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Based on four reanalysis datasets including CMA-RA, ERA5, ERA-Interim, and FNL, this paper proposes an improved intelligent method for shear line identification by introducing a second-order zonal-wind shear. Climatic characteristics of shear lines and related rainstorms over the Southern Yangtze River Valley (SYRV) during the summers (June-August) from 2008 to 2018 are then analyzed by using two types of unsupervised machine learning algorithm, namely the /-distributed stochastic neighbor embedding method (/-SNE) and the ?-means clustering method. The results are as follows: (1) The reproducibility of the 850 hPa wind fields over the SYRV using China ' s reanalysis product CMARA is superior to that of European and American products including ERA5, ERA-Interim, and FNL. (2) Theory and observations indicate that the introduction of a second-order zonal-wind shear criterion can effectively eliminate the continuous cyclonic curvature of the wind field and identify shear lines with significant discontinuities. (3) The occurrence frequency of shear lines appearing in the daytime and nighttime is almost equal, but the intensity and the accompanying rainstorm have a clear diurnal variation: they are significantly stronger during daytime than those at nighttime. (4) Half (47%) of the shear lines can cause short-duration rainstorms (?20 mm (3h)-1), and shear line rainstorms account for one-sixth (16%) of the total summer short-duration rainstorms. Rainstorms caused by shear lines are significantly stronger than that caused by other synoptic forcing. (5) Under the influence of stronger water vapor transport and barotropic instability, shear lines and related rainstorms in the north and middle of the SYRV are stronger than those in the south.

The 2013 subseasonal asymmetry in tropical cyclone (TC) genesis over the western North Pacific (WNP) was investigated by using the 1979–2013 RSMC best track dataset. The genesis frequency of the 2013 WNP TCs between June–August (summer) and September–November (fall) manifested an abnormal temporal asymmetry: fewer typhoons (more tropical storms) in summer and more typhoons (normal tropical storms) in fall. The 2013 active summer-tropical storm genesis arose from both a failure of eastward extension of monsoon confluence region, especially in August and a lack of moisture supply for TC genesis over the eastern part of WNP, and consequently from fewer probability to reach typhoon intensity due to the westward movement of favorable location for genesis. Thereafter, the eastward extension of monsoon shear line in September and the establishment of monsoon gyre in October induced the eastward movement of favorable location for genesis which increased probability to reach typhoon intensity. The relative contribution of mid-level relative humidity to the positive GPI change played a major role in favorable condition for typhoon genesis in September (45.2%) and October (50.9%). The monsoon gyre pattern played a leading role in the most active fall-typhoon in 2013 contributing to the highest number of October-typhoon. The eastward-migration of convection mainly contributed to the subseasonal shift of TC genesis location following eastward movement of local SST warming from summer to fall under the La Nina-like neutral state. The enhanced active boreal summer intraseasonal oscillation (BSISO) in fall provided more favorable conditions for TC genesis showing about twice as many TCs occurred regarding BSISO in fall than those in summer. This spatiotemporal asymmetry in the large-scale circulations and moisture conditions between summer and fall accounted for the subseasonal shift of genesis location of TCs, and consequently for the active summer-tropical storm genesis and the active fall-typhoon genesis in 2013.

Based on ERA-Interim data from June to July during 1981–2016 and daily meteorological dataset of China Surface Meteorological Stations (V3.0), 10 typical Yangtze-Huaihe cold shear lines (YCSL) over eastern China (28°~34° N, 110°~122° E) in summer are selected, and the structural characteristics of the YCSL during the evolution process are investigated by the composite analysis. The results indicate that the YCSL is horizontally in a northeast–southwest direction and vertically inclines northward from the lower layer to the upper layer. The vertical extension of the YCSL can reach 750 hPa, and its life time is about 54 h. The evolution process of the YCSL is affected by the comprehensive configuration of the high-level, medium-level, and low-level weather systems. The southward advancement, strengthening, and eastward movement of the north branch low-pressure trough over the Yangtze-Huaihe region at 850 hPa is a key factor for the evolution of the YCSL. Because the structural characteristics of the YCSL have obvious changes in the evolution process, the evolution process can be divided into the development stage, strong stage, and weakening stage. In terms of dynamic structures, the YCSL corresponds well with the axis of the positive vorticity belt, whose center is located at 850 hPa, and reaches the maximum in the strong stage. The YCSL is located in the non-divergence zone, and there are strong convergence centers located on its south side. The YCSL also locates in the ascending motion zone between two secondary circulations on the north and south sides, with the maximum ascending velocity in the strong stage, and its large-value area presents an upright structure. In the development stage, there is an ascending motion along the YCSL, but in the strong and weakening stages there are an ascending motion below 800 hPa and a descending motion above 800 hPa along the YCSL. In terms of thermal structures, the YCSL is located in the low temperature zone of the lower layer, and there is a high temperature zone around 500 hPa. Due to the dominant role of dry and cold airflow from the north, the YCSL locates in the dry and cold air during the development and strong stages, and then the warm and moist airflow from the south invades, resulting in the weakening of the YCSL. There is a convective unstable layer on the south side of the YCSL and a neutral layer on the north side. The water vapor gathers near the YCSL, and there are two water vapor convergence centers on the east and west sides of the YCSL, respectively. The water vapor convergence zone is mainly below 600 hPa in the low troposphere and the convergence center is located at around 900 hPa. The atmospheric baroclinicity is one of the reasons for the northward inclination of the YCSL.

In this paper, European Center for Medium-Range Weather Forecasts (ECMWF) Reanalysis-Interim (ERAInterim) data and daily precipitation data in China from May to October during 1981-2016 are used to study the climatic characteristics of the meridionally oriented shear lines (MSLs) over the Tibetan Plateau (TP). The relationship between the MSL and rainstorms in the eastern TP and neighboring areas of the TP during the boreal summer half-year is also investigated. An objective method, which uses a combination of three parameters, i.e. the zonal shear of the meridional wind, the relative vorticity and the zero line of meridional wind, is adopted to identify the shear line. The results show that there are two high-occurrence centers of MSL. One is over the central TP (near 90°E) and the other is over the steep slope area of the eastern TP. Fewer MSLs are found along the Yarlung Zangbo River over the western TP and the southern Tibet. There are averagely 42.2 MSL days in each boreal summer half-year. The number of MSL days reaches the maximum of 62 in 2014 and the minimum of 22 in 2006. July and October witness the maximum of 10.2 MSL days/ year and the minimum of 4.2 MSL days/year, respectively. The annual number of the MSL days shows periodicities of 24 4 4-6 years, which is quite similar to those of the MSL rainstorm days. In the neighboring areas of the TP, nearly 56% of the MSLs lead to rainstorms, and nearly 40% of rainstorms are caused by the MSLs, indicating a close relationship between the MSLs and rainstorms in this region.

This study classifies 407 developing disturbances (DEV) and 2309 nondeveloping disturbances (NONDEV) over the western North Pacific into five large-scale circulation patterns, namely the pre-existing cyclone (PC), easterly wave (EW), zonal wind convergence (CON), zonal wind shear line (SL), and mixed zonal wind convergence and shear line (CON-SL) patterns. The SL pattern has the highest TC yield percentage, followed by the CON-SL, while the EW is the least favorable pattern. The composite analysis shows that upper-level divergence, midlevel relative humidity, and surface heat flux (SHF) growth are crucial to the disturbance development in all the five patterns. Besides, large lower-level barotropic kinetic energy conversion and a well-developed primary circulation are good indicators for disturbance development in the PC, EW, and CON rather than in the SL and CON-SL patterns. Furthermore, for the PC, EW and CON patterns, the DEV features strong and rapidly growing SHF and mesoscale convective systems (MCS) closer to the disturbance center, which allows deep-layer warming and moistening, and drives a deep secondary circulation. Interestingly, due to an environment with high lower-level vorticity, the SL and CON-SL patterns typically foster a relatively mature primary circulation with strong SHF and MCS concentrated close to the center, especially for the NONDEV at the pre-genesis stage. However, a drier mid-to-upper-level environment for the NONDEV inhibits deep convection, which may explain its shallow secondary circulation and therefore poor potential to develop further.

Science would not have progressed without the tenacity and sacrifice of many dedicated scientists, among who some go on to be unnoticed and unremembered. Scientists in this category include physicists, earth scientists, and meteorologists. The fields of meteorology and oceanography, for example, evolved from basic observations and were developed by gifted scientists into physically based theories that currently serve as foundations for these disciplines. In particular, synoptic meteorology greatly benefited from the weather map analyses of Vilhelm Bjerknes, which served as the observational basis for transforming the Norwegian cyclone model. These midlatitude cyclones are formed along the polar front, a surface separating cold polar air and warm subtropical air, a feature that was further examined by Gustav Rossby and Francis Reichelderfer. In this context, Rossby considered the link between upper-air movements, surface fronts, and ensuing downstream cyclone development. In particular, Rossby was interested in the large-scale features embedding synoptic systems, which partly inspired him to develop the theoretical concept of what is now referred to as Rossby waves. It is relevant to mention here that Bjerknes and Rossby’s investigations were essential for Jule Charney and Arnt Eliassen’s groundbreaking development of the quasi-geostrophic models of the atmosphere and the oceanic circulation, which served as the basis for the first model-based weather predictions. These great pioneering meteorologists opened the gate of scientific-based advance in meteorology by training students and young scientific researchers and fostering them to carry forward the knowledge’s flame and enlighten the road to the following generations. One student from this early generation was Chester Newton, who became a recognized expert in synoptic meteorology. Newton was mentored and advised by Gustav Rossby and Erik Palmen. Initially, Newton worked with Rossby at the University of Chicago and then later also at the International Meteorological Institute (IMI) of Stockholm University. In his early research, Newton was inspired by Rossby’s idea regarding the three-dimensional structure of the polar front and synoptic scale features, including the jet stream, which serves as a key factor linking upper-level Rossby waves and surface fronts.

Influences of intraseasonal–interannual oscillations on tropical cyclone (TC) genesis are evaluated by productivity of TC genesis (P TCG) from the developing (TCd) and nondeveloping (TCn) precursory tropical disturbances (PTDs). A PTD is identified by a cyclonic tropical disturbance with a strong-enough intensity, a large-enough maximum center, and a long-enough lifespan. The percentage value of PTDs evolving into TCd is defined as P TCG. The analysis is performed over the western North Pacific (WNP) basin during the 1990–2014 warm season (May–September). The climatological P TCG in the WNP basin is 0.35. Counted in a common period, mean numbers of PTDs in the favorable and unfavorable conditions of climate oscillations for TC genesis [such as equatorial Rossby waves (ERWs), the Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO)], all exhibit a stable value close to the climatological mean [∼31 (100 days)−1]. However, P TCG increases (decreases) during the phases of positive-vorticity (negative-vorticity) ERWs, the active (inactive)MJO, and El Niño (La Niña) years. P TCG varies from 0.17 in the most unfavorable environment (La Niña, inactive MJO, and negative-vorticity ERW) to 0.56 in the most favorable environment (El Niño, active MJO, and positive-vorticity ERW). ERWs are most effective in modulating TC genesis, especially in the negative-vorticity phases. Overall, increased P TCG is facilitated with strong and elongated 850-hPa relative vorticity overlapping a cyclonic shear line pattern, while decreased P TCG is related to weak relative vorticity. Relative vorticity acts as the most important factor to modulate P TCG, when compared with vertical wind shear and 700-hPa relative humidity.

Tibetan Plateau shear line (TPSL), categorized into two types, i.e., transverse shear line (TSL) and vertical shear line (VSL), is one of the major precipitation systems over the Tibetan Plateau (TP). Previous studies on evolution mechanism of TPSLs mainly focused on individual cases of TSLs, and did not exclude the influence of TP vortices existing on the TPSLs. In this work, 120 cases, including 60 TSLs and 60 VSLs with absence of TP vortices, are selected. The mechanisms underlying the genesis and maintenance of both TSLs and VSLs are investigated. Analyses based on the environmental conditions and potential vorticity (PV) budgets reveal that, for both types of TPSLs, in both genesis and maintenance stages, the horizontal PV flux convergence associated with the wind convergence, and the vertically uneven condensational latent heating contribute to the genesis and maintenance of TPSLs. Particularly, for TSLs, the contribution of heating effect is larger than that of the horizontal PV flux convergence, indicating the dominant role of heating effect in the evolution of TSLs; on the contrary, for VSLs, the contribution of horizontal PV flux convergence is greater than that of the heating effect, demonstrating that the evolution of VSLs is primarily determined by the horizontal PV flux convergence .

The intraseasonal variability of tropical cyclogenesis in the western North Pacific (WNP) basin is explored in this study. The relation of cyclogenesis in each of the five large-scale patterns identified in recent work by Yoshida and Ishikawa is associated with the Madden–Julian oscillation (MJO). Confirming previous results, more events of cyclogenesis are found during the active MJO phase in the WNP. Furthermore, results indicate that most of the tropical cyclogenesis is associated with the monsoon shear line large-scale pattern during the active phase. The genesis potential index (GPI) and its individual components are used to evaluate the environmental factors that most contribute toward cyclogenesis under the different phases of the MJO. GPI exhibits a large positive anomaly during the active phase of the MJO, and such an anomaly is spatially correlated with the events of cyclogenesis. The analysis of each factor indicates that low-level relative vorticity and midlevel relative humidity are the two dominant contributors to the MJO-composited GPI anomalies. The positive GPI anomalies during the active phase are partially offset by the negative contributions from vertical wind shear and potential intensity. This is valid for all five large-scale patterns. It is noteworthy that the easterly wave (EW) large-scale pattern, while exhibiting the same influence of relative vorticity and midlevel humidity contributing toward positive GPI anomalies, presents slightly more cyclogenesis events under the inactive phase of the MJO. This unexpected result suggests that other factors not included in the definition of the GPI and/or changes in environmental flows on other time scales contribute to the tropical cyclogenesis associated with the EW large-scale pattern.

Extreme precipitation is one of the most frequent and catastrophic extreme weather events in China. In the last few decades, the Yangtze-Huai Region (YHR) has experienced a number of regional extreme precipitation events (REPE) that cause serious damage to human society and natural environment. In order to better understand these events, this study identified the weather patterns responsible for REPE using spectral clustering method considering the multi-level and multi-scale characteristics of weather systems, and further investigated the circulation configurations, main formation conditions and water vapor process of REPE based on the daily precipitation data and reanalysis data from 1979 to 2018 in the YHR. The results showed that there were four weather patterns of REPE, namely low vortex shear weather pattern (Pattern 1), extratropical cyclone weather pattern (Pattern 2), surface cold front weather pattern (Pattern 3) and landfalling typhoon weather pattern (Pattern 4). The four patterns showed different features for the intensity, location and coverage of the western Pacific subtropical high and the South Asian high. For the four patterns, vertical upward motion extended from near the surface to 200 hPa, and three types of moist potential vorticity configurations were favorable for the occurrences of REPE over the YHR. Further analysis of water vapor processes showed that the water vapor flowed in via the west and south boundaries in Pattern 1–3 and the east boundary in Pattern 4. And transient water vapor transport played an important role during REPE in comparison to stationary components. Despite differences between these patterns, the precipitation conversion efficiency was significantly higher near the shear line and coastal areas, and the areas of high-value precipitation conversion efficiency did not correspond to the areas of high-value water vapor content, which needs further investigation in the future.