Probabilistic prediction of climate using multi-model ensembles: from basics to applications.

Philos Trans R Soc Lond B Biol Sci 360 (2005) 1991-1998

TN Palmer, FJ Doblas-Reyes, R Hagedorn, A Weisheimer

The development of multi-model ensembles for reliable predictions of inter-annual climate fluctuations and climate change, and their application to health, agronomy and water management, are discussed.

Changing frequency of occurrence of extreme seasonal temperatures under global warming

Geophysical Research Letters 32 (2005) 1-5

A Weisheimer, TN Palmer

Using a multi-model multi-scenario ensemble of integrations made for the forthcoming fourth assessment report of the Intergovernmental Panel on Climate Change, the frequency of occurrence of extreme seasonal temperatures at the end of the 21st Century is estimated. In this study an extreme temperature is defined as lying above the 95 percentile of the simulated temperature distribution for 20th Century climate. The model probability of extreme warm seasons is heterogeneous over the globe and rises to over 90% in large parts of the tropics. This would correspond to an average return period of such anomalous warm seasons of almost one year. The reliability of these results is assessed using the bounding box technique, previously used to quantify the reliability of seasonal climate forecasts. It is shown that the dramatic increase in extreme warm seasons arises from the combined effect of a shift and a broadening of the temperature distributions. Copyright 2005 by the American Geophysical Union.

Recurrent climate winter regimes in reconstructed and modelled 500 hPa geopotential height fields over the North Atlantic/European sector 1659-1990

CLIMATE DYNAMICS 24 (2005) 809-822

C Casty, D Handorf, CC Raible, JF Gonzalez-Rouco, A Weisheimer, E Xoplaki, J Luterbacher, K Dethloff, H Wanner

Improved radio occultation sounding of the Arctic atmosphere using simulations with a high resolution atmospheric model

Physics and Chemistry of the Earth 29 (2004) 277-286

V Kunitsyn, V Zakharov, K Dethloff, A Weisheimer, M Gerding, R Neuber, A Rinke, I Hebestadt

Radio occultation experiments have been simulated for the Arctic region on the basis of the regional atmospheric model HIRHAM4. Irregular structures in the atmosphere produce a violation of the quasi-sphericity in the radio signal propagation and exert a strong influence on the accuracy of atmospheric profiles retrieved by the radio occultation technique. Errors in radio occultation data are spatially localised and associated with gradients in atmospheric structures. Local errors reach 2% in retrieved profiles of refractivity corresponding to an error of 6 K in temperature. Therefore mesoscale variations in atmospheric parameter gradients in a specified region must be taken into account when interpreting radio occultation data. We show, that a correction functional can be developed using the refractivity index field calculated from the regional model in order to improve the radio occultation retrieval of atmospheric parameters. This functional is constructed from instantaneous model outputs, as well as from temporally averaged fields of refractivity using data of the HIRHAM4 model for the Arctic atmosphere. The correction functional derived from monthly averaged data reduced the retrieval errors of refractivity, temperature, and pressure in the troposphere, in particular, temperature retrieval errors are reduced up to 1 K. Application of this kind of functional depends on whether the model used for the construction of the functional is able to simulate the real mesoscale atmospheric structures. © 2004 Elsevier Ltd. All rights reserved.

Internal climate variability in global and regional climate models


D Handorf, W Dorn, K Dethloff, A Rinke, A Weisheimer,

Gradient free descent: Shadowing, and state estimation using limited derivative information

Physica D: Nonlinear Phenomena 190 (2004) 153-166

K Judd, L Smith, A Weisheimer

Shadowing trajectories can play an important role in assessing the reliability of forecasting models, they can also play an important role in providing state estimates for ensemble forecasts. Gradient descent methods provide one approach for obtaining shadowing trajectories, which have been shown to have many useful properties. There remains the important question whether shadowing trajectories can be found in very high-dimensional systems, like weather and climate models. The principle impediment is the need to compute the derivative (or adjoint) of the system dynamics. In this paper we investigate gradient descent methods that use limited derivative information. We demonstrate the methods with an application to a moderately high-dimensional system using no derivative information at all. © 2003 Elsevier B.V. All rights rserved.

Nonlinear dynamics of the climate system


K Dethloff, A Rinke, D Handorf, A Weisheimer, W Dorn,

Numerical simulation of cloud- clear air interfacial mixing

Journal of the Atmospheric Sciences 61 (2004) 1726-1739

M Andrejczuk, WW Grabowski, SP Malinowski, PK Smolarkiewicz

This paper discusses results from a series of direct numerical simulations of the microscale cloud-clear air mixing, set forth in the idealized scenario of decaying moist turbulence. In the moist case, kinetic energy of microscale motions comes not only from the classical downscale energy cascade, but it can also be generated internally due to the evaporation of cloud droplets. Three sets of numerical simulations are performed for three intensities of initial large-scale eddies. In each set, a control dry simulation is performed, as well as two moist simulations applying either bulk or detailed representation of cloud microphysics. Model results suggest that, as far as the evolutions of enstrophy and turbulent kinetic energy are concerned, the most significant impact of moist processes occurs at the low intensity of initial large-scale eddies (the input turbulent kinetic energy of 2 X 10-4 m2 s-2 resulting in the maximum eddy dissipation rate of 5 X 10-4 m2 s-3). In such a case, mixing and homogenization are dominated by the kinetic energy generated as a result of evaporation of cloud water and its impact on the microscale buoyancy. Detailed microphysics, which explicitly treat the size dependence of cloud droplet sedimentation and evaporation, appear to have a comparatively small effect, although this result might be an artifact of a coarse grid resolution used in the simulations. High anisotropy, also observed in laboratory experiments with mixing, between cloudy and cloud-free air, prevails even at the high intensity of initial large-scale eddies (the input turbulent kinetic energy of 2 X 10-2 m2 s-2, the maximum eddy dissipation rate of 7 × 10-3 m2 s-3), despite the fact that mixing and homogenization proceed in a similar manner in moist and dry simulations. Impact on cloud microphysics is also quantified. Cloud droplet spectra at the end of simulations correspond to neither the extremely inhomogeneous nor homogeneous mixing scenarios-the two asymptotic limits where, respectively, either the cloud droplet size or the number of cloud droplets remain constant. The shift from low to high intensity of initial large-scale eddies shifts the mixing scenario toward the homogeneous case, corroborating the classical argument based on scale analysis. © 2004 American Meteorological Society.

Radar-echo tracking by use of invariant moments.

Appl Opt 42 (2003) 5891-5896

M Andrejczuk, S Moszkowicz, KE Haman, T Szoplik

A number of techniques to track rainfall patterns by use of radar observations have been developed over the years. We present a method for radar-echo tracking based on Hu invariant moments. The method has been tried on several sequences of test images, and the derived displacement fields were in good agreement with the real motions of the tested objects. For the real data obtained from the conventional meteorological radar in Legionowo the method occasionally failed when changes in the radar echo between observations were too large.

Extratropical low-frequency variability in a three-level quasi-geostrophic atmospheric model with different spectral resolution

Journal of Geophysical Research D: Atmospheres 108 (2003)

A Weisheimer, MV Kurgansky, K Dethloff, D Handorf

Apart from variations of external forcing components and interactions between climate subsystems, natural atmospheric fluctuations with periods of years, decades and centuries can also be generated by inherent atmospheric dynamical instabilities of the flow. The objective of this study is to investigate the spatial and temporal structure of internal low-frequency atmospheric variability of the Northern Hemisphere using a minimum-complexity model of the extratropical circulation. Here, the main focus is the influence of varying spectral horizontal resolution on the formation of dominant patterns of variability. For this purpose, a three-level quasi-geostrophic atmospheric model with idealized thermal and orographic forcing has been integrated over 1,000 years under perpetual winter conditions with T5, T10, T15, and T21 resolutions. It has been shown that for the crude resolution T5 a rather strong bias occurs, whereas starting with T1O resolution, the nonlinear feedback between large- and small-scale features is reasonably well described. At this resolution a sort of plateau in the model performance has been reached, in respect to both the model climatology and the spatiotemporal structure of variability. Ultralow-frequency variability is most pronounced in the model's stratosphere and is associated with changes in the polar vortex strength and shape caused by vertically propagating planetary waves. Rossby wave trains in the lee of the model large-scale orography are the most dominant structures of long-period fluctuations in the middle troposphere. The results show that interannual- and decadal-scale variations can, in substantial part, be considered as a manifestation of the natural variability of the extratropical atmosphere. The inclusion of a seasonal cycle of the model's diabatic heating increases the interannual and interdecadal variability.

Validation of water vapour profiles from GPS radio occultations in the Arctic


M Gerding, A Weisheimer

On the structure and variability of atmospheric circulation regimes in coupled climate models


A Weisheimer, D Handorf, K Dethloff

On the structure and variability of atmospheric circulation regimes in coupled climate models

Atmospheric Science Letters 2 (2001)

A Weisheimer, D Handorf, K Dethloff

In order to investigate whether climate models of different complexity have the potential to simulate natural atmospheric circulation regimes, 1000-year-long integrations with constant external forcing have been analysed. Significant non-Gaussian uni-, bi-, and trimodal probability density functions have been found in 100-year segments. © 2001 Royal Meteorological Society.

Niederfrequente Variabilität großräumiger atmosphärischer Zirkulationsstrukturen in spektralen Modellen niederer Ordnung. Reports on Polar Research.

AWI (2000) 356

A Weisheimer

Arctic and Antarctic ozone layer observations: Chemical and dynamical aspects of variability and long-term changes in the polar stratosphere

Polar Research 19 (2000) 193-204

M Rex, K Dethloff, D Handorf, A Herber, R Lehmann, R Neuber, J Notholt, A Rinke, P von der Gathen, A Weisheimer, H Gernandt

The altitude dependent variability of ozone in the polar stratosphere is regularly observed by balloon-borne ozonesonde observations at Neumayer Station (70°S) in the Antarctic and at Koldewey Station (79°N) in the Arctic. The reasons for observed seasonal and interannual variability and long-term changes are discussed. Differences between the hemispheres are identified and discussed in light of differing dynamical and chemical conditions. Since the mid-1980s, rapid chemical ozone loss has been recorded in the lower Antarctic stratosphere during the spring season. Using coordinated ozone soundings in some Arctic winters, similar chemical ozone loss rates have been detected related to periods of low temperatures. The currently observed cooling trend of the stratosphere, potentially caused by the increase of anthropogenic greenhouse gases, may further strengthen chemical ozone removal in the Arctic. However, the role of internal climate oscillations in observed temperature trends is still uncertain. First results of a 10 000 year integration of a low order climate model indicate significant internal climate variability, on decadal time scales, that may alter the effect of increasing levels of greenhouse gases in the polar stratosphere.

North Atlantic oscillation: Diagnosis and simulation of decadal variability and its long-period evolution

Izvestiya - Atmospheric and Ocean Physics 36 (2000) 555-565

II Mokhov, AV Eliseev, D Handorf, VK Petukhov, K Dethloff, A Weisheimer, DV Khvorost'yanov

Two 1000-year numerical experiments based on the IFA RAN global climate model, the first with completely interacting atmosphere and ocean and the second with a fixed climatic mean annual cycle of sea surface temperature, are analyzed. In both cases, a quasi-decadal cyclicity (QDC), but with substantially different amplitude-frequency characteristics, is detected for the North Atlantic Oscillation (NAO) in winter. Significant changes in the QDC regimes from one century to another are observed in the model. A comparison of the numerical results with empirical data and reconstructions reveal a fairly good agreement of the QDC amplitude and periods for winter NAO regimes in the model with completely interacting atmosphere and ocean for individual model subperiods on the order of a century. The model results suggest that interdecadal NAO variations of natural origin can be noticeably strengthened in the climate system without any influence of external, in particular, anthropogenic factors. In the case of a fixed annual cycle of SST, the QDC amplitudes are underestimated several times by the model, and no positive correlation is observed between the amplitudes and periods of the NAO QDC in contrast to the empirical data, reconstructions, and the model with completely interacting atmosphere and ocean.