The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.

This study explores synoptic‐scale circulation conditions that favor vertical motion on heavy rainfall days (HRDs) over North China (NC) and analyzes the quantitative contribution to vertical motion by different forcings using the quasi‐geostrophic (QG) omega equation. On HRDs, moisture is brought from the South China Sea and East China seas by surface cyclonic vortex activity. When South Asian High is located anomalously northward, NC is on the south side of the upper‐level jet (ULJ) stream entrance region. Diagnosis of the QG omega equation shows that vertical motion on HRDs is caused by dynamic forcing, diabatic forcing, and topographic forcing. The greatest contribution to vertical motion is from diabatic forcing, while topographic contributes to vertical motion at low levels. The dynamic forcing, which is manifested by the Q‐vector, indicates that both the ULJ and baroclinic trough generate the vertical motion. Furthermore, the effect of the baroclinic trough is larger than that of the ULJ. On medium rainfall days, the contributions from dynamic forcing, diabatic forcing, and topographic forcing are clearly weaker than on HRDs, with reductions of 62.3%, 71.3%, and 73.8%, respectively.

No abstract

During July 17-22, 2021, a record-breaking extreme rainfall occurred across Henan Province, with 6-day accumulated rainfall and the maximum daily rainfall reaching 1122.6 and 624.1 mm, respectively. In particular, the first-ranked Zhengzhou National Automatic Weather Station has an hourly rainfall intensity of up to 201.9 𝐴𝐴 𝐴𝐴𝐴𝐴 𝐴 −1 (at 0800-0900 UTC July 20), which broke the historical record in the Chinese inland (198.5 𝐴𝐴 𝐴𝐴𝐴𝐴 𝐴 −1 , Linzhuang, 5 August 1975). Such a historic rainfall event in North China (NC) resulted in devastating floods in Zhengzhou, Xinxiang, Pingdingshan, and other cities, leading to nearly 14 million people affected and 90 thousand houses collapsing. In total, the flooding caused 302 fatalities and up to $18 billion in directed economic losses (Yin et al., 2022). As the sustainable abundant water vapor supply is one of the essential factors determining the genesis of a heavy rainfall event in addition to sufficient lifting induced by front, orography (

Urbanization effects on rainfall induced by landfalling tropical cyclones have rarely been studied. Here high-resolution numerical simulations with the Weather Research and Forecasting/Noah/Single-layer urban canopy model system (WRF/SLUCM) are conducted to investigate impacts of urban land cover and building heights on heavy rainfall induced by landfalling Typhoon Lekima (2019) over the Megacity Shanghai. The default single urban category in WRF was updated to a new land cover data with three urban categories. Results indicate that WRF/SLUCM captures the typhoon intensity, track and total rainfall amount quite well. Urbanization has a small positive effect on rainfall amount for this event. However, urbanization has a significant impact on the spatial distribution of the accumulated rainfall with enhancement not confined over the urban area but mainly to the southwest of Shanghai possibly due to the changes of the typhoon tracks. With the impact of typhoon Lekima, the urban heat island disappears, indicating that the thermal effect of urbanization has limited influence on the rainfall processes. The model performance is very sensitive to the building height. More realistic building height values can noticeably improve simulations of the diurnal patterns of rainfall, urban heat island and the urban wind speed stilling effect. With the rising of building heights, the surface frictional dynamic effect and vertical uplift is enhanced, but seems not enough to evidently intensify the rainfall. The simulated lower level large moisture flux convergence corresponds well to rainfall peaks. This study has important scientific significance for the accuracy of rainfall forecast of landfalling typhoons and disaster mitigation in cities.