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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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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 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 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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William B. Rossow and Leonid Garder

Abstract

Aggregation of atmospheric data using the common equal-angle (latitude-longitude) map grid is shown to introduce an unnecessary degradation of data quality compared with aggregation using an equal-area grid. Analysis of this problem shows that the analysis grids can be different from the archival grids for convenience.

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George Tselioudis and William B. Rossow

Abstract

The recent analysis of Rossow et al. used a clustering technique to derive six tropical weather states (WS) based on mesoscale cloud-type patterns and documented the spatial distribution of those WS and the modes of variability of the convective WS in the tropical western Pacific. In this study, the global tropics are separated into 30° × 30° regions, and a clustering algorithm is applied to the regional WS frequency distributions to derive the dominant modes of weather state variability (or the climate state variability) in each region. The results show that the whole tropical atmosphere oscillates between a convectively active and a convectively suppressed regime with the exception of the eastern parts of the two ocean basins, where the oscillation is between a stratocumulus and a trade cumulus regime. The dominant mode of both those oscillations is the seasonal cycle with the exception of the eastern Indian and western–central Pacific region, where El Niño frequencies dominate. The transitions between the convectively active and suppressed regimes produce longwave (LW) and shortwave (SW) top-of-atmosphere (TOA) radiative differences that are of opposite sign and of similar magnitude, being of order 20–30 W m−2 over ocean and 10–20 W m−2 over land and thus producing an overall balance in the TOA radiative budget. The precipitation differences between the convectively active and suppressed regimes are found to be of order 2.5–3 mm day−1 over ocean and 1–2.4 mm day−1 over land. Finally, the transitions between the stratocumulus and shallow cumulus regimes produce noticeable TOA SW differences of order 10–20 W m−2 and very small TOA LW and precipitation differences. The potential climate feedback implications of the regime radiation and precipitation differences are discussed.

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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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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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Eric Tromeur and William B. Rossow

Abstract

To better understand the interaction between tropical deep convection and the Madden–Julian oscillation (MJO), tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud-top pressure—optical thickness joint distributions from the D1 dataset covering 21.5 yr. An MJO index based solely on upper-level wind anomalies is used to study variations of the tropical cloud regimes. The MJO index shows that MJO events are present almost all the time; instead of the MJO event being associated with “on or off” deep convection, it is associated with weaker or stronger mesoscale organization of deep convection. Atmospheric winds and humidity from NCEP–NCAR reanalysis 1 are used to characterize the large-scale dynamics of the MJO; the results show that the large-scale motions initiate an MJO event by moistening the lower troposphere by horizontal advection. Increasingly strong convection transports moisture into the upper troposphere, suggesting a reinforcement of the convection itself. The change of convection organization shown by the cloud regimes indicates a strong interaction between the large-scale circulation and deep convection. The analysis is extended to the complete atmospheric diabatic heating by precipitation, radiation, and surface fluxes. The wave organizes stronger convective heating of the tropical atmosphere, which results in stronger winds, while there is only a passive response of the surface, directly linked to cloud radiative effects. Overall, the results suggest that an MJO event is an amplification of large-scale wave motions by stronger convective heating, which results from a dynamic reorganization of scattered deep convection into more intense mesoscale systems.

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