Daily to submonthly weather and climate characteristics of the summer 1998 extreme rainfall over the Yangtze River Basin

Abstract

[1] In this study, the daily to submonthly weather and regional climate characteristics associated with the extreme rainfall events over the middle to lower reaches of the Yangtze River Basin (YRB-ML) during the summer of 1998 are examined using the National Centers for Environmental Prediction/Department of Energy Reanalysis-2 data and a 54-day high-resolution (i.e., Dx = 4 km) simulation with a regional climate model (RCM). As verified against various observations, the RCM reproduces reasonably well the accumulated daily to biweekly rainfall events, their generation and subsequent evolution along the Meiyu front, and especially the rainfall events over the YRB-ML during the later stage of the 1998 floods. It is found that the early stage of the 1998 floods resembles in many aspects that occurred in normal years, except for a moister-than-climate state and stronger-than-normal low-level jets (LLJs) in Southeast China, whereas the later stage exhibits two abnormal height anomalies to the northeast and southwest of the YRB-ML, facilitating convergence of northeasterly and southwesterly flows along the Meiyu front. It is also found that the Meiyu front weakens in depth and intensity as it migrates northward across and then retreats to the YRB-ML, playing less important roles in rain production. Results show different daily to biweekly rainfall characteristics at the different stages, e.g., from eastward propagation to local generation of mesoscale convective systems and periodical rain production along the Meiyu front, most of which reach their maximum intensities over the YRB-ML during the morning to early afternoon hours. Results indicate that the northeasterly moisture supply enhances the production of steady heavy rain at the later stage of the 1998 floods. It is concluded that the West Pacific subtropical high and the Meiyu front control the general areas of rain production, the lowlevel moisture supply through LLJs and mesoscale disturbances determines whether or not sustaining flooding rains would occur, and what rainfall characteristics are at the daily to biweekly timescales.

Numerical Simulation of the Heavy Rainfall in the Yangtze-Huai River Basin during Summer 2003 Using the WRF Model

“…The core speed is nearly 4 to 6 m s 1 greater than the value obtained from the FNL analysis. This result is similar to that obtained from the low-level wind simulation for the summer of 1998 using the Pennsylvania State University-National Center for Atmospheric Research mesoscale model (i.e., MM5) (Liu et al, 2008(Liu et al, , 2010. The relative humidity distributions shown by the FNL data and the WRF simulation are similar, but the region north of the Huai River Basin in the simulation is relatively dry.…”

“…The atmosphere and soil initial conditions are both taken from the six-hourly global analysis data provided by the 1.0×1.0-degree grids of the National Centers for Environmental Prediction (NCEP) Final (FNL) operational model. As in Liu et al (2008Liu et al ( , 2010, we adopt a buffer zone of approximately 1000 km (30 grid intervals) along each lateral boundary to keep the incoming large-scale flows reasonably well defined. The FNL data are also used as the atmospheric lateral boundary conditions for the model.…”

“…They have been adopted in the fields of meteorology, hydrology, ecology, and air chemistry (Liu et al, 2008;Hurk et al, 2002;Qian et al, 2009;Lu et al, 2001;Wu and Han, 2011). Compared to the coarse-resolution general circulation models, the primary advantage of the RCM approach is that it furnishes a better description of orographic forcing, based on smaller grid sizes (Yu et al, 2010).…”

“…For example, in the summer of 1998, two different periods of heavy rain associated with the mei-yu frontal system occurred over the Yangtze River Basin, lasted for approximately 31 days, and caused an unusual episode of catastrophic flooding . Several regional climate simulations of this event have been performed with different RCMs (e.g., Wang et al, 2003;Lee et al, 2004;Liu et al, 2008), and most of these simulations successfully reproduced the evolution of the heavy rainfall.…”

Simulation of the western North Pacific subtropical high in El Niño decaying summers by CMIP5 AGCMs

“…An anomalous shift of the WNPSH away from its normal location can influence the transport of moisture to East Asia, resulting in an abnormal summer climate in the region. For instance, following the 1997-1998 super El Niño event, the WNPSH was located westwards and southwards compared to its climatology, contributing to heavy rainfall and catastrophic flooding over the middle to lower reaches of the Yangtze River Valley in East China (Liu, Zhang, and Wang 2008). Thus, understanding the variability of the WNPSH is of central importance to East Asian scientists.…”