Urbanization Effects on Rainfall Processes Induced by Landfalling Typhoon Lekima (2019) over the Shanghai Metropolitan Area

Abstract

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.

Sensitivity of boundary layer schemes in simulating the asymmetric rainfall of landfalling typhoon Lekima (2019)

“…The water vapor flux in the coastal area north of Zhejiang is also relatively larger, which is generally above 8 × 10 −4 kg m −2 s −1 . It is indicated that the outer spiral rainbands concentrated on the north side of the typhoon center simulated in CTL agree well with the observation (Figure 3; Ao et al, 2022;He et al, 2021). While in PBL1, the water vapor flux is axisymmetrically distributed.…”

“…According to the radar observation (Figures 3A,B), concentric eyewalls developed before the landfall, accompanied by an eyewall replacement, which is also identified by the satellite TBB analysis (Figures 3C,D). It can be seen that along the shear at these times, the major convection in the second concentric eyewall as well as the outer spiral rainbands concentrated on the north/northwest side of the typhoon center, which has seriously impacted the coastal areas of Zhejiang (He et al, 2021;Ao et al, 2022).…”

Effect of urbanization on gust wind speed in summer over an urban area in Eastern China

“…On average, the daily mean friction velocity increases by 0.05-0.15 m s −1 (15%-30%) over the urban areas. The increase of friction velocity illustrates that the surface frictional and dragging effects are obviously increased with the increase of building density and building heights (Zhang et al 2018, Ao et al 2022. Since the friction velocity is proportional to the standard deviation of the wind speed fluctuations in the surface layer similarity relation, the increase of the friction velocity also represents the increase of the turbulence intensity characterizing the strength of turbulent motion (see supplementary information).…”

“…Thus in the urban areas, the diurnal variation of the friction velocity also partly reflects and is similar to that of the mean wind speed. In addition, the averaged increase in the friction velocity after the urbanization is largest in COI urban areas while least in LIR urban areas, suggesting the turbulent motion is stronger in COI urban areas with greater surface roughness (Zhang et al 2018, Ao et al 2022, which also contributes to stronger wind gust enhancement in COI urban areas. In general, the minimum increase of friction velocity in COI, HIR and LIR urban areas is approximately 0.020, 0.010 and 0.005 m s −1 at 08:00 LST, while the maximum increase of friction velocity in the three urban land categories is approximately 0.100, 0.095 and 0.080 m s −1 at 19:00 LST, respectively.…”

“…Some key climatic effects of urbanization have been widely investigated, such as the urban heat islands (UHI) effect (Oke 1982, Zhao et al 2013, the slow-down of near-surface wind by enhanced drag force and greater roughness length (Liao et al 2015, Wang et al 2019, the thermally-driven local atmospheric circulation (Lin et al 2008, Chen et al 2009, and the possible precipitation changes (Miao et al 2011, Georgescu et al 2014, Holst et al 2016. Urbanization can also modify the characteristics/ structures of the planetary boundary layer (PBL) including the boundary layer height in urban areas (Ryu et al 2013), decrease the surface evaporation (Wienert and Kuttler 2005) and relative humidity (Zhao et al 2013), and increase the friction velocity because of the enhanced surface frictional dynamic effect (Zhang et al 2018, Ao et al 2022.…”