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John M. Davis and Stephen K. Cox

Abstract

Analyses of bidirectional reflectance data are presented with implications regarding the spatial scales appropriate for inferring irradiances from radiances reflected by various surface–atmosphere scenes. Multiple-angle radiance data collected in a nearly simultaneous manner during the 1979 Summer Monsoon Experiment are analyzed using the squared coherency statistic to suggest a method to deduce the minimum spatial scale appropriate for irradiance inferences. Spatial convergence of the irradiances inferred from the component radiances is presented as a function of averaging distance to imply magnitudes of errors that may result from use of“similar scene” bidirectional reflectance models. The reduction in the inference errors with an increasing number of angular viewing positions is also presented. The data are analyzed in search of preferred viewing directions with the result that little improvement is imparted to the inference by viewing the scenes from any specific view direction.

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John M. Davis and Stephen K. Cox

Abstract

A set of bi-directional reflectance models is presented for various atmospheric scene types. The models were composited from data collected from an aircraft platform in May-July 1979 during Summer MONEX. The space scale of the composited models is generally from 250 to 1000 km, which corresponds to the scale of interest in climate monitoring and modeling. Composite models for the following scene types are presented: the desert sands of the Saudi Arabian Empty Quarters, the Himalayan mountains, the Arabian Sea with the ever-present fair weather cumulus cloudiness, the semi-arid agricultural land surface of the Indian subcontinent under pre-monsoon conditions, broken middle and low level clouds over ocean, an altostratus cloud deck, and the broken pack-ice fields of Hudson Bay. Nearly all the models display a degree of anisotropy such that serious errors (10–100%) would result in the reflected flux density isotropically inferred from some of the reflected radiances. The features of many of the models are discussed, and all of the models are tabulated in the Appendix. One of the models for altostratus is explicitly compared with theory, and differences between the altostratus and broken cloud models agree with the differences between infinite and finite cloud theory. The models are also compared with models from previous studies. The agreement is generally good (∼100%rms) except in a few cases in which the disagreement may have resulted from natural scene variability or differences between the methods of data collection.

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John M. Davis and Stephen K. Cox

Abstract

The results of a laboratory experiment are presented that provide additional verification of the methodology adapted for simulation of the radiances reflected from fields of optically thick clouds using the Cloud Field Optical Simulator (CFOS) at Colorado State University. The comparison of these data with their theoretically derived counterparts indicates that the crucial mechanism of cloud-to-cloud radiance field interaction is accurately simulated in the CFOS experiments and adds confidence to the manner in which the optical depth is scaled.

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Ronald M. Welch, Stephen K. Cox, and John M. Davis

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No Abstract available.

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John M. Davis, Steven K. Cox, and Thomas B. McKee

Abstract

A model which includes the effects of water vapor and droplet absorption in finite cloud radiative transfer calculations is described. The effect of absorption on the directional reflectance values of finite clouds and infinite clouds of equal optical thickness is examined in the 0.8–8.0 μm portion of the solar spectrum. Absorption of solar radiation in finite clouds is compared to absorption in equal volume elements of horizontally infinite clouds. The 0.8–8.0 μm values of directional reflectance and absorption are combined with previous research, in order to examine the impact of finite cloud radiative characteristics on the total spectral (0.3–0.8 μm), space-time averaged radiative budget of a region partially covered by finite clouds. The added contribution of the vertical cloud dimension of the finite cloud in this type of calculation may be expressed as an “effective cloud cover,” which is greater than the geometric cloud cover by the factor (1+tanθ), where θ is the solar zenith angle. Additionally, it is shown from purely geometrical considerations that the vertical extent of finite clouds, through the exaggeration of cloud-cover estimates obtained from satellite imagery, may have a significant impact on the calculated radiative budget of a region partially covered by finite clouds.

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Gordon H. Beck, John M. Davis, and S. K. Cox

Abstract

Beam transmittance, emittance, reflectance, and outgoing radiance are inferred from interferometric measurements in the infrared window region for 14 temperate continental and 12 subtropical cirrus cloud cash observed during FIRE II at Parsons, Kansas (37°18′N, 95°07′W), and the ASTEX at Porto Santo, Madeira (33°5′N, 16°21′W). Cirrus emittances were found to span nearly the entire range from 0 to 1 for cloud systems in each location. Spectrally averaged volume extinction coefficients of 0.19 and 0.62 km−1 were found for the respective continental and subtropical samples. A delta-Eddington routine was incorporated into the inference technique to examine the sensitivity of the inferences to the upwelling surface and subcloud-layer emission reflected by the cloud assuming spherical and nonspherical cloud particles. Including reflectance had only a small effect on the spectrally averaged values of the radiative parameters; however, the slope of outgoing longwave radiation across the window region was altered with the introduction of smaller particles. The iterative method is structured in a manner that does not constrain the transmittances of the clear atmosphere to line-by-line model results. Inferred emittances and extinction coefficients are compared to previously published results.

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John M. Davis, Stephen K. Cox, and Thomas B. McKee

Abstract

A band absorption model is used in conjunction with a Monte Carlo scattering model to calculate the amount of solar radiation absorbed above, below, within and adjacent to cubic, finite clouds. Horizontally and vertically nonhomogeneous values of absorption within the finite cloud range from 0.16 to 6.4 times the corresponding values in horizontally infinite clouds of the same optical thickness, which were calculated using the same model. Absorption values in the regions adjoining the finite cloud on the solar and anti-solar sides, converge to clear sky values within a distance of two cloud dimensions from the side walls of the cloud.

Absorption below the finite cloud ranges from 1.4 to 4.5 times that below the infinite cloud volume element. Values of absorption above the two cloud types are nearly identical when normalized to the cross-sectional area of the incident beam. If the absorption in the atmospheric column containing the finite cloud is normalized with respect to the horizontal area of the parallel radiation incident on the top plus the side of the cloud, the resulting value is within 3% of the absorption value in the column containing an element of infinite cloud. Thus, infinite cloud total column absorption values may be used to compute areal averages of absorption for a region partially covered by widely separated finite clouds, whose height to width ratio is near unity, if the fractional cloud cover is adjusted in the appropriate manner.

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Jonathan M. Lilly, Peter B. Rhines, Martin Visbeck, Russ Davis, John R. N. Lazier, Friedrich Schott, and David Farmer

Abstract

A 12-month mooring record (May 1994–June 1995), together with accompanying PALACE float data, is used to describe an annual cycle of deep convection and restratification in the Labrador Sea. The mooring is located at 56.75°N, 52.5°W, near the former site of Ocean Weather Station Bravo, in water of ∼3500 m depth. This is a pilot experiment for climate monitoring, and also for studies of deep-convection dynamics. Mooring measurements include temperature (T), salinity (S), horizontal and vertical velocity, and acoustic measurement of surface winds. The floats made weekly temperature–salinity profiles between their drift level (near 1500 m) and the surface.

With moderately strong cooling to the atmosphere (∼300 W m−2 averaged from November to March), wintertime convection penetrated from the surface to about 1750 m, overcoming the stabilizing effect of upper-ocean low-salinity water. The water column restratifies rapidly after brief vertical homogenization (in potential density, salinity, and potential temperature). Both the rapid restratification and the energetic high-frequency variations of T and S observed at the mooring are suggestive of a convection depth that varies greatly with location. Lateral variations in T and S exist down to very small scales, and these remnants of convection decay (with e-folding time ∼170 day) after convection ceases. Lateral variability at the scale of 100 km is verified by PALACE profiles. The Eulerian mooring effectively samples the convection in a mesoscale region of ocean as eddies sweep past it; the Lagrangian PALACE floats are complementary in sampling the geography of deep convection more widely. This laterally variable convection leaves the water column with significant vertical gradients most of the year. Convection followed by lateral mixing gives vertical salinity profiles the (misleading) appearance that a one-dimensional diffusive process is fluxing freshwater downward.

During spring, summer, and fall the salinity, temperature, and buoyancy rise steadily with time throughout most of the water column. This is likely the result of mixing with the encircling boundary currents, compensating for the escape of Labrador Sea Water from the region. Low-salinity water mixes into the gyre only near the surface.

The water-column heat balance is in satisfactory agreement with meteorological assimilation models. Directly observed subsurface calorimetry may be the more reliable indication of the annual-mean air–sea heat flux. Acoustic instrumentation on the mooring gave a surprisingly good time series of the vector surface wind.

The three-dimensional velocity field consists of convective plumes of width ∼200 to 1000 m, vertical velocities of 2 to 8 cm s−1, and Rossby numbers of order unity, embedded in stronger (∼20 cm s−1) lateral currents associated with mesoscale eddies. Horizontal currents with timescales of several days to several months are strongly barotropic. They are suddenly energized as convection reaches great depth in early March, and develop toward a barotropic state, as also seen in models of convectively driven geostrophic turbulence in a weakly stratified, high-latitude ocean. Currents decay through the summer and autumn, apart from some persistent isolated eddies. These coherent, isolated, cold anticyclones carry cores of pure convected water long after the end of winter.

Boundary currents nearby interact with the Labrador Sea gyre and provide an additional source of eddies in the interior Labrador Sea. An earlier study of the pulsation of the boundary currents is supported by observations of sudden ejection of floats from the central gyre into the boundary currents (and sudden ingestion of boundary current floats into the gyre interior), in what may be a mechanism for exchange between Labrador Sea Water and the World Ocean.

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Gabriele G. Pfister, Sebastian D. Eastham, Avelino F. Arellano, Bernard Aumont, Kelley C. Barsanti, Mary C. Barth, Andrew Conley, Nicholas A. Davis, Louisa K. Emmons, Jerome D. Fast, Arlene M. Fiore, Benjamin Gaubert, Steve Goldhaber, Claire Granier, Georg A. Grell, Marc Guevara, Daven K. Henze, Alma Hodzic, Xiaohong Liu, Daniel R. Marsh, John J. Orlando, John M. C. Plane, Lorenzo M. Polvani, Karen H. Rosenlof, Allison L. Steiner, Daniel J. Jacob, and Guy P. Brasseur

ABSTRACT

To explore the various couplings across space and time and between ecosystems in a consistent manner, atmospheric modeling is moving away from the fractured limited-scale modeling strategy of the past toward a unification of the range of scales inherent in the Earth system. This paper describes the forward-looking Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICA), which is intended to become the next-generation community infrastructure for research involving atmospheric chemistry and aerosols. MUSICA will be developed collaboratively by the National Center for Atmospheric Research (NCAR) and university and government researchers, with the goal of serving the international research and applications communities. The capability of unifying various spatiotemporal scales, coupling to other Earth system components, and process-level modularization will allow advances in both fundamental and applied research in atmospheric composition, air quality, and climate and is also envisioned to become a platform that addresses the needs of policy makers and stakeholders.

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Clark Evans, Kimberly M. Wood, Sim D. Aberson, Heather M. Archambault, Shawn M. Milrad, Lance F. Bosart, Kristen L. Corbosiero, Christopher A. Davis, João R. Dias Pinto, James Doyle, Chris Fogarty, Thomas J. Galarneau Jr., Christian M. Grams, Kyle S. Griffin, John Gyakum, Robert E. Hart, Naoko Kitabatake, Hilke S. Lentink, Ron McTaggart-Cowan, William Perrie, Julian F. D. Quinting, Carolyn A. Reynolds, Michael Riemer, Elizabeth A. Ritchie, Yujuan Sun, and Fuqing Zhang

Abstract

Extratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. This process is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, modeling the direct impacts of ET-related wind, waves, and precipitation are noted. Improved understanding of structural evolution throughout the transformation stage of ET fostered in large part by novel aircraft observations collected in several recent ET events is highlighted. Predictive skill for operational and numerical model ET-related forecasts is discussed along with environmental factors influencing posttransition cyclone structure and evolution. Operational ET forecast and analysis practices and challenges are detailed. In particular, some challenges of effective hazard communication for the evolving threats posed by a tropical cyclone during and after transition are introduced. This review concludes with recommendations for future work to further improve understanding, forecasts, and hazard communication.

Open access