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STEPHEN K. COX

Abstract

Balloon-borne radiation sonde measurements during 1964 and 1965 are used to form composite, three-dimensional radiative cooling models for the following midlatitude synoptic features: stationary front; nascent cyclone; warm sector cyclone; occluded cyclone; and anticyclone. Composite water vapor distributions for the same synoptic features are used to model the pattern of atmospheric warming by solar radiation.

Thickness tendency analyses of the 1000-500-mb layer for four synoptic features show that radiative cooling and warming may account for 10–30 percent of the observed maximum thickness tendency. The radiative thickness change components are of the same order of magnitude as the latent and the sensible heating terms.

The nascent cyclone case shows a radiatively induced vorticity tendency of 6 × 10−10 sec−2. This compares with a total expected vorticity tendency between 10−9 and 10−10 sec−2. The nascent cyclone, warm sector cyclone, and anticyclone cases show positive cyclonic development from radiative effects, while the occluded cyclone case shows negative cyclonic development.

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Gregory P. Byrd and Stephen K. Cox

Abstract

Tropospheric radiative convergence profiles from Cox and Griffith are used to assess the radiative forcing upon a tropical cloud cluster located in the vicinity of the GATE A/B-scale array during 4–6 September 1974. A background discussion summarizes some of the previous investigations that served as motivation for the present study. The atmospheric response to differential radiative cooling between the cluster and its surrounding environment is examined by means of “slab” and cross section analyses over the Cox-Griffith array. A radiatively derived vertical motion model is constructed to investigate the role of radiation with respect to larger-scale dynamics during a daytime (0600–1200 LST 5 September) and nighttime (1800–2400 LST 5 September) period of the cluster life cycle.

Radiative forcing is found to be strongest during the initial stages of cluster development. Throughout the cluster life cycle, the radiative forcing is consistently strongest in the middle troposphere (400–700 mb). As the cluster system intensifies, daytime shortwave warming superimposed upon the longwave cooling lessens the total radiative cooling in the surrounding cloud-free region, resulting in a lessening of the differential radiative cooling. Increased amounts of middle and high cloud remnants also contribute to the observed weakening of radiative forcing during the mature and dissipating disturbance stages. Cross section analyses reveal that E-W gradients of radiative convergence between the cluster and its surroundings are comparable in magnitude to the N-S gradients.

The radiatively derived vertical motion model yields a qualitatively realistic total area of cluster influence for a nighttime case, 1800–2400 GMT on 5 September. The model assumption of a closed mass system breaks down during the daytime (0600–1200 LST, 5 September) period, yielding an unrealistically 1arge total area of cluster influence. This suggests the occurrence of significant cluster-scale interactions with large-scale circulations during the daytime period. Radiative forcing appears to play a more significant role in dynamical interactions during the nighttime period, when circulations seem to be somewhat more localized.

The maximum in-cluster precipitation intensity lags the incidence of strong radiative forcing by 6–8 h, in general agreement with GATE composite observations. Continental oceanic differential beating must also play a significant role in modulating cluster- and large-scale dynamical interactions, accounting for the anomalously long precipitation lag observable in the GATE cluster. The interpretations presented herein are based solely upon this single case study and may not necessarily be representative of cluster disturbances as a whole.

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Steven A. Ackerman and Stephen K. Cox

Abstract

Tropospheric radiative convergence profiles are derived for an easterly wave composite during Phase III of the GATE. The easterly waves observed during this period were generally well developed. The profiles also represent the magnitudes and the spatial distribution of atmospheric radiative convergence of the Intertropical Convergence Zone in the GATE area. The 12 h mean daytime and nighttime profiles are presented. Cloud-top pressure distributions as a function of wave position are also presented.

The results of this research indicate three possible radiative induced mechanisms which contribute to the observed diurnal cycle in large-scale mass convergence: 1) radiative convergence differences between the ITCZ and the surrounding regions; 2) mesoscale radiative convergence differences between clear and cluster regions, and 3) a nighttime upper level tropospheric cooling maximum that is centered one-half a wavelength from the region of maximum convective activity.

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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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William L. Smith Jr., Paul F. Hein, and Stephen K. Cox

Abstract

On 28 October 1986 the NCAR Sabreliner observed a cirrus cloud layer in the vicinity of Green Bay, Wisconsin. A portion of each flight leg was conducted over western Lake Michigan and over the adjacent western shore. The cirrus layer would be qualitatively described as optically thin and tenuous, yet broadband infrared effective emittances were found between about 0.4 and 0.6 while broadband shortwave extinction values ranged from as low as 5% to 32%. This investigation examines the bulk radiative properties of the cirrus layer and the horizontal variability of these radiative properties. In addition, the microphysical characteristics and the dynamic properties of the layer are presented and analyzed. The broadband infrared volume absorption coefficients were deduced for the cirrus layer and found to be very similar in terms of a dependence on temperature to results recently presented by other authors. Infrared radiative heating rates were calculated and found to be typical of the optically thin cirrus layer examined here. The horizontal structures of the radiative properties of the cirrus cloud layer and the vertical velocity observations were very similar. Both showed a smaller scale variation at the top of the cirrus layer which merged into larger scale common elements near the bases of the layer. Power spectra analyses of along-wind and cross-wind components near the base of the clouds sampled exhibited a steep spectral slope of k −3 at the smaller wave numbers (scalelengths greater than 1 km). This k −3 slope is characteristic of two-dimensional eddies. The same k −3 slope is present in the power spectra of the radiative properties. It is probable that these radiative properties, which are modulated by the cloud elements, have their scales determined by the eddies detected in the analysis of wind components.

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