Diurnal Cycle Of Deep Convection Research Articles

The impact of surface heterogeneity on the diurnal cycle of deep convection

AbstractDespite some recent improvements, major deficiencies remain in model simulations using parameterised convection in capturing both the phase and amplitude of the daily cycle of precipitation in tropical regions. The difficulties are particularly acute in regions of heterogeneous surface conditions, since the simulations need not only to respond appropriately to the local forcing from surface fluxes but also to capture the interactions with near‐surface mesoscale circulations. Here, we examine such a situation by means of idealised cloud‐resolving simulations of deep convection over a heterogeneous surface, performed using the cloud‐resolving simulation known as the Met Office NERC Cloud model (MONC). In these simulations, we show that precipitation forms preferentially over dry and warm patches (“DRY”) compared with wet and cold patches (“WET”), with the peak precipitation rates differing by a factor of approximately 4. The initiation of precipitation occurs approximately 1.5 hr earlier in the DRY patches compared with the WET patches. Moreover, within the WET and DRY patches there are marked differences in the spatial distribution of the precipitation. These cloud‐resolving simulations are then used as a benchmark to assess the behaviour of simulations using parameterised convection, performed using the idealised configuration of the Met Office Unified Model (MetUM). The MetUM simulations produce a response with some qualitative similarities to the cloud‐resolving simulations. In particular, although the simulations with parameterised convection initiate precipitation too early, they are capable of capturing the relative amounts of daily‐mean precipitation in the DRY and WET patches. We propose that the cloud‐resolving simulations could be used further to investigate the impact of fully interactive surface schemes and as benchmark simulations to evaluate new parameterisation schemes.

Diurnal Differences in Tropical Maritime Anvil Cloud Evolution

AbstractSatellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud-top cooling and cloud-base heating dipole cannot compensate for the effect of diurnal shortwave radiative heating. Radiative–convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts.Significance StatementDeep convective activity and rainfall peak at night over the tropical oceans. However, anvil clouds that originate from the tops of deep convective clouds reach their largest extent in the afternoon hours. We study the underlying physical mechanisms that lead to this discrepancy by simulating the evolution of anvil clouds with a high-resolution model. We find that the absorption of sunlight by ice crystals lofts and spreads the daytime anvil clouds over a larger area, increasing their lifetime, changing their properties, and thus influencing their impact on climate. Our findings show that it is important not only to simulate the correct onset of deep convection but also to correctly represent anvil cloud evolution for skillful simulations of the tropical energy balance.

Diurnal variation of deep convective clouds over Indian monsoon region and its association with rainfall

Diurnal cycle of deep convective clouds (DCC) over the Indian Summer Monsoon Region is studied. A harmonic analysis has been applied on the 3 hourly deep convective cloud fraction estimated from satellite observations to derive diurnal and semi-diurnal amplitudes of deep convective clouds. The diurnal cycle of deep convective clouds shows seasonal variations and amplitudes which are stronger during the northern hemispheric summer than winter and exhibits prominence over land as compared to ocean. Irrespective of seasons, deep convection peaks in the late evening hours over land whereas the peak is in the afternoon hours over majority of the oceanic regions considered in our study. The reason for the difference is attributed to the semi-diurnal oscillation which determines the oceanic diurnal cycle of deep convection. Various parameters which represent the tropospheric instability are equally affected by the semi-diurnal oscillation as in deep convective clouds over ocean. These results suggest that diurnal tides could play a key role in governing the daily cycle of deep convective clouds over oceanic regions. The relation between diurnal amplitudes of DCC and rainfall is also explored and it is shown that the strength of the daily cycle of DCC has a strong bearing on the amount of rainfall observed over the Indian summer monsoon region.

Memory Properties in Cloud‐Resolving Simulations of the Diurnal Cycle of Deep Convection

AbstractA series of high‐resolution three‐dimensional simulations of the diurnal cycle of deep convection over land are performed using the new Met Office NERC cloud‐resolving model. This study features scattered convection. A memory function is defined to identify the effects of previous convection in modifying current convection. It is based on the probability of finding rain at time t0 and at an earlier time t0−Δt compared to the expected probability given no memory. The memory is examined as a function of the lag time Δt. It is strongest at gray‐zone scales of 4–10 km, there is a change of behavior for spatial scales between 10 and 15 km, and it is reduced substantially for spatial scales larger than 25 km. At gray‐zone scales, there is a first phase of the memory function which represents the persistence of convection and it is maintained for about an hour. There is a second phase which represents the suppression of convection in regions which were raining 1 to 3 hr previously, and subsequently a third phase which represents a secondary enhancement of precipitation. The second and third phases of the memory function develop earlier for weaker forcing. When thermodynamic fluctuations resulting from the previous day are allowed to influence the development of convection on the next day, there are fewer rainfall events with relatively large sizes, which are more intense, and thus decay and recover more slowly, in comparison to the simulations where feedback from previous days is removed. Further sensitivity experiments reveal that convective memory attributed to these thermodynamic fluctuations resides in the lower troposphere.

Memory properties in cloud--resolving simulations of the diurnal cycle of deep convection

Memory properties in cloud--resolving simulations of the diurnal cycle of deep convection

Evaluation of Machine Learning Classifiers for Predicting Deep Convection

AbstractThe realistic representation of convection in atmospheric models is paramount for skillful predictions of hazardous weather as well as climate, yet climate models especially suffer from large uncertainties in the parameterization of clouds and convection. In this work, we examine the use of machine learning (ML) to predict the occurrence of deep convection from a state‐of‐the‐art atmospheric reanalysis (ERA5). Logistic regression, random forests, gradient‐boosted decision trees, and deep neural networks were trained with lightning data to predict thunderstorm occurrence (TO) in Central and Northern Europe (2012–2017) and in Sri Lanka (2016–2017). Up to 40 input variables were used, representing, for example, instability, humidity, and inhibition. Feature importances derived for the various models emphasize the high importance of conditional instability for deep convection in Europe, while in Sri Lanka, TO is more strongly regulated by humidity. The Precision‐Recall curve indicates more than a twofold improvement in skill over convective available potential energy for short‐term (0–45 min) predictions of TO in Europe by using neural networks or gradient‐boosted decision tree and a larger improvement in the tropical domain. The diurnal cycle of deep convection is closely reproduced, suggesting that ML could be used to trigger convection in climate models. Finally, a strong relationship was found between area‐mean monthly TO and ML predictions, with correlation coefficients exceeding 0.94 in all domains. Convective available potential energy has a similar level of correlation with monthly thunderstorm activity only in Northern Europe. The results encourage the use of reanalyses and ML to study climate trends in convective storms.

Spatial Variability of the Background Diurnal Cycle of Deep Convection around the GoAmazon2014/5 Field Campaign Sites

AbstractThe isolation of the Amazon rain forest makes it challenging to observe precipitation forming there, but it also creates a natural laboratory to study anthropogenic impacts on clouds and precipitation in an otherwise pristine environment. Observations were collected upwind and downwind of Manaus, Brazil, during the “Observations and Modeling of the Green Ocean Amazon 2014–2015” experiment (GoAmazon2014/5). Besides aircraft, most of the observations were point measurements made in a spatially heterogeneous environment, making it hard to distinguish anthropogenic signals from naturally occurring spatial variability. In this study, 15 years of satellite data are used to examine the spatial and temporal variability of deep convection around the GoAmazon2014/5 sites using cold cloud tops (infrared brightness temperatures colder than 240 K) as a proxy for deep convection. During the rainy season, convection associated with the inland propagation of the previous day’s sea-breeze front is in phase with the diurnal cycle of deep convection near Manaus but is out of phase a few hundred kilometers to the east and west. Convergence between the river breezes and the easterly trade winds generates afternoon convection up to 10% more frequently (on average ~4 mm day−1 more intense rainfall) at the GoAmazon2014/5 sites east of the Negro River (T0e, T0t/k, and T1) relative to the T3 site, which was located west of the river. In general, the annual and diurnal cycles of precipitation during 2014 were similar to climatological values that are based on satellite data from 2000 to 2013.

Intense convection over West Africa during AMMA SOP3 experiment

ERA-Interim product from the European Center for Medium-Range Weather Forecast (ECMWF) assimilation of African Monsoon Multidisciplinary Analysis (AMMA) resources, Meteosat satellite images, and synoptic observations were used to study local- and regional-scale environments associated with intense convective systems during the AMMA-SOP3 experiment over West Africa in the Northern Hemisphere of summer 2006. The convective system, from the 21st to 23rd of August 2006, was more active at 0000 and 1800UTC showing diurnal cycle of deep convection over West Africa where the African easterly waves (AEWs) are developed downstream. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. From a dynamical viewpoint, the low-level vorticity presents strong positive values over the sea and Sahara zone, indicating that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. The 925-hPa equivalent potential temperature structure show a maximum over the Sahara which corresponds to the depression observed in this region. A mosaic of three hourly infrared (IR) satellite images, depicts a very distinct signal from an initial region of convection, developing through several stages and moving off the African coast. These observations, along with those available from the World Weather Watch, provide an opportunity to carry out numerical weather prediction (NWP) studies over West Africa utilizing high resolution limited area models.

A Numerical Investigation of Wet and Dry Onset Modes in the North American Monsoon Core Region. Part I: A Regional Mechanism for Interannual Variability

In this study the results of two regional fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) simulations forced at their boundaries with low-pass-filtered North American Regional Reanalysis (NARR) composite fields from which synoptic-scale variability was removed are presented. The filtered NARR data are also assimilated into the inner domain through the use of field nudging. The purpose of this research is to investigate wet and dry onset modes in the core region of the North American monsoon (NAM). Key features of the NAM that are present in the NARR fields and assimilated into the regional simulations include the position of the midlevel anticyclone, low-level circulation over the Gulf of California, and moisture flux patterns into the core monsoon region, for which the eastern Pacific is the likely primary source of moisture. The model develops a robust diurnal cycle of deep convection over the peaks of the Sierra Madre Occidental (SMO) that results solely from its radiation scheme and internal dynamics, in spite of the field nudging. The wet onset mode is related to a regional land–sea thermal contrast (LSTC) that is ~2°C higher than in the dry mode, and is further characterized by a northward-displaced midlevel anticyclone, a stronger surface pressure gradient along the Gulf of California, larger mean moisture fluxes into the core region from the eastern Pacific, a stronger diurnal cycle of deep convection, and the more northward distribution of precipitation along the axis of the SMO. A proposed regional LSTC mechanism for NAM onset interannual variability is consistent with the differences between both onset modes.

Entrainment processes in the diurnal cycle of deep convection over land

AbstractIn this article we show that entrainment during the developing stages of deep convection over land is much higher than convection at equilibrium. A series of idealised cloud‐resolving model simulations are performed for a range of environmental conditions, and these show that the interaction with the environment via the entrainment and detrainment rates gradually decreases as the day progresses, reverting to the values found at equilibrium. The entrainment and detrainment rates are themselves found to depend on the environmental humidity and stability, and are also strongly linked to cloud size, suggesting that the representation of the horizontal growth of clouds in convective parametrizations is important for the representation of the diurnal cycle. We propose a simple new entrainment and detrainment formulation to take account of these findings, and show that this improves the representation of developing convection in a single‐column model, providing a more gradual transition towards deep convection. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Diurnal cycle of deep convection in super clusters embedded in the Madden‐Julian Oscillation

The diurnal cycle of deep convection in the convectively active phase (CAP) and convectively suppressed phase (CSP) of the Madden‐Julian Oscillation (MJO) was investigated using global infrared‐equivalent blackbody temperature (TBB) data from October 2000 to April 2005. We particularly focused on super clusters (SCs) that occurred during the CAP of the MJO. To analyze the behavior of SCs and the CSP of the MJO statistically, we created composites of TBB for each case. Four regions were examined: (1) the Indian Ocean (IO), (2) the ocean area of the Maritime Continent (MCO), (3) the land area of the Maritime Continent (MCL), and (4) the western Pacific (WP). The amplitude (phase) of the diurnal cycle of deeper convection in SCs was larger than (similar to) that of the CSP of the MJO. This result is consistent with that found in a previous study. However, the diurnal cycle of deep convection in the CSP of the MJO had a clear maximum peak, especially over the MCL, but that in SCs had a broader maximum peak of deep convection, especially in oceanic regions, although the diurnal phase was similar between SCs and the CSP of the MJO. This result statistically suggests that the organization of a convective system affects the diurnal cycle of its deep convection, as described in a previous case study. We also demonstrated that the difference in the diurnal cycle of deep convection in the IO and WP regions arose from sea surface temperature differences in those regions.

Implications of the day versus night differences of water vapor, carbon monoxide, and thin cloud observations near the tropical tropopause

There are some interesting day versus night differences in the water vapor and carbon monoxide concentrations near the tropopause over tropical land and ocean from 4 years of EOS Microwave Limb Sounder (MLS) observations. To interpret these differences, the diurnal cycle of deep convection reaching near tropical tropopause summarized from a decade of tropical rainfall measuring mission (TRMM) observations. We also present the diurnal cycle of the cold point tropopause temperature and height derived from 2 years of constellation observing system for meteorology ionosphere and climate (COSMIC) GPS temperature profiles, the day versus night differences of occurrence of thin clouds from 2 years of cloud‐aerosol lidar and infrared pathfinder satellite observations (CALIPSO) and 16 years of stratospheric aerosol and gaseous experiment (SAGE) II. In the tropical upper troposphere, day versus night differences of water vapor and carbon monoxide are consistent with the diurnal cycle of the vertical transport of water vapor and carbon monoxide–rich air from the surface by deep convection. However, in the tropical tropopause layer (TTL) over land, day versus night differences of water vapor concentration are more consistent with the diurnal variations of temperature in a saturated TTL, which is related to the diurnal cycle of cooling in the TTL induced by deep convection. The day versus night differences of occurrences of thin clouds in the TTL are also consistent with the freeze drying, controlled by the diurnal cycles of temperature in the TTL.

Shifting the diurnal cycle of parameterized deep convection over land

In most atmospheric circulation models used for climate projections, cloud and convective processes are not explicitly resolved but parameterized. Such models are known to produce a diurnal cycle of continental thunderstorms in phase with insolation, while observed precipitation peaks in late afternoon. We propose a new approach which corrects this long standing bias of parameterized convection. In this approach, deep convection triggering and intensity are controlled by sub‐cloud processes: here boundary layer thermals and gust fronts, and potentially orography or surface heterogeneities. The representation of the diurnal cycle of deep convection is greatly improved in 1D mode, with rainfall maximum delayed from midday to late afternoon, provided parameterizations account for the key role played by shallow cumulus in preconditioning deep convection and by gust fronts in the self‐sustaining of thunderstorms in the afternoon.

On the diurnal cycle of deep convection, high‐level cloud, and upper troposphere water vapor in the Multiscale Modeling Framework

The Multiscale Modeling Framework (MMF), also called “superparameterization”, embeds a cloud‐resolving model (CRM) at each grid column of a general circulation model to replace traditional parameterizations of moist convection and large‐scale condensation. This study evaluates the diurnal cycle of deep convection, high‐level clouds, and upper troposphere water vapor by applying an infrared (IR) brightness temperature (Tb) and a precipitation radar (PR) simulator to the CRM column data. Simulator results are then compared with IR radiances from geostationary satellites and PR reflectivities from the Tropical Rainfall Measuring Mission (TRMM). While the actual surface precipitation rate in the MMF has a reasonable diurnal phase and amplitude when compared with TRMM observations, the IR simulator results indicate an inconsistency in the diurnal anomalies of high‐level clouds between the model and the geostationary satellite data. Primarily because of its excessive high‐level clouds, the MMF overestimates the simulated precipitation index (PI) and fails to reproduce the observed diurnal cycle phase relationships among PI, high‐level clouds, and upper troposphere relative humidity. The PR simulator results show that over the tropical oceans, the occurrence fraction of reflectivity in excess of 20 dBZ is almost 1 order of magnitude larger than the TRMM data especially at altitudes above 6 km. Both results suggest that the MMF oceanic convection is overactive and possible reasons for this bias are discussed. However, the joint distribution of simulated IR Tb and PR reflectivity indicates that the most intense deep convection is found more often over tropical land than ocean, in agreement with previous observational studies.

Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

Abstract This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date. The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet. Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

Diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere: Satellites versus a general circulation model

Global high‐resolution (3‐hourly, 0.1° × 0.1° longitude‐latitude) water vapor (6.7 μm) and window (11 μm) radiances from multiple geostationary satellites are used to document the diurnal cycle of upper tropospheric relative humidity (UTH) and its relationship to deep convection and high clouds in the whole tropics and to evaluate the ability of the new Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land model (AM2/LM2) to simulate these diurnal variations. Similar to the diurnal cycle of deep convection and high clouds, coherent diurnal variations in UTH are also observed over the deep convective regions, where the daily mean UTH is high. In addition, the diurnal cycle in UTH also features a land‐sea contrast: stronger over land but weaker over ocean. UTH tends to peak around midnight over ocean in contrast to 0300 LST over land. Furthermore, UTH is observed to lag high cloud cover by ∼6 hours, and the latter further lags deep convection, implying that deep convection serves to moisten the upper troposphere through the evaporation of the cirrus anvil clouds generated by deep convection. Compared to the satellite observations, AM2/LM2 can roughly capture the diurnal phases of deep convection, high cloud cover, and UTH over land; however, the magnitudes are noticeably weaker in the model. Over the oceans the AM2/LM2 has difficulty in simulating both the diurnal phase and amplitude of these quantities. These results reveal some important deficiencies in the model's convection and cloud parameterization schemes and suggest the lack of a diurnal cycle in SST may be a shortcoming in the boundary forcing for atmospheric models.

Multistage moisture transport into the interior of northern Mexico during the North American summer monsoon

The summertime sources and transport pathways of water vapor into the interior of southwestern North America are examined in a high‐resolution mesoscale model simulation. A complex multi‐stage transport pathway is associated with a pronounced diurnal cycle of deep convection and low‐level wind systems. Daytime thermal pumping of moisture from the Gulf of California towards the Sierra Madre Occidental (SMO) leads to widespread, deep convection. At night, land surface and atmospheric cooling generate drainage flows down both sides of the SMO. A nocturnal, low‐level jet forms above the western foothills and transports moisture north into the Sonoran Desert. Moist air draining to the east mixes with onshore flow from Gulf of Mexico and flows into the northern Chihuahuan Desert. The diurnal flow of moisture up and down the SMO and the formation of nocturnal, low‐level jets transport moisture into the core regions of the NA monsoon in NW Mexico.

The Diurnal Cycle of West Pacific Deep Convection and Its Relation to the Spatial and Temporal Variation of Tropical MCSs

Abstract Infrared (IR) and visible satellite data from the Japanese Geostationary Meteorological Satellite (GMS-4) with 5-km spatial and 1-h temporal resolution were used to examine the diurnal cycle of deep convection over a sector of the tropical west Pacific warm pool bounded by 0°–20°N, 140°E–180°. Data were analyzed for 45 days of summer from 22 June to 5 August 1994 and for 65 days of winter between 28 November 1994 and 31 January 1995. Deep convective clouds were identified in IR imagery using brightness temperature (TBB) threshold techniques. Based on previous studies, a −65°C cloud-top TBB threshold was chosen to isolate pixels containing active, deep convection. Spectral analysis of time series constructed from hourly cold cloud (⩽−65°C) pixel counts revealed a powerful diurnal cycle of deep convection significant at the 95% confidence level during summer and winter. Composited hourly statistics of fractional areal cloud cover documented a 0500–0600 local standard time (LST) maximum with a 1500–...

Impact of Interactive Radiative Transfer on the Macroscopic Behavior of Cumulus Ensembles. Part II: Mechanisms for Cloud-Radiation Interactions

Abstract The two-dimensional UCLA cumulus ensemble model is used to examine the impact of cloud-radiation interactions on the macroscopic behavior of cumulus ensembles. Two sets of simulations are performed with noninteractive (NI) and fully interactive (FI) radiative transfer, and with prescribed large-scale advective effects. The time-varying horizontally averaged radiative heating rates QR from the FI simulators are used to prescribe the time-varying, horizontally homogeneous QR in the NI simulations. The effects of both longwave radiation and diurnally varying solar radiation are examined from these two sets of simulations. The diurnally varying solar radiation can drive a diurnal cycle of deep convection over the tropical oceans by stabbing the large-scale environment during the daytime relative to the nighttime. The results presented in this study confirm the dominant role of the direct radiation-convection interaction mechanism for the diurnal cycle of oceanic precipitation. Comparison of the resul...

Daily surface wind variations over the equatorial Pacific Ocean

Daily surface wind variations over the equatorial eastern Pacific Ocean during summer 1992 are documented, using hourly observations from the tropical atmosphere ocean moored buoy array. Diurnal and semidiurnal variations are apparent in the zonal wind component, with peak‐to‐peak amplitudes of a few tenths of a meter per second. The phase of the semidiurnal cycle in zonal wind is approximately uniform across the equatorial eastern Pacific Ocean, with westerly wind maxima at ∼0300 and 1500 LT. The diurnal cycle dominates the daily march of meridional wind. The range of the diurnal meridional wind variations is ∼0.6–0.8 ms−1 at most locations: more than twice as large as the daily zonal wind changes. The low‐level flow is southward across the equator at night (relative to the daily mean), regardless of whether the mean winds are southerly or northerly. The diurnal meridional wind variations along the equator may be related to the diurnal cycle of deep convection in the Intertropical Convergence Zone (ITCZ) to the north. In particular, surface wind divergence along the equator (which is dominated by the meridional component) exhibits a pronounced diurnal cycle, with the strongest divergence in the early morning when deep convection in the ITCZ is at a maximum. The average daily range of the equatorial surface wind divergence is 1.5×10−6 s−1, or ∼30% of the daily mean. The semidiurnal zonal wind variations are dynamically consistent with the well‐known semidiurnal cycle in surface pressure, which is thought to be a manifestation of the atmospheric thermal tide.