1. Introduction

During the snow season of northeastern China (NEC), which mainly occurs from November through March, widespread snowfall (WS) is one of the most important synoptic phenomena, impacting local agriculture, the economy, and social activities to a large degree.

A number of studies have been conducted that examine the characteristics and mechanisms of snowstorm systems in China (Sun et al. 2009; Wen et al. 2009; Sun et al. 2010; Xie et al. 2010; Wang et al. 2011). The East Asian winter monsoon and its interannual–interdecadal variability are closely associated with the snowstorm activities (Wang and He 2012, 2013). Sun et al. (2009) searched for the causes of a heavy snowstorm over the northeastern parts of China in March 2007 and suggested that strong anomalous southerly water vapor transport (WVT) before the event greatly favored its occurrence. The anomalous WVT was mainly caused by anomalous atmospheric circulations, which were associated with anomalous Arctic Oscillations (AO; Thompson and Wallace 1998; Ho et al. 2011), Antarctic Oscillations (Thompson and Wallace 2000), and North Pacific Oscillations (Wallace and Gutzler 1981). Xie et al. (2010) analyzed the heavy snowfall over south China in 2008, finding that one of the important factors influencing snowfall is the active southern branch of currents over low latitude ocean areas, which provided plenty of water vapor for south China, collided with the enhanced northerly cold airflow, and finally caused the disaster. More recently, Wang et al. (2011) focused on an exceptionally heavy snowfall event in NEC that occurred on 12–13 April 2010. They claimed that this heavy snowfall primarily results from large convergences of water vapor and a strong rising motion over eastern NEC, which are mainly caused by a previously intensified and southeastward-shifted Siberian high (SH). Among these heavy snowfall events, WVT associated with anomalous East Asian winter monsoon circulation has no doubt played an important role in water vapor supply. However, as most of the aforementioned studies only examined specific cases, little information has been learned about the general features of WVT during WS events in NEC, particularly with respect to its influence on precipitation from a quantitative angle. As the global climate system is a chaotic system, no specific synoptic system could be exactly the same as any other; large discrepancies may thus exist among WS systems in NEC. Nevertheless, the common features that distinguish WS systems from other systems deserve more consideration. In a general way, the common features of WS systems could be identified through a composite analysis of multiple WS cases, whereby the most important characteristics of the WS process could be captured.

In general, precipitation over a region comes from three sources: moisture transported into the region by atmospheric advection, local evaporation, and the water vapor that is originally present over the region. Atmospheric moisture advection (i.e., the WVT) depends on the atmospheric flow field and the spatial distribution of water vapor; the local evaporation depends on surface conditions such as land cover; the steady state of water vapor present over a region depends on the storage capacity of the atmosphere and the surface moisture that evaporates into the atmosphere. Over the long term, WVT is the dominant contributor of moisture to precipitation around the globe, while evaporation makes a smaller contribution (Trenberth 1999). However, the ratios of WVT and evaporation to precipitation can vary from region to region and from season to season. In addition, few studies have addressed the contribution of the original water vapor to precipitation. The partitioning of precipitation into the three moisture sources during WS events is therefore of considerable interest, although WVT appears to be the most important influence on precipitation under most circumstances.

Numerous studies have examined summertime WVT over China and its association with precipitation (Simmonds et al. 1999; Zhou and Yu 2005; Jiang et al. 2009; Shen et al. 2010; Sun et al. 2011; Zhou et al. 2011; Wang and Chen 2012; Wei et al. 2012). It has been widely recognized that the summer rainfall over China, including NEC, is directly related to a southerly and southwesterly warm-moist airflow, dominated by the Indian and East Asian summer monsoons and the western Pacific subtropical high, which also characterizes the climatology of summer WVT (Ding and Chan 2005). On the other hand, northwesterly cold airflow prevails as the dominant water vapor supplier over NEC during winter (Zhang et al. 1997; Jhun and Lee 2004). Nevertheless, it remains unclear what the regime and general paths of WVT are like during WS events, and whether they are similar to or very different from the climatology of winter WVT. Moreover, from the view of climatology, WVT and precipitation are associated synchronously; in other words, winter precipitation is solely connected with winter WVT, and has nearly nothing to do with WVT in other seasons. However, WS systems are generally daily- or diurnal-scale synoptic systems. It is doubtful whether the intensity of water vapor gained by WVT is synchronous with precipitation at such a scale. In fact, several studies have suggested the importance of water vapor accumulation before snowstorms in NEC (Sun et al. 2009; Wang et al. 2011). Nevertheless, little effort has been made to quantitatively compare the contributions of preaccumulated water vapor and synchronous WVT to WS.

In this paper, the general WVT features of 50 WS events in NEC during 1980–2009 are identified using composite analysis to address above questions. The main purposes of the current study is to provide a general view of the anomalous paths and mechanisms of water vapor supply when WS hits NEC and to obtain a deeper understanding of the role and significance of WVT at different stages of WS. Moreover, the reason why the WVT budget and precipitation are not in sync has also been preliminarily examined.

The outline of this paper is as follows: section 2 describes the employed datasets and 50 WS events in NEC; section 3 discusses the climatology of atmospheric circulation, WVT, and precipitation over NEC during winter; section 4 analyzes the temporal evolution of WVT during different stages of WS and its effect on precipitation in regards to water vapor supply; section 5 extends the discussion on the lag between WVT budget and precipitation; and section 6 provides a brief discussion and conclusions.

2. Data and methods

The station data used in this study were derived from the China Meteorological Administration observation archives, which contains daily records of 756 gauge stations in China. The gridded data were derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) (1.5° × 1.5°) (Dee et al. 2011).

Among the 756 stations in China, 92 stations north of 40°N and east of 120°E were extracted as the data sources for NEC; the distribution of these stations is shown in Fig. 1. Because the atmospheric circulation over East Asia, as well as around the globe, transitioned in the late 1970s (Trenberth and Hurrell 1994; Wang 2001; Gong and Ho 2002; Wang et al. 2009), datasets from the period of 1980–2009 were used.

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Spatial distribution of the 92 gauge stations (solid dots) in NEC and the topography (colors, m). The blue area represents water. The topography data were derived from the Global Land One-km Base Elevation Project database (Hastings et al. 1999, available online at http://www.ngdc.noaa.gov/mgg/topo/globe.html).

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Given that Beijing time (BT) is eight hours ahead of coordinated universal time (UTC), the gauged precipitation data include the cumulative precipitation at 2000–0800 BT (1200–2400 UTC), 0800–2000 BT (0000–1200 UTC), and 2000–2000 BT (1200–1200 UTC). For insight into the diurnal variation of WVT and related processes, gauge data recorded at 12-h intervals better identifies short-duration WS events than data recorded at 24-h intervals; this method maintains consistency in time at the diurnal scale for the composite analyses that involve multiple cases. In this study, the WS events in NEC during winter are defined based on the following three criteria: 1) the surface air temperature and the ground temperature should both be below the freezing point; 2) the gauged 12-h precipitation should be greater than 0.1 mm; and 3) more than 60% of the 92 stations in NEC (exclusive of stations with missing records) should meet the previous two criteria. Fifty WS cases were identified based on the three criteria (Table 1), most of which occurred in December and January. Of these cases, 31 occurred between 1200 and 2400 UTC (2000–0800 BT), while the other 19 cases occurred between 0000 and 1200 UTC (0800–2000 BT).

Table 1.

The 50 target WS events in NEC between 1980 and 2009 that were identified based on gauged 12-h cumulative precipitation, including 31 events that occurred between 1200 and 2400 UTC and 19 events that occurred between 0000 and 1200 UTC.

The 6-h ERA-Interim reanalysis data were also used, including precipitation, the vertically integrated water vapor flux, evaporation, sea level pressure (SLP), total column water (TCW), convective available potential energy (CAPE), and vertical velocity, winds and specific humidity at nine levels (1000, 925, 850, 775, 700, 600, 500, 400, and 300 hPa). In addition, monthly ERA-Interim data and the Global Precipitation Climatology Project (GPCP) precipitation (Adler et al. 2003) were employed to compute the winter climatology of relevant parameters.

Climatically, the SH and AO are among the most important factors exerting direct and significant influence on the East Asian winter monsoons and the winter climate of China (Gong et al. 2001; Gong and Ho 2002; Wu and Wang 2002). The current paper gives a brief view of the high-frequency variation of the SH and AO associated with WS. In this study, the intensity of the SH is given by a regionally averaged SLP of 40°–60°N, 80°–120°E, following Wu and Wang (2002). The daily AO is constructed by projecting the daily 1000-hPa geopotential height anomalies poleward of 20°N onto the loading pattern of the winter AO from 1980 to 2009, which is defined as the first leading mode from the empirical orthogonal function analysis of monthly mean height anomalies at 1000 hPa.

WVT and its budgets were computed following the methods of Sun et al. (2011).

3. The climatology of winter WVT

Geographically, the SH lies west of NEC and is strongly coupled with the large-scale circulation and air temperature over East Asia (Fig. 2a). During the snow season, NEC is under the control of robust cold-dry airflow and northwesterly WVT (Fig. 2b). Although the neighboring regions south and southeast of NEC are seas, over which moisture is abundant, the water vapor transported into NEC during the wintertime appears more likely to be from the high-latitude Eurasian continent. Climatically, there is a predominant westerly water vapor influx of 4.22 × 10⁷ kg s⁻¹ into NEC and a second influx of 1.08 × 10⁷ kg s⁻¹ across the northern boundaries of NEC; additionally, water vapor flows out across the eastern boundaries at 4.16 × 10⁷ kg s⁻¹ and across the southern boundaries at 9.11 × 10⁶ kg s⁻¹, as shown in Fig. 2c. The corresponding net budget of water vapor flux is a gain of 2.39 × 10⁶ kg s⁻¹, indicating that NEC is a sink region for water vapor during the winter. Precipitation exhibits a gradient from the southeastern coastal regions to the northwestern inland (Fig. 2d). This might be partly due to a foehn effect along the foot of the Greater Khingan Mountains (Fig. 1) and the fact that air over the southeastern parts of NEC that are close to the seas is more humid, as revealed by the TCW shown in Fig. 2b. However, the climatology map provides little evidence for the connection between WVT, the TCW, and precipitation.

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Winter (November–March) climatology of (a) SLP (colors, hPa) and geopotential height (contours, gpm) at the 500-hPa level, (b) vertically integrated WVT (vectors, kg m⁻¹ s⁻¹) and TCW (colors, kg m⁻²), (c) WVT budgets over NEC (10⁶ kg s⁻¹), and (d) gauged precipitation (colored dots, mm day⁻¹) and GPCP precipitation (colors, mm day⁻¹).

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Figure 3 shows the interannual variation of winter WVT budget and precipitation over NEC. It can be observed that the winter WVT budget and precipitation are evidently in phase with each other, and both increased in the early twenty-first century. The corresponding correlation coefficient is as strong as 0.76, significant at the 99% confidence level. This indicates that WVT, as well as its budget, tends to exert a direct influence on the variation of precipitation in NEC at the interannual scale. Moreover, although zonal WVT is much more intense than the meridional components, it seems that meridional WVT is a more influential factor in determining the WVT budget and further precipitation over NEC, particularly the WVT across the southern boundaries of NEC (Table 2). As shown in Table 2, the interannual variations of winter WVT budget and precipitation over NEC are significantly correlated with meridional WVT across the southern and northern boundaries, while their correlation with zonal WVT across the western and eastern boundaries are lower and not significant. Specifically, the correlation coefficient between precipitation and the WVT influx (outflux is denoted by negative influx) across the southern boundaries reaches up to 0.68, implying that precipitation during the snow season highly relies on the moisture conveyed through the southern boundaries.

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Interannual variations of areal mean winter precipitation (dashed line, mm day⁻¹) and the WVT budget (solid line, 10⁶ kg s⁻¹) over NEC from 1980 to 2009.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Table 2.

The correlation coefficients between winter WVT influx across the southern, northern, western, and eastern boundaries, the net flux, the SH, the AO, and precipitation as well as the WVT net budget during 1980–2009 (the WVT outflux across the southern and eastern boundaries was regarded as negative influx for computation). Bolded values are significant at the 99% confidence level.

As mentioned previously, robust northwesterly WVT prevails over NEC during the snow season, and no evidence can be drawn for the importance of WVT across the southern boundaries from the climatic map. In other words, the climatic-scale precipitation over NEC during the snow season cannot be directly explained by the corresponding climatic-scale WVT. In addition, although the AO and SH have been alleged to influence the East Asian winter monsoons to a noticeable extent (Gong et al. 2001; Gong and Ho 2002; Wu and Wang 2002), it seems that precipitation and the corresponding WVT budget over NEC during the snow season are not closely connected to these two factors, as they have low and nonsignificant correlation coefficients at the interannual scale (Table 2). All of these issues require deeper insight and clarification. Ultimately, the exertion of the AO and SH on circulation and the coupling between WVT and precipitation are all performed in a synoptic system, wherein the interrelation between them are directly reflected. Hence, it is necessary to step further into the synoptic scale to uncover the direct relationship between these variables. It should be noted that here “synoptic scale” does not mean spatial scale, but rather the temporal scale of synoptic systems in contrast to the “climatic scale.”

4. Evolution of WS and water vapor supply

a. Evolution of WS

As described in section 2, the 50 WS cases were identified based on gauged 12-h cumulative precipitation and consist of two categories: 1) those events that occurred between 0000 and 1200 UTC and 2) those that occurred between 1200 and 2400 UTC. The temporal evolution of WVT, precipitation, TCW, and CAPE were calculated for both categories. The calculations indicated that variations of these parameters before, during, and after the WS are essentially the same in the two categories (figure omitted). Therefore, it is feasible to perform a composite analysis for the 50 cases in the two categories, wherein the time axes of the two categories are modified to be consistent with respect to the start times of the WS events. However, parameters that have strong daily cycles are not appropriate to be directly composited in this way, such as evaporation; in these data, the daily cycle must be removed before the composite analysis is performed. All of the parameters used in this study were examined to ensure that they are appropriate for the composite analysis. The results indicate that most of the parameters are eligible, except for evaporation, in which the daily cycle was removed using a 24-h running mean.

The evolution of the areal mean precipitation and the WVT budget over NEC and the SH and AO for 72 h before and after the WS events are shown in Fig. 4, where time step 0 represents the beginning of the WS event. Figure 4a shows that the areal mean precipitation of NEC was steady until 6 h before the WS. After time step 0, the areal mean precipitation increased dramatically, peaked at time step 18 and then declined rapidly. Although these WS cases were selected based on 12-h precipitation records, it appears that WS events generally last longer than 24 h. In contrast to the sharp increase of precipitation within a short time, the WVT budget begins slowly increasing at time step −24 (Fig. 4b). This implies that water vapor accumulation began 24 h before the beginning of the WS event. Interestingly, the increase in the WVT budget ended at time step 0, after which it began descending while remaining above climatology before time step 12. Thus, it appears that the WVT in the 24 h before the WS event plays a key role in moisture accumulation over NEC, which is crucial for triggering the WS. On the other hand, the negative WVT budgets after time step 12 should not be interpreted as meaning that little water vapor is supplied for the mature and later stages of WS by the synchronous WVT; instead, these negative values only indicate that the WVT outflux was greater than the influx. Even so, the leading phase of the WVT budget ahead of the precipitation may indicate that WS events in NEC do not merely rely on synchronous WVT but rather rely more on preaccumulated moisture. This may be primarily attributed to the anomalous circulation that is associated with the anomalous SH and AO. The composite results show that the SH tended to be strengthened continually from 36 h before WS and retained a relatively high value during WS (Fig. 4c), implying a gradually enhanced cold airflow striking NEC. In comparison to the SH, the composite AO showed less variation at such a synoptic scale, but maintained a weakly negative phase throughout the entire process (Fig. 4d). Statistically, the SH is higher than its winter climatology at the 99% confidence level between time steps 0 and 24 and has an anomaly of approximately 4 hPa, while the AO is lower than its winter climatology at the 93% confidence level and has an anomaly of −0.2. Thus, it seems that the WS events are more or less related with an enhanced SH and a negative-phase AO. In addition, the SH appears to exert its influence on high-frequency processes at the synoptic scale, while the AO tends to impact processes of relatively lower frequency.

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Evolution of (a) areal mean 6-h cumulative precipitation (mm) and (b) WVT budget (10⁸ kg s⁻¹) over NEC, (c) SH (hPa), and (d) AO in the 72 h before and after the occurrence of WS events in NEC. Time step 0 represents the start of the WS. Black lines represent the 50 cases, the red lines represent their averages, and the blue horizontal lines represent the climatology.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Before WS hit NEC, an anomalous high pressure system occupied high-latitude Eurasia, while a strong low pressure system developed over Mongolia (Figs. 5a,b). This implies a strong convergence over Mongolia, as well as southwesterly WVT along the southeast margin of the low pressure system into NEC. As the low pressure system shifted southeastward, the SH was gradually intensified, exerting robust cold airflow. In the meantime, a low pressure system dominated NEC and the East Asian Trough began to deepen (Figs. 5c–f), which would result in the intensification of the convergence of northwesterly cold-dry advection and southeasterly warm-wet advection, as well as vertical motion. Consequently, WS occurred over NEC from west to east, and lasted until the low pressure system marched out (Figs. 5g,h). Throughout the entire process, high-latitude Eurasia was enveloped by anomalous high pressure, allowing more cold airflow to attack East Asia. This could partly be regarded as an embodiment of a negative-phase AO. Additionally, Liu et al. (2012) suggested that the decline of Arctic sea ice in recent years favored more snowfall in the Northern Hemisphere, which was linked to atmospheric circulation changes that resembled the negative phase of the winter AO. Thus, there may be other important factors associated with the anomalous weather system of WS in NEC, except for the SH and AO.

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Evolution of SLP anomalies (colors, hPa) and geopotential height (contours, gpm) at the 500-hPa level from time step −18 to 24.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

The above discussion on large-scale circulation has given a brief overview on the process of WS in NEC. However, it should be noted that without a sophisticated WVT regime, WS could hardly be triggered and maintained. As shown in Figs. 6a–d, from time step −18 to time step 0 the westerly WVT gradually turned to be southwesterly and increased in magnitude. During this short period, relatively warm-wet southwesterly moisture flow prevailed over NEC, and there was an increasing convergence of water vapor, especially over the southern part of NEC. Although WS had not yet begun, a large mass of water vapor had been accumulated. As the SH strengthened and the East Asia Trough deepened, the cyclonic pattern of WVT over NEC was enhanced, accompanied by robust northwesterly WVT into NEC across its western boundary (Figs. 6e–h). The corresponding WS was triggered as the cold flow attacked NEC and first emerged over the western and southern parts of NEC, then shifted southeastward and spread over nearly the entire area of NEC and was most intense in its southern part. What should be noted is that the center of the WS lagged behind the center of the WVT convergences throughout the process and was mostly located at the border between positive and negative WVT divergences, where a cold front should exist. Generally, a large amount of moisture would be consumed during the process of WS, for which the supply from synchronous WVT might not be sufficient. In contrast, the lag between precipitation and WVT budget would guarantee an accumulation of moisture immediately before WS emerged over each part of NEC. A discussion in section 5 will provide a deeper insight into what factors may lead to the observed lag between precipitation and WVT budget.

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Evolution of 6-h cumulative precipitation (colored shadings, mm), vertically integrated WVT (vectors, kg m⁻¹ s⁻¹), and its divergences (colored contours, kg m⁻² s⁻¹) from time step −18 to 24. The contours for negative, zero, and positive WVT divergences are in red, yellow, and purple, respectively.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

In general, atmospheric moisture primarily lies in the lower layers of atmosphere, as specific humidity decreases significantly with altitude. WVT over NEC is robust at levels below 700 hPa and decreases rapidly in the upper levels (Fig. 7). As the major contributor to vertically integrated WVT, WVT at low levels exhibits more specific and realistic details of the moisture supply regime. Figure 8 shows the evolution of WVT at 925 hPa, at an altitude of approximately 750 m. At the moisture preaccumulation stage, the southwesterly WVT into NEC was mainly from the eastern Chinese mainland and its adjacent seas south of NEC (Figs. 8a–d). At the WS stage, northwesterly WVT from the East Asian continent, southerly WVT from over the Bohai and the Yellow Seas, and southeasterly WVT off of the Sea of Japan seem to be the main paths of moisture supply (Figs. 8e–h). Therefore, different WVT branches have made contributions to the water vapor supply for WS at different stages. Furthermore, compared with the abundant moisture conveyed from the south, particularly from off of the seas, the moisture from the high-latitude Eurasian continent conveyed by the northwesterly WVT might account for a smaller fraction of the moisture consumed by WS. However, this needs to be quantitatively verified.

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Vertical profiles of winter specific humidity (solid line, g kg⁻¹) and the magnitude of WVT (dashed line, 10⁻³ m s⁻¹) over NEC during 1980–2009.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

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Evolution of WVT (vectors, m s⁻¹) at the 925-hPa level from time step −18 to 24.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

b. Moisture contribution

The net WVT budget decreased significantly after the WS event began (Fig. 4b), implying that the synchronous WVT could not provide sufficient moisture for the increasing precipitation during the WS stage. This discrepancy should be nearly compensated for by the preaccumulated moisture. A question then arises: to what extent does the preaccumulated moisture compensate for the discrepancy between precipitation and the synchronous WVT budget in the WS stage? The answer to this question would directly determine the significance of the preaccumulated moisture to the WS event, which should be addressed quantitatively.

Therefore, the WVT budget was divided by the approximate area of NEC to convert its units from kilograms per second to the same units as precipitation [i.e., mm (6 h)⁻¹] to make the two parameters more comparable. Figure 9 shows that the WVT budget was higher than the precipitation from time step −24 to time step 6, causing the TCW over NEC to increase significantly during this period. The net accumulation of atmospheric moisture (i.e., WVT budget minus precipitation) during this period was 1.6 mm. After time step 6, the net WVT budget dropped steeply and was lower than precipitation until time step 60. The corresponding net moisture accumulation for this period was −3.7 mm, indicating that the synchronous WVT cannot maintain the WS. Nevertheless, even if the 1.6 mm of preaccumulated moisture is taken into account, 2.1 mm of moisture is not compensated for. This remaining discrepancy might be substantially compensated for by the original atmospheric moisture over NEC and partly compensated for by local evaporation, although the magnitude of evaporation remained low. As the WS faded out (time step 24 and afterward), the TCW fell to a level that is below that prior to the WS, indicating that the WS also consumed a considerable portion of the original atmospheric moisture over NEC. The relationship between precipitation, WVT, evaporation, and TCW can be given by

where ΔP is the cumulative precipitation, ΔWVT is the cumulative WVT budget, ΔO is the consumption of original atmospheric moisture, and ΔE is the accumulative evaporation.

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Evolution of the areal mean TCW (black, mm), 6-h cumulative evaporation [E, green, mm (6 h)⁻¹], WVT budget [red, mm (6 h)⁻¹], and precipitation [P, blue, mm (6 h)⁻¹] over NEC from time step −72 to 72. The WVT budget was divided by the approximate area of NEC to convert its units from kg s⁻¹ to mm (6 h)⁻¹. To distinguish them from TCW, the units of 6-h cumulative precipitation are also given in mm (6 h)⁻¹. The strong daily cycle of evaporation was removed using a 24-h running mean before the composite analysis.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

To make this comparison more intuitive, the percentages of the WVT budget, evaporation, and the original atmospheric moisture of different stages accounting for the total precipitation are shown in Fig. 10. The period between time steps −24 and 0 is subjectively defined as the moisture preaccumulation stage, and the period between time steps 0 and 48 is defined as the entire WS stage, including the main WS stage (time steps 0–24) and its residues (time steps 24–48), as illustrated in Fig. 9. The cumulative consumption of original atmospheric moisture was computed based on a transformation of Eq. (1):

where the cumulative precipitation, WVT budget, and evaporation have been given. As shown by Fig. 10, WVT plays a predominant role in supplying moisture for the WS events in NEC, accounting for nearly 50% of the total moisture consumption. The original atmospheric moisture plays a secondary role, accounting for approximately 36% of the total, while evaporation has a relatively small contribution. In addition, the preaccumulated moisture from the WVT is much greater than the synchronous WVT during the WS stage in sustaining the WS events. However, this illustration principally demonstrates the significance of the preaccumulated moisture and does not decrease the significance of the synchronous WVT. In fact, the results shown in Fig. 10 may be fairly crude. The relationship between precipitation, the WVT budget, evaporation, and TCW is given by

where ΔTCW is the net change of TCW. According to Eqs. (2) and (3), the consumption of original atmospheric moisture should equal the net change of TCW. Based on the given cumulative precipitation, the WVT budget, and evaporation, the net change of TCW should be 1.5 mm. Nevertheless, as shown by the TCW line in Fig. 9, the net change of TCW between time steps −24 and 48 is only approximately 1.0 mm. This discrepancy may be attributed to several factors, including the limited precision of our computation and the lack of agreement among the ERA-Interim parameters. Despite these discrepancies, the diagram provides a relatively reliable comparison between the components mentioned above with respect to their contributions to the moisture supply for the WS.

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(a) Ratios of the WVT budgets, evaporation, and the consumed original atmospheric moisture to the cumulative precipitation between time steps −24 and 48 (%). and denote the cumulative WVT budget and evaporation between time steps −24 and 0, respectively, while and denote those values between time steps 0 and 48, respectively. The denotes the consumed original atmospheric moisture between time steps −24 and 48. (b) Ratios of cumulative precipitation between time steps −24 and 0 () and between time steps 0 and 48 () to their total (%).

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Because of the importance of WVT, especially that preceding WS, for WS has been recognized, it is a further step to find out which branch of WVT has contributed more to the moisture supply. Figure 11 shows the evolution of moisture flux across each boundary. It is evident that the significant increase in the net WVT budget between time steps −24 and 0 is substantially attributed to the sharply increasing southerly influx across the southern boundaries, and secondarily attributed to the gently increasing westerly influx across the western boundary. On the other hand, the strength of the westerly influx remained above that of the southerly influx throughout this stage. Even so, considering the nearly equivalent zonal westerly influx and eastward outflux and their relatively smaller changes, the significantly varying meridional WVT should be the actual dominator of this stage, changing from weak northerly flow into strong southerly flow with amplified discrepancy between the influx and outflux, which would lead to a prominent accumulation of moisture. Thus, it can be inferred that the preaccumulated moisture is mainly contributed by southerly WVT from lower-latitude eastern China and its adjacent seas (Figs. 8a–d); in contrast, the moisture from over high-latitude Eurasia, which is closely related to large-scale zonal WVT (Figs. 5a–d), may have a strong flow but makes a less important contribution. As WS began, the westerly influx began to decline, while the southerly influx remained for a time and then also declined. It is worth noting that from time step 6 to time step 18, the southerly influx was superior to the westerly influx. More importantly, this period was the most booming period of WS, wherein the areal mean precipitation remarkably increased and reached its peak. This indicates that the southerly moisture influx remained as the dominant supplier of moisture during the booming stage of WS. However, it should be noted that the moisture sources of southerly influx at this stage are not the same as during the prior stage, mostly off of seas (Figs. 8e–h).

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Evolution of the WVT influx (10⁸ kg s⁻¹) across the western (W, triangle-marker line), eastern (E, plus-marker line), southern (S, star-marker line), and northern (N, circle-marker line) boundaries of NEC and the net budget (Net, solid line) from time step −72 to time step 72. Outflux is regarded as negative influx.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Therefore, the anomalous southerly WVT seems to be the main moisture supplier to NEC, whether during the moisture preaccumulation stage or during the WS stage. This is quite different from the climatic state of winter WVT over NEC. Hence, it is important to note that when we investigate the general water vapor sources and transport paths for precipitation over a specific region, it is not enough to only focus on the climatic scale, even using long-term pentad or daily means; instead, it is also necessary to actually take the WVT that is directly related to precipitation into account, while taking out the WVT that is irrelevant to precipitation.

5. The lag between the WVT budget and precipitation

Ultimately, the WS events are a result of anomalous atmospheric circulation. The causal relationship between atmospheric circulation and precipitation should be nearly instantaneous; that is to say, precipitation and the anomalous flow field should essentially be in phase at the diurnal scale. Figures 12a and 12b show the evolution of divergence and the vertical motion of the flow field. During the WS stage (time steps 0–24), the lower troposphere over NEC was controlled by convergence (Fig. 12a), and ascending motion dominated through almost the entire troposphere (Fig. 12b). This situation is completely opposite from that before or after the WS. It is worth noting that the areal mean convergence and ascending motion were most intense at around time step 12, while the 6-h cumulative precipitation peaked at time step 18. This is mainly because as the WS system shifted southeastward, western and northern NEC were occupied by northwesterly cold flow and accompanied by divergence in the lower layer and descending motion through troposphere. These factors would impact the areal mean results to a certain degree. In the meantime, however, strong convergence, ascending motion, and intense precipitation characterized southeastern NEC (Fig. 8g). Moreover, the CAPE over NEC, which is a valuable indicator of atmospheric instability and convection, exhibited evolution features that were similar to precipitation and also peaked at time step 18 (Fig. 12c). These variations in the flow field, including both advection and vertical motion, demonstrate that there is no obvious lag between precipitation and the atmospheric flow field at the diurnal scale.

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Evolution of (a) the areal mean divergences (10⁻⁶ s⁻¹), (b) vertical velocity (hPa s⁻¹), and (c) CAPE (J kg⁻¹) over NEC from time step −72 to 72. The black lines in (c) denote the 50 cases, while the red line denotes their average.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

The peak of the WVT budget led that of precipitation by approximately 18 h (Fig. 9) and resulted in the accumulation of a large mass of moisture over NEC before the WS event. WVT is mainly regulated by the wind field. Nevertheless, although the most intense convergence occurred during the WS stage, most of the moisture accumulated in the 24 h before the WS, implying that other important factors may operate.

The WVT field consists of the wind field and the water vapor field. In general, the wind field has a large influence on WVT variations, while the water vapor field is a relatively minor factor. The climatology of the water vapor field is principally determined by latitude and the underlying surface conditions, while its high-frequency temporal variations are largely governed by the variable atmospheric flow field. Even so, the impact of the water vapor field on the variations in the WVT and the corresponding moisture budget over NEC should not be ignored. To provide insight into the possible impacts of the two components of WVT on the accumulation of moisture before the WS event, the budgets of WVT flux and wind flux were computed and compared (Fig. 13). The distinction between the two fluxes is that the WVT flux is calculated by multiplying the wind field by the specific humidity field, while the wind flux simply involves the wind field. As shown in Fig. 13a, positive WVT budgets dominated almost the entire troposphere during time steps from −24 to 0, especially below the 500-hPa level, and intensified with time. During time steps 0–24, positive WVT budgets and negative WVT budgets occupied the lower layers and upper layers, respectively, as a result of the convergence in the lower layers and the divergence in the upper layers. Moreover, the upper negative budgets extended downward, while the lower positive budgets shrank with time. These results show why the vertically integrated WVT budget increased prominently during time steps from −24 to 0 and rapidly decreased afterward. In contrast, the wind budgets were mostly negative below the 500-hPa level during time steps from −24 to 0 (Fig. 13b), which prevented mass accumulation. Because this distinction between the WVT budgets and the wind budgets is caused by the water vapor field, it can be inferred that the spatial distribution of specific humidity is a key factor in the accumulation of moisture before the WS.

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Evolution of (a) the WVT budget (10³ m² s⁻¹) and (b) the wind budget (10⁶ m² s⁻¹) profiles from time step −72 to 72.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

Furthermore, the specific humidity field can be divided into two parts: the climatology field () and the anomaly field (). Correspondingly, the wind field can be divided into and . Based on

WVT can be given as a superposition of four components:

where

and

The budgets of , , , and were computed, respectively, to determine the influences of and as well as their cooperation with and , as shown in Fig. 14. The four component budgets are different in both the moisture pre-accumulation stage and the WS stage. Constant negative values characterize the budgets (Fig. 14a), due to northwesterly flow field and southeast–northwest graded atmospheric water field (Fig. 2b). It seems that is strongly against water vapor gaining during the WS stage (Fig. 14b). In contrast, the positive budgets during time steps 0–24 indicate that and tend to play crucial roles in the synchronous moisture supply for the WS event (Figs. 14c,d). On the other hand, it is interesting to see that these three components all have been conducive to the moisture accumulation during time steps from −24 to 0, especially and (Figs. 14b–d). This implies that the cooperation between and as well as is a key factor in the preaccumulation of moisture.

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Evolution of the (a) , (b) , (c) , and (d) budget profiles (10³ m² s⁻¹) from time step −72 to 72.

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

It is important to recognize that different cooperation relationships among , , , and may exert distinct effects, whereby a mass of moisture could be gained or carried off at different stages of WS process. To give a comprehensible interpretation, Fig. 15 shows the cooperation between specific humidity fields and wind fields at the 850-hPa level during time steps from −24 to 0 and from 0 to 24, with respect to , , and . The corresponding effects can be envisioned from the direction of winds and the gradient of specific humidity field. During time steps from −24 to 0, northwesterly was approximately parallel to the isolines of field over NEC (Fig. 15a), while southerly blew from higher and to lower values (Figs. 15b,c), resulting in an increase of moisture over NEC, despite the absence of convergence in the flow field. During time steps from 0 to 24, was against the southeast–northwest-graded field, especially over the western NEC (Fig. 15d), which would impede a sufficient water vapor supply for the WS. Conversely, southeasterly blew nearly along the gradient of and over the eastern NEC (Figs. 15e,f), conveying moisture into NEC efficiently. It should be noted that the and fields shown in Fig. 15 are averages of the 50 WS cases, which were calculated by

thus, what is shown in Figs. 15c and 15f is actually the cooperation between and . However, the real cooperation between and fields with respect to should be calculated by

Considering that positive q_a would increase to a higher value with enhanced southerly wind_a (the blue areas in Figs. 15c,f), and vice versa, the following formula can be given as

Thus, the real cooperation between and fields should have stronger effects than that exhibited in Figs. 15c,f. Besides, it is also noticeable that, as mentioned above, is a passive factor subject to wind field and thus the gradient direction of the field substantially relies on the direction of wind field. Over all, the lag between WVT budget and precipitation could be essentially attributed to the various cooperation among , , , and .

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Cooperation between (a),(d) and ; (b),(e) and ; (c),(f) and at the 850-hPa level during time steps from (top) −24 to 0 and from (bottom) 0 to 24. and are denoted by vectors (m s⁻¹); and are denoted by colors (g kg⁻¹).

Citation: Journal of Climate 26, 13; 10.1175/JCLI-D-12-00300.1

6. Conclusions and discussion

Based on a composite analysis of 50 WS events over NEC, this study investigated and revealed the corresponding characteristics of WVT at the diurnal scale, including variations of its paths and budgets at different stages. Some meaningful results and inferences were obtained, one of which is that although northwesterly WVT prevails over NEC during the snow season, southerly WVT seems to be the key contributor to moisture for WS by carrying water vapor from eastern China and its adjacent seas, as well as the Sea of Japan, into NEC. Moreover, it has been recognized that the large mass of water vapor accumulated in the 24 h before WS tends to make the most important contribution to the moisture supply for WS, superior to the synchronous WVT and the original water vapor over NEC. This lag between the WVT budget and precipitation is actually due to a good cooperation between the atmospheric flow field and the water vapor field. Although there was no strong convergence in the atmospheric flow field at the moisture preaccumulation stage, the uniform southwesterly winds pushed a large amount of moisture from the southwest into NEC, whereby NEC became warmer and wetter; in contrast, although there was intense convergence in the lower layers at the WS stage, the net budget of WVT decreased rapidly due to the cold-dry northwesterlies striking NEC and the strong divergence in the middle and upper layers, unable to compensate for the large mass of moisture consumed by WS on its own.

As a composite analysis, this study is able to provide a general view of WVT during WS events in NEC. However, a specific WS case may differ from the composite results to a certain degree, in aspects such as the intensity of WVT and precipitation and the WVT paths. In addition, the WS cases employed in this study were all selected based on a subjective set of criteria, which, although it has been carefully considered, may still be inappropriate in some ways. Thus, a better understanding of the WVT regimes of specified WS events, which is important for the prediction of various WS events, requires more detailed knowledge of related dynamical and thermodynamic mechanisms.

The East Asian winter monsoon circulation, which has direct influences on WVT over East Asia, has been suggested to be related to many factors, including the AO, SH, El Niño–Southern Oscillation, and North Atlantic Oscillation (Zhang et al. 1997; Russell and Rind 1999; Wang et al. 2000; Wu and Wang 2002; Sakai and Kawamura 2009; Zeng et al. 2011). Most studies have focused on the interannual and interdecadal variability of the East Asian winter monsoon and its influencing factors mentioned above. However, the WS in NEC is more related to their high-frequency variability. Although this study has dipped into the possible impacts of the AO and SH at the diurnal scale, the relevant results are superficial and crude. Moreover, all of the results and conclusions of this study were obtained based on the WS events that occurred during 1980–2009. However, under global warming, widespread and severe weather systems and the related WVT regimes may undergo perceivable decadal changes. Circulation associated with the East Asian monsoon weakened significantly after the late 1970s, and clear interdecadal transitions of precipitation and WVT patterns over eastern China occurred in the late 1990s (Wang 2001; Sun et al. 2011; Zhu et al. 2011). Thus, it is unclear whether the findings of this study are applicable to WS events that occurred before the 1970s or to future events. All of these issues call for further research.