Sodar for precipitation measurements

A 6.4 kHz Doppler sodar has been used in a feasibility study to retrieve rain rate and drop size distribution from sodar Doppler spectra. For rain rates above about 0.5 mm/h raindrop and turbulence spectra can be separated, thus allowing a drop size distribution measurement in the Prandtl layer without assumptions on the size distribution type for drop diameters above 1 mm. The noise of the sodar used mostly forced us to 10 min averages for the rather light rain episodes (< mm/h rain rate) we have enountered in January 1993 at our experimental site in Northern Germany. In order to arrive at consistent rain parameter retrievals we have corrected both the mean vertical wind influence and the drop size dependent reaction to turbulent motion. The comparison to a drop counter at the site gives good results only for the periods with rain rates above 1 mm/h at the absence of drizzle. Assuming a height independent rain rate in the Prandtl layer even gives the possibility for a near surface temperature profile retrieval. For higher sodar frequencies above 15 kHz a clearer separation of turbulence and rainspectra would lower the threshold drop size and thus improve rain parameter retrievals but be more strongly dependent on the near surface temperature and humidity profiles. (orig.)Available from TIB Hannover: RR 1347(160) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Cloud-turbulence interactions: sensitivity of a general circulation model to closure assumptions

Several approaches to parameterize the turbulent transport of momentum, heat, water vapour and cloud water for use in a general circulation model (GCM) have been tested in one-dimensional and three-dimensional model simulations. The schemes differ with respect to their closure assumptions (conventional eddy diffusivity model versus turbulent kinetic energy closure) and also regarding their treatment of cloud-turbulence interactions. The basis properties of these parameterizations are discussed first in column simulations of a stratocumulus-topped atmospheric boundary layer (ABL) under a strong subsidence inversion during the KONTROL experiment in the North Sea. It is found that the K-models tend to decouple the cloud layer from the adjacent layers because the turbulent activity is calculated from local variables. The higher-order scheme performs better in this respect because internally generated turbulence can be transported up and down through the action of turbulent diffusion. Thus, the TKE-scheme provides not only a better link between the cloud and the sub-cloud layer but also between the cloud and the inversion as a result of cloud-top entrainment. In the stratocumulus case study, where the cloud is confined by a pronounced subsidence inversion, increased entrainment favours cloud dilution through enhanced evaporation of cloud droplets. In the GCM study, however, additional cloud-top entrainment supports cloud formation because indirect cloud generating processes are promoted through efficient ventilation of the ABL, such as the enhanced moisture supply by surface evaporation and the increased depth of the ABL. As a result, tropical convection is more vigorous, the hydrological cycle is intensified, the whole troposphere becomes warmer and moister in general and the cloudiness in the upper part of the ABL is increased. (orig.)Available from TIB Hannover: RR 1347(117) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A process-based, climate-sensitive model to derive methane emissions from natural wetlands: application to 5 wetlands sites, sensitivity to model parameters and climate

Methane emissions from natural wetlands constitutes the largest methane source at present and depends highly on the climate. In order to investigate the response of methane emissions from natural wetlands to climate variations, a 1-dimensional process-based climate-sensitive model to derive methane emissions from natural wetlands is developed. In the model the processes leading to methane emission are simulated within a 1-dimensional soil column and the three different transport mechanisms diffusion, plant-mediated transport and ebullition are modeled explicitly. The model forcing consists of daily values of soil temperature, water table and net primary productivity, and at permafrost sites the thaw depth is included. The methane model is tested using observational data obtained at 5 wetland sites located in North America, Europe and Central America, representing a large variety of environmental conditions. It can be shown that in most cases seasonal variations in methane emissions can be explained by the combined effect of changes in soil temperature and the position of the water table. Our results also show that a process-based approach is needed, because there is no simple relationship between these controlling factors and methane emissions that applies to a variety of wetland sites. The sensitivity of the model to the choice of key model parameters is tested and further sensitivity tests are performed to demonstrate how methane emissions from wetlands respond to climate variations. (orig.)76 refs.Available from TIB Hannover: RR 1347(293) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

4th International conference on modelling of global climate change and variability Abstracts

This documents contains the about 200 abstracts of the talks and poster presentations given at the 4th International Conference on Modelling of Global Climate Change and Variability. The seasonal to interannual variability is modelled as well as decadal and centennial climate variability. Other presentations deal with the prediction and detection of anthropogenic climate change (KW)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

3. International conference on modelling of global climate change and variability Abstracts

The book contains the summaries of oral and poster presentations. The topics are: Development of comprehensive climate models, modelling of atmospheric chemistry and aerosols, modelling of natural climate variability and prediction, detection and specification of anthropogene climate change. (KW)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Marine Palaeoklimatologie: Rapiden Klimawechseln auf der Spur. Teilprojekt: Modellierung eiszeitlicher Klimaschwankungen Abschlussbericht

Available from TIB Hannover: DtF QN1(34,15) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Modes of ocean variability in the tropical Pacific as derived from GEOSAT altimetry

Satellite-derived (GEOSAT) sea surface height anomalies for the period November 1986 to August 1989 were investigated in order to extract the dominant modes of climate variability in the tropical Pacific. Four modes are identified by applying the POP technique. The first mode has a time scale of about 3 months and can be identified with the first baroclinic equatorial Kelvin wave mode. The second mode has a time scale of about six months and describes the semi-annual cycle in tropical Pacific sea level. Equatorial wave propagation is also crucial for this mode. The third mode is the annual cycle which is dominated by Ekman dynamics. Wave propagation or reflection are found to be unimportant. The fourth mode is associated with the El Nino/Southern Oscillation (ENSO) phenomenon. The ENSO mode is found to be consistent with the 'delayed action oscillator' scenario. The results are substantiated by a companion analysis of the sea surface height variability simulated with an oceanic general circulation model (OGCM) forced by observed wind stresses for the period 1961 to 1989. The modal decomposition of the sea level variability is found to be similar to that derived from the GEOSAT data. The high consistency between the satellite and the model data indicates the high potential value of satellite altimetry for climate modeling and forecasting. (orig.)SIGLEAvailable from TIB Hannover: RR 1347(103) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The role of the individual air-sea flux components in CO_2-induced changes of the ocean's circulation and climate

In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO_2 concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO_2 concentrations have been performed. In a set of sensitivity experiments either freshwater and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem to almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes36 refs.Available from TIB Hannover: RR 1347(263) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Towards a contrail climatology from NOAA-satellite images over Europe

Contrail cloudiness over Europe and the eastern part of the North Atlantic Ocean was analyzed for the two periods Sept. 1979-Dec. 1981 and Sept. 1989-Aug. 1992 by visual inspection of quicklook photographic prints of NOAA/AVHRR infrared images. The averaged contrail cover exhibits maximum values along the transatlantic flight corridor around 50 N (of almost 2%) and over western Europe resulting in 0.5% contrail cloudiness on the average. A strong yearly cycle appears with a maximum (<2%) in spring and summer over the Atlantic and a smaller maximum (<1%) in winter over southwestern Europe. Comparing the two time periods of one decade separation there is a significant decrease in contrail cloudiness over western Europe and a significant increase over the North Atlantic observable between March and July. Contrail cloud cover during daytime is about twice as high as during nighttime. Contrails are preferably found in larger fields of 1000 km diameter which last usually for more than one day. Causes, possible errors and consequences are discussed. (orig.)SIGLEAvailable from TIB Hannover: RR 1347(118) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model

The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and slowly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the Atlantic intertropical convergence zone is displaced southward and the westerlies in the northern hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface waters in high northern latitudes, which allows them to accumulate more precipitation and runoff from the continents, which results in an increased stability in the North Atlantic61 refs.Available from TIB Hannover: RR 1347(188) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman