A key component necessary to improve the performance of climate and weather forecasting models is understanding the physical mechanisms controlling tropical deep convection. In this study, the thermodynamic variables linked to deep convection within this equatorial sea-breeze convective regime are analyzed. A range of data sets are employed: GNSS-based PWV and surface precipitation data, lightning and daily radiosonde observations, and GOES-13/16 and GPM satellite products. Our results indicate that the convective indices of CAPE and CIN, often used as predictors of deep convection, do not clearly distinguish deep-convective and non-convective days. In contrast, the variables representative of the atmospheric water vapor content, PWV and vertical water vapor distribution as well as an entrainment-based buoyancy measure, are better markers of potential deep convection. For this region, however, the water vapor/deep convection relationship with precipitation does not appear as strong as over tropical oceans and tropical continental regions. Finally, our results show that there is no strong link between daily average precipitation intensity and daily lightning count. However, deep-convective days without lightning had higher water vapor at the beginning of the day, as compared to days with lightning, which resulted in convective showers earlier in the day.

Abstract We develop a diagnostic framework for assessing systematic biases in tropical precipitation to support the improvement of weather forecast systems. The approach is demonstrated through its application to a suite of global models associated with various stages of NOAA’s Unified Forecast System (UFS) development. The diagnostics are based on observed relationships between precipitation and lower-tropospheric buoyancy, estimated offline using a plume model. This buoyancy metric serves as a proxy for convective instability, incorporating the effects of dry air entrainment, a key factor in tropical convection. Among the models examined, tropical convection biases primarily arise during two convective regimes: the transition from shallow to deep convection, involving cumulus congestus clouds, and periods of widespread deep convection, dominated by mesoscale convective systems (MCSs). When large-scale conditions favor enhanced cumulus congestus activity in observations, the NOAA models analyzed here tend to overproduce precipitation and develop a dry bias in the lower troposphere. This leads to rapid stabilization of the convective environment compared to observations, suppressing the frequency of high convectively unstable conditions that typically support active MCS development. During periods when large-scale conditions favor MCS activity in observations, the models often underpredict precipitation, partly due to this artificially stabilized model environment. Systematic coupled biases in precipitation, humidity, and lower-tropospheric buoyancy emerge rapidly, within less than a day, and persist over longer timescales. Building on previous applications to other models, these results underscore the value of plume model diagnostics as a powerful tool for evaluating how convection scheme modifications influence tropical precipitation biases, providing actionable insights that can directly inform operational model development.

Representation of tropical convection in an advanced process-oriented diagnostic framework

Abstract. Secondary ice production (SIP) plays an important role in tropical deep convection. This study implements multiple SIP mechanisms, including droplet fragmentation and ice–ice collisional breakup, into the CASIM microphysics scheme of the UK Met Office Unified Model, and evaluates their impacts through a real-case simulation of a Hector thunderstorm. SIP enhances ice number concentration in upper cloud layers, with values up to 3 orders of magnitude higher than the no-SIP case, particularly above −10 °C. Ice water content (IWC) increases by a factor of 3–5 in the anvil region, contributing to more extensive upper-level cloud coverage. These microphysical changes reduce outgoing longwave radiation (OLR) by ∼ 3.2 W m−2 (1.3 %) and increase outgoing shortwave radiation (OSR) by ∼ 4.5 W m−2 (1.8 %) over a 6 h analysis period and a 110 km × 110 km domain. SIP modifies precipitation spatially, yielding a more localized, compact rainfall pattern near the convective core, while reducing domain-averaged precipitation by ∼ 8 %. Peak rainfall rates remain only slightly affected, consistent with the minor changes (< 1 m s−1) in maximum updraft velocity. Among the tested mechanisms, ice–ice collisional breakup shows negligible impact on simulated ice concentration, consistent with limited graupel-involved collision energetics under warm profiles. Ensemble experiments confirm that these effects are robust and exceed the influence of meteorological variability. These results highlight the importance of representing SIP processes in cloud-resolving models of tropical convection and accounting for their environmental dependence.

According to the scientific consensus, tropical convection must decrease with global warming. This decrease is manifested by a decrease of the mass transported in the upward branch of the atmospheric overturning circulation – the convective mass flux – and a connected decrease of high clouds in the tropics, with implications for climate sensitivity. By using kilometer-scale simulations in radiative-convective equilibrium and a convective tracking algorithm, we show that no such decrease occurs in storms when taken individually and that the mass transport per storm increases instead. Storms can achieve this result by aggregating more surface of the convective cores – the inner part of the storm doing the vertical transport – so that the decrease of tropical convection is actually explained by a decrease in the total number of storms. There is little variation of the mean pressure velocity in the cores of the storms, a robust finding of this study. This remarkable invariance of the mean pressure velocity points to an emerging property of convection that should receive more attention in future studies.

Abstract Global climate models project an increase in global‐mean precipitation in response to increases in global‐mean surface temperature; that is, a positive “hydrological sensitivity.” However, there are hints in the literature that global‐mean precipitation is sensitive to the pattern of warming in addition to global‐mean warming. Here we leverage previous theoretical insights into tropical dynamics and radiative cooling to connect clear‐sky longwave radiative cooling, a key component of hydrological sensitivity, to sea‐surface temperature (SST) patterns in the tropics. We use this theory to explain why hydrological sensitivity is about 25% larger in uniform warming scenarios compared to abrupt‐ runs. This discrepancy is driven by different changes in clear‐sky longwave radiative cooling in the tropics, which we quantitatively attribute to differences in the rate of SST warming in regions of tropical convection.

Abstract This study analyzes the thermodynamic effects of atmospheric vertical motion recorded in satellite observations to investigate the processes behind the evolution of tropical convection. These processes, difficult to observe directly, are diagnosed from satellite retrievals of precipitation ( P ) and atmospheric cloud radiative effect (ACRE) with the aid of a simple theory. It is found that hourly changes of P and ACRE projected onto the P ‐ACRE plane have a tendency of pointing toward a settling point at LP 700 W and ACRE 75 W , corresponding to a state of midheavy ascent. These physically driven transitions are counteracted largely by inverse transitions ascribed mainly to the stochastic effects due to a sample density gradient, except where samples are very few. The net effect of these mutually competing transitions is an evolutionary path qualitatively reminiscent of the known Lagrangian life cycle of a traveling convective system, whereas the P ‐ACRE trajectory stays in the close vicinity of the settling point when spatially averaged over the system.

Abstract Poleward heat flux driven by large‐scale eddies is a widely recognized component of the general circulation. Although less known, a weak but persistent equatorward eddy heat flux (EEHF) also exists in the subtropical upper troposphere–lower stratosphere (UTLS). In this study, we solve a Transformed Eulerian Mean (TEM) form of the Kuo–Eliassen equation and find that in the subtropical UTLS, the EEHF alone weaken the boreal winter TEM circulation. This EEHF fluctuates on daily time scales. When the EEHF strength exceeds 1 standard deviation, the UTLS TEM circulation substantially weakens. This EEHF cannot be explained by background temperature gradients. Instead, the evolution of the Eliassen–Palm flux and precipitation rate indicates that it is likely driven by horizontal propagation of planetary‐scale waves which, at least in part, is triggered by tropical convection over the western tropical Pacific.

Abstract Mesopause‐region (87 km) gravity waves (GWs) generated by tropical convection are investigated within the four longitude sectors encompassing Africa, the Indian Ocean, the Intertropical Convergence Zone, and South America during the Dec 2023–Feb 2024 Southern Hemisphere monsoon season. Variances () in the OH Q‐line emission measured by the Atmospheric Waves Experiment (AWE) capture GW activity, and precipitation rates (PR) from the Global Precipitation Measurement (GPM) Mission identify regions of convective activity. The zonal component of GWs comprising the between 10S‐10N primarily propagate eastward. The distributions are latitudinally shifted and more confined in local solar time (LST) compared with those of PR. Mesospheric winds (including tides) appear to induce the latitude‐longitude‐LST variability seen in through critical‐level filtering and Doppler‐shifting of the GWs. These new insights into the variability of the GW spectrum entering the ionosphere‐thermosphere system further our understanding of the dynamical connections between tropospheric and space weather.

Abstract This study examines the evolution of “top‐heaviness” in tropical convection during extreme precipitation events. Top‐heaviness describes the extent to which ascent peaks in the upper (top‐heavy) versus lower (bottom‐heavy) troposphere. Reanalysis vertical velocity profiles are projected onto two sinusoidal basis functions, representing the first and second baroclinic modes, that together characterize top‐heaviness. Two distinct modes are found following the peak in rainfall: stratiform decay and convective decay. Stratiform‐decay events transition rapidly from bottom‐heavy to top‐heavy to stratiform‐like ascent profiles and experience sharp reductions in instability, moisture and precipitation after the peak of the event. In contrast, convective‐decay events sustain bottom‐heavy ascent profiles with a gradual decline in instability and moisture and prolonged precipitation; they contribute over 55% of the rainfall during extreme events. These findings emphasize the significant role of convective decay in shaping extreme precipitation compared to conventional stratiform decay.

Abstract In the summer of 2022, the Yangtze River basin (YRB) experienced the most intense and prolonged extreme high-temperature event since 1961, featured by three successive episodes of 18–29 June (P1), 6–17 July (P2), and 3–26 August (P3). An assessment of two operational subseasonal-to-seasonal models reveals that the spatial pattern of surface air temperature (SAT) anomalies over the YRB can be predicted with lead times of 11–12, 8–9, and 15–20 days for P1, P2, and P3, respectively. However, both models underestimate the intensity of maximum SAT anomalies, especially during P3. In P1 and P2, both models fail to reproduce the propagation of Eurasian transient waves at the lead time of 15 days, hindering the development of high-pressure anomalies over East Asia. In P2, additional biases in reproducing the boreal summer intraseasonal oscillation (BSISO)-like convection anomalies promote the further northward shift of the high-pressure anomaly. These model errors result in poor simulations of local diabatic heating, which is the primary contributor to SAT anomalies over the YRB in P1 and P2. In P3, well-reproduced mid- to high-latitude Eurasian transient waves and BSISO-like convection are favorable for simulating SAT anomalies over YRB. Nonetheless, the underestimation of regional adiabatic heating associated with vertical motion in P3 limits the skill in predicting the intensity of SAT anomalies over the YRB. Thus, the ability of accurately capturing the two predictability sources of Eurasian transient waves and tropical Indo-Pacific convections is the key to improve the subseasonal prediction of this prolonged extreme high-temperature event over the YRB. Significance Statement A record-breaking prolonged extreme high-temperature event occurred over the Yangtze River basin (YRB) in the summer of 2022, featuring three distinct subseasonal episodes. Evaluation of two subseasonal-to-seasonal models from the European Centre for Medium-Range Weather Forecasts and China Meteorological Administration shows that the distribution patterns of surface air temperature anomalies over the YRB can be predicted with lead times of 11–12 days for P1, 8–9 days for P2, and 15–20 days for P3, whereas the intensity of maximum anomalies is consistently underestimated. Mid- to high-latitude Eurasian transient waves and tropical Indo-Pacific convections are identified as key sources of subseasonal predictability for this extreme event. These results offer valuable insights for improving subseasonal prediction skills for future extreme high-temperature events over the YRB.

ABSTRACTENSO exerts a strong influence on global mean surface temperature. Using reanalysis data and CMIP6 AMIP simulations, this study examined distinct late winter (February–March) Northern Hemisphere (NH) mid‐to‐high latitude surface air temperature (SAT) responses to Eastern Pacific (EP) and Central Pacific (CP) ENSO types. EP El Niño induces a “north cold, south warm” SAT dipole pattern over Eurasia, contrasting sharply with the “north warm, south cold” pattern observed during CP El Niño. EP La Niña produces SAT anomalies nearly opposite to EP El Niño, whereas the CP La Niña responses are comparatively weak. Enhanced low pressure and cyclonic circulation over Eurasia during EP El Niño drive meridional temperature advection, establishing the SAT dipole. Conversely, anomalous high pressure and anticyclonic circulation dominate during CP El Niño and both La Niña types. Observational analysis reveals that the divergent Eurasian SAT patterns stem from differences in tropical Pacific convection anomalies. These anomalies excite distinct poleward‐propagating Rossby wave trains and modulate the variability of the polar front jet (PFJ), thereby influencing extratropical circulation. CMIP6 AMIP models realistically simulate these contrasting late winter NH SAT responses.

Abstract. Aerosol modulation of atmospheric convection remains an important topic in ongoing research. A key challenge in evaluating aerosol impacts on cumulus convection is isolating their effects from environmental influences. This work investigates aerosol effects on maritime tropical convection using airborne observations from NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). Eight environmental parameters with known physical connections to cloud and storm formation were identified from dropsonde data, and 92 dropsondes were matched with corresponding CAMP2Ex flight “scenes.” To constrain environmental conditions, scenes were binned based on their association with “low,” “medium,” or “high” values for each dropsonde-derived parameter. In each scene and environmental bin, eight radar- and radiometer-based parameters with physical implications for convective intensity and/or prevalence were correlated with lidar-derived aerosol concentrations to examine trends in convective characteristics under different aerosol conditions. Threshold values used to stratify the environments were varied across four sensitivity tests to examine how the convective–aerosol correlations within each environmental bin responded. The results were generally inconclusive, with relatively weak correlations observed with limited statistical significance in many cases. Some interesting and potentially impactful comparisons identified in the convective–aerosol analyses support the idea of warm-phase convective invigoration trends and suggest that higher aerosol concentrations were correlated with stronger and/or more-prevalent convection in some cases, while other cases saw a “Goldilocks” zone of medium aerosol concentration favoring enhanced convection. Our results also stress the importance of considering environmental conditions when evaluating aerosol impacts.

Strengths and limitations of the Gálvez-Davison Index in forecasting tropical and subtropical convection over South America

Abstract Arctic sea ice reduction has the potential to cause climatic change and extreme weather in the mid- and high latitudes of the Northern Hemisphere. The causes of sea ice variation, such as the influence of tropical sea surface temperature (SST) variation due to El Niño–Southern Oscillation, are actively studied. The notion that tropical SSTs influence atmospheric conditions in mid- and high-latitude regions is widely accepted. Nonetheless, the extent to which SSTs in the eastern tropical Atlantic drive sea ice variability has not been extensively investigated. Here, we show that the autumn eastern tropical Atlantic convective cloud activity, independent of underlying SST, influences interannual variation in wintertime sea ice via atmospheric teleconnection with a lag of 2 months. In years characterized by the heightened convective activity in October, Scandinavian anticyclonic anomalies emerge, driven by a wave train originating from the tropical Atlantic. This contrasts with years marked by inactive convection, which may be related to water vapor transport from the African Sahel. These anticyclonic anomalies facilitate the influx of warm, moist air into the Atlantic Arctic, thereby warming SST in the region and impeding the refreezing of sea ice. The convective activity affects not only the interannual variability of sea ice but also the decadal variability. Since convective activity in the tropical Atlantic is not driven by SST, SST may not serve as a reliable predictor for sea ice forecasting. Clarifying the mechanisms underlying tropical atmospheric convection that are independent of SST is crucial not only for sea ice forecasting but also for predicting extreme weather in midlatitudes. Significance Statement Given the recent rapid loss of sea ice and its impact on global climate, understanding the causes of sea ice variability has become increasingly important. Atmospheric impacts significantly influence sea ice variation, and numerous studies have examined the impact of warm tropical sea surface temperature (SST)-driven atmospheric convection on sea ice through atmospheric teleconnection, i.e., high–low pressure systems from the tropics to the Arctic. Our data analysis shows that autumn convective cloud activity in the eastern tropical Atlantic, which is not driven by SST, affects wintertime Arctic sea ice following a 2-month time lag through atmospheric teleconnections. This discovery challenges the prevalent belief linking tropical SSTs and convective activity, prompting reconsideration of the impact of non-SST-driven tropical convection activity on extreme extratropical weather.

The geographical response of western North Pacific subtropical high (SH) to environmental conditions such as the El Niño-Southern Oscillation (ENSO) and global warming has been one of the main concerns with respect to extreme events induced by tropical convections. By considering observed outgoing longwave radiation (OLR) as the strength of subtropical high, this study attempts to further understand the geographical response of SH strength to ENSO and global warming. Here, “SH strength” is defined as the inhibition of regional convections under SH environment. A meridional seesaw pattern among SH strength anomalies is found at 130°–175° E. In addition, the La Niña environment with weaker convections at lower latitudes is characterized by farther westward expansion of SH but with a weaker strength. Conversely, the El Niño environment with stronger convections at lower latitudes leads to shrunken SH but with a greater strength. The influence of the seesaw mechanism appears to be modulated by global warming. The western North Pacific subtropical high strengthens overall under warming in both the La Niña and El Niño environments. This suggests that the weakening effect by drier tropics is largely offset by anomalous highs induced by a warming atmosphere. It is most remarkable that the highest SH strengths appear in a warmer El Niño environment. The finding implies that every new El Niño environment may experience the driest atmosphere ever in the subtropics under global warming. The value of this study lies in the fact that OLR effectively illustrates how the ENSO variation and global warming bring the zonally undulating strength of boreal-summer SH.

Abstract Notable interdecadal variability in tropical cyclone (TC) genesis frequency within western North Pacific (WNP) has been well-documented, yet its physical mechanisms remain unclear. This study demonstrates that the interdecadal variation in the WNP TC genesis frequency during the peak TC season (July–October) is strongly influenced to the boreal winter–spring (January–may) North Atlantic Oscillation (NAO) on interdecadal timescales, with the Pacific Meridional Mode (PMM) acting as a crucial intermediary. Specifically, the NAO triggers a Eurasian wave train, resulting in positive temperature anomalies over northeastern Asia. These anomalies are subsequently transported to the Kuroshio Extension region by the midlatitude westerlies, which weakens the meridional temperature gradient and reduces the strength of the subtropical westerly jet. As the jet weakens, it induces anomalous negative vorticity and anticyclonic circulation to its north. The anomalous northerly winds east of the anticyclone transport cold and dry air southward to the southeastern side of the anomalous anticyclone, where the air accumulates and sinks. This subsidence then alters the local meridional circulation, promoting anomalous deep convection over the central tropical Pacific. The enhanced tropical convection further excites a northward-propagating Pacific–North American (PNA) wave train, leading to the development of an anomalous low-pressure system over the North Pacific. Anomalous westerly winds on the southeastern flank of this system subsequently amplify the PMM through wind–evaporation–sea surface temperature (SST) feedback. The PMM-related SST anomalies ultimately induce anomalous cyclonic circulation over WNP through the Gill response, creating favorable conditions for TC genesis. This result reveals a novel remote subtropical modulator of interdecadal variability in WNP TC genesis, offering valuable insights for improving TC activity predictions.

Abstract Reliable projections of tropical cyclones (TCs) and associated impacts remain hampered by both climate model resolution and simulation length. To address this, here we present updated projections of TCs for the southwest Pacific from a high‐resolution downscaled ensemble of CMIP6 models. The downscaling implements a variable‐resolution atmospheric model enhancing resolution over the southwest Pacific and New Zealand (∼12–30 km). We assess future changes in TC frequency, changes in large‐scale environmental conditions, and associated extreme precipitation and winds across tropical and ex‐tropical storm phases. Changes in TC track pathways are also investigated through cluster analysis. Across the downscaled simulations, robust changes in TC frequency were not found, including for a high‐emissions scenario at end‐of‐century. Projections of the background environmental conditions are shown to be a significant source of uncertainty, owing to diverging projections of relative SST and tropical convection across the region in the host GCMs. However, very strong TCs (category 4 and above) show greater consensus for an increase in frequency, with 16 of 18 simulations across models and scenarios projecting an increase. Cluster analysis of TC tracks indicates a slight decrease in tracks that often impact northern parts of Australia. Extreme precipitation associated with TCs under a high‐emissions scenario is projected to increase by ∼30%–35% averaged across models, both for storms in the tropics and ex‐TCs impacting New Zealand. This increase exceeds Clausius‐Clapeyron scaling in five of six simulations. These projected increases in associated extreme precipitation pose significant societal risks despite the remaining uncertainty in TC frequency changes.

Abstract. Palaeoclimate proxies reveal a significant precessional impact on the low-latitude hydrological cycle. Classical theory suggests that precession modulates the inter-hemisphere summer insolation difference and hence controls the meridional displacement of the Intertropical Convergence Zone (ITCZ). Accordingly, low-latitude precipitation variations are expected to be in phase (for the Northern Hemisphere) or anti-phase (for the Southern Hemisphere) with the Northern Hemisphere summer insolation. However, increasing numbers of proxies, particularly those that are absolutely dated, reveal that variations in terrestrial precipitation at different low latitudes follow distinct precession rhythms that are very often out of phase with hemispheric summer insolation. The mechanism underlying such spatial–temporal complexity remains elusive. In this study, we performed theoretical analysis, climate simulations, and synthesis of geological records to hypothesise that the low-latitude hydrological cycle is paced by shifting perihelion rather than by the hemispheric summer insolation. More specifically, precession of the Earth’s rotation axis shifts the season and latitude of perihelion. Here, the latitude of perihelion is introduced as the latitude of Earth's subsolar point during perihelion, which is the location where the most intense solar radiation is concentrated. At the time of perihelion, intense solar radiation heats the land faster than the ocean due to differing thermal inertia. This thermodynamically moves the tropical convection from the ocean to the land, contributing to enhancing the terrestrial precipitation around the perihelion latitude. As the precessional phase changes, perihelion moves toward different latitudes, causing asynchronous maximums in terrestrial precipitation at different latitudes. Perihelion can occur in any season; therefore, the insolation in individual seasons is equally important in shaping the orbital-scale climate changes at low latitudes. This offers new insight into the Milankovitch theory, which highlights summer insolation's role in shaping orbital-scale climate change.

Tropical convection anomaly could serve as a crucial driver of global atmospheric teleconnections and weather extremes around the world. However, quantifying the dominances of convection anomalies with regional discrepancies, relevant for the variations of global atmospheric circulations, remains challenging. By using a network analysis of observation-based rainfall and ERA5 reanalysis datasets, our study reveals that El Niño-like convection is the most primary rainfall pattern driving the global atmospheric circulation variations. High local concurrences of above-normal rainfall events over equatorial central-eastern Pacific amplify their impacts, even though the most intense rainfall anomalies are observed near the Maritime Continent. Furthermore, we find that the impacts of El Niño-like convection will be tripled by the end of this century, as projected consistently by 23 climate models. Such “rich nodes get richer” phenomenon is probably attributable to the dipolar rainfall changes over the equatorial western-central Pacific. This study highlights the dominant role of El Niño-like convection on the global climate variations, especially under the future changing climate.