In 21 consecutive patients with confirmed Covid-19, the median times from symptom onset to negative viral culture and negative real-time RT-PCR were 7 days and 34 days, respectively. The longest ...interval from symptom onset and from resolution of fever to positive culture was 12 days and 3 days, respectively.
Phase Two of the Integrative Monsoon Frontal Rainfall Experiment (IMFRE-II) was conducted over the middle and lower reaches of the Yangtze River during the period 16 June to 19 July 2020. This paper ...provides a brief overview of the IMFRE-II field campaign, including the multiple ground-based remote sensors, aircraft probes, and their corresponding measurements during the 2020 mei-yu period, as well as how to use these numerous datasets to answer scientific questions. The highlights of IMFRE-II are: (1) to the best of our knowledge, IMFRE-II is the first field campaign in China to use ground-based, airborne, and spaceborne platforms to conduct comprehensive observations over the middle and lower reaches of the Yangtze River; and (2) seven aircraft flights were successfully carried out, and the spectra of ice particles, cloud droplets, and raindrops at different altitudes were obtained. These in-situ measurements will provide a “cloud truth” to validate the ground-based and satellite-retrieved cloud and precipitation properties and quantitatively estimate their retrieval uncertainties. They are also crucial for the development of a warm (and/or cold) rain conceptual model in order to better understand the cloud-to-rain conversion and accretion processes in mei-yu precipitation events. Through an integrative analysis of ground-based, aircraft, and satellite observations and model simulations, we can significantly improve our cloud and precipitation retrieval algorithms, investigate the microphysical properties of cloud and precipitation, understand in-depth the formation and dissipation mechanisms of mei-yu frontal systems, and improve cloud microphysics parameterization schemes and model simulations.
The summer precipitation over the southeastern Tibetan Plateau (SETP) exhibits significant diurnal variability, with a notable peak before midnight. This study investigates how thermally forced ...circulations regulate the diurnal precipitation over the SETP, using surface rain‐gauge observations and high‐resolution reanalysis data from June to August during 2015–2019. Water vapor supply and precipitation onset in the afternoon are associated with the upslope winds and upward motions of the mountain–plains solenoid (MPS) between the SETP and nearby lowlands (the Sichuan Basin and Yunnan–Guizhou Plateau). After sunset, the sustained easterly and stronger southerly winds below 500 hPa transport abundant moisture from the rainy lowlands to the SETP. The findings suggest that these winds are mainly due to the larger zonal continental‐scale thermally forced circulation combined with the meridional large‐scale MPS before midnight and the meridional small‐scale MPS after midnight.
Plain Language Summary
The summer rainfall over the southeastern Tibetan Plateau (SETP) is crucial to the hydrological cycle of both the SETP and nearby regions. However, it remains unclear why the rainfall over the SETP mostly occurs at night due to limited observations. This study aims to investigate the role of thermally driven winds in diurnal rainfall during summer (June–August) over the SETP, using high‐resolution reanalysis data and surface rain‐gauge observations. The results show that the strong upward motions and low‐level upslope winds of the thermally forced mountain‐valley wind systems (MVWS) between the SETP and the lowlands (Sichuan Basin and Yunnan–Guizhou Plateau) help to trigger the precipitation and transport moisture in the afternoon. After sunset, thermally forced circulations, including the east–west continental‐scale thermal circulation between the Asian continent and its adjacent ocean, the north–south large‐scale MVWS between the Yunnan–Guizhou Plateau and its southern plain, and the north–south small‐scale MVWS between the SETP and its southern slope, help maintain horizontal winds from the rainy lowlands, transporting abundant moisture to facilitate the rainfall over the SETP at night.
Key Points
Thermally forced circulations greatly impact the diurnal cycle of summer precipitation over the southeastern Tibetan Plateau (SETP)
Daytime rainfall over this region is associated with the upward motions and upslope winds of the mountain–plains solenoid
Multi‐scale thermal circulations are responsible for moisture transport of nighttime rainfall over the SETP
Mei-yu fronts are often accompanied with prominent diabatic heating due to the development of frontal clouds and rain bands. The direct effect and relative importance of diabatic heating on the ...spatiotemporal morphology of Mei-yu fronts however remain unclear. Here a new frontogenesis function is derived to isolate the effect of diabatic heating and this function is then applied to the latest high resolution reanalysis product ERA5 to conduct an initial survey of dynamic and thermodynamic processes driving the intensity and structure evolutions of three typical Mei-yu fronts. It is found that the direct effect of latent heating (moisture depletion) is always frontogenetical (frontolytical) in the pre-frontal and frontal zone throughout the lifecycle of the front with latent heating (moisture depletion) in general dominates the front intensification (dissipation). Tilting is another critical process that turns the vertical gradient of equivalent potential temperature into horizontal gradient leading to frontogenesis during the front development stage, and, after the release of convective instability ahead of the front, flattens the front surface leading to frontolysis during the front decaying stage. Therefore titling maintains consistently positive contributions to the evolution of a Mei-yu front and its importance depends highly on the convective intensity near the front. The deformation effect appears frontogenetical but carries a much smaller weight compared to diabatic heating, moisture depletion and tilting. The structure evolutions of the three Mei-yu fronts studied here exhibit two distinct patterns: “bending and breaking”, and “moving and rotating”. Both patterns are dominated by the front propagation (i.e., frontogenesis through air parcels), further highlighting the critical roles played by diabatic heating, moisture depletion and titling in determining the spatiotemporal characteristics of Mei-yu fronts. As these three processes are all closely tied to updrafts, clouds and precipitation near the fronts, improved representations of clouds and convection are needed to accurately depict their feedbacks to frontal evolutions and ultimately achieve better predictions of heavy rainfall and floods associated with Mei-yu fronts.
Mei-yu fronts are accompanied with the formation of frontal clouds. In addition to its direct effect on Mei-yu fronts through the modification of atmospheric temperature, diabatic heating related to ...clouds formation also influences frontogenesis indirectly by changing local atmospheric circulation. To quantify such indirect effects, piecewise potential vorticity (PV) inversion is adopted to decompose the process of deformation frontogenesis into several parts associated with distinct PV anomalies. The balanced flow associated with the interior-level diabatic PV anomaly emerges as the most stable and important contributor to the total deformation frontogenesis with the effect of local diabatic PV anomaly in the frontal zone outweighing the effect of remote diabatic PV anomaly. Lower-boundary thermal anomaly (i.e., surface cooling associated with frontal clouds formation) and mean flow provide weak negative and positive contributions to the deformation frontogenesis, respectively. The balanced flow associated with the upper-level PV perturbations is weak at lower-levels, especially in the vicinity of the front zone and thus has negligible contributions to the Mei-yu frontogenesis. The indirect effect of diabatic heating on Mei-yu frontogenesis is generally weaker in magnitude compared to the direct effect of temperature modification as well as the impact of moisture depletion that is also tied to clouds formation. The results presented here add further evidences about the importance of cloud feedback to the evolution of Mei-yu fronts and suggest the necessity of improved model representations of cloud processes in achieving a better simulation and prediction of Mei-yu rainfall.
Here, we analyze the characteristics and the formation mechanisms of low-level jets (LLJs) in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data ...and the fifth generation of the European Centre for Medium-Range Weather Forecasts (ERA5) reanalysis dataset. Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets (BLJs) concentrated at heights of 900–1200 m. The BLJs occur most frequently at 2300 LST (LST=UTC+ 8 hours) but are strongest at 0200 LST, with composite wind velocities >14 m s
−1
. Synoptic-system-related LLJs (SLLJs) occur most frequently at 0800 LST but are strongest at 1100 LST, with composite wind velocities >12 m s
−1
. Both BLJs and SLLJs are characterized by a southwesterly wind direction, although the wind direction of SLLJs is more westerly, and northeasterly SLLJs occur more frequently than northeasterly BLJs. When Wuhan is south of the mei-yu front, the westward extension of the northwest Pacific subtropical high intensifies, and the low-pressure system in the eastern Tibetan Plateau strengthens, favoring the formation of LLJs, which are closely related to precipitation. The wind speeds on rainstorm days are greater than those on LLJ days. Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation. BLJs were shown to develop earlier than SLLJs.
The mei-yu season, typically occurring from mid-June to mid-July, on average, contributes to 32% of the annual precipitation over the Yangtze–Huai River Valley (YHRV) and represents one of the three ...heavy-rainfall periods in China. Here, we briefly review the large-scale background, synoptic pattern, moisture transport, and cloud and precipitation characteristics of the mei-yu frontal systems in the context of the ongoing Integrative Monsoon Frontal Rainfall Experiment (IMFRE) field campaign. Phase one of the campaign, IMFRE-I, was conducted from 10 June to 10 July 2018 in the middle reaches of the YHRV. Led by the Wuhan Institute of Heavy Rain (IHR) with primary support from the National Natural Science Foundation of China, IMFRE-I maximizes the use of our observational capacity enabled by a suite of ground-based and remote sensing instruments, most notably the IHR Mesoscale Heavy Rainfall Observing System (MHROS), including different wavelengths of radars, microwave radiometers, and disdrometers. The KA350 (Shanxi King-Air) aircraft participating in the campaign is equipped with Ka-band cloud radar and different probes. The comprehensive datasets from both the MHROS and aircraft instruments are combined with available satellite observations and model simulations to answer the three scientific questions of IMFRE-I. Some highlights from a previously published special issue are included in this review, and we also briefly introduce the IMFRE-II field campaign, conducted during June–July 2020, where the focus was on the spatiotemporal evolutions of the mei-yu frontal systems over the middle and lower reaches of the YHRV.
2020 and 1998 are the strongest Meiyu years in recent decades. The characteristics of the super-strong Meiyu precipitation and water vapor sources in 2020 and 1998 were compared, and the atmospheric ...circulation anomalies and the forcing factor SST were examined. (1) In 2020, the Meiyu duration, accumulated precipitation, and number of rainstorm days were greater than in 1998, and the highest since 1961. The Meiyu period in 2020 experienced 11 rainstorm processes. In 1998, a typical “second Meiyu” phenomenon occurred, and the area of heavy rainfall in 1998 was located further southward than that in 2020 (2) The contribution of the Bay of Bengal-South China Sea (BOB-SCS) to the total supply of water vapor in 2020 and 1998 was 43.0% and 42.0%, respectively, i.e., much higher than that of the climatological mean (25.5%). In 2020, the sources that provide most water vapor were the BOB, SCS, and central Pacific Ocean, while in 1998 were the Arabian Sea, BOB, and the western Pacific Ocean. (3) During the Meiyu period in 2020 and 1998, the position of atmospheric circulation pattern “two ridges and one trough” are different. Analysis of the vertical structure revealed that the specific humidity intensity above the area of heavy rainfall in 1998 was weaker than that in 2020, and the low-level convergence zone was further south and not as strong as in 2020. The positions of the western Pacific subtropical high (WPSH) and the western North Pacific anticyclone (WNPAC) in 1998 were both further south than those in 2020, which resulted in the more southerly locations of the southwesterly jet stream and rain belt. It should be pointed out that, the important contributions of the SST anomalies in the equatorial central eastern Pacific and the tropical Indian Ocean to the anomalous WNPAC in 1998 and 2020, respectively.
Eastward-moving cloud clusters from the Tibetan Plateau (TP) often trigger heavy rainfall events in the Yangtze River basin in summer. Forecasting these events in an operational environment remains a ...challenging task. Here, dynamical diagnosis and a Lagrangian trajectory model are used to analyze the background atmospheric circulation, maintenance mechanism, and moisture transport of two Meiyu front rainstorms (MYFR) during 30 June–2 July 2016 and 17–19 June 2018 associated with eastward-moving cloud clusters from the TP. It is shown that in both cases heavy rainfall is characterized by semi-continuous rainbelts extending from the eastern TP to the Yangtze River valleys with eastward-spreading convective clouds weakening and strengthening alternately from the eastern TP to downstream regions. Following the track of positive water vapor advection, centers of positive vorticity propagate downstream through the Sichuan basin. The baroclinic thermodynamic—dynamical interaction and the barotropic non-equilibrium force work against each other in the development of the MYFR. Specifically, during the early stage of precipitation development, the barotropic non-equilibrium force dominates, while during the period of heavy precipitation the baroclinic thermodynamic—dynamical interaction dominates. The convergence associated with the baroclinic thermodynamic—dynamical interaction guarantees the persistence of heavy precipitation. Compared to the climate mean state (1988–2018), both MYFR events associated with eastward-moving cloud clusters from the eastern TP are characterized by increased moisture transport from the southwest. One of the major paths of moisture transport in both cases is along the south side of the TP, directly connected to the eastward movement of cloud clusters.
From 29 June to 6 July 2016, China's Yangtze River Basin experienced heavy precipitation, causing more than 200 deaths and affecting tens of millions of people. Using ERA5 reanalysis, soundings, and ...satellite observations from FY‐2 and Global Precipitation Measurement (GPM), we reviewed this heavy rainfall event from the perspective of its meteorological triggers and the cloud microphysics. As the cloud system moved eastward from the Tibetan Plateau, precipitation particles got larger and denser, and the enhancement of stratiform precipitation contributed the most precipitation. The riming and aggregation processes, the dominant growth modes of particles, were significantly enhanced between 5.5 and 7 km. The increase in the echo‐top height had a considerable positive effect on the near‐surface particle size (from ~1.0 mm at 4 km to ~1.5 mm at 10 km), but this feature was not significant over the Tibetan Plateau. We suggest that the microphysics of this event was dominated by the combination of the eastward moving cloud system, which acted as continuous “seeder” cloud, and the increase in water vapor associated with the low‐level vortex and jet, which generated enhancing “feeder” cloud. The particles from the “seeder” cloud grew by the lower “feeder” cloud, thus increasing the particle size to enhance the precipitation. Overall, the generation and movement of such a cloud system is an important atmospheric disturbance for the generation of heavy precipitation downstream, and could be an early warning signal in the forecasting of heavy rainfall in China during the Meiyu period.
Key Points
An event that produced a historical amount of rainfall in Wuhan during the Meiyu period is reviewed
Riming and aggregation processes were enhanced as the cloud system moved out of the Tibetan Plateau
The event was dominated by an eastward moving system acting as “seeder” cloud, together with the increasing water vapor acting as “feeder” cloud
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