Lower Middle Troposphere Research Articles

Propagation and Maintenance of the Quasi-Biweekly Oscillation over the Western North Pacific in Boreal Winter

Abstract This study investigates the propagation and maintenance mechanisms of the dominant intraseasonal oscillation over the western North Pacific in boreal winter, the quasi-biweekly oscillation (QBWO). The wintertime QBWO over the western North Pacific is characterized by the westward-northwestward movement from the tropical western Pacific to the western North Pacific and resembles the n = 1 equatorial Rossby wave. Its westward migration is primarily driven by the seasonal-mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. Its northward movement is preconditioned by the vorticity dynamics of the beta effect, the low-level vertical moisture variation, and the local air–sea interaction. The latter involves the atmospheric forcing on the underlying ocean by changing the surface heat flux fluctuations and the sea surface temperature feedback on the low-level atmospheric instability. Its maintenance is primarily through atmospheric external energy sources from diabatic heating, which first generates eddy available potential energy and then converts it to eddy kinetic energy. Significance Statement Atmospheric quasi-biweekly oscillation (QBWO) is an important climate phenomenon over the western North Pacific in boreal winter. It can trigger significant influences both in the tropical and extratropical regions. Given the importance, it is necessary to investigate the propagation and maintenance mechanisms of the QBWO over the western North Pacific in boreal winter. The QBWO usually propagates northwestward from the tropical western Pacific to the western North Pacific. The westward propagation is driven by the seasonal mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. The northward propagation is caused jointly by several processes, including the atmospheric forcing on the ocean via the surface heat flux fluctuations, the feedback of ocean on the low-level atmospheric instability, the vorticity dynamics of the beta effect, and the vertical variation of low-level moisture. In addition, converting from eddy available potential energy to eddy kinetic energy is essential in maintaining the QBWO’s development. The disclosure of these crucial processes deepens the dynamics of the QBWO over the western North Pacific in boreal winter.

Extreme dry advection dominates the record-breaking Yangtze River heatwave in midsummer of 2022

The Yangtze River Valley (YRV) experienced an unprecedented heatwave in midsummer of 2022. Still, the detailed physical processes involved in the influence of abnormal large-scale atmospheric circulation on the heatwave remain unexplored. Here, we show that the positive meridional gradient of anomalous atmospheric moisture at the middle-lower troposphere and associated extreme dry air advection over the YRV are vital prerequisites for forming the 2022 YRV heatwave. The 2022 YRV heatwave is dominated by interannual variability, contributing 72.7% to the total temperature anomalies. Diagnosis of the surface heat budget equation indicates that the surface cloud radiative forcing is the most critical process in driving the 2022 YRV heatwave, which is dominated by the positive surface short-wave cloud radiative forcing associated with the suppressed precipitation and the middle-low clouds. The suppressed precipitation is induced by the vertical dynamical processes of anomalous moisture advection caused by the abnormal descending flows over the YRV, which are driven by the negative advection of anomalous latent heat energy by climatological meridional wind (anomalous dry air advection) according to the atmospheric moist static energy equation. Simulations from the Lagrangian model FLEXPART further indicate that the moisture anomaly over the north of YRV mainly originated from the surface evaporation in the YRV, implying that there is a positive land-air feedback during the life cycle of the YRV heatwave. Our study enriches the mechanism understanding of the 2022 YRV heatwave from the perspective of surface energy budget and land-air feedback.

AMO modulation of interdecadal background of persistent heavy rainfall in summer over the Huaihe River Basin

This study used observed rainfall, ERA5 reanalysis, and CMIP6 model datasets to investigate the interdecadal variation and underlying mechanism of persistent heavy rainfall (PHR) over the Huaihe River Basin (HRB) in China during July–August, and to examine the role of the Atlantic Multidecadal Oscillation (AMO) in modulating such rainfall. The results indicate that PHR over the HRB exhibited a marked interdecadal variation. The interdecadal increase in PHR was found attributable primarily to interdecadal enhancement of ascending motion, which might have been due to external forcing by the AMO. The AMO can trigger upper-tropospheric mid–high-latitude Rossby wave trains that can lead to an upper-tropospheric anticyclone and a lower-tropospheric cyclone configuration over the HRB. Increased warm advection and greater transport of water vapor from lower latitudes, caused by enhanced southerlies in the middle–lower troposphere over the HRB, can also lead to enhanced ascending motion over the HRB. Meanwhile, an anomalous cyclone over northeastern China triggered by AMO can transport cold air from higher latitudes to the HRB, boosting convection and promoting the development and duration of PHR. By affecting sea surface temperature in the western North Pacific Ocean, the AMO can also indirectly cause a meridional teleconnection pattern in the lower troposphere, which increases the transport of water vapor to the HRB. Numerical model experiments can reproduce both the interdecadal variations in PHR and the mechanisms of the influence of the AMO on PHR, providing a reliable foundation for understanding and forecasting PHR over the HRB.

The relationship between interannual variability of winter surface solar radiation over the Southeast Asian low-latitude highlands and the circumglobal teleconnection

Using ERA5 reanalysis data, this study reveals the relationship between variations of winter surface solar radiation (SSR) over the Southeast Asian low-latitude highlands (SEALLH) and the circumglobal teleconnection (CGT). The results indicate that the winter SSR shows significant interannual variability with a monopole pattern over the SEALLH. The winter CGT significantly modifies the interannual variability of the simultaneous SSR over the SEALLH. When the CGT is in its positive phase, a wave train appears over the central-eastern Mediterranean, Arabian Peninsula, Indian Peninsula, and East Asia with anomalous “northerly, southerly, northerly, southerly” nondivergent meridional wind. The associated southerly wind anomalies around the SEALLH favor greater transport of water vapor from the Bay of Bengal that converges over the SEALLH. The consequent increase of cloud cover in the lower–middle troposphere and aerosol optical depth (AOD) reduce the winter SSR over the SEALLH. When the CGT is in its negative phase, the anomalous northerly wind with anomalous divergence increases the winter SSR over the SEALLH by decreasing both cloud cover in the lower–middle troposphere and the AOD.

Factors responsible for the increasing trend of mei-yu season rainfall during 1979–2020 over the western and eastern mei-yu domain

The mei-yu season (June–July) rainfall over the mei-yu monitoring domain (MMD) in the Yangtze–Huaihe Basin has shown an increasing trend in recent decades. This study examines the dominant factors responsible for this increasing trend for the period 1979–2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation. Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD, the magnitude of the trends is slightly larger over the eastern MMD (EMMD) than over the western MMD (WMMD). Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD. The increasing rainfall trend over the WMMD (EMMD) is attributable to increased evaporation (enhanced vertical moisture advection), which is dependent on an anomalous cyclonic circulation in the middle-lower troposphere over the MMD. Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 600 hPa over the EMMD, leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa. By contrast, the anomalous cyclone favors increasing local evaporation over the WMMD.摘要近几十年来, 江淮流域梅雨监测区 (MMD) 的梅雨期 (6–7月) 降水呈增加趋势. 本文基于1979–2020年台站观测降水资料和ERA5再分析数据, 从大气环流变异的角度揭示了这种长期增加趋势的主要影响因素. 发现在MMD范围内, 梅雨期降水趋势的增幅东部大于西部. 水汽收支定量诊断表明, 异常的蒸发和水汽平流对MMD西部和东部降水增加趋势的相对贡献是不同的. MMD西部 (东部) 的降水趋势主要归咎于增强的局地蒸发 (增强的垂直水汽平流) , 后者又取决于MMD对流层中, 低层的异常气旋环流. 这种位于气候平均的西太平洋副热带高压西北侧的异常气旋有助于MMD东部600 hPa以上的水汽辐散增加, 伴随加强的850 hPa水汽辐合, 从而导致垂直水汽平流的增强. 相反, 该异常气旋则有利于增强MMD西部的局地蒸发.

Characteristics and circulation patterns for wet and dry compound day-night heat waves in mid-eastern China

Compound day-night heat waves (CohotEs) have more adverse effects on human health and ecosystem, and the consequences vary with humidity. This study classifies the CohotEs over Chinese mainland into wet and dry CohotEs, and compares their characteristics and associated circulation anomalies. Here, a populated region of mid-eastern China (MEC) with higher occurrence of both types of CohotEs is mainly focused. Results show that, during past six decades, wet CohotEs have higher frequency, longer duration and broader spatial scale, while dry CohotEs are less, shorter and more regional. Both types of CohotEs are associated with anticyclonic anomalies accompanied by the strengthening and northwestern expansion of western Pacific subtropical high (WPSH), leading to adiabatic heating with anomalous subsidence. The anomalies in wet CohotEs are influenced by the eastward expansion of East Asian westerly jet stream and northeastward expansion of the south Asia high. Anomalous southwesterlies transport additional water vapor from the tropics, leading to above-normal humidity that could reduce the amplitudes of anomalous subsidence but favor heat maintenance. By contrast, dry CohotEs are related to a more stable barotropic anticyclonic structure, and the anomalous signals are stronger in the middle-lower troposphere. Meanwhile, tropical cyclones appear over the South China Sea below 500 hPa, promoting the intensification of northwestward shifts of WPSH and anomalous northeasterlies over South China. So, lower humidity results from weakened northward water vapor transport could lead to faster heat dissipation and thus shorter duration for dry CohotEs. Our results emphasize the diversity of CohotEs, indicating that different types of CohotEs should be analyzed separately and multiple local and large-scale circulations need to be considered for better forecasts and predictions of CohotEs. • Characteristics of wet and dry compound hot extremes (CohotEs) are analyzed. • Circulation anomalies of wet and dry CohotEs are compared over mid-eastern China. • Anomalous anticyclone and western Pacific subtropical high (WPSH) cause CohotEs. • Wet CohotEs are influenced by eastward (westward) shifts of south Asia high (WPSH). • Dry CohotEs are related to shifts of WPSH and tropic cyclonic anomaly.

Effect of circulation variation associated with East Asian jet on spring rainfall over North China and Yangtze-Huaihe River Valley

The present study investigates the association of East Asian westerly jet (EAWJ) variations with spring rainfall anomalies in Northern China and Yangtze-Huaihe River Valley (NC-YHV) and the dynamics using reanalysis datasets. Based on the climatology and interannual variation in 200-hPa zonal winds, the index EAWJI is defined as the average 200-hPa zonal wind velocity over a zonal 10-degree-width belt centered around the seasonal-mean jet axis between 105°E and 145°E. Associated with anomalously strengthened EAWJ, significant negative rainfall anomalies are observed over NC-YHV. The dynamics are as follows. When the EAWJ is anomalously intensified, a quasi-barotropic Pacific-Japan-like (PJ) teleconnection along coastal China and an associated anomalous westerly flow over NC-YHV are observed. In middle-lower troposphere, Tibetan Plateau (TP) drastically reduces the anomalous southwesterly momentum transported into NC-YHV, turning the westerly anomalies into northwesterly anomalies. The anomalous northwesterly winds over NC-YHV advect cold and dry air southeastward toward NC-YHV, which induce downward motion (diabatic heating feedback is weak) and negative moisture anomalies, respectively, and thus cause reduced rainfall anomalies over NC-YHV. Anomalous winter SSTAs in western Pacific and tropical Indian Ocean associated with ENSO are sustained until spring, inducing barotropic waves that propagate northwards to cause EAWJ-associated circulation anomalies and thus bring about spring rainfall anomalies in NC-YHV. The quasi-barotropic features of the EAWJ-associated circulation anomalies and their association with the northward propagation of tropical SSTA-induced barotropic waves together suggest that EAWJ-associated circulation variations are at least partly among the external forcings responsible for spring rainfall anomalies in NC-YHV.

Summer Regional Pentad Extreme Precipitation in Eastern China and Their Possible Causes

Persistent extreme precipitation covering a large area usually causes severe flooding disasters in China, but how to depict it and what are the possible causes are still open questions. With Climate Prediction Center global unified gauge-based analysis of daily precipitation and NCEP/NCAR daily reanalysis dataset from 1979 to 2019, summer regional pentad extreme precipitation (RPEP) is defined according to the threshold of the 95th percentile of pentad precipitation with more than 5% land grids coverage in eastern China. While the definition of RPEP highlights the climate features of both the persistence and the regionality of extreme precipitation, it is distinctly different from the previous definitions that mainly reflect the synoptic aspects with daily data and have strictly temporal-spatial constraints. Four categories of RPEPs are objectively identified by K-means cluster analysis, i.e., South China (SC), South of Yangtze River (SYR), Jianghuai River (JHR), and North China (NC). Along the Yangtze River (SYR and JHR), intensity and area of RPEP are positively correlated with each other, and with the increase of RPEP intensity, its center of gravity tends to move eastward in all the four cluster regions and southward in Jianghuai River and North China, respectively, and vice versa. The RPEPs mostly persist for one pentad but can reach up to two to three pentads at most, and along with the duration of RPEP, its intensity and area are both enhanced accordingly. Furthermore, the frequency of RPEP increased significantly since the late 1990s in SYR, JHR, and SC. Associated with RPEP, strong pentad-mean convergence and ascending motion occur in the middle-lower troposphere, and except for SC that is dominated by the local low-pressure and cyclone anomalies, the other three cluster regions are all forced by the western Pacific subtropical high to the southeast and weak low-pressure trough to the north, and the low-level anticyclone anomaly to the southeast transports abundant water vapors to the RPEP regions accordingly. Besides, all the RPEPs are closely in accordance with obvious subseasonal oscillations, especially the 10–30-day and 30–60-day oscillations, which can be regarded as the potential sources of RPEP predictability in eastern China.

Environmental Controls on Tropical Mesoscale Convective System Precipitation Intensity

AbstractUsing multiple independent satellite and reanalysis datasets, we compare relationships between mesoscale convective system (MCS) precipitation intensity Pmax, environmental moisture, large-scale vertical velocity, and system radius among tropical continental and oceanic regions. A sharp, nonlinear relationship between column water vapor and Pmax emerges, consistent with nonlinear increases in estimated plume buoyancy. MCS Pmax increases sharply with increasing boundary layer and lower free tropospheric (LFT) moisture, with the highest Pmax values originating from MCSs in environments exhibiting a peak in LFT moisture near 750 hPa. MCS Pmax exhibits strikingly similar behavior as a function of water vapor among tropical land and ocean regions. Yet, while the moisture–Pmax relationship depends strongly on mean tropospheric temperature, it does not depend on sea surface temperature over ocean or surface air temperature over land. Other Pmax-dependent factors include system radius, the number of convective cores, and the large-scale vertical velocity. Larger systems typically contain wider convective cores and higher Pmax, consistent with increased protection from dilution due to dry air entrainment and reduced reevaporation of precipitation. In addition, stronger large-scale ascent generally supports greater precipitation production. Last, temporal lead–lag analysis suggests that anomalous moisture in the lower–middle troposphere favors convective organization over most regions. Overall, these statistics provide a physical basis for understanding environmental factors controlling heavy precipitation events in the tropics, providing metrics for model diagnosis and guiding physical intuition regarding expected changes to precipitation extremes with anthropogenic warming.

Relationships Between Jianghuai Meiyu Anomaly and the Collaborative Evolution of Wave Trains in the Upper and Lower Troposphere in Mid-July of 2020

Based on the ECMWF reanalysis dataset (ERA5), SST , OLR and the daily precipitation data, the relationship between the Meiyu anomaly in the Yangtze-Huaihe river basin (YHB) and the collaborative evolution of wave trains in the upper and lower troposphere in the mid-July of 2020 is analyzed. The results show that the key circulation background for the long-lasting Meiyu season in the summer of 2020 is the coexistence and collaborative evolution of the Silk-Road (SR) wave train in the upper troposphere and the – + Pacific-Japan (PJ) wave train in the middle-lower troposphere, which is in favor of the movement towards each other of the South Asia high (SAH) and the western Pacific subtropical high (WPSH), resulting in the upper-level divergence over the YHB where is the right side of the jet entrance area. The WPSH strengthens and extends westward, transporting abundant water vapor to the YHB from the Western Pacific and South China Sea. Meanwhile, the meridional circulation develops at the middle and high latitudes, which is in favor of cold-dry air and warm-moist air strongly interact over the YHB, leading to the long-lasting Meiyu season. The coordinative evolution of the SR and PJ wave trains not only affects the persistence and intensity of precipitation in the YHB, but also can be regarded as a precursor signal for the occurrence of persistent heavy rainfall to some extent. The wave-activity flux dispersing eastward along the upper-level jet is beneficial to the development and maintenance of the SR wave train, and the poleward dispersing wave-activity flux is related to the development and maintenance of the PJ wave train. Additionally, along with the weakening El Nino in 2019, the anomalous convection caused by the warm SST in the Western Pacific warm pool is conducive to the formation and maintenance of the PJ wave train. The abnormal convection caused by the warm SST in the Indian Ocean and the east coast of the tropical Atlantic may affect the SR wave train. These are also one of the reasons for the long Meiyu season in 2020.

Anomalous holiday precipitation over southern China

Abstract. The Chinese Spring Festival (CSF, also known as the Chinese New Year or Lunar New Year) is the most important festival in China. Lunar New Year's Day (LNYD) is the first day of the Lunar New Year. Traditionally, the CSF holiday begins a couple of days before LNYD and ends on lantern day, lasting for approximately 2 weeks. In this paper, based on the long-term station observations from 1979 to 2012, the precipitation during the holiday over southern China (108–123° E and 21–33° N, 155 stations) tends to be lower than that before and after the holiday. The mean precipitation frequency anomaly from the fourth day to the sixth day after LNYD (i.e., days [+4, +6]) decreases by −7.4 %. Simultaneously, the daily precipitation amount experiences a reduction of −0.62 mm day−1 during days [+2, +5]. The holiday precipitation anomalies are strongly linked to the anomalies of relative humidity (ΔRH) and cloud cover. The station observations of the ΔRH show an evident decrease from day +2 to day +7, and a minimum appears on days [+4, +6], with a mean of −3.9 %. The ΔRH vertical profile displays significant drying below approximately 800 hPa. Between 800 and 1000 hPa, the mean ΔRH is −3.9 %. The observed station daytime low cloud cover (LCC) evidently decreases by −6.1 % during days [+4, +6]. Meanwhile, the ERA-Interim daily LCC also shows a comparable reduction of −5.0 %. The anomalous relative humidity is mainly caused by the decreased water vapor in the lower-middle troposphere. Evident negative specific humidity anomalies persist from day −3 to day +7 in the station observations. The average specific humidity anomaly for days [+4, +6] is −0.73 g kg−1. When the precipitation days are excluded, the anomaly remains significant at −0.46 g kg−1. A significant water vapor deficit is observed in the lower troposphere below 700 hPa. Between 800 and 1000 hPa, the mean specific humidity drops by −0.70 g kg−1. This drier lower-middle troposphere is due to anomalous northerly winds, which are closely related to the cyclonic circulation anomaly over the northwestern Pacific. The time-lag correlation demonstrates that approximately 1 week after a lower temperature occurs over eastern China, a stronger cyclone is observed over the western Pacific. The possible mechanism needs further clarification through elaborate observation and numerical modeling.

Increasing persistent haze in Beijing: potential impacts of weakening East Asian winter monsoons associated with northwestern Pacific sea surface temperature trends

Abstract. Over the past decades, Beijing, the capital city of China, has encountered increasingly frequent persistent haze events (PHE). While the increased pollutant emissions are considered as the most important reason, changes in regional atmospheric circulations associated with large-scale climate warming also play a role. In this study, we find a significant positive trend of PHE in Beijing for the winters from 1980 to 2016 based on updated daily observations. This trend is closely related to an increasing frequency of extreme anomalous southerly episodes in North China, a weakened East Asian trough in the mid-troposphere and a northward shift of the East Asian jet stream in the upper troposphere. These conditions together depict a weakened East Asian winter monsoon (EAWM) system, which is then found to be associated with an anomalous warm, high-pressure system in the middle–lower troposphere over the northwestern Pacific. A practical EAWM index is defined as the seasonal meridional wind anomaly at 850 hPa in winter over North China. Over the period 1900–2016, this EAWM index is positively correlated with the sea surface temperature anomalies over the northwestern Pacific, which indicates a wavy positive trend, with an enhanced positive phase since the mid-1980s. Our results suggest an observation-based mechanism linking the increase in PHE in Beijing with large-scale climatic warming through changes in the typical regional atmospheric circulation.

The three-dimension intensity index for western Pacific subtropical high and its link to the anomaly of rain belt in eastern China

Western Pacific subtropical high (WPSH) is a deep circulation system located at middle-lower troposphere over the East Asia. The anomalies of its position and intensity determine the shift of climates in eastern China, especially the summer rainfall belt. However, previous definitions of WPSH intensity index were limited mainly by a single threshold of 5880-gpm geopotential height contour at 500-hPa, which cannot denote the rain pattern in eastern China reasonably, in particular the floods and droughts along the Yangtze River valley. In this paper, the link between the intensity of East Asian summer monsoon (EASM) and the geopotential heights at each level were analyzed by correlation analyses. The results indicate that the significant negative correlations spread obviously from the upper to lower levels with longitude changed from west to east. At the upper levels over the Asian continent, the most significant negative correlation was found at the central region of the South Asia High (SAH). This implies that the EASM would be weaker if the SAH was stronger, and vice versa. Over the western Pacific warm pool, the most significant negative correlation was found in the middle-lower troposphere. The correlation coefficients at 500-hPa were much weaker than those at 700-hPa and 850-hPa, indicating that the EASM was easier influenced by the subtropical high at lower levels. Similar, the relationship between the WPSH and the precipitation along the Yangtze River valley was more significant at lower levels indicated by the correlation coefficient. In this study, the representativeness of the threshold of 5880 gpm at 500-hPa level over the western Pacific subtropical regions was analyzed at different grids and seasons, and the limitations of previous definitions were also discussed. Considering the three-dimension (3D) structures of the subtropical high, a new WPSH intensity index was defined by integrated the standardized geopotential heights from 500-hPa to 1000-hPa levels over the region (10°–30°N, 110°–150°E). Compared with the two traditional WPSH indices, the new 3D WPSH intensity index can better represent the subtropical summer monsoon and the precipitation along the middle and lower reaches of the Yangtze River. Although all the three indices could reflect the primary correlation patterns between WPSH and the precipitation in eastern China, only the correlation coefficient between the 3D intensity index and precipitation is significant at the 95% confidence level to pass the student t -test. Compared to the other two indices, the 3D WPSH index can better reflect the anomalous of both meridional and zonal water vapor transportation between the tropical and subtropical regions of East Asia. During the transition seasons of boreal spring and autumn, the mechanisms of anomalous precipitation are more complicated, which are obviously different from EASM. Preliminary analysis shows that even in these two transition seasons, the 3D WPSH index defined in this paper also has better link with the precipitation anomalies in eastern China. As mentioned before, the WPSH is a deep circulation system over the western Pacific. Besides its intensity, the position also impacts on the climates over East Asia, especially its west boundary and meridional axis. In the future study, the definitions of other parameters of WPSH need to be studied from its 3D structure.

Cause and predictability for the severe haze pollution in downtown Beijing in November–December 2015

Based on the hourly PM2.5 concentrations, meteorological variable records and ERA-Interim reanalysis data, a series of diagnostic analyses were conducted to explore the possible meteorological causes for the severe haze pollution that occurred in Beijing in November–December 2015. Using the online-coupled WRF-Chem model and GFS data, the predictability of hourly and daily PM2.5 concentrations was evaluated. The results showed that, in the context of pollutant emission, the severe haze pollution in downtown Beijing in November–December 2015 was primarily attributed to anomalous local meteorological conditions, which were caused and strengthened by anomalous large-scale atmospheric circulations. The abnormal changes in the upper troposphere appeared to trigger the anomalies in the middle-lower troposphere and the local conditions. The numerical simulations can capture the spatial distribution patterns of the PM2.5 concentrations for predictions of 1 to 10days in advance. The PM2.5 concentration trends in downtown Beijing were generally consistent with the predictions on both daily and hourly time-scales, although the predictability decreased gradually as the lead times prolonged. The predictability of the daily mean PM2.5 concentration was slightly higher than that of the hourly concentration. The statistical indices suggested that the predictions of daily and hourly mean PM2.5 concentrations were generally skillful and reliable for maximum lead times of 8 and 5days, respectively.

Evaluation of atmospheric precipitable water from reanalysis products using homogenized radiosonde observations over China

Many multidecadal atmospheric reanalysis products are available now, but their consistencies and reliability are far from perfect. In this study, atmospheric precipitable water (PW) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), NCEP/Department of Energy (DOE), Modern Era Retrospective-Analysis for Research and Applications (MERRA), Japanese 55 year Reanalysis (JRA-55), JRA-25, ERA-Interim, ERA-40, Climate Forecast System Reanalysis (CFSR), and 20th Century Reanalysis version 2 is evaluated against homogenized radiosonde observations over China during 1979–2012 (1979–2001 for ERA-40). Results suggest that the PW biases in the reanalyses are within ∼20% for most of northern and eastern China, but the reanalyses underestimate the observed PW by 20%–40% over western China and by ∼60% over the southwestern Tibetan Plateau. The newer-generation reanalyses (e.g., JRA25, JRA55, CFSR, and ERA-Interim) have smaller root-mean-square error than the older-generation ones (NCEP/NCAR, NCEP/DOE, and ERA-40). Most of the reanalyses reproduce well the observed PW climatology and interannual variations over China. However, few reanalyses capture the observed long-term PW changes, primarily because they show spurious wet biases before about 2002. This deficiency results mainly from the discontinuities contained in reanalysis relative humidity fields in the middle-lower troposphere due to the wet bias in older radiosonde records that are assimilated into the reanalyses. An empirical orthogonal function (EOF) analysis revealed two leading modes that represent the long-term PW changes and El Nino–Southern Oscillation-related interannual variations with robust spatial patterns. The reanalysis products, especially the MERRA and JRA-25, roughly capture these EOF modes, which account for over 50% of the total variance. The results show that even during the post-1979 satellite era, discontinuities in radiosonde data can still induce large spurious long-term changes in reanalysis PW and other related fields. Thus, more efforts are needed to remove spurious changes in input data for future long-term reanalyses.

Water Vapor Transport and Moisture Budget over Eastern China: Remote Forcing from the Two Types of El Niño

Abstract The water vapor transport and moisture budget over eastern China remotely forced by the cold-tongue (CT) and warm-pool (WP) El Niño show striking differences throughout their lifetime. The water vapor transport response is weak in the developing summer but strong in the remaining phases of CT El Niño, whereas the opposite occurs during WP El Niño. WP El Niño causes a moisture deficit over the Yangtze River valley (YZ) in the developing summer and over southeastern China (SE) in the developing fall, whereas CT El Niño induces a moisture surplus first over SE during the developing fall with the influential area expanding in the decaying spring and shifting northward in the decaying summer. It is the divergence of meridional water vapor transport that dominates the total water vapor divergence anomaly, with the divergence of zonal transport showing an opposite pattern with smaller magnitude. Investigation of the vertical profile of moisture budget shows a great baroclinicity, with the strongest abnormal moisture budget occurring in different levels. The moisture transport via the southern boundary plays a crucial role in the regional moisture budget anomalies and is located near the surface over SE, in the lower troposphere over the YZ, and at the lower-middle troposphere over the eastern part of northern China. The enhanced moisture surplus near the surface forced by WP El Niño over SE in the mature winter and decaying spring is offset by a moisture deficit within the lower-middle troposphere due to a diverse response circulation at different vertical levels.

Effects of Monsoon Trough Intraseasonal Oscillation on Tropical Cyclogenesis over the Western North Pacific*

Abstract The effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough on tropical cyclone (TC) formation were investigated using the Advanced Research Weather Research and Forecasting (ARW) Model. A weak vortex was specified initially and inserted into the background fields containing climatological-mean anomalies associated with active and inactive phases of monsoon trough ISOs. The diagnosis of simulations showed that monsoon trough ISO can modulate TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the lower–midtropospheric large-scale vorticity associated with monsoon trough ISO. Interactions between cyclonic vorticity in the lower middle troposphere during the active ISO phase and a vortex lead to the generation of vortex-scale outflow at the midlevel, which promotes the upward penetration of friction-induced ascending motion and thus upward moisture transport. In addition, the low-level convergence associated with active ISO also helps the upward moisture transport. Both processes contribute to stronger diabatic heating and thus promote a positive convection–circulation–moisture feedback. On the other hand, the large-scale flow associated with inactive ISO suppresses upward motion near the core by inducing the midlevel inflow and the divergence forcing within the boundary layer, both inhibiting TC development. The thermodynamic impact comes from greater background specific humidity associated with active ISO that allows a stronger diabatic heating. Experiments that separated the dynamic and thermodynamic impacts of the ISO showed that the thermodynamic anomaly from active ISO contributes more to TC development, while the dynamic anomalies from inactive ISO can inhibit vortex development completely.

Discrepancies in tropical upper tropospheric warming between atmospheric circulation models and satellites

Recent studies have examined tropical upper tropospheric warming by comparing coupled atmosphere–ocean global circulation model (GCM) simulations from Phase 3 of the Coupled Model Intercomparison Project (CMIP3) with satellite and radiosonde observations of warming in the tropical upper troposphere relative to the lower-middle troposphere. These studies showed that models tended to overestimate increases in static stability between the upper and lower-middle troposphere. We revisit this issue using atmospheric GCMs with prescribed historical sea surface temperatures (SSTs) and coupled atmosphere–ocean GCMs that participated in the latest model intercomparison project, CMIP5. It is demonstrated that even with historical SSTs as a boundary condition, most atmospheric models exhibit excessive tropical upper tropospheric warming relative to the lower-middle troposphere as compared with satellite-borne microwave sounding unit measurements. It is also shown that the results from CMIP5 coupled atmosphere–ocean GCMs are similar to findings from CMIP3 coupled GCMs. The apparent model-observational difference for tropical upper tropospheric warming represents an important problem, but it is not clear whether the difference is a result of common biases in GCMs, biases in observational datasets, or both.

Analyzing and forecasting climate change in Harbin City, Northeast China

Based on sounding data from 1975 to 2005 and TM/ETM+ remote sensing images in 1989, 2001 and 2007, the climate changes in Harbin City, Northeast China in recent 30 years were analyzed and forecasted. Results show that in the lower troposphere the meridional wind speed and mean annual wind speed decrease, and in the lower stratosphere the temperature decreases while the meridional wind speed increases significantly. In the study area, the climate is becoming warmer and wetter in the middle lower troposphere. The expansion of urban area has great effects on the surface air temperature and the wind speed, leading to the increase of the surface air temperature, the decrease of the surface wind speed, and the increase of the area of urban high temperature zone. The quantitative equations have been established among the surface air temperature, the carbon dioxide (CO2) concentration and the specific humidity (the water vapor content). It is predicted that the future increasing rate of the surface air temperature is 0.85°C/10yr if emission concentration of CO2 remains unchanged; if emission concentration of CO2 decreases to 75%, 50% and 25%, respectively, the surface air temperature will increase 0.65°C/10yr, 0.46°C/10yr and 0.27°C/10yr, respectively. The rise of the surface air temperature in the study area is higher than that of the global mean temperature forecasted by IPCC.

A Simple Multicloud Parameterization for Convectively Coupled Tropical Waves. Part II: Nonlinear Simulations

Abstract Observations in the Tropics point to the important role of three cloud types, congestus, stratiform, and deep convective clouds, besides ubiquitous shallow boundary layer clouds for both the climatology and large-scale organized anomalies such as convectively coupled Kelvin waves, two-day waves, and the Madden–Julian oscillation. Recently, the authors have developed a systematic model convective parameterization highlighting the dynamic role of the three cloud types through two baroclinic modes of vertical structure: a deep convective heating mode and a second mode with lower troposphere heating and cooling corresponding respectively to congestus and stratiform clouds. The model includes both a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep convective precipitation and also a nonlinear switch that favors either deep or congestus convection depending on whether the lower middle troposphere is moist or dry. Here these model convective parameterizations are applied to a 40 000-km periodic equatorial ring without rotation, with a background sea surface temperature (SST) gradient and realistic radiative cooling mimicking a tropical warm pool. Both the emerging “Walker cell” climatology and the convectively coupled wave fluctuations are analyzed here while various parameters in the model are varied. The model exhibits weak congestus moisture coupled waves outside the warm pool in a turbulent bath that intermittently amplify in the warm pool generating convectively coupled moist gravity wave trains propagating at speeds ranging from 15 to 20 m s−1 over the warm pool, while retaining a classical Walker cell in the mean climatology. The envelope of the deep convective events in these convectively coupled wave trains often exhibits large-scale organization with a slower propagation speed of 3–5 m s−1 over the warm pool and adjacent region. Occasional much rarer intermittent deep convection also occurs outside the warm pool. The realistic parameter regimes in the multicloud model are identified as those with linearized growth rates for large scale instabilities roughly in the range of 0.5 K day−1.