Convective Activity Research Articles

Human‐Induced Climate Change Intensifies Extreme Precipitation Events in Central China's Urban Areas

AbstractUnderstanding the drivers behind extreme precipitation is crucial for predicting and mitigating the impacts of climate change globally, yet it remains little known how anthropogenic factors contribute to these phenomena. This study investigates the impact of human‐induced climate change on circulation patterns conducive to extreme precipitation over the Central Plains Urban Agglomeration in China, a region frequently experiencing severe flooding and home to a dense population with significant economic and agricultural activities. Using advanced techniques such as deep learning and optimal fingerprinting, this study identifies and analyzes the physical mechanisms behind the extreme precipitation. The findings reveal that greenhouse gas emissions play a pivotal role in altering atmospheric circulation patterns, specifically promoting the westward extension of western North Pacific subtropical high and northwestward shift of South Asian High, along with tropical cyclones. These changes enhance moisture transport and convective activity, leading to more frequent and intense precipitation events.

Analysis of the Bay of Bengal cyclone and its rejuvenation in the Arabian Sea after passing over peninsular India

Tropical Cyclone (TC) Gulab-Shaheen in the Northern Indian Ocean in September 2021 marked an extraordinary and rare climatic event that formed during the monsoon season, making it uncommon. The western North Pacific subtropical High (WNPSH) branch over the northeastern part of India is responsible for the westward movement of the Gulab cyclone in the Bay of Bengal (BoB). The extension of WNPSH and availability of soil moisture influences the cyclone’s movement over the Indian land, and it eventually emerges as a westward-moving cyclone Shaheen in the Arabian Sea (AS). Furthermore, the fourth stage of the Madden-Julian Oscillation propagated over southeast Asia is responsible for aiding cyclogenesis by increasing convective activities and maintaining higher humidity with lower wind shear. Due to this, TC Gulab remains a basic cyclonic flow over the land and moves westward after landfall. In the case of Shaheen, higher tropical cyclone heat potential and SST assisted the remnant of Gulab in rejuvenating over the AS, and lower VWS lets the storm’s center line up vertically, which helps it get more robust. Therefore, this study demonstrates the causes of the cyclone formation in BoB and its rejuvenation in the AS after surviving over the peninsular India.

Seasonal, regional, and vertical characteristics of high-carbon-monoxide plumes along with their associated ozone anomalies, as seen by IAGOS between 2002 and 2019

Abstract. In situ measurements from the In-service Aircraft for a Global Observing System (IAGOS) are used to characterise extreme values of carbon monoxide (CO) in large regions of the globe in the troposphere between 2002 and 2019. The SOFT-IO model, combining the FLEXPART Lagrangian dispersion model with emission inventories over the footprint region, is used to identify the origins of the CO in the sampled plumes. The impact of biomass burning and anthropogenic emissions on such CO plumes is characterised through CO mixing ratios and simultaneously recorded ozone (O3) ones. In the Northern Hemisphere, CO reaches its maximum values in DJF in the lower troposphere, which can be attributed to elevated anthropogenic emissions and reduced convective activity during the season. Due to the low photochemistry and the fresh age of the air masses, the O3 values of these plumes are low. CO plumes in the upper troposphere (UT) result from intense emissions and efficient vertical transport, peaking during JJA. The largest values of CO in the Northern Hemisphere are found in eastern Asia in the lower troposphere (LT) and middle troposphere (MT) and in Siberia in the upper troposphere. Among the anomalies detected in the upper troposphere in JJA, the ones with higher associated O3 values are the ones associated with biomass-burning emissions. The middle troposphere is a combination of the characteristics of the LT and the UT, with contributions from both local emissions and long-range transport. Among the studied regions, the troposphere above the Middle East and the UT above Siberia presented extremely high O3 values. Indian CO anomalies have different characteristics depending on the season, as the wet and dry phases of the monsoon have a strong impact on the transport of the pollutant in this region. Similarly, the shift in the intertropical convergence zone (ITCZ) strongly impacts the seasonality of the emissions and the transport patterns above Africa. In that region, convection is no longer the limiting factor, and the transport of the CO plumes is driven by the ITCZ shift, trade winds, and the upper branch of the Hadley cell redistributing the pollution to higher latitudes.

Delayed Impact of Biomass Burning in the Indochinese Peninsula on the Bay of Bengal Monsoon

Abstract The Indochinese Peninsula experiences a dry season with extensive biomass burning peaking from March to April. As the monsoon arrives, rainfall significantly removes aerosols through deposition, ending the emission season. However, the biomass burning aerosols exert an influence on the atmospheric circulation prior to the monsoon onset. This study employed statistical methods and a regional atmosphere–chemistry model [WRF Model coupled with chemistry (WRF-Chem)] to investigate the delayed impact of biomass burning from the Indochinese Peninsula on the monsoon onset. The results indicate that the cold sea surface temperature anomaly caused by the aerosols during the emission season can be stored in the ocean and inhibit convective activities over the adjacent sea regions in the postemission season, suppressing the southwestward cross-equatorial flow over the southern Bay of Bengal. This suppression delays the westward extension and northward shift of the upper-level South Asian high pressure system, along with its divergence and subsidence effects, thereby postponing the breakdown and retreat of the subtropical high-pressure belt. Simultaneously, the cold sea temperature also suppresses the development of a warm pool in the southeastern Bay of Bengal, which is associated with the generation of a monsoon onset vortex. Consequently, the onset of the Bay of Bengal monsoon is delayed. Due to the decreasing delayed effects of aerosols over time and the counteractive warming from the accelerated abnormal anticyclonic circulation in the upper-level Bay of Bengal, which results in accelerated disappearance of the cold signal in sea surface, the delayed influence of aerosols diminishes gradually after the onset of the Bay of Bengal monsoon until it disappears. Significance Statement This study explores the correlation between spring biomass burning in the Indochinese Peninsula and the onset of the Bay of Bengal and South China Sea monsoons. The research reveals that biomass burning during the emission season might postpone the initiation of the Bay of Bengal monsoon, while it has minimal influence on the South China Sea monsoon. By employing sensitivity experiments using models, the study elucidates the underlying mechanisms responsible for these observed effects.

Combined Influence of 10–30‐Day Tropical and Mid–High‐Latitude Intraseasonal Oscillations on the Rapid Increases of Humid Heatwaves in Southern China

AbstractAs a typical hotspot of heatwaves, southern China experienced a marked shift from dry to humid heatwaves in the recent decade of 2013–2022. Significant 10–30‐day intraseasonal oscillation (ISO) in temperature and humidity is a major contributor to the humid heatwaves. Configuration of the 10–30‐day quasi‐barotropic wave train at mid–high latitudes with the dipolar pattern of tropical convection leads to the concurrence of strong moisture convergence and anomalous descents over southern China. The increased downward longwave radiation in association with moisture enhancement, shortwave radiation and adiabatic heating in association with descents bring about the persisting hot and humid conditions in humid heatwaves. Due to stronger barotropic energy conversion from mean flow to 10–30‐day circulation at mid–high latitudes, and stronger tropical convection activities in the recent decade, the occurrence of 10–30‐day circulation configuration induced humid heatwaves has increased by 13.3%, thus leading to the predominance of humid heatwaves.

Role of QBO and MJO in Sudden Stratospheric Warmings: A Case Study

The impact of the quasi-biennial oscillation (QBO) and Madden–Julian oscillation (MJO) on the dynamics of major sudden stratospheric warmings (SSWs) observed in the winters of 2018, 2019, and 2021 is investigated. Using data from the MERRA-2 reanalysis, we analyze the daily mean variability of critical atmospheric parameters at the 10 hPa level, including zonal mean polar cap temperature, zonal mean zonal wind, and the amplitudes of planetary waves 1 and 2. The results reveal dramatic increases in polar cap temperature and significant wind reversals during the SSW events, particularly in 2018. The analysis of planetary wave (PW) amplitudes demonstrates intensified wave activity coinciding with the onset of SSWs, underscoring the pivotal role of PWs in these stratospheric disruptions. Further examination of outgoing long-wave radiation (OLR) anomalies highlights the influence of QBO phases on tropical convection patterns. During westerly QBO (w-QBO) phases, enhanced convective activity is observed in the western Pacific, whereas the easterly QBO (e-QBO) phase shifts convection patterns to the maritime continent and central Pacific. This modulation by QBO phases influences the MJO’s role during SSWs, affecting tropical and extra-tropical weather patterns. The day-altitude variability of upward heat flux reveals distinct spatiotemporal patterns, with pronounced warming in the polar regions and mixed heat flux patterns in low latitudes. The differences observed between the SSWs of 2017–2018 and 2018–2019 are likely related to the varying QBO phases, emphasizing the complexity of heat flux dynamics during these events. The northern annular mode (NAM) index analysis shows varied responses to SSWs, with stronger negative anomalies observed during the e-QBO phase compared to the w-QBO phases. This variability highlights the significant role of the QBO in shaping the stratospheric and tropospheric responses to SSWs, impacting surface weather patterns and the persistence of stratospheric anomalies. Overall, the study demonstrates the intricate interactions between stratospheric dynamics, QBO, and MJO during major SSW events, providing insights into the broader implications of these atmospheric phenomena on global weather patterns.

Outflowing of a high potential vorticity air mass and enhanced convection near South Asia in August 2022

AbstractIn August 2022, high potential vorticity (PV) air masses significantly outflowed near South Asia along the southern periphery of the intensified Tibetan high in the upper troposphere, contributing to enhanced convection near South Asia through intensified thermodynamically unstable conditions. Convective activities near South Asia have relationships with the upper‐level PV and the advection over the region. The upper‐level northward divergent wind anomalies from South Asia due to the enhanced convection cross the Asian jet, contributing to the intensified Tibetan high. This suggests a feedback process between the intensified Tibetan high and enhanced convection near South Asia through the high PV intrusion.

A Coupled Quadrupole Mode in the South Pacific

Abstract Although links between the atmospheric convergence zone and the local ocean dipole in the South Atlantic are well established, relationships between the South Pacific convergence zone (SPCZ) and the South Pacific quadrupole (SPQ) remain largely unexplored. Based on maximum covariance analysis applied to a 110-yr monthly coupled atmosphere–ocean reanalysis, we describe a coupled quadrupole mode (CQM) that connects the SPCZ and SPQ during austral summer [December–February (DJF)]. The CQM is linked to the “enhanced SPCZ” mode in the atmosphere and the SPQ in the ocean, with the atmospheric signal leading the ocean signal by about 1 month. This coupled mode essentially represents the atmospheric and oceanic responses to a stationary Rossby wave train that propagates from low- to high latitudes before reflecting back toward lower latitudes around 150°E. Coupled atmosphere–ocean feedbacks help to maintain anomalous convective activity in the SPCZ and related circulation anomalies. The stationary waves that organize the CQM are often rooted in anomalous convection over the Maritime Continent and have close connections with the atmospheric wavenumber-4 mode in the midlatitude Southern Hemisphere. Significance Statement In this study, we investigate the relationships between coherent large-scale patterns in the South Pacific Ocean and the overlying atmosphere. These patterns, which we refer to as a coupled quadrupole for their four centers of action, impact both local communities and the global climate by shaping rainfall and temperature anomalies across the “four corners” of the South Pacific: east–west and north–south. We show that this coupled quadrupole arises as the joint atmospheric and oceanic response to a large-scale wave that arcs across the entire South Pacific basin more than 10 km above the surface and that feedbacks from the ocean to the atmosphere help it to last longer.

Characteristics of the Mediterranean Cyclone IANOS in Convection‐Permitting Simulations: Unraveling Model Sensitivity to Microphysics and Cumulus Parameterization

AbstractThis study quantified the relative impact of microphysics parameterization (MP) and cumulus parameterization (CP) schemes on storm prediction using a non‐hydrostatic weather research and forecasting model for an intense Medicane: IANOS. All the simulations agreed with the main observed characteristics of this storm, with some discrepancies in magnitude and location, which vary with CP and MP schemes. These discrepancies were larger during the mature phase of the storm. In the CP‐on simulations, the non‐scale aware Kain–Fritsch (KF) scheme typically resulted in higher precipitation, while the Betts‐Miller‐Janjic (BMJ) scheme led to lower precipitation compared to the CP‐off simulations (C0). The tendency of the KF scheme to consume available convective potential energy at a relatively high rate (i.e., short time scale) resulted in high mass fluxes and latent heat release, leading to strengthened convective activity and, hence, precipitation. The multi‐scale aware (MSKF) substantially reduces the precipitation compared to KF due to the reduced contribution of convective scale precipitation. It also modulates the spatial structure precipitation compared to KF, especially light precipitation over the outer bands, and the contribution of grid‐scale precipitation to total precipitation. KF shows the lowest contribution, around 50%, whereas BMJ exhibits a slightly higher contribution, and MSKF versions nearly reach 100%, making it closer to CP‐off. The improved performance in MSKF compared to KF highlights the importance of MSKF convection parameterization for gray zone (~1–4 km) simulation. The persistent discrepancy in landfall location in CP‐on and CP‐off simulation underscored the need of further investigating other physics parameterizations and dynamical mechanisms.

Opposite summer precipitation anomalies over the Maritime Continent in fast and slow decaying El Niño cases

Summer precipitation over the Maritime Continent (MC) is tightly connected to livelihoods of the locals, and the corresponding strong convective activities are also crucial to Asian regional climate variations remotely. Using observational and reanalysis data from 1958 to 2020, we found that El Niño with different decay rates can cause different summer precipitation anomalies in the MC area. In the fast decaying (FD) El Niño summer, the MC area shows a precipitation surplus, while the MC precipitation decreases during the slow decaying (SD) El Niño summer. For the FD El Niño, the equatorial central-eastern Pacific warm sea surface temperature (SST) anomaly quickly turns into cold in the decaying summer, while the SST over MC region sustains a warm anomaly owing to the Indo-western Pacific Ocean capacitor effect. This local warm SST anomaly leads to anomalous ascent and then excessive precipitation, further stimulating low-level easterly wind to east of the MC with increased water vapor transport. For the SD El Niño, the central-eastern Pacific warm SST anomaly sustains till the decaying summer, continuously adjusting the Walker circulation. An anomalous local descent motion associated with the weakened Walker circulation region hampers the MC precipitation in summer. Our findings benefit the local researches of the MC summer climate variations, and potentially favor the regional climate predictions.

The Response of Cloud Precipitation Efficiency to Warming in a Rainfall Corridor Simulated by WRF

Due to model errors caused by local variations in cloud precipitation processes, there are still significant uncertainties in current predictions and simulations of short-duration heavy rainfall. To tackle this problem, the effects of warming on cloud-precipitation efficiency was analyzed utilizing a weather research and forecasting (WRF) model. The analysis focused on a rainstorm corridor event that took place in July 2020. Rainstorm events from 4–6 July formed a narrow rain belt with precipitation exceeded 300 mm in the middle and lower reaches of the Yangtze River. Temperature sensitivity tests revealed that warming intensified the potential temperature gradient between north and south, leading to stronger upward motion on the front. It also strengthened the southwest wind, which resulted in more pronounced precipitation peaks. Warming led to a stronger accumulation and release of convective instability energy. Convective available potential energy (CAPE) and convective inhibition (CIN) both increased correspondingly with the temperature. The precipitation efficiency increased sequentially with 2 °C warming to 27.4%, 31.2%, and 33.1%. Warming can affect the cloud precipitation efficiency by both promoting and suppressing convective activity, which may be one of the reasons for the enhancement of extreme precipitation under global warming. The diagnostic relationship between upward moisture flux and lower atmospheric stability during precipitation evolution was also revealed.

High‐Frequency Gravity Waves and Kelvin‐Helmholtz Billows in the Tropical UTLS, as Seen From Radar Observations of Vertical Wind

AbstractThe present study analyzes novel observations of vertical wind in the tropical upper troposphere‐lower stratosphere obtained from a radar wind profiler in Cochin, India. Between December 2022 and April 2023, 63 consecutive 4 hr curtains of were measured with a vertical spacing of 180 m and a sampling time step of 44 s, thus resolving almost the whole spectrum of vertical motions. Spectra of strongly vary with altitude. They are generally flat up to the local Brunt‐Väisälä frequency (BVF), but sometimes exhibit a peak of variance closer to BVF, a feature which may be attributed to trapped gravity waves. At other times and locations, the profiles reveal Kelvin‐Helmholtz billows. Finally, the variability of variance over the 4 month campaign period is investigated. Using brightness temperature from geostationary satellites as a convective proxy, it is found that variance is highly correlated with fluctuations in convective activity.

The cause of an extreme sea surface warming in the midlatitude western North Pacific during 2012 summer

This study investigated an extreme sea surface warming in the midlatitude western North Pacific (MLWNP) during the summer of 2012. The 2012 extreme event was characterized by warm sea surface temperature anomaly (SSTA) extending from the East/Japan Sea to central North Pacific. The SSTA box–averaged over the MLWNP (130–180°E, 33–50°N) in 2012 ranked as the third warmest in recent four decades, which has caused intense marine heatwaves in this region. During the summer of 2012, a positive Indian Ocean Dipole event co-occurred with El Niño, favoring anomalous moisture transport between the two basins that caused enhanced convection in the South China and Philippine Seas and western–to–central subtropical Pacific. The enhanced convective activities triggered two meridional atmospheric Rossby wave trains to form strong atmospheric blocking high–pressure systems in the MLWNP. This reduced the total cloud cover and surface wind speed, enhancing insolation and reducing the release of latent heat flux. In addition, the weakened wind strengthened the stratification and shoaled the mixed layer. As a result, the increased net heat flux into the ocean accompanied by a shallower mixed layer contributed to the upper ocean warming in the MLWNP. Meanwhile, the North Pacific was dominated by a negative phase of Pacific Decadal Oscillation (PDO), significantly contributing to warm SSTAs in the MLWNP in 2012. Consequently, the 2012 extreme warming in the MLWNP was the results of the combination of atmospheric Rossby waves and PDO. Our study highlighted the roles of high–frequency atmospheric teleconnection and low–frequency PDO in extreme sea surface warming in the MLWNP.

The role of diabatic heating/cooling in outer rainbands in the secondary eyewall formation and evolution in a numerically simulated tropical cyclone

In this study, the role of diabatic heating/cooling in outer rainbands (ORBs) in the formation and evolution of the secondary eyewall of a numerically simulated tropical cyclone (TC) is investigated. This is done through a series of sensitivity experiments under idealized conditions using a high-resolution cloud-resolving atmospheric model. The results show that artificially increasing diabatic heating in rainbands enhances convective activities in ORBs and leads to an earlier secondary eyewall formation (SEF), and later the faster weakening and earlier dissipation of the primary eyewall. Reducing diabatic heating in ORBs weakens the rainbands and delays the SEF but prolongs the duration of the double eyewall structure if the SEF occurs. Reducing diabatic cooling in ORBs enhances convective activity in rainbands but has little effect on convection in the primary eyewall prior to the SEF. However, it results in a widened eyewall structure and a stronger TC after the eyewall replacement. Increasing diabatic cooling in ORBs largely suppresses convection in rainbands and prohibits the SEF. These results demonstrate that diabatic heating/cooling in ORBs plays important roles in the SEF and evolution. Since diabatic heating/cooling in rainbands is sensitive to the near-core environmental relative humidity, our results demonstrate the critical importance of large-scale environmental moist condition to the formation and evolution of secondary eyewall in TCs. In addition, it is also found that when the area-averaged diabatic heating rate in ORBs becomes similar in magnitude to that in the primary eyewall, the secondary eyewall forms. Plain language summaryPrevious studies have demonstrated the importance of diabatic heating/cooling in outer rainbands to the structure and intensity changes of tropical cyclones (TCs) with a single eyewall. It is unclear whether and how diabatic heating/cooling in outer rainbands may affect the formation and evolution of the secondary eyewall in TCs. These issues have been addressed based on a series of sensitivity experiments under idealized conditions using a high-resolution atmospheric model. Results show that diabatic heating in outer rainbands is favorable for the secondary eyewall formation (SEF). Increasing diabatic heating in outer eyewall can lead to faster weakening and thus earlier dissipation of the primary eyewall. Diabatic cooling in outer rainbands suppresses convection in outer rainbands and prohibits the SEF. Since diabatic heating/cooling in outer rainbands is sensitive to the near-core environmental relative humidity, our results demonstrate the importance of the large-scale environmental moist condition to the SEF of TCs. We also found that when the area-averaged diabatic heating rate in outer rainbands becomes similar in magnitude to that in the primary eyewall, the secondary eyewall would form, which can be considered as a measure of the SEF in TCs. Key points1.Increasing diabatic heating in outer rainbands leads to earlier SEF, and faster weakening and earlier dissipation of the primary eyewall.2.Increasing diabatic cooling in outer rainbands suppresses convective activity in outer rainbands and is unfavorable for the SEF.3.When the area-averaged diabatic heating rate in outer rainbands and the eyewall become similar in magnitude, the secondary eyewall forms.

Measurement report: Intra-annual variability of black carbon and brown carbon and their interrelation with meteorological conditions over Gangtok, Sikkim

Abstract. Black carbon (BC) and brown carbon (BrC) both have a versatile nature, and they have an apparent role in climate variability and changes. As anthropogenic activity is surging, BC and BrC are also reportedly increasing. So, the monitoring of BC and BrC and observations of land use land cover change (LULCC) at a regional level are necessary for the changes in various interconnected meteorological phenomena. The current study investigates BC, BrC, CO2, BC from fossil fuels (BCff), BC from biomass burning (BCbb), and LULCC and their relationship to the corresponding meteorological conditions over Gangtok in the Sikkim Himalayan region. The concentration of BC (BrC) was found to be highest during March 2022 (April 2021) at 43.5 µg m−3 (32.0 µg m−3)​​​​​​​. Surface pressure exhibits a significant positive correlation with BC, BCff, BCbb, and BrC. Higher surface pressure results in a calmer and more stable boundary layer, which effectively retains deposited contaminants. Conversely, the wind appears to facilitate the dispersion of pollutants, showing a strong negative correlation. The fact that all pollutants and precipitation have been shown to behave similarly points to moist scavenging of the pollutants. Despite the dense cloud cover, it is clear that the area is not receiving convective precipitation, implying that orographic precipitation is occurring over the region. Most of Sikkim receives convective rain from May to September, indicating that the region has significant convective activity contributed from the Bay of Bengal during the monsoon season. Furthermore, monsoon months have the lowest concentrations of BC, BCbb, BCff, and BrC, suggesting the potential of convective rain (as rainout scavenging) to remove most of the pollutants.

Impacts of Local and Remote SST Warming on Summer Circulation Changes in the Western North Pacific

Abstract This study explores how future SST warming in remote ocean basins may affect the western North Pacific (WNP) wet season climate by applying a high-resolution atmospheric general circulation model to conduct a series of numerical experiments. A marked precipitation and tropical cyclone (TC) activity reduction, as well as enhanced anticyclonic circulation, in the WNP is projected in AMIP experiments forced by SST change in a future warming scenario. The sensitivity experiments reveal that various SST warming phenomena (e.g., in the global SST warming pattern, the tropical ocean belt, the Indian Ocean, the tropical Atlantic, and the subtropical northeast Pacific) and the increase in greenhouse gas concentration could weaken the precipitation, TC activity, and circulation. By contrast, the SST warming in the WNP and eastern equatorial Pacific has opposite and mixed effects, respectively, and tends to weakly offset the dominant influences of remote ocean warming. These results indicate that the WNP, being the epicenter of the global teleconnection of divergent and rotational flow, is susceptible to the influence of the SST warming in remote ocean basins. The remote forcing as projected in future scenarios would overwhelm the enhancing effect of local SST warming and weaken the circulation, convection, and TC activity in the WNP. These findings further the understanding of how the decreased precipitation and enhanced subtropical high in the WNP may be easily triggered by remote SST warming as revealed in the AMIP-type simulations. How this effect would be affected by air–sea coupling needs further investigation.

Comparing MJO Intensity over the Tropical Western Pacific during Mega and Equatorial La Niña Winters

Abstract This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO’s diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast.

Cloud characteristics and their role in the 2024 United Arab Emirates extreme rainfall events

ABSTRACT Intensified by climate change, extreme rainfall events are frequently increasing and have become severe particularly in tropical and subtropical regions. On 16 April 2024, the United Arab Emirates (UAE) experienced a record-breaking flood event, characterized by ~ 250 mm of rain within a 24-hour period and surpassed the previous record of 1949. Later, on 2 May 2024, another rainfall event occurred but with low rainfall (~20 mm in 12 hours). These consecutive incidents raised a need to understand convective mechanism, so present study focus to analyse it using various cloud properties over Dubai. Our findings inferred that the April event was mostly driven by extensive cloud such as cumulonimbus which stretched up to ~ 16 km, with prominent features of high optical thickness (~150), small-sized cloud droplets (~23.89 µm), and considerable liquid water content (~2185 g/m−2). Further, we found high (~400–1800 J kg−1) and comparatively low (~0–700 J kg−1) potential energies, which trigerred convection on April 16 and May 02 respectively. Thus our study gained insights related to occurrence of convective activities at regional level using atmospheric and cloud parameters. These outcomes can be used as an input in climate models to improve weather patterns even in arid environments like Dubai, where cloud seeding is a regular practice.

The impact of the QBO vertical structure on June extreme high temperatures in South Asia

Using observation data and numerical simulations, we have demonstrated that the stratospheric Quasi-Biennial Oscillation (QBO) can predict extreme high temperatures (EHTs) in South Asia in June. The vertical structure of the QBO plays a crucial role in this prediction. When the QBO in June shows easterlies (westerlies) at 50 hPa and westerlies (easterlies) at 70 hPa, more (fewer) EHT events occur. This likely results from the QBO’s vertical structure causing positive (negative) temperature anomalies in the lower stratosphere and negative (positive) static stability anomalies near the tropical tropopause. These anomalies enhance (weaken) convective activity over the equatorial Indian Ocean, leading to anomalous circulation with ascending (descending) air over the equatorial Indian Ocean and descending (ascending) air over northern and central South Asia. This suppresses (promotes) convection over northern and central South Asia, affecting cloud formation and precipitation. Consequently, more (less) solar radiation reaches the region, along with weaker (stronger) evaporative cooling effects, warming (cooling) the surface and creating a background state conducive to (against) EHT events. Additionally, the opposite zonal winds at 30 hPa and 50 hPa in April may serve as a reference factor for predicting the probability of EHT events in northern and central South Asia. This study provides a potential approach for forecasting tropospheric extreme weather events based on stratospheric signals.

Microphysical characteristics of torrential predecessor rain events over the Yangtze River Delta Area and the related tropical cyclones

The precipitation structures and microphysical characteristics of predecessor rain events (PREs) over the Yangtze River Delta area and related tropical cyclones (TCs) from 2014 to 2019 were investigated using Dual-frequency Precipitation radar data from Global Precipitation Measurement (GPM) for drop size distributions (DSDs). Results showed that the total mean rain rate of PREs was larger than that of TCs, primarily due to higher convective and stratiform rain rates, with enhanced fractional coverage of convective rain in PREs. Examination of microphysical characteristics revealed that a greater quantity of small-sized droplets and a smaller quantity of medium- as well as large-sized droplets contributed towards PREs in comparison with TCs. The conclusion still holds when partitioning DSDs based on different precipitation rate categories. Further investigation of DSDs using gamma functions illustrated that precipitation in PREs had lower average mass-weighted diameters (Dm) and enhanced normalized number concentration (Nw) compared with TCs; partitioning precipitation into convective and stratiform components illustrated a larger Dm and lower Nw in TCs than PREs. The analysis of microphysical and thermodynamical processes using the reanalysis data indicates that relatively intense convective activity with drier conditions may be favorable to enhancing raindrop growth through collision-coalescence processes, as a result of larger Dm in TCs than PREs. The empirical relations (Z–R algorithms) applied in different rain regimes (stratiform, convective, and total PREs) revealed significant diversities, relying on weather conditions and geographical locations. Plain language summaryThe Yangtze River Delta area is an important economic belt in China. Under climate change, observed frequent occurrences of weather extremes of heavy rainfall and tropical cyclones (TCs) exert adverse effects on economic development in this region. Thus, a deep understanding of the mechanism of TCs torrential rainfall in the Yangtze River Delta area is urgently necessary. In this study, we investigated the precipitation patterns and microphysical characteristics of predecessor rain events (PREs) in the Yangtze River Delta region, and their association with TCs in the South China Sea-Western North Pacific Ocean (SCS-WNPO) area from 2014 to 2019. We found that PREs had a higher total mean precipitation rate than TCs. Further examination using gamma functions demonstrated that PREs exhibited lower average mass-weighted diameters (Dm) and higher normalized intercept parameters (Nw) than TCs. This pattern persisted when distinguishing between convective and stratiform precipitation components. We believe that our study makes a significant contribution to the literature because these results provide valuable insights into the distinct precipitation characteristics of PREs and TCs in the study region and contribute to a better understanding of tropical cyclone-related rainfall patterns, and act as a scientific basis for disaster mitigation.