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William B. Rossow

Abstract

A three-dimensional general circulation model of a slowly rotating, massive atmosphere is forced with an axisymmetric radiative heating/cooling distribution to explore the heat and momentum budgets established in this type of atmosphere. In the model lower atmosphere, the mean meridional circulation, as suggested by Stone (1974), balances the differential radiative heating and maintains a statically stable, quasi-barotropic thermal state. However, the nature of this balance depends crucially on the momentum budget established. Although small-scale convection and eddies also play a role in maintaining the static stability of the lower atmosphere, eddy horizontal heat transport is completely negligible. The meridional circulation takes the form of multiple equator-to-pole cells, one above the other. The correlation of this vertical structure with the vertical distribution of radiative and convective/eddy heating suggests that the net heating vertical distribution produces this multicellular structure. The model results confirm the proposals of Gierasch (1975) and Rossow and Williams (1979) in a fully three-dimensional circulation. The mean meridional circulation, despite its multicellular form, interacts with quasi-barotropic eddies produced by zonal flow shell instability to produce a weak superrotation of the entire model atmosphere. This process is general enough to conclude that it will occur in all slowly rotating atmospheres; but, whether it can accelerate wind speeds as large as those observed on Venus, cannot be determined yet.

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William B. Rossow

Abstract

Using an approximate numerical technique, we investigate the influence of coagulation, sedimentation and turbulent motions on the observed droplet size distribution in the upper layers of the Venus clouds. If the cloud mass mixing ratio is <10−5 at 250 K or the eddy diffusivity throughout the cloud is >106 cm2 s−1, then coagulation is unimportant. In this case, the observed droplet size distribution is the initial size distribution produced by the condensation of the droplets. We find that all cloud models with droplet formation near the cloud top (e.g., a photochemical model) must produce the observed droplet size distribution by condensation without subsequent modification by coagulation. We find, however, that neither meteoritic or surface dust can supply sufficient nucleating particles to account for the observed droplet number density. If, on the other hand, the cloud droplets are formed near the cloud bottom, the observed droplet size distribution can be produced solely by the interaction of coagulation and dynamics; all information about the initial size distribution is lost. The eddy diffusivity is ∼5×105 cm2 s−1. If droplet formation occurs near the cloud bottom, then the lower atmosphere of Venus is oxidizing rather than reducing.

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William B. Rossow

Abstract

A brief review of observations of clouds using satellites highlights open issues and directions for future studies. The key one is improved treatment of the effects of small-scale spatial inhomogeneity in remote sensing data analyses and in the treatment of radiation in climate models, though studies and observations of the spectral dependence of cloud-radiation interactions are also limited. Significant progress in understanding the role of clouds in climate, especially regarding cloud-radiation budget relationships, is expected in the next several years because of an unprecedented suite of global and regional observation and analysis programs.

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Elaine Matthews and William B. Rossow

Abstract

A global series of seasonal visible surface reflectance maps derived from NOAA-5 Scanning Radiometer observations is presented. Methods for isolating clear-sky observations from satellite data are evaluated and the magnitude of atmospheric effects (Rayleigh scattering and ozone absorption) are presented. A preliminary analysis of digital vegetation and soils data bases which were analyzed in conjunction with the satellite observations, is discussed. Regional and global reflectance homogeneity of land-cover types, and snow brightening for types, are presented. Results demonstrate that the statistical approach for isolating clear-sky radiances used in this study obtains accurate enough values for each location to allow meaningful measurements of seasonal, spatial and ecosystem variations in surface reflectance.

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David Rind and William B. Rossow

Abstract

The Hadley cell is involved in the energy, momentum and moisture budgets in the atmosphere; it may be expected to change as sources and sinks of these quantities are altered due to climate perturbations. The nature of the Hadley cell change is complicated since alterations in one budget generally result in alterations in the others. Thus, Hadley cell sensitivity needs to be explored in an interactive system. In the GISS GCM (model I), a number of experiments are performed in which physical processes in each of the three budgets are omitted, the system adjusts, and the resultant circulation is compared to that of the control run. This procedure highlights which effects are most important and reveals the nature of the various interactions.

The results emphasize the wide variety of processes that appear capable of influencing the mean circulation. The intensity of the circulation is related to the coherence of the thermal forcing, and to the thermal opacity of the atmosphere. When all frictional forcing is removed, the circulation is restricted to the equatorial region. The latitudinal extent appears to be controlled primarily by eddy processes (Ferrel cell intensity). The implications for climate modeling and climate projections (e.g., rainfall changes) are discussed.

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Zhengzhao Luo and William B. Rossow

Abstract

Tropical cirrus evolution and its relation to upper-tropospheric water vapor (UTWV) are examined in the paper by analyzing satellite-derived cloud data, UTWV data from infrared and microwave measurements, and the NCEP–NCAR reanalysis wind field. Building upon the existing International Satellite Cloud Climatology Project (ISCCP) data and the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) product, a global (except polar region), 6-hourly cirrus dataset is developed from two infrared radiance measurements at 11 and 12 μm. The UTWV is obtained in both clear and cloudy locations by developing a combined satellite infrared and microwave-based retrieval. The analysis in this study is conducted in a Lagrangian framework. The Lagrangian trajectory analysis shows that the decay of deep convection is immediately followed by the growth of cirrostratus and cirrus, and then the decay of cirrostratus is followed by the continued growth of cirrus. Cirrus properties continuously evolve along the trajectories as they gradually thin out and move to the lower levels. Typical tropical cirrus systems last for 19–30 ± 16 h. This is much longer than cirrus particle lifetimes, suggesting that other processes (e.g., large-scale lifting) replenish the particles to maintain tropical cirrus. Consequently, tropical cirrus can advect over large distances, about 600–1000 km, during their lifetimes. For almost all current GCMs, this distance spans more than one grid box, requiring that the water vapor and cloud water budgets include an advection term. Based on their relationship to convective systems, detrainment cirrus are distinguished from in situ cirrus. It is found that more than half of the tropical cirrus are formed in situ well away from convection. The interaction between cirrus and UTWV is explored by comparing the evolution of the UTWV along composite clear trajectories and trajectories with cirrus. Cirrus are found to be associated with a moister upper troposphere and a slower rate of decrease of UTWV. Moreover, the elevated UTWV has a longer duration than cirrus. The amount of water in cirrus is too small for evaporation of cirrus ice particles to moisten the upper troposphere significantly (but cirrus may be an important water vapor sink). Rather, it is likely that the same transient motions that produce the cirrus also transport water vapor upward to maintain a larger UTWV.

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William B. Rossow and Carl Sagan

Abstract

The dielectric properties of H2O and H2SO4 at microwave frequencies have been calculated from the Debye equations. The derived frequency and temperature dependence agrees wed with existing data. The dielectric properties of H2O/H2SO4 mixtures are deduced and, for a well-mixed atmosphere, the structure of H2O and H2O/H2SO4 clouds is calculated. With the COSPAR model atmosphere and the calculated cloud models, the microwave properties of the atmosphere and clouds are determined. The 3.8 cm radar reflectivity of the planet, the Mariner 5 S-band occultation profile, and the passive microwave emission spectrum of the planet together set an upper limit on the mixing ratio by number of H2O of ∼102 in the lower Venus atmosphere, and of H2SO4 of ∼10−5. The polarization value of the real part of the refractive index of the clouds, the spectroscope limits on the abundance of water vapor above the clouds, and the microwave data together set corresponding upper limits on H2O of ∼2 × 10−4 and on H2SO4 of ∼9 × 10−6. Upper limits on the surface density of total cloud constituents and of cloud liquid water are, respectively, ∼0.1 g cm−2 and ∼0.01 g cm−2. The infrared opacities of 90 bars of CO2, together with the derived upper limits to the amounts of water vapor and liquid H2O/H2SO4, may be sufficient to explain the high surface temperatures through the greenhouse effect.

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Junhong Wang and William B. Rossow

Abstract

A method is described to use rawinsonde data to estimate cloud vertical structure, including cloud-top and cloud-base heights, cloud-layer thickness, and the characteristics of multilayered clouds. Cloud-layer base and top locations are identified based on three criteria: maximum relative humidity in a cloud of at least 87%, minimum relative humidity of at least 84%, and relative humidity jumps exceeding 3% at cloud-layer top and base, where relative humidity is with respect to liquid water at temperatures greater than or equal to 0°C and with respect to ice at temperatures less than 0°C. The analysis method is tested at 30 ocean sites by comparing with cloud properties derived from other independent data sources. Comparison of layer-cloud frequencies of occurrence with surface observations shows that rawinsonde observations (RAOBS) usually detect the same number of cloud layers for low and middle clouds as the surface observers, but disagree more for high-level clouds. There is good agreement between the seasonal variations of RAOBS-determined top pressure of the highest cloud and that from the International Satellite Cloud Climate Project (ISCCP) data. RAOBS-determined top pressures of low and middle clouds agree better with ISCCP, but RAOBS often fail to detect very high and thin clouds. The frequency of multilayered clouds is qualitatively consistent with that estimated from surface observations. In cloudy soundings at these ocean sites, multilayered clouds occur 56% of the time and are predominately two layered. Multilayered clouds are most frequent (≈70%) in the Tropics (10°S–10°N) and least frequent at subtropical eastern Pacific stations. The frequency of multilayered clouds is higher in summer than in winter at low-latitude stations (30°S–30°N), but the opposite variation appears at the two subtropical stations. The frequency distributions of cloud top, cloud base, and cloud-layer thickness and cloud occurrence as a function of height are also presented. The lowest layer of multilayered cloud systems is usually located in the atmospheric boundary layer.

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William B. Rossow and Joseph Ferrier

Abstract

Two systematic calibrations have been compiled for the visible radiances measured by the series of AVHRR instruments flown on the NOAA operational polar weather satellites: one by the International Satellite Cloud Climatology Project (ISCCP), anchored on NASA ER-2 underflights in the 1980s and early 1990s and covering the period 1981–2009, and one by the PATMOS-x project, anchored on comparisons to the MODIS instruments on the Aqua and Terra satellites in the 2000s and covering the period 1979–2010 (this result also includes calibration for the near-IR channels). Both methods have had to extend their anchor calibrations over a long series of instruments using different vicarious approaches, so a comparison provides an opportunity to evaluate how well this extension works by cross-checking the results at the anchor points. The basic result of this comparison is that for the “afternoon” series of AVHRRs, the calibrations agree to within their mutual uncertainties. However, this retrospective evaluation also shows that the representation of the time variations can be simplified. The ISCCP procedure had much more difficulty extending the calibration to the “morning” series of AVHRRs with the calibrations for NOAA-15 and NOAA-17 exceeding the estimated uncertainties. Given the general agreement, a new calibration for all AVHRR visible radiances (except TIROS-N, NOAA-6, NOAA-19, and MetOp-A) is proposed that is based on the average of the best linear fits to the two time records. The estimated uncertainty of these calibrations is ±3% absolute (scaled radiance units).

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Ademe Mekonnen and William B. Rossow

Abstract

The interaction between deep convection and easterly waves over tropical North Africa is studied using a weather state (WS) dataset from the International Cloud Climatology Project (ISCCP) and reanalysis products from the European Centre for Medium-Range Weather Forecast, as well as radiative fluxes from ISCCP and a precipitation dataset from the Global Precipitation Climatology Project. Composite analysis based on 21 yr of data shows that stronger latent and radiative heating of the atmosphere are associated with stronger, more organized, convective activity than with weaker, less organized, convective activity, implying that any transition from less to more organized and stronger convection increases atmospheric heating. Regression composites based on a meridional wind predictor reveal coherent westward propagation of WS and large-scale wind anomalies from the Arabian Sea into East Africa and through West Africa. The analysis shows that enhanced, but unorganized, convective activity, which develops over the Arabian Sea and western Indian Ocean, switches to organized convective activity prior to the appearance of the African easterly wave (AEW) signature. The results also suggest that low-level moisture flux convergence and the upper-tropospheric wind divergence facilitate this change. Thus, the upper-level easterly waves, propagating into East Africa from the Indian Ocean, enhance one form of convection, which interacts with the Ethiopian highlands to trigger another, more organized, form of convection that, in turn, initiates the low-level AEWs.

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