Moisture Convection Feedback Research Articles

Role of a Pacific Easterly Wave in the Genesis of Hagupit (2008)

Abstract The Advanced version of the Weather Research and Forecasting (WRF-ARW) Model is used to investigate the influence of an easterly wave (EW) on the genesis of Typhoon Hagupit (2008) in the western North Pacific. Observational analysis indicates that the precursor disturbance of Typhoon Hagupit (2008) is an easterly wave (EW) in the western North Pacific, which can be detected at least 7 days prior to the typhoon genesis. In the control experiment, the genesis of the typhoon is well captured. A sensitivity experiment is conducted by filtering out the synoptic-scale (3–8-day) signals associated with the EW. The absence of the EW eliminates the typhoon genesis. Two mechanisms are proposed regarding the effect of the EW on the genesis of Hagupit. First, the background cyclonic vorticity of the EW could induce the small-scale cyclonic vorticities to merge and develop into a system-scale vortex. Second, the EW provides a favorable environment in situ for the rapid development of the typhoon disturbance through a positive moisture–convection feedback.

A Framework to Evaluate Convective Aggregation: Examples With Different Microphysics Schemes

AbstractThis study introduces a modeling framework to evaluate convective aggregation (CA) under radiative‐convective equilibrium simulations using the vector vorticity equation cloud‐resolving model (VVM) coupled to a mixed‐layer slab ocean. The framework introduces the competing effects between the convection‐sea surface temperature (SST) feedback (CSF) and the moisture‐convection feedback (MCF) by modifying the initial SST gradient and the mixed layer depth. Examples of applying this framework are demonstrated by comparing simulations with different microphysics schemes. The conventional five‐category scheme (VVM‐Lin) and the predicted particle properties scheme (P3) are examined by the matrix formed by these two factors. A clear bifurcation of the aggregated/non‐aggregated states can be identified in both sets of experiments. The change of the bifurcation between two sets of experiments suggests that the P3 experiments tend to develop CA due to the stronger MCF. The budget analysis of spatial frozen moist static energy variance is applied to quantify the stronger MCF in the P3 simulations. Convective systems in P3 simulations develop more organized structures, which enhances MCF and leads to CA. The proposed modeling framework provides a reconciled view of the process‐based evaluation of CA among cloud‐resolving models that use different dynamics and physical parameterizations.

Observed Soil Moisture Impact on Strong Convection over Mountainous Tibetan Plateau

AbstractConvection over the Tibetan Plateau (TP) has been linked to heavy rain and flooding in downstream parts of China. Understanding processes which influence the development of convection on the TP could contribute to better forecasting of these extreme events. TP scale (~1000 km) soil moisture gradients have been shown to influence formation of convective systems over the eastern TP. The importance of smaller-scale (~10 km) variability has been identified in other regions (including the Sahel and Mongolia) but has yet to be investigated for the TP. In addition, compared to studies over flat terrain, much less is known about soil moisture–convection feedbacks above complex topography. In this study we use satellite observations of cold cloud, land surface temperature, and soil moisture to analyze the effect of mesoscale soil moisture heterogeneity on the initiation of strong convection in the complex TP environment. We find that strong convection is favored over negative (positive) land surface temperature (soil moisture) gradients. The signal is strongest for less vegetation and low topographic complexity, though still significant up to a local standard deviation of 300 m in elevation, accounting for 65% of cases. In addition, the signal is dependent on background wind. Strong convective initiation is only sensitive to local (tens of kilometers) soil moisture heterogeneity for light wind speeds, though large-scale (hundreds of kilometers) gradients may still be important for strong wind speeds. Our results demonstrate that, even in the presence of complex topography, local soil moisture variability plays an important role in storm development.

Modulation of ENSO on Fast and Slow MJO Modes during Boreal Winter

AbstractThis study investigates modulation of El Niño–Southern Oscillation (ENSO) on the Madden–Julian oscillation (MJO) propagation during boreal winter. Results show that the spatiotemporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Niño, whereas the MJO during La Niña is very slow and tends to frequently “detour” via the southern Maritime Continent (MC). The westward group velocity of the MJO is also more significant during El Niño. Based on the dynamics-oriented diagnostics, it is found that, during El Niño, the much stronger leading suppressed convection over the EWP excites a significant front Walker cell, which further triggers a larger Kelvin wave easterly wind anomaly and premoistening and heating effects to the east. However, the equatorial Rossby wave to the west tends to decouple with the MJO convection. Both effects can result in fast MJO propagation. The opposite holds during La Niña. A column-integrated moisture budget analysis reveals that the sea surface temperature anomaly driving both the eastward and equatorward gradients of the low-frequency moisture anomaly during El Niño, as opposed to the westward and poleward gradients during La Niña, induces moist advection over the equatorial eastern MC–EWP region due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. The role of nonlinear advection by synoptic-scale Kelvin waves is also nonnegligible in distinguishing fast and slow MJO modes. This study emphasizes the crucial roles of dynamical wave feedback and moisture–convection feedback in modulating the MJO propagation by ENSO.

Noninstantaneous Wave-CISK for the Interaction between Convective Heating and Low-Level Moisture Convergence in the Tropics

Abstract The interaction between tropical convective heating and thermally forced circulation is investigated using a global dry primitive-equation model with the parameterization of wave-conditional instability of the second kind (CISK). It is demonstrated that deep convective heating can hardly sustain itself through the moisture convergence at low levels regardless of the fraction of immediate consumption of converged moisture. In contrast, when the fraction is large, shallow convective heating and its forced circulation exhibit preferred growth of small scales. As the “CISK catastrophe” mainly comes from the instantaneous characters of moisture–convection feedback in the conventional wave-CISK, a noninstantaneous wave-CISK is proposed, which highlights the accumulation–consumption (AC) time scale for the convective heating accumulation and/or the converged moisture consumption. In the new wave-CISK, once moisture is converged, the release of latent heat takes place gradually within an AC time scale. In this sense, convective heating is not only related to the instantaneous moisture convergence at the current time, but also to that which occurred in the past period of the AC time scale. The noninstantaneous wave-CISK could guarantee the occurrence of convective heating and/or moisture convergence at larger scales, and then favor the growth of long waves, and thus solve the problem of CISK catastrophe. With the new wave-CISK and AC time scale of 2 days, the simulated convective heating-driven system bears a large similarity to that of the observed convectively coupled Kelvin wave.

Spectral Signatures of Moisture–Convection Feedbacks over the Indian Ocean

Abstract Positive feedbacks between the cloud population and the environmental moisture field are central to theoretical expositions on the Madden–Julian oscillation (MJO). This study investigates the statistical incidence of positive moisture–convection feedbacks across multiple space and time scales over the tropical Indian Ocean. This work uses vertically integrated moisture budget terms from the ECMWF interim reanalysis [ERA-Interim (ERA-I)] in a framework proposed by Hannah et al. Positive moisture–convection feedbacks are primarily a low-frequency, low-wavenumber phenomenon with significant spectral signatures in the 32–48-day time scale. The efficacy of these feedbacks, however, is subject to horizontal moisture advection variations, whose relative importance varies with scale. Wave-filtered Tropical Rainfall Measuring Mission (TRMM) satellite precipitation is used to show that these moisture–convection feedbacks contribute more to moisture increases in the MJO than in other equatorial waves. A moving-window correlation analysis suggests that instances of moisture–convection feedbacks are more frequent in drier conditions, when column water vapor (CWV) is below its climatological mean value, with the implication that positive moisture–convection feedbacks shape the mean CWV field by moistening drier air columns, but that they are less effective in moistening already moist environments. Ground radar observations show that stratiform rain damps local CWV increases on short time scales (<2 days) and therefore precludes positive moisture–convection feedbacks in high-CWV environments. Vertical coherence structures from ERA-I confirm that relatively bottom-heavy cloud ensembles (i.e., peaks between 700 and 850 hPa) are more effective in inducing low-frequency positive moisture–convection feedbacks than ensembles with other vertical structures. Low-frequency horizontal advective drying damps moisture increases and is strongly coherent with upper-level rising motion.

Precipitation Partitioning, Tropical Clouds, and Intraseasonal Variability in GFDL AM2

AbstractA set of Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model version 2 (AM2) sensitivity simulations by varying an entrainment threshold rate to control deep convection occurrence are used to investigate how cumulus parameterization impacts tropical cloud and precipitation characteristics. In the tropics, model convective precipitation (CP) is frequent and light, while large-scale precipitation (LSP) is intermittent and strong. With deep convection inhibited, CP decreases significantly over land and LSP increases prominently over ocean. This results in an overall redistribution of precipitation from land to ocean. A composite analysis reveals that cloud fraction (low and middle) and cloud condensate associated with LSP are substantially larger than those associated with CP. With about the same total precipitation and precipitation frequency distribution over the tropics, simulations having greater LSP fraction tend to have larger cloud condensate and low and middle cloud fraction.Simulations having a greater LSP fraction tend to be drier and colder in the upper troposphere. The induced unstable stratification supports strong transient wind perturbations and LSP. Greater LSP also contributes to greater intraseasonal (20–100 days) precipitation variability. Model LSP has a close connection to the low-level convergence via the resolved grid-scale dynamics and, thus, a close coupling with the surface heat flux. Such wind–evaporation feedback is essential to the development and maintenance of LSP and enhances model precipitation variability. LSP has stronger dependence and sensitivity on column moisture than CP. The moisture–convection feedback, critical to tropical intraseasonal variability, is enhanced in simulations with large LSP. Strong precipitation variability accompanied by a worse mean state implies that an optimal precipitation partitioning is critical to model tropical climate simulation.

Convective Entrainment and Large-Scale Organization of Tropical Precipitation: Sensitivity of the CNRM-CM5 Hierarchy of Models

Abstract The spurious double intertropical convergence zone (ITCZ) is a systematic bias affecting state-of-the-art coupled general circulation models (GCMs). Modeling studies show that the ITCZ structure is very sensitive to moist convection parameterization and in particular, to the vertical profile of convective heating and free-tropospheric moistening. To further explore this sensitivity, the authors focus in this study on the influence of lateral entrainment in convective plumes on the simulated tropical precipitation and large-scale circulation. Sensitivity studies to the entrainment parameter were performed in a hierarchy of models (coupled ocean–atmosphere GCM, atmospheric GCM, and aquaplanet GCM), in order to mitigate the double ITCZ problem in the Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5 (CNRM-CM5). The sensitivity of the ITCZ structure to lateral entrainment is robust across our hierarchy of models. In response to increased entrainment, the realistic simulations exhibit a weakening of the southern side of the double ITCZ over the southeastern Pacific Ocean and a better representation of the South Pacific convergence zone (SPCZ). However, as a result of stronger moisture–convection feedbacks, precipitation is overestimated in the center of convergence zones. The change in ITCZ configuration is associated with a more realistic representation of the large-scale vertical regimes, explained by a decreased frequency of weak-to-moderate ascending regimes and an enhanced frequency of subsidence regimes. Mechanisms at play in this circulation change are examined by analyzing the vertically integrated dry static energy budget. This energetic analysis suggests that the feedback between large-scale dynamics and deep convection is crucial in controlling the probability distribution function (PDF) of midtropospheric vertical wind. This PDF, in turn, controls the precipitation distribution and, in particular, the double ITCZ bias.

The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part I: Characterization of Large-Scale Organization and Propagation*

Abstract High-resolution simulations over a large tropical domain (~20°S–20°N, 42°E–180°) using both explicit and parameterized convection are analyzed and compared to observations during a 10-day case study of an active Madden–Julian oscillation (MJO) event. The parameterized convection model simulations at both 40- and 12-km grid spacing have a very weak MJO signal and little eastward propagation. A 4-km explicit convection simulation using Smagorinsky subgrid mixing in the vertical and horizontal dimensions exhibits the best MJO strength and propagation speed. Explicit convection simulations at 12 km also perform much better than the 12-km parameterized convection run, suggesting that the convection scheme, rather than horizontal resolution, is key for these MJO simulations. Interestingly, a 4-km explicit convection simulation using the conventional boundary layer scheme for vertical subgrid mixing (but still using Smagorinsky horizontal mixing) completely loses the large-scale MJO organization, showing that relatively high resolution with explicit convection does not guarantee a good MJO simulation. Models with a good MJO representation have a more realistic relationship between lower-free-tropospheric moisture and precipitation, supporting the idea that the moisture–convection feedback is a key process for MJO propagation. There is also increased generation of available potential energy and conversion of that energy into kinetic energy in models with a more realistic MJO, which is related to larger zonal variance in convective heating and vertical velocity, larger zonal temperature variance around 200 hPa, and larger correlations between temperature and ascent (and between temperature and diabatic heating) between 500 and 400 hPa.

The Apparent Water Vapor Sinks and Heat Sources Associated with the Intraseasonal Oscillation of the Indian Summer Monsoon

Abstract The possibility of using remote sensing retrievals to estimate apparent water vapor sinks and heat sources is explored. The apparent water vapor sinks and heat sources are estimated from a combination of remote sensing, specific humidity, and temperature from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and wind fields from the National Aeronautics and Space Administration (NASA)’s Goddard Space Flight Center (GSFC)’s Modern Era Retrospective-Analysis for Research and Applications (MERRA). The intraseasonal oscillation (ISO) of the Indian summer monsoon is used as a test bed to evaluate the apparent water vapor sink and heat source. The ISO-related northward movement of the column-integrated apparent water vapor sink matches that of precipitation observed by the Tropical Rainfall Measuring Mission (TRMM) minus the MERRA surface evaporation, although the amplitude of the variation is underestimated by 50%. The diagnosed water vapor and heat budgets associated with convective events during various phases of the ISO agree with the moisture–convection feedback mechanism. The apparent heat source moves northward coherently with the apparent water vapor sink associated with the deep convective activity, which is consistent with the northward migration of the precipitation anomaly. The horizontal advection of water vapor and dynamical warming are strong north of the convective area, causing the northward movement of the convection by the destabilization of the atmosphere. The spatial distribution of the apparent heat source anomalies associated with different phases of the ISO is consistent with that of the diabatic heating anomalies from the trained heating (TRAIN Q1) dataset. Further diagnostics of the TRAIN Q1 heating anomalies indicate that the ISO in the apparent heat source is dominated by a variation in latent heating associated with the precipitation.

The Role of Moisture–Convection Feedbacks in Simulating the Madden–Julian Oscillation

AbstractThe sensitivity of a simulated Madden–Julian oscillation (MJO) was investigated in the NCAR Community Atmosphere Model 3.1 with the relaxed Arakawa–Schubert convection scheme by analyzing the model’s response to varying the strength of two moisture sensitivity parameters. A higher value of either the minimum entrainment rate or rain evaporation fraction results in increased intraseasonal variability, a more coherent MJO, and enhanced moisture–convection feedbacks in the model. Changes to the mean state are inconsistent between the two methods. Increasing the minimum entrainment leads to a cooler and drier troposphere, whereas increasing the rain evaporation fraction causes warming and moistening. These results suggest that no straightforward correspondence exists between the MJO and the mean humidity, contrary to previous studies.Analysis of the mean column-integrated and normalized moist static energy (MSE) budget reveals a substantial reduction of gross moist stability (GMS) for increased minimum entrainment, while no significant changes are found for an increased evaporation fraction. However, when considering fluctuations of the normalized MSE budget terms during MJO events, both methods result in negative GMS prior to the deep convective phase of the MJO. Intraseasonal fluctuations of GMS, rather than the mean, appear to be a better diagnostic quantity for testing a model’s ability to produce an MJO.

Multiple Equilibria of the Hadley Circulation in an Intermediate-Complexity Axisymmetric Model

Abstract A model of intermediate complexity based on quasi-equilibrium theory—a version of the Quasi-Equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature—is used in a zonally symmetric aquaplanet configuration to study the sensitivity of the Hadley circulation to the sea surface temperature (SST) latitudinal distribution. For equatorially symmetric SST forcing with large SST gradients in the tropics, the model simulates the classical double Hadley cell with one equatorial intertropical convergence zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilibria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror images of each other) with an off-equatorial ITCZ. Further investigation of the feedbacks at play in the model shows that the assumed vertical structure of temperature variations is crucial to the existence and stability of the asymmetric equilibria. The free-tropospheric moisture–convection feedback must also be sufficiently strong to sustain asymmetric equilibria. Both results suggest that the specific physics of a given climate model condition determine the existence of multiple equilibria via the resulting sensitivity of the convection to free-tropospheric humidity and the vertical structure of adiabatic heating. The symmetry-breaking mechanism and resulting multiple equilibria have their origin in the local multiple equilibria that can be described by a single-column model using the weak temperature gradient approximation. An additional experiment using an SST latitudinal distribution with a relative minimum at the equator shows that the feedbacks controlling these multiple equilibria might be relevant to the double-ITCZ problem.

Impact of soil moisture on the development of a Sahelian mesoscale convective system: a case‐study from the AMMA Special Observing Period

AbstractInteractions between the land and atmosphere play an important role in the precipitation of the Sahel. The African Monsoon Multidisciplinary Analysis Special Observing Period provided observations with which to illuminate potential feedback mechanisms. This case‐study highlights a major storm which developed over northern Mali in an area where a research aircraft was surveying the atmospheric response to soil moisture features. Soil moisture variability is characterized using satellite land‐surface temperature data whilst cloud images illustrate the evolution of the storm and its relationship to the surface.Measurements in the planetary boundary layer (PBL) indicate mesoscale variations in pre‐storm humidity and temperature consistent with high evaporation from wet soils. The storm developed above a dry surface within a wetter region with cells first appearing along a wet–dry soil boundary. This suggests that the storm was triggered in association with low‐level convergence driven by the soil moisture pattern. A gravity wave propagating away from a remote mature storm also appears to have played an important role in the initiation, though only in the region of the soil moisture contrast did deep convection become established. Once organised into a Mesoscale Convective System, convection developed over wet areas as well as dry, and indeed at this stage, convection became more intense over wetter soils. This behaviour is consistent with the large gradients in PBL humidity. The study illustrates the complexity of soil moisture–convection feedback loops and highlights the mechanisms which may operate at different stages of a storm's life cycle. Copyright © 2009 Royal Meteorological Society

Convectively Coupled Equatorial Waves Simulated on an Aquaplanet in a Global Nonhydrostatic Experiment

Abstract Large-scale tropical convective disturbances simulated in a 7-km-mesh aquaplanet experiment are investigated. A 40-day simulation was executed using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Two scales of eastward-propagating disturbances were analyzed. One was tightly coupled to a convective system resembling super–cloud clusters (SCCs) with a zonal scale of several thousand kilometers (SCC mode), whereas the other was characterized by a planetary-scale dynamical structure (40 000-km mode). The typical phase velocity was 17 (23) m s−1 for the SCC (40 000 km) mode. The SCC mode resembled convectively coupled Kelvin waves in the real atmosphere around the equator, but was accompanied by a pair of off-equatorial gyres. The 40 000-km mode maintained a Kelvin wave–like zonal structure, even poleward of the equatorial Rossby deformation radius. The equatorial structures in both modes matched neutral eastward-propagating gravity waves in the lower troposphere and unstable (growing) waves in the upper troposphere. In both modes, the meridional mass divergence exceeded the zonal component, not only in the boundary layer, but also in the free atmosphere. The forcing terms indicated that the meridional flow was primarily driven by convection via deformation in pressure fields and vertical circulations. Moisture convergence was one order of magnitude greater than the moisture flux from the sea surface. In the boundary layer, frictional convergence in the (anomalous) low-level easterly phase accounted for the buildup of low-level moisture leading to the active convective phase. The moisture distribution in the free atmosphere suggested that the moisture–convection feedback operated efficiently, especially in the SCC mode.

Tropical Cyclogenesis Associated with Rossby Wave Energy Dispersion of a Preexisting Typhoon. Part II: Numerical Simulations*

Abstract The cyclogenesis events associated with the tropical cyclone (TC) energy dispersion are simulated in a 3D model. A new TC with realistic dynamic and thermodynamic structures forms in the wake of a preexisting TC when a large-scale monsoon gyre or a monsoon shear line flow is present. Maximum vorticity generation appears in the planetary boundary layer (PBL) and the vorticity growth exhibits an oscillatory development. This oscillatory growth is also seen in the observed rainfall and cloud-top temperature fields. The diagnosis of the model output shows that the oscillatory development is attributed to the discharge and recharge of the PBL moisture and its interaction with convection and circulation. The moisture–convection feedback regulates the TC development through controlling the atmospheric stratification, raindrop-induced evaporative cooling and downdraft, PBL divergence, and vorticity generation. On one hand, ascending motion associated with deep convection transports moisture upward and leads to the discharge of PBL moisture and a convectively stable stratification. On the other hand, the convection-induced raindrops evaporate, leading to midlevel cooling and downdraft. The downdraft further leads to dryness and a reduction of equivalent potential temperature. This reduction along with the recharge of PBL moisture due to surface evaporation leads to reestablishment of a convectively unstable stratification and thus new convection. Sensitivity experiments with both a single mesh (with a 15-km resolution) and a nested mesh (with a 5-km resolution in the inner mesh) indicate that TC energy dispersion alone in a resting environment does not lead to cyclogenesis, suggesting the important role of the wave train–mean flow interaction. A proper initial condition for background wind and moisture fields is crucial for maintaining a continuous vorticity growth through the multioscillatory phases.

On the Role of Moist Processes in Tropical Intraseasonal Variability: Cloud–Radiation and Moisture–Convection Feedbacks

Abstract Recent observations of the tropical atmosphere reveal large variations of water vapor and clouds at intraseasonal time scales. This study investigates the role of these variations in the large-scale organization of the tropical atmosphere, and in intraseasonal variability in particular. For this purpose, the influence of feedbacks between moisture (water vapor, clouds), radiation, and convection that affect the growth rate and the phase speed of unstable modes of the tropical atmosphere is investigated. Results from a simple linear model suggest that interactions between moisture and tropospheric radiative cooling, referred to as moist-radiative feedbacks, play a significant role in tropical intraseasonal variability. Their primary effect is to reduce the phase speed of large-scale tropical disturbances: by cooling the atmosphere less efficiently during the rising phase of the oscillations (when the atmosphere is moister) than during episodes of large-scale subsidence (when the atmosphere is drier), the atmospheric radiative heating reduces the effective stratification felt by propagating waves and slows down their propagation. In the presence of significant moist-radiative feedbacks, planetary disturbances are characterized by an approximately constant frequency. In addition, moist-radiative feedbacks excite small-scale disturbances advected by the mean flow. The interactions between moisture and convection exert a selective damping effect upon small-scale disturbances, thereby favoring large-scale propagating waves at the expense of small-scale advective disturbances. They also weaken the ability of radiative processes to slow down the propagation of planetary-scale disturbances. This study suggests that a deficient simulation of cloud radiative interactions or of convection-moisture interactions may explain some of the difficulties experienced by general circulation models in simulating tropical intraseasonal oscillations.

Moisture–convection feedback in the tropics

AbstractThis paper discusses the large‐scale moisture–convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free‐tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free‐tropospheric humidity change explains why moisture–convection feedback is particularly relevant for tropical intraseasonal oscillations.The large‐scale dynamical context for moisture–convection feedback is investigated in idealized rotating constant‐sea‐surface‐temperature (‘tropics everywhere’) aquaplanet using cloud‐resolving convection parametrization (CRCP; super‐parametrization) and a traditional convective parametrization (the Emanuel scheme). The large‐scale organization of convection within the equatorial waveguide takes the form of MJO‐like (Madden–Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large‐scale free‐tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO‐like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetary‐scale moisture perturbation in the free troposphere are conducted. The ensuing large‐scale velocity perturbations have e‐folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture–convection feedback; an enhanced large‐scale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively‐generated moisture and cloudiness. Additional support for the role of moisture–convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free‐tropospheric humidity and results in weak MJO‐like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free‐tropospheric humidity dramatically improves the simulated MJO‐like coherences. Copyright © 2004 Royal Meteorological Society