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STEPHEN K. COX and STEFAN L. HASTENRATH

Abstract

During the Line Islands Experiment in spring 1967, surface shortwave and net radiation was continuously recorded at Palmyra, and Snomi-Kuhn infrared radiationsondes were released daily at the islands of Palmyra, (5°53′ N., 162°05′ W.) and Christmas (1°59′ N., 157°22′ W.) as part of an extensive surface and upper air observation program. Data are evaluated in terms of the diurnal march of the surface radiation balance and the radiation budget characteristics of the troposphere-ocean system. These direct measurements indicate a substantially larger surface net radiation than is expected from available climatic mean charts based on empirical formulas. Implications for the tropical heat budget are pointed out.

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Thomas B. McKee and Stephen K. Cox

Abstract

A theoretical model of the scattering of shortwave radiation is applied to clouds finite in horizontal extent. The resulting irradiance patterns are then compared with calculations for horizontally semi-infinite clouds. This analysis shows, that the irradiance fields are dramatically dependent upon energy passing through the vertical sides of the finite sized clouds.

Directional reflectance of individual cubic clouds is shown to be approximately 25% less than for semi-infinite clouds of optical depths ranging from 20 to 80. Directional reflectance from the top of cubic clouds for small solar zenith angle continues to increase at large optical depths (∼70) while the infinite cloud becomes nearly asymptotic at this point. It is shown that for a solar zenith angle of 60°, the directional reflectance for a 2/10 sky cover of cubic clouds is 0.29 while for 2/10 coverage of semi-infinite cloud the directional reflectance is 0.185.

Implications of differences between the cubic cloud results and the semi-infinite cloud case are discussed. These implications include: the effect on calculated planetary albedo; a possible explanation for reported correlations of cloud brightness, cloud height and precipitation; and effects on the surface energy budget.

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Thomas B. McKee and Stephen K. Cox

Abstract

Stlratospheric temperature profiles have been inferred from limb radiance data. The limb radiance observations were made from an Aerobee rocket flown on 7 February 1970 from White Sands Missile Range, N.M. The inferred temperatures were similar to those available from rocket sounding data. Horizontal variations of temperature at the 10-mb surface agreed with radiosonde data; however, the inferred temperatures were systematically warmer at 10 mb with larger increases of temperature from 30 to 10 nab than comparative radiosonde data. Two of the inferred temperature profiles provide an excellent example in which two measurements are sufficiently alike to separate real vertical temperature structure from effects of random radiance noise. Vertical resolution was about 2 km and the largest lapse noted was near dry adiabatic.

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Steven A. Ackerman and Stephen K. Cox

Abstract

Obsemations of temperature moisture, cloud amount, cloud height and soil-derived aerosols are incorporated into radiative transfer models to yield estimates of the tropospheric and surface radiative energy budgets for the summer Monsoon of 1979. Results are presented for six phases of the monsoon for the region 30°S to 40°N latitude and 30°E to 100°E longitude. The derived radiative fields are significantly different from climatological estimates. The evolution of the radiative energy budgets are discussed in relation to monsoon activity. Total tropospheric convergence (TTC) for the January and February phases exhibits a minimum cooling over the southern Indian Ocean and a maximum tropospheric radiative energy loss over the Arabian Sea and Bay of Bengal. The early May, pre-onset, onset and post-onset periods exhibit cellular patterns in TTC, with maximum cooling over the cloud-free oceanic regions, and minimum cooling associated with continental regions and areas with large amounts of cloud. This cellular structure is still evident when TTC is averaged over 10° regions. Large seasonal variations in TTC are observed over the deserts, due to the presence of dust in the summer. Regions with large seasonal variations in cloud cover (e.g., the Arabian Sea) also display large variations in TTC. Regionally averaged radiative heating profiles also change significantly with period. These variations result primarily from changes in the cloud distribution associated with the evolution of the monsoon.

The net surface radiative flux varies markedly from period to period, and within the same period. As expected, all six periods have a maximum surface radiative energy gain for the cloud-free oceanic regions, while cloudy and continental regions tend to have relative minimae. Large spatial and temporal variations exist in the net surface flux.

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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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David O'C. Starr and Stephen K. Cox

Abstract

The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (∼20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable environmental conditions. Five times more cloud water is maintained in the altostratus case where the updraft wind speed are greater by a factor of 10. Ale role of the large-wale ascent or descent is also examined. Inferences are drawn from these results with respect to the parameterization of cirrus in large-scale forecast or climate models.

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Stephen K. Cox and Keith T. Griffith

Abstract

The methodology used to generate the GATE A/B-scale radiative divergence budget estimates is described. The technique consists of compositing radiative divergence profiles on the basis of synoptic observations and satellite and radar inferred cloud structure. Cloud-top distributions were generated from SMS infrared brightness data observed hourly over the entire GATE A/B array. Cloud-clear thresholds were determined using concurrent visible brightness and infrared brightness observations. Corrections to cloud-top height were made for water vapor extinction and for non-blackness of clouds. Cloud-base distributions were inferred statistically from a limited sample of concurrent radar and satellite data. Temperature and moisture distributions from 6 h synoptic analyses of rawinsonde data were used in the radiative divergence computations.

The computational algorithms used to generate the shortwave and longwave radiation divergence estimates were compared with aircraft observations; average cloud properties of upper level clouds inferred from the observations were used in the computational algorithms. The area-average radiative divergence profiles were then generated by computational algorithms.

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Stephen K. Cox and Keith T. Griffith

Abstract

The GATE Phase III radiative divergence profiles generally show less upper tropospheric radiative divergence and more middle level divergence than previous cilimatological estimates suggest. These differences are due primarily to the extensive middle and upper tropospheric cloudiness in the GATE area, the large mean values of total precipitable water vapor (∼5.1 cm), and the inclusion in the present study of the effects of the water vapor pressure broadened continuum.

Averages for the 6 h local time periods 0000–0600, 0600–1200, 120–1800 and 1800–2400 show all layers of the GATE Phase III B-scale atmosphere experience a net radiative loss of energy. However, actual radiative heating of some layers is evident near midday. For a convectively suppressed composite case all levels above 700 mb show heating for the 1000–1400 LST period. The total troposphere shows a net radiative gain for the same 6 h interval (0900–1500 LST). For the enhanced convection case absolute warming is generally confined to the 100–400 mb layer and the 0800–1600 LST time interval with no net heating of the entire troposphere occurring during the day. The diurnal variability of the horizontal gradients in the radiative divergence fields appears adequate to explain at least some of the diurnal variations in cloud cover and precipitation suggested by other authors.

The daytime tropospheric total radiative divergence is remarkably stable for all observed cloud-top distributions during Phase III over the A/B- and B-scale arrays. This characteristic constancy of the daytime total tropospheric divergence (TTD) values is a potentially useful tool in the inference of maritime tropical surface energy budgets from satellite data.

Average TTD values computed over various tune and space scales are examined. It is shown that for an area the size of the B-scale array the 6 day averages do not vary more than 5 W m−2 (912 mb)−1

Cross sections of the Phase III mean and the disturbed composite radiative divergence values for the A/B-scale array suggest a north–south radiative forcing caused by east–west oriented cloud bands centered around 8–9°N latitude. Coupled with the analysis of the diurnal radiative effect of clouds and adjacent clear areas, this suggests the possibility of a diurnal radiative forcing on the basic Hadley circulation.

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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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Steven A. Ackerman and Stephen K. Cox

Abstract

Horizontal cloud coverages derived from a geostationary satellite and all-sky cameras were compared for a 3-month period of the GARP Atlantic Tropical Experiment (GATE). Estimates of total cloud cover using the satellite and all-sky camera were similar for the daytime period. The all-sky cameras also gave reasonable estimates of the 24 h cloud cover due to the small difference in the satellite determined daytime and nighttime total cloud cover in the vicinity of the all-sky cameras. However, other regions in the area of study which were not covered by an all-sky camera revealed large diurnal variations. In these areas the daytime total cloud amount did not yield an accurate representation of the 24 h cloud cover.

A method is presented which enables one to construct a three-dimensional representation of cloud structure by combining surface and satellite observations. The disadvantages of this technique are that it assumes no overlapping cloud tops or cloud bases, as well as the limitations of the satellite and all-sky camera in estimating cloud cover.

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