Symbiotic Relationship between Meiyu Rainfall and the Morphology of Meiyu Front

Observational evidences from a heavy precipitation event of the 2020 extreme Meiyu season are presented here to reveal a symbiotic relationship between Meiyu rainfall and the morphology of Meiyu front. The two influence each other through dynamical and thermodynamic feedbacks and evolve in a coherent way to generate cyclic behaviors. Specifically, an intense and band-shaped Meiyu front leads to symmetrical instability in the lower atmospheric layer and convective instability in the middle atmospheric layer, forming a rain band along the front. The Meiyu front and its associated instability subsequently weakens as a result of rainfall and the front is bent by the process of tilting frontolysis. Deep convective instability in the middle and lower layers develops in the warm-humid prefrontal area, and triggers isolated heavy rainfall replacing the original rain band south of the bent front. This warm sector precipitation then strengthens the front through tilting and diabatic heating frontogenesis. A stronger front recovers its initial band shape and the associated rainfall also resumes the form of rain band along the front. Analyses of potential energy associated with instability, water vapor convergence, and cross-frontal circulation are carried out to illustrate key processes of this Meiyu front-rainfall cycle. The implications of this symbiotic relationship for simulating and predicting extreme rainfall associated with Meiyu fronts are presented.

A Study on the Severe Precipitation Induced by the Meiyu Front in Taiwan

<p>Along the coast of northwestern Taiwan, when the Meiyu front passed, there were occasional rapids caused by the terrain, and after convergence with the prevailing wind field, it caused severe precipitation. In 1987, some studies conducted by Taiwan mesoscale experiment (TAMEX) found that low-level jet (about 1 km high) under certain conditions, known as barrier jet, strongly affected the heavy rainfall in northern Taiwan. On the morning of June 2, according to the results of the study in the Meiyu frontal contact north Taiwan, in just 12 hours later to Keelung and north coast down to the super heavy rain of reason, may be related to frontal subject in northern Taiwan snowy mountains, the enhancement of barrier jets occurring near the surface height is related to the increase in barrier jets during the movement of the frontal body. The results of this study show that when the Meiyu front moves southeast, the enhancement of the convective system on the front may cause by a larger inclination angle with its front, with appropriate movement speed, through the southwest airflow in front of the Meiyu front, especially a barrier of about 1 km in height. The warm and humid air introduced by the jet is related. Strong convection will develop in a forced convergence zone off the northwestern part of Taiwan, and the convergence zone is mainly caused by a combination of sub-synoptic forcing such as low-level wind shear convergence, barrier jets, and convective feedback of non-adiabatic terms. In addition, due to the existence of the barrier jet and the frontal wind shear zone, cyclonic circulation around the jet area was generated. With the instability of the temperature gradient and the enhancement of the abnormal zone of the positive potential vorticity, it is speculated that it should be the cause of the severe precipitation in this case. Because the biggest difference between the first and the second strong precipitation may come from the complicated topographic effect and limited space, this research focuses on the reasons for the development of the first severe weather development and heavy precipitation.</p>

A Case Study of Severe Precipitation Caused by Meiyu Front in Northwest Taiwan

<p>From May to June in Southeast Asia, the cold high pressure on the mainland gradually weakens and the Pacific high pressure gradually increases. These two cold and warm pressure systems will form confrontations near Taiwan and South China. The stable "front" system is called "Meiyu front" in Taiwan. In previous studies, when the Meiyu front passed, it had the opportunity to converge with the prevailing wind field in front of the terrain in the northwestern part of Taiwan, resulting in a fast-moving airflow and the intensity of the jet, which is usually concentrated in the lower layers. It is therefore called a low-level jet. Low-level jets under certain conditions, known as barrier jets, can cause severe rainfall in northern Taiwan when they occur. The results of this study show that in the early morning of June 2, 2017, the Meiyu front approached northern Taiwan. When the main body of the front moved toward the Snow Mountain Range in northern Taiwan, a barrier jet appeared at an altitude of about 1 km. After the emergence of the barrier jets, sever precipitation occurred in Keelung and the northern coast of Taiwan in just 12 hours. Our research found that the emergence of barrier jets resulted in the increase of temperature gradients and vertical velocities in local areas; horizontal vortex tubes were twisted in the vicinity, and the horizontal wind shear on both sides of the jets enhanced the cyclonic circulation above the jets. And through the non-adiabatic effect, the stability of the release part was caused, resulting in a severe precipitation event in northern Taiwan. In this study, the observation data and model simulation results are compared with each other to analyze the main cause and physical mechanism of the severe precipitation in the northwest region in this case, and then to infer the dynamic and thermal processes of such weather phenomena over time.</p>

Assimilation of Radar Data, Pseudo Water Vapor, and Potential Temperature in a 3DVAR Framework for Improving Precipitation Forecast of Severe Weather Events

An improved approach to derive pseudo water vapor mass mixing ratio and in- cloud potential temperature was developed in this paper to better initialize numerical weather prediction (NWP) and build convective-scale predictions of severe weather events. The process included several steps. The first was to identify areas of deep moist convection, utilizing Vertically Integrated Liquid water (VIL) derived from a mosaicked 3D radar reflectivity field. Then, pseudo- water vapor and pseudo- in- cloud potential temperature observations were derived based on the VIL. For potential temperature, the latent heat initialization for stratiform cloud and moist adiabatic initialization for deep moist convection were used based on a cloud analysis method. The third step was to assimilate the derived pseudo- water vapor and potential temperature observations, together with radar radial velocity and reflectivity into a convective-scale NWP model during data assimilation cycles spanning several hours. Finally, 3-h forecasts were launched each hour during the data assimilation period. The effects of radar data and pseudo- observation assimilation on the prediction of rainfall associated with convective systems surrounding the Meiyu front in 2018 were explored using two real cases. Two sets of experiments, each including several experiments in each real case, were designed to compare the effects of assimilation radar and pseudo- observations on the ensuing forecasts. Relative to the control experiment without data assimilation and radar experiment, the analyses and forecasts of convections were found to be improved for the two Meiyu front cases after pseudo- water vapor and potential temperature information was assimilated.

Evaluating the Forecast Performance of the Meiyu Front Rainbelt Position: A Case Study of the 30 June to 4 July 2016 Extreme Rainfall Event

An impactful and poorly forecasted heavy rainfall event was observed in association with the Meiyu front over the Yangtze River valley of China from 30 June–4 July 2016. Operational global numerical weather prediction models for almost all forecast lead times beyond 24 h incorrectly forecasted the location and intensity of the precipitation associated with this event. This study presents the first examination of this poleward bias in the operational models for the Meiyu front, which has been frequently noted by meteorologists at the Chinese Meteorological Administration, and explores areas of forecast error and uncertainty in the prediction of the position of the primary frontal rainbelt that is crucial to the placement and intensity of the heavy rainfall. A new zonal mean maximum accumulated precipitation index is introduced and utilized to identify members in the European Centre for Medium-Range Forecasts (ECMWF) Ensemble Prediction System (EPS) that either perform well or perform poorly in forecasting the location of the Meiyu front. Using this new precipitation metric, five-member subgroups representing the EPS members that were most accurate and those that incorrectly displace the Meiyu front the furthest north were identified. An analysis of composite mean fields for the EPS subgroups and the correlation between the rain band placement and the 500 hPa heights was performed for several EPS model runs. We showed that a successful prediction of the location of the Meiyu front rainbelt position by the EPS is most sensitive to the intensity of the 500 hPa trough located over eastern China for the event. The ensemble members that had the largest northward error in the location of the rain band were found to have a more intense 500 hPa trough than the members that more accurately predicted the rainbelt. The more intense upper level trough was found to have enhanced the lower tropospheric southerly flow equatorward of the front and led to a less zonal-oriented Meiyu front, resulting in a northward displacement of both the rainbelt and the regions of more intense precipitation rates. Finally, an examination of the evolution of the differences between the subgroups shows that the primary differences in 500 hPa intensity propagate in-phase with the 500 hPa trough. We show that it is the intensity of the trough, rather than the rate of propagation, that is the most important source of forecast dissimilarities between the successful and failed forecasts.