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Diurnal variations in the shallow and deep meridional overturning over West Africa during the premonsoon (April–May) and monsoon (July–September) seasons were analyzed using ECMWF Reanalysis 5th Generation (ERA5) reanalysis and Tropical Rainfall Measuring Mission (TRMM)/Global Precipitation Measurement (GPM) satellite radar data from 1998 to 2019. We found that a daytime sea breeze and two nocturnal low‐level jets have varying impacts on the diurnal cycle of convection and the meridional circulations over West Africa, depending on the season. During the premonsoon, the zonal rain belt is centered over the Gulf of Guinea and the sea breeze initiates intense deep convection (and thus deep overturning) over the coastal region in the afternoon that propagates northward into the early evening. The nocturnal Sahel/Sahara low‐level jet (SLLJ) then drives low‐level convergence into the West African heat low, producing a strong, dry shallow overturning over land at night. During the monsoon, the rain belt is centered over land (10∘$$ 1{0}^{\circ } $$N) and the sea breeze is forced by weaker temperature gradients, producing less intense convection that does not propagate as far inland as during the premonsoon. However, organized convection inland maintains deep daytime overturning. At night, a coastal low‐level jet strengthens northward moisture transport from the Gulf of Guinea into the active monsoon region after the sea breeze dissipates, helping maintain convective systems with large stratiform components and continued deep meridional overturning over land. The nocturnal SLLJ still drives low‐level convergence into the West African heat low, but the dry shallow meridional overturning is farther north and weaker than in the premonsoon. While ERA5 winds and moisture‐flux convergence are generally consistent with diurnal variations in the TRMM/GPM precipitation, ERA5 precipitation is not. The reanalysis indicates significant biases in the timing and magnitude of rainfall over the Gulf of Guinea and West Africa in each season, with the convective and large‐scale rain fields both contributing to the disparate precipitation patterns. [ABSTRACT FROM AUTHOR]

An 8‐year climatology of raintype observations from the Global Precipitation Measurement dual‐frequency precipitation radar highlights two stratiform rain producing storm regimes differentiated by the relative importance of convection. The more convectively active regime occurs in the tropics and over warm‐season midlatitude land, where warm‐ and cold‐topped convection accounts for 55% (40%) of the rain (rain area). The less convectively active regime dominates over midlatitude ocean and cold‐season midlatitude land, where convective rain only accounts for 15% (8%) of the rain (rain area). The ratio between cold‐topped convective and stratiform rain area is highly distinct between the two regimes (22% vs. 5.5%), with different precipitation amounts (and thus heating) for similar stratiform rain areas. A third tropics‐only warm‐topped convection regime exists, but is not associated with major stratiform rain production. Plain Language Summary: We separate global precipitation into convective and stratiform types using a spaceborne weather radar, where convective rain is intense and short‐lived and stratiform rain is weaker and more homogeneous in space and time. We show that there are two unique regimes between these precipitation types that are dictated by how important convective processes are to a storm's evolution, that is, cold‐topped convection is either essential to sustaining the storm system or more of a bystander to large‐scale atmospheric processes that prefer to produce stratiform precipitation. The more convective regime is dominant in the tropics but can also occur over land in the midlatitudes during summer. This regime requires strong interaction between the convective and stratiform rainy regions and is more likely to produce large rain amounts. The less convective regime is predominant over higher latitude oceans and over land in the midlatitudes during winter. While convective and stratiform rain occurrence is well correlated in this regime, convective rain does not contribute significantly to the overall precipitation amounts. We believe that these regimes support a reevaluation of how precipitation physics and their interactions are handled in climate models. Key Points: Areal coverage and echo‐top heights of stratiform rain and warm‐ and cold‐topped convection indicate strong geographic variabilityStratiform rain fraction is 45% and 85% in the tropics and midlatitudes, respectively, with a rapid transition in the subtropicsStatistical analysis suggests unique interactions between cold‐top convection and stratiform precipitation in the tropics and extratropics [ABSTRACT FROM AUTHOR]

This work investigates impacts of the Madden‐Julian Oscillation (MJO) on the daily and diurnal rainfall over the Maritime Continent, with emphasis on the influences of topography over Sumatra, Borneo and New Guinea. Eighty‐nine MJO events were identified during 2001–2019 using IMERG satellite data. Daily and diurnal rainfall maxima on the east side of topography lag the west side as the MJO passes. In addition, the island vanguard (pre‐MJO) rain is more convective, while the island rearguard (behind the main MJO body) rain is more stratiform. The timing and magnitude of diurnal rainfall is defined using the maximum hourly rain rate instead of the first diurnal harmonic to avoid smoothing. While a single sharp peak is observed over the mountains around 19 LT, a much broader delayed peak occurs over land to the west of topography and two peaks are observed over land to the east of topography at 15 and 2 LT. Rain peaks offshore from 3 to 7 LT. Cluster analysis shows that the afternoon and nighttime rainfall peaks are highest before the MJO arrives, then gradually decrease and become earlier on the west side of the topography, whereas the afternoon (nighttime) peak east of the topography is enhanced before (after) the MJO arrives. The contrasting east‐west features can be attributed to topographic influences on the moisture flux convergence of the mean column moisture by MJO‐induced winds. The MJO wind modulation of diurnal rainfall over most of the open ocean areas is insignificant. Plain Language Summary: The Indo‐Pacific Maritime Continent (MC) has complex geography and strong variations in rainfall throughout the day. The Madden‐Julian Oscillation (MJO) is a slow, eastward‐moving envelope of storminess in the tropics, initiating over the Indian Ocean and propagating across the MC into the West Pacific over a month's time. The passage of the MJO can strongly impact when and how much it rains over the MC islands. Using a 19‐year satellite data set, we show that higher rain amounts occur on the west side of the mountains before the east side as the MJO passes. There is also a strong dependence when rain occurs during the day on whether you are to the west, over, or to the east of the mountains when the MJO passes. The east‐west rainfall differences can be attributed to the influence of the mountains on the mean moisture distribution and MJO‐induced winds. This study highlights the role of mountains in modulating the impacts of the MJO on rainfall over the MC islands, and thus the importance of accurately representing topographic effects in numerical models. Key Points: The first harmonic and the time of maximum hourly rain rate produce different diurnal rain patterns over the Maritime ContinentDaily and diurnal rain anomalies during the Madden‐Julian Oscillation (MJO) are asymmetrical around topography with similar convective and stratiform rain contributionsThe asymmetry of rainfall around topography is determined by convergence by MJO‐scale winds while MJO moisture plays less of a role [ABSTRACT FROM AUTHOR]

Reanalysis and satellite data sets disagree on the vertical structure of the meridional overturning circulation over the East Pacific (EP) intertropical convergence zone (ITCZ). Recent observations from the Organization of Tropical East Pacific Convection (OTREC) 2019 field campaign show a meridional slope of vertical motion with a shallow mode from 3° to 7°N, over a strong meridional sea surface temperature (SST) gradient, and a deeper mode from 7° to 10.5°N, where the SST is predominantly warm. However, reanalysis fields from the OTREC period show a predominant shallow mode from 3° to 9°N unless OTREC dropsondes are assimilated; then a weak deep mode is found at 8°N. Vertical motion over the OTREC region derived from Global Precipitation Measurement (GPM) precipitation radar latent heating retrievals has the opposite problem in that the deep mode is predominant from 3° to 10°N. A shallow mode is present with the addition of cloud radar data. Plain Language Summary: There is no consensus on where air rises most strongly and to what heights over the tropical East Pacific Ocean because there are very few available in situ observations. Thus, we rely on numerical models or satellite observations to make estimates of where air rises and where storms are deepest and strongest. In this study, we show recent field campaign observations that indicate the existence of two modes of upward motion: a shallow mode (i.e., ascending motion up to 4 km AMSL) in the southern part of the main precipitation band just north of the equator, and a deep mode (i.e., ascending motion up to 10 km AMSL) a few hundred km farther north. While numerical models only produce the shallow mode, satellite algorithms suggest only a deep mode. Improvements were made when the numerical models used in situ data for guidance and the satellite algorithms were supplemented with observations from other sensors. In situ observations were crucial to conclude that currently available numerical model and satellite data sets, independently, are problematic for describing the large‐scale air motions in the East Pacific. Key Points: Organization of Tropical East Pacific Convection field campaign observations indicate shallow (5°N) and deep (8°N) ascending motion over the East Pacific intertropical convergence zone (ITCZ)Reanalysis (satellite) products show predominant shallow (deep) ascending motion across the East Pacific ITCZReanalysis and satellite products by themselves are insufficient for characterizing convection over this region [ABSTRACT FROM AUTHOR]

The ability of an observationally‐constrained cloud‐system resolving model (Weather Research and Forecasting; WRF, 4‐km grid spacing) and a global climate model (Energy Exascale Earth System Model; E3SM, 1‐degree grid spacing) to represent the precipitation diurnal cycle over the Amazon basin during the 2014 wet season is assessed. The WRF model coupled with a 3‐D variational data assimilation scheme reproduces the spatial variability of the precipitation diurnal cycle over the basin and the lifecycle of westward propagating MCSs initiated by the coastal sea‐breeze front. In contrast, a single morning peak in rainfall is produced by E3SM for simulations despite the nudging of large‐scale winds toward global reanalysis, indicating precipitation in E3SM is largely controlled by local convection associated with diurnal heating. The role of propagating MCS on the environment are discussed by using a multivariate perturbation analysis. We also find that the advection of moisture perturbations from ocean to inland regions have a higher correlation with the occurrence of MCSs in the Amazon than the intensity of colder air intrusion associated with sea breezes along the coast. Moreover, the presence of large cold pools over the central Amazon basin are responsible for the maintenance of propagating deep convection. Plain Language Summary: The Amazon basin in South America is one of the regions over land that has the highest occurrence of large‐size and deep cloud systems (also called "Mescoscale Convective System" [MCS]). Since they have a wide coverage and produce much heavier rainfall than the other types of cloud, the regional climate and even the earth system are tied closely with their behaviors. However, current global atmospheric models are unable to reproduce realistic diurnal variation of precipitation in the Amazon and the poor representation of those MCSs is responsible for the deficiency. We use various observations as the reference to understand how accurate the physical processes related to MCS are represented by both the cloud‐system resolving (higher‐resolution) and global climate (lower‐resolution) models. The results show the diurnal variation of local precipitation in the basin is mostly reproduced by cloud‐system resolving model but not the global climate model, because the propagating MCSs and many related processes can only be simulated by using higher‐resolution model. We also found the advection of moisture perturbation from ocean to inland regions have a higher correlation with the occurrence of MCS in the Amazon than intensity of colder air intrusion associated with sea breezes along the coast. Key Points: Spatial variability of the precipitation diurnal variation in the Amazon is mostly reproduced by WRF but not E3SMRepresentation of propagating convective systems is the key to simulate accurate diurnal cycle of precipitationOcean to inland advection of moisture perturbations into the Amazon control the occurrence of propagating MCS [ABSTRACT FROM AUTHOR]

Convective system tracking was performed using 30‐min GOES‐13 infrared imagery over the Amazon region during 2014 and 2015. A total of 116,701 convective systems were identified and statistics on the probability of occurrence of track area, lifetime, and system velocity were analysed. Maps of the total and seasonal geographic density of trajectories and the geographic density of clusters at genesis, during propagation, and at dissipation were also assessed. The mean area and lifetime of the tracked systems was 4 × 104 km2 and 3 hr, respectively. The top 10% largest systems had areas >8 × 104 km2 and the top 10% longest lived systems lasted >7 hr. The geographical distribution of clusters identified on the coast and within the Amazon basin varied seasonally and their life cycle tracking showed that they are typically distinct from one another (i.e., it is relatively rare for systems to start at the coast and propagate 1,500 km to the centre of the basin). Although the average system velocity indicated a predominantly westward motion, a large spread in the direction of propagation was found. In particular, the probability of a meridional component of motion was generally the same for northward or southward directions and 35% of the zonal propagation was associated with eastward movement. The presence of Kelvin waves accounted for some of the eastward system motion, in addition to increasing the area and lifetime of storms compared to when Kelvin waves were not present. [ABSTRACT FROM AUTHOR]

The objective of this study is to determine the relationship between lightning observed by the Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) and seven large‐scale environmental variables obtained from the 3‐hourly Modern‐Era Retrospective analysis for Research and Application version 2 (MERRA‐2) reanalysis in the tropics and subtropics. The large‐scale environmental variables used are: convective available potential energy (CAPE), normalized CAPE (nCAPE), lifting condensation level (LCL), column saturation fraction (r), 700‐hPa omega, low‐level wind shear (LS) from 900 to 700 hPa, and deep wind shear (DS) from 900 to 300 hPa. All environmental variables show a significant shift toward larger values when lightning is present except for shear. DS decreases when lightning is present, while LS shows little mean change. However, strong geographical differences exist in the relationship between the environmental variables and lightning occurrence, particularly between land and ocean and the tropics and subtropics. Using a logistic regression, a lightning parameterization for global climate models (GCMs) is created using the above environmental variables as predictors while also adding geographic indicators (coast, slope, and latitude) and terms representing interactions between the predictors. The logistic regression predicts lightning occurrence accurately up to 86% of the time and is further applied to MERRA‐2. While there are regions of overprediction and underprediction, the lightning parameterization performance shows promising potential for use in GCMs. Key Points: CAPE, LCL, and column‐integrated humidity are the most important environmental variables for predicting lightning in the tropics & subtropicsA logistic regression using only large‐scale environmental variables and interactions accurately predicts lightning up to 86% of the timeA parameterization based on the logistic regression captures the general pattern of lightning occurrence over land and ocean [ABSTRACT FROM AUTHOR]

The Green Ocean Amazon (GoAmazon) 2014/5 field campaign data are used to study the diurnal cycle of clouds and precipitation. Through a careful classification of days with shallow cumulus, congestus and deep convection, we investigate the major differences among locally generated convection regimes and the most important environmental factors governing the shallow‐to‐deep convection transition. On shallow cumulus days, a greater sensible heat flux drives deeper boundary layer growth, which entrains drier free‐tropospheric air and lowers the relative humidity, thus leading to a significantly higher cloud base than those on days with deeper convection. Congestus and deep convection regimes exhibit distinct cloud top height distributions with noticeable differences in the vertical wind shear in the mid‐troposphere, suggesting an important role of wind shear in limiting the vertical extent of convection. On deep convection days, with preexisting nocturnal convection or cold‐pools from external disturbances, the timing of peak surface precipitation (12:00–13:00 LST) tends to be in‐phase with the diurnal variation in surface fluxes. However, it takes longer for local deep convection to develop without these disturbances. A plume model with thermodynamic and dynamical constraints is developed to explore the relative importance of various convection‐controlling factors. Initial cloud‐base vertical velocity and buoyancy are important in helping parcels ascend to the level of free convection (LFC). After parcels reach the LFC, entrainment of environmental air and lower free troposphere humidity become crucial in determining cloud top. Entrainment rate differentiates among convection regimes, which may be tied to the cloud size distribution at cloud base. Key Points: Greater surface sensible heat flux on shallow cumulus days leads to higher cloud bases than on deeper convection daysDays with external disturbances trigger earlier convection, making the peak rainfall timing more in phase with surface fluxesEntrainment of environmental air and free tropospheric humidity play crucial roles in determining the shallow to deep convection transition [ABSTRACT FROM AUTHOR]

Previous studies suggested that cloud longwave radiation contributes to the development and maintenance of the Madden‐Julian Oscillation (MJO) and model‐based convection is highly sensitive to the radiation scheme. However, currently used radiation schemes do not take cloud longwave scattering into account, resulting in an overestimation of the outgoing longwave radiation (OLR) and an underestimation of the downward longwave flux at the surface. We use combined active and passive satellite cloud property retrievals to quantify the one‐layer cloud OLR and heating rate (HR) biases introduced by neglecting cloud longwave scattering in the Indian Ocean and Maritime Continent in the context of MJO, with a focus on its phases 3, 5, and 6. The results show that the satellite‐detected one‐layer cloud area consists primarily of ice clouds, particularly during the boreal winter in the 4‐year study period. An increased ice cloud area fraction of one‐layer cloud groups is present up to 5 days before the onset of MJO events. If longwave scattering is neglected, the composite mean OLR overestimation over the one‐layer ice cloud area from 5 days before to 4 days after the MJO passage is approximately 3.5 to 5.0 W m−2. Neglecting longwave scattering also leads to a HR underestimation at cloud base and an overestimation at cloud top, making the base‐to‐top heating gradient less sharp at the cloud‐resolving scale. Key Points: Duration of one‐layer ice cloud coverage increases up to 5 days before the Madden‐Julian Oscillation (MJO) passageNeglecting longwave scattering leads to a 3.5 to 5.0 W m−2 overestimation of the outgoing longwave radiation (OLR)Neglecting longwave scattering leads to a less sharp heating gradient from cloud base to cloud top [ABSTRACT FROM AUTHOR]

Abstract: The interactions and teleconnections between the tropical and midlatitude regions on intraseasonal time scales are an important modulator of tropical and extratropical circulation anomalies and their associated weather patterns. These interactions arise due to the impact of the tropics on the extratropics, the impact of the midlatitudes on the tropics, and two‐way interactions between the regions. Observational evidence, as well as theoretical studies with models of complexity ranging from the linear barotropic framework to intricate Earth system models, suggest the involvement of a myriad of processes and mechanisms in generating and maintaining these interconnections. At this stage, our understanding of these teleconnections is primarily a collection of concepts; a comprehensive theoretical framework has yet to be established. These intraseasonal teleconnections are increasingly recognized as an untapped source of potential subseasonal predictability. However, the complexity and diversity of mechanisms associated with these teleconnections, along with the lack of a conceptual framework to relate them, prevent this potential predictability from being translated into realized forecast skill. This review synthesizes our progress in understanding the observed characteristics of intraseasonal tropical‐extratropical interactions and their associated mechanisms, identifies the significant gaps in this understanding, and recommends new research endeavors to address the remaining challenges. [ABSTRACT FROM AUTHOR]

Three months of profiler observations from Darwin, Australia are separated into seven cloud types (shallow, mid-level, and deep convective; weak and robust stratiform rain; and transitional and ice-only anvil) to provide vertical velocity statistics on the full spectrum of tropical convective clouds over the course of a monsoon season. Consistent with past studies, convective updraughts increase in height and magnitude as the convective cloud height increases. Shallow/mid-level/deep convection has a mean maximum value of 0.3 m s-1 at 3 km/0.6 m s-1 at 5 km/2.5 m s-1 at 8 km. Deep convective extremes approach 18 m s-1 above 8 km while downdraughts in all convective cloud types are maximum below 4 km. Stratiform vertical velocities are weaker and less varied than in active convection, with maximum mean values <0.25 m s-1. However, stratiform rain regions associated with larger near-surface reflectivities (and thus rain rates) have stronger mesoscale up- and downdraughts than weaker stratiform rain regions. Anvil cloud with little or no rain near the surface also exhibits mesoscale up- and downdraughts, but the vertical velocity profile is shifted up in height by at least 2 km. In addition, anvil updraughts were only 20-50% of the magnitudes in the stratiform rain region. Overall, the vertical motion statistics were similar across the pre-, active and suppressed monsoon periods. The vertical velocity distributions for each cloud type were also generally consistent with the reflectivity distributions (i.e. the strongest updraughts were often linked to the largest reflectivities), allowing for some linkages between the dynamical and microphysical cloud properties. For example, strong updraughts above the 0 °C level in deep convection are associated with strongly sloping reflectivity profiles supporting the importance of cold rain growth processes, and stronger vertical motions are associated with stratiform rain that has a robust bright band. [ABSTRACT FROM AUTHOR]