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D. D. Turner

Abstract

A new approach to retrieve microphysical properties from mixed-phase Arctic clouds is presented. This mixed-phase cloud property retrieval algorithm (MIXCRA) retrieves cloud optical depth, ice fraction, and the effective radius of the water and ice particles from ground-based, high-resolution infrared radiance and lidar cloud boundary observations. The theoretical basis for this technique is that the absorption coefficient of ice is greater than that of liquid water from 10 to 13 μm, whereas liquid water is more absorbing than ice from 16 to 25 μm. MIXCRA retrievals are only valid for optically thin (τ visible < 6) single-layer clouds when the precipitable water vapor is less than 1 cm. MIXCRA was applied to the Atmospheric Emitted Radiance Interferometer (AERI) data that were collected during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment from November 1997 to May 1998, where 63% of all of the cloudy scenes above the SHEBA site met this specification. The retrieval determined that approximately 48% of these clouds were mixed phase and that a significant number of clouds (during all 7 months) contained liquid water, even for cloud temperatures as low as 240 K. The retrieved distributions of effective radii for water and ice particles in single-phase clouds are shown to be different than the effective radii in mixed-phase clouds.

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D. D. Turner and U. Löhnert

Abstract

The Atmospheric Emitted Radiance Interferometer (AERI) observes spectrally resolved downwelling radiance emitted by the atmosphere in the infrared portion of the electromagnetic spectrum. Profiles of temperature and water vapor, and cloud liquid water path and effective radius for a single liquid cloud layer, are retrieved using an optimal estimation–based physical retrieval algorithm from AERI-observed radiance data. This algorithm provides a full error covariance matrix for the solution, and both the degrees of freedom for signal and the Shannon information content. The algorithm is evaluated with both synthetic and real AERI observations. The AERI is shown to have approximately 85% and 70% of its information in the lowest 2 km of the atmosphere for temperature and water vapor profiles, respectively. In clear-sky situations, the mean bias errors with respect to the radiosonde profiles are less than 0.2 K and 0.3 g kg−1 for heights below 2 km for temperature and water vapor mixing ratio, respectively; the maximum root-mean-square errors are less than 1 K and 0.8 g kg−1. The errors in the retrieved profiles in cloudy situations are larger, due in part to the scattering contribution to the downwelling radiance that was not accounted for in the forward model. Scattering is largest in one of the spectral regions used in the retrieval, however, and removing this spectral region results in a slight reduction of the information content but a considerable improvement in the accuracy of the retrieved thermodynamic profiles.

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D. Bruce Turner

Abstract

Three methods to calculate wind direction standard deviation are evaluated. Although eight hours of wind data show no significant differences between the methods, synthetically generated data having standard deviations near the maximum possible show the Yamartino method to perform well. Also, the wind direction for periods such as an hour is shown to be well represented by the arctangent of the mean sines and cosines.

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D. S. Turner

Abstract

The effect of changes in the mixing ratio of C02 on the uniformly mixed gases component of the TIROS-N Observational Vertical Sounder radiances in High-Resolution Infrared Sounder (HIRS) channels is examined using a line-by-line radiative transfer model. A 9% increase in C02 concentration is found to cause up to a 3.5% change in the clear-sky radiance in some channels. When simple black clouds are introduced in the simulations, the magnitude of such radiance changes decreases with increasing cloud-top height.

A simple correction term to account for C02 mixing ratio changes is developed that can be applied to the current uniformly mixed gases component of the National Environmental Satellite Data and Information Service fast transmittance model. The correction term is a linear function of the difference between the C02 mixing ratio and a reference mixing ratio and is applied as an exponent to the uniformly mixed gas transmittance obtained from the fast transmittance model. The method requires the determination of a single coefficient, β¯, for each of the 19 HIRS channels. These coefficients are tabulated for the NOAA-9 through NOAA-12 satellites.

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D. Bruce Turner

Abstract

The suggestions of Pasquill as set forth in his landmark 1961 Meteorological Magazine paper are briefly reviewed. These methods are viewed from the perspective of the requirements placed upon air agencies after the passage of the Clean Air Act in 1970. Pasquill’s clarification of the use of his methods in a technical report for the Environmental Protection Agency (EPA) is discussed in relation to the incorporation of these methods into numerous air quality dispersion models. The shifts in the problem areas faced by the EPA and the relation of these to the methods of Pasquill are discussed. Reasons are suggested for the long persistence of the Pasquill methods in the models used to address U.S. EPA regulations. Current trends in the incorporation of newer techniques into regulatory modeling and their relation to the methods of Pasquill are briefly stated.

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D. Bruce Turner

Abstract

The purpose of this paper is to present a working model for the diffusion of gases from multiple sources in an urban area. Since 24-hr sulfur dioxide measurements were made at 32 sampling stations in Nashville, Tenn., during a 12-month study, this was the city chosen for investigation. A diffusion equation modified to use area instead of point sources was used with a source inventory of sulfur dioxide emissions to calculate 24-hr concentrations at 1-mile intervals. Wind velocity and stability were averaged by 2-hr periods to evaluate the diffusion equation. One average effective source height was used. Sulfur dioxide was assumed to be oxidized or adsorbed on particulates exponentially with a half-life of 4 hr. By averaging twelve 2-hr calculated concentrations, 24-hr concentrations were determined. Maps with lines of equal concentration were drawn from the calculated point concentrations for 35 test periods. From these, concentrations were determined for the locations having observed concentrations. Fifty-eight per cent of all calculated concentrations were within ±1 pphm of the observed concentrations. Excluding zero values of both calculated and observed concentrations, 70 per cent of the calculated values are within a factor of 2 of the observed values. There is a general tendency toward overcalculation (calculated values greater than observed) especially downwind of major sources. Undercalculation which was noted upwind of major sources is probably because calculated concentrations at the center of a square-mile area received no contribution from sources within that area but only from other areas. Results show that 24-hr concentrations and their areal extent in urban areas from multiple sources may be estimated for a variety of meteorological conditions using the source inventory—diffusion approach.

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D. S. Turner

Abstract

Radiative transfer models used in processing TIROS-N sounder data via Physical retrieval schemes do not explicitly account for changes in CO2 mixing ratio. However, CO2 mixing ratios have increased by approximately 30 ppmv over the past 17 years. A simulation study has been conducted using a line-by-line radiative transfer model to study the effect of increasing CO2 on the seven HIRS 15-µm channels used for temperature sounding. The mutts show that the brightness temperature differences can he as large as 1 K for a 30-ppmv CO2 increase and that a seasonal variation of a few tenths of a kelvin may exist. These deviations are of the same order as the standard deviations of brightness temperature expected from errors in a 6-h numerical weather prediction forecast in a data assimilation cycle.

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D. D. Turner and R. G. Ellingson
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Ulrich Löhnert, D. D. Turner, and S. Crewell

Abstract

Two independent ground-based passive remote sensing methods are used to retrieve lower-tropospheric temperature and humidity profiles in clear-sky cases. A simulation study for two distinctly different climatic zones is performed to evaluate the accuracies of a standard microwave profiler [humidity and temperature profiler (HATPRO)] and an infrared spectrometer [Atmospheric Emitted Radiance Interferometer (AERI)] by applying a unified optimal estimation scheme to each instrument. Different measurement modes for each instrument are also evaluated in which the retrieval uses different spectral channels and observational view angles. In addition, both instruments have been combined into the same physically consistent retrieval scheme to evaluate the differences between a combined retrieval relative to the single-instrument retrievals. In general, retrievals derived from only infrared measurements yield superior RMS error and bias to retrievals derived only from microwave measurements. The AERI retrievals show high potential, especially for retrieving humidity in the boundary layer, where accuracies are on the order of 0.25–0.5 g m−3 for a central European climate. In the lowest 500 m the retrieval accuracies for temperature from elevation-scanning microwave measurements and spectral infrared measurements are very similar (0.2–0.6 K). Above this level the accuracies of the AERI retrieval are significantly more accurate (<1 K RMSE below 4 km). The inclusion of microwave measurements to the spectral infrared measurements within a unified physical retrieval scheme only results in improvements in the high-humidity tropical climate. However, relative to the HATPRO retrieval, the accuracy of the AERI retrieval is more sensitive to changes in the measurement uncertainty. The discussed results are drawn from a subset of “pristine” clear-sky cases: in the general case in which clouds and aerosols are present, the combined HATPRO–AERI retrieval algorithm is expected to yield much more beneficial results.

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D. D. Turner and E. J. Mlawer

Accurately accounting for radiative energy balance between the incoming solar and the outgoing infrared radiative fluxes is very important in modeling the Earth's climate. Water vapor absorption plays a critical role in the radiative heating rate profile in the midtroposphere by strongly absorbing both infrared and solar radiation in several absorption bands throughout the electromagnetic spectrum. One of the most important of these absorption bands is in the far-infrared portion of the spectrum, where the far-infrared is defined here to be wavelengths longer than 15 microns. A large fraction (~40%) of the outgoing infrared flux is emitted by water vapor in the far-infrared. Errors in the radiative transfer models associated with the far-infrared and other strong water vapor absorption bands will therefore affect the calculation of the planet's total outgoing radiative flux and its vertical distribution of the radiant energy; these errors may result in inaccurate modeling of the general circulation of the planet.

A set of field experiments, called the Radiative Heating in Underexplored Bands Campaigns (RHUBC), has been conducted as part of the Atmospheric Radiation Measurement (ARM) program. The RHUBC campaigns deployed spectrally resolved far-infrared spectrometers alongside other ARM observations in extremely dry environments to provide a robust and complete dataset that allows radiative transfer models to be evaluated in the far-infrared and other spectral regions where water vapor absorbs strongly. RHUBC I was conducted in February–March 2007 in Barrow, Alaska, and RHUBC II was conducted in August–October 2009 in the Atacama Desert region of Chile at an altitude of 5.3 km. The motivation for and initial results from these experiments are described, as well as the implications for global climate models.

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