Surface Wind Convergence Research Articles

Frontal effects on the rapid formation of a deep layer of marine fog and cloud in the NW Atlantic

AbstractRapid formation of a deep layer of fog and clouds extending to a height of 2 km was observed over Sable Island off Nova Scotia on 13–14 July 2019. This fog and cloud event was dominantly caused by the rapid intrusion of a trough from a cyclone over NE Canada. The trough quickly moved from SW–W to E–NE and encroached on the coastal waters of Nova Scotia. Due to the warm front in the trough, the air temperature increased up to 4 km in height with increased stability, while the sea‐level pressure at Sable Island dropped by 11 hPa within 12 hours. The moderate to strong winds turned counterclockwise from the N to the SE, converging with the SE winds maintained by the ridge located to the east. There was no indication of an advection‐type fog mechanism because the surface air temperature was not adjusted to the colder sea‐surface temperature that could have resulted in saturation of the surface air. The leading portion of a massive cloud band in the trough passing over Sable Island included precipitation before fog formation. The rain that evaporated in the unsaturated and warm subcloud layer effectively reduced the air temperature and significantly increased the wet‐bulb temperature in this layer. The cloud extended into the moist subcloud layer, while fog formed at 1400 UTC 13 July and lasted until 2327 UTC 13 July (with two brief periods of mist lasting less than two hours). The development of this deep layer of fog and cloud in the NW Atlantic was caused by evolving synoptic conditions through the simultaneous effects of a rapid trough intrusion over coastal waters with a massive cloud band, induced precipitation causing cloud deepening into the subcloud layer, and surface wind convergence forcing vertical mixing in a stable marine boundary layer.

Atmospheric Fronts Shaping the (Sub)Mesoscale SST‐Wind Coupling Over the Southern Ocean: Observational Case

AbstractSurface wind divergence is largely modulated by the sea surface temperature (SST) gradient through vertical momentum mixing and pressure adjustment. Here, the two mechanisms affecting the coupling strength between SST gradient and surface wind divergence are examined during an atmospheric front passage in the Southern Ocean. This event is also recorded by an uncrewed surface vehicle (USV). The reanalysis product (ERA5) revealed that downward momentum mixing is the dominant mechanism on the daily time scale. The coupling strength during the day when the atmospheric front passed over declined by 75%, compared to the adjacent days. This implies that the atmospheric front can partially attenuate the SST gradient effect on the surface wind divergence. Furthermore, a decade‐long statistic also showed a decreasing trend of SST‐wind coupling when the atmospheric fronts occur more. Additionally, after removing the mesoscale weather variation, the USV observations showed a remarkable SST imprint on the atmospheric boundary layer in the oceanic submesoscale regime, which denotes the scale below the deformation radius (∼16 km). The submesoscale air‐sea interaction processes also displayed decreased air‐sea coupling strength during atmospheric front passage. This is possible as the vertical velocity induced by the atmospheric front can compensate for the daily averaged uprising vertical velocity due to surface wind convergence. This analysis indicates that the atmospheric front can diminish the coupling between the SST gradient and surface wind divergence, which contrasts the existing statistical results showing that atmospheric fronts tend to enhance such coupling.

The role of the Kuroshio Current on the diurnal cycle of Meiyu-Baiu rainband in mid-June

This study investigated the mechanism of the strong diurnal cycle of precipitation over the Kuroshio Current (KC) in mid-June when the climatological location of the Meiyu-Baiu (MB) front is aligned with the KC. The diurnal cycle of precipitation exhibits double peaks, with the primary peak in the morning (03–12 LST) and the secondary peak in the afternoon (12–15 LST). It was revealed that the morning peak is induced by convective instability associated with the large temperature difference between the sea surface and the surface air (TDiff), whereas the cross-frontal circulation is responsible for the afternoon peak. In the morning, heating from the KC intensifies as the TDiff maximizes. Intensified convective instability due to the enhanced heating from the KC increases the mean amount of convective precipitation (CP) as well as the probability of stronger CP. The surface wind convergence and the confluence sustaining the convection intensify the frontogenesis, which leads to the maximization of the frontal intensity in the afternoon. The afternoon peak of precipitation is in accordance with the intensified MB front and the associated cross-frontal circulation. However, direct contribution of diabatic heating to the frontogenesis is relatively weak. This study provides a physical understanding of the intensified diurnal cycle of MB rainfall in terms of the interaction between oceanic and atmospheric fronts, the KC and MB, respectively.

A Mesoscale Ocean–Atmosphere Coupled Pathway for Decadal Variability of the Kuroshio Extension System

Abstract The Kuroshio Extension (KE) system has been observed to experience a decadal cycle between dynamically stable and unstable states. However, divergent conclusions on its interaction with the atmosphere obfuscate the understanding of its oscillatory nature at the preferred decadal time scale. Here, using satellite observations and the ERA-Interim reanalysis in 2002–16, physical process-oriented diagnoses suggest that the wintertime finer-scale thermodynamic response to mesoscale oceanic surface conditions and slow oceanic Rossby wave adjustment frame a coupled ocean–atmosphere delayed oscillator for the decadal KE variability. During the stable state of the KE system, the downstream KE transition region is rich in mesoscale oceanic warming associated with warm eddies, which induces surface wind convergence and upward motion, probably via the enhanced turbulent mixing. Meanwhile, increased finer-scale diabatic heating in the lower troposphere with abundant moisture supply from warmer water likely facilitates the deep-reaching updraft that adiabatically cools the middle troposphere. The background northwesterly wind helps to spread out the cooling, leading to southward deflection of local atmospheric eddy available potential energy (EAPE) production by baroclinic conversion. Consequently, the synoptic eddy activity displaces southward across the basin with additional energy supply from the increased diabatic production of EAPE downstream. Anomalous synoptic eddy thermal and vorticity forcing eventually fosters the basin-scale equivalent-barotropic cyclonic circulation anomaly, which is further maintained by energy conversion from the background state. The resultant wind-driven negative sea surface height anomalies propagate westward into the upstream KE region with a delay of ∼4 years and can trigger the unstable state of the KE system.

Surface Wind Responses to Mesoscale Sea Surface Temperature over Western Boundary Current Regions Assessed by Spectral Transfer Functions

Abstract Satellite observations have revealed that mesoscale sea surface temperature (SST) perturbations can exert distinct influence on sea surface wind by modifying the overlying atmospheric boundary layer. Recently, spectral transfer functions have been shown to be useful to elucidate the wind response features. Spectral transfer functions can represent spatially lagged responses, their horizontal scale dependence, and background wind speed dependence. By adopting the transfer function analysis, the present study explores seasonality and regional differences in the wind response over the major western boundary current regions. Transfer functions estimated from satellite observations are found to be largely consistent among seasons and regions, suggesting that the underlying dominant dynamics are ubiquitous. Nevertheless, the wind response exhibits statistically significant seasonal and regional differences depending on background wind speed. When background wind is stronger (weaker) than 8.5 m s−1, the wind response is stronger (weaker) in winter than in summer. The Agulhas Retroflection region exhibits stronger wind response typically by 30% than the Gulf Stream and Kuroshio Extension regions. Although observed wind distributions are reasonably reconstructed from the transfer functions and observed SST, surface wind convergence zones along the Gulf Stream and Kuroshio Extension are underrepresented. The state-of-the-art atmospheric reanalysis and regional model represent well the structure of the transfer functions in the wavenumber space. The amplitude is, however, underestimated by typically 30%. The transfer function analysis can be adapted to many other atmospheric responses besides sea surface wind, and thus provide new insights into the climatic role of the mesoscale air–sea coupling.

Surface Wind Convergence During Localized Heavy Rainfall in Tokyo in Summer: Evaluation and Utilization of the Amount of Divergence Using High-Density Observation Data

Surface Wind Convergence During Localized Heavy Rainfall in Tokyo in Summer: Evaluation and Utilization of the Amount of Divergence Using High-Density Observation Data

Low-Level Atmospheric Responses to the Sea Surface Temperature Fronts in the Chukchi and Bering Seas

Atmospheric responses to ocean surface temperature (ST) fronts related to western boundary currents have been extensively analyzed over the last two decades. However, the organized near-surface response to ST, which is defined as the temperature of open water and sea ice, excluding land surface, at higher latitudes where sea ice exists has been rarely investigated due to the difficulties of observations. Here, 32 years of high-resolution atmospheric reanalysis data are analyzed to determine the atmospheric responses to ST fronts in the Bering Sea and Chukchi Sea. In the Chukchi Sea, the convergence of 10-m-high wind increases in October and November, when the horizontal gradient and Laplacian of ST become noticeable. On the other hand, an ST contrast between the continental shelf and the southwestern deep basin develops in winter in the Bering Sea. In both seas, the spatial distribution of surface wind convergence and the Laplacians of ST and sea level pressure agree well with each other, demonstrating the pressure adjustment mechanism. The vertical mixing mechanism is also confirmed in both seas. Ascending motion and diabatic heating develop over the Chukchi Sea in late autumn, but are confined to the lower troposphere. Turbulent heat fluxes at the surface become especially large in this season, resulting in an increase of diabatic heating and low-level clouds. Low-level clouds and downward shortwave radiation exhibit contrasting behavior across the shelf break in the Bering Sea that corresponds to the ST distribution, which is regulated by the bottom topography.

Physical processes controlling the diurnal cycle of convective storms in the Western Ghats

Diurnal variation of convective storms (CSs) during monsoon season and associated physical mechanisms are significantly important for accurate forecast of short-time and extreme precipitation. The diurnal cycle of CSs is investigated using ground-based X-band radar, Tropical Rainfall Measuring Mission Precipitation Radar, and reanalysis data during the summer monsoon (June–September of 2014) over complex mountain terrain of Western Ghats, India. Diurnally, CSs show a bimodal distribution in the coastal areas, but this bimodality became weak along the upslope regions and on the mountain top. The first occurrence mode of CSs is in the afternoon–evening hours, while the second peak is in the early-morning hours. The diurnal cycle’s intensity varies with location, such that it reaches maximum in the afternoon–evening hours and early morning on the mountain top and coastal areas, respectively. Two possible mechanisms are proposed for the observed diurnal variation in CSs (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between land-sea breeze, local topography and large-scale monsoon winds. It is also observed that the CSs developed on the mountain top during afternoon–evening hours are deeper than those along the coast. The higher moisture in the lower- and mid-troposphere, higher instability and strong upward motion facilitate deeper CSs during afternoon–evening hours.

Summer afternoon precipitation associated with wind convergence near the Himalayan glacier fronts

Little is known about the effects of glacier-air interactions on the Himalayan glacier mass balance. Until this knowledge gap is filled, a reliable projection of the future changes in the Himalayan glaciers is hardly possible. Here, we describe the drying effect of the katabatic winds on the up-valley summer monsoon flows by creating favorable conditions for local convergence-induced precipitation to occur near the glacier fronts. We postulate that this retarding effect on the up-valley monsoon flows results in a negative feedback mechanism mediated by glacier-air interactions, in which glacial retreat pushes precipitation upwards as the down-valley katabatic winds weaken, resulting in greater local precipitation and enhanced snow accumulation across the upper parts of the Himalayan glaciers. Our analyses are based on the exclusive data recorded in the Khumbu valley and the Langtang valley in the Nepalese Himalayas. These data revealed higher afternoon precipitation in summer associated with surface wind convergence near the glacier fronts and a sharp decrease in the temperature lapse rate over the glacier surfaces. The principle of the observed phenomena was proven by our high-resolution modeling sensitive experiment, which involved two simulations, one with the present glaciers and the other without. This numerical experiment also supports the proposed negative feedback. Furthermore, we report a low deuterium excess near the glacier fronts, indicating below-cloud re-evaporation facilitated by the local convergence induced by the dry katabatic winds. Our study suggests that current models may overestimate the retreat of Himalayan glaciers because they have completely ignored the glacier-air interactions.

Spatial and temporal variability of deep convective clouds over the tropics using multi-year Megha - Tropiques – Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie (SAPHIR) observations

ABSTRACT Multi-year seasonal mean diurnal variation of the occurrence frequency of Deep Convective Clouds (DCCs) over the tropical belt for various climatic regions including land and ocean are studied, using Level-1 brightness temperature () data from Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie (SAPHIR) payload, aboard the Megha-Tropiques (MT) satellite for the period 2011 to 2018. The low inclination orbit of MT which provides the measurements at different local times makes the study of the diurnal cycle of DCCs possible from a single platform over the entire tropics. The spatial and temporal distribution of DCCs are derived for different seasons including measurements at all possible local time (LT) unlike measurements from a payload aboard polar-orbiting satellites. The seasonal variation of DCCs is in concurrence with the position of sun with maximum occurrence frequency in the summer months of the respective hemisphere. The largest occurrence of DCCs are found over the west Pacific and the northeast Bay of Bengal with a maximum occurrence frequency of around 4.0% during Northern Hemispheric summer. Central African landmasses, the South Pacific Convergence Zone, and the Amazon region also show a higher occurrence during their summer seasons. From the occurrence frequency maps, different regions are identified to estimate the seasonal mean diurnal variation of those regions. Over the continental regions, convective activity peaks during 15:00 to 18:00 LT and over the open ocean it peaks during 00:00 to 06:00 LT. Similarly, the convective activity is minimum over the oceanic region during 12:00 to 18:00 LT and over the land during 06:00 to 12:00 LT. The specific features associated with the distribution of DCC such as double Inter-Tropical Convergence Zone (ITCZ), the mean zonal diurnal variation of DCCs, the effect of El-Niño, the role of surface temperature and surface wind convergence on the occurrence of DCC are also presented.

A foehn-induced haze front in Beijing: observations and implications

Abstract. Despite frequent foehns in the Beijing–Tianjin–Hebei (BTH) region, there are only a few studies of their effects on air pollution in this region, or elsewhere. Here, we discuss a foehn-induced haze front (HF) event using observational data to document its structure and evolution. Using a dense network of comprehensive measurements in the BTH region, our analyses indicate that the foehn played an important role in the formation of the HF with significant impacts on air pollution. Northerly warm–dry foehn winds, with low particulate concentration in the northern area, collided with a cold–wet polluted air mass to the south and formed an HF in the urban area. The HF, which is associated with a surface wind convergence line and distinct contrasts of temperature, humidity and pollutant concentrations, resulted in an explosive growth of particulate concentration. As the plain–mountain wind circulation was overpowered by the foehn, a weak pressure gradient due to the different air densities between air masses was the main factor forcing advances of the polluted air mass into the clean air mass, resulting in severe air pollution over the main urban areas. Our results show that the foehn can affect air pollution through two effects: direct wind transport of air pollutants, and altering the air mass properties to inhibit boundary layer growth and thus indirectly aggravating air pollution. This study highlights the need to further investigate the foehn and its impacts on air pollution in the BTH region.

Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Gulf Stream and Agulhas Return Current in Winter

ABSTRACTHigh-resolution satellite observations and numerical experiments have revealed local enhancement of time-mean surface wind convergence along the axes of warm western boundary currents and divergence slightly poleward. A recent study has suggested that frequent occurrence of persistent atmospheric fronts and sea level pressure (SLP) troughs along a sea surface temperature (SST) front are responsible for shaping the frontal-scale wind convergence and divergence contrast as seen in the wintertime climatology near the Kuroshio Extension (KE). These events tend to induce surface wind convergence with moderate magnitude. Through atmospheric reanalysis with high-resolution SST, the present study reveals that, as in the vicinity of the KE, surface wind convergence with moderate magnitude and divergence with moderate-to-extreme magnitude are found to play a primary role in shaping the climatological-mean wind convergence–divergence contrasts across the SST fronts near the Gulf Stream (GS) and Agulhas Return Current (ARC) in winter. In contrast, strong-to-extreme convergence events associated with synoptic-scale atmospheric disturbances are found to yield horizontally uniform time-mean wind convergence. Furthermore, cluster analysis and case studies suggest that persistent atmospheric fronts and SLP troughs are responsible for inducing moderate wind convergence also near the GS and ARC. Thus, these features are consistent with their counterpart near the KE, but the impacts of the ARC tend to be substantially weaker, probably due to its cooler SST among other potential factors.

Transition of Low Clouds in the East China Sea and Kuroshio Region in Winter: A Regional Atmospheric Model Study

AbstractBias in simulating the stratocumulus‐to‐cumulus transition remains a main source of uncertainties in climate projection and can significantly affect the energy budget in climate models. To gain insights into the transition, this study investigates the cloud transition forced by the sea surface temperature (SST) front and synoptic disturbances in the East China Sea and Kuroshio region in winter based on both observations and regional atmospheric model simulations. The Kuroshio SST front greatly accelerates cloud transition by enhancing surface latent heat flux, marine atmospheric boundary layer (MABL) dynamical adjustment, and cloud top entrainment. With the sharp SST increase from the cold flank to the Kuroshio SST warm tongue (KWT), surface wind convergence (SWC) over the KWT induced by the SST front and synoptic disturbances enhances the coupling between the cloud layer and subcloud layer. An underlying positive feedback between the SWC and latent heating in the cloud layer can enhance abrupt change in cloud properties and maintain cloud band over the KWT against the decoupling through the so‐called “Deepening‐Warming” mechanism induced by latent heating. From the KWT downwind southward, the surface layer turbulent mixing weakens, while latent heating in the cloud layer and cloud top longwave radiative cooling enhance buoyancy and vertical mixing in the cloud layer. This difference in vertical mixing between the cloud layer and subcloud layer facilitates the MABL decoupling and impedes upward moisture transport. Meanwhile, decreasing lower tropospheric stability is conducive to the entrainment of drier and warmer air from above into the cloud layer, strengthening cloud evaporation.

Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

Impact of Lake Surface Temperature Variations on Lake Effect Snow Over the Great Lakes Region

AbstractA high‐resolution three‐dimensional Weather Research and Forecasting (WRF) model is used to investigate the coupled impact of lake surface temperature (LST) and surface wind on the lake effect snow (LES) over the Great Lakes region. A set of twin WRF simulations, with and without resolving LST spatial variations in the model's surface boundary condition, is performed to quantify the impact of LST variation on LES. Both observations and model results reveal a positive correlation between the downwind LST gradient and surface wind convergence over the Great Lakes region. Furthermore, model simulations show that resolving the spatial variation of LST increases the surface wind convergence, correspondingly enhances local vertical motions in the atmospheric boundary layer, and creates favorable conditions for the LES formation on the lee sides of the Great Lakes. The contribution of LST spatial variations to the increase in precipitation on the lee sides of the lakes varies between 5% and 30% in individual LES events. The increase in the winter‐mean snow water equivalent due to LST spatial variations is between 3% and 15%. The most significant impact of LST variation on the winter‐mean snow water equivalent is on the lee side of Lake Huron.

Relationship between the Interannual Variations of Summer Convective Afternoon Rainfall Activity in Taiwan and SSTA(Ni\xf1o3.4) during 1961\u20132012: Characteristics and Mechanisms

This study examines the interannual variation of the Convective Afternoon Rainfall (CAR) activity (including frequency and intensity) in Taiwan during the summers (JJA) of 1961–2012 with a focus on identifying its relationship with the changes in sea surface temperature anomalies over the Niño3.4 region [SSTA(Niño3.4)] and the underlying physical mechanisms. Our analyses show that during the colder (warmer) phase of SSTA(Niño3.4), the subtropical high system over the region east of Taiwan is enhanced (weakened), the local surface wind convergence is enhanced (weakened), and the local thermal instability is enhanced (weakened), which facilitates (suppresses) the formation of CAR in Taiwan. This consistent negative relationship between the interannual variation of CAR frequency in Taiwan and SSTA(Niño3.4) occurs throughout 1961–2012. In contrast, the relationship between the interannual variation of CAR intensity in Taiwan and SSTA(Niño3.4) changed from positively correlated to negatively correlated in the late 1980s. This change is attributed to the change in the moisture supply for maintaining the CAR intensity in Taiwan from an increase (decrease) in the warmer (colder) phase of SSTA(Niño3.4) before the mid-1980s to the opposite after the late 1980s. These findings highlight how the rainfall characteristics in East Asia may change in response to changes in SSTA(Niño3.4).

Atmospheric energetics over the tropical Indian Ocean during Indian Ocean dipole events

The evolution of atmospheric perturbation potential energy (PPE) over the tropical Indian Ocean is analyzed during a composite positive IOD event using reanalysis datasets for the period 1948–2015. The IOD modulates the variation in PPE, which then affects perturbation kinetic energy (PKE) through energy conversion. The PPE anomalies in the lower troposphere (1000–850 hPa) as the dominant layer of the PPE in the whole troposphere (1000–150 hPa) present a dipole pattern corresponding to the anomalous variation in SST during the IOD event. When cold SST anomalies (SSTAs) first appear in the eastern Indian Ocean (IOD-E), they reduce the atmospheric PPE in the lower troposphere rapidly. The negative PPE anomalies lead to less energy conversion to PKE, restraining the surface wind convergence over the IOD-E and weakening the climatological Walker circulation. Meanwhile, the surface easterly wind anomalies strengthen, which depresses the thermocline to the west and gives warmer SSTAs in the western Indian Ocean (IOD-W). The PPE anomalies and energy conversion (CK) over the IOD-W are opposite to those over the IOD-E, the anomalous easterly wind continues to develop, and the positive SSTAs in the IOD-W reach a peak. Thus, the response of the Walker circulation over the Indian Ocean provides a positive feedback during the IOD event and explains the delayed effect of IOD-E SSTAs on IOD-W SSTAs.

A New Framework for Near‐Surface Wind Convergence Over the Kuroshio Extension and Gulf Stream in Wintertime: The Role of Atmospheric Fronts

AbstractIt is well known that the wintertime time‐mean surface wind convergence patterns over the Kuroshio Extension and Gulf Stream show significant imprints of the underlying oceanic fronts. Previous studies have suggested that this collocation results from a time‐mean response to sea level pressure forcing from sea surface temperature gradients. However, more recent work has illustrated this phenomenon is heavily influenced by extratropical cyclones, although exact mechanisms are still debated. The purpose of this study is to introduce a new framework that explicitly distinguishes between two separate components in their contribution to the time‐mean surface wind convergence, that associated with and without atmospheric fronts. It is then argued that this distinction can help better explain the mechanisms driving the Kuroshio Extension and Gulf Stream influence on the atmosphere.

Role of SST Patterns on Surface Wind Modulation of a Heavy Midlatitude Precipitation Event

AbstractThe mechanisms controlling the influence of the sea surface temperature (SST) structure on the surface winds are studied by means of numerical simulations run with a nonhydrostatic fully compressible state‐of‐the‐art numerical model in a realistic midlatitudes setup, leading to the 9 October 2014 Genoa heavy rainfall event. Starting from a simulation with high‐resolution submesoscale eddy‐permitting SST field, the surface temperature boundary conditions are changed to enhance, reduce, or smooth the SST gradient. It is found that the marine atmospheric boundary layer responds to the submesoscale SST forcing structures over time scales of the order of hours. In particular, through the downward momentum mixing mechanism, the presence of SST horizontal gradients impacts the spatial structure of the surface wind convergence, which can displace the convective heavy rain bands that develop over the sea.

Characteristics and synoptic environment of torrential rain in the warm sector over South China: a composite study

The characteristics and synoptic environment of an extreme rainfall event (maximum 24-h accumulated rainfall, 464 mm) in the warm sector over South China on 20 May 2016 are investigated. This warm sector torrential rain (WSTR) was occurring in the warm zone over the trumpet-shaped topography of Xinyi in the west of Guangdong Province. Large-scale analysis shows that the WSTR was continuously influenced by strong warm horizontal temperature advection from the Beibu Gulf and South China Sea accompanying with a cold front in the northwest. Water vapor fluxes in the lower troposphere reached 500 kg m−1 s−1 and total column water vapor exceeded 20 g kg−1 in the early stage of the WSTR. The topographic analysis showed that the WSTR was caused by the mesoscale convective boundary between the down-valley winds and the outbreak of the south winds from the south plain regions. The convections were strengthened and maintained due to the new generations of the convections by cold outflows and the south winds, as well as blocking effects caused by the trumpet-shaped topography over Xinyi. The strong convections were alleviated due to the weakening of south wind from the plain regions. Climatology statistical analysis confirms that the torrential rains are mainly concentrated during the night and early morning at 1200–2100 UTC and reach the peak at 1700–1800 UTC with a consistent high frequency of the surface wind convergence over Xinyi.