Meridional Wind Field Research Articles

Phase-lagged wave-CISK

The conventional formulation of the wave-CISK mechanism involves a quasi-equilibrium assumption whereby the instantaneous large-scale diabatic heating that is induced by cloud-scale motions is assumed to be related to the instantaneous distribution of the large-scale flow, and in particular to the low-level moisture convergence of the large-scale flow. Thus in this context the deep convective clouds of the tropical atmosphere are viewed as instantaneous processors of the large-scale moisture supply. Herein a conventional, inviscid wave-CISK model is developed and an examination undertaken of the properties of the analytically determined solutions. The quasi-equilibrium assumption is then discarded by incorporating into the CISK model's formulation a simple time-dependent model of the dynamics of a cumulus cloud ensemble. The latter model has a finite adjustment time (TR) to changes in the large-scale forcing field. Hence the apparent diabatic heat source experienced by the large-scale fields becomes a function of the time history of that same forcing field. Neutral solutions of the refined wave-CISK model are derived as functions of the diabatic heating amplitude and the adjustment time. These solutions illustrate the frequency-selective stabilizing effect of the hypothesized cloud-scale dynamics. Again, the marginally unstable waves of the refined model are shown to exhibit properties that overcome some of the debilitating features of conventional wave-CISK. These solutions also reproduce some of the observed features of tropical wave motions. In particular, for Tr ∼ 12 h and with a physically reasonable value of the cumulus diabatic heating amplitude, it is shown that all the inertia-gravity modes are stable, and that the most unstable Kelvin wave, mixed Rossby-gravity wave and Rossby (n = 1 and n = 2) waves have finite preferred spatial length scales corresponding respectively to zonal wavenumbers (m) of 2, 4, 6 and 8. The periods of these four waves correspond approximately to 10, 4/1/2 12/1/2 and 15 days but the growth rates are comparatively small. The agreement with tropical wave data is not unreasonable when allowance is made for the Doppler shift of the Rossby waves. Also the vertical structures of these preferred modes are different: the Kelvin and mixed Rossby-gravity waves are associated respectively with the shortest and longest vertical wavelengths, and the meridional wind field of the Rossby wave (n = 1) has a maximum in both the upper and lower troposphere. The degree of mutual compensation in the model solutions between diabatic heating due to cumulus clouds and cooling due to large-scale ascent is not as strong as that observed in the atmosphere.

Phase-lagged wave-CISK

The conventional formulation of the wave-CISK mechanism involves a quasi-equilibrium assumption whereby the instantaneous large-scale diabatic heating that is induced by cloud-scale motions is assumed to be related to the instantaneous distribution of the large-scale flow, and in particular to the low-level moisture convergence of the large-scale flow. Thus in this context the deep convective clouds of the tropical atmosphere are viewed as instantaneous processors of the large-scale moisture supply. Herein a conventional, inviscid wave-CISK model is developed and an examination undertaken of the properties of the analytically determined solutions. The quasi-equilibrium assumption is then discarded by incorporating into the CISK model's formulation a simple time-dependent model of the dynamics of a cumulus cloud ensemble. The latter model has a finite adjustment time (TR) to changes in the large-scale forcing field. Hence the apparent diabatic heat source experienced by the large-scale fields becomes a function of the time history of that same forcing field. Neutral solutions of the refined wave-CISK model are derived as functions of the diabatic heating amplitude and the adjustment time. These solutions illustrate the frequency-selective stabilizing effect of the hypothesized cloud-scale dynamics. Again, the marginally unstable waves of the refined model are shown to exhibit properties that overcome some of the debilitating features of conventional wave-CISK. These solutions also reproduce some of the observed features of tropical wave motions. In particular, for Tr ∼ 12 h and with a physically reasonable value of the cumulus diabatic heating amplitude, it is shown that all the inertia-gravity modes are stable, and that the most unstable Kelvin wave, mixed Rossby-gravity wave and Rossby (n = 1 and n = 2) waves have finite preferred spatial length scales corresponding respectively to zonal wavenumbers (m) of 2, 4, 6 and 8. The periods of these four waves correspond approximately to 10, 4/1/2 12/1/2 and 15 days but the growth rates are comparatively small. The agreement with tropical wave data is not unreasonable when allowance is made for the Doppler shift of the Rossby waves. Also the vertical structures of these preferred modes are different: the Kelvin and mixed Rossby-gravity waves are associated respectively with the shortest and longest vertical wavelengths, and the meridional wind field of the Rossby wave (n = 1) has a maximum in both the upper and lower troposphere. The degree of mutual compensation in the model solutions between diabatic heating due to cumulus clouds and cooling due to large-scale ascent is not as strong as that observed in the atmosphere.

Eastern Tropical Ocean Response to Changing Wind Systems: with Application to El Niño

El Niño is an anomalous condition that historically has been studied in the coastal region of the eastern tropical South Pacific. The phenomenon is commonly characterized there by the rapid appearance of anomalously high sea level and sea surface temperatures, and southward transport of the warmer, fresher equatorial waters. Observational evidence also suggests that El Niño is associated with a large-scale weakening of the equatorial wind systems over a large extent of the ocean interior. The problem investigated here is whether these ocean and atmosphere events are dynamically connected. A model is used to follow the initial baroclinic response of the eastern Pacific Ocean to different distributions of interior wind stress change. Model results suggest the following conclusions. Changes in the meridional wind field cannot cause an El Niño event. Changes in the zonal wind field outside of the equatorial band (roughly ±5° of latitude) are not important in generating El Niño. A symmetric weakening of the equatorial zonal wind field can cause rapid lowering of the thermocline in the eastern Pacific; moreover, this lowering spreads all along the eastern boundary well out of the region of direct forcing and at higher latitudes is associated with poleward quasi-geostrophic coastal jets. An anti-symmetric change of the equatorial zonal wind field forces cross-equatorial transport; in this case, the anomaly fields do not spread along the eastern boundary but remain confined to the equatorial region. An asymmetric change of the equatorial zonal wind field has commonly observed aspects of El Niño; this response is defined to be a “model” El Niño event. The model is of interest in itself. It is simple and analytic, and therefore its response can be understood in terms of familiar linear wave theory. After the swiftly propagating coastal Kelvin waves have passed through the model, the solution in the eastern Pacific can be viewed as a superposition of three parts: 1) a locally forced response, 2) packets of nondispersive Rossby waves excited at the eastern boundary, and 3) packets of equatorially trapped Kelvin waves arriving from the western boundary.

Effects of heating at high latitudes on global thermospheric dynamics

Three dimensional numerical calculations of thermospheric wind, density and temperature fields generated by solar EuV heating were described in a recent paper by Straus et al. (1975). These model calculations indicated that many of the global characteristics of the thermosphere are generated by EUV heating alone. However, several observed features were found to be poorly represented by the model results. These include the amplitude of the diurnal temperature variation and properties of the diurnal variations of the meridional wind and pressure gradient fields. The present paper describes investigation of the extent to which heating of the thermosphere at high latitudes would contribute to the resolution of these difficulties. This heating lowers the day to night temperature ratio, since it provides a heat source at night. It brings calculated meridional winds and pressure gradients into better agreement with observations, since it raises isobaric surfaces at high latitudes. The amount of heating at high latitudes required to make these modifications is consistent with that due to quiet time joule heating.

A Diagnostic Model for Equatorial Wave Disturbances: The Role of Vertical Shear of the Mean Zonal Wind

Abstract A model is developed to diagnose the response of the atmosphere to a known distribution of diabatic heating. The linearized primitive equations in spherical coordinates are reduced to a single partial differential equation relating the perturbation geopotential to the diabatic heating pattern. The model diagnoses the atmospheric perturbation due to a heating pattern of specified zonal wavenumber, frequency, and distribution in the meridional plane. The model is used to test the hypothesis that the observed westward propagating wave disturbances in the equatorial Pacific are Rossby waves driven by the latent heat release in the cloud clusters embedded within the waves. For a diabatic heating pattern designed to model the heating in cloud clusters the model duplicates many features of the observed waves. The computed perturbation meridional wind field has maxima in the upper and lower troposphere, separated by a relatively undisturbed region in the mid-troposphere. The structure of the disturbance ...