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The Role of Cumulus Heating in the Development and Evolution of Meiyu Frontal Systems

George Tai-Jen Chen Department of Atmospheric Sciences, National Taiwan University, Taipei,, Taiwan

[email protected]

Meiyu is a unique feature of East Asia, and the Meiyu frontal system is the key synoptic feature which causes the maximum seasonal rainfall. Numerous studies have been focused on various aspects of the Meiyu frontal system. The main purpose of this article is to present an overview on the recent research on the Meiyu frontal system, which includes the structure and dynamics of the related phenomena, such as Meiyu frontogenesis, frontal movement, frontal disturbances, and low-level jets (LLJ's) during the Meiyu season. Particularly, the recent studies of the role of convective latent heating in frontogenesis, cyclogenesis, and LLJ formation using the piecewise PV inversion technique will be emphasized.

1. Introduction

The Meiyu (Baiu) rainy season starts concurrently with the East Asian summer monsoon onset in the South China Sea, which tends to occur before the South Asian monsoon development over the Bay of Bengal during late May and over the western coast of the Indian subcontinent in early June (Ding, 1992; Tao and Chen, 1987). It is a unique climatological feature in East Asia, and occurs in the period of mid-May to mid-June over South China and Taiwan, late May to late June in Japan (Baiu), and mid-June to mid-July over the Yangtze River Valley. The seasonal rainfall maximum during the Meiyu period is caused by the organized distribution of upward motion and the moisture flux convergence along the Meiyu frontal zone (G. Chen, 1977a, 1979; G. Chen and Tsay, 1978). The occurrence and variation of this climatological phenomenon has significant impacts on regional agriculture, water resources, and human activities.

Satellite pictures usually reveal a long stratiform cloud band along the Meiyu front with vigorous convection embedded within the band (G. Chen, 1978a,b). Figure 1 presents a case of a Meiyu frontal cloud band with stratiform and convective clouds on 9 June 2006 as revealed by JMA MTSAT satellite infrared imagery. The organized mesoscale convective systems (MCS's) developed along and to the south of the Meiyu front over the southern China coast, the northern South China Sea, Taiwan and the vicinity. The MCS's produced more than 300 mm daily rainfall over central and southern Taiwan on the same day in this case. Synoptically, the rainfall in Meiyu season is associated with the repeated occurrence of a front which develops in the deformation wind field between a migratory high to the north, and the subtropical Pacific high to the south (G. Chen 1977b, 1983). The Meiyu front often moves southward or southeastward slowly in the early stage of its lifetime, and appears as a quasi-stationary front in the late stage, with an average lifetime of 8 days (G. Chen and Chi, 1980). The characteristics of different types of Meiyu fronts,

Figure 1. JMA MTSAT-1R infrared satellite image at 0000 UTC 9 June 2006. The surface Meiyu front is indicated.

as compared to the polar front occurring in the midlatitudes, can be found in a review paper by G. Chen (2004).

One of the very important features accompanying a Meiyu front was the existence of a low-level jet (LLJ) to the south or southeast of the 850/700 hPa trough (or shear line) (G. Chen, 1977a, 1978a; Ding, 1992; Tao and Chen, 1987). A close relationship between extremely heavy rainfall and an LLJ in the 850-700 hPa layer during the Meiyu season was found in many observational case studies over various geographic locations (Akiyama, 1973; G. Chen, 1979, 1983; G. Chen and Chi, 1978; G. Chen and Yu, 1988; G. Chen et al., 2005; Matsumoto, 1972; Ninomiya and Akiyama, 1974; Tao and Chen, 1987; Tsay and Chain, 1987). Over China, the heavy rainfall during the Meiyu period is mainly generated by the meso-a- and meso-,3-scale disturbances, which are embedded within and propagated along the Meiyu cloud and rain band or frontal zone with a horizontal length scale of several thousand kilometers (Ding, 1992). One example of these disturbances is the intermediate-scale cyclone which forms along the Meiyu front with a horizontal scale of 10003000 km.

In view of the important roles of the Meiyu front, LLJ, and the frontal disturbances in producing the seasonal maximum rainfall in the Meiyu period, this article intends to present an overview of the recent studies of the synoptic and dynamical aspects of these features. Particularly, studies of the role of latent heat release from cumulus convection in Meiyu fron-togenesis, frontal movement, development of frontal disturbances, and LLJ formation from a potential vorticity (PV) perspective will be emphasized. Earlier research on these features can be found in the review papers by G. Chen (1992, 2004), Ding (1992), Ding and Chan (2005), Tao and Chen (1987), and others.

2. Meiyu Frontogenesis

Frequency distribution of Meiyu frontogenesis during the Meiyu season of South China and Taiwan as presented in Fig. 2 revealed that the Meiyu front tends to form over the subtropical latitudes to the south of 35°N (G. Chen and Chi, 1980). The front that forms in the midlatitudes to the north of 35°N is the polar front, which does not penetrate into the subtropical latitudes. Different frontal characteristics were found before and after the seasonal transition in mid-June for Meiyu fronts over southern China. Before the transition, it was common for Meiyu fronts to process appreciable baroclinicity and penetrate into the subtropics (e.g. Chen et al., 1989; Trier et al., 1990; Chen and Hui, 1990; G. Chen, Wang, and Wang, 2007). After the transition, Meiyu fronts in southern China usually had strong moisture, but only weak temperature gradients (e.g. G. Chen and Chang, 1980; G. Chen et al., 2003; G. Chen et al., 2006).

Theoretical studies by Cho and G. Chen (1994, 1995) suggested that the frontogenetic process is initiated and maintained by the conditional instability of the second kind (CISK) mechanism (G. Chen and Chang, 1980) through

Figure 2. Frequency of frontogenesis at 1° lat X 1° long grid intervals during the Meiyu season of 15 May— 15 June 1968-77 (1970 and 1975 excluded) over South China and Taiwan. The heavy solid line marks the boundary between the formation of the polar front and the Meiyu front (from G. Chen and Chi, 1980).

Figure 2. Frequency of frontogenesis at 1° lat X 1° long grid intervals during the Meiyu season of 15 May— 15 June 1968-77 (1970 and 1975 excluded) over South China and Taiwan. The heavy solid line marks the boundary between the formation of the polar front and the Meiyu front (from G. Chen and Chi, 1980).

the interaction between the PV anomaly and the convection induced by Ekman layer pumping, as the Meiyu front over South China is characterized by a positive low-level PV anomaly with a weak baroclinicity. The scale contraction produced by the convergence flow associated with convection provides the basic frontoge-netic forcing. This is quite different from the classical frontal theory, in that frontogenesis is primarily due to the deformation field and nonlinear positive feedback mechanism from the induced thermally direct secondary circulation to enhance the temperature gradient and scale contraction. The crucial role of the convective latent heating in the Meiyu frontogenesis was also demonstrated in a modeling case study by Chen et al. (1998) and recently in a diagnostic case study using the piecewise PV inversion technique by G. Chen et al. (2003).

Figure 3 presents the total PV at 850 hPa from 1200 UTC 12 June to 0000 UTC 13 June 1990 of the case studied by G. Chen et al. (2003).

(a)

Figure 3. Total 850-hPa potential vorticity (10~2 PVU) at (a) 1200 UTC 12 June, and (b) 0000 UTC 13 June 1990. Contour intervals are 10 X 10"2 PVU

Figure 3. Total 850-hPa potential vorticity (10~2 PVU) at (a) 1200 UTC 12 June, and (b) 0000 UTC 13 June 1990. Contour intervals are 10 X 10"2 PVU

L). The dashed line indi-

cates the location of the 850-hPa Meiyu front from synoptic analysis (from G. Chen et al., 2003).

The Meiyu front was accompanied by a clear PV maximum with a weak baroclinicity, and remained stationary near 29.5°N to the west of about 117°E. Meiyu frontogenesis was indicated by the intensification of PV along the front. In that study, G. Chen et al. (2003) partitioned PV perturbations (q') into those from different physical processes, largely following the piecewise PV inversion technique of Davis and Emanuel (1991) and Davis (1992). The schematic diagram is presented in Fig. 4 for partitioning the PV anomaly (perturbation) q'. The potential temperature perturbation (6r) at 925 hPa (interpolated from values at 1000 and 850 hPa) was defined as the component at the lower boundary (denoted by 1b). The upper-level (ul) component included q' and 6f from 400 to 150 hPa. In the lower to middle troposphere between 850 and 500 hPa, q' was further partitioned into perturbations related to latent heat release (saturated; denoted by ms) and those not related (unsaturated, mu). They further partitioned the ms perturbations into two parts; one from deep convection (denoted as msd) and the other from shallower stratiform clouds (mss).

Partitioning results among individual processes [Fig. 5(a)] of G. Chen et al. (2003)

Figure 4. Scheme for partitioning the potential vor-ticity anomaly q' (or potential temperature perturbation 0') into perturbations associated with different processes using piecewise inversion. Data levels and types are indicated along the vertical axis. See text for details. (From G. Chen et al2003).

Figure 4. Scheme for partitioning the potential vor-ticity anomaly q' (or potential temperature perturbation 0') into perturbations associated with different processes using piecewise inversion. Data levels and types are indicated along the vertical axis. See text for details. (From G. Chen et al2003).

indicate that the contribution from PV perturbations related to midlevel latent heat release (ms) was nearly identical to the total contribution. Therefore, latent heating was suggested to be the major contributing process in fron-togenesis and explained almost all the frontal intensity in this case, while the remaining processes had a combined contribution of nearly zero. Over the 18 h period, both upper-level (ul)

Figure 5. Time series of averaged relative vorticity (Z) over the frontal region (29.25°-30.375°N, 109.125°-117° E) at 850 hPa. (a) Partitioning of contributions from different processes (lb, ul, ms, and mu) toward total Z and (b) partitioning of contribution from ms between deep convection (msd) and stratiform clouds (mss). The abscissa indicates date and time (UTC), and the ordinate indicates vorticity (10_5 s_1 ) (from G. Chen et al., 2003).

Figure 5. Time series of averaged relative vorticity (Z) over the frontal region (29.25°-30.375°N, 109.125°-117° E) at 850 hPa. (a) Partitioning of contributions from different processes (lb, ul, ms, and mu) toward total Z and (b) partitioning of contribution from ms between deep convection (msd) and stratiform clouds (mss). The abscissa indicates date and time (UTC), and the ordinate indicates vorticity (10_5 s_1 ) (from G. Chen et al., 2003).

and lower boundary (1b) perturbations had weak negative contributions toward frontal vor-ticity, and their effects were roughly canceled by the positive contribution from midlevel perturbations not related to latent heating (mu). The major contributor to 850-hPa frontal vor-ticity, ms, was further partitioned into perturbations associated with deep convections (msd) and those associated with shallower stratiform clouds [mss, Fig. 5(b)]. During the 18 h period analyzed, it is clear that the significant intensification of the Meiyu front at 0000 UTC 13 June was mainly attributable to the heating associated with deep convection. Apparently, the convective latent heating played a major role in Meiyu frontogenesis in this case. Similar diagnosis was carried out by G. Chen et al. (2006) on a retreating Meiyu front with a weak barocli-nicity over the Taiwan area. Figure 6 presents the temporal variations of averaged relative vor-ticity (Z) over the region of maximum frontal Z and the partition of different components from the piecewise PV inversion. Again, the latent heating effect associated with the organized MCS appeared to be the primary mechanism for strengthening the Meiyu front.

Meiyu frontogenesis was also studied recently by G. Chen, Wang, and Wang (2007) for a Meiyu front case with a relatively strong barocli-nicity using the 2 D frontogenetical function of Ninomiya (1984). The frontogenetical function F can be written as follows:

where the four forcing terms on the right hand side, respectively, are

0 0

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