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End date: 2019 × Start date: 2019 × Start date: 1996 × Subject: atmospheric circulation ×

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Now showing 1 - 4 of 4
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    Regional modelling of Saharan dust and biomass-burning smoke, Part 2: Direct radiative forcing and atmospheric dynamic response
    (Milton Park : Taylor & Francis, 2017) Heinold, Bernd; Tegen, Ina; Bauer, Stefan; Wendisch, Manfred
    The direct radiative forcing and dynamic atmospheric response due to Saharan dust and biomass-burning aerosol particles are presented for a case study during the SAMUM-2 field campaign in January and February 2008. The regional model system COSMO-MUSCAT is used. It allows online interaction of the computed dust and smoke load with the solar and terrestrial radiation and with the model dynamics. Model results of upward solar irradiances are evaluated against airborne radiation measurements in the Cape Verde region. The comparison shows a good agreement for the case of dust and smoke mixture. Dust and smoke particles influence the atmospheric dynamics by changing the radiative heating rates. The related pressure perturbations modify local and synoptic scale air-flow patterns. In the radiative feedback simulations, the Hadley circulation is enhanced and convergence zones occur along the Guinea coast. Thus, the smoke particles spread more than 5◦ further north and the equatorward transport is reduced. Within the convergence zones, Saharan dust and biomass-burning material are more effectively advected towards the Cape Verdes. Given the model uncertainties, the agreement between the modelled and observed aerosol distribution is locally improved when aerosol–radiation interaction is considered.
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    The feedback of midlatitude waves onto the Hadley cell in a simple general circulation model
    (Milton Park : Taylor & Francis, 1996) Becker, E.; Schmitz, G.; Geprägs, R.
    We present self-consistent comparisons of axissymmetric and 3-dimensional simulations of the tropospheric circulation under idealized physical conditions. These reveal a feedback of transient eddies onto the Hadley circulation which, first, strongly depends on equatorial heating and, second, is in case of realistic heating quite different from prescribed eddy forcing. A quantitative estimation for the eddy-induced mixing of vorticity into the poleward flow of the Hadley cell is given. The proposed relation is in accordance with observations. It is derived from the computational result that eddy momentum flux convergence is of the same order as the equatorward flux of relative vorticity generated by the Hadley circulation. Evaluation of the local budgets of sensible heat gives rise to a clear picture of how the poleward heat transports due to Hadley circulation and transient eddies interlock. This mechanism is found to be essential for an interpretation of the eddy feedback.
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    Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks
    (München : European Geopyhsical Union, 2017) Franke, Jasper G.; Werner, Johannes P.; Donner, Reik V.
    Obtaining reliable reconstructions of long-term atmospheric circulation changes in the North Atlantic region presents a persistent challenge to contemporary paleoclimate research, which has been addressed by a multitude of recent studies. In order to contribute a novel methodological aspect to this active field, we apply here evolving functional network analysis, a recently developed tool for studying temporal changes of the spatial co-variability structure of the Earth's climate system, to a set of Late Holocene paleoclimate proxy records covering the last two millennia. The emerging patterns obtained by our analysis are related to long-term changes in the dominant mode of atmospheric circulation in the region, the North Atlantic Oscillation (NAO). By comparing the time-dependent inter-regional linkage structures of the obtained functional paleoclimate network representations to a recent multi-centennial NAO reconstruction, we identify co-variability between southern Greenland, Svalbard, and Fennoscandia as being indicative of a positive NAO phase, while connections from Greenland and Fennoscandia to central Europe are more pronounced during negative NAO phases. By drawing upon this correspondence, we use some key parameters of the evolving network structure to obtain a qualitative reconstruction of the NAO long-term variability over the entire Common Era (last 2000 years) using a linear regression model trained upon the existing shorter reconstruction.
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    The rôle of orographically and thermally forced stationary waves in the causation of the residual circulation
    (Milton Park : Taylor & Francis, 1999) Becker, E.; Schmitz, G.
    Several experiments performed with an idealized troposphere–stratosphere GCM are comparedto estimate the impact of orography and prescribed local heat sources on the residual circulationin the northern winter stratosphere. It is found that only the combined action of orographicand thermal wave forcing in northern midlatitudes is capable of inducing a residual circulationreaching continuously from tropical to polar latitudes at stratospheric altitudes. Intensificationsof the residual circulation in response to modified forcing of stationary waves are generallyassociated with, firstly, a reduced polar night jet accompanied by enhanced easterlies inlow and summer hemispheric latitudes and, secondly, substantial warming of the polar nightstratosphere completely compensated by cooling in the tropics and subtropics.