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Figure 3. Cross section of the simulated (a) 9v, (b) total water content qw, (c) vertical velocity w. The intrusion of the moisture is much deeper than the potential temperature field along the dryline (Sun and Wu, 1992).

Figure 3. Cross section of the simulated (a) 9v, (b) total water content qw, (c) vertical velocity w. The intrusion of the moisture is much deeper than the potential temperature field along the dryline (Sun and Wu, 1992).

Figure 4. (a) Locations of the cyclones at 6-hr interval. Solid circles represent observation from ECMWF/TOGA analysis. Open from PRCM. The cyclones over Nevada (eastern Idaho) first appeared at 1200 UTC1 (0000 UTC 2) March 1985. (b) Observed (left) and simulated (right) surface pressure at low pressure centers in Nevada (filled bar) and Idaho (hatched bar) (Chern, 1994).

Figure 4. (a) Locations of the cyclones at 6-hr interval. Solid circles represent observation from ECMWF/TOGA analysis. Open from PRCM. The cyclones over Nevada (eastern Idaho) first appeared at 1200 UTC1 (0000 UTC 2) March 1985. (b) Observed (left) and simulated (right) surface pressure at low pressure centers in Nevada (filled bar) and Idaho (hatched bar) (Chern, 1994).

The results show that the PRCM is capable of reproducing the observed mesoscale/synoptic systems, clouds, and precipitation.

2.3. Applications to Taiwan and East Asia

Sun et al. (1991) applied the PRCM with a freeslip surface to generate a lee vortex and an area of strong wind in northwestern Taiwan under an easterly flow, and a lee vortex to the southeast of the island under a southwesterly flow. The budget of the vertical vorticity component showed that not only the tilting term, as proposed by Smolarkiewicz and Rotunno (1989), but also the stretching and friction terms are important to the formation of lee vortices. With detailed physics and surface parametrizations,

Figure 5. (a) Observed sea surface pressure, (b) observed streamline at 900 mb by airplane (from Kuo and Chen, 1990), (c) simulated sea surface pressure, and (d) simulated streamline at z = 1km at 0200 LST 17 May 1987 (Sun and Chern, 1993).

Sun and Chern (1993) reproduced the observed mesolow, lee vortices, and downslope wind on the lee of the Central Mountain Range (CMR) during TAMEX (Taiwan Area Mesoscale Experiment) IOP(intensive observation period)-2. The lee vortex and the mesolow moved northeastward with time, and were located to the east of Taiwan at 0200LST, 17 May 1987, as shown in Fig. 5. It is also noted that the air moving over the CMR became drier and warmer than the surrounding environment and formed the mesolow to the south of the lee vortex. Hence, no meso-^ scale front, which was suggested by Kuo and Chen (1990), developed around the vortex, as shown by smooth streamlines in Fig. 5(d). The model also captured the diurnal oscillation of the land-sea breeze and the formation of the clouds on both sides of the CMR, as observed during fair weather. Furthermore, the existence of mountain waves explains the observed light precipitation on the windward side with a clear sky near the peak of the CMR. Hence, the numerical model can provide plausible physical explanations of phenomena which have otherwise been misinterpreted according to incomplete observational data, such as the mesofront associated with a mesovortex and the distribution of rainfall in Taiwan in that particular situation.

Sun and Chern (1994) further investigated the formation of lee vortices and vortex shedding for low Froude number flow (the Froude number is defined as Fr = U/NH, where U is the mean flow in the upstream, N is the buoyancy frequency, and H is the height of the mountain in a rotational fluid. A pair of symmetric vortices forms on the lee side of the mountain in an irrotational, symmetric flow; but vortex shedding develops in a rotating system, because of the buildup of high pressure from the adiabatic cooling of the ascending motion on the windward side of the mountain, which enhances the anticyclonic circulation and produces a stronger wind on the left side (facing downstream) of the mountain. The anticy-clonic flow associated with the relative high on the windward side of the mountain was also observed by Sun and Chern (1993, 2006). Vortex shedding can also be developed by any asymmetry in the wind field, mountain shape, surface stress, etc.

The PRCM also faithfully reproduced the observed severe front during TAMEX IOP-2 (Sun et al., 1995). The well-developed front extended from Japan and the Japan Sea through Taiwan into the South China Sea and Vietnam. The segment of the front between Taiwan and Japan is characteristic of midlatitude fronts, but it is a typical Mei-yu front in the lower latitudes, as reported by Hsu and Sun (1994) and many others: the southern segment has a large moisture gradient but a weak temperature gradient with an accompanying low-level jet on the moist side, which transports moisture from the warm ocean into the system.

In addition to the formation of a lee vortex, the front can be deformed by the CMR, as shown in many satellite images (Chen et al., 2002) and surface maps (Chen and Hui, 1990). Figure 6(a) shows a simulated deformation of the cold front after a 30-hour integration (Sun and Chern, 2006), and Fig. 6(b) is the ECWMF

Figure 6. (a) Simulated streamline from the PRCM (after 30 h integration) at 0600 UTC15 June 1987 at z ~ 25m; (b) streamline from ECMWF reanalysis (from Sun and Chern, 2006).

reanalysis. The large-scale patterns are quite comparable, except that the deformation of the front by the CMR is missing in Fig. 6(b) due to a lack of observations in the Pacific and/or the coarse resolution used in the ECWMF. The momentum budget shows that the friction, ageostrophic forcing, and nonlinear advection terms are important for the propagation of the front, which has been frequently misinterpreted according to the theory of density current in the irrotational fluid (Chen and Hui, 1990) or trapped Kelvin waves. For more discussion, see Sun and Chern (2006).

2.4. Regional climate studies

The results discussed in the previous sections show that the PRCM is a useful tool for forecasting and studying short-term mesoscale and synoptic disturbances. The model has also been applied to study the monthly and seasonal variations of regional climate and the hydrological cycle. Bosilovich and Sun (1999a,b) applied the PRCM to study the 1993 summer flood in the Mississippi Valley. The model reproduced the observed precipitation associated with the transient synoptic waves in June and the Mesoscale Convective System (MCS) in July (Fig. 7) due to a change in atmospheric stability. Sensitivity tests showed that without the local surface source of water vapor, the flooded region's atmospheric hydrological budget reduced to an approximate balance between precipitation and moisture flux convergence. Comparisons with the control simulation hydrology estimated that 12% and 20% of precipitation had a local source for June and July 1993, respectively (Bosilovich and Sun, 1999b).

Many scientists have applied numerical models to study the cause(s) of this drought. However, discrepancies exist among the different hypotheses. For example, Oglesby and Erikson (1989) and Oglesby (1991) demonstrated the persistence of an imposed soil moisture anomaly in the US using National Center for Atmospheric Research Community Climate Model 1 (NCAR-CCM1). However, also using NCAR-CCM1, Sun et al. (1997) carried out four experiments, including using the climatological SSTs (control case), 1988 SSTs, dry soil moisture anomaly (25% of soil moisture in May in the normal year between 35 and 50°N in the US), and 1988 SSTs and dry soil moisture anomaly, to study the effect of SST and soil moisture. For each experiment, three model simulations were performed and were initialized from arbitrary conditions. The results show that the 1988 SST did not cause the simulated weather pattern over the US to be drier than the control case with cli-matological SST. The ensemble mean with perturbed soil moisture experiment did show less precipitation than the control; however, due to the large variance in the data, the reduction in precipitation was not statistically significant.

Figure 7. Simulated from PRCM and NCDC observation of daily integrated precipitation over flooding area in the midwestern US for: (a) June associated with synoptic waves and (b) July 1993 associated with mesoscale convective system (Bosilovich and Sun, 1999a).

The experiment with the soil moisture anomaly and 1988 SST obtained the most significant differences, specifically in the reduction of precipitation in the US. This may indicate that the 1988 severe drought may have been caused by a combination of dry soil and a special large scale weather pattern related to the SST, rather than by either the SST or dry soil alone.

Using the observed SST and the ECMWF analysis as initial and lateral boundary conditions, the PRCM (Sun et al., 2004) reproduced a strong warm ridge at 500 hPa in North America over a hot, dry land. The monthly precipitation and soil moisture were far below the normal values in the Midwest and the Gulf states, but above normal in the Rocky Mountains, consistent with observations. It is noted that the PRCM reproduces not only the waves passing through the lateral boundaries, but also the development and decay of the disturbances inside the domain, which are not shown here. A sensitivity test also shows that monthly precipitation could significantly increase using the saturated soil moisture as the initial condition. However, the soil would dry up eventually, because the wet soil does not provide a positive feedback with the low-level jet. Hence, the large-scale weather pattern in the early summer of 1988 might have triggered the dry episode by forming a ridge in North America. This ridge gradually cut down precipitation in the Gulf states and Midwestern region. The soil became dry and hot, which further intensified the blocking of the warm ridge in the US. Hence, dry soil had a positive feedback on the severe drought in the summer of 1988. But the dry soil alone was not the cause of the drought, for in spite of even less soil moisture in early July, observed precipitation amounts returned to normal after mid-July 1988.

Min (2005) recently applied the PRCM with the new snow-land-surface module developed by Sun and Chern (2005) to study the 1997 spring flooding in Minnesota and the Dakotas due to spring snowmelt. With a wet soil, the north-central US experienced horrific conditions over the winter of 1996-97. An enormous snow pack was built up by blizzard after blizzard from the second half of November through January; many areas had more than 250 cm of snowfall. These amounts were as much as 2-3 times the normal annual amount. Although February and March were quite dry, frigid conditions throughout much of the winter ensured that as much as 25 cm of snow water equivalent remained on the ground when the spring melt period began in March. Early March brought temperatures below normal, delaying the onset of snowmelt. Significant melt of the deep snow started with particularly warm conditions at the end of March and in early April. At this time, many rivers in South Dakota, southern Minnesota, and southern North Dakota were rising, in some cases well above the flood stage. With a comprehensive snow-soil physics included in the PRCM, the simulated results of snow depth and precipitation rate can be dramatically improved (not shown in the figures). Our study revealed the importance of the detailed physics/parameters in a model, which can influence results in faraway regions. In addition, we also showed that the land properties can affect the flow pattern in the upper level as high as 200 hPa.

The PRCM has also been applied to study the East Asia Monsoon and short-term regional climate, by Sun and Chern (1999), Sun (2002), Hsu et al. (2003), Yu et al. (2004a,b), Hsu et al. (2004), etc. Figure 8 shows that the jet at 850 mb migrated northward over the summer of 1998 (Sun and Chern, 1999). Heavy precipitation occurred in Central China in midsummer, moving to Korea and northern and northeastern China in August. The front and the low-level jet dissipated after mid-August, as observed.

In simulations of the heavy flooding over Korea and China between 30 July and 18 August 1998, the model results show that the heavy rainfall along the Baiu/Mei-yu front was due to a combination of: (1) an anomalous

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