Organized Tropical Convection Research Articles

An Object-Based Approach to Assessing the Organization of Tropical Convection

Abstract The organization of tropical convection is assessed through an object-based analysis of satellite brightness temperature data Tb, a proxy for convective activity. The analysis involves the detection of contiguous cloud regions (CCRs) in the three-dimensional space of latitude, longitude, and time where Tb falls below a given threshold. A range of thresholds is considered and only CCRs that satisfy a minimum size constraint are retained in the analysis. Various statistical properties of CCRs are documented including their zonal speed of propagation, which is estimated using a Radon transformation technique. Consistent with previous studies, a majority of CCRs are found to propagate westward, typically at speeds of around 15 m s−1, regardless of underlying Tb threshold. Most of these zonally propagating CCRs have lifetimes less than 2 days and zonal widths less than 800 km, implying aggregation of just a few individual mesoscale convective systems. This object-based perspective is somewhat different than that obtained in previous Fourier-based analyses, which primarily emphasize the organization of convection on synoptic and planetary scales via wave–convection coupling. To reconcile these contrasting views, an object-based data reconstruction is developed that objectively demonstrates how the spectral peaks of synoptic- to planetary-scale waves can be attributed to the organization of CCRs into larger-scale wave envelopes. A novel method based on the randomization of CCRs in physical space leads to an empirical background spectrum for organized tropical convection that does not rely on any smoothing in spectral space. Normalization by this background reveals spectral peaks associated with synoptic- and planetary-scale waves that are consistent with previous studies.

The Tropical Subseasonal Variability Simulated in the NASA GISS General Circulation Model

Abstract The tropical subseasonal variability simulated by the Goddard Institute for Space Studies general circulation model, Model E2, is examined. Several versions of Model E2 were developed with changes to the convective parameterization in order to improve the simulation of the Madden–Julian oscillation (MJO). When the convective scheme is modified to have a greater fractional entrainment rate, Model E2 is able to simulate MJO-like disturbances with proper spatial and temporal scales. Increasing the rate of rain reevaporation has additional positive impacts on the simulated MJO. The improvement in MJO simulation comes at the cost of increased biases in the mean state, consistent in structure and amplitude with those found in other GCMs when tuned to have a stronger MJO. By reinitializing a relatively poor-MJO version with restart files from a relatively better-MJO version, a series of 30-day integrations is constructed to examine the impacts of the parameterization changes on the organization of tropical convection. The poor-MJO version with smaller entrainment rate has a tendency to allow convection to be activated over a broader area and to reduce the contrast between dry and wet regimes so that tropical convection becomes less organized. Besides the MJO, the number of tropical-cyclone-like vortices simulated by the model is also affected by changes in the convection scheme. The model simulates a smaller number of such storms globally with a larger entrainment rate, while the number increases significantly with a greater rain reevaporation rate.

Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans

Abstract The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types—congestus, deep, and stratiform—that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature θe connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer θe and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day−1) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day−1 at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.

Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part II: The Continental Regime

Abstract The variation of precipitation over land due to the diurnal cycle of solar heating is examined here in the context of a simple multicloud model for tropical convection with bulk atmospheric boundary layer (ABL) dynamics. The model utilizes three cloud types (congestus, deep, and stratiform) that are believed to characterize organized tropical convection based on the first two baroclinic modes of vertical structure in the free troposphere, coupled to the ABL through full bulk boundary layer (FBBL) dynamics, that allow a careful separation between sensible and latent heat surface fluxes. In a land parameter regime, characterized by a strong inversion profile, a large Bowen ratio of 0.4, and active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL, the model supports a stable 1-day periodic solution that is characterized by a pronounced (7 K day−1) afternoon peak in precipitation consistent with observations of tropical precipitation over continental regions. The current study suggests a division of the diurnal cycle of precipitation over land into a cycle of five phases: 1) an overnight phase of a radiative–convective equilibrium (RCE) state between 2000 and 0600 LST; 2) an early morning CAPE buildup accompanied by a sudden rise in precipitation that quickly dries the middle troposphere occurs between 0600 and roughly 1000 LST; 3) a moistening phase between roughly 1000 and 1600 LST; 4) a phase of maximum precipitation between 1600 and 1800 LST that dries the middle troposphere and quickly consumes CAPE; and 5) a rapid remoistening phase that restores the moisture level to sustain the overnight RCE precipitation and connects to phase 1 in the cycle. Sensitivity tests in the model confirm that the late afternoon precipitation maximum over land depends crucially on a strong inversion, the large Bowen ratio, and the active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL.

The MJO and Convectively Coupled Waves in a Coarse-Resolution GCM with a Simple Multicloud Parameterization

Abstract The adequate representation of the dominant intraseasonal and synoptic-scale variability in the tropics, characterized by the Madden–Julian oscillation (MJO) and convectively coupled waves, is still problematic in current operational general circulation models (GCMs). Here results are presented using the next-generation NCAR GCM—the High-Order Methods Modeling Environment (HOMME)—as a dry dynamical core at a coarse resolution of about 167 km, coupled to a simple multicloud parameterization. The coupling is performed through a judicious choice of heating vertical profiles for the three cloud types—congestus, deep, and stratiform—that characterize organized tropical convection. Important control parameters that affect the types of waves that emerge are the background vertical gradient of the moisture and the stratiform fraction in the multicloud parameterization, which set the strength of large-scale moisture convergence and unsaturated downdrafts in the wake of deep convection, respectively. Three numerical simulations using different moisture gradients and different stratiform fractions are considered. The first experiment uses a large moisture gradient and a small stratiform fraction and provides an MJO-like example. It results in an intraseasonal oscillation of zonal wavenumber 2, moving eastward at a constant speed of roughly 5 m s−1. The second uses a weaker background moisture gradient and a large stratiform fraction and yields convectively coupled Rossby, Kelvin, and two-day waves, embedded in and interacting with each other; and the third experiment combines the small stratiform fraction and the weak background moisture gradient to yield a planetary-scale (wavenumber 1) second baroclinic Kelvin wave. While the first two experiments provide two benchmark examples that reproduce several key features of the observational record, the third is more of a demonstration of a bad MJO model solution that exhibits very unrealistic features.

Convectively Coupled Waves in a Sheared Environment

Abstract A linear stability analysis, about a radiative–convective equilibrium in a sheared environment, on an equatorial beta plane, for a simple multicloud model for organized tropical convection is presented here. Both vertical/baroclinic and meridional/barotropic zonal wind shears are considered separately in a parameter regime for which the shear-free multicloud model exhibits synoptic-scale instability of Kelvin and n = 0 eastward inertio-gravity [eastward mixed Rossby–gravity (MRG)] waves only, with moderate growth rates. The maximum growth rates appear to increase significantly with the strength of the background wind shear, and new wave instabilities appear and/or disappear depending on the strength and type of the wind shear. It is found here that both high- and low-level vertical shears have a strong impact on the stability of convectively coupled waves (CCWs), consistent with the fact that the multicloud instability mechanism is controlled by both stratiform heating and low-level moisture and congestus heating. Typically, vertical shears with high-level easterly wind destabilize westward moving waves and stabilize eastward waves, whereas westerly winds aloft and on bottom tend to destabilize eastward moving and stabilize westward moving waves. In the mixed situation of high-level easterlies and low-level westerlies both eastward and westward waves are unstable, while in the case of high-level westerlies and low-level easterlies only eastward waves are unstable. In the presence of a barotropic/meridional shear, synoptic-scale convectively coupled westward MRG and Rossby waves emerge, when the shear strength is large enough, due essentially to pure shear instability of the dry dynamics. The meridional shear has also an important impact on the horizontal structure of the waves. Owing to the meridional shear, the Kelvin wave displays a nonzero meridional velocity that induces a significant contribution toward the horizontal convergence. The two-day waves adopt a crescentlike shape while the westward MRG, and somewhat the Rossby waves, become less trapped in the vicinity of the equator.

Multiscale cloud system modeling

The central theme of this paper is to describe how cloud system resolving models (CRMs) of grid spacing ∼1 km have been applied to various important problems in atmospheric science across a wide range of spatial and temporal scales and how these applications relate to other modeling approaches. A long‐standing problem concerns the representation of organized precipitating convective cloud systems in weather and climate models. Since CRMs resolve the mesoscale to large scales of motion (i.e., 10 km to global) they explicitly address the cloud system problem. By explicitly representing organized convection, CRMs bypass restrictive assumptions associated with convective parameterization such as the scale gap between cumulus and large‐scale motion. Dynamical models provide insight into the physical mechanisms involved with scale interaction and convective organization. Multiscale CRMs simulate convective cloud systems in computational domains up to global and have been applied in place of contemporary convective parameterizations in global models. Multiscale CRMs pose a new challenge for model validation, which is met in an integrated approach involving CRMs, operational prediction systems, observational measurements, and dynamical models in a new international project: the Year of Tropical Convection, which has an emphasis on organized tropical convection and its global effects.

On the radiative effects of dust on tropical convection

Radiative‐convective equilibrium experiments with a two‐dimensional cloud resolving model illustrate the influence of a lofted absorbing dust layer on the organization of tropical convection. At quasi‐equilibrium, the dust‐covered region of the model exhibits increased occurrence of deep convection compared to the dust‐free region but with reduced convection in the dust‐free region controlled in part by a large‐scale monsoon‐like circulation forced by the aerosol radiative heating. The dry air associated with the dust layer inhibits convection initially over most of the dust‐covered region with convection occurring predominantly at the lateral boundaries of the layer. This behavior reproduces features which have been observed in cases of Saharan dust transport over the tropical Atlantic.

From Hot Towers to TRMM: Joanne Simpson and Advances in Tropical Convection Research

Joanne Simpson began contributing to advances in tropical convection about half a century ago. The hot tower hypothesis jointly put forth by Joanne Simpson and Herbert Riehl postulated that deep convective clouds populating the “equatorial trough zone” were responsible for transporting heat from the boundary layer to the upper troposphere. This hypothesis was the beginning of a 50-year quest to describe and understand near-equatorial deep convection. Tropical field experiments in the 1970s [Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) and the Monsoon Experiment (MONEX)] in which Joanne participated documented the mesoscale structure of the convective systems, in particular the deep, stratiform, dynamically active mesoscale clouds that are connected with the hot towers. In the 1980s these new data led to better understanding of how tropical mesoscale convective systems vertically transport heat and momentum. The role of the mesoscale stratiform circulation in this transport was quantified. Tropical field work in the 1990s [especially the Coupled Ocean-Atmosphere Response Experiment (COARE), in which Joanne again participated] showed the importance of a still larger scale of convective organization, the “supercluster.” This larger scale of organization has a middle-level inflow circulation that appears to be an important transporter of momentum. The mesoscale and supercluster scale of organization in tropical convective systems are associated with the stratiform components of the cloud systems. Joint analysis of satellite and radar data from COARE show a complex, possibly chaotic relationship between cloud-top temperature and the size of a stratiform precipitation area. The Tropical Rainfall Measuring Mission (TRMM) satellite, for which Joanne served as project scientist for nearly a decade, is now providing a global census of mesoscale and supercluster-scale organization of tropical convection. The TRMM dataset should therefore provide some closure to the question of the nature of deep convection in the equatorial trough zone.

Large‐scale organization of tropical convection in two‐dimensional explicit numerical simulations: Effects of interactive radiation

AbstractWe examine interactions among radiative processes, water vapour, clouds, and the multi‐scale organization of tropical convection in two‐dimensional idealized cloud‐resolving simulations using planetary‐scale horizontal domains. We extend our earlier study where radiative cooling was prescribed. Therein, deep convection spontaneously organized into two primary scales: westward travelling mesoscale convective systems on a scale of a few hundred kilometres and the eastward‐propagating envelopes of convection spanning thousands of kilometres. These envelopes represented large‐scale convectively coupled gravity waves, two‐dimensional non‐rotating analogues of equatorially trapped Kelvin waves.Interactive radiation introduces a new mechanism of large‐scale convective organization. Weak overturning circulations, steered by the mean wind and of a few thousand kilometres in scale, gradually develop. This overturning is a manifestation of the baroclinic response to horizontal gradients of radiative heating established between the moist and dry regions, which causes a spontaneous positive feedback among large‐scale dynamics, water vapour, clouds, and radiation. The mechanism is as follows: the convective systems maintain the humidity (vapour and cloud) in the ascending branch which, in turn, maintains the differential radiative heating. Conversely, the large‐scale overturning provides the large‐scale dynamical forcing that maintains the deep convection. This quantifies recent idealized studies that applied convective parametrizations. The radiatively driven overturning and the deep convection are modulated by large‐scale convectively coupled gravity waves responsible for the large‐scale envelopes reported earlier. A series of sensitivity tests, including the effects of environmental shear, demonstrate the robustness of these results. Copyright © 2002 Royal Meteorological Society

Large‐scale organization of moist convection in idealized aquaplanet simulations

AbstractThis paper discusses results from idealized simulations using a non‐hydrostatic general circulation model with Cloud‐Resolving Convection Parameterization (CRCP, the ‘super parameterization’) pertinent to the large‐scale organization of tropical convection. The essence of tropical dynamics is the intricate balance between large‐scale processes—such as radiative transfer, large‐scale waves, monsoons, Hadley and Walker circulations—and the convective dynamics. Traditional approaches to this problem consider either large‐scale models using convection parameterization or cloud‐resolving models in which large‐scale effects are prescribed. The CRCP merges the two approaches. It uses a 2D cloud‐resolving model to represent the impact of cloud‐scale processes—such as convective motions, precipitation formation and fallout, interaction of clouds with radiative and surface processes—in every column of a large‐scale or global model. The global CRCP model is applied to the idealized problem of a rotating constant‐SST aquaplanet in convective–radiative equilibrium. The aquaplanet has the size and rate of rotation of the Earth. The global CRCP simulations feature pronounced large‐scale organization of convection within the equatorial waveguide. Prominent equatorial ‘super cloud clusters’ spontaneously develop in CRCP global simulations and bear a strong resemblance to the Madden–Julian Oscillation observed in the terrestrial tropics. Model results suggest that convective transport of zonal momentum, supposedly due to the impact of organized convection, plays a significant role in the large‐scale organization of tropical deep convection. Copyright © 2002 John Wiley & Sons, Ltd.

Aspects of the parametrization of organized convection: contrasting cloud‐resolving model and single‐column model realizations

AbstractCloud‐resolving model (CRM) simulations of organized tropical convection observed in the Tropical Ocean/Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to evaluate versions of the European Centre for Medium‐Range Weather Forecasts convection and cloud schemes in single‐column model simulations. Emphasis is placed upon the ability of the convection scheme to represent ‘convective‐scale’ processes with typically mode‐1 heating structures through the troposphere, together with a cloud scheme representing the ‘stratiform (mesoscale) component’ with upper‐level heating and low‐level cooling due to the evaporation of precipitation. While diagnosis of convective and stratiform precipitation is sensitive to the sampling criteria applied to the CRM, vertical structures of the mass and heat budgets are robust. Using diagnostics from the CRM simulations as a guide, revisions to the convection and cloud schemes are suggested in order to enable the parametrization to represent the two scales. The study suggests that a mass‐flux convection scheme linked via detrainment to a prognostic treatment of cloud can represent organized convection, provided that the upward motion in the upper‐level stratiform cloud is considered. Copyright © 2002 Royal Meteorological Society

Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Cold Pools

Abstract An investigation is conducted to document the role convectively generated cold pools play in determining the spatial organization of tropical deep convection. Using a high-resolution cloud-resolving model, the evolution of cold pools produced by deep convection is examined, in the situation of limited large-scale wind shear, and a homogeneous underlying sea surface temperature. Ignoring the cold pools resulting from multiple deep convective events, the mean model cold pool attained a minimum temperature and water vapor mixing ratio depression of 1 K and 1.5 g kg−1, respectively; a horizontal velocity increase of 4.8 m s−1; and the latent and sensible heat fluxes are increased by a factor of 1.9 and 2.6, respectively. The cold pools had a mean lifetime of approximately 2.5 h and attained maximum radii ranging from 3 to 18 km, with a mean of 8.6 km. Taking the organization of convection into account, these figures are consistent with observational studies of convective wakes. The composite cold poo...

Organization of tropical convection in a GCM with varying vertical resolution; implications for the simulation of the Madden-Julian Oscillation

Experiments using a GCM with two different vertical resolutions show differences in the amount of variability in the tropical upper tropospheric zonal wind component associated with the Madden-Julian Oscillation (MJO). The GCM with lower vertical resolution shows very little variability in this quantity whereas when the vertical resolution is doubled in the free troposphere, the GCM produces variability which is of the same strength as observations. However, the eastward propagation of an enhanced convective region from the Indian Ocean into the west Pacific is not well represented in either simulation of this atmospheric GCM. A water-covered or “aqua-planet” version of the same GCM is used to investigate the behaviour of tropical convection when the vertical resolution is doubled. When the vertical resolution is increased, the spectrum of tropical cloud types changes from a bimodal distribution with peaks representing shallow cumulus and deep cumulonimbus clouds to a trimodal distribution with a third peak in mid-troposphere near the melting level. Associated with periods when these mid-level congestus clouds are dominant, the detrainment from these clouds significantly moistens the mid-troposphere. The appearance of these congestus clouds is shown to be partly due to improved resolution of the freezing level and the convective processes occurring at this level. However, due to the way in which convective detrainment is parametrized in this model, the vertical profile becomes rather noisy and this too contributes to the change in the nature of the convective clouds. The resulting cloud distribution more closely resembles observations, particularly during the suppressed phase of the MJO when cumulus congestus is the dominant cloud type.

The organization of tropical convection by intraseasonal sea surface temperature anomalies

The organization of tropical convection by intraseasonal sea surface temperature anomalies

The organization of tropical convection by intraseasonal sea surface temperature anomalies

AbstractThe organization of convection by intraseasonal sea surface temperature (SST) anomalies is investigated to examine the role that atmosphere‐ocean coupling may play in the maintenance of the Madden‐Julian Oscillation. An atmospheric general‐circulation model with an ocean‐covered surface is used to investigate the response of tropical convection to idealized imposed intraseasonal SST anomalies, and the sensitivity of this response to their propagation speed. The convection is found to be organized on the spatial and temporal scales of the imposed SST anomalies and the location of the maximum in precipitation relative to the SST anomaly is in good agreement with observations, and different from that for a stationary SST anomaly. The magnitude of the precipitation anomalies increases with decreasing propagation speed of the SST anomalies. The role of the free‐tropospheric humidity is crucial for determining the location and magnitude of the precipitation response.

Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Water Vapor

A modeling study is conducted to gain insight into the factors that control the intensity and organization of tropical convection, and in particular to examine if organization occurs in the absence of factors such as vertical wind shear or underlying sea surface temperature (SST) gradient. The control experiment integrates a cloud-resolving model for 15 days using a 3D domain exceeding 1000 km in length, with no imposed winds, and horizontally uniform SST and forcing for convection. After 2 days of random activity, the convection organizes into clusters with dimensions of approximately 200 km. Convective systems propagate through the clusters at speeds of 2–3 m s−1, while the clusters themselves propagate at minimal speeds of around 0.5 m s−1. Examining the thermodynamic structure of the model domain, it is found that the convective free bands separating the clusters are very dry throughout the troposphere, and due to virtual temperature effects, are correspondingly warmer in the lower troposphere and boundary layer. This suggests a positive feedback between convection and water vapor, where convective moistening of the local atmosphere renders it more favorable to future convection. The existence of this feedback is demonstrated by experiments in which the free-tropospheric water vapor is perturbed in convective regions, and it is found that the lower-atmospheric water vapor is most critical in controlling convection, most likely through the role of downdrafts. Examination of the boundary layer in the control experiment also indicated that convectively generated cold pools also play a key role in the organization of convection, possibly by their influence on the boundary layer water vapor field. In order to see how the water vapor feedback modifies established convective organization, a further experiment was conducted with an SST gradient imposed, which established a mock Walker cell type circulation, with ascending motion over the warmest SSTs. After 5 days, the SST gradient is reversed to see how the convection would establish itself over the new SST maximum. This highly idealized experiment therefore represents a surrogate for the atmospheric response to SST “hotspots,” that observations have shown to form under the descending branch of large-scale tropical circulations such as the Madden–Jullian oscillation, due to increased incident solar radiation and decreased latent heat fluxes at the surface. It is found that the convection does not spontaneously initiate over the new SST maximum, but instead must propagate toward it. After a further 5 days, much longer than the boundary layer adjustment timescale, the warmest SSTs are still completely free from convection. This is directly due to the dryness of the atmosphere caused by the initial period of subsidence. A further set of experiments examines the robustness of the feedback in cases of imposed vertical wind shear. It is found that strong wind shears prevent the feedback by effectively mixing water vapor. However, the feedback is still very important in cases of weak wind shears.

Large-scale organization of tropical convection in two-dimensional explicit numerical simulations

Large-scale organization of tropical convection in two-dimensional explicit numerical simulations

Large‐scale organization of tropical convection in two‐dimensional explicit numerical simulations

AbstractThe large‐scale organization of tropical deep convection is investigated in idealized two‐dimensional cloud‐resolving simulations. A 20 000 km periodic computational domain allows interactions among moist convection, mesoscale organization, and surface exchange on a wide range of scales. A uniform 10 m s−1 easterly wind and a uniform sea surface temperature of 30°C are assumed. A prescribed temperature tendency mimicking the mean radiative cooling of the tropical troposphere is used in this pilot study. The large‐scale organization of convection resembles the observed tropical atmosphere as well as results from some previous numerical studies using parametrized convection. The simulation highlights mesoscale convective systems with horizontal scales of several hundred kilometres moving east to west with speeds similar to the mean flow, and an envelope of convection with a horizontal extent of a few thousand kilometres and propagating west to east. The propagation speed of the large‐scale envelope compares well with the phase velocitites of convectively coupled Kelvin waves observed in the equatorial waveguide. Convective momentum transport and the impact of convective systems on temperature and moisture near the surface are key processes responsible for the large‐scale organization of convection. These results are discussed in the context of recent observations and numerical studies of convection organization in the tropics.

Mass divergence in tropical weather systems Paper II: Large‐scale controls on convection

AbstractComposited summertime deep cumulus convective weather systems are investigated over three tropical oceans. The occurrence and intensity of the convection are governed by various large‐scale forcing mechanisms, such as ITCZ convergence, easterly waves, and differences in diabatic heating between the disturbance and its clear or mostly cloud‐free surrounding region. Vertical profiles of divergence and vertical velocity for the weather systems are analysed from the point of view of trying to establish the relative importance of each large scale control on convection.The dominant forcing in the Western Pacific and the GATE region is ITCZ convergence, with important secondary roles being played by differential radiative‐convective heating, easterly waves and convective feedback. In the Western Atlantic trade wind region there is no contribution from ITCZ convergence; the vertical motion of a typical weather system is consequently much weaker than in the other regions.In all three regions the contribution to vertical velocity from frictional convergence is shown to be quite small.A large diurnal modulation of convection is observed. In the GATE region a diurnal variation is documented for the low‐level convergence into the ITCZ. A physical mechanism is proposed to explain this phenomenon.It is shown that the actual observed vertical motion of the summer‐time tropical oceanic weather systems comes from a combination of forcing mechanisms and does not result from any individual large‐scale control acting by itself. It is important that any idealized picture of tropical weather systems be broad and flexible enough to account for the large regional and multi‐scale forcing mechanism differences which exist. A simple ‘easterly wave’ or ‘ITCZ’ model of organized tropical convection is inadequate.