Convectively coupled equatorial waves (original) (raw)
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Journal of the Atmospheric Sciences, 2020
The companion of this paper, Part I, discovered the characteristics of the rainfall progression in tropicaldepression (TD)-type waves over the western North Pacific. In Part II, the large-scale controls on the convective rainfall progression have been investigated using the ERA-Interim data and the TRMM 3B42 precipitation-rate data during June-October from 1998 to 2013 through budgets of moist static energy (MSE) and moisture. A buildup of columnintegrated MSE occurs in advance of deep convection, and an export of MSE occurs following deep convection, which is consistent with the MSE recharge-discharge paradigm. The MSE recharge-discharge is controlled by horizontal processes, whereby horizontal moisture advection causes net MSE import prior to deep convection. Such moistening by horizontal advection creates a moist midtroposphere, which helps destabilize the atmospheric column, leading to the development of deep convective rainfall. Following the heaviest rainfall, negative horizontal moisture advection dries the troposphere, inhibiting convection. Such moistening and drying processes explain why deep convection can develop without preceding shallow convection. The advection of moisture anomalies by the mean horizontal flow controls the tropospheric moistening and drying processes. As the TD-type waves propagate northwestward in coincidence with the northwestward environmental flow, the moisture, or convective rainfall, is phase locked to the waves. The critical role of the MSE import by horizontal advection in modulating the rainfall progression is supported by the anomalous gross moist stability (AGMS), where the lowest AGMS corresponds to the quickest increase in the precipitation rate prior to the rainfall maximum.
Large-Scale Dynamical Fields Associated with Convectively Coupled Equatorial Waves
Journal of the Atmospheric Sciences, 2000
Convectively coupled equatorial waves, as previously detected in studies of wavenumber-frequency spectra of tropical clouds, are studied in more detail. Composite dynamical structures of the waves are obtained using linear regression between selectively filtered satellite-observed outgoing longwave radiation (OLR) data, and various fields from a global reanalysis dataset. The selective filtering of the OLR was designed to isolate the convective variations contributing to spectral peaks that lie along the equatorial wave dispersion curves for equivalent depths in the range of 12-50 m. The waves studied are the Kelvin, n ϭ 1 equatorial Rossby (ER), mixed Rossby-gravity, n ϭ 0 eastward inertio-gravity, n ϭ 1 westward inertio-gravity (WIG), and n ϭ 2 WIG waves. The horizontal structures of the dynamical fields associated with the waves are all generally consistent with those calculated from inviscid equatorial -plane shallow water theory. In the vertical, there are statistically significant structures spanning the depth of the troposphere, and for all but the ER wave there are associated vertically propagating signals extending into the equatorial stratosphere as well. In zonal cross sections, the vertical structure of the temperature anomaly field appears, for all but the ER wave, as a ''boomerang''-like shape, with the ''elbow'' of the boomerang occurring consistently at the 250-hPa level. The tilts of the boomerang imply upward phase propagation throughout most of the troposphere, and downward phase propagation above. The deep convection of the waves occurs in regions of anomalously cold temperatures in the lower troposphere, warm temperatures in the upper troposphere, and cold temperatures at the level of the tropopause. Such a vertical structure appears to indicate that waves of relatively short vertical wavelengths (L z ϳ 10 km) are indeed important for the coupling of large-scale dynamics and convection. The deeper structure of the convectively coupled ER wave, on the other hand, is thought to be an indication of the effects of basic-state vertical shear. Finally, the scales of the waves in the equatorial lower stratosphere that are forced by the convectively coupled equatorial waves are quite consistent with those found in many previous studies.
Tropical Multiscale Convective Systems: Theory, Modeling, and Observations
Bulletin of the American Meteorological Society, 2009
fundamental advance in the field of tropical meteorology was the discovery by that the dominant component of intraseasonal variability in the tropics is a 40-50-day oscillation of enhanced and suppressed rainfall, now known as the Madden-Julian oscillation (MJO). Nakazawa (1988) suggested that these tropical intraseasonal oscillations are actually space-time envelopes of multiple organized clusters and superclusters of convective clouds occurring on shorter scales than the longwave oscillation. While the MJO envelope propagates eastward, the embedded clusters and superclusters move at faster speeds in both directions. Recent studies (e.g., Wheeler and Kiladis 1999), using outgoing longwave radiation (OLR) spectrum power to produce empirical estimates of the frequency-wavenumber structure of propagating features, have identified these clusters and superclusters as the moist equivalents of linear shallow-water equatorially trapped waves, whereas the MJO signal appears on the bottom of the empirical dispersion diagram, separate from the dispersion relation curves of the linear waves. Organized convection and convectively coupled waves in the tropics have a significant impact on midlatitude weather and climate through atmospheric and oceanic teleconnection patterns (e.g., Zhang 2005). While a broad range of mechanisms has been proposed to explain the MJO, it is typically only poorly represented in contemporary general circulation models (GCMs), apparently due to the inadequate treatment across multiple spatial scales of the interaction of the associated hierarchy of organized structures.
Journal of the Atmospheric Sciences, 2003
Convectively coupled equatorial waves are fundamental components of the interaction between the physics and dynamics of the tropical atmosphere. A new methodology, which isolates individual equatorial wave modes, has been developed and applied to observational data. The methodology assumes that the horizontal structures given by equatorial wave theory can be used to project upper-and lower-tropospheric data onto equatorial wave modes. The dynamical fields are first separated into eastward-and westward-moving components with a specified domain of frequency-zonal wavenumber. Each of the components for each field is then projected onto the different equatorial modes using the y structures of these modes given by the theory. The latitudinal scale y o of the modes is predetermined by data to fit the equatorial trapping in a suitable latitude belt y ϭ ϮY. The extent to which the different dynamical fields are consistent with one another in their depiction of each equatorial wave structure determines the confidence in the reality of that structure. Comparison of the analyzed modes with the eastward-and westward-moving components in the convection field enables the identification of the dynamical structure and nature of convectively coupled equatorial waves. In a case study, the methodology is applied to two independent data sources, ECMWF Reanalysis and satelliteobserved window brightness temperature (T b) data for the summer of 1992. Various convectively coupled equatorial Kelvin, mixed Rossby-gravity, and Rossby waves have been detected. The results indicate a robust consistency between the two independent data sources. Different vertical structures for different wave modes and a significant Doppler shifting effect of the background zonal winds on wave structures are found and discussed. It is found that in addition to low-level convergence, anomalous fluxes induced by strong equatorial zonal winds associated with equatorial waves are important for inducing equatorial convection. There is evidence that equatorial convection associated with Rossby waves leads to a change in structure involving a horizontal structure similar to that of a Kelvin wave moving westward with it. The vertical structure may also be radically changed. The analysis method should make a very powerful diagnostic tool for investigating convectively coupled equatorial waves and the interaction of equatorial dynamics and physics in the real atmosphere. The results from application of the analysis method for a reanalysis dataset should provide a benchmark against which model studies can be compared.
Journal of the Atmospheric Sciences
The companion of this paper, Part I, discovered the characteristics of the rainfall progression in tropical-depression (TD)-type waves over the western North Pacific. In Part II, the large-scale controls on the convective rainfall progression have been investigated using the ERA-Interim data and the TRMM 3B42 precipitation-rate data during June–October from 1998 to 2013 through budgets of moist static energy (MSE) and moisture. A buildup of column-integrated MSE occurs in advance of deep convection, and an export of MSE occurs following deep convection, which is consistent with the MSE recharge–discharge paradigm. The MSE recharge–discharge is controlled by horizontal processes, whereby horizontal moisture advection causes net MSE import prior to deep convection. Such moistening by horizontal advection creates a moist midtroposphere, which helps destabilize the atmospheric column, leading to the development of deep convective rainfall. Following the heaviest rainfall, negative horiz...
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.
2020
A large scale perturbation by the Convectively Coupled Equatorial Waves (CCEW) is often observed in the tropics as a precursor to influence weather condition, for example over the Indonesian archipelago (Maritime Continent (MC)). This study examines the interaction between local factors and CCEW with regard to convection and vertical interferences on a local scale over Indonesia during extreme Western North Pacific (WNP) and Australian (AU) monsoon phases. Through space-time spectra analysis of a 15 year (2001-2015) Tropical Rainfall Measuring Mission (TRMM) 3B42 dataset, the propagation of CCEW, i.e. Kelvin, Equatorial Rossby (ER) and Mixing Rossby-Gravity (MRG) waves was assessed. An Empirical Orthogonal Function (EOF) 1 and 2 for each wave evolution across the region of Indonesia, was then compared with daily precipitation anomalies and multilevel wind observations from seven locations in Indonesia to assess the interaction between local factors and CCEW. Results suggest there is...
Precipitation from African Easterly Waves in a Coupled Model of the Tropical Atlantic
Journal of Climate, 2008
A regional coupled climate model is configured for the tropical Atlantic to explore the role of synopticscale African easterly waves (AEWs) on the simulation of mean precipitation in the marine intertropical convergence zone (ITCZ). Sensitivity tests with varying atmospheric resolution in the coupled model show that these easterly waves are well represented with comparable amplitudes on both fine and coarse grids of the atmospheric model. Significant differences in the model simulations are found in the precipitation fields, however, where heavy rainfall events occur in the region of strong cyclonic shear of the easterly waves only on the higher-resolution grid. This is because the low-level convergence due to the waves is much larger and more realistic in the fine-resolution simulation, which enables heavier precipitation events that skew the rainfall distributions toward longer tails. The variability in rainfall on these time scales accounts for more than 60%-70% of the total variability. As a result, the simulation of mean rainfall in the ITCZ and its seasonal migration improves in the higher-resolution case. This suggests that capturing these transient waves and the resultant strong low-level convergence is one of the key ingredients for improving the simulation of precipitation in global coupled climate models.