Mid-latitude Trough Research Articles

A Tale of Two Novembers: Confounding Influences on La Niña’s Relationship with Rainfall in Australia

Abstract Despite common background La Niña conditions, Australia was very dry in November 2020 and wet in November 2021. This paper aims to provide an explanation for this difference. Large-scale drivers of Australian rainfall, including El Niño–Southern Oscillation, Indian Ocean dipole, Southern Annular Mode, and Madden–Julian oscillation, were examined but did not provide obvious clues for the differences. We found that the absence (in 2020) or presence (in 2021) of an enhanced thermal wind and subtropical jet over the Australian continent contributed to the rainfall anomalies. In general, La Niña sets up warm sea surface temperatures around northern Australia, which enhances the meridional temperature gradient over the continent and hence thermal wind and subtropical jet. In November 2021, these warm sea surface temperatures, coupled with a persistent midlatitude trough, which advected cold air over the Australian continent, led to an enhanced meridional temperature gradient and subtropical jet over Australia. The enhanced jet provided favorable conditions for the development of rain-bearing weather systems across Australia. In 2020, the continent was warm, displacing the latitude of maximum meridional temperature gradient south of the continent, resulting in fewer instances of the subtropical jet over Australia, and little development of weather systems over the continent. We highlight that although La Niña tilts the odds to wetter conditions for Australia, in any given month, variability in temperatures over the continent can contribute to subtropical jet variability and resulting rainfall in ways which confound the normal expectation from La Niña. Significance Statement Forecasts of El Niño–Southern Oscillation are eagerly awaited, as the state of this climate driver has profound impacts on the likelihood of rainfall in regions around the world. While El Niño and La Niña do change rainfall likelihoods, the actual outcomes of these events are sometimes counter to expectation. This work explores one of the confounding factors to those expectations in the Australian context—the role of the meridional temperature gradient over the continent in modifying the storm track over Australia, which can disrupt the expected El Niño and La Niña teleconnections. We present case studies for two La Niña springs, highlighting that the Australian continent can help shape its own weather toward wetter or drier outcomes.

The Relation Among the Ring Current, Subauroral Polarization Stream, and the Geospace Plume: MAGE Simulation of the 31 March 2001 Super Storm

AbstractThe geospace plume, referring to the combined processes of the plasmaspheric and the ionospheric storm‐enhanced density (SED)/total electron content (TEC) plumes, is one of the unique features of geomagnetic storms. The apparent spatial overlap and joint temporal evolution between the plasmaspheric plume and the equatorial mapping of the SED/TEC plume indicate strong magnetospheric‐ionospheric coupling. However, a systematic modeling study of the factors contributing to geospace plume development has not yet been performed due to the lack of a sufficiently comprehensive model including all the relevant physical processes. In this paper, we present a numerical simulation of the geospace plume in the 31 March 2001 storm using the Multiscale Atmosphere‐Geospace Environment model. The simulation reproduces the observed linkage of the two plumes, which, we interpret as a result of both being driven by the electric field that maps between the magnetosphere and the ionosphere. The model predicts two velocity channels of sunward plasma drift at different latitudes in the dusk sector during the storm main phase, which are identified as the sub‐auroral polarization stream (subauroral polarization streams (SAPS)) and the convection return flow, respectively. The SAPS is responsible for the erosion of the plasmasphere plume and contributes to the ionospheric TEC depletion in the midlatitude trough region. We further find the spatial distributions of the magnetospheric ring current ions and electrons, determined by a delicate balance of the energy‐dependent gradient/curvature drifts and the E × B drifts, are crucial to sustain the SAPS electric field that shapes the geospace plume throughout the storm main phase.

Moisture sources and isotopic composition of the 2020 extraordinary and persistent Meiyu rainfall in the Yangtze River valley modulated by large-scale circulations

Meiyu rainfall is an important climate phenomenon in East Asia, but its interannual variability has amplified in recent decades. To gain a better understanding of the underlying atmospheric processes of torrential Meiyu rainfall, we investigated the moisture sources and isotopic composition of the 2020 extraordinary and persistent Meiyu rainfall in the Yangtze River valley from June 19 to July 10 using an isotopic regional spectral model. During the Meiyu period, significant amounts of Indian Ocean moisture were transported to Yangtze River valley by Indian monsoonal southwesterlies, especially in the middle levels between 850 hPa and 600 hPa. Whereas moderate amounts of East Asian continent moisture were distributed principally within the boundary layer below 850 hPa. Besides, tiny amounts of South China Sea moisture were transported by the western Pacific subtropical high to the middle and lower reaches of Yangtze River valley within the boundary layer below 900 hPa. With respect to isotopic characteristics, the lowest δ2H and the highest d-excess values of the Indian Ocean moisture were attributable to more rainout and below-cloud evaporation during long-distance transport. In contrast, the isotopic signals of the East Asian continent moisture and the South China Sea moisture were relatively preserved. The 2020 Meiyu rainfall exhibited significant intraseasonal variability and was classified into heavy and light Meiyu periods based on rainfall amount. Heavy Meiyu rainfall with northward migration were characterized by enhanced Indian Ocean moisture with lower δ2H and higher d-excess, resulting from the westward expansion of the western Pacific subtropical high and the deepened East Asian mid-latitude trough.

First Detection of Midlatitude Plasma Bubble by SuperDARN During a Geomagnetic Storm on May 27 and 28, 2017

AbstractWe used the global navigation satellite system‐total electron content (TEC) and Super Dual Auroral Radar Network (SuperDARN) radar to elucidate the characteristics of the plasma bubble extending to midlatitudes over North America during a geomagnetic storm on 27 and 28 May 2017. To identify plasma bubbles, we analyzed the rate of the TEC index (ROTI), which is a good indicator of the occurrence of plasma bubbles. The enhanced ROTI region expanded up to 50°N (geomagnetic latitude), and the upper limit of this region coincided with the equatorward wall of the midlatitude trough. Two‐dimensional ROTI maps show that the enhanced midlatitude ROTI region moved westward at a bulk speed of approximately 310 m/s. The Fort Hays East SuperDARN radar also detected the radar echo, showing the existence of plasma density irregularities at ∼10‐m scales within the midlatitude plasma bubble. The radar echo had a westward velocity of ∼300 m/s, which is almost consistent with the westward motion of the enhanced midlatitude ROTI region. We deduced that the westward propagation of the plasma bubble could be caused by a poleward sub‐auroral polarization stream electric field. The observed radar echo overlapping with the enhanced ROTI region had a narrower spectral width (<50 m/s) than that of the auroral activity. This feature resembled that of the type 1 echo, although the Doppler velocity was smaller than the ion acoustic speed at the F‐region height. This is the first case in which plasma density irregularities within plasma bubbles were captured by the SuperDARN radar.

Multi-scale characteristics of an extreme rain event in Shandong Province, produced by Typhoon Lekima (2019)

Super Typhoon Lekima (2019) is the fifth strongest typhoon to make landfall in mainland China since 1949. After its landfall, Typhoon Lekima moved northward along the coastline, resulting in an extreme rain event in Shandong Province that caused the highest precipitation in available meteorological records. A Weather Research and Forecasting (WRF) model simulation that properly produces the track and intensity of Typhoon Lekima and the spatio-temporal evolution of rainfall is used to analyze the multi-scale characteristics of the extreme rain event. The results show that different from the typhoon precipitation which occurred at low latitudes, the extreme rain event that occurred in mid-latitudes was influenced by the interactions of mid-latitude synoptic systems and typhoon circulation, especially with five mesoscale rainbands. The mid-latitude synoptic systems, mainly including the upper-tropospheric jet, the western North Pacific subtropical high, the mid-latitude trough, the low-level jets, and Typhoon Krosa (2019), allowed Typhoon Lekima to maintain its intensity after landfall and provided favorable kinematic, thermodynamic, and moisture conditions for the heavy rainfall in Shandong. Based on the evolution of the mesoscale rainbands, the extreme rain event can be divided into three stages. The first stage can be classified as distant rainfall, which was affected by two convective rainbands associated with boundary-layer processes. The second stage had the highest precipitation, featuring the formation of a frontal zone in Shandong, interacting with Typhoon Lekima. The third stage had weakened rainfall and was directly influenced by the spiral rainband of Typhoon Lekima.

Mechanism of the late summer monsoon onset in Bay of Bengal and South China Sea during 2021: Impacts of intraseasonal oscillation and local thermal contrast

The process of Bay of Bengal (BOB) summer monsoon (BOBSM)- and South China Sea (SCS) summer monsoon (SCSSM)-onset in 2021 is investigated by using the NCEP/NCAR reanalysis, NOAA OLR and ERA5 SST data from 1981 to 2021. Results show that BOBSM and SCSSM are built up two pentads later than climatology, on May 18 (pentad 28) and May 29 (pentad 30), respectively. For the BOBSM, the late onset is attributed to the late-originating 10–30-day intraseasonal oscillation (ISO), of which the wet phase strengthens the vortex and local convection over BOB in pentad 28, providing dynamical and thermodynamical conditions for the establishment of BOBSM. For the SCSSM, however, the onset is impeded by the anomalously weak Indochina Peninsula (ICP)-SCS thermal contrast before pentad 30 and the lack of the mid-latitude trough (a trigger of SCSSM confirmed in previous studies) in pentad 29. We propose a possible trigger of SCSSM in 2021, i.e., land wind effect, based on the sudden strengthening of ICP-SCS thermal contrast, which connects BOBSM convection and SCSSM onset. Specifically, a sub-vortex is formed over the ICP in pentad 29 following the vigorous BOBSM convection. This sub-vortex structure results in a drastic surface temperature decrease in ICP, causing fairly significant thermal contrast between ICP and SCS. Then, the dynamical response to the low-level baroclinicity, i.e., land wind effect, triggers the onset of SCSSM in pentad 30.

An unprecedented high temperature event in southern China in autumn 2021 and the essential role of the mid-latitude trough

Studies about high temperatures in China rarely touched the period beyond summer. But in September–early October 2021, a record-breaking high temperature event occurred in southern China, and affected agriculture in many provinces. Both the daily maximum temperature and the number of high temperature days were the highest in September since 1951. To better understand the different features and possible causes of the extreme event in autumn from those in summer, the analysis of atmospheric circulation patterns was conducted. The western Pacific subtropical high (WPSH) was quite stronger and extended much more westward in most of the days in September 2021 than its climatology, which directly caused this unprecedented autumn high temperature event. Besides, the mid-latitude trough over the east of Japan may also play an essential role in this high temperature event. Due to the southward extension of the trough, a cyclonic circulation anomaly appeared at 850 hPa, and its northerly wind component prevailed in the west of the trough and blocked the eastward retreat of the WPSH. As a result, southern China was stably controlled by the WPSH, which led to the persistence of this high temperature event. The conclusion is further confirmed by the statistical results from the analysis of the multi-year data and analogue analysis. Compared to the previous studies, this study reveals the essential role of the mid-latitude trough and highlights the joint impacts of the WPSH and the trough in causing the unprecedented autumn high temperature event.

Large‐Scale Depletion of Nighttime Oxygen Ions at the Low and Middle Latitudes in the Winter Hemisphere

AbstractBy analyzing the latitudinal distributions of ionospheric ions observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions satellite at 670 km altitude in different seasons, we found that the large‐scale depletion of nighttime oxygen ions is prevalent at the low and middle latitudes of winter hemisphere (center ∼ 30°) under different geomagnetic conditions. The latitudinal width of the winter oxygen ion depletion (WOD) region is about 20°–60° at different longitudes, and its upper boundary latitudes are mostly lower than the midlatitude trough (|λ| ≥ 50°) of nighttime hydrogen ions (H+) near the plasmapause. In the WOD region, the density ratio of oxygen and hydrogen ions and that of their neutral densities (O and H) approximately agree with the chemical equilibrium relationship of charge exchange reactions (H + O+ ↔ H+ + O) predicted by previous theory, indicating that the charge exchange reactions are mainly responsible for the large‐scale oxygen ion depletion at the low and middle latitudes in winter hemisphere.

Upper‐level midlatitude troughs in boreal winter have an amplified low‐latitude linkage over Africa

AbstractIn boreal winter, strong upper‐level midlatitude troughs across the Atlantic–Africa–southwestern Asia sector generate substantial tropical–extratropical interaction and have become recognized as important factors in some extreme weather events. As such, they represent important dynamic features to understand and capture in weather forecasts, as well as in climate models for projections on longer timescales. Here, we empirically study the 20% of winter days with strongest trough signatures during 1982–2020 at each longitude across the sector, and show that the trough impact over northern Africa, most notably in central parts, is particularly strong in magnitude, low‐latitude extent and persistence, leading to the characterization of a northern Africa mode of several‐days weather fluctuation. Weather conditions that follow strong troughs from the eastern Atlantic to the Central Mediterranean include: (i) a warming tendency across much of northern Africa, generally of several Celsius magnitude ahead of the trough, and >1°C even extending to the south of 10° N in central parts and continuing eastward until the Ethiopian Highlands; (ii) precipitation development further north than normal across northern tropical Africa, especially strong over longitudes corresponding to a northward extension of the main Congo rain belt. The intertropical discontinuity and low‐level heat low are also shifted significantly north, with the complex of anomalies persisting for several days, beyond the timescale of the trough. For context, at all other trough longitudes across the sector, a warming signal does emerge (statistically significant), but with much shorter persistence (2–3 days), smaller magnitude and extending southward clearly only to 15–20° N. Mid‐level tropical plumes of moisture are also typically present for strong troughs from the eastern Atlantic to southwestern Asia, and these alone can lead to weather extremes. However, low‐level warming and mid‐level moistening are uniquely juxtaposed at low latitudes over central Africa, where a near‐equatorial signature develops.

The Ionospheric Responses from Satellite Observations within Middle Latitudes to the Strong Magnetic Storm on 25–26 August 2018

The multi observations from the China Seismo-Electromagnetic Satellite (CSES) were presented and analyzed during the biggest magnetic storm on 25–26 August in the quiet solar activity year of 2018, together with the Swarm satellite and GNSS TEC (Global Navigation Satellite System, Total Electron Content). The whole tempo-spatial evolutional process was demonstrated in electromagnetic fields and in-situ plasma parameters within the whole magnetic storm time period of three phases, the main phase with quick decrease in SYM-H, the quick recovery phase, and the slow recovery phase. Strong correlations were revealed in time and space between electric fields and electron density. During the main phase, the penetrated electric field was the major factor to induce the injection of electric fields to low latitudes even to the equator and contribute to constructing the double peaks of Ne at altitudes above 500 km of CSES in daytime. In the quick recovery phase, Ne depletion was found in low middle and low latitudes in the daytime, associated with a quick decrease in solar wind dynamic pressure, but in the nightside Ne maintained or increased. Due to the high solar wind speed following the quick recovery phase, it controlled the enhancements in an electric field below 1125 Hz at medium and low latitudes in daytime and produced similar structures in a 225 Hz electric field with the mid-latitude trough of Ne in local nighttime and maintained their equator-ward movements in this time period. Ne/TEC showed typical local time-dependence in this magnetic storm, which illustrated that although the electron density in the ionosphere was mainly caused by this solar activity event, local background environments must also not be ignored for their final evolutional modes.

Continuous Enhancements of Electron Temperature in the Subauroral Ionosphere Over Eight Days During the 2015 St. Patrick’s Day Storm

AbstractWe have used measurements of the Defense Meteorological Satellite Program (DMSP) satellites to study variations of electron temperature in the subauroral ionosphere during the magnetic storm on 17–25 March 2015. This magnetic storm had a long recovery phase of 7 days, and the ionospheric behavior over the entire storm time was seldom investigated. In this study, we find that the electron temperature at subauroral latitudes was continuously enhanced for 8 days, from the storm onset to the end of the recovery phase. The maximum electron temperature during the storm times was 1000–4000 K higher than the maximum electron temperature during quiet times. This long‐lasting enhancement of subauroral electron temperature was attributed to energy transfer among the solar wind, magnetosphere, ring current, plasmasphere, and ionosphere driven by high‐speed solar wind streams and fluctuating interplanetary magnetic field during the entire 8‐day period of the storm. The electron temperature enhancements were quite symmetric in the post‐midnight sector but became strongly asymmetric near dawn between the southern and northern hemispheres. The asymmetric enhancements of electron temperature near dawn may be related to the magnetic declination and the daytime midlatitude trough in the southern hemisphere. Large daily variations of maximum electron temperature in the post‐midnight sector were observed and may be related to the offset between geomagnetic and geographic latitudes. These DMSP observations provide new insight on ionospheric response to intense magnetic storms.

Revisiting a Mei-Yu Front Associated with Heavy Rainfall over Taiwan during 6–7 June 2003

During 6–7 June 2003, a Mei-Yu jet/front system over Southern China is characterized by appreciable horizontal temperature contrast below the 850 hPa level (>8 K), where the cold, dry, postfrontal northeasterlies converge with the warm, moist southwesterly flow, and above the 400–hPa level (>18 K) associated with an upper-level front. The frontal baroclinic zone tilts northward with a slope of ~1/100. During the passage of a midlatitude trough, the upper-level jet/front system advances southeastward. The thermally direct circulation across the subsynoptic low-level jet (SLLJ)/Mei-Yu front system, coupled with dynamic forcing aloft on the equatorial side of the entrance region of a subsynoptic upper-level jet (SULJ), provides a favorable environment for the development of a frontal cyclone over Southern China. A southwesterly marine boundary layer jet (MBLJ) develops between the deepening Mei-Yu frontal cyclone and the West Pacific Subtropical High (WPSH). The MBLJ transports moisture from the northern South China Sea (NSCS) to Southern China. All three jets (SULJ, SLLJ, and MBLJ) interact together during the deepening of the Mei-Yu frontal cyclone with positive feedback effects of latent heat release. On 7 June 2003, as the Mei-Yu front arrives near the Taiwan area, the warm, moist, and unstable air associated with the MBLJ decelerates as it approaches the Central Mountain Range (CMR). The warm, moist, and unstable air is orographically lifted by the CMR and enhances the vertical motion already present with the frontal zone. A region of widespread heavy rainfall develops, with a maximum of more than 350 mm/day, over a region extending from the southwestern coast of Taiwan to the windward slopes of the CMR.

Spatiotemporal Variation and Circulation Characteristics of Extreme Maximum Temperature Events in East China (1961–2020)

This study analyzed the spatiotemporal variation characteristics of extreme maximum temperature events (EMTEs) in East China in the last 60 years and investigated the relationship between EMTEs and atmospheric circulation. The arithmetic mean, linear trend, and the Mann–Kendall test were applied to daily maximum temperature (DMT) data (1961–2020) from 345 meteorological observation stations with complete observation records in East China to compile four characteristic indexes of EMTEs: intensity, consecutive days, first days, and last days. The analysis of these indexes revealed the following: (1) The annual number of days with a DMT ≥ 35 °C increased at the rate of 1.45 d/decade (p ≤ 0.05); the mutation occurred in 2009 with a growth rate before and after the mutation of 0.4 and 2.8 d/10a, respectively. Most of the region showed an increasing trend, with the most significant increase to the east of the Yangtze River Delta, in coastal areas of Zhejiang and Fujian, and south of Jiangxi. (2) The EMTE intensity rose at the rate of 0.15 °C/decade (p ≤ 0.05). Most areas showed a significant upward trend, and the historical extreme values of EMTEs mostly appeared in the 21st century. (3) The annual mean growth rate of consecutive EMTE days was 0.24 d/10a, which increased significantly after 2003. In comparison with 1961–2002, consecutive EMTE days increased by 35% during 2003–2020. The rate of increase was significant (p ≤ 0.05) for most areas east of the Yangtze River Delta, coastal areas of Zhejiang and Fujian, and areas south of Jiangxi. The mean mutation time was 2003, and the growth rate before and after the mutation was 0.4 and 1.4 d/10a, respectively. (4) The mean first EMTE day advanced and the mean last EMTE day became delayed, especially in the 21st century. Over the study period, the mean first EMTE day advanced by 12 days and the mean last EMTE day became delayed by 7 days. (5) The analysis of National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data indicated that an increasing number of EMTEs have occurred in East China. The reason is that this region experiences atmospheric subsidence resulting from the intensification and westward extension of the subtropical high coupled with the weakening and northward displacement of the mid-latitude westerly trough.

The Impact of Intra-Seasonal Oscillation on Westward Track Deflection of Super Typhoon Fitow (2013)

Typhoon Fitow (2013) took an unusual westward track deflection after a lengthy northward movement over the western North Pacific (WNP). Based on observation and wave analysis, it is found that the track deflection of Fitow is attributed to the transition of environmental flow from meridional to zonal orientation, which is closely associated with a low-frequency intra-seasonal oscillation (ISO). Furthermore, the impact of ISO on tropical cyclone (TC) unusual movement is investigated using the Advanced Research version of Weather Research and Forecasting (WRF-ARW) model. The control simulation (CTL) reproduces well the synoptic pattern and track deflection of the TC. The TC moves straightly westward and northwestward without track deflection in the sensitivity experiments with the removal of total ISOs and the west-propagating ISO component, while keeping the recurving track with the removal of east-propagating ISO, which suggests that the west-propagating ISO plays a dominant role in the westward track deflection. In the experiment of removing west-propagating ISOs, an anomalous southeast–northwest-oriented wave train around the TC is modified, the mid-latitude trough decays, and the enhanced zonally elongated subtropical high is responsible for the straight northwestward motion of the TC. However, after removing a weaker convection anomaly associated with east-propagating ISOs in the form of a southwest–northeast oriented dipole circulation, the TC is affected by a sustained shallow mid-latitude trough and a west-extended ridge of subtropical high to keep the cyclonic track turning analogous to the counterpart in CTL. The piecewise potential vorticity inversion diagnosis further assesses the contribution of the different ISO components to TC track deflection.

Multiscale Processes of Heavy Rainfall over East Asia in Summer 2020: Diurnal Cycle in Response to Synoptic Disturbances

Abstract Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The Type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the Type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary-layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of Type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of Type-2 episodes. The NLLJs strengthened the mesoscale mean ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced Meiyu rainband was re-established in Central China during the Type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall.

The Feature of Ionospheric Mid-Latitude Trough during Geomagnetic Storms Derived from GPS Total Electron Content (TEC) Data

This study aims to investigate the features of the ionospheric mid-latitude trough over North America by using the MIT total electron content data obtained during three geomagnetic storms that occurred in August 2018, September 2017, and March 2015. The mid-latitude trough position sharply moves equatorward from the quiet-time subauroral latitude to mid-latitude with the decrease in SYM-H during geomagnetic storms. We find that the ionospheric behavior of TEC around the mid-latitude trough position displays three kinds of ionospheric storm effect: negative ionospheric storm effect, unchanged ionospheric behavior, and positive ionospheric storm effect. These ionospheric storm effects around the mid-latitude trough position are not always produced by the mid-latitude trough. The ionospheric storm effects produced by the mid-latitude trough are limited in the narrow mid-latitude trough regions, and are transmitted to other regions with the movement of the mid-latitude trough.

Diurnal Variations of Southerly Monsoon Surge and Their Impacts on East Asian Summer Rainfall

AbstractMonsoon southerlies can be particularly active for days and produce substantial rainfall over East Asia. These multiday episodes of southerly monsoon surge may exhibit distinct diurnal variations due to regional forcings under given large-scale conditions. This study categorizes the southerly surges into two types with different wind diurnal variations to clarify their influence on rainfall over East Asia. In the summers of 1998–2019, there are 63 episodes of southerly surges with large wind diurnal cycles and 55 episodes with small diurnal cycles. The first type of southerly surges usually occurs with anomalous low-level warming over southeastern China related to the westward extension of the western Pacific subtropical high. The second type of southerly surges instead occurs with anomalous cooling due to the deepened midlatitude trough. They thus represent the different mechanisms downscaling from large-scale conditions to regional diurnal forcings. After the onset of the first type, the intensified monsoon southerlies at night lead to the northward displacement of large-scale ascent and northward water vapor transport with warm moist energy. The monsoon rainband tends to move to the north of 35°N with a robust response in precipitation systems, especially in the meso-α-scale rain events from midnight to morning. As a comparison, the rainband stays at 30°–35°N after the onset of the second type when the strengthened large-scale ascent and water vapor convergence are located relatively south. These differences between the two types of southerly monsoon surges highlight that the multiday large-scale conditions interact with subdaily regional forcings and greatly regulate the detailed evolution of summer rainband over East Asia.

Linking ECMWF 2 m Temperature Forecast Errors with Upper-Level Circulation Situation: A Case-Study for China

Using the observational data and the forecast and reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) during 2015–2018, the temporal and spatial distribution characteristics of the 2 m temperature forecast errors of ECMWF in China, as well as their attribution to upper-level circulation, are analyzed. Results show that positive 2 m temperature forecast errors mainly occur in northwestern, northern and northeastern China, and gradually increase from January to December. This kind of error is attributed to the circulation errors associated with the circumfluence of the low-level differential winds along the Mongolian Plateau, and influenced by the changes in the mid-latitude trough and ridge with the seasons. In contrast, the negative 2 m temperature forecast errors mainly occur in the southeastern part of the Qinghai-Tibet Plateau, with the largest errors around March and October, and the smallest errors around June and December. This kind of error is associated with a series of cyclonic and anticyclonic differential circulations generated by the detouring of the mid-level differential winds along the terrain near the south side of the Plateau. The positions and intensity of these differential circulations are also influenced by the variation in the mid-level circulation with the seasons.

Magnetosphere‐Ionosphere‐Thermosphere (M‐I‐T) Coupling Leading to Equatorial Upward and Westward Drifting Supersonic Plasma Bubble Development and Amplified Subauroral Polarization Streams (SAPS) During the January 21, 2005 Moderate Storm

AbstractBy utilizing multipoint observations, we investigate the January 21, 2005 moderate storm's prompt penetration electric field (PPEF) effects in the American sector, Rayleigh‐Taylor (R‐T) instability driven polarization electric (E) field effects in the African sector, and disturbance dynamo electric field (DDEF) effects in the Northern Hemisphere. We study how these E field effects impacted the equatorial ionization anomaly (EIA) and underlying forward fountain, the midlatitude trough, and the subauroral polarization streams (SAPS). In the daytime sector during the initial phase, the PPEF development was produced by solar wind pressure enhancements. In the dusk sector, the undershielding PEFs triggered R‐T instability mechanisms, which impacted the EIA during a prereversal enhancement (PRE) like scenario and afterwards. These led to the development of supersonic plasma bubbles drifting upward and westward. Supersonic bubbles are characteristics of superstorms, and their detections further verify the January 21, 2005 moderate storm's turbulent and superstorm nature. While the disturbance dynamo‐driven anti‐Sq current system operated, the net subauroral westward currents became enhanced and the net zonal (E‐ and F‐region) drift created a deep O+ trough that deepened the midlatitude trough and led to amplified SAPS flows via positive feedback mechanisms. We conclude that strong magnetosphere‐ionosphere‐thermosphere coupling was present at equatorial latitudes where forward fountain plasma drift‐signatures were seen in the thermospheric neutral wind speed data and where the supersonic bubbles developed, and at subauroral latitudes where the midlatitude trough became deepened during the anti‐Sq current system's operation amplifying the SAPS flows by positive feedback mechanisms.

VHF Imaging Radar Observations and Theory of Banded Midlatitude Sporadic E Ionization Layers

AbstractObservations of backscatter from field‐aligned plasma density irregularities in sporadic E (Es) layers made with a 30‐MHz coherent scatter radar imager in Ithaca, New York are presented and analyzed. The volume probed by the radar lies at approximately 54° geomagnetic latitude, under the midlatitude trough and at the extreme northern edge of the zone where Es layers are prevalent. Nonetheless, the irregularities exhibit many of the characteristics of quasiperiodic echoes observed commonly at lower middle latitudes. These include a tendency to occur in elongated bands stretching from the northwest to southeast in the Northern hemisphere separated by tens of kilometers and propagating to the southwest. In addition, the irregularities were found to exhibit finer‐scale structures with secondary bands oriented nearly normally to the primary bands. We investigate the proposition that the primary bands are telltale of Es‐layer structuring caused by neutral Kelvin Helmholtz (KH) instability in the lower thermosphere and that the secondary bands signify secondary KH instability. Results from a 3D numerical simulation of KH support this proposition.