Southeasterly Flow Research Articles

Downstream and upstream effects of urban chains on precipitation in Beijing

A summer convective rainfall event in Beijing on June 26, 2018 was investigated though a series of numerical simulations. The system moved from northwest to southeast, passing through Yanqing (YQ), Changping (CP), and the central urban area (CUA) of Beijing. The combined influences of the multiple urban areas and the mechanisms of the mutual impacts of upstream and downstream cities on precipitation were investigated. Results showed that the urban surfaces led to an increase in near-surface temperature, a decrease in lower-level wind speed, and airflow triggered convergence and updrafts. The resulting upward moisture transport and latent heat release in the cloud were promoted during the subsequent precipitation. The thermal and dynamic impacts of CUA and CP urban areas propagated downstream along the low-level southeasterly flow, resulting in increased temperatures, strengthened updrafts, and latent heat release in the cloud along YQ-CP-CUA region. The urban effects propagated downstream under the southeasterly wind, leading to enhanced precipitation over YQ and CP. The YQ also, however, contributed to the increased precipitation in CUA. The “city chain” comprised of YQ, CP, and CUA thus promoted the progression and advancement of the precipitation system. This study highlights the importance of the correct identification of the “upwind” and “downwind” directions when studying urban impacts on precipitation.

Influence of upper-tropospheric cold anomalies on heavy rainfall along the Eastern Foothills of Taihang Mountains

Using hourly rain records and reanalysis data, the climatic composite analysis and two case studies of heavy rainfall along the eastern foothills of Taihang Mountains are used to examine the influence of the upper troposphere cold anomalies (UTCA) on heavy rainfall. Three UTCAs, i.e., the cold anomaly located upstream of North China (upNC), the cold anomaly dominating the Japan Sea (JS), and the cold anomaly near the East China Sea (ES) are revealed to play positive roles in the heavy rainfall. The upNC cold anomaly and JS cold anomaly both stretch southeastward from the upper to the lower troposphere. The upNC cold anomaly can trigger a negative geopotential height anomaly accompanied by cyclonic circulation anomaly in the upper troposphere to strengthen upper-layer divergence, and a negative geopotential height anomaly in the middle and lower troposphere to enhance the low-level extratropical cyclone (ETC), resulting in heavy rainfall. JS cold anomaly can enhance the downstream trough to prevent the eastward propagation of the upstream convective systems, and further, promote significant southeasterly flows in the northeast of ETC in the middle and lower troposphere with more moist air transported to the eastern foothills of Taihang Mountains. The two heavy rainfall events indicate that ES cold anomaly stretches northwestward to the lower troposphere with a smaller horizontal scale. ES cold anomaly can increase the precipitation by enhancing the easterly anomaly to its north, which supplies a large amount of water vapor from the Northwest Pacific and amplifies the uplift of topography. The evolutions of upNC, JS, and ES cold anomalies 12-h in advance are found to be closely connected to hourly precipitation, which could be precursor signals to provide helpful information in forecasting heavy rainfall.

Study on O3 Variations in Nanjing and the Surrounding Source Analysis

To understand the transport patterns and major sources of ozone (O3) in Nanjing, this study carried out the 48-hour backward trajectories of air masses in Nanjing from March 2021 to March 2022, based on the HYSPLIT backward trajectory model driven by GDAS global reanalysis data. The primary transmission routes and putative source locations of O3 pollution in Nanjing were determined through the integration of trajectory clustering analysis, potential source contribution function (PSCF), and concentration-weighted trajectory (CWT) analysis with meteorological data and O3 concentration data. The results showed that the high O3 concentrations and exceedance rates in Nanjing were in late spring and early summer, with the highest in June. The diurnal variation of O3 concentrations in all seasons exhibited a single peak with a maximum from 13:00 to 16:00. The southeasterly flow passing through Zhenjiang, Changzhou, Wuxi, Suzhou, and Shanghai dominated the O3 pollution in Nanjing. The PSCF and CWT presented a high consistency of O3 potential sources in Nanjing. Zhenjiang, Ma’anshan, Changzhou, Wuxi, Suzhou, and Huzhou were identified as the main potential source regions of O3 pollution in Nanjing. This study provides accurate theoretical references for regional joint prevention and control of O3 pollution in Nanjing.

North American Monsoon Impacts Southern California's Coastal Low Clouds

AbstractLow‐level stratiform clouds modulate California's coastal climate during the warm season. Previous work describing the seasonal and daily variability of coastal low cloudiness (CLC) suggests that in July, August, and September southern California's CLC is under the influence of an additional driver, which has less impact in northern California. In this work, we introduce the link in which free‐tropospheric moisture dictated by North American Monsoon (NAM) processes can impact southern California CLC. We use in situ and remote sensing observations, as well as reanalysis and single column model simulations to identify and investigate this previously missing component. We find that monsoonal moisture advected by southeasterly flow from the core NAM region into southern California reduces CLC by diminishing cloud‐top longwave cooling. To add to an already complex brew of known factors influencing coastal cloudiness, another one is hereby introduced and should be accounted for in future work.

Multiscale Processes Leading to Heavy Precipitation in the Eastern Nepal Himalayas

Abstract The processes underlying heavy rainfall in the higher elevations of the Himalayas are still not well known despite their importance. Here, we examine the detailed process causing a heavy rainfall event, observed by our rain gauge network in the Rolwaling valley, eastern Nepal Himalayas, using ERA5 and a regional cloud-resolving numerical simulation. Heavy precipitation (112 mm day−1) was observed on 8 July 2019 at Dongang (2790 m above sea level). Most of the precipitation (81 mm) occurred during 1900–2300 local time (LT). The synoptic-scale environment is characterized by a monsoon low pressure system (LPS) over northeastern India. The LPS lifted moisture upward from the lower troposphere and then horizontally transported it into the eastern Nepal Himalayas within the middle troposphere, increasing the content of the water vapor around Dongang. A mesoscale convective system passed over Dongang around the time of the intense precipitation. The numerical simulation showed that surface heat fluxes prevailed under the middle tropospheric (∼500 hPa) southeasterly flow associated with the LPS around a mountain ridge on the upwind side of Dongang until 1900 LT, enhancing convective instability. Topographic lifting led to the release of the enhanced instability, which triggered the development of a mesoscale precipitation system. The southeasterly flow pushed the precipitation system northward, which then passed over Dongang during 2000–2200 LT, resulting in heavy precipitation. Thus, we conclude that the heavy precipitation came from the multiscale processes such as three-dimensional moisture transport driven by the LPS and the diurnal variation in heat fluxes from the land surface. Significance Statement Precipitation in the Himalayas is closely related to the hydrological cycle, floods, and landslide disasters in South Asia. Thus, elucidating the features of precipitation in the Himalayas is important. This study explored multiscale processes leading to a heavy precipitation event that was observed on 8 July 2019 at Dongang in the Rolwaling valley of the eastern Nepal Himalayas. We identified new processes producing heavy precipitation in the Himalayas: the three-dimensional synoptic-scale moisture transport driven by a monsoon low pressure system and the effect of the diurnal variation in heat fluxes from the land surface on the development and movement of a mesoscale precipitation system causing heavy precipitation. These findings broaden our understanding of heavy precipitation in the Himalayas.

Pattern of Wintertime Southern Rainfall and Northern Pollution over Eastern China: The Role of the Strong Eastern Pacific El Niño

Abstract Air quality over the North China Plain (NCP) and rainfall over southern China exhibit robust and correlated interannual variations during winter from 1979 to 2018. A correlation coefficient of 0.7 between the PM2.5 concentration over the NCP and rainfall in southern China indicates that poor air quality and rainfall are linked (referred to as the south rainfall–north pollution pattern), which is likely due to the strong eastern Pacific (EP) El Niño. In the mature phase of the strong EP El Niño, the Northeast Asian anomalous anticyclone (NAAA) is strengthened by two Rossby wave trains modulated by warmer sea surface temperatures. One wave train is directly related to the strong EP El Niño and originates in the tropical eastern Pacific, propagating eastward to East Asia along the middle and high latitudes, which weakens the Ural ridge and strengthens the NAAA. The other wave train is derived from the northern Indian Ocean and propagates into East Asia along the great circle route, which strengthens an anomalous cyclone in southern China and the NAAA. This results in an anomalous horizontal trough of 850 hPa being captured in eastern China at ∼30°N. The southwesterly flow along the southern margin of the trough transports abundant moisture to southern China, which leads to heavy rainfall in southern China combined with an anomalous ascending motion related to the anomalous cyclone. The southeasterly flow to the north of the trough weakens the East Asia winter monsoon due to a weakened Ural ridge, which then leads to a high PM2.5 concentrations over the NCP with the help of an anomalous descending motion related to the NAAA.

Nocturnal Convection Initiation over Inland South China during a Record-Breaking Heavy Rainfall Event

Abstract A local long-lived convective system developed at midnight over inland South China, producing record-breaking rainfall in Guangzhou on 7 May 2017. This study examines the physical processes responsible for nocturnal convection initiation (CI) and growth. Observational analyses show that the CI occurs in the warm sector under weakly forced synoptic conditions at 500 hPa, while moderate but nocturnally enhanced low-level southeasterlies with a mesoscale moist tongue at 925 hPa intrude inland from the northern South China Sea. Convection-permitting model results show that mesoscale low-level convergence and increased moisture at the leading edge of the southeasterlies are favorable for CI dynamically and thermodynamically. Local ascent and potential instability are further enhanced by orographic lifting and warm moist air from the urban surface, respectively, which trigger convection in northern Guangzhou. The mesoscale moist tongue of southeasterly flows then meets convectively generated outflows, thereby maintaining strong updrafts and continuously triggering back-building convective cells in eastern Guangzhou. Sensitivity tests are conducted to estimate the relative roles of ambient southeasterly moist tongue and urban thermal effects. The southeasterly moist tongue provides moisture that is crucial for CI, while warm moist air from the urban surface is lifted at the leading edge of the southeasterlies and locally facilitates convection. Therefore, the mesoscale processes of lifting and moistening due to nocturnal southeasterlies and their strong interaction with the local factors (orographic lifting, urban heating, and cold-pool-related ascent) provide the sustained lifting and instability crucial for triggering the local long-lived convective systems. The multiscale processes shed light on the understanding of the nocturnal warm-sector heavy rainfall inland.

Characteristics of tropical cyclones affecting Taiwan in different phases of intraseasonal oscillations

This study examines the relationship between eight 30–60-day intraseasonal oscillation (ISO) phases and three tropical cyclone (TCs) tracks affecting Taiwan during July–September. These tracks include two northwestward tracks around northern Taiwan (NW-N) and southern Taiwan (NW-S) and a northward track around eastern Taiwan. In phases 7 and 8, the ISO features a north-south heating dipole and a major cyclonic circulation centering near Taiwan. TCs are steered by anomalous southeasterly flows with a NW-N track from the eastern and southeastern sections of the anomalous cyclone toward the central region. When this anomalous cyclone displaces eastward in phase 7, TCs appearing in the southern section of the anomalous cyclone are hindered by anomalous northeasterly flows from the north to move with a NW-S track. In phase 8, a weakened and westward-displaced anomalous cyclone is paired with an anomalous anticyclone on its eastern side. Anomalous southeasterly and southerly flows between this circulation pair drive TCs with a northward track along the oceans east of Taiwan. In phases 4 and 5, the ISO features major convections over the South China Sea and western North Pacific and an associated north-south circulation dipole. TCs are driven by anomalous southeasterly flows between this dipole. TCs undergo a NW-N track when the southern anomalous cyclone extends northward across Taiwan, but a NW-S track when the northern anomalous anticyclone expands southward across Taiwan. In phase 1, a meridional wave train consisting of an anomalous cyclone centering over Taiwan and an anomalous anticyclone on its northeastern side guides TCs along a NW-N track. Variability features of real-time large-scale circulations associated with various TC-ISO relationship types are discussed.

Effects of mantle flow on the chemistry of Coriolis Troughs backarc magmas

Ridge subduction along convergent plate margins can cause a tear to develop in the underlying slab, which allows the influx of hot sub-slab mantle materials into the mantle wedge. Such a process has a profound influence on the chemistry of Vanuatu arc magmas, but its effect on the genesis of Vanuatu backarc magmas remains to be assessed. Here we present new geochemical data for samples from the Coriolis Troughs (part of the Vanuatu backarc) and combine our new data with published analyses of samples from the entire Vanuatu arc. Many samples from the Coriolis Troughs have major element systematics, chalcophile element systematics and trace element ratios (i.e., Ba/Nb, Th/Nb) that are more similar to the global MORB array than Vanuatu arc samples. These systematics can be attributed to slab rollback prior to ridge subduction, which caused a progressive decline in the proportion of slab fluxes to the mantle source of the Coriolis Trough magmas. As rollback progressed, deep upwelling of enriched mantle components beneath the North Fiji Basin influenced the compositions of the erupted backarc basin magmas. Melting of this enriched mantle source generated the most recent Nifonea Caldera Floor magmas. In contrast to the other recent Coriolis Trough magmas, the Young Lava Plain samples have geochemical characteristics that are comparable to Vanuatu arc magmas. The Young Lava Plain magmas are situated to the southeast of a slab tear. We therefore suggest that influx of hot sub-slab mantle materials through a slab tear caused southeast flow (i.e., mantle flow away from the trench) of sub-arc mantle (i.e., mantle that was enriched by hydrous and oxidising components released from the slab at sub-arc depths) and that this sub-arc mantle source generated the Young Lava Plain magmas. In particular, we suggest that this oxidised source can account for the anomalous chalcophile element signatures of the Young Lava Plain samples compared to the other Coriolis Trough samples. Because all of the Coriolis Trough samples have similar depth-to-slab, we propose that changes in mantle flow patterns caused by ridge subduction and the development of a slab tear can account for the genesis of proximal magmas with comparable ages but distinct compositions.

Temperature and moisture transport during atmospheric blocking patterns around the Antarctic Peninsula

We assess temperature and moisture transport in and around the Antarctic Peninsula (AP) associated with atmospheric blocking over two domains, one located to the west (150–90°W, 50–70°S, Western AP (WAP)) and the other over and to the east (90–30°W, 50–70°S, Eastern AP (EAP)) of the AP. We make use of surface meteorological observations, ERA5 reanalysis data, and a state-of-the-art atmospheric river (AR) database. Observed temperature anomalies indicate that the WAP and EAP blocking patterns are characterized by significant cold and warm anomalies over the AP, respectively, particularly in austral autumn, winter and spring. Consistent with these changes, cold anomalies depicted by ERA5 are associated with the transport of cold and dry air from the Antarctic continent by southerly and southeasterly flow over the eastern flank of the WAP blocking. ERA5 results highlight the importance of blocking days over the EAP domain (largely centered over the Drake Passage) to the occurrence of warm events associated with northerly and northwesterly warm air transport. Significant increases in integrated vapor transport (IVT) and AR frequency are also evident during the EAP blocking, particularly on the windward side of the AP. During the most extreme blocking days in this domain, there exist significant increases in latent and sensible heat fluxes on the windward side of the AP and the Larsen C Ice Shelf, respectively, indicating the contribution of foehn events to warm anomalies, especially in austral autumn and winter. The co-occurrences between landfalling ARs and blocking are found to amplify foehn effect due to higher IVT and associated latent heat condensation compared to blocking days without ARs. We conclude that blocking patterns are important to understand the occurrence of extremely warm events and landfalling ARs in the AP and their potential impacts on the surface cryospheric processes.

Interdecadal Increase in Summertime Extreme Precipitation over East China in the Late 1990’s

This study focuses on the interdecadal increase of summertime extreme precipitation over East China in the late 1990s and physical mechanisms behind. The results show that summer extreme precipitation over East China during 1979–2020 demonstrates an upward trend and a significant interdecadal increase occurs around 1997/1998. Since 1997, extreme precipitation anomalies turn from less than normal to more than normal, corresponding to a strong upward movement in the lower troposphere over East China. A cyclonic circulation with positive vorticity controlled by a strong southeasterly flow appears in the lower level over South China. The reasons for the interdecadal increase of summer extreme precipitation over East China are analyzed from the perspective of the abnormally strong South Asian subtropical high (SAH) and the atmospheric circulation anomalies caused by zonal sea surface temperature (SST) gradient in the tropical oceans. After 1997, positive sensible heat anomalies appear over the Tibetan Plateau in spring, which is the major factor that maintains the intensity of the SAH. Besides, the SST of the tropical oceans presents an obvious “high-low-high” zonal gradient distribution with positive sea surface temperature anomaly (SSTA) in the Indian Ocean and western Pacific, and negative SSTA over the Maritime Continent. The zonal SST gradient results in an anomalous downdraft and boundary layer divergence over the Maritime Continent. The tropical zonal SST gradient triggers a local Hadley circulation, and its ascending branch is located in East China. This is a circulation condition favorable for the development of extreme precipitation. In addition, since the late 1990s, the SAH has strengthened in the upper troposphere above the southeastern China. The anomalous divergence at the upper level effectively maintains the upward movement, which in turn facilitates the occurrence of extreme precipitation. As a result, the sensible heat changes in the Tibetan Plateau in spring and the tropical SST zonal gradient jointly affect summer extreme precipitation over East China, leading to the interdecadal increase of extreme precipitation in the late 1990s.

The strengthening relationship between summer rainfall over North China and PDO since the mid-2000s

The interdecadal change of the summer (July–August) rainfall over North China (SRNC) is of great concern to the climate research community. Based on the sliding correlation and interdecadal difference analysis, the interdecadal change of SRNC was studied. Results show a significantly wetter period of SRNC since the mid-2000s, which was associated with strengthening negative relationship between SRNC and PDO. PDO affected the SRNC through the jet stream in the upper-level and the East Asia–Pacific (EAP) teleconnection pattern. In the negative phase of the PDO since the mid-2000s, the contractive upper-level jet stream in the west of North China resulted in the abnormal updraft over the southern region of North China. Meanwhile, the positive phase of the EAP teleconnection pattern brought warm and wet southeast flow entering North China from its eastern boundary, both bringing more rainfall in North China. The abnormal water vapor transport from the western Pacific was very different from that accompanying the East Asian summer monsoon.

Indirect Effects of Binary Typhoons on an Extreme Rainfall Event in Henan Province, China From 19 to 21 July 2021: 1. Ensemble‐Based Analysis

AbstractThe extreme rainfall from 19 to 21 July 2021, which caused massive flooding and loss of life, is the second heaviest rainfall that has occurred in Henan province, central China. To identify the key factors controlling this rainfall event, we conducted an ensemble‐based analysis using ECMWF operational global ensemble forecasts. The forecasts of extreme rainfall had a relatively large spread and there was a large bias in the ensemble mean precipitation, indicating uncertainties in the forecast. The extreme rainfall was closely related to the Huang‐Huai cyclone and the southerly and southeasterly flows. Although they had little influence on the southeasterly flow, the uncertainty and predictability of typhoons In‐fa and Cempaka probably caused the variation in the southerly flow and the maintenance of the Huang‐Huai cyclone, which determined the amount of precipitation for this extreme event in the ensemble forecast model. When typhoon In‐fa was located further northwest, the northeasterly airflow generated by the binary interaction between typhoons In‐fa and Cempaka weakened the intensity of the southerly flow, reduced the transport of water vapor to the rainstorm area and weakened the Huang‐Huai cyclone, thus reducing precipitation in the control area. The results of this study indicate that ensemble‐based analysis can improve our understanding and forecasting of extreme precipitation events under the influence of multiple remote tropical cyclones.

Case studies of the wind field around Ny-Ålesund, Svalbard, using unmanned aircraft

The wind field in Arctic fjords is strongly influenced by glaciers, local orography and the interaction between sea and land. Ny-Ålesund, an important location for atmospheric research in the Arctic, is located in Kongsfjorden, a fjord with a complex local wind field that influences measurements in Ny-Ålesund. Using wind measurements from UAS (unmanned aircraft systems), ground measurements, radiosonde and reanalysis data, characteristic processes that determine the wind field around Ny-Ålesund are identified and analysed. UAS measurements and ground measurements show, as did previous studies, a south-east flow along Kongsfjorden, dominating the wind conditions in Ny-Ålesund. The wind measured by the UAS in a valley 1 km west of Ny-Ålesund differs from the wind measured at the ground in Ny-Ålesund. In this valley, we identify a small-scale catabatic flow from the south to south-west as the cause for this difference. Case studies show a backing (counterclockwise rotation with increasing altitude) of the wind direction close to the ground. A katabatic flow is measured near the ground, with a horizontal wind speed up to 5 m s-1. Both the larger-scale south-east flow along the fjord and the local katabatic flows lead to a highly variable wind field, so ground measurements and weather models alone give an incomplete picture. The comparison of UAS measurements, ground measurements and weather conditions analysis using a synoptic model is used to show that the effects measured in the case studies play a role in the Ny-Ålesund wind field in spring.

Preliminary investigation on the extreme rainfall event during July 2021 in Henan Province and its multi-scale processes

<p indent="0mm">During July 17–22, 2021, a disastrous extreme rainfall event occurred in Henan Province, China, with the maximum 6-day accumulated rainfall obtained from meteorological observations being <sc>1122.6 mm</sc> (Hebi City). The maximum hourly rainfall reached <sc>201.9 mm</sc> (Zhengzhou city), which broke the record in the inland of China. The daily rainfall of 1/6 of the national rain gauge stations in Henan Province exceeded their respective records. In this study, a comprehensive review and a preliminary investigation on the multi-scale atmospheric processes are conducted by using rain gauge observations, sounding data, Doppler weather radar observations and the ERA5 reanalysis data. Results show that this extreme rainfall event was caused by the joint effect of multi-scale atmospheric systems in the middle and low latitudes, and the effect of local topography: (1) The western Pacific subtropical high is abnormally northerly and strong, as compared with the mean circulation in July during the 30-year period of 1991–2020. It prevented the tropical cyclone “In-fa” from moving northward, and was conducive to the westward movement of “In-fa” along its southern periphery. An easterly low-level jet formed between the subtropical high and “In-fa”, which strengthened the water vapor transport to East China. (2) The water vapor was transported from the South China Sea to the Northwest Pacific by the southwest monsoon, and then was carried northwards by the tropical cyclone “In-fa”. The water vapor over the Northwest Pacific was transported to Henan Province by the easterly jet on the north of “In-fa” and the southeast flow around the southwestern periphery of the western Pacific subtropical high. This abnormal remote water vapor transport associated with the tropical cyclone caused a positive anomaly in precipitable water in Henan. (3) In the upper troposphere, the location of Henan Province was in front of an upper-troposphere trough. In the lower troposphere, there was a low-level vortex or an inverted trough over Henan and its periphery. The synoptic conditions over Henan were conducive to the convergence and upward motion of lower-tropospheric atmosphere. In addition, the Funiu and Taihang mountain ranges favored the convergence of water vapor and the uplift of the approaching easterly or southeasterly airflow. (4) The subtropical high and the abnormally strong continental high connected and formed a “high-pressure dam”, which prevented mid-latitude cold air from moving southward. The extreme rainfall in Zhengzhou occurred in a warm-humid environment which resulted in a low-centroid precipitation resembling tropical precipitation. During the two days with the heaviest rainfall in Zhengzhou, convection was initiated continuously and merged into the main convective cluster from the southeast, south and southwest respectively, which helped maintain the mesoscale convective system in Henan for a long time.

Late Weichselian ice-sheet flow directions in the Russian northern Barents Sea from high-resolution imagery of submarine glacial landforms

Abstract The locations and orientations of more than 1000 late Quaternary subglacial and ice-marginal landforms, including streamlined sedimentary bed forms, glacitectonic hill-hole pairs, meltwater channels, and eskers, were mapped from blocks of multibeam data (area of 4861 km2) in the little-known Russian Barents Sea. Between Sentralbanken and Admiralty Bank, at ∼75°N, there is evidence for southward ice flow. Ice-flow indicators between Franz Josef Land and Novaya Zemlya show northeast flow into the head of St. Anna Trough. There is also evidence of southeast flow off the bank to the south of Franz Josef Land, and of flow convergence with northeast-flowing ice in Sedov Trough. Northeast flow of ice between Novaya Zemlya and Franz Josef Land suggests that the latter archipelago was not overrun by ice flowing north from the Barents Sea and, therefore, that a subsidiary ice dome was likely on Franz Josef Land. A major ice divide was also present at ∼76°N –77°N in the Russian Barents Sea.

Synopsis of the dramatic climate change in Iran: a seasonal synoptic analysis

This study examines the NCEP (National Centers for Environmental Prediction)/NCAR (National Center for Atmospheric Research) reanalysis model datasets to display the existence of the anomaly periodic time with the significant atmospheric changes in Iran. Datasets such as surface air temperatures, relative humidity, surface soil moisture, precipitation rate, mean sea level pressure, 500 hPa geopotential height, near-surface vorticity and surface scale winds have been used. The anomalies’ departures from climate normal for the meteorological datasets are investigated, and synoptically patterns have been analysed. The yearly seasonal mean surface air temperature time series during 1948–2018 displays a dramatic warming shift with an abnormal pattern in Iran from 1994 onwards. The analysis of the other datasets resulted from ERA-Interim and MERRA-2 database also confirm a warming shift nearly from 1994 over Iran. The statistical analysis of t-tests shows the existence of a significant difference in seasonal mean surface air temperatures during the anomaly period (1994–2018) and the mean values during climatology normal periodic time (1981–2010). This suggests remarkable warming over all parts of Iran with the highest changes in surface temperatures ranging ~0.1–1 °C in spring and the lowest change in fall ranging ~0.1–0.7 °C. Significant changes have resulted in precipitation rate, atmospheric humidity, soil moistures and surface wind speeds. Also, due to the strengthened northern high-pressure system along with the intensification of the 500 hPa ridge and less atmospheric humidity, a rather stable, warmer and drier atmosphere with less precipitation (respect to climate normal) over the most part of Iran has been indicated. The low-level vorticity anomalies have displayed the intensified flows from the regional desert areas to the central and north-east of Iran and from Iraq to the west and south-west of Iran for all seasons (with the higher gradient in summer rather than other seasons). Also, mean sea level pressure anomalies have exposed the increase in pressure gradient mostly over the western and central parts in all seasons except winter (with decreased wind speeds leads to haze or accumulated pollutants). This may cause instability in these parts with the possibility of frequent dust storm because of wind acceleration and warmer and dry atmospheric conditions rather normal mean. However, this study shows that during the fall seasons, humidity and precipitation values in the south-west and south-east area have been increased in comparison with the normal mean values. The increase in relative humidity during fall in the south-east areas can be associated with the impact of the intensified south-easterly flows (southern low-pressure tongue). This is accompanying by rather warmer sea surface temperatures in the southern marine areas, especially in the Oman sea and the northern Indian Ocean. Overall, this study presents some synoptic abnormal patterns associated with the recent Iran significant atmospheric climate changes.

Modulation of El Niño events on summertime westward‐propagating wave patterns and tropical cyclone movement in the western North Pacific

AbstractThis study investigated the association between westward‐propagating wave patterns and associated tropical cyclones (TCs), and eastern Pacific (EP) and central Pacific (CP) El Niño events. The wave pattern was more enhanced in the EP El Niño events than it was in the CP El Niño events, whereas the monsoon trough was deeper in the CP El Niño events than it was in the EP El Niño events. This enhanced monsoon trough together with the formation of a significantly stronger subtropical high strengthened the southeasterly flow, which caused TCs to detour northwestward to Japan in the CP El Niño events. An analysis of kinetic energy indicated that barotropic conversion provided more kinetic energy following the axis of the monsoon trough from the background flow to the westward‐propagating wave pattern in EP El Niño events, except near the centre of the monsoon trough where the background flow was more favourable for the wave pattern during CP El Niño events. The advection term by the southeasterly flow became more important in supporting the perturbation kinetic energy during CP El Niño events. Large‐scale circulation changes can influence smaller‐scale phenomena through barotropic conversion and advection.

Impacts of Three Gorges Dam on Regional Circulation: A Numerical Simulation

AbstractThe Three Gorges Dam (TGD) is the largest hydropower project in the world. The resulting Three Gorges Reservoir (TGR) extends 660 km along the waterway of the Yangtze River in surrounded by complicated terrain. The dam and its impounded reservoir has triggered large‐scale land use and land cover change, which can potentially impact the environment and climate significantly. In this study, the impact of the TGD on regional‐scale circulation is explored through high‐resolution simulation using the weather research and forecast model. It is found that the construction of the TGD has little influence on the near‐surface temperature in the TGR region even when extreme hot weather occurs in the region. The numerical experiments performed in this study indicate that the construction of the TGD can alter the atmospheric circulation on the regional scale when the weak southeasterly wind in the lower troposphere prevails in the region. Due to the blocking effects of the TGD and the expanding water surface area of the TGR, the low‐level southeasterly flow is turned into northeasterly and southeasterly flows before reaching the dam, leading to an anomalous cyclone over 112–115°E, 33–34°N, and an anomalous anticyclone over 110–112°E, 28–29°N. It is also found that the anomalous cyclone can evidently enhance precipitation locally under some special atmospheric conditions. This study suggests that the impact of the TGD can be amplified due to the complicated terrain in the surrounding areas of the TGD.

Observational analyses of topographic effects on convective systems in an extreme rainfall event in Northern China

An extreme rainfall event occurred in northern China between 18 and 21 July 2016, with the strongest precipitation of 783.4 mm occurring within the rainfall zone of Taihang Mountain. In this study, mesoscale convective systems (MCSs) resulting in heavy rainfall are studied using high-resolution surface observations, soundings and radar data. The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall. The synoptic low-pressure environmental circulation promotes mesocyclone and convection development in northern China, where a strong and moist low-level jet is observed. The interaction between the mesocyclone and mesoscale topography produces two strong, separate precipitation centers, which are clearly the direct results of three MCSs. Orographic blocking plays a crucial role in the development of MCSs. Diabatic cooling due to raindrop evaporation in association with MCS1 forms a cold pool on the plain east of the central Taihang Mountain area. Convective cell initiation and development in MCS2 is due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin. Convective cells that are successively generated and shifted along a mesoscale shear line comprise the MCS2 linear distribution and a northwestward rain belt. The northern extreme rainfall center is caused by MCS3 along the southern slopes of an eastward-opening valley. Convective cells are triggered along a shear line, which is affected by a combination of topography, a cold pool in the boundary layer and mesocyclone outer flow. Together, the orographic forcing and cold pool enhance the Taihang Mountain precipitation event.