattern and other aspects of western U.S. precipitation. The so-called drizzle problem persists in CCSM4 but is significantly improved compared to CCSM3. In particular, it is found that CCSM4 has substantially less precipitation duration bias than is present in CCSM3. Both the overall and extreme intensity of wintertime precipitation over the western United States show statistically significant linkages with ENSO and PDO in CCSM4. This analysis provides a basis for future studies using greenhouse gas (GHG)-forced CCSM4 runs.

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