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Adrian J. Matthews and Jason Lander

Abstract

“Nonlinear Kelvin wave-CISK modes” are critically assessed as a possible mechanism for the Madden–Julian Oscillation (MJO) with a global spectral transform model and a one-dimensional analog. Convection is parameterized using a simple “positive-only CISK” scheme, where tropospheric diabatic heating is proportional to the low-level convergence, and is set to zero in regions of low-level divergence. Although the modes have many properties that are consistent with the MJO, they also have a serious drawback. The growth rate of unstable modes depends crucially on the width of the heating region, which is resolution dependent. The “CISK catastrophe” has not been averted, and the heating region collapses to the smallest localized scale that the model can support. This scale is larger than the model resolution, as measured by both the gridpoint scale and the inverse wavenumber or half-zonal-wavelength of the highest wavenumber basis function, and is associated with the appearance of negative Gibbs fringes, which are then cut off by the positive-only CISK parameterization.

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Adrian J. Matthews and George N. Kiladis

Abstract

Equatorward-propagating wave trains in the upper troposphere are observed to be associated with deep convection over the eastern tropical Pacific on the submonthly timescale during northern winter. The convection occurs in the regions of ascent and reduced static stability ahead of cyclonic anomalies in the wave train. In this study an atmospheric primitive equation model is used to examine the roles of the dry wave dynamics and the diabatic heating associated with the convection.

Many features of a dry integration initialized with a localized wave train in the African–Asian jet on a three-dimensional climatological basic state quantitatively agree with the observations, including the zonal wavenumber 6–7 scale of the waves, the time period of approximately 12 days, and the cross-equatorial Rossby wave propagation over the eastern Pacific. There is ascent and reduced static stability ahead of the cyclonic anomalies, consistent with the interpretation that the waves force the convection. The spatial scale of the waves appears to be set by the basic state; baroclinic growth upstream in the Asian jet favors waves with zonal wavenumber 6. On reaching the Pacific sector, lower-wavenumber components of the wave train are not refracted so strongly equatorward, while higher-wavenumber components are advected quickly along the Pacific jet before they can propagate equatorward. Once over the Pacific, the wave train approximately obeys barotropic Rossby wave dynamics.

The observed lower-tropospheric anomalies include an equatorial Rossby wave that propagates westward from the region of cross-equatorial wave propagation and tropical convection. However, this equatorial Rossby wave is not forced directly by the dry equatorward-propagating wave train but appears in a separate integration as a forced response to the observed diabatic heating associated with the tropical convection.

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Adrian J. Matthews and Roland A. Madden

Abstract

The structure of the 33-h Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP–NCAR reanalysis data. Cross-spectral analysis of 6 yr (1993–98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 h. The phase angles are consistent with an eastward-propagating zonal-wavenumber-1 structure, implying an equatorial phase speed of approximately 340 m s−1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 yr (1968–98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-1 equatorial Kelvin wave with an equatorial trapping scale of approximately 34° lat. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two datasets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation dataset.

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Alexandros P. Poulidis, Ian A. Renfrew, and Adrian J. Matthews

Abstract

Intense rainfall over active volcanoes is known to trigger dangerous volcanic hazards, from remobilizing loose volcanic surface material into lahars or mudflows to initiating explosive activity including pyroclastic flows at certain dome-forming volcanoes. However, the effect of the heated volcanic surface on the atmospheric circulation, including any feedback with precipitation, is unknown. This is investigated here, using the Weather Research and Forecasting (WRF) Model. The recent activity at the Soufrière Hills Volcano (SHV), Montserrat, is a well-documented case of such rainfall–volcano interaction and is used as a template for these experiments. The volcano is represented in the model by an idealized Gaussian mountain, with an imposed realistic surface temperature anomaly on the volcano summit. A robust increase in precipitation over the volcano is simulated for surface temperature anomalies above approximately 40°C, an area-average value that is exceeded at the SHV. For wind speeds less than 4 m s−1 and a range of realistic atmospheric conditions, the precipitation increase is well above the threshold required to trigger volcanic hazards (5–10 mm h−1). Hence, the thermal atmospheric forcing due to an active, but nonerupting, volcano appears to be an important factor in rainfall–volcano interactions and should be taken account of in future hazard studies.

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