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Figure 7. 925-hPa ECMWF analyses of geopoential height (gpm; solid), temperature (° C; dashed), and horizontal winds (ms^1) over the domain of 15°-35°N, 105°-125°E at (a) 0000 UTC 8 June, (b) 1200 UTC 10 June, and (c) 0000 UTC 13 June, 2000. Contour intervals are 15 gpm for geopotential height and 2° C for temperature, respectively. Thick dashed lines indicate the position of the 925-hPa Meiyu front based on temperature gradient and winds (from G. Chen, Wang, and Wang, 2007).

Figure 7. 925-hPa ECMWF analyses of geopoential height (gpm; solid), temperature (° C; dashed), and horizontal winds (ms^1) over the domain of 15°-35°N, 105°-125°E at (a) 0000 UTC 8 June, (b) 1200 UTC 10 June, and (c) 0000 UTC 13 June, 2000. Contour intervals are 15 gpm for geopotential height and 2° C for temperature, respectively. Thick dashed lines indicate the position of the 925-hPa Meiyu front based on temperature gradient and winds (from G. Chen, Wang, and Wang, 2007).

remained quite strong until after 12 June. The along-front averages of the frontogenetical function and contributing terms over 108°-120°E from 5.625° south to 7.875° north of the 925 hPa front are presented in Fig. 8. During the initial stage [Fig. 8(a)], all three terms, including diabatic processes, horizontal convergence, and deformation, were in phase with the frontal zone, with the front mainly maintained through diabatic effects. During the intensification stage

[Fig. 8(b)], the frontal 0 gradient strengthened and maximized about 120 km north of the front. Diabatic effects become strongly fronto-lytic, while convergence frontogenesis nearly collocated with the frontal zone. The deformation, while also frontogenetic with roughly the same contribution as convergence, had a tendency to peak slightly to the south of the maximum 0 gradient. During the decaying stage [Fig. 8(c)], the front entered the South China Sea. The

Figure 8. Averaged values of frontogenetical function (d|Vh0\/dt, F), its contributing terms FG1 (diabatic processes), FG2 (horizontal convergence), and FG3 (deformation) (all in 10_10 K m_1 s_1, scale on left side), and the magnitude of the horizontal potential temperature gradient (|VhGT, shaded, scale on right side) at 925 hPa from -5.625° (south) to 7.875° (north) relative to the 925-hPa front (at 0°) at (a) 0000 UTC 8 June, (b) 1200 UTC 10 June, and (c) 0000 UTC 13 June, 2000. The average is performed over 108°-120°E, and curves for frontogenetical terms (F, FG1, FG2, and FG3) are smoothed (from G. Chen, Wang, and Wang, 2007).

Figure 8. Averaged values of frontogenetical function (d|Vh0\/dt, F), its contributing terms FG1 (diabatic processes), FG2 (horizontal convergence), and FG3 (deformation) (all in 10_10 K m_1 s_1, scale on left side), and the magnitude of the horizontal potential temperature gradient (|VhGT, shaded, scale on right side) at 925 hPa from -5.625° (south) to 7.875° (north) relative to the 925-hPa front (at 0°) at (a) 0000 UTC 8 June, (b) 1200 UTC 10 June, and (c) 0000 UTC 13 June, 2000. The average is performed over 108°-120°E, and curves for frontogenetical terms (F, FG1, FG2, and FG3) are smoothed (from G. Chen, Wang, and Wang, 2007).

frontogenetical effects of convergence and deformation were reduced, the frontal 6 gradient weakened mainly owing to the continuous sensible heat flux over warmer water into the postfrontal cold air. Overall, the distribution of the frontogenetical function in this case indicated that the Meiyu frontogenesis and the maintenance of the front were attributed to both deformation and convergence.

In summary, the latent heat release and the associated CISK mechanism were suggested to play major roles in Meiyu frontogenesis for one type of Meiyu front, which is characterized by a weak baroclinicity and a strong PV anomaly. For the Meiyu front with a relatively strong barocli-nicity, on the other hand, it was suggested that the diabatic effects become strongly frontolytic and the horizontal convergence as well as deformation wind fields play major roles in Meiyu frontogenesis similar to what would be expected from the classical frontal theory. Indeed, further studies will be needed to clarify the role of latent heat release in frontogenesis for the Meiyu front with a strong baroclinicity.

3. Movement of the Meiyu Front

As pointed out in the introduction, Meiyu fronts often move southward or southeastward slowly and then become stationary. Clima-tologically, about 20% of the Meiyu fronts retreat northward after they become stationary in the vicinity of Taiwan. G. Chen, Wang, and Wang (2007) studied the southward movement of the Meiyu front case as presented in Fig. 7. They analyzed the distribution of frontal strength (\VH0\, GT), frontogenetical function (d\VH0\/dt, F), local tendency of \Vh0\, (d\Vh0\/dt, LT), and horizontal advection of \Vh0\ (-V •Vh\VhQ\, ADV). The local tendency of the frontal strength d \VH 6\/dt can be written as follows:

Results are presented in Fig. 9. Note that the earth-relative frontal motion exists when there is a phase difference between LT and the frontal

Figure 9. Same as Fig. 8 except for averaged values of frontogenetical function (d\V h0\/dt, F), local tendency (d|Vh0\/dt, LT), and horizontal advection (-V • Vh\Vh0\, ADV) of the magnitude of the horizontal potential temperature graident (all in 10- 10 Km-1 s-1, scale on left side), and the magnitude of the horizontal potential temperature gradient (\Vh0\, GT, shaded, scale on right side). Curves for F, LT, and ADV are smoothed (from G. Chen, Wang, and Wang, 2007).

Figure 9. Same as Fig. 8 except for averaged values of frontogenetical function (d\V h0\/dt, F), local tendency (d|Vh0\/dt, LT), and horizontal advection (-V • Vh\Vh0\, ADV) of the magnitude of the horizontal potential temperature graident (all in 10- 10 Km-1 s-1, scale on left side), and the magnitude of the horizontal potential temperature gradient (\Vh0\, GT, shaded, scale on right side). Curves for F, LT, and ADV are smoothed (from G. Chen, Wang, and Wang, 2007).

zone, and is related to the propagation represented by F and the transport by advection. On 8 June [Fig. 9(a)], the total F contributed toward a positive LT that was roughly in phase with the frontal gradient, resulting in intensification of the front, as discussed in Sec. 2. When the front grew stronger [Fig. 9(b)], the effect of ADV was negative near the front, especially immediately ahead of and behind the zone of the maximum 0 gradient. The total F peaked ahead of the frontal zone, and resulted in a positive local tendency also slightly ahead of the front, and thus the forward propagation of the front was contributing toward its total southward movement. On 13 June [Fig. 9(c)], the advection became positive ahead of and negative behind the front, indicating that the postfrontal cold air advection was dominant at this time. The local tendency was roughly 90° ahead of the front, in agreement with the rapid southward movement during the period. Apparently, for the Meiyu frontal movement, the total fronto-genetical function (F) that peaked ahead of the frontal zone contributed toward the southward propagation of the Meiyu front, in addition to the advection by the postfrontal cold air.

G. Chen et al. (2006) studied a case of a retreating Meiyu front which caused heavy rainfall over the Taiwan area. Synoptic analyses at 850 hPa (Fig. 10) indicate the northward movement of the Meiyu front, as defined by the zone of maximum relative vorticity. The nonli-nearly balanced fields of geopotential height and wind associated with PV perturbation of mid-level latent heat release (ms) are presented in Fig. 11. Based on the overall analyses using the piecewise PV inversion technique, it was suggested that the convective latent heating was the primarily cause of the formation and intensification of the LLJ and the subsequent northward retreat of the Meiyu front. They also studied the vorticity budget for this retreating Meiyu front case. Figure 12 presents the time variations of vorticity budget terms across the front at 850 hPa. The stationary nature of the front before 1200 UTC 7 June is clearly illustrated by the near-zero local tendency along the front, while its rapid northward movement afterward is indicated by the large positive tendency north

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