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This study presents some explicit exact solutions for nonlinear geophysical ocean waves in the β-plane approximation near the equator. The solutions are provided in Lagrangian coordinates by describing the path of each particle. The unidirectional equatorially trapped waves are symmetric about the equator and propagate eastward above the thermocline and beneath the near-surface layer to which wind effects are confined. At each latitude the flow pattern represents a traveling wave. [ABSTRACT FROM AUTHOR]

Abstract: An improved cloud tracking method for deriving wind velocities from successive planetary images was developed. The new method incorporates into the traditional cross-correlation method an algorithm that corrects for erroneous cloud motion vectors by re-determining the most plausible correlation peak among all of the local maxima on the correlation surface by comparing each vector with its neighboring vectors. The newly developed method was applied to the Venusian violet images obtained by the Solid State Imaging system (SSI) onboard the Galileo spacecraft during its Venus flyby. Although the results may be biased by the choice of spatial scale of atmospheric features, the cloud tracking is the most practical mean of estimating the wind velocities with extensive spatial and temporal coverage. The two-dimensional distribution of the horizontal wind vector field over 5 days was obtained. It was found from these wind maps that the solar-fixed component in 1990 was similar to that in 1982 obtained by the Pioneer Venus orbiter. The deviation of the instantaneous zonal wind field from the solar-fixed component shows a distinct wavenumber-1 structure in the equatorial region. On the assumption that this structure is a manifestation of an equatorial Kelvin wave, the phase relationship between the zonal wind and the cloud brightness suggests a short photochemical lifetime of the violet absorber. The momentum deposition by this Kelvin wave, which is subject to radiative damping, would induce a westward mean-wind acceleration of ∼0.3ms−1 per Earth day. [Copyright &y& Elsevier]

Multiscale convective wave disturbances with structures broadly resembling observed tropical waves are found to emerge spontaneously in a nonrotating, two-dimensional cloud model forced by uniform cooling. To articulate the dynamics of these waves, model outputs are objectively analyzed in a discrete truncated space consisting of three cloud types (shallow convective, deep convective, and stratiform) and three dynamical vertical wavelength bands. Model experiments confirm that diabatic processes in deep convective and stratiform regions are essential to the formation of multiscale convective wave patterns. Specifically, upper-level heating (together with low-level cooling) serves to preferentially excite discrete horizontally propagating wave packets with roughly a full-wavelength structure in troposphere and “dry” phase speeds cn in the range 16–18 m s-1. These wave packets enhance the triggering of new deep convective cloud systems, via low-level destabilization. The new convection in turn causes additional heating over cooling, through delayed development of high-based deep convective cells with persistent stratiform anvils. This delayed forcing leads to an intensification and then widening of the low-level cold phases of wave packets as they move through convecting regions. Additional widening occurs when slower-moving (∼8 m s-1) “gust front” wave packets excited by cooling just above the boundary layer trigger additional deep convection in the vicinity of earlier convection. Shallow convection, meanwhile, provides positive forcing that reduces convective wave speeds and destroys relatively small-amplitude-sized waves. Experiments with prescribed modal wind damping establish the critical role of short vertical wavelengths in setting the equivalent depth of the waves. However, damping of deep vertical wavelengths prevents the clustering of mesoscale convective wave disturbances into larger-scale envelopes, so these circulations are important as well. [ABSTRACT FROM AUTHOR]

Atmospheric general circulation model experiments are conducted to quantify the contribution of internal oceanic variability in the form of tropical instability waves (TIWs) to interannual wind and rainfall variability in the tropical Pacific. It is found that in the tropical Pacific, along the equator, and near 25°N and 25°S, TIWs force a significant increase in wind and rainfall variability from interseasonal to interannual time scales. Because of the stochastic nature of TIWs, this means that climate models that do not take them into account will underestimate the strength and number of extreme events and may overestimate forecast capability. [ABSTRACT FROM AUTHOR]

The origin, initiation, and dispersion behavior of the observed summertime synoptic-scale disturbances in the tropical western Pacific are studied. These westward-propagating disturbances have the strongest growth rate over the region of ∼130°–160°E off the equator. The three-dimensional wave activity flux associated with a wave packet in the vicinity of this region is computed. In general, wave activity is directed westward. There is accumulation of activity flux, which gives rise to the amplification of waves. In the low levels, such accumulation can be attributed to the convergence of both the mean flow and the intrinsic group velocity. Diabatic forcing also contributes to the growth of disturbances and is most important in the 500–600-hPa layer. Along the east–west-oriented “storm tracks” of the synoptic-scale disturbances, there are two different dynamical regimes. West of ∼150°E, enhanced convection is associated with increased specific humidity at the top of the planetary boundary layer and is in phase with positive low-level vorticity anomalies. To the east of 150°E the vorticity leads the convection by about one-quarter of a wavelength. This phase relationship can be explained by adiabatic dynamics and is related to the positive vertical shear of the mean zonal flow in the latter region. Near and to the east of the date line where disturbances are initiated in the low levels, the heat flux associated with the synoptic-scale eddies is negative (i.e., υ′T ′ < 0) from about 300 to 700 hPa. This implies downward-directed wave activity. In the upper troposphere at the same geographical location, there is southward wave activity from the extratropics penetrating into the Tropics. These findings suggest that summertime synoptic-scale disturbances may originate from extratropical forcing. This hypothesis is supported by a case study. Intrusion of high potential vorticity into the Tropics was seen to be followed by downward development, resulting in low-level disturbances that subsequently moved westward in the western Pacific and grew. [ABSTRACT FROM AUTHOR]

A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2°S–6°N, 160°–90°W, where instabilities are most active. TIW-induced horizontal advection leads to a warming of 0.84°C month-1, which is of the same order as the 0.77°C month-1 warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by -0.59°C month-1 and -1.06°C month-1, respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution. [ABSTRACT FROM AUTHOR]

Simultaneous measurements of Ku-band attenuation and rain-rate give A0.01 and R0.01 9.7 dB and 54 mm/hr respectively, Unlike the monotonous variation predicted by the ITU-R model, breakpoints wore observed in both the rain-rate and Ku-band attenuation exceedances. Similar observations were reported from other stations in the tropics. The accumulation time factor measured at the breakpoints remained almost a constant in these stations. Using this it is predicted that for stations with high elevation angle receivers, at breakpoint rain-rates, one rain-cell intercepts the propagation path of the signal. It is estimated that, in the tropics, the diameter of a rain-cell is ∼1.5 km when the rain-rate begin to saturate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]