Harald Lejenäs

Traveling planetary-scale waves and blocking

Abstract The possible relation between blocking-type flow patterns in the atmosphere and large-scale traveling waves has been investigated. A 30-yr time series of observational 500-hPa geopotential-height data was used to study the relation between westward-moving planetary-scale waves 1 and 2 and blocked flow. It was found that, depending on longitude, 20%–40% of blocks were related to traveling wave 1, whereas the percentage was smaller for wave 2. The study confirms results of earlier studies that suggest a possible important role for large-scale, westward-moving waves in many blocking episodes.

Mountain Torques Caused by Normal-Mode Global Rossby Waves, and the Impact on Atmospheric Angular Momentum

Abstract Planetary-scale free Rossby waves present in the earth’s atmosphere propagate toward the west. Pressure torques varying in time then arise as a consequence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain torques, have an influence on the exchange of angular momentum between the atmosphere and the earth. The authors investigated the impact of all identified planetary-scale free Rossby waves on atmospheric angular momentum by computing the contribution from mountain torques to the rate of change of total atmospheric angular momentum for each wave. Comparing contributions from individual waves, the authors found that for the average wave amplitudes the maximum torque for a particular wave is around 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenumber 2 causes the largest global torques. Changes in angular momentum depend on both the amplitude of the changing torque and on its period...

Global atmospheric angular momentum and Earth‐atmosphere exchange of angular momentum simulated in a general circulation model

The atmospheric angular momentum budget as described by the National Center for Atmospheric Research community climate model has been studied. Model data from a 20-year climate simulation have been used to study time variations of atmospheric angular momentum and Earth-atmosphere exchange of angular momentum. Computed values of frictional torques, mountain torques, and atmospheric angular momentum as determined by the model were compared with observed values already published in the literature, as well as with values based on data obtained from the National Center for Environmental Prediction. Computed values of gravity wave drag torques are also presented. These values cannot be directly compared with observational data, but they give some idea of the possible role of wave drag on the angular momentum budget. A detailed comparison shows that the general circulation model simulates the seasonal variation of the angular momentum and frictional and mountain torques reasonably well. We also found that the model simulates the seasonal global atmospheric pressure variations owing to global redistributions of mass in a realistic way.

Initialization of Moisture in Primitive Equation Models

Abstract An explicitly integrated primitive equation grid-point model is used as a tool to study the impact of some variables on the hemispheric average precipitation rate in the model. The experiments show that the model needs 15 h to create vertical velocities and build up a moisture field which gives a constant rate of precipitation, starting from an initial state adjusted by the balance equation. Two kinds of moisture initializations are discussed, and it is shown that if the moisture field is perfectly initialized, i.e., if the moisture can be specified in the most consistent manner, it still takes several hours for the rate of precipitation to become constant, again starting from an initial state adjusted by the balance equation. The reason for this is the lack of initial vertical velocities. The results suggest that vertical velocities initially are of equal or higher importance than details in the moisture.

On the breakdown of the westerlies

Abstract The dynamics of the ultra‐long Rossby waves are studied with the aid of a primitive equation numerical model. The investigation is focused on a study of the breakdown of a high‐index circulation. Different idealized flow patterns are used as initial conditions and forecasts are made with the numerical model. The influences of oceans and mountains are studied by incorporating them into the model. The results indicate that barotropic and baroclinic instabilities as well as the structure of the earths surface are of importance for the development, but the experiments do not clearly indicate which one of these factors is most important.

Semidiurnal Variations in the Budget of Angular Momentum in a General Circulation Model and in the Real Atmosphere

Abstract Diurnal and semidiurnal variations in the budget of atmospheric angular momentum are evident in a simulation by the NCAR Community Climate Model (CCM2). These variations depicted with 20-min time resolution (each time step) are used as guides to study similar variations determined from 6-hourly NCEP/NCAR Reanalysis data. A semidiurnal variation in relative angular momentum and in angular momentum related to solid body rotation of the atmosphere is found in the reanalysis. Although there is evidence that both frictional and gravity-wave drag torques play roles, the effects of semidiurnal variations in mountain torque, resulting from the migrating semidiurnal pressure wave, are most thoroughly documented.

On the Influence of the Technique of Nonlinear Normal Mode Initialization on the Nonconvective Precipitation Rate

Abstract A primitive equation grid-point model is used as a tool to study the impact of nonlinear normal mode initialization on the nonconvective precipitation rate in the model. It is found that the precipitation rate is affected if a simplified precipitation model is used, but no influence is found when a more sophisticated model is used.

A new format and a new layout for Tellus

The format of Tellus has remained unchanged since its inception in 1949. Modern techniques and the publisher’s need for a standard format have stimulated an ongoing discussion on how to improve Tellus. As you have already noticed, the format of Tellus has now increased. The new format, named sub-A4, is 276x210 mm. The journal’s layout has been modernized. We hope that you will find the new Tellus attractive. One big advantage is that figures can now be reproduced in a larger format.

Tellus launches an electronic submission and peer-review system

Since 2001 Tellus has been available in electronic form. Recently all volumes from 1990 through 2000 werealso made available in electronic form at http://www.blackwelI-synergy.com.