Impact of land–sea breezes at different scales on the diurnal rainfall in Taiwan
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- Huang, W. & Wang, S. Clim Dyn (2014) 43: 1951. doi:10.1007/s00382-013-2018-z
The formation mechanism of diurnal rainfall in Taiwan is commonly recognized as a result of local forcings involving solar thermal heating and island-scale land–sea breeze (LSB) interacting with orography. This study found that the diurnal variation of the large-scale circulation over the East Asia-Western North Pacific (EAWNP) modulates considerably the diurnal rainfall in Taiwan. It is shown that the interaction between the two LSB systems—the island-scale LSB and the large-scale LSB over EAWNP—facilitates the formation of the early morning rainfall in western Taiwan, afternoon rainfall in central Taiwan, and nighttime rainfall in eastern Taiwan. Moreover, the post-1998 strengthening of a shallow, low-level southerly wind belt along the coast of Southeast China appears to intensify the diurnal rainfall activity in Taiwan. These findings reveal the role of the large-scale LSB and its long-term variation in the modulation of local diurnal rainfall.
KeywordsDiurnal rainfallLand–sea breezeCirculation changeLow-level jet
The large-scale circulation covering the EAWNP region also exhibits a marked diurnal variation (e.g. Yu et al. 2009). Huang et al. (2010) found that the LSB circulation over much of the East Asian coastal areas is coupled with the global-scale atmospheric pressure tide; this produces a planetary-scale LSB with a spatial scale of ~1,000 km. Later, Huang and Chan (2011, 2012) utilized a newer generation of global reanalysis, namely the Modern-Era Retrospective Analysis for Research and Applications (MERRA; Rienecker et al. 2011), to reveal further details of the large-scale LSB in EAWNP and the South China Sea. Huang and Chan (2011, 2012) showed that the interaction between the monsoonal southwesterlies and the continental LSB over East Asia contributes to the formation of morning convection in the South China Sea and afternoon convection in southern China. Because Taiwan is located within EAWNP’s diurnal flow regime, it is possible that the formation mechanism of diurnal rainfall in Taiwan also undergoes certain modulation by such a large-scale LSB. This aspect has not been documented and is examined herein.
It is anticipated that the much increased temporal and spatial resolutions of modern reanalyses like MERRA can provide an observational depiction of the interaction between the island-scale LSB (referred to as Taiwan-LSB) and the large-scale LSB (referred to as EAWNP-LSB), as well as their impact on the formation of local diurnal rainfall. Both the Taiwan-LSB and EAWNP-LSB exhibit a marked seasonal variation (Li et al. 2008; Huang et al. 2010; Kerns et al. 2010; Huang and Chan 2012). Here, our season of interest is May and June (MJ), a time period when the diurnal rainfall in Taiwan experiences a strong interaction between Taiwan-LSB and EAWNP-LSB (explained later). Analyses in this paper were performed using various sources of observational datasets introduced in Sect. 2. Results are shown in Sect. 3. A summary and conclusion are provided in Sect. 4.
2 Data sources
Observed precipitation is derived from (1) 21 conventional meteorological stations operated by Taiwan’s Central Weather Bureau as indicated in Fig. 1 and (2) 3-hourly TRMM (Tropical Rainfall Measuring Mission) 3B42 satellite precipitation (Simpson et al. 1996) beginning in 1998. The TRMM 3B42 dataset provides rain rate at the spatial resolution of 0.25° longitude × 0.25° latitude, comparable with the common spacing of meteorological station network in EAWNP (e.g. Zhou et al. 2008). Satellite observed rain rate has proven to be a good proxy for diurnal convection (e.g. Dai et al. 2007; Mao and Wu 2012). Kishtawal and Krishnamurti (2001) found that the TRMM 3B42 rain rate replicates the gauge-depicted diurnal convection features in Taiwan and provides additional information not observable at the surface, such as vertical profile or convection nearby surrounding oceans. Hong et al. (2005) also showed that the TRMM precipitation represents well the diurnal rainfall variability in EAWNP.
Meteorological data including wind fields, humidity, and vertical velocity are extracted from the 3-hourly MERRA reanalysis (Rienecker et al. 2011) at the spatial resolution of 0.667° longitude × 0.5° latitude. MERRA’s spatial and temporal resolutions represent a leap forward when compared to older reanalyses with the common 6-hourly, 2.5-deg resolutions. The analysis here covers the time period from 1998 to 2012 for the MJ months, when diurnal variation is becoming predominant in the region (Wang and Chen 2008). The following analyses refer to the local time in Taiwan which is universal time (UTC) + 8 h, e.g. 0800 h is 0000 UTC, and so forth.
3.1 Rainfall characteristics
3.2 LSB scale interaction
3.3 Impact on diurnal rainfall
The consequence from such a scale interaction between the local and regional LSBs can be visualized by the 925-hPa vertical motion superimposed with Fig. 5. Overall, areas with surface convergent anomalies are associated with ascending motion with the strength corresponding to the magnitude of convergence; areas of divergence are association with descending motion. Examining the early-morning rainfall in Taiwan, Chen et al. (1999) suggested that the nocturnal inversion along Taiwan’s western plains is broken by downslope flow of cooled air, triggering shallow early morning convection. Using MERRA and based upon the regional perspective, we find that the early morning convergence initiated in the Taiwan Strait also plays a role as it propagates over coastal western Taiwan, forming the morning convergence there and the subsequent morning rainfall (Fig. 2b).
The formation mechanism of Taiwan’s afternoon convection is rather straightforward, i.e. sea breeze interacting with terrain in conjunction with heated slopes creating thermal lifting (Wang and Chen 2008). However, additional effect of EAWNP-LSB on Taiwan’s afternoon convection can be observed from Fig. 5c–e, in which the development of the local convergence in Taiwan is coupled with the passage of the regional surface convergence zone. Soon after the regional surface convergence zone propagates over Taiwan, a relatively strong island convergence is initiated at 1100 h (Fig. 5d) and persists through 1700 h under the strong local thermal lifting. As the surface convergence zone moves away from Taiwan (Fig. 5e, f), easterly winds develop over the ocean as part of the sea breeze of EAWNP-LSB (Huang et al. 2010; Huang et al. 2013) and encounter the land breeze in eastern Taiwan; this forms a subsequent narrow band of convergence offshore eastern Taiwan (~0200 h, Fig. 5a).
3.4 Synoptic modulations
The frequency and intensity of Taiwan’s diurnal rainfall activity are modulated by the synoptic conditions, such as the quasi-biweekly (QBW) mode observed by Wang et al. (2013a). When the cyclonic circulation of the QBW mode develops to the west of Taiwan followed by an anticyclonic circulation to the east, the diurnal convection can intensify and the intensification can persist for several days. This synoptic setting reflects increased monsoonal southwesterly winds and moistened lower troposphere while the enhanced subtropical anticyclone favors fair weather conditions with increased solar radiative heating. To examine the impact of EAWNP-LSB under such synoptic conditions, we categorize the active (inactive) diurnal rainfall days in Taiwan, defined as the daily amplitude of diurnal rainfall is greater (smaller) than 0.2 mm h−1. In this composite analysis, dates with the influences from tropical cyclones and fronts are eliminated by following the criteria used in Wang et al. (2013a).
3.5 Change in the recent decade
4 Summary and conclusion
The impact of the large-scale diurnal circulations on Taiwan’s diurnal rainfall formation was examined for the MJ seasons during 1998–2012 using satellite data and the high-resolution MERRA reanalysis. The analyses showed that the diurnal rainfall in Taiwan consists of three regimes: a noticeable early-morning rainfall in western Taiwan, a predominant afternoon rainfall in central Taiwan, and a marked nighttime rainfall in eastern Taiwan. Analyses conducted by MERRA suggested that such sub-island differences are caused by the interaction between the two land–sea breeze systems: Taiwan-LSB (i.e. island-scale LSB) and EAWNP-LSB (i.e. large-scale LSB). In the early morning, easterly winds from the land breeze of Taiwan-LSB meet the westerly winds from the large-scale land breeze of EAWNP-LSB; their interaction initiates surface convergence in the Taiwan Strait. When coupled with the large-scale land breezes that migrate eastward across the Taiwan Strait, the shallow convergence facilitates the formation of early morning rainfall in western Taiwan. Later in the day, the late-afternoon rainfall in central Taiwan coincides with the passage of the large-scale surface convergence zone, which contributes to the local thermal forcing in the initiation of diurnal convection. At the nighttime, the shallow outflows in eastern Taiwan collide with easterly winds from the remnant of the large-scale sea breeze of EAWNP-LSB, triggering the formation of nighttime rainfall over the coastal ocean.
Additional synoptic analyses post-1998 indicated an intensification of a narrow band of southwesterly flows coming from the South China Sea towards Taiwan. Cross-sections of such circulation changes revealed a wind pattern resembling the LLJ—i.e. shallow southerly winds with a near-ground, high wind speed jet core to the immediate west of Taiwan. The increased LLJ provides favorable synoptic conditions of lift and instability for diurnal convection to develop over Taiwan. These findings may provide guidance to weather forecasting, and pointed out the need for further studies using sophisticated climate models in projecting future changes of Taiwan’s frequent diurnal rainfall.
The authors thank anonymous reviewers for their comments and suggestions which greatly improved the manuscript. This research was supported by the National Science Council of Taiwan under NSC 101-2119-M-003-006-MY2 and NSC 102-2621-M-492-001. SYW was supported by Grant MOTC-CWB-101-M-15, Grant NNX13AC37G, and the Utah Agricultural Experiment Station, Utah State University as Journal Paper # 8464.