Essential Role of Synoptic Environment on Rainfall Distribution of Landfalling Tropical Cyclones Over China

Abstract

How the rainfall characteristics of landfalling tropical cyclones (TCs) over China change with the dry-air intrusion is explored through analyzing Tropical Rainfall Measuring Mission (TRMM) and environmental fields. It is found that the rainfall area of landfalling TC is positively correlated to the midlevel environmental relative humidity: the larger the surrounding relative humidity before TC landfall, especially in the southern quadrants of the TC, the larger the rainfall coverage at landfall. Even when situated in a dry environment, the TC may produce severe concentrated rainfall with stronger intensity than in a moist environment. Results show that interaction between synoptic environment and TC is essential for influencing rainfall distribution for landfalling TCs over China. As a TC moves with northward component under two subtropical highs and westerly trough, it is under the influence of significant dry-air intrusion, which results in limited rainfall area. The increasing northwesterly vertical wind shear that is nearly opposite to the TC movement, on the one hand enhances the upshear-side subsidence, which offsets the friction-induced ascent ahead of TC. On the other hand, it strengthens the downshear-side updraft with the corporation of increasing synoptic convergence and results in severe asymmetric rainfall there. When a TC moves under the influences of enhanced subtropical high and monsoonal southwesterlies, it is under the moist environment that causes a larger rainfall area. Influenced by the weaker vertical wind shear with a similar direction as the TC movement, the rainfall distributes relatively symmetrically with heavy rain over the downshear-friction convergence overlapping region at landfall.When a TC makes landfall, its surrounding condition changes dramatically. First of all, the underlying surface changes from ocean to land. Previous observational studies have found that TC rainfall generally has a maximum ahead of the TC center, due to surface friction-induced lower level convergence induced by different magnitudes of radial momentum advection in different quadrants of the TC. Shapiro (1983) used a simple slab boundary layer model to analyze the effect of storm movement on boundary frictional convergence against the observations of radar and wind for Hurricanes Frederic (1979) and Allen (1980) and found that for slow-moving storms, the convergence was located in the front right quadrant, while for fasting-moving storms, it was concentrated more directly ahead of TC. A long-term statistical study also showed that the rainfall maximum tends to locate in the front-to-front-right quadrant of the track, especially in the western North Pacific (WNP;Chen et al., 2006).As the most active basin for TC genesis, the WNP has a favorable environment with warm sea surface temperature and high relative humidity (RH; e.g., Gray, 1968). In addition, a climatological weather systemthe western North Pacific subtropical high (WNPSH)-affects East Asia during the highly active TC season over the WNP. Observ...

On the flood peak distributions over China

“…There is no strong preference for the spatial distribution of annual flood peaks with respect to storm tracks (i.e., left or right of the track), even though the records floods caused by tropical cyclones tend to be frequently observed in the leftfront quadrant (typically the down-shear side) of the circulations. This is related to the preferable distribution of extreme tropical-cyclone rainfall, due to enhanced moisture conver-gence and updraft on the down-shear side of the circulation (e.g., Atallah et al, 2007;Shu et al, 2018).…”

On the Flood Peak Distributions over China

“…The other 6 tropical cyclones in Table 1 are categorized into the second group (Figure 12 There are no strong preferences for the spatial distribution of annual flood peaks with respect to storm tracks (i.e., left or right of the track), even though the records floods caused by tropical cyclones tend to be frequently observed in the leftfront quadrant (typically the down-shear side) of the circulations. This is related to the preferable distribution of extreme tropical cyclone rainfall, due to enhanced moisture convergence and updraft on the down-shear side of the circulation (e.g., Atallah et al, 2007;Shu et al, 2018). Future studies need to investigate variabilities in the physical properties of river basins (i.e., drainage area, slope, shape) and their relationships with flood peaks (i.e., frequency and magnitude) caused by tropical cyclones, to shed more light on flood hazards associated with landfalling tropical cyclones over China.…”

“…Previous studies show that landfalling tropical cyclones can make great contributions to extreme rainfall in inland regions, even though the frequency of occurrence is not as comparative as coastal regions (e.g., Zhang et al, 2018a). This is closely linked to a couple of factors, such as the interplay of tropical cyclone and baroclinic disturbances (i.e., known as extratropical transition, Hart and Evans, 2000), interactions with mid-latitude systems (e.g., easterly, Shu et al, 2018;Feng and Shu, 2018), and impact of regional topography (as demonstrated by Typhoon Nina, Yang et al, 2017). The devastating consequences of Typhoon Nina and the August 1975 flood expose inadequacies of conventional approaches for flood frequency analysis (e.g., fitting historical flood records with assumed distribution function), and highlight the importance of hydrometeorological approaches for Probable Maximum Precipitation (PMP) / Probable Maximum Flood (PMF) analyses for better designs of flood-control infrastructures (e.g., Smith and Baeck, 2015;Yang et al, 2017).…”

Observed Shear-Relative Rainfall Asymmetries Associated with Landfalling Tropical Cyclones

“…Asymmetries in the TC rainfall associated with landfalling can be modulated by not only VWS [30][31][32][33][34], but also some other factors, such as nonuniform surface features including land-sea contrast and local topographic effect [31,32,[34][35][36][37][38][39] and interaction between TCs and synoptic weather systems [38,40,41]. In general, the landfalling TCs regularly had an asymmetry with a downshear to downshear-left rainfall maximum near the coastal regions, which is consistent with the studies for TCs over the ocean [3,19,42].…”

“…Wingo and Cecil [43] noted that strong westerly shear was likely to form more asymmetric rainfall patterns than those of strong easterly shear. Some studies found that the dry air intruded to the right side of the shear vector was more adverse to the TC symmetric circulation [21,38,44,45]. Xu et al [42] presented that the rainfall percentage towards the right quadrant relative to the coastline would exhibit an obvious increase in prelandfall TCs near landfall in the eastern part of USA, which might be induced by the northward recurving of the storm towards the coast where the land-sea roughness gradient is present.…”