Generation of Extreme Precipitation over the Southeastern Tibetan Plateau Associated with TC Rashmi (2008)

Wei Ye aState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China

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Ying Li aState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China

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Da-Lin Zhang aState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China
bDepartment of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland

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https://orcid.org/0000-0003-1725-283X
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Abstract

In this study, the development of an extreme precipitation event along the southeastern margin of the Tibetan Plateau (TP) by the approach of Tropical Cyclone (TC) Rashmi (2008) from the Bay of Bengal is examined using a global reanalysis and all available observations. Results show the importance of an anomalous southerly flow, resulting from the merging of Rashmi into a meridionally deep trough at the western periphery of a subtropical high, in steering the storm and transporting tropical warm–moist air, thereby supplying necessary moisture for precipitation production over the TP. A mesoscale data analysis reveals that (i) the Rashmi vortex maintained its TC identity during its northward movement in the warm sector with weak-gradient flows; (ii) the extreme precipitation event occurred under potentially stable conditions; (iii) topographical uplifting of the southerly warm–moist air, enhanced by the approaching vortex with some degree of slantwise instability, led to the development of heavy to extreme precipitation along the southeastern margin of the TP; and (iv) the most influential uplifting of the intense vortex flows carrying ample moisture over steep topography favored the generation of the record-breaking daily snowfall of 98 mm (in water depth), and daily precipitation of 87 mm with rain–snow–rain changeovers at two high-elevation stations, respectively. The extreme precipitation and phase changeovers could be uncovered by an unusual upper-air sounding that shows a profound saturated layer from the surface to upper troposphere with a moist adiabatic upper 100-hPa layer and a bottom 100-hPa melting layer. The results appear to have important implications to the forecast of TC-related heavy precipitation over high mountains.

Significance Statement

This study attempts to gain insight into the multiscale dynamical processes leading to the development of an extreme precipitation event over the southeastern margin of the Tibet Plateau as a Bay of Bengal tropical cyclone (TC) approached. Results show (i) the importance of an anomalous southerly flow with a wide zonal span in steering the relatively large-sized TC and transporting necessary moisture into the region; and (ii) the subsequent uplifting of the warm and moist TC vortex by steep topography, producing the extreme precipitation event under potentially stable conditions, especially the record-breaking daily snowfall of 98 mm (in water depth). The results have important implications to the forecast of TC-related heavy precipitation over the Tibet Plateau and other high mountainous regions.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Da-Lin Zhang, dalin@umd.edu; Ying Li, yli@cma.gov.cn

Abstract

In this study, the development of an extreme precipitation event along the southeastern margin of the Tibetan Plateau (TP) by the approach of Tropical Cyclone (TC) Rashmi (2008) from the Bay of Bengal is examined using a global reanalysis and all available observations. Results show the importance of an anomalous southerly flow, resulting from the merging of Rashmi into a meridionally deep trough at the western periphery of a subtropical high, in steering the storm and transporting tropical warm–moist air, thereby supplying necessary moisture for precipitation production over the TP. A mesoscale data analysis reveals that (i) the Rashmi vortex maintained its TC identity during its northward movement in the warm sector with weak-gradient flows; (ii) the extreme precipitation event occurred under potentially stable conditions; (iii) topographical uplifting of the southerly warm–moist air, enhanced by the approaching vortex with some degree of slantwise instability, led to the development of heavy to extreme precipitation along the southeastern margin of the TP; and (iv) the most influential uplifting of the intense vortex flows carrying ample moisture over steep topography favored the generation of the record-breaking daily snowfall of 98 mm (in water depth), and daily precipitation of 87 mm with rain–snow–rain changeovers at two high-elevation stations, respectively. The extreme precipitation and phase changeovers could be uncovered by an unusual upper-air sounding that shows a profound saturated layer from the surface to upper troposphere with a moist adiabatic upper 100-hPa layer and a bottom 100-hPa melting layer. The results appear to have important implications to the forecast of TC-related heavy precipitation over high mountains.

Significance Statement

This study attempts to gain insight into the multiscale dynamical processes leading to the development of an extreme precipitation event over the southeastern margin of the Tibet Plateau as a Bay of Bengal tropical cyclone (TC) approached. Results show (i) the importance of an anomalous southerly flow with a wide zonal span in steering the relatively large-sized TC and transporting necessary moisture into the region; and (ii) the subsequent uplifting of the warm and moist TC vortex by steep topography, producing the extreme precipitation event under potentially stable conditions, especially the record-breaking daily snowfall of 98 mm (in water depth). The results have important implications to the forecast of TC-related heavy precipitation over the Tibet Plateau and other high mountainous regions.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding authors: Da-Lin Zhang, dalin@umd.edu; Ying Li, yli@cma.gov.cn

1. Introduction

An extreme precipitation event (EPE) occurred over the southeastern margin of the Tibetan Plateau (TP), in association with the approach of Tropical Cyclone (TC) Rashmi, during the 2-day period of 0000 UTC 26 October–0000 UTC 28 October 2008. Specifically, four surface stations along the southeastern margin of the TP reported accumulated precipitation of ≥90 mm (Fig. 1a), with intense snowfall1 rates occurring mostly at the stations of higher than 4-km elevation (Fig. 1c). Daily precipitation (i.e., from 1200 UTC 26 October to 1200 UTC 27 October) reached 98 mm (all in snow) at the Cona station, and 87 mm (with phase changeovers from rain to snow and then back to rain) at the Bomi station (Fig. 2d). Moreover, daily precipitation at 17 stations far exceeded their respective extreme values by the 99th percentile thresholds (Pendergrass 2018), based on their 40-yr (1981–2020) daily precipitation records. The then operational forecasts with 48-h lead times by the National Meteorological Center of China indicated much less than half of the observed amounts at Cona, Bomi, and the other stations. As a result, widespread severe property damage, communication blockage and casualties took place over the southeastern TP (Tang 2008). Although there have been many studies of EPEs associated with landfalling TCs, few have been conducted to understand the multiscale processes leading to the generation of extreme precipitation over the TP in relation to the approach of TCs that were formed over the Bay of Bengal (BoB). Moreover, the BoB TCs bring often local rainstorms and snowstorms with variable environmental and societal impacts as they move northward onto the TP (e.g., Zhang et al. 1988; Lin et al. 2015). Thus, it is of particular interest to examine how the Rashmi vortex interacted with mesoscale disturbances and local complex topography in generating the present EPE, and gain insight into the forecasts of heavy precipitation over the TP as TCs formed over the BoB approach.

Fig. 1.
Fig. 1.

(a) Two-day accumulated precipitation (sum of rainfall and snowfall; mm; colored dots), (b) rainfall amount (mm; colored dots), and (c) snowfall amount (mm; snowflake symbols) from 0000 UTC 26 Oct to 0000 UTC 28 Oct 2008. The inner box denotes the study area of heavy precipitation (28°–33°N, 89°–98°E) with the daily amount of ≥20 mm. The yellow line in (a) represents the southeastern margin of the TP and is similar in the rest of the figures. In (a), the locations (latitudes, longitudes, and terrain elevations) of Cona (27.98°N, 91.95°E, 4281 m MSL), Bomi (29.87°N, 95.77°E, 2737 m MSL), and Zayu (28.65°N, 97.47°E, 2331 m MSL) are indicated by triangles; the Linzhi station (29.57°N, 94.47°E, 3001 m MSL), at which an upper-air sounding is shown in Fig. 10, is denoted by an “L” above a triangle; and symbols, “QH,” “SC,” and “TB,” denote provinces of Qinghai and Sichuan, and Tibet Autonomous Region of China, respectively. The 2-, 3-, and 4-km terrain elevations over the TP in (b) and (c) are denoted by different gray contours.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

Fig. 2.
Fig. 2.

The center location of Rashmi, as denoted by the TC symbol (similar in the rest of the figures), and its track, the 30-min delayed equivalent blackbody brightness temperature (°C), and the subsequent 6-h accumulated precipitation (mm; filled circles: rainfall; snowflake symbols: snowfall) from surface rain gauges at (a) 1200 UTC 26 Oct, (b) 0000 UTC 27 Oct, and (c) 0600 UTC 27 Oct 2008. (d) Time series of rainfall (mm; solid color fill) and snowfall (mm; pattern fill) at 6-h intervals at Cona, Bomi, and Zayu (see Fig. 1a for their locations) during the 2-day period of 0000 UTC 26 Oct–0000 UTC 28 Oct 2008.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

Previous studies show that the genesis frequency of the BoB TCs has obvious seasonal variations, with the peaks from April to May and from October to November (Gray 1968; Camargo et al. 2007; Bhardwaj and Singh 2020). They resulted mainly from barotropic instability associated with horizontal wind shear during the northward displacement of the monsoon trough in spring or the southward retreat in autumn (Krishnamurti et al. 1981; Mao and Wu 2011). Their associated influences over the TP are mainly reflected in precipitation, which also have the same dual peaks as the frequency of the BoB TCs, i.e., with mainly rainfall in May and snowfall in October (Li et al. 2013). Of interest is that the spatial distribution of the daily mean precipitation over the TP in the months is basically similar to the climatological mean, regardless of whether there is a TC. This indicates that TCs cannot cause large-scale changes in precipitation pattern on a monthly or seasonal scale, but it is the mesoscale EPEs that could (Xiao and Duan 2015). Zhu et al. (1998) find that heavy convective rainfall from a BoB TC was only limited to the southern TP, whereas its associated cirrus clouds could extend northward, producing widespread precipitation over distant areas, e.g., Qinghai Province, and northwestern Sichuan (see their locations in Fig. 1). Dai (1984) notes that TC-related precipitation could cover the entire TP in the presence of cold air activity in the northern TP. These results imply that the interaction between the cold air activity and TCs plays an important role in determining the precipitation coverage and intensity.

Wang and Wang (1989) indicate the following three large-scale patterns associated with TC-related precipitation over the TP: (i) the merging of a TC with a midlevel trough on the south of the TP, (ii) a TC sandwiched by an Indian high on its west and a subtropical high over the South China Sea, and (iii) a northward moving TC dominated by meridional flows. If a midlevel trough in the first scenario could be maintained over the BoB, the south- to southwesterly flows ahead of the trough axis are conducive to transporting the water vapor carried by a BoB TC northward to the TP (Lv et al. 2013; Suo and Ding 2014; Duan and Zhang 2015; Liu et al. 2015). On the other hand, two recent studies by Ci et al. (2019) and Kumar et al. (2020) show the generation of heavy precipitation by a BoB TC over a long journey to the TP in the absence of a large-scale trough. Clearly, such a longevity can be attributed to the presence of inertial stability associated with the TC’s cyclonic rotation.

The interaction of TCs with topography could also play a critical role in determining the generation of local heavy rainfall (Parrish et al. 1982; Wu and Kuo 1999; Yang et al. 2008). Previous studies indicate the importance of the following factors in producing extreme precipitation over high mountains (e.g., the Central Mountain Range of Taiwan island): the presence of a low-level jet, steep topography, high moisture content, more potential instability, and slow translational motion of TCs (Ge et al. 2010; Lin et al. 2011; Tao et al. 2011; Yu and Cheng 2013; Agyakwah and Lin 2021). Similarly, the TC-related precipitation intensity over the TP depends more or less on the strength of approaching winds due partly to its role in transporting moisture, and partly to its lifting of moist air over sloping topography (Rasmussen and Houze 2012; Prokop and Walanus 2015; De et al. 2015).

The above studies reveal that the impact of BoB TCs on the TP precipitation could involve complicated multiscale interactions of middle and lower latitude disturbances with complex topography. However, our understanding of the multiscale interactions still remains elusive. Therefore, the purpose of the present EPE study seeks to answer the following three questions: How could Rashmi (2008) travel a long journey inland, while keeping its TC identity, until reaching the southeastern margin of the TP? What were the relative roles of the Rashmi vortex, local topography, and larger-scale flows in generating the present EPE? Why did the EPE take place mainly at the Cona, and Bomi regions?

The next section describes the data and analysis methods used for the present study. Section 3 provides an overview of Rashmi activity and its associated extreme precipitation along the southeastern margin of the TP. Section 4 examines what important aspects of the large-scale flows that accounted for the northward movement of the Rashmi vortex, while keeping its TC identity, until reaching the TP. Section 5 explores how certain mesoscale processes, especially uplifting of the moist-warm air associated with the Rashmi vortex versus larger-scale flows along the southeastern margin of the TP, led to the generation of the EPE at the Cona and Bomi regions. The formation of a warm front and its roles in producing widespread precipitation over the TP are also discussed. A summary and conclusions are given in the final section.

2. Data and methodology

In this study, the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data for the global climate and weather (ERA5) with 0.25° resolution at 6-h intervals is used to diagnose the environmental conditions associated with the present EPE. The best track data over the north Indian Ocean, provided by the Joint Typhoon Warning Center (JTWC), are used to analyze the track, intensity and size of TC Rashmi (2008). The FY-2D cloud top equivalent blackbody brightness temperature (TBB) data from the China Meteorological Data Service Centre with 0.1° resolution at 1-h intervals are used to examine the distribution of clouds. The coverage of the TP (denoted by yellow lines in most figures) is determined by shapefile data from the Institute of Geographic Sciences and Natural Resources Research, the Chinese Academy of Sciences (Y. L. Zhang et al. 2014). The 2′-resolution terrain elevation data from the National Centers for Environmental Information (NCEI) of the National Oceanic and Atmospheric Administration (NOAA) are used to specify terrain heights mentioned herein.

Due to large regional differences in precipitation across China, the probability of extreme precipitation, either rain or snow, in the TP area is much lower than that in plain areas according to the general precipitation scaling thresholds (Peng et al. 2018; Wu et al. 2018). Thus, it is necessary to develop a separate precipitation scaling threshold for the TP area. That is, 6-h surface rain gauge data, provided by the China Meteorological Administration, are used to show the spatiotemporal distribution of precipitation, but excluding precipitation of ≤0.1 mm. The mean 99th threshold of 20 mm is used herein to define the study area of heavy precipitation over the TP that was more associated with the approaching Rashmi vortex, as indicated by a black box in Fig. 1.

The vertical integral of moisture flux vectors, QFLUX, is calculated to determine the sources of moisture associated with the generation of the EPE along the southeastern margin of the TP. It is calculated as
QFLUX=p=P_bottomp=100hPa(Vq/g)dp,
where V is the horizontal wind vector (m s−1), q is the specific humidity of water vapor (g kg−1), g is the gravity, and P_bottom is generally treated as the surface pressure. However, in the presence of topographical blocking, P_bottom is treated as the lowest pressure level, where air parcels could be lifted to elevated terrain surfaces (see section 5b), and it needs to be determined by the unity moist Froude number as defined below.
The moist Froude number (Frm) is calculated to examine to what extent moist but unsaturated air parcels could be lifted by horizontal flows over local topography (Chen and Lin 2005; Reeves and Lin 2006; Xia and Zhang 2019). It is given by
Frm=UNmh
where U is the incoming flow speed, i.e., the meridional wind speeds at about 150 km to the south of the surface station of concern; Nm = [(g/θυ)(∂θυ/∂z)]1/2 is the moist Brunt–Väisälä frequency using the virtual potential temperature (θυ); and h is the mountain elevation MSL. It should be mentioned that the three stations of concern are all located on the downstream side of a major west–east-oriented mountain ridge of higher than 4 km (cf. Figs. 1a,b). So, h in Eq. (2) is set to 4 km in calculating Frm and the associated QFLUX in this study.

3. Overview

TC Rashmi was originated from a tropical depression in the northwestern BoB (i.e., 16.0°N, 85.0°E) at 0600 UTC 24 October 2008 (not shown). It was strengthened to the maximum surface wind (VMAX) of 17.5 m s−1 and the minimum sea level pressure (PMIN) of 996 hPa at 0000 UTC 26 October during its northeastward movement. Its VMAX kept increasing until 1800 UTC 26 October when its VMAX reached 22.5 m s−1, and its PMIN dropped to 989 hPa. Shortly after, the TC made landfall in the southern Bangladesh, and then moved along the direction of north by east, with its intensity weakening rapidly. The rapid intensity decreases after landfall could also be inferred from Figs. 2a and 2b, showing marked reduction in the coverage of deep convection (through equivalent blackbody brightness temperature) in the core region of the TC. By 0600 UTC 27 October, it was weakened to a tropical depression at Purbadhala, Bangladesh (25°N, 90.6°E), and then its positioning was stopped. This was also the time when the EPE began to take place along the southeastern margin of the TP (Fig. 2d). As compared to typical BoB TCs (Bhardwaj and Singh 2020), Rashmi was short-lived at tropical storm intensity (i.e., <24 h), and its peak intensity was relatively weak, but it was relatively larger sized. At the time of its tropical storm intensity at 0000 UTC 26 October, Rashmi had the radius of maximum wind (RMW) of 92.6 km, which is much larger than the average RMW of 55 km for all BoB storms from 2001 to 2018 (Fan et al. 2020). Based on the concept of TC fullness (TCF = 1 − RMW/R17, where R17 ≈ 230 km, estimated from the ERA5 data, is the radius of 17 m s−1; Guo and Tan 2017), Rashmi’s TCF was about 0.8 prior to its landfall, which was close to the Guo–Tan FS4 scale that is necessary for an intense TC. This means that the large fullness of Rashmi has a high potential to intensify if it would continue to move over the ocean. On the other hand, this TCF value indicates to some extent the relatively long-lived nature of the vortex under favorable larger-scale flows, which could be understood in terms of inertial stability, and its important influences of precipitable water carried by a large-sized cyclonic circulation on this EPE.

Figures 2a and 2c also show the evolution of surface precipitation in relation to Rashmi’s clouds prior to and after landfall. At 1200 UTC 26 October (Fig. 2a), the cloud shields of Rashmi extended from the northern BoB into far inland. Its central TBB was colder than −80°C, indicating the development of deep convection, and the cloud activity appeared to be more vigorous on the northeast side than that on the southwest side. Meanwhile, some lower elevated clouds developed in the southern portion of the TP, even though Rashmi was still located over the BoB. Subsequently, the area of TBB below −70°C decreased sharply, suggesting the weakening of the TC cloud system. As it moved close to the TP, Rashmi’s northward displacement slowed, due likely to the blocking of the TP (cf. Figs. 2a–c). By 0600 UTC 27 October, a mesoscale convective cloud band extended from Rashmi’s core region to the southeastern margin of the TP, in contrast to a long and narrow cloud band with a large coverage of lower to midlevel clouds distributed over the northern TP, where the minimum TBB temperature was lower than −52°C (Figs. 2b,c). Subsequently, the major cloud system began to dissipate as it moved rapidly northeastward. The above analysis indicates that the clouds and precipitation over the northern TP prior to 1200 UTC 26 October did not appear to be related to the approaching Rashmi vortex, but the heavy to extreme precipitation along the southern margin of the TP and the long and narrow cloud band over the northern TP afterward was closely related. The long and narrow cloud band might be aided by upward motion at the right entrance region and subsequent horizontal advection of an upper-level jet stream (Fig. 3b). A further analysis of Figs. 2a–c indicates that the rain (snow) areas roughly coincided with the influence area of the TC cloud system, especially with snowfall mostly occurring in areas with TBB ≤ −52°C.

Fig. 3.
Fig. 3.

(a) The 500-hPa geopotential heights (blue contoured at intervals of 4 dagpm), isotherms (dash contoured at intervals of 4°C), and wind barbs (a full barb is 5 m s−1); (b) the 200-hPa geopotential heights (blue contoured at intervals of 5 dagpm), and horizontal wind vectors (m s−1); and (c) the 500-hPa geopotential height anomaly (contoured; intervals of 1 dagpm) and 200-hPa geopotential height anomaly (shading) at 0000 UTC 27 Oct 2008 with respect to the 30-yr climatic mean height during 1979–2008. In (c), dashed contours and blue colors are for negative values.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

Figure 2d shows the time series of 6-h accumulated precipitation at Cona, Bomi, and Zayu that were the top three stations receiving extreme precipitation with different precipitation types, which are distributed from the west to east along the southeastern margin of the TP (Fig. 1a). Results indicate that Cona and Zayu experienced snowfall and rainfall throughout the EPE period, respectively, whereas Bomi underwent rainfall first, followed by snowfall, but with a changeover back to rainfall after 0000 UTC 27 October. The different precipitation types at Cona and Zayu could be clearly attributed to large differences in their local terrain elevations, while the phase changeovers at Bomi involved some complicated cloud microphysics processes, in addition to mesoscale variations, which will be explored in section 5d. Of our concern is that the 6-h precipitation rates at Cona and Bomi reached their peak values of 57 and 35 mm (6 h)−1, respectively, during the period of 0600–1200 UTC 27 October when the Rashmi vortex moved close to the southeastern margin of the TP. The precipitation decreased rapidly afterward, which coincided with the dissipation of the TC cloud system (not shown). As compared to the extreme precipitation at Cona and Bomi, heavy precipitation began at Zayu prior to Rashmi’s landfall, and its 6-h precipitation even decreased before 0600 UTC 27 October. This indicates the different influences of the Rashmi vortex and larger-scale flows interacting with local topography on the precipitation generation at these stations, as will be seen in section 5. Note that although heavy precipitation occurred at three stations on the east of Zayu, which are also located at the southeastern margin of the TP (see Fig. 1a), they are excluded from the study area. This is because the heavy precipitation amounts were more or less determined by southwesterly flows on the northwest of the WPSH interacting with the general south–north-oriented mountain ridges and valleys, rather than directly influenced by Rashmi’s circulation (K. Zhang et al. 2014; Yu et al. 2018), as will also be seen in section 5. Thus, our attention will be given hereafter to the multiscale processes leading to the EPE over the study area, as shown in Fig. 1, with more focus on the generation of the daily precipitation of >85 mm at the Cona and Bomi stations.

4. Large-scale conditions

Figure 3 shows the large-scale flows, in which the EPE occurred, at 0000 UTC 27 October 2008 when Rashmi made landfall. At 500 hPa, a meridionally deep shortwave trough extended southeastward from the west of the TP to the BoB where Rashmi with its surrounding warm-moist air was merged into the southeastern bottom of the trough. Meanwhile, the southeastern TP was located on the northwestern periphery of a semipermanent west Pacific subtropical high (WPSH). As a result, a dominant warm, moist southerly flow, in which Rashmi was embedded, appeared ahead of the trough axis, i.e., from the southern coast of Bangladesh to the TP, which played an important role in steering the Rashmi vortex northward. In addition, the quasi-steady southerly flow would help transport more moist air of tropical origin northward, providing necessary moisture for the precipitation generation over the TP. Note that a thermal trough lagged behind the height trough, with cold and warm advection occurring behind and ahead of the trough axis, respectively, except in the vicinity of the Rashmi vortex. As will be seen in the next section, the thermal wave structures were indicative of frontogenesis, whereas the relative weak-gradient environment allowed the Rashmi vortex to maintain its vortex identity until reaching steep topography along the southeastern margin of the TP.

Since most BoB TCs do not produce extreme precipitation over the TP, it is of interest to examine what anomalous signal(s) in the larger-scale flows accounted for the present EPE. For this purpose, Fig. 3c displays the geopotential height anomaly field with respect to its 30-yr climatic mean field at 500 hPa. Results show a southeast–northwest-elongated negative anomaly from the tropical BoB to higher latitudes, corresponding to the distribution of the abovementioned trough, and a positive anomaly on the east, corresponding to the distribution of the semipermanent WPSH. As a result, a southerly flow regime with a wide span in the zonal direction formed between the positive and negative anomalies. This tended to increase the northward steering of Rashmi, and the transport of moisture to the southern TP, as mentioned above. This result indicates the importance of the anomalous southerly flow, resulting from the merging of the Rashmi vortex into a meridionally deep trough, in the generation of this EPE compared to the climatology of 500-hPa circulation on the south of the TP.

The abovementioned height trough with the WPSH to its east could also be seen at 200 hPa (Fig. 3b), i.e., with the former tilted northwestward from that at 500 hPa; similarly for the 200-hPa negative and positive height anomalies (see shadings in Fig. 3c). Of relevance was the distribution of an upper-level southwesterly jet stream ahead of the trough axis that extended from the southwest through the study area at 0000 UTC 27 October. This upper-level jet stream appeared to account for the distribution of the long and narrow cloud band and light to moderate precipitation over the TP prior to and after the approach of Rashmi near 0600 UTC 27 October (cf. Figs. 3b and 2a–c), as mentioned in section 3. They were also associated with the quasigeostrophic ascent ahead of the midlevel trough axis and the advection of clouds and moisture in the upper-level anticyclonic outflow layer of Rashmi.

5. Mesoscale processes

As the Rashmi vortex carrying moist and warm air was steered toward the TP by the dominant southerly flow, its structural evolution and interaction with complex topography must play an important role in generating the EPE along the southeastern margin of the TP. Thus, in this section, we examine the mesoscale processes leading to the present EPE, and particularly attempt to understand why it took place mainly over the Cona and Bomi regions.

a. Structural evolution of the Rashmi vortex

Figure 4 shows the zonal and meridional vertical cross sections of temperature and geopotential height deviations through the center of the Rashmi vortex at two different stages: one close to its landfall and the other at the occurrence of the EPE. It is evident that deep warm columns are overlaid by lower heights in the inner region of the storm during both stages. In particular, the warm core centered at 400 hPa and the lower height peaked at the surface are vertically upright, with little evidence of extratropical transitional features (Hart 2003). The lower heights only showed slightly northward tilt as the storm moved close to the TP at 0600 UTC 27 October (cf. Figs. 4b,d). This implies that Rashmi maintained its near-tropical characters under the larger-scale environment with relatively weak vertical wind shear on the south of the TP, in contrast to the presence of intense vertical wind shear associated with an upper-level jet over the TP (cf. Figs. 3 and 4). A calculation of the vertical wind shear over a 600 km × 600 km area across the low-level TC center gave 2.4, and 11.6 m s−1 between 400 and 850 hPa at 1200 UTC 26 October, and 0000 UTC 27 October, respectively. The weak vertical shear prior to 0000 UTC 27 October could be attributed to the fact that the storm was propagating in the warm sector ahead of a cold front to be shown in section 5c, where relative weak-gradient flows were present, as discussed in the preceding section. The increased vertical shear, mostly southerly, after 0000 UTC 27 October was attributable to the approach of Rashmi with its upper portion (i.e., above 500 hPa) tilting toward the upper-level jet stream; the westward vortex tilt could be seen from Fig. 4c, albeit at a later time. The blocking effect of the TP on the southerly wind could help reduce the vertical wind shear, at least below 500 hPa, as evidenced by weak flows occurring between the storm and TP, as indicated by rectangular areas in Figs. 4b,d. The negative height deviations over the TP just reflected the distribution of the midlevel trough with colder air below in the north (cf. Figs. 4b,d and 3a). Note a deep layer of northwesterly cold air that appeared on the west (i.e., in the outer region) of Rashmi by 0600 UTC 27 October, as indicated by the northward tilted negative temperature and height deviations up to 450 hPa (Figs. 4c,d). The negative deviations coincided with the vertical distribution of a cold front to be shown in section 5c. Based on the above analysis, we may view the Rashmi vortex with equivalent barotropic characters that was surrounded by larger-scale baroclinic flows (i.e., the trough/frontal system) over the TP and on the west, but weak-gradient flows in the zonally wide southerly flow regime on the east, especially below 500 hPa. The larger-scale flows differed significantly from the extratropical transition of typical TCs or landfalling TCs (e.g., Klein et al. 2000; Jones et al. 2003; Shin and Zhang 2017).

Fig. 4.
Fig. 4.

Vertical cross section of temperature deviations (color shading; °C), geopotential height deviations (contoured at 2-dagpm intervals), and in-plane flow vectors with vertical component multiplied by 100 that is taken through Rashmi’s center zonally along the latitudes of (a) 19.8°N and (c) 25.0°N; and meridionally along the longitudes of (b) 88.8°E and (d) 90.6°E. Shown are (top) 1200 UTC 26 Oct and (bottom) 0600 UTC 27 Oct 2008. All deviations at individual levels are calculated by subtracting their averaged values over an area of 1500 km × 1500 km centered at the storm.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

Negative temperature deviations also occurred below 700 hPa in the inner region of Rashmi and on the south of the TP, which were more or less associated with evaporative (and melting) cooling of precipitation, as indicated by weak downdrafts beneath slantwise updrafts (e.g., Figs. 4c,d); further evidence of the weak downdrafts can be seen in section 5c. In addition, pronounced slantwise upward motion appeared at 0600 UTC 27 October, with intense upward motion reaching at least 200 hPa (Fig. 4d). Evidently, this intense upward motion resulted from the generation of tremendous latent heat release associated with extreme precipitation during this stage (Fig. 2d), as the southerly moist air was forced to ascend over the southern sloping surface of the TP, which will be further examined in section 5c.

b. Moisture source and topographical effects

After seeing the vertical structures of the Rashmi vortex and larger-scale flows, we can now quantify their relative importance of moisture supply and the topographical effects in generating extreme precipitation (Doswell et al. 1996; Xia and Zhang 2019). For this purpose, the vertical integrated water vapor flux vectors are given in Fig. 5a, showing the presence of two distinct moisture channels just at the time the Rashmi vortex reached the southeastern margin of the TP and the EPE was about to begin: an intense south to southeasterly flow of the vortex toward Cona, and a southerly flow of moderate intensity with a wide zonal span ahead of the midlevel trough axis on the western periphery of the WPSH, as mentioned before (Figs. 3a,c). Figure 5b presents time series of the vertical integrated water vapor flux at the three stations of interest, after considering the impact of topographical blocking at individual times (see section 2). Results show similar increasing trends and magnitudes among the three stations until 0000 UTC 27 October. However, a sharp 6-h increase at Cona occurred afterward as the moist vortex approached the steep topography, followed by a rapid drop as the associated moist air was advected eastward on the upstream side of the mountain ridge. The sharp variations in moisture flux around 0600 UTC 27 October were in marked contrast to moderate changes at the other two stations. Nevertheless, these temporal variations were consistent with those of precipitation at the three stations (cf. Figs. 5b and 2d). With the topographical blocking effects in mind, we may conjecture that the moderate southerly moist air was responsible for the general distribution of light to moderate precipitation over the TP, including areas near Zayu, prior to the approaching Rashmi, while the EPE did not occur until the moist Rashmi vortex moved close to the southeastern margin of the TP, especially to Cona (cf. Figs. 5, 1, and 2). The abrupt northward decreases in moisture flux on the downstream of the major mountain ridge can be understood by the fact that much moisture was condensed to precipitation as moist air parcels were forced to ascend moist adiabatically over the steep sloping surface, which will be seen in section 5c.

Fig. 5.
Fig. 5.

(a) Vertical integral of moisture flux vectors and shading, QFLUX (kg m−1 s−1), with P_bottom set to the surface pressure [see Eq. (1)], at 0600 UTC 27 Oct 2008. (b) Time series of QFLUX at about 150 km to the south of Cona (orange), Bomi (green), and Zayu (blue), with P_bottom determined by the unity moist Froude number at 6-h intervals (see section 2 for the calculation).

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

The pressure–time cross sections of relative humidity (RH) and vertical motion that are averaged over the study area prior to and during the approaching Rashmi vortex are given in Fig. 6a, showing two distinct cloudy layers: one appeared in the 150–250-hPa layer with weak ascent that was associated with the upper-level jet stream during the early stages and then under the influences of anticyclonic outflows of Rashmi, as mentioned before, and the other occurred in the 700–300-hPa layer with stronger ascent that increased in both RH and upward motion with time. The more rapid increases in RH after 1200 UTC 26 October was more closely related to the approaching Rashmi vortex and its interaction with local topography (cf. Figs. 6a and 4d), as will be further discussed in section 5c. The maximum intensities in both quantities occurred at 0600 UTC 27 October, when Cona and Bomi began to experience extreme precipitation (cf. Figs. 6a and 2d).

Fig. 6.
Fig. 6.

(a) The pressure (700–100 hPa)–time cross section of vertical velocity (contoured at 0.1 Pa s−1 intervals; dashed are negative and solid are positive) and relative humidity (shaded; %) that are averaged over the study area; and (b) the moist Froud number values at individual pressure levels at about 150 km to the south of Cona (red), Bomi (green), and Zayu (blue) at 0600 UTC 27 Oct 2008, assuming an elevation of 4 km for the major mountain ridge (at about 625 hPa); See section 2 for their calculations.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

The topographic blocking of unsaturated air can be roughly evaluated through the moist Froude number (Frm) for air parcels at individual pressure levels climbing the upstream mean ridge elevation of 4 km at the time of strong ascent (Fig. 6b). Results show that the moist Froude numbers at the abovementioned three stations are less than unity for moist air parcels below roughly the 750-hPa level. This means that only (unsaturated) moist air above the 750-hPa level could contribute to the generation of extreme precipitation at the three stations during the EPE stage. Of importance is that at 0600 UTC 27 October, lower-level air at both Cona and Bomi than that at Zayu could be lifted across the 4-km high mountain ridge, due to the approach of Rashmi with enhanced southerly flows and reduced moist static stability to be shown in Figs. 7 and 8. This result helps reveal why much less precipitation was produced at Cona and Bomi than that at Zayu before 1200 UTC 26 October (Fig. 2d). That is, it is attributable to the presence of weaker southerly flows with larger moist static stability, as can also be inferred from Figs. 7 and 8 below.

Fig. 7.
Fig. 7.

Horizontal distribution of the equivalent potential temperature distribution (θe; contoured at 2-K intervals), and horizontal wind vectors (m s−1) at 500 hPa, and the precipitable water (PW; mm) at (a) 1200 UTC 26 Oct, (b) 0000 UTC 27 Oct, (c) 0600 UTC 27Oct, and (d) 1800 UTC 27 Oct 2008. The PW is calculated with the ERA5 data as the column-integrated water vapor content divided by water density. Note that PW outside the study area and on the south of the southeastern margin of the TP is not shown. Red stars from the left to right denote the locations of Cona, Bomi, and Zayu, respectively.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

Fig. 8.
Fig. 8.

South–north vertical cross section of the equivalent potential temperature (contoured at 2-K intervals), in-plane flow vectors (m s−1) with the vertical component multiplied by 100, and relative humidity (shadings only for ≥90%), along the longitudes of (a) 94°E (i.e., between Cona and Bomi) at 1200 UTC 26 Oct; (b) 97.5°E through Zayu at 1200 UTC 26 Oct; (c) 92°E through Cona at 0600 UTC 27 Oct; and (d) 95.8°E through Bomi at 0600 UTC 27 Oct 2008. Gray shading denotes terrain. Red-dashed lines in (b) and (c) indicate the layer of slantwise instability. One may note the elevations of Zayu and Bomi that differ from those denoted, which could be attributed to the coarse terrain resolution used; they were all located on the downstream of higher mountains.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

c. Interaction of the Rashmi vortex with steep topography

In this subsection, we focus on the interaction of the warm and moist south to southwesterly flows with complex terrain, especially as the Rashmi vortex reached the southeastern margin of the TP, during the EPE stage. Figure 7 shows the evolution of a warm/cold front system at 500 hPa, as defined herein by pronounced gradients in the equivalent potential temperature (θe). Specifically, at 1200 UTC 26 October when moderate precipitation was observed at the abovementioned three stations (cf. Figs. 7a and 2d), an elongated southwest–northeast-oriented cold frontal zone with large θe gradients was distributed over lower-elevated regions, i.e., on the south of the high TP, and a warm frontal zone with much weak θe gradients over the southern portion of the study area (i.e., the TP). The Rashmi vortex with higher-θe air in its core area moved northward ahead of the cold front that coincided closely with the westward tilted temperature and height deviations shown in Fig. 4b. Apparently, the confluence of the warm, moist southerly flow to the east and the cold, dry southwesterly flow to the west accounted for the cold frontogenesis, while the advection of high-θe air by the south to southwesterly flows played a role in the formation of the warm front over the southern portion of the study region (also see Fig. 3a). As shown in Fig. 8, the topographical lifting of high-θe air from the layers below the TP could also play an important role in forming the warm front. Like the vertically integrated moisture flux (QFLUX), sharp gradients in precipitable water (PW) occurred along the southeastern margin of the TP, where steep topography is located (cf. Figs. 7, 5a, and 1). Note that PW values were about 10, 12.5, and 15 mm near Cona, Bomi, and Zayu, respectively, which were more or less determined by their elevations, given the relatively uniform moisture distribution in the south to southwesterly flows. In general, the PW values regulate to some extent the magnitude of precipitation rate; namely, higher rainfall rates should be expected at Zayu, but less amounts at the other two stations. It is evident from Fig. 7 that it was the high-θe air overrunning the warm front that assisted the generation of light to moderate precipitation over the TP. However, the occurrence of the EPE at the southeastern margin involved the important uplifting by local steep topography, which could not be inferred from Fig. 7.

As the Rashmi vortex moved close to the southeastern margin of the TP, its southeast to easterly flows, more evident in the lower troposphere, advected the cold frontal zone westward at 0000 UTC 27 October (Fig. 7b), while its northwest to westerly flows helped displace the cold front to the south of vortex by 0600 UTC 27 October (Fig. 7c). Meanwhile, the general southerly flows were enhanced by Rashmi’s cyclonic flow. As a result, increasing advection of higher-θe air occurred along the southern border of the study region with enhanced south to southwesterly flows; for example, the 500-hPa southerly wind component at Cona, Bomi, and Zayu increased from 11, 12, and 14 m s−1 at 1200 UTC 26 October to 20, 26, and 25 m s−1 at 0600 UTC 27 October, respectively (cf. Figs. 7a,c). The enhanced flows coincided well with the extreme precipitation production at Cona and Bomi (cf. Figs. 7c and 2d). In contrast, despite the enhanced flows at Zayu, it received much less precipitation than that at the other two stations during 0600–1200 UTC 27 October. This could be attributed to the reduced frontal lifting as the warm front moved northward away from this station (cf. Figs. 7a–c). Clearly, the Rashmi vortex produced the most significant influences on the generation of extreme snowfall at Cona due to their closest distance. Subsequently, precipitation at Cona and Bomi was markedly reduced after the Rashmi vortex collapsed upon reaching steep topography (with 4–5-km altitudes) of the TP. Shortly after 1200 UTC 27 October, its higher-θe air became occluded with some fluctuating structures along the southeastern margin of the TP (Fig. 7d). This led to the eastward acceleration of the cold front (cf. Figs. 7c,d), and the termination of snowfall at Cona and the marked reduction of rainfall at Bomi after 1200 UTC 27 October (Fig. 2d). This result is also consistent with rapid drops in the vertically integrated moisture flux (Fig. 5b).

As compared to precipitation at Cona and Bomi, rainfall at Zayu began earlier, i.e., at 0000 UTC 26 October, due likely to the less blocking of the southerly flow of high-θe air. Its rainfall rate intensified rapidly from 2 to 19 mm (6 h)−1 by 0600 UTC 26 October (Fig. 2d). Because this station experienced much less direct influences of the Rashmi vortex, its 6-h peak rainfall rate was much smaller than that at both Cona and Bomi (cf. Figs. 7d and 2d). Rainfall at Zayu was terminated after the cold frontal zone moved to the east of the study region (not shown). Its 2-day accumulated amount was about 90 mm.

To illustrate how the moist southerly flow interacted with complex topography, Figs. 8a shows the south–north vertical cross sections of θe, in-plane flow vectors, and relative humidity at the very early stage through a location between Cona and Bomi, while Figs. 8b–d present those through Zayu, Cona, and Bomi, respectively, near their peak precipitation times. First of all, the EPE under study occurred under potentially stable conditions, as indicated by positive vertical θe gradients, with slantwise upward motion along northwestward-tilted warm-frontal surfaces over the TP, and an incoming southerly flow along near-flat, albeit slightly fluctuating, θe surfaces above 600 hPa on the south of the TP. Clearly, the frontal lifting of the southerly moist air in association with the quasigeostrophic ascent from the midlevel trough played an important role in generating light to moderate precipitation with favorable upward motion and high RH (e.g., >98%) columns over the TP, as mentioned before. Second, lower-θe air pools with weak descending motion and smaller RH (i.e., <90%) were seen over the foothills of the TP, especially in its southern valleys, some of which extended from 700 hPa down to the surface layer along the sloping topography (e.g., Figs. 8a,b). The cold and dry pools could be considered as a result of cooling by evaporation (and melting) of precipitation particles in the moist columns with RH > 98% above; this evaporative cooling accounted for the generation of moist downdrafts, as mentioned in section 5a. Third and more importantly, we see a weak vertical θe gradient layer, albeit slightly positive, centered at 700 hPa (Figs. 8a–c), and even a slantwise unstable layer with negative vertical θe gradients below 700 hPa (Bennetts and Hoskins 1979; Zhang and Cho 1992) to the south of 25°N, as indicated by red-dashed lines in Figs. 8b and 8c. They were indicative of the approaching high-θe air associated with Rashmi (cf. Figs. 8 and 7a–c). More importantly, this layer of high-θe air could be lifted to contribute to the extreme precipitation production at both Cona and Bomi, as explored before (Fig. 6b). Farther to the south was potentially unstable atmospheric columns, as indicated by negative vertical θe gradients below 600 hPa (not shown). Fourth, given the weak potential stability and relatively stronger southerly flow approaching, only air parcels above 750-hPa level could generally be lifted to the TP surface, as discussed in section 5b, unless under moist ascent (i.e., with RH > 98%) in the presence of latent heat release. The latter could be seen by sloping θe surfaces on the southern terrain of the TP that was superimposed by cold pools.

With the above general understanding of cross-sectional flows, we may now discuss more detailed flow structures associated with the EPE at individual stations. As indicated by Fig. 2d, Zayu had experienced heavy rainfall for several hours by 1200 UTC 26 October, while the other two stations had received light to moderate precipitation. This could be attributed to the presence of more favorable conditions at Zayu, including the supply of slantwise unstable air, less blocking of the southerly moist air in terms of Frm, and the mesoscale concaved terrain shape (Fig. 1a) that facilitated confluence of airflows. By this time, the midlevel high-θe air associated with the Rashmi vortex had also arrived at near 26°N, i.e., between Cona and Bomi. Of importance to note is the lifting of the midlevel high-θe air by the southerly flow over the sloping topography that facilitated the formation of the warm front (Fig. 8a). In particular, there was little evidence of the warm front over low-elevated regions on the south of the TP, where little horizontal θe gradients were present (Fig. 7). The higher topography and stronger incoming flows, the larger was the horizontal θe gradients generated along the southeastern margin of the TP (cf. Figs. 8a–c). In this regard, the warm front over the TP was more or less produced by the uplifting of southerly higher-θe air at the levels lower than the TP surface.

At 0600 UTC 27 October, the EPE with record-breaking precipitation began to occur at both Cona and Bomi, which coincided well with the approaching high-θe air associated with the Rashmi vortex (cf. Figs. 8c,d and 2d). Because of its close distance from Rashmi, Cona experienced the largest uplifting, with near-upright θe contours, and the peak 6-hourly precipitation rate. Again, a boundary layer of large θe gradients was superimposed on the sloping topography, confirming the role of topographical lifting of high-θe air by intense southerly flows in developing the warm front over the TP. Clearly, this in turn helped extend clouds and precipitation northward into the central portion of the TP, as the southerly high-θe air in the layers above the TP overran the warm frontal surfaces. Precipitation weakened rapidly once the intense southerly high-θe airstream moved eastward away from the two stations. However, the occluded frontal system produced some notable precipitation at Bomi but little at Cona during the period of 1200 UTC 27 October–0000 UTC 28 October. This is again consistent with the rapid drop in the vertically integrated moisture flux.

d. A upper-air sounding related to extreme precipitation and phase changeover

In the present EPE, extreme snowfall and rainfall were both observed, depending mostly on local terrain elevations along the southeastern margin of the TP, i.e., snowfall at locations of above 4-km elevation, and rainfall at much lower elevations. It is well known that a small amount of rain can turn, roughly at a ratio of 1:10, into a severe snowstorm with significant damage and high societal impact. Thus, it is desirable to discuss the generation of the two different precipitation types in relation to ambient temperatures and associated weather phenomena, especially when a changeover from rain to extreme snow occurs.

As mentioned in section 3, extreme snowfall and rainfall occurred at Cona (z = 4281 m MSL) and Zayu (z = 2331 m MSL), respectively, while Bomi (z = 2737 m MSL) underwent changeovers from rain to snow, and later back to rain again. The single-phased precipitation types at Zayu and Cona can also be readily seen from Fig. 9, showing that the 0°C isotherm at Zayu always maintained at 1500–1800 m above the ground with 4°–7°C surface temperatures, whereas surface temperatures at Cona were mostly below 0°C due to its high terrain elevation, only slightly above 0°C during 0600–1200 UTC 27 October.

Fig. 9.
Fig. 9.

Temporal variations of (a) the 0°C isotherm height (m), (b) the surface air temperature (°C) at Cona (in red) where only snowfall occurred; Bomi (in green) where rainfall changed to snowfall and to rainfall again; and Zayu (in blue) where only rainfall occurred. The filled circles, snowflake symbols, and open circles indicate rainfall, snowfall, and no precipitation, respectively.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

In contrast, the altitude of 0°C isotherm at Bomi was on average more than 600 m lower than that at Zayu (Fig. 9a). It changed little between 1200 and 1800 UTC 26 October, when a changeover from rain to snow took place, but became more elevated during the subsequent 6-h period of snowfall. The increased altitude of 0°C isotherm after 1800 UTC 26 October suggests the presence of enhanced warm advection by the approaching Rashmi vortex, as could be inferred from the northward displacement of θe = 340 K contour in Figs. 7a and 7b. This increased altitude (and enhanced warm advection) was consistent with the changeover from snowfall back to rainfall after 0000 UTC 27 October. Of interest is that the surface temperature at Bomi dropped from 5° to 1°C during the first 6 h and then to 0°C at 0000 UTC 27 October, despite the presence of warm advection (Fig. 9b). When considering the altitude of 0°C isotherm and the surface melting temperature in Figs. 9a and 9b, we may view the surface temperature drops as a result of cooling due to melting snowflakes and frozen precipitation, which started from the melting level, layer by layer downward to the surface, during the first 6-h rainfall period. This gave rise to the formation of an unusually deep melting layer of 1000–1100 m at 0000 UTC 27 October (cf. Figs. 9a,b).

The above conjecture is confirmed by a skew T–logp diagram of sounding data taken at 0000 UTC 27 October at Linzhi (Fig. 10), which is close to Bomi. Indeed, this sounding exhibited a melting layer from the surface, at about 700–600 hPa. Such a deep melting layer is not common because the melting cooling is typically much smaller in magnitude than other latent heating effects (Stewart et al. 1984; Stewart and Macpherson 1989). Its formation was often associated with the persistent extreme snowfall, especially its changeover from rain, which developed in convectively and symmetrically stable columns (cf. Figs. 10 and 8c). Its formation also requires the presence of a saturated layer allowing little evaporative cooling and having weak thermal advection and vertical motion (Szeto et al. 1988a,b; Stewart and Macpherson 1989; Stewart et al. 1996). Moreover, the melting cooling appeared to account for the generation of lower-θe pools and facilitate frontogenesis at the foothill near Bomi (Fig. 8c). The above analysis suggests that Bomi and its neighboring regions could be a transition zone between snowfall on the west and rainfall on the east, though still depending on local terrain elevations.

Fig. 10.
Fig. 10.

Skew T–logp diagram taken at 0000 UTC 27 Oct 2008 at the Linzhi station (29.6°N, 94.5°E; z = 3001 m MSL; see Fig. 1a for its location), where snowfall occurred from 1800 UTC 26 Oct to 0000 UTC 27 Oct 2008. This is the nearest upper-air station to Bomi.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

It is worthwhile exploring the development of a profound saturated layer from the surface to the upper troposphere, especially with a moist-adiabatic layer between 400 and 300 hPa (Fig. 10). This upper-air sounding was obtained in a potentially and conditionally stable column, and it was unlikely a result of mesoscale slantwise ascent over the warm front. We may assume that such a deep saturated layer with an upper moist-adiabatic layer would also appear in extreme-precipitation-producing storms along the southeastern margin of the TP, at least in the vicinity of Cona and Bomi. Clearly, such an unusual upper-air sounding resulted from intense upward motion driven by enormous latent heat released in those storms, with ample moisture supplied mostly by the approaching Rashmi vortex. In particular, the development of the upper moist-adiabatic layer reveals the importance of enhanced latent heating through depositional growth associated with extreme snow production directly from water vapor. Some of these phenomena could be seen from Figs. 8c and 8d, showing strong updrafts with near-upright θe surfaces below 400 hPa, and near-saturated layers from 700 to 200 hPa at the leading mountain ridges. Apparently, the development of the deep saturated columns would in turn facilitate more effective release of latent heat, maintain intense updrafts and moisture convergence, and amplify precipitation production, particularly in the upper moist-adiabatic layer. In short, the large-sized TC vortex with ample moisture and some degree of slantwise instability, the larger-scale southerly flow enhanced by the TC vortex flow, the local steep topography and its close interaction with the approaching vortex favored the generation of the EPE at Cona, and less favored at Bomi due to their reduced influences far away from the vortex.

6. Summary and conclusions

In this study, an extreme precipitation event along the southeastern margin of the TP associated with the approaching TC Rashmi from the Bay of Bengal on 26–27 October 2008 is examined by using the ECMWF global reanalysis data (i.e., ERA5) and available surface, upper-air and remote sensing observations. Figure 11 shows a conceptual model of the three key processes associated with the generation of extreme precipitation over the southeastern TP by the approach of Rashmi. They are described as follows.

Fig. 11.
Fig. 11.

A conceptual model showing the main ingredients associated with the generation of extreme precipitation (snow: snowflake symbol in blue; rain: raindrop symbol in blue) over the southeastern TP (beige background) by the approaching TC Rashmi (light orange shading) with its track (yellow) originating from the BoB. The large-scale environment, in which Rashmi was embedded, consisted of a westerly northwestward-tilted trough with descending airflows (blue arrows) behind the midlevel and upper-level trough axis (dashed lines in brown), a surface cold front over the plain region (green background) and a warm front over the southeastern TP, an upper-level jet stream (green arrow), and the WPSH (yellow). Rashmi was steered by southerly flows (orange arrows) ahead of the midlevel trough axis and at the western periphery of the WPSH. The extreme precipitation occurred at Cona and Bomi (hollow triangles) as the intense warm, moist vortex flows (white arrow) were topographically lifted (dark orange arrows) along the major mountain ridge of the southeastern TP. The figure topographical background is modified from https://www.shutterstock.com/zh-Hant/g/antartis.

Citation: Weather and Forecasting 37, 12; 10.1175/WAF-D-22-0067.1

First, an anomalous southerly warm-moist airstream with weak thermal gradients, resulting from the merging of Rashmi into a meridionally deep (baroclinic) trough along the western periphery of the semipermanent WPSH, played an important role in steering the TC vortex and transporting tropical warm and moist air northward, thereby supplying necessary moisture for the development of precipitation over the TP.

Second, topographical lifting of the southerly warm and moist air assisted the formation of a warm front superimposed on complex topography over the TP. The quasigeostrophic ascent ahead of the midlevel trough axis, together with the presence of an upper-level jet stream and the low-level warm front, appeared to account for extensive clouds and light to moderate precipitation generated over the TP prior to the approach of the Rashmi vortex. The above weak-gradient environment allowed the Rashmi vortex to maintain its equivalent-barotropic warm-cored structures during its northward movement until reaching the southeastern margin of the TP. In addition, its relatively large-sized rotation, as indicated by its relatively high fullness prior to landfall, tended to be more inertially stable with slower dissipation.

Third, the uplifting of the southerly warm, moist airstream, upon approaching topography, triggered cloud condensation and latent heat release, and induce intense upward motion, thereby leading to the generation of heavy to extreme precipitation along the southeastern margin of the TP. Calculation of the moist Froude number indicates that only the southerly airstream roughly above 750 hPa could be uplifted over the mean elevated topography. The most influential topographical uplifting of the intense vortex flows carrying ample moisture and some degree of slantwise instability occurred at Cona, where the record-breaking daily snowfall of 98 mm was observed; and then at Bomi, where the daily precipitation of 87 mm with rain–snow–rain changeovers was produced.

The extreme precipitation and the phase changeovers could be uncovered by an observed skew T–logp diagram, showing a profound saturated layer from the surface to the upper troposphere with a deep upper moist-adiabatic layer, and a deep bottom melting layer. In particular, it reveals the importance of the steep topographical uplifting of the vortex flows in inducing intense updrafts that were driven by enormous latent heat release in the absence of significant dynamic instability. The presence of the deep saturated columns would in turn facilitate more effective latent heat release, maintain intense updrafts, and enhance precipitation production, especially in the upper moist-adiabatic layer.

In conclusion, we may state that (i) the anomalous southerly flow under a weak-gradient environment allowed the Rashmi vortex to maintain its TC identity during its movement toward the TP and helped supply necessary moisture for precipitation production along the southeastern margin of the TP; and (ii) the close interaction of the large-sized vortex flows carrying ample moisture with the steep topography favored the generation of the EPE more at Cona and nearby regions, and less at Bomi, and the other stations that were far away from the vortex influences. Of course, there are some caveats with the above results, especially on the interaction of the intense vortex flows with local steep topography, extreme precipitation production over such a high plateau, and phase changeovers and their impacts on mesoscale circulations over complex topography, including the warm front and mountain valley flows. Most of them could be attributed to the lack of high-resolution observations over the sparsely populated mountainous regions in the TP. A series of high-resolution model simulations of this EPE appears to be much needed in order to provide a more complete understanding of the multiscale processes leading to the generation of extreme precipitation along the southeastern margin of the TP. Nevertheless, the above results appear to have important implications to the weather analysis and forecast of heavy precipitation associated with BoB TCs approaching the southern margin of the TP, and they are also applicable to the TC-related heavy precipitation over complex topography over the other regions of the globe. In particular, given the pronounced underprediction of the present EPE, more work is needed to increase the numerical model grid resolutions in order to better resolve local topography and TC structures, and improve the model initial conditions with more upper-air observations over the mountainous TP, and the model cloud microphysical processes by incorporating multiphase hydrometeors.

1

The amount of snowfall is measured in millimeters as snow collected in a standard container at a surface station is converted by melting it into water. For the convenience of the subsequent description, the term of “precipitation” is used herein without distinguishing its phase as rainfall or snowfall, while “accumulated precipitation” denotes the sum of rainfall and snowfall.

Acknowledgments.

We thank three anonymous reviewers for their constructive comments that helped improve the presentation of this study. This work was supported by the National Natural Science Foundation of China (41930972, 51778617, and 42005141), U.S. NSF Grant AGS2202766, and the Science and Technology Development Funds of the Chinese Academy of Meteorological Sciences (2020KJ019 and 2020KJ022).

Data availability statement.

The ECMWF global reanalysis data (i.e., ERA5) were downloaded from online archive system at https://cds.climate.copernicus.eu/cdsapp#!/search?type=dataset. The best track data were derived from http://www.metoc.navy.mil/jtwc/jtwc.html. The terrain elevation data are available online at https://www.ngdc.noaa.gov/mgg/global/etopo2.html. The FY-2D cloud top equivalent blackbody brightness temperature data are openly available at http://satellite.nsmc.org.cn/portalsite/Data/Satellite.aspx. Due to confidentiality agreements, ground observation data can only be made available to bona fide researchers subject to a nondisclosure agreement. Details of the data and how to request access are available from datacenter@cma.gov.cn at the China Meteorological Administration.

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    • Export Citation
  • Shin, J.-H., and D.-L. Zhang, 2017: The impact of moist frontogenesis and tropopause undulation on the intensity, size, and structural changes of Hurricane Sandy (2012). J. Atmos. Sci., 74, 893913, https://doi.org/10.1175/JAS-D-15-0362.1.

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    • Export Citation
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    • Export Citation
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  • Fan, X., Y. Li, A. Lyu, and L. Liu, 2020: Statistical and comparative analysis of tropical cyclone activity over the Arabian Sea and Bay of Bengal (1977–2018). J. Trop. Meteor., 26, 441452, https://doi.org/10.46267/j.1006-8775.2020.038.

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  • Guo, X., and Z. M. Tan, 2017: Tropical cyclone fullness: A new concept for interpreting storm intensity. Geophys. Res. Lett., 44, 43244331, https://doi.org/10.1002/2017GL073680.

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    • Export Citation
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  • Jones, S. C., and Coauthors, 2003: The extratropical transition of tropical cyclones: Forecast challenges, current understanding, and future directions. Wea. Forecasting, 18, 10521092, https://doi.org/10.1175/1520-0434(2003)018<1052:TETOTC>2.0.CO;2.

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  • Kumar, S., P. Lal, and A. Kumar, 2020: Turbulence of tropical cyclone ‘Fani’ in the Bay of Bengal and Indian subcontinent. Nat. Hazards, 103, 16131622, https://doi.org/10.1007/s11069-020-04033-5.

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  • Lin, C. Y., H. M. Hsu, Y. F. Sheng, C. H. Kuo, and Y. A. Liou, 2011: Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over southern Taiwan. Atmos. Chem. Phys., 11, 345361, https://doi.org/10.5194/acp-11-345-2011.

    • Search Google Scholar
    • Export Citation
  • Lin, Z., Z. Bianba, S. Wen, and Z. Zhou, 2015: Objective classification of the tracks of tropical storms in the Bay of Bengal. J. Trop. Meteor., 21, 222231, https://doi.org/10.16555/j.1006-8775.2015.03.002.

    • Search Google Scholar
    • Export Citation
  • Liu, L., Y. Li, and Y. Zhao, 2015: Impact of Storm Phailin (1302) over the Bay of Bengal on one snowstorm process in southern Tibetan Plateau (in Chinese). Meteor. Mon., 41, 10791085, https://doi.org/10.7519/j.issn.1000-0526.2015.09.004.

    • Search Google Scholar
    • Export Citation
  • Lv, A., Y. Wen, and Y. Li, 2013: Study of the impact of Tropical Cyclone Akash (0701) over the Bay of Bengal on a heavy rainfall event in Southwest China (in Chinese). Chin. J. Atmos. Sci., 37, 160170, https://doi.org/10.3878/j.issn.1006-9895.2012.12040.

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  • Mao, J., and G. Wu, 2011: Barotropic process contributing to the formation and growth of Tropical Cyclone Nargis. Adv. Atmos. Sci., 28, 483491, https://doi.org/10.1007/s00376-010-9190-4.

    • Search Google Scholar
    • Export Citation
  • Parrish, J. R., R. W. Burpee, F. D. Marks, and R. Grebe, 1982: Rainfall patterns observed by digitized radar during the landfall of Hurricane Frederic (1979). Mon. Wea. Rev., 110, 19331944, https://doi.org/10.1175/1520-0493(1982)110<1933:RPOBDR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Pendergrass, A. G., 2018: What precipitation is extreme? Science, 360, 10721073, https://doi.org/10.1126/science.aat1871.

  • Peng, Y., X. Zhao, D. Wu, B. Tang, P. Xu, X. Du, and H. Wang, 2018: Spatiotemporal variability in extreme precipitation in China from observations and projections. Water, 10, 1089, https://doi.org/10.3390/w10081089.

    • Search Google Scholar
    • Export Citation
  • Prokop, P., and A. Walanus, 2015: Variation in the orographic extreme rain events over the Meghalaya Hills in northeast India in the two halves of the twentieth century. Theor. Appl. Climatol., 121, 389399, https://doi.org/10.1007/s00704-014-1224-x.

    • Search Google Scholar
    • Export Citation
  • Rasmussen, K. L., and R. A. Houze, 2012: A flash-flooding storm at the steep edge of high terrain: Disaster in the Himalayas. Bull. Amer. Meteor. Soc., 93, 17131724, https://doi.org/10.1175/BAMS-D-11-00236.1.

    • Search Google Scholar
    • Export Citation
  • Reeves, H. D., and Y. L. Lin, 2006: Effect of stable layer formation over the Po Valley on the development of convection during MAP IOP-8. J. Atmos. Sci., 63, 25672584, https://doi.org/10.1175/JAS3759.1.

    • Search Google Scholar
    • Export Citation
  • Shin, J.-H., and D.-L. Zhang, 2017: The impact of moist frontogenesis and tropopause undulation on the intensity, size, and structural changes of Hurricane Sandy (2012). J. Atmos. Sci., 74, 893913, https://doi.org/10.1175/JAS-D-15-0362.1.

    • Search Google Scholar
    • Export Citation
  • Stewart, R. E., and S. R. Macpherson, 1989: Winter storm structure and melting‐induced circulations. Atmos.–Ocean, 27, 523, https://doi.org/10.1080/07055900.1989.9649326.

    • Search Google Scholar
    • Export Citation
  • Stewart, R. E., J. D. Marwitz, J. C. Pace, and R. E. Carbone, 1984: Characteristics through the melting layer of stratiform clouds. J. Atmos. Sci., 41, 32273237, https://doi.org/10.1175/1520-0469(1984)041<3227:CTTMLO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Stewart, R. E., R. W. Crawford, K. K. Szeto, and D. R. Hudak, 1996: Horizontal aircraft passes across 0°C regions within winter storms. Atmos.–Ocean, 34, 133159, https://doi.org/10.1080/07055900.1996.9649560.

    • Search Google Scholar
    • Export Citation
  • Suo, M., and Y. Ding, 2014: A case study on the effect of southern branch trough in the subtropical westerlies combined with storm over the Bay of Bengal on plateau snowstorm (in Chinese). Meteor. Mon., 40, 10331047.

    • Search Google Scholar
    • Export Citation
  • Szeto, K. K., C. A. Lin, and R. E. Stewart, 1988a: Mesoscale circulations forced by melting snow. Part I: Basic simulations and dynamics. J. Atmos. Sci., 45, 16291641, https://doi.org/10.1175/1520-0469(1988)045<1629:MCFBMS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Szeto, K. K., R. E. Stewart, and C. A. Lin, 1988b: Mesoscale circulations forced by melting snow. Part II: Application to meteorological features. J. Atmos. Sci., 45, 16421650, https://doi.org/10.1175/1520-0469(1988)045<1642:MCFBMS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Tang, W., 2008: Thousands of people were trapped and three people were dead in dozens of counties in central and eastern Tibet (in Chinese). ChinaNews, 30 October 2008, https://www.chinanews.com/gn/news/2008/10-30/1430563.shtml.

  • Tao, W. K., and Coauthors, 2011: High-resolution numerical simulation of the extreme rainfall associated with Typhoon Morakot. Part I: Comparing the impact of microphysics and PBL parameterizations. Terr. Atmos. Oceanic Sci., 22, 673696, https://doi.org/10.3319/TAO.2011.08.26.01(TM).

    • Search Google Scholar
    • Export Citation
  • Wang, Y., and S. Wang, 1989: Tropical cyclones in the North Indian Ocean and its relationship with precipitation in Tibet (in Chinese). Meteor. Mon., 15, 3843.

    • Search Google Scholar
    • Export Citation
  • Wu, C. C., and Y. H. Kuo, 1999: Typhoons affecting Taiwan: Current understanding and future challenges. Bull. Amer. Meteor. Soc., 80, 6780, https://doi.org/10.1175/1520-0477(1999)080<0067:TATCUA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Wu, X., S. Guo, J. Yin, G. Yang, Y. Zhong, and D. Liu, 2018: On the event-based extreme precipitation across China: Time distribution patterns, trends, and return levels. J. Hydrol., 562, 305317, https://doi.org/10.1016/j.jhydrol.2018.05.028.

    • Search Google Scholar
    • Export Citation
  • Xia, R., and D.-L. Zhang, 2019: An observational analysis of three extreme rainfall episodes of 19–20 July 2016 along the Taihang Mountains in North China. Mon. Wea. Rev., 147, 41994220, https://doi.org/10.1175/MWR-D-18-0402.1.

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  • Fig. 1.

    (a) Two-day accumulated precipitation (sum of rainfall and snowfall; mm; colored dots), (b) rainfall amount (mm; colored dots), and (c) snowfall amount (mm; snowflake symbols) from 0000 UTC 26 Oct to 0000 UTC 28 Oct 2008. The inner box denotes the study area of heavy precipitation (28°–33°N, 89°–98°E) with the daily amount of ≥20 mm. The yellow line in (a) represents the southeastern margin of the TP and is similar in the rest of the figures. In (a), the locations (latitudes, longitudes, and terrain elevations) of Cona (27.98°N, 91.95°E, 4281 m MSL), Bomi (29.87°N, 95.77°E, 2737 m MSL), and Zayu (28.65°N, 97.47°E, 2331 m MSL) are indicated by triangles; the Linzhi station (29.57°N, 94.47°E, 3001 m MSL), at which an upper-air sounding is shown in Fig. 10, is denoted by an “L” above a triangle; and symbols, “QH,” “SC,” and “TB,” denote provinces of Qinghai and Sichuan, and Tibet Autonomous Region of China, respectively. The 2-, 3-, and 4-km terrain elevations over the TP in (b) and (c) are denoted by different gray contours.

  • Fig. 2.

    The center location of Rashmi, as denoted by the TC symbol (similar in the rest of the figures), and its track, the 30-min delayed equivalent blackbody brightness temperature (°C), and the subsequent 6-h accumulated precipitation (mm; filled circles: rainfall; snowflake symbols: snowfall) from surface rain gauges at (a) 1200 UTC 26 Oct, (b) 0000 UTC 27 Oct, and (c) 0600 UTC 27 Oct 2008. (d) Time series of rainfall (mm; solid color fill) and snowfall (mm; pattern fill) at 6-h intervals at Cona, Bomi, and Zayu (see Fig. 1a for their locations) during the 2-day period of 0000 UTC 26 Oct–0000 UTC 28 Oct 2008.

  • Fig. 3.

    (a) The 500-hPa geopotential heights (blue contoured at intervals of 4 dagpm), isotherms (dash contoured at intervals of 4°C), and wind barbs (a full barb is 5 m s−1); (b) the 200-hPa geopotential heights (blue contoured at intervals of 5 dagpm), and horizontal wind vectors (m s−1); and (c) the 500-hPa geopotential height anomaly (contoured; intervals of 1 dagpm) and 200-hPa geopotential height anomaly (shading) at 0000 UTC 27 Oct 2008 with respect to the 30-yr climatic mean height during 1979–2008. In (c), dashed contours and blue colors are for negative values.

  • Fig. 4.

    Vertical cross section of temperature deviations (color shading; °C), geopotential height deviations (contoured at 2-dagpm intervals), and in-plane flow vectors with vertical component multiplied by 100 that is taken through Rashmi’s center zonally along the latitudes of (a) 19.8°N and (c) 25.0°N; and meridionally along the longitudes of (b) 88.8°E and (d) 90.6°E. Shown are (top) 1200 UTC 26 Oct and (bottom) 0600 UTC 27 Oct 2008. All deviations at individual levels are calculated by subtracting their averaged values over an area of 1500 km × 1500 km centered at the storm.

  • Fig. 5.

    (a) Vertical integral of moisture flux vectors and shading, QFLUX (kg m−1 s−1), with P_bottom set to the surface pressure [see Eq. (1)], at 0600 UTC 27 Oct 2008. (b) Time series of QFLUX at about 150 km to the south of Cona (orange), Bomi (green), and Zayu (blue), with P_bottom determined by the unity moist Froude number at 6-h intervals (see section 2 for the calculation).

  • Fig. 6.

    (a) The pressure (700–100 hPa)–time cross section of vertical velocity (contoured at 0.1 Pa s−1 intervals; dashed are negative and solid are positive) and relative humidity (shaded; %) that are averaged over the study area; and (b) the moist Froud number values at individual pressure levels at about 150 km to the south of Cona (red), Bomi (green), and Zayu (blue) at 0600 UTC 27 Oct 2008, assuming an elevation of 4 km for the major mountain ridge (at about 625 hPa); See section 2 for their calculations.

  • Fig. 7.

    Horizontal distribution of the equivalent potential temperature distribution (θe; contoured at 2-K intervals), and horizontal wind vectors (m s−1) at 500 hPa, and the precipitable water (PW; mm) at (a) 1200 UTC 26 Oct, (b) 0000 UTC 27 Oct, (c) 0600 UTC 27Oct, and (d) 1800 UTC 27 Oct 2008. The PW is calculated with the ERA5 data as the column-integrated water vapor content divided by water density. Note that PW outside the study area and on the south of the southeastern margin of the TP is not shown. Red stars from the left to right denote the locations of Cona, Bomi, and Zayu, respectively.

  • Fig. 8.

    South–north vertical cross section of the equivalent potential temperature (contoured at 2-K intervals), in-plane flow vectors (m s−1) with the vertical component multiplied by 100, and relative humidity (shadings only for ≥90%), along the longitudes of (a) 94°E (i.e., between Cona and Bomi) at 1200 UTC 26 Oct; (b) 97.5°E through Zayu at 1200 UTC 26 Oct; (c) 92°E through Cona at 0600 UTC 27 Oct; and (d) 95.8°E through Bomi at 0600 UTC 27 Oct 2008. Gray shading denotes terrain. Red-dashed lines in (b) and (c) indicate the layer of slantwise instability. One may note the elevations of Zayu and Bomi that differ from those denoted, which could be attributed to the coarse terrain resolution used; they were all located on the downstream of higher mountains.

  • Fig. 9.

    Temporal variations of (a) the 0°C isotherm height (m), (b) the surface air temperature (°C) at Cona (in red) where only snowfall occurred; Bomi (in green) where rainfall changed to snowfall and to rainfall again; and Zayu (in blue) where only rainfall occurred. The filled circles, snowflake symbols, and open circles indicate rainfall, snowfall, and no precipitation, respectively.