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

Impacts of horizontal resolution on simulated climate statistics in ECHAM 4

Available from TIB Hannover: RR 1347(253) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Introduction of a prognostic cloud ice scheme in the ECHAM general circulation model: Impact on climate and climate sensitivity

A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. A precipitation formation which can distinguish between maritime and continental clouds by considering the cloud droplet number concentration in addition to the cloud water content is introduced. Based on different observational data sets, the cloud droplet number concentration is proportional to the sulfate aerosol mass concentration as given from the simulated sulfer cycle with ECHAM. Results obtained from the new scheme are compared to satellite observations and in-situ measurements of cloud physical and radiative properties. (orig./KW)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Klimavariabilitaet und Signalanalyse. Modellrechnungen zum Klima des 20. Jahrhunderts Abschlussbericht

In den letzten 100 Jahren hat sich die Erdoberflaechentemperatur um etwa 0.5 C erhoeht. Im gleichen Zeitraum erhoehte sich der atmosphaerische Treibhauseffekt infolge der Zunahme von Kohlendioxid (CO_2), Methan (CH_4), Distickstoffoxid (N_2O), troposphaerischem Ozon (O_3) sowie der Fluorchlorkohlenwasserstoffe (FCKW's) um etwa 2 W/m"2 (IPCC, 1996). Ein ursaechlicher Zusammenhang zwischen der beobachteten Erwaermung und den anthropogenen Emissionen von Treibhausgasen erscheint daher plausibel. Modellrechnungen zur Gleichgewichtsreaktion auf eine vorgegebene Stoerung der globalen Strahlungsbilanz von etwa 2 W/m"2 sind mit der beobachteten Erwaermung jedoch inkompatibel, d.h. die simulierte Erwaermung ist etwa 3-5 mal hoeher als die beobachtete. Moegliche Gruende fuer diese Diskrepanzen sind: (1) Die Klimasensitivitaet der Modelle ist unrealistisch hoch. (2) Die heute realisierte Erwaermung ist aufgrund der thermischen Traegheit der Ozeane weit von der Gleichgewichtsreaktion entfernt. (3) Die natuerliche Klimavariabilitaet infolge interner Wechselwirkungen und externer Stoerungen der Strahlungsbilanz (Vulkanismus, solare Aktivitaet z.B.) verdeckt das anthropogene Signal. (4) Anthropogene Emissionen koennen moeglicherweise auch den Aerosolgehalt der Atmosphaere erhoehen und damit der globalen Erwaermung entgegenwirken. Hauptziel des Projektes war die Untersuchung der Punkte (2) bis (4) mit Hilfe eines gekoppelten Modells von Atmosphaere und Ozean. (orig)In the course of the last 100 years the Earth's temperature has increased by around 0.5 C. As a result of the simultaneous increase in carbon dioxide (CO_2), methane (CH_4), nitrous oxide (N_2), tropospheric ozone (O_3), and fluorochlorocarbons (CFC), atmospheric heat retention in that period increased by 2W/m"2 (IPCC, 1996). A causal connection between the observed warming and the anthropogenic emission of greenhouse gases therefore appears plausible. However, model calculations have shown the observed warming to be at variance with the equilibrium reaction that would theoretically result from a disturbance of the global radiation balance on the order of 2 W/m"2. Specifically, the simulated warming is around 3 to 5 times greater than the warming observed. This discrepancy may be due to any of the following: the climate sensitivity of the models is unrealistically high; (1) the warming observed today does not nearly represent an equilibrium state due to the thermal inertia of the oceans; (2) the anthropogenic signal is overlaid by natural climate variability resulting from internal interactions and external disturbances (volcanism, solar activity etc.); (3) anthropogenic emissions may possibly lead to an increased aerosol content of the atmosphere, thus counteracting global warming. The chief aim of the present project was to examine hypotheses two to four by means of a coupled model of the ocean and atmosphereSIGLEAvailable from TIB Hannover: F99B198 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Modellierung der troposphaerischen Luftchemie mit dem Hamburger Klimamodell 'ECHAM' Abschlussbericht

Available from TIB Hannover: DtF QN1(48,53) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Tropical stabilisation of the thermohaline circulation in a greenhouse warming simulation

Most global climate models simulate a weakening of the North Atlantic thermohaline circulation (THC) in response to enhanced greenhouse warming. Both surface warming and freshening in high latitudes, the so-called sinking region, contribute to the weakening of the THC. Some models simulate even a complete breakdown of the THC at sufficiently strong forcing. Here we present results from a state-of-the-art global climate model that does not simulate a weakening of the THC in response to greenhouse warming. Large-scale air-sea interactions in the tropics, similar to those operating during present-day El Ninos, lead to anomalously high salinities in the tropical Atlantic. These are advected into the sinking region, thereby increasing the surface density and compensating the effects of the local warming and freshening. (orig.)Available from TIB Hannover: RR 1347(299) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Oceanic control of decadal North Atlantic sea level pressure variability in winter

The predictability of winter-time North Atlantic sea level pressure (SLP) variability has been investigated by means of an ensemble of integrations with an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures (SSTs) for the period 1951-1994. The results imply that the SLP variations on timescales of several years to decades may be predictable, provided the SST anomalies themselves used to drive the AGCM can be predicted. Our results reveal also that in addition to SST anomalies in the North Atlantic SST anomalies in the equatorial and North Pacific Oceans drive low-frequency variability in SLP over the North Atlantic. (orig.)SIGLEAvailable from TIB Hannover: RR 1347(292) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Predicting the '97 El Nino event with a global climate model

A global climate model is used for hindcasting the '97 El Ni no event. For this purpose a simple data assimilation technique has been developed which generates the thermal state of the low-latitude oceans by forcing the model with observed sea surface temperature (SST) anomalies. In this assimilation experiment, the vertical temperature profiles in the subsurface equatorial Pacific measured by the TOGA-TAO array were successfully simulated for the period 1996-1997. In a further series of eight hindcast experiments, initialized between January 1996 and October 1997, the predictive skill of the model was tested. All experiments starting in 1997 correctly simulated the evolution of the '97 El Ni no, although the amplitude was slightly underestimated. The model also predicted a dry period in the Indonesian region during fall 1997 when extensive forest fires occurred in this area. While the ocean was preconditioned to create an El Ni no already in 1996, the model correctly stayed in the cold (La Ni na) phase. All experiments initialized in '97 forecast a La Ni na event for the middle of '98. This is also suggested by the subsurface thermal structure in the tropical equatorial Pacific with a strong cold anomaly propagating eastward according to both, TOGA-TAO data and the data assimilation experiment. (orig.)18 refs.Available from TIB Hannover: RR 1347(254) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Why is the global warming proceeding much slower than expected?

Upper air observations from radiosondes and microwave satellite instruments do not indicate any global warming during the last 19 years in contrary to surface measurements where a warming trend is supposedly being found. This result is somewhat difficult to reconcile, since climate model experiments do indicate a reverse trend, namely that upper tropospheric air should warm faster than the surface. To contribute towards an understanding of this difficulty, we have here undertaken some specific experiments to study the effect on climate due to the decrease in stratospheric ozone and the Mt. Pinatubo eruption in 1991. The associated forcing was added to the forcing from greenhouse gases, sulfate aerosols (direct and indirect effect) and tropospheric ozone, which was investigated in a separate series of experiments. Furthermore, we have undertaken an ensemble study in order to explore the natural variability of an advanced climate model exposed to such a forcing over 19 years. The result shows that the reduction of stratospheric ozone does not only cool the lower stratosphere but also the troposphere, in particular the upper and middle part. In the upper troposphere the cooling from stratospheric ozone leads to a significant reduction of the greenhouse warming. The stratospheric aerosols from Mt. Pinatubo generate a climate response (stratospheric warming and tropospheric cooling) in good agreement with microwave satellite measurements. Finally, the analysis of a series of experiments with both stratospheric ozone and the Mt. Pinatubo effect shows a considerably variability in its climate response49 refs.SIGLEAvailable from TIB Hannover: RR 1347(256) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Interpretation of interbasin exchange in an isopycnal ocean model

This paper reports an analysis of interbasin and interlayer exchanges in the component ocean part of the coupled ECHAM4/OPYC3 general circulation model, aimed at documenting the simulation of North Atlantic deep water (NADW) and related thermohaline circulations in the Indian and Pacific Oceans. The modeled NADW is formed mainly in the Greenland-Iceland-Norwegian Seas through a composite effect of deep convection and downward cross-isopycnal transport. The modeled deep-layer outflow of NADW can reach 16 Sv near 30 S in the South Atlantic, with the corresponding upper-layer return flow mainly coming from the ''cold water path'' through Drake Passage. Less than 4 Sv of the Agulhas ''leakage'' water contribute to the replacement of NADW along the ''warm water path''. In the South Atlantic Ocean, the model shows that some intermediate isopycnal layers with potential densities ranging between 27.0 and 27.5 are the major water source for compensating the NADW return flow and for enhancing the circumpolar deep water (CDW) flowing from the Atlantic into Indian Ocean. The modeled thermohaline circulations in the Indian and Pacific Oceans indicate that the Indian Ocean may play the major role in converting deep water into intermediate waterAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman