The Influence of the Spectral Truncation on the Simulation of Waves in the Tropical Stratosphere

Abstract Convectively triggered waves are the main driver of the tropical stratospheric circulation. In atmospheric models, the model’s resolution limits the length of the simulated wave spectrum. In this study, the authors compare the tropical tropospheric wave sources, their projection on the wave field in the lower stratosphere, and the circumstances of their upward propagation in the atmospheric model ECHAM6 with three spectral truncations of T63, T127, and T255. The model internally generates the quasi-biennial oscillation (QBO), which dominates the variability in the tropical stratosphere. This analysis focuses on two opposite phases of the QBO to account for the influence of the background wind field on the wave filtering. It is shown that, compared to the high-resolution model versions, the T63 version has less convective variability and less wave momentum in the lower stratosphere at wavenumbers larger than 20, well below the version’s truncation limit. In the low-resolution version, the upward propagation of the waves is further hindered by the highly active (relative to the high-resolution versions) horizontal diffusion scheme. However, even in the T255 version of ECHAM6, the convective variability is too small compared to TRMM observations at periods shorter than 2 days and wavelengths shorter than 1000 km. Hence, to model a realistic tropical wave activity, the convective parameterization of the model has to improve to increase the day-to-day precipitation variability.

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Towards Direct Simulation of Future Tropical Cyclone Statistics in a High-Resolution Global Atmospheric Model

Advances in Meteorology ◽

10.1155/2010/915303 ◽

2010 ◽

Vol 2010 ◽

pp. 1-13 ◽

Cited By ~ 23

Author(s):

Michael F. Wehner ◽

G. Bala ◽

Phillip Duffy ◽

Arthur A. Mirin ◽

Raquel Romano

Keyword(s):

High Resolution ◽

Radiative Forcing ◽

General Circulation ◽

Circulation Model ◽

Atmospheric Model ◽

Tropical Storm ◽

Model Studies ◽

The North ◽

Resolution Model ◽

High Resolution Model

We present a set of high-resolution global atmospheric general circulation model (AGCM) simulations focusing on the model's ability to represent tropical storms and their statistics. We find that the model produces storms of hurricane strength with realistic dynamical features. We also find that tropical storm statistics are reasonable, both globally and in the north Atlantic, when compared to recent observations. The sensitivity of simulated tropical storm statistics to increases in sea surface temperature (SST) is also investigated, revealing that a credible late 21st century SST increase produced increases in simulated tropical storm numbers and intensities in all ocean basins. While this paper supports previous high-resolution model and theoretical findings that the frequency of very intense storms will increase in a warmer climate, it differs notably from previous medium and high-resolution model studies that show a global reduction in total tropical storm frequency. However, we are quick to point out that this particular model finding remains speculative due to a lack of radiative forcing changes in our time-slice experiments as well as a focus on the Northern hemisphere tropical storm seasons.

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On the presence of equatorial waves in the lower stratosphere of a general circulation model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-22607-2013 ◽

2013 ◽

Vol 13 (8) ◽

pp. 22607-22637 ◽

Cited By ~ 1

Author(s):

P. Maury ◽

F. Lott

Keyword(s):

Gravity Waves ◽

General Circulation ◽

Lower Stratosphere ◽

Circulation Model ◽

Atmospheric Model ◽

Kelvin Waves ◽

Flux Analysis ◽

Equatorial Waves ◽

Quasi Biennial Oscillation ◽

The Waves

Abstract. To challenge the hypothesis that equatorial waves in the lower stratosphere are essentially forced by convection, we use the LMDz atmospheric model extended to the stratosphere and compare two versions having very different convection schemes but no quasi biennial oscillation (QBO). The two versions have realistic time mean precipitation climatologies but very different precipitation variabilities. Despite these differences, the equatorial stratospheric Kelvin waves at 50 hPa are almost identical in the two versions and quite realistic. The Rossby-gravity waves are also very close but significantly weaker than in observations. We demonstrate that this bias on the Rossby-gravity waves is essentially due to a dynamical filtering occurring because the model zonal wind is systematically westward: during a westward phase of the QBO, the Rossby-gravity waves in ERA-Interim compare well with those in the model. These results suggest that in the model the effect of the convection scheme on the waves is in part hidden by the dynamical filtering and the waves are produced by other sources than equatorial convection. For the Kelvin waves, this last point is illustrated by an Eliassen and Palm flux analysis, showing that in the model they come more from the subtropics and mid-latitude regions whereas in the ERA-Interim reanalysis the sources are more equatorial. We also show that non-equatorial sources are significant in reanalysis data, and we consider the case of the Rossby-gravity waves. We identify situations in the reanalysis where here are large Rossby-gravity waves in the middle stratosphere, and for dates when the stratosphere is dynamically separated from the equatorial troposphere. We refer to this process as a "stratospheric reloading".

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On the presence of equatorial waves in the lower stratosphere of a general circulation model

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-1869-2014 ◽

2014 ◽

Vol 14 (4) ◽

pp. 1869-1880 ◽

Cited By ~ 7

Author(s):

P. Maury ◽

F. Lott

Keyword(s):

Gravity Waves ◽

General Circulation ◽

Lower Stratosphere ◽

Circulation Model ◽

Atmospheric Model ◽

Kelvin Waves ◽

Flux Analysis ◽

Equatorial Waves ◽

Quasi Biennial Oscillation ◽

The Waves

Abstract. To challenge the hypothesis that equatorial waves in the lower stratosphere are essentially forced by convection, we use the LMDz atmospheric model extended to the stratosphere and compare two versions having very different convection schemes but no quasi-biennial oscillation (QBO). The two versions have realistic time mean precipitation climatologies but very different precipitation variabilities. Despite these differences, the equatorial stratospheric Kelvin waves at 50 hPa are almost identical in the two versions and quite realistic. The Rossby gravity waves are also very similar but significantly weaker than in observations. We demonstrate that this bias on the Rossby gravity waves is essentially due to a dynamical filtering occurring because the model zonal wind is systematically westward. During a westward phase of the QBO, the ERA-Interim Rossby gravity waves compare well with those in the model. These results suggest that (i) in the model the effect of the convection scheme on the waves is in part hidden by the dynamical filtering, and (ii) the waves are produced by other sources than equatorial convection. For the Kelvin waves, this last point is illustrated by an Eliassen and Palm flux analysis, showing that in the model they come more from the subtropics and mid-latitude regions, whereas in the ERA-Interim reanalysis the sources are more equatorial. We show that non-equatorial sources are also significant in reanalysis data sets as they explain the presence of the Rossby gravity waves in the stratosphere. To illustrate this point, we identify situations with large Rossby gravity waves in the reanalysis middle stratosphere for dates selected when the stratosphere is dynamically separated from the equatorial troposphere. We refer to this process as a stratospheric reloading.

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Impact of the Atmospheric Mean State on Tropical Instability Wave Activity

Journal of Climate ◽

10.1175/2011jcli4244.1 ◽

2012 ◽

Vol 25 (7) ◽

pp. 2341-2355 ◽

Cited By ~ 9

Author(s):

Yukiko Imada ◽

Masahide Kimoto ◽

Xianyan Chen

Keyword(s):

High Resolution ◽

Negative Feedback ◽

Mean Field ◽

Surface Current ◽

Atmospheric Model ◽

Instability Wave ◽

Tropical Instability Waves ◽

Resolution Model ◽

The Difference ◽

High Resolution Models

Abstract The features of simulated tropical instability waves (TIWs) in the Pacific Ocean are compared between atmospheric models of two different resolutions coupled with a uniform oceanic model. Results show that TIWs are more active in the high-resolution model, even though it includes atmospheric negative feedback. Such negative feedback is not identified in the low-resolution atmospheric model because of the absence of atmospheric responses. Comparison of the energetics between the two models shows that the large TIW activity in the higher-resolution model is due to the difference in barotropic energy sources near the surface. A high-resolution atmosphere results in a tighter intertropical convergence zone and associated stronger wind curl and shear. This causes a stronger surface current shear between the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), which is one of the main sources of TIW kinetic energy. These results indicate the important role of the atmospheric mean field on TIW activity and the advantage of using high-resolution models to represent coupling among multiscale phenomena.

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