2007): Large-scale modes of the tropical atmosphere. Part II: Analytical modeling of Kelvin waves using the CAPE closure, Geofizika (original) (raw)
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Geofizika, 2006
One way of modeling the convectively coupled Kelvin waves in an equatorial non-rotating atmosphere is presented. It implements a simple linear, analytical model using the boundary-layer quasiequilibrium approximation and wind-induced surface heat exchange. The dynamics of the model are based on the assumption that the vertical heating profile has the shape of the first baroclinic mode. The vertical velocity has two sinusoidal components of different vertical wavelengths. One component corresponds to deep convection and the imposed heating profile while the other component, with shallower vertical wavelength, defines the phase speed of the convectively coupled Kelvin wave. The results of the model show fast Kelvin waves that resemble adiabatic modes with the vertical wavelength being twice the depth of the troposphere and convectively coupled Kelvin waves that are damped and propagate with phase speed of 18 m/s. Wind-induced surface heat exchange causes the instability of the convect...
2007
The thermal assumption of the model is based on the convective available potential energy (CAPE) closure, i.e. increased CAPE, represented by decreased midlevel potential temperature, results in increased precipitation. The dynamic assumption of the model is that the vertical heating profile has the shape of the first baroclinic mode, while the vertical dependence of modeled fields is calculated, i.e. the model is vertically resolved. The modeled modes are free Kelvin waves and convectively coupled Kelvin waves. It is shown that the CAPE closure is not sufficient to produce the observed destabilization of the Kelvin mode, but that the dynamical properties of the model give the observed phase speeds.
The Role of Convectively Coupled Atmospheric Kelvin Waves on African Easterly Wave Activity
Monthly Weather Review, 2013
The role of convectively coupled atmospheric Kelvin waves (CCKWs) on African easterly wave (AEW) activity is explored over tropical Africa during boreal summer. Examination of the pre-Alberto AEW in 2000 highlights the observation that the convective trigger for the initiation of the AEW was generated by a strong CCKW and that the subsequent intensification of the AEW at the West African coast was associated with a second CCKW. Composite analysis shows that, generally, AEW activity increases during and after the passage of the convectively active phase of strong CCKWs. The increase in AEW activity is consistent with convective triggering at the leading edge of the convective phase of the CCKW. This convective triggering occurs in a region where the background low-level easterly vertical wind shear is increased by the CCKW. As the AEW propagates westward through the convectively active phase of the CCKW, it can develop in an environment favorable for convection. It is also shown that...
2008
The structure and variability of convectively coupled Kelvin waves during the boreal summer are explored using satellite-observed brightness temperature data and ECMWF reanalyses. Kelvin wave activity is most prominent between the central and eastern Pacific, across Africa, and the Indian Ocean. Composite analysis shows that over sub-Saharan Africa Kelvin wave convection is peaked north of the equator, while the dynamical fields tend to be symmetric with respect to the equator. Convectively coupled Kelvin waves propagate faster over the Pacific and western Atlantic (ϳ24 m s Ϫ1 ), and slow down over tropical Africa (ϳ14 m s Ϫ1 ), consistent with stronger coupling between the dynamics and convection over tropical Africa. The Kelvin waves observed over Africa generally propagate into the region from anywhere between the eastern Pacific and the Atlantic, and decay over the eastern Indian Ocean basin.
Behavior of Kelvin waves for two low latitude stations
IJRSP Vol.49(4) [December 2020], 2020
The wind profiles of Jakarta (Indonesia) and Hyderabad (India) have been analyzed from the radiosonde data for 12 months (January to December 2019) and the data have been used to investigate the behavior of Kelvin waves in the equatorial atmosphere. A non-dispersive wave of zonal wind easterlies are recorded between 15 and 25 km altitude in January over the Jakarta region with zero perturbation of meridional wind velocity at the same altitude. Similar pattern with less magnitude of easterlies has also been recorded over Hyderabad region at the same period. This might be an apparent evidence for the decay of Kelvin waves as they move away from the equator and the two regions have a nearly opposite trend of wind profiles. Further effect of Kelvin waves on the atmospheric pressure has been investigated for both the regions at three different altitudes including 5, 15 and 25 km, respectively. The observed effect indicate the existence of kelvin waves depend on the decrease of pressure. Therefore, there is nearly four months interval for the shifting of an intertropical conventional zone (ITCZ) from Jakarta to Hyderabad latitude.
Dynamics of the Tropical Middle Atmosphere: A Tutorial Review
Atmosphere-Ocean, 1998
The general circulation of the tropical stratosphere, mesosphere and lowermost thermosphere is discussed at a tutorial level. Observations of the quasi-biennial and semiannual oscillations by both in situ and satellite techniques are first reviewed. The basic dynamics controlling the zonal-mean component of the circulation are then discussed. The role of radiative diabatic cooling in constraining the zonal-mean circulation in the middle atmosphere is emphasized. It is shown that the effectiveness of this radiative constraint is reduced at low latitudes, allowing for the sustained mean flow accelerations over long periods of time characteristic of the quasi-biennial and semiannual oscillations in the tropics.
The Interaction of Waves and Convection in the Tropics
Journal of the Atmospheric Sciences, 2003
Interest in tropical waves and their interaction with convection has been rekindled in recent years by the discovery, using satellite infrared data to track high clouds, that such waves closely display the dispersive properties of linear, inviscid wave theory for an atmosphere with a resting basic state and equivalent depths between 12 and 60 m. While several current approaches focus on internal modes in the atmosphere, this is inconsistent with the absence of internal modes in the atmosphere, which is characterized by a single isolated eigenmode and a continuous spectrum. It will be shown, using an extremely simple approach to convection, that the observed properties of waves are consistent with a continuous spectrum. The approach assumes that the total convection is determined by mean evaporation, but that the convection is patterned by zero-averaged perturbations to triggering energy following the recent approach of Mapes. This is, perhaps, the simplest hypothesis that can be applied. The observed convection associated with the migrating semidiurnal tide is used to calibrate the time scale for the convective response to patterning, which is the only adjustable parameter in this formulation. It is shown that this time scale leads to not only the observed phase of the semidiurnal heating but also the observed phase lead of low-level convergence in tropical waves vis-à-vis the convective heating. Finally, it is shown that this phase is sensitive to the equivalent depth, which it is suggested is the basis for the selection of equivalent depth. Reasonable simulations of observed waves are readily obtained.
Vertically Propagating Kelvin Waves and Tropical Tropopause Variability
Journal of the Atmospheric Sciences, 2008
The relationship between local convection, vertically propagating Kelvin waves, and tropical tropopause height variability is examined. This study utilizes both simulations of a global primitive-equation model and global observational datasets. Regression analysis with the data shows that convection over the western tropical Pacific is followed by warming in the upper troposphere (UT) and cooling in lower stratosphere (LS) over most longitudes, which results in a lifting of the tropical tropopause. The model results reveal that these UT–LS temperature anomalies are closely associated with vertically propagating Kelvin waves, indicating that these Kelvin waves drive tropical tropopause undulations at intraseasonal time scales. The model simulations further show that regardless of the longitudinal position of the imposed heating, the UT–LS Kelvin wave reaches its maximum amplitude over the western Pacific. This result, together with an analysis based on wave action conservation, is us...
Monthly Weather Review, 2012
High-amplitude convectively coupled equatorial atmospheric Kelvin waves (CCKWs) are explored over the tropical Atlantic during the boreal summer (1989–2009). Focus is given to the atmospheric environmental conditions that are important for tropical cyclogenesis. CCKWs are characterized by deep westerly vertical wind shear to the east of its convectively active phase and easterly vertical wind shear to the west of it. This dynamical signature increases vertical wind shear over the western tropical Atlantic ahead of the convectively active phase, and reduces vertical wind shear after its passage. The opposite is true over the eastern tropical Atlantic where the climatological vertical wind shear is easterly. Positive total column water vapor (TCWV) anomalies progress eastward with the convectively active phase of the CCKW, whereas negative TCWV anomalies progress eastward with the convectively suppressed phase. During the passage of the convectively active phase of the CCKW, a zonally...
Interaction of equatorial waves through resonance with the diurnal cycle of tropical heating
Tellus A, 2010
In this work, a new theoretical mechanism is presented in which equatorial Rossby and inertio-gravity wave modes may interact with each other through resonance with the diurnal cycle of tropical deep convection. We have adopted the two-layer incompressible equatorial primitive equations forced by a parametric heating that roughly represents deep convection activity in the tropical atmosphere. The heat source was parametrized in the simplest way according to the hypothesis that it is proportional to the lower-troposphere moisture convergence, with the background moisture state function mimicking the structure of the ITCZ. In this context, we have investigated the possibility of resonant interaction between equatorially trapped Rossby and inertio-gravity modes through the diurnal cycle of the background moisture state function. The reduced dynamics of a single resonant duo shows that when this diurnal variation is considered, a Rossby wave mode can undergo significant amplitude modulations when interacting with an inertio-gravity wave mode, which is not possible in the context of the resonant triad non-linear interaction. Therefore, the results suggest that the diurnal variation of the ITCZ can be a possible dynamical mechanism that leads the Rossby waves to be significantly affected by high frequency modes.