Rain Belt and Flood Peak: A Study of the Extreme Precipitation Event in the Yangtze River Basin in 1849

Floods caused by extreme precipitation events, in the context of climate warming, are one of the most serious natural disasters in monsoon region societies. The great flood in the Yangtze River Basin in 1849, in Eastern China, was a typical extreme flood event. According to historical archives, local chronicles, diaries, and historical hydrological survey data, this study reconstructed the temporal and spatial patterns of extreme precipitation in 1849, and the flood process of the Yangtze River. We found four major precipitation events at the middle and lower reaches of the Yangtze River, from 18 May to 18 July 1849. The torrential rainfall area showed a dumbbell-like structure along the Yangtze River, with two centers distributed separately in the east and west. For the specific flood process of the Yangtze River, many tributaries of the Yangtze River system entered the flood season consecutively since April, and the mainstream of the Yangtze River experienced tremendous pressure on flood prevention with the arrival of multiple rounds of heavy rainfall. In mid-to-late July, the water level and flow rate of many stations along the mainstream and tributaries had reached their record high. The record-breaking peak flow rate at many stations along the mainstream and tributaries in the middle reaches of the Yangtze River indicated intense precipitation in the area. The heavy rainfall disaster in the Yangtze River Basin could be driven by these reasons. First, the cold air in North China was extraordinary active in 1849, which made it difficult for the subtropical high pressure to move northward. Second, the rain belt stagnated in the Yangtze River Basin for a long time, and the Meiyu period reached 42 days, 62% longer than normal years. Third, the onset of a southwest monsoon was earlier and more active, which provided abundant moisture to the Yangtze River Basin. The great flood disaster was caused by heavy precipitation at the middle reaches, which made it quite different from the other three great floods in the Yangtze River in the 20th century. At present, the large water conservancy projects in the Yangtze River are mainly designed for flood problems caused by rainstorms in the upper reaches of the Yangtze River. The middle reaches of the Yangtze River, however, are facing the weakening of flood diversion capacity, caused by social and economic development. Therefore, future flood prevention measures in the Yangtze River should pay great attention to the threat of this flood pattern.

Effects of Suppressed Transient Eddies by the Tibetan Plateau on the East Asian Summer Monsoon

It is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP.The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, mid-tropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to the south-more-north-less rainfall change over East Asia.

Effects of Assimilating Clear-Sky FY-3D MWHS2 Radiance on the Numerical Simulation of Tropical Storm Ampil

A new advanced microwave humidity sounder FY-3D MWHS2 radiance has been assimilated under the clear-sky conditions by implementing its data assimilation interface. The case of the tropical storm Ampil in 2018 is selected to address the effectiveness of the new-built module in the initialization and forecast of typhoons. Apart from the experiment assimilating both the Global Telecommunications System (GTS) data and the FY-3D MWHS2 radiance data, an experiment with only GTS data is also conducted for comparison. The results show that the bias correction of this humidity sounder is effective, and the analysis field after assimilating its radiance data matches well with the observation. The increment of specific humidity below the middle layers is evident after the assimilation of the radiance data. Besides, the geopotential height increment and the specific humidity increment at 500 hPa and 850 hPa, respectively, are favorable, resulting in more accurate rain belt distribution and a higher fraction skill score (FSS). In the deterministic forecast, the track error of the FY-3D MWHS2 experiment is consistently less than 90 km.

Large-scale synoptic drivers of co-occurring summertime ozone and PM_2.5 pollution in eastern China

Surface ozone (O3) pollution during summer (June–August) over eastern China has become more severe in recent years, resulting in a co-occurrence of surface O3 and PM2.5 (particulate matter with aerodynamic diameters ≤ 2.5 µm in the air) pollution. However, the mechanisms regarding how the synoptic weather pattern (SWP) might influence this compound pollution remain unclear. In this study, we applied the T-mode principal component analysis (T-PCA) method to objectively classify the occurrence of four SWPs over eastern China, based on the geopotential heights at 500 hPa during summer (2015–2018). These four SWPs over eastern China were closely related to the western Pacific subtropical high (WPSH), exhibiting significant intra-seasonal and interannual variations. Based on ground-level air quality observations, remarkable spatial and temporal disparities of surface O3 and PM2.5 pollution were also found under the four SWPs. In particular, there were two SWPs that were sensitive to compound pollution (Type 1 and Type 2). Type 1 was characterized by a stable WPSH ridge with its axis at about 22∘ N and the rain belt located south of the Yangtze River Delta (YRD); Type 2 also exhibited WPSH dominance (ridge axis at ∼ 25∘ N) but with the rain belt (over the YRD) at a higher latitude compared to Type 1. In general, SWPs have played an important role as driving factors of surface O3–PM2.5 compound pollution in a regional context. Our findings demonstrate the important role played by SWPs in driving regional surface O3–PM2.5 compound pollution, in addition to the large quantities of emissions, and may also provide insights into the regional co-occurring high levels of both PM2.5 and O3 via the effects of certain meteorological factors.

Downscaling of a seasonal ensemble forecast at the convection-permitting resolution over the Horn of Africa using the WRF model

<p>Studies have shown the benefits of convection-permitting downscaling at the seasonal scale using limited-area models. To evaluate the performance with real forecasts as boundary conditions, four members of the SEAS5 global ensemble were dynamically downscaled over Ethiopia during June, July, and August 2018 at a 3-km resolution. We used a multi‐physics ensemble based on the WRF model to compare the effects of boundary conditions and physics <span><span>parametrization</span></span> producing 16 ensemble members. With ECMWF analyses as a reference, SEAS5 averaged to a +0.17°C bias over Ethiopia whereas WRF resulted in +1.14°C. With respect to precipitation, the WRF model simulated 264 mm compared to 248 mm for SEAS5 and 236 mm for GPM-IMERG. The maximum northward extension of the tropical rain belt decreased by about 2° in both models. Downscaling enhanced the ensemble spread in precipitation by 60% on average, correcting the SEAS5 underdispersion. The WRF ensemble spread over Ethiopia was mostly generated by the perturbed boundary conditions, as their effect is often 50% larger than the physics‐induced variability. The results indicate that boundary condition perturbations are necessary, although not always sufficient, to generate the right amount of ensemble spread in a limited-area model with complex topography. The next step is to use specific methods to calculate the added value provided by the downscaling.</p>