Cold Air Intrusion Research Articles

Comparative analysis of multiple observation data during two typhoons affecting Hainan Island

Based on different observation data, the wind and rain processes of two typhoons that continuously affected Hainan Island in October 2021 were compared and analyzed. The analysis shows that the two typhoons are similar in weather conditions, water vapor transport conditions, cold air intrusion, sea surface temperature providing energy, and so on. The spatial distribution of rainfall and wind field exhibits an evidently asymmetrical structure. Before the typhoon landed, the gale area was located mainly in the northeast of the typhoon center. Furthermore, different detection data have advantages and disadvantages, which should be verified and used each other. For example, there are also differences between radar echo reflectivity, wind profile radar data and MTCSWA data. By means of analysis and research, the ability to analyze and articulate the typhoon impact process with high spatial-temporal resolution data is further enhanced, thereby aiding in the forecast and service work during the typhoon period.

Green Campus Transformation in Smart City Development: A Study on Low-Carbon and Energy-Saving Design for the Renovation of School Buildings

In the context of increasingly deteriorating global ecological conditions and rising carbon emissions from buildings, campus architecture, as the primary environment for youth learning and living, plays a crucial role in low-carbon energy-efficient design, and green environments. This paper takes the case of Yezhai Middle School in Qianshan, Anhui Province, to explore wind environment optimization and facade energy-saving strategies for mountainous campus buildings under existing building stock renovation. In the context of smart city development, integrating advanced technologies and sustainable practices into public infrastructure has become a key objective. Through wind environment simulations and facade energy retrofitting, this study reveals nonlinear increases in wind speed with building height and significant effects of ground roughness on wind speed variations. Adopting EPS panels and insulation layers in facade energy retrofitting reduces energy consumption for winter heating and summer cooling. The renovated facade effectively prevents cold air intrusion and reduces external heat gain, achieving approximately 24% energy savings. This research provides a scientific basis and practical experience for low-carbon energy retrofitting of other campus and public buildings, advancing the construction industry towards green and low-carbon development goals within the framework of smart city initiatives.

Combined Impacts of Autumn Snow Cover on the Tibetan Plateau and Northeast Asia on the Winter Eurasian Temperature

ABSTRACTThis study explores the combined effects of autumn snow cover anomalies on the Tibetan Plateau (TP) and Northeast Asia (NEA) on winter Eurasian temperature using observational data for the period 1972–2021 and a linear baroclinic model (LBM). Distinctive wintertime temperature patterns are found across the Eurasian continent corresponding to increased autumn snow cover in NEA when TP experiences normal, above‐normal, or below‐normal snow cover condition. In the scenario with an anomalous increase in autumn snow cover in NEA in combination with normal snow cover condition in TP, the overall winter temperature anomalies tend to be generally weak across the Eurasian continent. In years when autumn TP snow cover is above normal, the spatial distribution of winter temperature anomalies over the Eurasian continent associated with more NEA snow cover exhibits a ‘cold Arctic‐warm Eurasia’ (CAWE) pattern. The emergence of this CAWE pattern can be attributed to the low‐pressure system induced by the intensified NEA snow cover, which is further reinforced by the atmospheric wave train generated by negative North Atlantic sea surface temperature anomalies (SSTAs) in winter. This low‐pressure system amplifies the polar vortex and causes cooling in polar regions. Simultaneously, southeasterly winds along the southwestern flank of the North Pacific high‐pressure system contribute to warming in the mid‐latitude regions of Eurasia. While in years when autumn snow cover in TP is less than normal, more snow cover over NEA is accompanied by a ‘warm Arctic‐cold Eurasia’ (WACE) temperature anomaly pattern prevalent during the winter season. The decrease in autumn Barents‐Kara Sea ice is accompanied by a circulation pattern akin to the negative phase of the Arctic Oscillation during winter, favouring the southward intrusion of cold air, thus contributing to this WACE temperature anomaly pattern. Further analysis reveals that the impact of snow cover on the WACE temperature pattern is, for the most part, independent of the Arctic sea ice.

Deep Penetrating Cooling in the Black Sea As a Response to Cold Air Intrusions in Winter

Deep Penetrating Cooling in the Black Sea As a Response to Cold Air Intrusions in Winter

Differentiated influences of anomalous subtropical high on extreme persistent precipitation and heatwave events in the Yangtze River Valley

AbstractIn the summers of 2020 and 2022, the Western Pacific Subtropical High (WPSH) intensified extremely and extended westward. However, in summer 2020, the Yangtze River Valley (YRV) witnessed record‐breaking floods, while in 2022, an unprecedented and prolonged heatwave occurred. Distinctly, these two extreme events were caused by different effects of the WPSH: one is enhancement of the transportation of water vapor and the other is adiabatic heating caused by the descending airflow. In June–July 2020, the stable extension of the WPSH ridge line to South China directed a southwesterly airflow along its northwest flank, leading to sustained precipitation in the YRV. Additionally, the midlatitude circulation pattern featured two troughs and two ridges. Such a circulation configuration, combined with the strong and westward WPSH, enabled the continuous southward intrusion of cold air and northward transport of warm moist air, converging over the YRV, and thus influenced extreme persistent precipitation. In contrast, the WPSH covered the YRV almost entirely during summer 2022. Under this influence, the clear‐sky condition and descending airflow through adiabatic warming directly resulted in the heatwave. In addition, local land–atmosphere feedback was crucial in its development and persistence. The soil moisture deficit induced by high temperatures increased the sensible heat flux between the soil and atmosphere upward, further enhanced the surface air temperature and strengthened the heat dry condition.

Comparative analysis of heavy rainfall area between landfalling typhoon LUPIT (2109) and typhoon LISA (9610)

Based on the ERA5 reanalysis data and the surface observations from automatic weather stations, a comparative analysis has been conducted to investigate the differences in heavy rainfall distributions caused by two landfalling tropical cyclones (TCs): LUPIT (2109) and LISA (9610). The two TCs have similar tracks, intensity and landing points, but show different asymmetric features in their rainstorm location relative to their tracks. The results indicate that the TC rainfall differences are mainly caused by different rainstorm formation mechanisms. The wind shear contributes most to the rainstorm of LISA, while land-sea contrast and topographical effect are the main factors of LUPIT rainstorm. Under the influence of strong environmental vertical wind shear and the weak cold air invasion from the west, the circulation center of LISA tilts westward with height, which cooperates with the low-level water vapor convergence and vertical ascending movement on the western side of the TC center to jointly cause the heavy rainstorm to the west of LISA center. In contrast, LUPIT has weak environmental vertical wind shear and no obvious structure tilting with height. Topographic effect plays a crucial role in causing the heavy rainstorm on the north of TC center. The southeasterly jet is blocked by the Taimu Mountain in the northeastern Fujian Province, and the strong ascending motion caused by the terrain-induced convergence appears to the north of LUPIT center. In addition, the moisture convergence is more pronounced in the north and weaker in the south. The intrusion of weak cold air from the east to the coastal areas of central-northern Fujian, and the moisture-convergence distribution, jointly cause the heavy rainstorm to the north of LUPIT.

The passive optimization mechanism of winter thermal performance in commercial complex based on coupled multi-spatial parameters

In cold regions, public buildings often suffer from suboptimal thermal performance due to cold air intrusion. This study explores the synergistic impact of multiple spatial parameters on winter thermal environments to discover the passive optimization potential in commercial complexes. Airflow analysis covers the entire connected space from the entrance to the large atrium, integrating the collective effects of all critical spatial parameters. Based on 4500 Computational Fluid Dynamics (CFD) simulation, the study establishes how these parameters influence wind resistance and thermal comfort. Key findings reveal that the “Atrium-plan Ratio,” “Transitional Space-width,” and “Atrium-height,” are most impactful on wind resistance, contributing over 53 % collectively. For thermal comfort, “Atrium-volume,” “Atrium-section Form,” and “Atrium-height,” are most significant, with a combined weight exceeding 57 %. As wind speed increases, the effect of single parameters gradually diminishes, highlighting the synergistic effect of spatial combinations. A case study demonstrates an 80.5 % improvement in thermal performance score following optimization, underscoring the potential for low-carbon, high-efficiency design in commercial complexes.

Analysis of a Rainstorm Process in Nanjing Based on Multi-Source Observational Data and Lagrangian Method

Using multi-source observation data including automatic stations, radar, satellite, new detection equipment, and the Fifth Generation European Centre for Medium-Range Weather Forecasts Reanalysis (ERA-5) data, along with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform, an analysis was conducted on a rainstorm process that occurred in Nanjing on 15 June 2020, with the aim of providing reference for future urban flood control planning and heavy rainfall forecasting and early warning. The results showed that this rainstorm process was generated under the background of an eastward-moving northeast cold vortex and a southward retreat of the Western Pacific Subtropical High. Intense precipitation occurred near the region of large top brightness temperature (TBB) gradient values or the center of low TBB values on the northern side of the convective cloud cluster. During the heavy precipitation period, the differential propagation phase shift rate (KDP), differential reflectivity factor (ZDR), and zero-lag correlation coefficient (ρHV) detected by the S-band dual-polarization radar all increased significantly. The vertical structure of the wind field detected by the wind profile radar provided a good indication of changes in precipitation intensity, showing a strong correspondence between the timing of maximum precipitation and the intrusion of upper-level cold air. The abrupt increase in the integrated liquid water content observed by the microwave radiometer can serve as an important indicator of the onset of stronger precipitation. During the Meiyu season in Nanjing, convective precipitation was mainly composed of small to medium raindrops with diameters less than 3 mm, with falling velocities of raindrops mainly clustering between 2 and 6 m·s−1. The rainstorm process featured four water vapor transport channels: the mid-latitude westerly channel, the Indian Ocean channel, the South China Sea channel, and the Pacific Ocean channel. During heavy rainfall, the Pacific Ocean water vapor channel was the main channel at the middle and lower levels, while the South China Sea water vapor channel was the main channel at the upper level, both accounting for a trajectory proportion of 34.2%.

Top-Down Phenomenon Observed in the Outer Rainbands of Typhoon Maysak (2020): Insights from the Comparative Analysis of Cloud and Precipitation Microphysics with a Typical Precipitation Event in Northeast China

Abstract Cloud and precipitation microphysics in tropical cyclones (TCs) moving toward Northeast China may exhibit significant distinctions compared with the typical precipitation of this region. In this study, observations from ground-based particle size velocity (Parsivel) disdrometers in Liaoning Province, China, and cloud property data from the Himawari geostationary satellite were utilized to analyze the raindrop size distribution (DSD) characteristics and cloud vertical evolution associated with the outer rainbands of Typhoon Maysak (2020). A comparative analysis was conducted with a typical precipitation event in Northeast China induced by a cold vortex (cold-core low). Our findings reveal distinctive DSD characteristics related to the TC, where medium-sized raindrops dominate, with a smaller diameter but higher concentration in the TC case compared to the typical cold-vortex-induced precipitation case in Northeast China. Convective precipitation falls between maritime-like and continental-like patterns, leaning more toward continental convection. This varies significantly with TCs in Southeast China but is similar to that observed in coastal-front-like rainbands, suggesting extratropical influence. A detailed analysis of the vertical profile of cloud droplets shows a unique “top-down” phenomenon during the extratropical transition process of the TC, where the development of lower-level clouds follows that of upper-level clouds, inconsistent with previous studies and the case for comparison. Further investigation indicates the significant role of the intrusion of dry and cold air from upper levels and the presence of high humidity in low levels in driving this phenomenon. Our results will provide novel insights into cloud and precipitation microphysics associated with TCs in midlatitude regions.

Analysis of the Mesoscale Asymmetric Rainbands during the Slow Northward-Moving Period of Typhoon In-Fa (2021)

After making landfall, Typhoon In-Fa (2021) moved slowly, resulting in heavy rainfall and flooding across fourteen provinces in China. This extreme precipitation was primarily linked to the evolution of active mesoscale convective systems. This study analyzes the characteristics and causes of mesoscale rainbands during In-Fa’s slow northward-moving period, aiming to identify the key factors that influence the detailed evolution of typhoon rainbands and to enhance typhoon quantitative precipitation forecasting skill. In-Fa’s mesoscale asymmetric rainbands can be categorized into three types: mesoscale spiral rainbands, a convective rainband to the east of In-Fa, and a rainband to the north of In-Fa. Mesoscale low-level jets are a critical factor in the development of mesoscale spiral rainbands. The wind speed gradient near these jets, along with the convergence of wind directions between two jets, fosters low-level convergence and upward motion, triggering the evolution of several mesoscale rainbands. The convective rainband to the east of In-Fa flourishes under conditions of high humidity and energy, displaying distinct diurnal variations. This is due to the strengthening of low-level jets at night, which enhances both dynamic convergence and water vapor availability. The presence of moderate to strong convective available potential energy (600–1500 J kg−1), substantial whole-layer water vapor (relative humidity exceeding 90–95%), and a high 0 °C-layer favors the development of efficient warm-cloud convective precipitation, leading to intense hourly rainfall. The rainband to the north of In-Fa is primarily associated with cold air intrusion in the lower troposphere, although the interaction between typhoon and mid-latitude systems has not yet occurred. At the interface between cold and warm air, the colder air to the north side sinks while the warmer air to the south side rises, forming a secondary circulation that supports the development and persistence of precipitation on the north side of the typhoon. These findings offer a conceptual model for accurately predicting precipitation associated with typhoons that move slowly northward after landfall.

Latent Heat Flux Trend and Its Seasonal Dependence over the East China Sea Kuroshio Region

Investigating latent heat flux (LHF) variations in the western boundary current region is crucial for understanding air–sea interactions. In this study, we examine the LHF trend in the East China Sea Kuroshio Region (ECSKR) from 1959 to 2021 using atmospheric and oceanic reanalysis datasets and find that the LHF has a significant strengthening trend. This strengthening can be attributed to sea surface warming resulting from the advection of sea surface temperatures. More importantly, the LHF trend has an apparent seasonal dependence: the most substantial increasing trend in LHF is observed in spring, while the trends are weak in other seasons. Further analysis illustrates that the anomaly of air–sea humidity difference plays a pivotal role in controlling the seasonal variations in LHF trends. Specifically, as a result of the different responses of the East Asian Trough to global warming across different seasons, during spring, the East Asian Trough significantly deepens, resulting in northerly winds that facilitate the intrusion of dry and cold air into the ECSKR region. This intensifies the humidity difference between the sea and air, promoting the release of oceanic latent heat. These findings can contribute to a better understanding of the surface heat budget balance within western boundary currents.

Observation of Ice Pellets and its Association with Meteorological Conditions in the Yeongdong Region of Korea

The microphysical properties of ice pellets (IP) are analyzed, and associated relevant thermodynamic conditions are investigated using rawinsonde soundings and model reanalysis data in the Yeongdong region of Korea. During the intensive observation campaign of snowfall, two distinctive IP events of 1 March 2021 (IP1) and 15 March 2018 (IP2) were observed when strong cold advection was prevalent below about 2 km as accompanied by distinctive inversion strength (4.7 ~ 9.3 ℃) above the cold layers. Cold air intrusion along the eastern side of Taebaek mountains appeared to abruptly decrease low level (850 hPa) temperature up to -4.7 ~ -3.4 ℃, but warmer than 8-year average (-9.5 ℃), respectively. Both episodes had smaller maximum size (1.8 mm in average) of ice pellets with greater fallspeed (4.2 m s−1) in comparison to general snow crystals. Ice pellets occurred in the synoptic condition of the High in the north and the Low passing by the south, which resulted in cold northeasterly over the Yeongdong region. Rawinsonde soundings show a melting layer between 800 and 700 hPa just above the freezing layer of 900 ~ 800 hPa existed, such as a reversed S temperature profile, which is also consistent with the model reanalysis. The IPs’ life time was short within a couple of hours since it occurred along with low-level strong cold advection (IP1) or rapidly-moving squall line (IP2).

Interdecadal changes in the frequency of winter extreme cold events in North China during 1989–2021

How extreme weather and climate events change is an intriguing issue under global warming. By investigating the frequency of extreme cold events (Frexces) in winter over North China (NC), this paper presents robust interdecadal changes in Frexces in NC since the late 1980s. Two shift points are detected at about 2003 and 2013 through a Mann–Kendall test. Three periods are then identified as 1989–2002 (P1), 2003–2012 (P2), and 2013–2021 (P3). Frexces increases from P1 to P2 and then decreases from P2 to P3. Correspondingly, the winter mean Siberian–Ural High (SUH), polar jet stream (PJS), and North Atlantic Oscillation (NAO) show interdecadal changes. The winter SUH gets stronger and the PJS and NAO weaker during P2, while the SUH is weakened and the PJS and NAO strengthened in P3. The stronger SUH and weaker PJS is favourable for cold-air intrusion into NC in P2, and the opposite is true for the weaker SUH and stronger PJS in P1 and P3. The weaker NAO in P2 relates to long-distance wave propagation to Eurasia to strengthen the SUH, while wave activity accompanying the stronger NAO in P3 is confined to western Eurasia. For the all-extreme cold events composite, the area affected by the cold air inducing extreme cold events in NC enlarges northwestward to the West Siberian Plain, and the intensity of the cold air also gets strengthened from P1 to P3.摘要全球变暖背景下, 极端天气气候事件的变化受到关注. 本文研究发现, 1989–2021年期间, 华北地区极端冷日数在2003和2013年发生了年代际变化. 极端冷日数先增加后减少. 2003–2012年, 西伯利亚–乌拉尔高压偏强, 极地西风急流偏弱, 有利于冷空气南下入侵华北地区, 华北极端冷日数偏多. 而在1989–2002年和2013–2021年, 情况相反. 虽然三个时段华北极端冷日的强度没有显著差异, 但与其相联系的冷空气强度变得更强, 2013–2021年冷空气中心区域往西北扩张到了贝加尔湖以西地区.

Analyzing urban influence on extreme winter precipitation through observations and numerical simulation of two South China case studies

This study investigates the impact of urbanization on extreme winter rainfall in the South China Greater Bay Area (GBA) through the analysis of hourly station observations and simulations using the Weather Research and Forecasting Model with the Single Layer Urban Canopy Model (WRF-SLUCM). Data from 2008 to 2017 reveal that urban areas in the GBA experience lower 99th percentile hourly winter rainfall intensity compared to surrounding rural regions. However, urban locations exhibit higher annual maximum hourly rainfall (Rmax) and very extreme rainfall events (99.99th percentile) in winter, suggesting a positive influence of urbanization on extreme winter precipitation. A case study further underscores the role of the Urban Heat Island (UHI) effect in enhancing extreme rainfall intensity and probability in the GBA urban areas. Additionally, two extreme cases were dynamically downscaled using WRF-SLUCM, involving four parallel experiments: replacing urban land use with cropland (Nourban), using historical urban land use data from 1999 (99LS), projecting near-future urban land use for 2030 (30LS), and considering 2030 urban land use without anthropogenic heat (AH) (30LS-AH0). Synoptic analysis demonstrates that cold air intrusion suppresses the GBA UHI in Case 2013 but not in Case 2015. Reduced evaporation and humidity induced by urban surfaces significantly decrease urban precipitation in Case 2013. In contrast, the persistent UHI in Case 2015 enhances local convection and land–ocean circulation, leading to increased moisture flux convergence and amplified urban precipitation intensity and probability in 30LS compared to Nourban. This amplification is primarily attributed to AH, while the change in 99LS remains insignificant. These findings suggest that urban influences on extreme precipitation in the GBA persist during winter, particularly when the UHI effect is maintained.

Subtropical SST improved the understanding of the subseasonal reversal of surface air temperature in winter over the mid-high latitudes of Asia

The mid-high latitudes of Asia are cold air gathering places and dust source areas, whose subseasonal reversal of surface air temperature (SAT) between early winter (December 1 to January 14) and late winter (January 15 to February 28) could trigger extreme temperature at mid-low latitudes. Based on the observed interannual variation of SAT over mid-high latitudes of Asia (SATMH-A) in early and late winter after detrending from 1979 to 2021, the across-latitude linkage between the SATMH-A subseasonal reversal and subtropical sea surface temperature (SST) was revealed. The southern Indian Ocean SST anomalies caused the Ural high weakened through meridional circulations and led to a warm SATMH-A in early winter, but had little effect on SATMH-A in later winter. Whereas the North Atlantic SST anomalies enhanced the cold air intrusion in late winter by exciting Rossby wave to strengthen the Ural blocking, while the SATMH-A in early winter was largely unresponsive. The SST factors in Indian Ocean and North Atlantic primarily impacted the SATMH-A in early and late winter respectively. Their synergies effectively modulated the subseasonal reversal, which was conducive to improving the explanation and prediction for the SATMH-A subseasonal reversal. Especially in the winter of 2020/21 and 2021/22, the mean prediction bias was only 0.03 °C and 0.20 °C in early and late winter contributed by the two SST factors. Moreover, the improved prediction ability on the subseasonal scale was also helpful for the accurate predictions of SATMH-A on winter-mean scale. Findings of this study also have the potential to be applied to more accurately predict temperature extremes in mid-low latitudes.

Upper‐level wind mode over the North Atlantic‐Arctic Ocean leading to coherent variations of Eurasian and North American winter temperatures and weakness in the CMIP6 models

AbstractThis study discovers that the interannual synchronization of winter temperature anomalies between northern Eurasia and North America is closely related to variation in the second leading mode of the 300 hPa meridional wind anomalies (i.e., V300‐EOF2) over the North Atlantic‐Arctic Ocean. The V300‐EOF2 mode modulates the large‐scale Rossby wave propagating across the mid‐high latitudes of the Eurasian and North American continents at the mid‐high level of troposphere, and it also interferes with the climatological quasi‐stationary wavenumber‐2 to affect the stratosphere circulation, which facilitates the southward intrusion of cold air into Eurasia and North America simultaneously. This study also evaluated whether the CMIP6 high‐top (HT) models could more accurately represent the V300‐EOF2 mode and the associated modulation mechanism on synchronized temperature variation between Eurasia and North America than the low‐top (LT) models. The results demonstrate that although the multi‐model ensemble means (MME) of both the HT and LT models can reproduce the V300‐EOF mode, they all underestimate its modulation mechanism on atmospheric circulation and coherent temperature changes over mid‐latitude Eurasia and North America, with minor differences in simulation skill capability. Most importantly, although containing a full stratosphere, the HT models do not significantly improve their simulation ability to depict the observed linear interference between the anomalous wave‐2 and the climatological stationary wave‐2 compared to the LT models. Therefore, comparing to the LT models, the CMIP6 HT models present no clear advantages in reproducing the V300‐EOF2 mode and its associated low‐frequency climate anomalies. These results also have implications for the evaluation of the predictability of concomitant Eurasian‐North American cooling in the Subseasonal‐to‐Seasonal Prediction Project.

Spatiotemporal Distributions and Related Large-Scale Environmental Conditions of Extreme Rainfall from Tropical Cyclones with Different Tracks and Seasons in Guangxi, South China: A Comparative Climatological Study

This study investigates the main climatological features of extreme precipitation (TCER) induced by tropical cyclones (TCs) affecting Guangxi (GX), South China using multiple datasets and a 99th percentile threshold during 1981–2020, with an emphasis on the rainfall diversities of different high-impact TC groups and their associated mechanisms. Results show that there are large regional differences and a seasonal imbalance in the climatological features of TCER in GX. In summer (fall), TCs with TCER events primarily move northward or eastward (northwestward or westward), namely, S-NWTCs and S-ETCs (F-WTCs and F-NWTCs). The rainfall centers exhibit asymmetrical features with S-NWTCs and F-NWTCs located in the northeast quadrant, but S-ETCs and F-WTCs in the southwest and northeast quadrants, respectively. Comparisons of atmospheric circulations and environmental factors indicate that the intense rainfall of F-WTCs is mainly attributed to the trough–TC interaction, which is accompanied by stronger upper-level westerly jet and cold air intrusion, thus increasing baroclinic energy and uplifting for the strongest rainfall among these four groups. This interaction is absent for other groups due to a greater South Asian high and western North Pacific subtropical high. Instead, the increased rainfall in S-NWTCs and F-NWTCs can mainly be attributed to the stronger low-level southwesterly jet, which, in combination with low-level warm advection and convergence induced by land–sea friction, promotes the release of latent heat through moisture condensation. S-ETCs differ from S-NWTCs and F-NWTCs in that moisture convergence is weaker due to the much-weakened TC circulation.

Role of the Thermodynamic Structure of the Inner Core in Predicting the Intensification of Hurricane Patricia (2015)

AbstractWe use short‐range ensemble forecasts and ensemble clustering analysis to study the factors affecting the intensification of Hurricane Patricia (2015). Convection‐permitting ensemble forecasts are classified into two groups: 10 spin‐down (SPD) members and 10 spin‐up (SPU) members with intensification rates of <0 and >0 m s−1 for the first 6 hr, respectively. Ensemble clustering analysis found that the wind–pressure relationship was incorrect in the SPD group, indicating that the SPD issue may be partly caused by an imbalance in the initial minimum sea‐level pressure (MSLP) and the maximum wind speed (MWS). The SPD issue appears to be related to three main points: (a) a weaker upper‐level warm core; (b) a drier inner core at low levels; and (c) a dry, cold air intrusion at mid‐levels. In contrast, the SPU group has a stronger upper‐level warm core and a relatively wet inner core at lower levels, as well as a relatively strong secondary circulation. These favorable initial conditions in the SPU group, combined with a greater updraft and stronger convection around the eyewall, result in more latent heating around or in the eyewall that favors the intensification of the tropical cyclone. Comparisons between the SPD and SPU groups suggest that the analyzed ensemble could not accurately capture the relationship between the initial MSLP and MWS, which, combined with the unfavorable thermodynamic conditions at the initial time, resulted in the incorrect evolution of the intensity. Therefore, improving the initial conditions appears to be an effective way to address the SPD issue.

Impact of tropospheric polar vortex on winter cold extremes over Northeast China

Using CN05.1 dataset and NCEP reanalysis, this study investigated the potential linkage between the tropospheric polar vortex and extreme winter cold events over Northeast China. Results show that Asian part of polar vortex (APV) is expanded and weakened before the cold events, this pattern manifests itself as a significant correlation a week before anomalous cold events. Moreover, these cold events could be classified into two types according to the polar vortex position, i.e., eastward-shifted and westward-shifted polar vortex events (EPVE and WPVE). There are significant differences in tropospheric circulation between these two types of events from 14 to 7 days before the cold event outbreak. More specifically, blockings over western Europe and the Bering strait during early-middle stage of EPVE are favourable for the shift of polar vortex towards the northern Eurasia. The southward-shifted polar vortex enhances Rossby wave activity, favourable for the development of Ural blocking, cold vortex in East Asia and cold air transport. APV is greatly weakened after the onset of cold outbreak, leading to cold air intrusion from the western side of cold vortex into Northeast China. During WPVE, the polar vortex is located over the Baffin Island. APV and cold vortex are weaker before the cold event outbreak. The cold air is mainly induced by the blocking high ridge over East Siberian Sea, with a longer transport path and weaker intensity compared with that during the eastward events. In conclusion, the cooling over Northeast China is more drastic due to stronger cold air transport when the polar vortex centre is shifted towards Eurasia.

Combined Modification of Urbanization and Monsoon in Meiyu Precipitation Changes in the Megacity Shanghai, China

The Meiyu season is a typical rainy season in East Asia that is controlled by summer monsoon. Despite extensive research on its impact, it is unclear how urbanization modifies precipitation during the Meiyu season in the background of the monsoon influence. To address this gap, this study investigated the effects of urbanization and monsoon on the modification of precipitation during the Meiyu season (PDM) in the megacity of Shanghai, China. Through homogenization analysis of the original observational data, we assessed the temporal and spatial variation in PDM in Shanghai during two stages of urbanization. Our findings revealed that both total precipitation and extreme daily precipitation during the Meiyu season in Shanghai have significantly increased since 1961. The spatial heterogeneity of PDM has also enhanced during the rapid urban process that has occurred since 1986. The long-term trend of increasing precipitation in Shanghai showed a synchronous variation with the East Asian subtropical summer monsoon (EASM) in 1961–2021. Over the interannual time scale, the significant positive correlation between PDM and EASM during the slow urbanization period (Stage 1: 1961–1985) changed to a non-significant correlation during the rapid urbanization period (Stage 2: 1986–2021), which was associated with the enhanced convective precipitation in Shanghai during the Meiyu season. Urbanization induced more convective precipitation and further weakened the association between PDM and EASM over the central city and nearby areas during Stage 2. The rapid urbanization process also resulted in increased differences in near-surface wind between urban and non-urban areas, which facilitated more PDM over the central city due to the urban friction effect and wind shear in Stage 2. Furthermore, our analysis suggests that the increase in precipitation may be associated with the enhanced coupling of cold air intrusion with the warmer climate background due to the urban heat effect occurring in Stage 2. These findings contribute to a better understanding of how urbanization and monsoons affect PDM in East Asian megacities and serve as a unique reference for climate prediction in this region.