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Isolating the effects of moisture entrainment on convectively coupled equatorial waves in an aquaplanet GCM

Peatman, S. C., Methven, J. and Woolnough, S. J. (2018) Isolating the effects of moisture entrainment on convectively coupled equatorial waves in an aquaplanet GCM. Journal of the Atmospheric Sciences, 75 (9). pp. 3139-3157. ISSN 1520-0469

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To link to this item DOI: 10.1175/JAS-D-18-0098.1

Abstract/Summary

The rate of humidity entrainment in the convective parametrization scheme in a general circulation model affects the simulation of convectively-coupled waves. However, it is unclear whether this is caused directly by the effects of entrainment on waves or indirectly through associated impacts such as on the basic state. Therefore, using an aquaplanet model, we employ a novel framework in which we entrain a weighted average of the resolved humidity field and a prescribed zonally symmetric field, with the weighting controlled by a decoupling parameter. Hence, we can vary the entrainment rate of basic state humidity independently of the entrainment of humidity perturbations, simultaneously minimizing changes in basic state. Thus we isolate the effect of moisture entrainment on the waves. Enhancing entrainment rate increases spectral power over all zonal wavenumbers and frequencies, with an increase in the ratio of eastward-to-westward power. The Kelvin wave speed decreases as entrainment increases, which can be partially accounted for by an associated change in basic state humidity. Increasing the decoupling parameter reduces spectral power in Kelvin waves relative to the background, with only long waves still prominent when entrainment is almost fully decoupled from the resolved moisture field, suggesting the wave structure in humidity is required for convection to organize into short wave structures. For long waves the increase in the ratio of eastward-to-westward power as entrainment rate increases cannot be explained by the changes in the coupling with the wave structure in humidity, but is consistent with the changes in the basic state.

Item Type:Article
Refereed:Yes
Divisions:Faculty of Science > School of Mathematical, Physical and Computational Sciences > NCAS
Faculty of Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
ID Code:77851
Publisher:American Meteorological Society

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