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Harald Lejenäs

The winter of 1941–42 is known as the coldest European winter of the 20th Century. The temperature was much below normal from the beginning of January until the end of March 1942. Blockings and cut-off lows were frequent, particularly during January and February 1942.

The role of quasi-stationary waves during this winter has been studied by decomposing the 500-mb geopotential height data in a low-pass, filtered, quasi-stationary part and a traveling part. The phase of the quasi-stationary wave was such that a ridge was present over the eastern Atlantic and a trough over western Russia throughout most of the winter. As a result, the majority of migratory cyclones that approached Europe from the west were steered either south toward the Mediterranian or north of Scandinavia.

The synoptic course of events during an outbreak of unusually cold air from the northeast at the end of January 1942 is described in some detail. Some comments are given on how the severe winter weather affected the war in the USSR.

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Harald Lejenäs

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.

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Harald Lejenäs

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.

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Harald Lejenäs
and
Roland A. Madden

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. As a result zonal wavenumbers 1 and 2 cause the largest angular momentum anomalies with peak-to-trough amplitudes of 2–5 × 1023 kg m2 s−1. The 16-day wave produces the largest amplitude, 4.9 × 1023 kg m2 s−1. These values refer to average amplitudes reported in the literature. Individual waves may cause anomalies five times as big.

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Harald Lejenäs
and
Roland A. Madden

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.

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Roland A. Madden
,
Harald Lejenäs
, and
James Hack

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.

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