131 research outputs found
Observational and Modeling Studies of Clouds and the Hydrological Cycle
Our approach involved validating parameterizations directly against measurements from field programs, and using this validation to tune existing parameterizations and to guide the development of new ones. We have used a single-column model (SCM) to make the link between observations and parameterizations of clouds, including explicit cloud microphysics (e.g., prognostic cloud liquid water used to determine cloud radiative properties). Surface and satellite radiation measurements were used to provide an initial evaluation of the performance of the different parameterizations. The results of this evaluation will then used to develop improved cloud and cloud-radiation schemes, which were tested in GCM experiments
Pattern recognition of satellite cloud imagery for improved weather prediction
The major accomplishment was the successful development of a method for extracting time derivative information from geostationary meteorological satellite imagery. This research is a proof-of-concept study which demonstrates the feasibility of using pattern recognition techniques and a statistical cloud classification method to estimate time rate of change of large-scale meteorological fields from remote sensing data. The cloud classification methodology is based on typical shape function analysis of parameter sets characterizing the cloud fields. The three specific technical objectives, all of which were successfully achieved, are as follows: develop and test a cloud classification technique based on pattern recognition methods, suitable for the analysis of visible and infrared geostationary satellite VISSR imagery; develop and test a methodology for intercomparing successive images using the cloud classification technique, so as to obtain estimates of the time rate of change of meteorological fields; and implement this technique in a testbed system incorporating an interactive graphics terminal to determine the feasibility of extracting time derivative information suitable for comparison with numerical weather prediction products
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FINAL REPORT (DE-FG02-97ER62338): Single-column modeling, GCM parameterizations, and ARM data
Our overall goal is the development of new and improved parameterizations of cloud-radiation effects and related processes, using ARM data at all three ARM sites, and the implementation and testing of these parameterizations in global models. To test recently developed prognostic parameterizations based on detailed cloud microphysics, we have compared SCM (single-column model) output with ARM observations at the SGP, NSA and TWP sites. We focus on the predicted cloud amounts and on a suite of radiative quantities strongly dependent on clouds, such as downwelling surface shortwave radiation. Our results demonstrate the superiority of parameterizations based on comprehensive treatments of cloud microphysics and cloud-radiative interactions. At the SGP and NSA sites, the SCM results simulate the ARM measurements well and are demonstrably more realistic than typical parameterizations found in conventional operational forecasting models. At the TWP site, the model performance depends strongly on details of the scheme, and the results of our diagnostic tests suggest ways to develop improved parameterizations better suited to simulating cloud-radiation interactions in the tropics generally. These advances have made it possible to take the next step and build on this progress, by incorporating our parameterization schemes in state-of-the-art three-dimensional atmospheric models, and diagnosing and evaluating the results using independent data. Because the improved cloud-radiation results have been obtained largely via implementing detailed and physically comprehensive cloud microphysics, we anticipate that improved predictions of hydrologic cycle components, and hence of precipitation, may also be achievable
Cloud Radiation Forcings and Feedbacks: General Circulation Model Tests and Observational Validation
Using an atmospheric general circulation model (the National Center for Atmospheric Research Community Climate Model: CCM2), the effects on climate sensitivity of several different cloud radiation parameterizations have been investigated. In addition to the original cloud radiation scheme of CCM2, four parameterizations incorporating prognostic cloud water were tested: one version with prescribed cloud radiative properties and three other versions with interactive cloud radiative properties. The authors' numerical experiments employ perpetual July integrations driven by globally constant sea surface temperature forcings of two degrees, both positive and negative. A diagnostic radiation calculation has been applied to investigate the partial contributions of high, middle, and low cloud to the total cloud radiative forcing, as well as the contributions of water vapor, temperature, and cloud to the net climate feedback. The high cloud net radiative forcing is positive, and the middle and low cloud net radiative forcings are negative. The total net cloud forcing is negative in all of the model versions. The effect of interactive cloud radiative properties on global climate sensitivity is significant. The net cloud radiative feedbacks consist of quite different shortwave and longwave components between the schemes with interactive cloud radiative properties and the schemes with specified properties. The increase in cloud water content in the warmer climate leads to optically thicker middle- and low-level clouds and in turn to negative shortwave feedbacks for the interactive radiative schemes, while the decrease in cloud amount simply produces a positive shortwave feedback for the schemes with a specified cloud water path. For the longwave feedbacks, the decrease in high effective cloudiness for the schemes without interactive radiative properties leads to a negative feedback, while for the other cases, the longwave feedback is positive. These cloud radiation parameterizations are empirically validated by using a single-column diagnostic model. together with measurements from the Atmospheric Radiation Measurement program and from the Tropical Ocean Global Atmosphere Combined Ocean-Atmosphere Response Experiment. The inclusion of prognostic cloud water produces a notable improvement in the realism of the parameterizations, as judged by these observations. Furthermore, the observational evidence suggests that deriving cloud radiative properties from cloud water content and microphysical characteristics is a promising route to further improvement
Trace gas emissions to the atmosphere by biomass burning in the west African savannas
Savanna fires and atmospheric carbon dioxide (CO2) detection and estimating burned area using Advanced Very High Resolution Radiometer_(AVHRR) reflectance data are investigated in this two part research project. The first part involves carbon dioxide flux estimates and a three-dimensional transport model to quantify the effect of north African savanna fires on atmospheric CO2 concentration, including CO2 spatial and temporal variability patterns and their significance to global emissions. The second article describes two methods used to determine burned area from AVHRR data. The article discusses the relationship between the percentage of burned area and AVHRR channel 2 reflectance (the linear method) and Normalized Difference Vegetation Index (NDVI) (the nonlinear method). A comparative performance analysis of each method is described
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Direct radiative effect of mineral dust and volcanic aerosols in a simple aerosol climate model
Airborne mineral dust can influence the climate by altering the radiative properties of the atmosphere, but the magnitude of the effect is uncertain. An idealized global model is developed to study the dust-climate system. The model determines the dust longwave and shortwave direct radiative forcing, as well as the resulting temperature changes, based on the specified dust distribution, height, and optical properties. Comparisons with observations and general circulation results indicate that the model produces realistic results for the present-day dust distribution as well as for volcanic aerosols. Although the model includes many simplifications, it can still provide insight into dust-climate system behavior. Recent observations suggest that dust may absorb less solar radiation than previously thought. Experiments with the model suggest that previous studies which used more absorbing dust may be underestimating the effect of dust. Increasing the solar single scattering albedo value from 0.85 to 0.97, corresponding to recent measurements, more than doubles the modeled global average top-of-the-atmosphere (TOA) shortwave direct forcing for the present-day dust distribution, while the surface shortwave forcing is halved. The corresponding temperature decreases are larger for the larger single scattering albedo, and the latent and sensible heat fluxes decreases are smaller. The dust forcing and climate response are approximately linear with respect to optical depth. However, the relationship depends on the relative magnitudes of shortwave versus longwave TOA forcing. Thus the net TOA forcing alone does not determine the steady state climate response
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Sensitivity of climate forcing and response to dust optical properties in an idealized model
An idealized global climate model is used to explore the response of the climate to a
wide range of dust radiative properties and dust layer heights. The top-of-the-atmosphere
(TOA) shortwave forcing becomes more negative as the broadband shortwave single
scattering albedo increases and the broadband shortwave asymmetry parameter decreases,
but the sensitivity is highly dependent on the location of the dust layer with respect to
clouds. The longwave TOA forcing is most affected by the height of the dust layer. The
net TOA forcing is most sensitive to the shortwave single scattering albedo and shortwave
asymmetry parameter. The surface and atmospheric temperature responses are
approximately linear with respect to the TOA forcing, as opposed to the surface or
atmospheric forcings. Thus the TOA forcing can be used to estimate both the surface and
atmospheric temperature responses to dust. The corresponding changes in latent and
sensible heat fluxes are essential for the close relationship of the surface temperature
response to the TOA forcing. Estimating the hydrological cycle response requires
knowledge of the vertical distribution of dust with respect to clouds or other reflective
particles. The sensitivity of the latent heat flux to variations in the shortwave single
scattering albedo changes sign with dust height. The latent heat flux change becomes less
negative as the shortwave single scattering albedo increases if the dust layer is below
clouds. However, when the dust is above clouds, the latent heat response becomes more
negative as the single scattering albedo increases.Keywords: climate model, mineral dust, hydrological cycleKeywords: climate model, mineral dust, hydrological cycl
Strong size evolution of the most massive galaxies since z~2
Using the combined capabilities of the large near-infrared Palomar/DEEP-2
survey, and the superb resolution of the ACS HST camera, we explore the size
evolution of 831 very massive galaxies (M*>10^{11}h_{70}^{-2}M_sun) since z~2.
We split our sample according to their light concentration using the Sersic
index n. At a given stellar mass, both low (n2.5)
concentrated objects were much smaller in the past than their local massive
counterparts. This evolution is particularly strong for the highly concentrated
(spheroid-like) objects. At z~1.5, massive spheroid-like objects were a factor
of 4(+-0.4) smaller (i.e. almost two orders of magnitudes denser) than those we
see today. These small sized, high mass galaxies do not exist in the nearby
Universe, suggesting that this population merged with other galaxies over
several billion years to form the largest galaxies we see today.Comment: MNRAS in press, 13 pages, 11 figures. Data Table will be published in
its integrity in the MNRAS online versio
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