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John H. E. Clark and Thomas G. Rogers

Abstract

An analytical model for the horizontal and vertical transport of a conservative trace gas by a stationary planetary wave is developed. The wave is associated with a constant background zonal flow. In the absence of dissipation, transports are zero if the wave is trapped, i.e., the background flow is easterly or strong westerly. Propagating waves can carry out appreciable transports. The vertical transport is proportional to the latitudinal gradient of the background mixing ratio and the horizontal transport is proportional to the vertical gradient.

With dissipation in the form of Newtonian cooling, transports are permitted by trapped waves. For strong westerlies, they are very small but for easterlies they can be appreciable. In fact, substantial southward ozone transports in the summertime lower stratosphere are suggested.

The simplicity of the flux expressions suggests that they might be exploited in two-dimensional models of ozone.

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Reginald E. Newell and Thomas G. Dopplick

Abstract

Estimates of the atmospheric temperature changes due to a doubling of CO2 concentrations have been with a static radiative flux model. They yield temperature changes >0.25 K. It appears that the much larger changes predicted by other models arise from additional water vapor evaporated into the atmosphere and not from the CO2 itself.

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Reginald E. Newell and Thomas G. Dopplick

Abstract

Estimates of the atmospheric temperature changes due to a doubling of CO2 concentrations have been with a static radiative flux model. They yield temperature changes >0.25 K. It appears that the much larger changes predicted by other models arise from additional water vapor evaporated into the atmosphere and not from the CO2 itself.

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Reginald E. Newell and Thomas G. Dopplick

Abstract

No abstract available.

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Gerald G. Mace, Eugene E. Clothiaux, and Thomas P. Ackerman

Abstract

The properties of midlatitude cirrus clouds are examined using one year of continuous vertically pointing millimeter-wave cloud radar data collected at the Atmospheric Radiation Measurement Program Southern Great Plains site in Oklahoma. The goal of this analysis is to present the cloud characteristics in a manner that will aid in the evaluation and improvement of cirrus parameterizations in large-scale models. Using a temperature- and radar reflectivity–based definition of cirrus, the occurrence frequency of cirrus, the vertical location and thickness of cirrus layers, and other fundamental statistics are examined. Also the bulk microphysical properties of optically thin cirrus layers that occur in isolation from other cloud layers are examined. During 1997, it is found that cirrus were present 22% of the time, had a mean layer thickness of 2.0 km, and were most likely to occur in the 8.5–10-km height range. On average, the cirrus clouds tended to be found in layers in which the synoptic-scale vertical velocity was weakly ascending. The mean synoptic-scale vertical motion in the upper troposphere as derived from Rapid Update Cycle model output was +0.2 cm s−1. However, a significant fraction of the layers (33%) were found where the upper-tropospheric large-scale vertical velocity was clearly descending (w < −1.5 cm s−1). Microphysical properties were computed for that subset of cirrus events that were optically thin (infrared emissivity < 0.85) and occurred with no lower cloud layers. This subset of cirrus had mean values of ice water path, effective radius, and ice crystal concentration of 8 g m−2, 35 μm, and 100 L−1, respectively. Although all the cloud properties demonstrated a high degree of variability during the period considered, the statistics of these properties were fairly steady throughout the annual cycle. Consistent with previous studies, it is found that the cloud microphysical properties appear to be strongly correlated to the cloud layer thickness and mean temperature. Use of these results for parameterization of cirrus properties in large-scale models is discussed.

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T. G. Owe Berg, Joyce E. Clutter, Thomas A. Gaukler, and Robert L. McDonald

Abstract

Experiments were conducted with sublimate AgI or AgI smoke in an expansion chamber at various temperatures. A condensate cloud of liquid droplets was formed at all temperatures. Below −5C ice crystals appeared in the cloud a few seconds after its formation. Photographs were taken of liquid droplets and of partially frozen droplets. The droplet diameter was 7 μ above 0C, but at lower temperatures it was larger, reaching 15 μ at −7C. The results show that the water vapor is converted to ice by condensation followed by freezing of the liquid droplets under these conditions. Impurities in the AgI and the air had no effect upon these observations.

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Richard E. Brandt, Stephen G. Warren, Anthony P. Worby, and Thomas C. Grenfell

Abstract

In three ship-based field experiments, spectral albedos were measured at ultraviolet, visible, and near-infrared wavelengths for open water, grease ice, nilas, young “grey” ice, young grey-white ice, and first-year ice, both with and without snow cover. From the spectral measurements, broadband albedos are computed for clear and cloudy sky, for the total solar spectrum as well as for visible and near-infrared bands used in climate models, and for Advanced Very High Resolution Radiometer (AVHRR) solar channels. The all-wave albedos vary from 0.07 for open water to 0.87 for thick snow-covered ice under cloud. The frequency distribution of ice types and snow coverage in all seasons is available from the project on Antarctic Sea Ice Processes and Climate (ASPeCt). The ASPeCt dataset contains routine hourly visual observations of sea ice from research and supply ships of several nations using a standard protocol. Ten thousand of these observations, separated by a minimum of 6 nautical miles along voyage tracks, are used together with the measured albedos for each ice type to assign an albedo to each visual observation, resulting in “ice-only” albedos as a function of latitude for each of five longitudinal sectors around Antarctica, for each of the four seasons. These ice albedos are combined with 13 yr of ice concentration estimates from satellite passive microwave measurements to obtain the geographical and seasonal variation of average surface albedo. Most of the Antarctic sea ice is snow covered, even in summer, so the main determinant of area-averaged albedo is the fraction of open water within the pack.

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Rym Msadek, William E. Johns, Stephen G. Yeager, Gokhan Danabasoglu, Thomas L. Delworth, and Anthony Rosati

Abstract

The link at 26.5°N between the Atlantic meridional heat transport (MHT) and the Atlantic meridional overturning circulation (MOC) is investigated in two climate models, the GFDL Climate Model version 2.1 (CM2.1) and the NCAR Community Climate System Model version 4 (CCSM4), and compared with the recent observational estimates from the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) array. Despite a stronger-than-observed MOC magnitude, both models underestimate the mean MHT at 26.5°N because of an overly diffuse thermocline. Biases result from errors in both overturning and gyre components of the MHT. The observed linear relationship between MHT and MOC at 26.5°N is realistically simulated by the two models and is mainly due to the overturning component of the MHT. Fluctuations in overturning MHT are dominated by Ekman transport variability in CM2.1 and CCSM4, whereas baroclinic geostrophic transport variability plays a larger role in RAPID. CCSM4, which has a parameterization of Nordic Sea overflows and thus a more realistic North Atlantic Deep Water (NADW) penetration, shows smaller biases in the overturning heat transport than CM2.1 owing to deeper NADW at colder temperatures. The horizontal gyre heat transport and its sensitivity to the MOC are poorly represented in both models. The wind-driven gyre heat transport is northward in observations at 26.5°N, whereas it is weakly southward in both models, reducing the total MHT. This study emphasizes model biases that are responsible for the too-weak MHT, particularly at the western boundary. The use of direct MHT observations through RAPID allows for identification of the source of the too-weak MHT in the two models, a bias shared by a number of Coupled Model Intercomparison Project phase 5 (CMIP5) coupled models.

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David A. Rutan, Seiji Kato, David R. Doelling, Fred G. Rose, Le Trang Nguyen, Thomas E. Caldwell, and Norman G. Loeb

Abstract

The Clouds and the Earth’s Radiant Energy System Synoptic (SYN1deg), edition 3, product provides climate-quality global 3-hourly 1° × 1°gridded top of atmosphere, in-atmosphere, and surface radiant fluxes. The in-atmosphere surface fluxes are computed hourly using a radiative transfer code based upon inputs from Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), 3-hourly geostationary (GEO) data, and meteorological assimilation data from the Goddard Earth Observing System. The GEO visible and infrared imager calibration is tied to MODIS to ensure uniform MODIS-like cloud properties across all satellite cloud datasets. Computed surface radiant fluxes are compared to surface observations at 85 globally distributed land (37) and ocean buoy (48) sites as well as several other publicly available global surface radiant flux data products. Computed monthly mean downward fluxes from SYN1deg have a bias (standard deviation) of 3.0 W m−2 (5.7%) for shortwave and −4.0 W m−2 (2.9%) for longwave compared to surface observations. The standard deviation between surface downward shortwave flux calculations and observations at the 3-hourly time scale is reduced when the diurnal cycle of cloud changes is explicitly accounted for. The improvement is smaller for surface downward longwave flux owing to an additional sensitivity to boundary layer temperature/humidity, which has a weaker diurnal cycle compared to clouds.

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C. A. Barth, R. W. Sanders, G. E. Thomas, G. J. Rottman, D. W. Rusch, R. J. Thomas, G. H. Mount, G. M. Lawrence, J. M. Zawodny, R. A. West, and J. London
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