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  • 1
    Electronic Resource
    Electronic Resource
    Ensemble formulation of surface fluxes and improvement in evapotranspiration and cloud parameterizations in a GCM (1984)
    Springer
    Boundary layer meteorology 29 (1984), S. 185-210 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The influence of some simple modifications to the physical parameterizations in the current GLAS climate GCM is examined. The aim of these modifications was to eliminate strong occasional bursts of 2 - δt oscillations in the PBL fluxes. The PBL of the current GLAS climate model was modified by invoking concepts of ensemble averaging of PBL eddies in a grid cell of the GCM. This resulted in smoothly varying bulk aerodynamic friction and heat transport coefficients. An arbitrary function to account for diffusion of moisture from stomatal cavities found in vegetation was also incorporated. Simultaneously some modifications to the cloud parameterizations were made. Two integrations, one with the old model and the other with the modified model, were made to simulate 47 days, starting from the NMC analysis for June 15, 1979. Their comparisons showed that the surface fluxes and cloudiness in the modified model simulations are far better. The planetary albedo in the modified model is also realistic. However, some weaknesses still persist, including an ITCZ (Inter-tropical convergence zone) that is too far northward in Sahelian Africa, polar regions that are too cold, and a rather strong ITCZ. It is pointed out that these weaknesses are primarily caused by model deficiencies, e.g., the cloud parameterization and the uniformly prescribed land surface roughness height. In another simulation with the modified model using a realistic value of surface roughness for deserts, the precipitation in the Sahara Desert reduced significantly, which effectively pushed the ITCZ southward to a more realistic location as compared to observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00206830
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    The influence of surface roughness of deserts on the July circulation (1985)
    Springer
    Boundary layer meteorology 33 (1985), S. 15-49 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The influence of low surface roughness of deserts on the July circulation is examined by employing numerical simulations with a GLAS GCM. Two identical sets of simulations were made with the model starting from the initial state of the atmosphere based on the NMC analysis of observations for June 15, at OOZ for the years 1979 and 1980. The first set, called the Control, had land surface roughness set to 45 cm, everywhere. The second set called the Experiment, had surface roughness set to 0.02 cm for deserts, but 45 cm everywhere else on land. All other prescribed boundary conditions were the same in both runs. A comparative analysis of these simulations showed that the rainfall in the Sahara desert was reduced significantly in both Experiments as compared to the corresponding Controls; the ITCZ (inter-tropical convergence zone) moved southward, to about 14° N, which is close to its observed location at about 10° N. This was primarily caused by the relative moisture divergence from the smoother Sahara. In other deserts, which anyway had little rainfall in the July simulation of the Control run, there was virtually no change. The differences in regional heat and moisture budgets, particularly for the Sahara desert, are significant as compared to the sample standard deviation for a set of three July simulations (i.e., Control runs for three different initial conditions). In a third simulation, in which the surface roughness was changed over all land, similar results were obtained in the Sahara desert region. The study reveals the influence of low surface-roughness of deserts on the July rainfall. For the Sahara desert, this influence is comparable to that of an increase in surface albedo. In nature, formation of deserts leads to reduction of surface roughness as the vegetation perishes and soil erosion ensues. It is inferred that the smoothness of land then causes reduction in rainfall and further promotes desertification.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00137034
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  • 3
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    Ensemble formulation of surface fluxes and improvement in evapotranspiration and cloud parameterizations in a GCM (1984)
    Springer
    Details
    Publication Date: 1984-06-01
    Print ISSN: 0006-8314
    Electronic ISSN: 1573-1472
    Topics: Geosciences , Physics
    Published by Springer
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  • 4
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    The influence of surface roughness of deserts on the July circulation (1985)
    Springer
    Details
    Publication Date: 1985-09-01
    Print ISSN: 0006-8314
    Electronic ISSN: 1573-1472
    Topics: Geosciences , Physics
    Published by Springer
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  • 5
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    Spatial Variability of Field-measured Soil-water Characteristics (1985)
    Wiley
    Details
    Publication Date: 1985-09-01
    Print ISSN: 0361-5995
    Electronic ISSN: 1435-0661
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Published by Wiley
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  • 6
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    Radiative impacts of cloud heterogeneity and overlap in an atmospheric General Circulation Model (2012)
    Copernicus
    Details
    Publication Date: 2012-10-04
    Description: The radiative impacts of horizontal heterogeneity of layer cloud condensate, and vertical overlap of both condensate and cloud fraction are examined with the aid of a new radiation package operating in the GEOS-5 Atmospheric General Circulation Model. The impacts are examined in terms of diagnostic top-of-the atmosphere shortwave (SW) and longwave (LW) cloud radiative effect (CRE) calculations for a range of assumptions and overlap parameter specifications. The investigation is conducted for two distinct cloud schemes, one that comes with the standard GEOS-5 distribution, and another used experimentally for its enhanced cloud microphysical capabilities. Both schemes are coupled to a cloud generator allowing arbitrary cloud overlap specification. Results show that cloud overlap radiative impacts are significantly stronger in the operational cloud scheme where a change of cloud fraction overlap from maximum-random to generalized results in global changes of SW and LW CRE of ~4 Wm−2, and zonal changes of up to ~10 Wm−2. This is an outcome of fewer occurrences (compared to the other scheme) of large layer cloud fractions and fewer multi-layer situations where large numbers of atmospheric layers are simultaneously cloudy, both conditions that make overlap details more important. The impact of the specifics of condensate distribution overlap on CRE is much weaker. Once generalized overlap is adopted, both cloud schemes are only modestly sensitive to the exact values of the overlap parameters. When one of the CRE components is overestimated and the other underestimated, both cannot be driven simoultaneously towards observed values by adjustments to cloud condensate heterogeneity and overlap specifications alone.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
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    Sensitivity of cirrus and mixed-phase clouds to the ice nuclei spectra in McRAS-AC: single column model simulations (2012)
    Copernicus
    Details
    Publication Date: 2012-11-16
    Description: The salient features of mixed-phase and ice clouds in a GCM cloud scheme are examined using the ice nucleation parameterizations of Liu and Penner (LP) and Barahona and Nenes (BN). The performance of both parameterizations was assessed in the GEOS-5 AGCM using the McRAS-AC cloud microphysics framework in single column mode. Four dimensional assimilated data from the intensive observation period of ARM TWP-ICE campaign was used to drive the fluxes and lateral forcing. Simulation experiments were established to test the impact of each parameterization in the resulting cloud fields. Three commonly used IN spectra were utilized in the BN parameterization to describe the availability of IN for heterogeneous ice nucleation. The results showed large similarities in the cirrus cloud regime between all the schemes tested, in which ice crystal concentrations were within a factor of 10 regardless of the parameterization used. In mixed-phase clouds there were some persistent differences in cloud particle number concentration and size, as well as in cloud fraction, ice water mixing ratio, and ice water path. Contact freezing in the simulated mixed-phase clouds contributed to the effective transfer of liquid to ice, so that on average, the clouds were fully glaciated at T 260 K, irrespective of the ice nucleation parameterization used. Comparison of simulated ice water path to available satellite derived observations were also performed, finding that all the schemes tested with the BN parameterization predicted average values of IWP within ±15% of the observations.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 8
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    Sensitivity of cirrus and mixed-phase clouds to the ice nuclei spectra in McRAS-AC: single column model simulations (2012)
    Copernicus
    Details
    Publication Date: 2012-06-12
    Description: The salient features of mixed-phase and ice clouds in a GCM cloud scheme are examined using the ice formation parameterizations of Liu and Penner (LP) and Barahona and Nenes (BN). The performance of LP and BN ice nucleation parameterizations were assessed in the GEOS-5 AGCM using the McRAS-AC cloud microphysics framework in single column mode. Four dimensional assimilated data from the intensive observation period of ARM TWP-ICE campaign was used to drive the fluxes and lateral forcing. Simulation experiments where established to test the impact of each parameterization in the resulting cloud fields. Three commonly used IN spectra were utilized in the BN parameterization to described the availability of IN for heterogeneous ice nucleation. The results show large similarities in the cirrus cloud regime between all the schemes tested, in which ice crystal concentrations were within a factor of 10 regardless of the parameterization used. In mixed-phase clouds there are some persistent differences in cloud particle number concentration and size, as well as in cloud fraction, ice water mixing ratio, and ice water path. Contact freezing in the simulated mixed-phase clouds contributed to transfer liquid to ice efficiently, so that on average, the clouds were fully glaciated at T~260 K, irrespective of the ice nucleation parameterization used. Comparison of simulated ice water path to available satellite derived observations were also performed, finding that all the schemes tested with the BN parameterization predicted average values of IWP within ±15% of the observations.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 9
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    Radiative impacts of cloud heterogeneity and overlap in an atmospheric General Circulation Model (2012)
    Copernicus
    Details
    Publication Date: 2012-05-12
    Description: The radiative impacts of introducing horizontal heterogeneity of layer cloud condensate, and vertical overlap of condensate and cloud fraction are examined with the aid of a new radiation package operating in the GEOS-5 Atmospheric General Circulation Model. The impacts are examined in terms of diagnostic top-of-the atmosphere shortwave (SW) and longwave (LW) cloud radiative effect (CRE) calculations for a range of assumptions and parameter specifications about the overlap. The investigation is conducted for two distinct cloud schemes, the one that comes with the standard GEOS-5 distribution, and another which has been recently used experimentally for its enhanced cloud microphysical capabilities; both are coupled to a cloud generator allowing arbitrary cloud overlap specification. We find that cloud overlap radiative impacts are significantly stronger for the operational cloud scheme for which a change of cloud fraction overlap from maximum-random to generalized results to global changes of SW and LW CRE of ∼4 W m−2, and zonal changes of up to ∼10 W m−2. This is because of fewer occurrences compared to the other scheme of large layer cloud fractions and of multi-layer situations with large numbers of atmospheric layers being simultaneously cloudy, conditions that make overlap details more important. The impact on CRE of the details of condensate distribution overlap is much weaker. Once generalized overlap is adopted, both cloud schemes are only modestly sensitive to the exact values of the overlap parameters. We also find that if one of the CRE components is overestimated and the other underestimated, both cannot be driven towards observed values by adjustments to cloud condensate heterogeneity and overlap alone.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 10
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    Effects of Cloud Microphysics on Tropical Atmospheric Hydrologic Processes and Intraseasonal Variability (2005)
    American Meteorological Society
    Details
    Publication Date: 2005-11-15
    Description: The sensitivity of tropical atmospheric hydrologic processes to cloud microphysics is investigated using the NASA Goddard Earth Observing System (GEOS) general circulation model (GCM). Results show that a faster autoconversion rate leads to (a) enhanced deep convection in the climatological convective zones anchored to tropical land regions; (b) more warm rain, but less cloud over oceanic regions; and (c) an increased convective-to-stratiform rain ratio over the entire Tropics. Fewer clouds enhance longwave cooling and reduce shortwave heating in the upper troposphere, while more warm rain produces more condensation heating in the lower troposphere. This vertical differential heating destabilizes the tropical atmosphere, producing a positive feedback resulting in more rain and an enhanced atmospheric water cycle over the Tropics. The feedback is maintained via secondary circulations between convective tower and anvil regions (cold rain), and adjacent middle-to-low cloud (warm rain) regions. The lower cell is capped by horizontal divergence and maximum cloud detrainment near the freezing–melting (0°C) level, with rising motion (relative to the vertical mean) in the warm rain region connected to sinking motion in the cold rain region. The upper cell is found above the 0°C level, with induced subsidence in the warm rain and dry regions, coupled to forced ascent in the deep convection region. It is that warm rain plays an important role in regulating the time scales of convective cycles, and in altering the tropical large-scale circulation through radiative–dynamic interactions. Reduced cloud–radiation feedback due to a faster autoconversion rate results in intermittent but more energetic eastward propagating Madden–Julian oscillations (MJOs). Conversely, a slower autoconversion rate, with increased cloud radiation produces MJOs with more realistic westward-propagating transients embedded in eastward-propagating supercloud clusters. The implications of the present results on climate change and water cycle dynamics research are discussed.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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