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E. Källén

Abstract

An analysis of a low-order barotropic system with orographic and momentum forcing is presented. The low-order expansion of the streamfunction is performed on a spherical geometry, the expansion functions thus being spherical harmonics. Two purely zonal components and two wave components with the same zonal wavenumber but different orders, or latitudinal wavenumbers, are described by the model. The nonlinear terms appearing in the six ODE's governing the time evolution of the system give rise to bifurcations into multiple steady-state solutions.

In order to retain as many nonlinear terms as possible, however, one must be careful in the choice of components. An analysis of the different possibilities is presented and two examples having somewhat different properties are investigated. Three of the components are the same in each example, while the three others differ in their symmetry properties around the equator.

For the example which is believed to be most representative of realistic conditions, it is shown that a combination of orographic forcing and zonally asymmetric momentum forcing is required to obtain multiple steady-state solutions for realistic parameter values. The forcing must exceed certain critical values for a bifurcation from one to three steady states to appeal. A stability investigation of the steady-state triplets shows that two are stable while one is unstable. Examining the energetics of the two stable steady states for a situation which corresponds to a wintertime forcing pattern, it is shown that one of the stable steady states is much more zonal than the other. The non-zonal circulation is similar to a “blocked” flow. Another significant difference in the energetics between the two flow types is the intensity of the energy transfer between zonal and eddy kinetic energy. Comparisons with observational studies of “blocked” and zonal flow confirms that this is a characteristic feature of the observed energetics.

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E. Källén, C. Crafoord, and M. Ghil

Abstract

A study of stable periodic solutions to a simple nonlinear model of the ocean-atmosphere-ice system is presented. The model has two dependent variables: ocean-atmosphere temperature and latitudinal extent of the ice cover. No explicit dependence on latitude is considered in the model. Hence all variables depend only on time and the model consists of a coupled set of nonlinear ordinary differential equations.

The globally averaged ocean-atmosphere temperature in the model is governed by the radiation balance (Budyko, 1969; Sellers, 1969). The reflectivity to incoming solar radiation, i.e., the planetary albedo, includes separate contributions from sea ice and from continental ice sheets. The major physical mechanisms active in the model are 1) albedo-temperature feedback, 2) continental ice-sheet dynamics (Weert-man, 1964, 1976) and 3) precipitation-rate variations.

The model has three equilibrium solutions, two of which are linearly unstable, while one is linearly stable. For some choices of parameters, the stability picture changes and sustained, finite-amplitude oscillations obtain around the previously stable equilibrium solution. The physical interpretation of these oscillations points to the possibility of internal mechanisms playing a role in glaciation cycles.

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D. P. Dee, E. Källén, A. J. Simmons, and L. Haimberger

Abstract

No Abstract available.

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Wayman E. Baker, Robert Atlas, Carla Cardinali, Amy Clement, George D. Emmitt, Bruce M. Gentry, R. Michael Hardesty, Erland Källén, Michael J. Kavaya, Rolf Langland, Zaizhong Ma, Michiko Masutani, Will McCarty, R. Bradley Pierce, Zhaoxia Pu, Lars Peter Riishojgaard, James Ryan, Sara Tucker, Martin Weissmann, and James G. Yoe

The three-dimensional global wind field is the most important remaining measurement needed to accurately assess the dynamics of the atmosphere. Wind information in the tropics, high latitudes, and stratosphere is particularly deficient. Furthermore, only a small fraction of the atmosphere is sampled in terms of wind profiles. This limits our ability to optimally specify initial conditions for numerical weather prediction (NWP) models and our understanding of several key climate change issues.

Because of its extensive wind measurement heritage (since 1968) and especially the rapid recent technology advances, Doppler lidar has reached a level of maturity required for a space-based mission. The European Space Agency (ESA)'s Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) Doppler wind lidar (DWL), now scheduled for launch in 2015, will be a major milestone.

This paper reviews the expected impact of DWL measurements on NWP and climate research, measurement concepts, and the recent advances in technology that will set the stage for space-based deployment. Forecast impact experiments with actual airborne DWL measurements collected over the North Atlantic in 2003 and assimilated into the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model are a clear indication of the value of lidar-measured wind profiles. Airborne DWL measurements collected over the western Pacific in 2008 and assimilated into both the ECMWF and U.S. Navy operational models support the earlier findings.

These forecast impact experiments confirm observing system simulation experiments (OSSEs) conducted over the past 25–30 years. The addition of simulated DWL wind observations in recent OSSEs performed at the Joint Center for Satellite Data Assimilation (JCSDA) leads to a statistically significant increase in forecast skill.

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