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1. Introduction 2. Model description and design of experiment by JasonDetriou

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									 Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
 Preferred Mode of Presentation: Oral


         MESOSCALE SIMULATION OF A HEAVY RAIN EVENT OVER
                        SOUTHERN TAIWAN IN MEIYU SEASON

                              Lisa T. C. Chang            George T. J. Chen
            Department of Atmospheric Sciences, National Taiwan University, Taiwan


1. Introduction                                           2. Model description and design of
                                                          experiment
       There are about 4-5 frontal systems
affecting Taiwan during Meiyu season each                       COAMPS is a nested, three-dimensional
year (Chen 1988). The Meiyu front is usually              model that solves the non-hydrostatic, fully
accompanied by a nearly continuous cloud                  compressible equations of motion on
band, which produces stratiform and/or                    terrain-following sigma coordinate £m z .
convective precipitation (Chen 1992). During              Horizontal grid increment of 45, 15 and 5 km
                                                          are used for the coarse, medium and
the period of 29-30 May 2001, a series of
                                                          fine-mesh grids, respectively (Fig. 1). There
mesoscale convective systems (MCSs)
                                                                  N01
developed in the vicinity of Taiwan along a
retrograde moving Meiyu front. The
development and northeastward propagation
of the MCSs produced heavy rainfall over                                N02
southern Taiwan, with a maximum 24-h
accumulation rainfall of 200 mm. This study                                   N03
concentrates on the most intense precipitation
that occurred during 12 UTC 29 May to 00
UTC 30 May. To further understand the
physical process related to the heavy rain
                                                             Fig.1. Domain configuration (N01, N02,
event, the atmospheric component of the
                                                             N03) for COAMPS model simulation.
Naval Research Laboratory’s (NRL) Coupled
                                                          are 30 layers in the vertical, 12 of which are
Ocean/Atmosphere Mesoscale Prediction
                                                          below 1.5 km for higher vertical resolution in
System (COAMPS; Hodur 1997) is                            the boundary layer. Assimilation cycles with
performed in this study. In order to include              12-h periods starting from 1200 UTC 28 May
mesoscale features in the model initial                   (hereafter 052812) to 053012 are performed
condition, satellite scatterometer QuikSCAT               in COAMPS (Fig. 2). The cycles begin with a
surface wind measurements (Graf et al. 1998)              cold start (CS) that refers to a COAMPS
are blended into the COAMPS analysis                      simulation that is initialized directly from the
procedure where the NOGAPS global                         NOGAPS analyses, while the later warm
analysis data were used as the first guess. The           starts (WS) refer to a COAMPS run that is
successful simulation also provides further               restarted from a previous 12-h forecast.
insight and understanding into the moisture               Throughout the simulation, NOGAPS
transport processes related to convective                 forecast fields are adopted as the lateral
precipitation.                                            boundary conditions.
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Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
Preferred Mode of Presentation: Oral


                                                           downstream toward southern Taiwan in the
                                                           subsequent hours. The 6-h accumulated
                                                           precipitation mainly produced by MCS A was
                                                           larger than 50 mm (Fig. 4). Localized rainfall
                                                           maximums are found near the central coastal
                                                           plain and over the southern tip of Taiwan




  Fig. 2. Initialization and data assimilation
  cycle of COAMPS.
     Two experiments, namely, the control
(CTNL) and QuikSCAT (QUIK), are
performed. Both simulations have the same
full physics, except the QUIK experiment is
conducted by blending QuikSCAT oceanic
surface wind in 50-km resolution using the
Multivariate Optimum Interpolation (MVOI)
scheme (Barker 1992) into the initial
                                                             Fig. 4 CWB observed 6-h accumulated
conditions of every 12-h assimilation cycle.                 precipitation (contour levels of 15, 30 and 50mm) at
                                                             18 UTC 29 May 2001. Model terrain height above
3. Case description and model
                                                             1500 m is shaded.
validation
                                                  The closest time to the simulation period
      GMS-5 satellite imagery indicates the with available TRMM rainfall estimations
MCS A first developed and enhanced over (Kummerow 2000) is at 00 UTC 30 May
southern Taiwan around 15-18 UTC 29 May
                                             2001 (Fig 5a). There were maximum
along the Meiyu frontal cloud band (Fig. 3).
                                             precipitation areas over the ocean to the
                                             northeast as well as to the southwest of
                                             Taiwan. COAMPS simulated precipitation in
                                             the 45-km grid from CTNL experiment valid
                                             at 00 UTC 30 May has a smaller amount of
            A                                precipitation over both the southern Taiwan
        B
                                             Strait and over the ocean northeast to Taiwan
                                             (Fig. 5b). On the other hand, the QUIK
                                             experiment generates better simulated
                                             precipitation in terms of both the amount and
                                             the spatial distribution (Fig. 5c).
  Fig. 3. GMS infrared image for 18 UTC 29 May
About the same time, MCS B initiated over
southern Taiwan Strait and propagated

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Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
Preferred Mode of Presentation: Oral

            a                                               portion of wind shift line from QUIK
                                                            experiment (Fig. 6c) is closer to that derived
                                                            from QuikSCAT in Fig. 6a.

                                                                      a



           b




                                                                      b

           c




    Fig. 5. (a) TRMM observed rainfall rate at 00 UTC                 c
    30 May 2001 (shading, in mm) (b) simulated
    precipitation in 45-km grid of CTNL (contour
    levels of 1, 2, 4 and 6 mm) valid at 00 UTC 30
    May 2001 ( c ) as in (b), except of QUIK.

     As in Yeh et al. (2002), kinematic
characteristics of the Meiyu front in the
15-km model gird of both the CTNL and
QUIK experiments are further examined in
order to study the impact of better resolved         Fig. 6. (a) QuikSCAT-derived relative vorticity at 12
surface winds. At 12 UTC 29 May, the                 UTC 29 May 2001 (b) the CTNL simulation in
surface wind shift line in CTNL experiment           15-km grid domain valid at 12 UTC 29 May 2001 at
                                                     1000 hPa for relative vorticity, (c) as in (b), except
shows a light shift to the north (Fig. 6b)           for QUIK simulation. Dash lines indicate frontal
compared to that in QuikSCAT (Fig. 6a),              wind shift line. Contours are analyzed at 5x10-5 s-1
whereas the corresponding areas from the
QUIK experiment are closer to the wind shift         Simulated 3-h accumulated precipitation in
line (Fig. 6c). In addition, the simulated area the 5-km grid from both experiments is also
of maximum cyclonic vorticity at the western studied. During the period of 12-18 UTC 29

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Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
Preferred Mode of Presentation: Oral


May, the CTNL experiment does not                          coordinate with the molecular diffusion term
reproduce the precipitation to the south of                neglected and portioned the variables into
Taiwan well (not shown). Comparatively, the                mean (bar) and turbulent (prime) part, can be
precipitation pattern is much better reproduced            written as,
by the QUIK. There are two areas of local
                                                             ∂q              ∂ wq               ∂(w ' q ' ) S
maximum in Taiwan in the three-hour                             = −∇ ⋅ V q −      − ∇ ⋅V 'q ' −            +
                                                             ∂t               ∂z                  ∂z         ρ
accumulated rainfall valid at 15 UTC and 18
UTC 29 May (Fig 7), which is quite similar                  LC        HFLX   VFLX THFLX TVFLX S
to the observed rainfall pattern (Fig. 4).        where q is specific humidity of water
Furthermore, the QUIK experiment also       vapor, V horizontal wind vector, w vertical
reproduces the precipitation successfully over
                                            velocity in height coordinate, £l air density,
the southern Taiwan Strait brought largely by
                                            and S sources and sinks of water vapor. LC
MCS B. Since the QUIK experiment            represents the local time change of mean total
reproduces the overall rainfall distributionmoisture, while HFLX and VFLX denotes
satisfactorily. It allows detailed calculations
                                            mean total horizontal and vertical moisture
of the mesoscale moisture budget, in        flux convergence, respectively. The mean
particular, in the boundary layer that are  quantities in terms LC, HFLX and VFLX are
essential to understanding the origin of    calculated from results in the 5-km resolution
moisture associated with the convection     grid. The sets of prime terms in THFLX and
initiated over the ocean (i.e. MCS B in this study)
                                            TVFLX represent the convergence of
The computation methodology and preliminary turbulent moisture flux, which are calculated
results are described in section 4.         implicitly by employing K-theory of the
                                            5-km resolution grid. The last term S is the
                                            source/sink term. It is computed as a residual
                                            term to balance the other terms. A simple
                                            centered finite-difference scheme is used to
                                            compute all spatial and time derivates.
                                                  Budget calculation is employed prior to
                                            and during the convective outbreak over
                         I                  Taiwan Strait in the subdomain I (Fig. 7),
                                            while MCS B within domain was related to a
                                            frontal convection translated from outside the
                                            southwestern corner of the domain as well as
                                            local development. The budget values then
   Fig. 7 Simulated 3-h accumulated         averaged over grids where precipitation
   precipitation in the 5-km grid of QUIK
                                            occurred that are defined as moisture
   at 18 UTC 29 May. The subdomain I is
   for calculating area-average moisture    variation in convective region. On the other
   budget in section 4.                     hand, values averaged over grids where
                                            precipitation is absent represent the processes
4. Diagnose of Moisture budget              related with water vapor fluctuations in the
     The moisture budget equation in height non-convective region.
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Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
Preferred Mode of Presentation: Oral


    Prior to and during the convection                     with vertical moisture divergence at
developed over the ocean, the non-convective               1000-2000 m were crucial for accumulating
region was characterized by strong horizontal              and transporting moisture into higher levels.
moisture flux convergence in the low levels                In the layer of 2000-4000 m, turbulent
and weak divergence aloft (not shown). Both                vertical moisture divergence appeared to
grid-resolvable and sub-grid scale turbulent               predominate over the grid-scale processes in
processes help to carry moisture upward                    transporting moisture upward into convective
away from the boundary layer to the middle                 cloud. Negative source/sink term above 4000
troposphere. On the other hand, results in                 m indicated condensation and precipitation
convective region (Fig. 8) during the                      occurred in the convective cloud. The
convection development indicated the                       strongest sub-grid turbulent vertical moisture
horizontal    moisture     divergence    and               convergence together with weak resolvable
evaporation below 1000 m were mainly                       vertical moisture divergence in the lower
associated with the downdraft. Strong                      cloud level lead to a moisture downward
horizontal moisture convergence together                   transport which may help moisture


      a                                                       b




       c                                                       d




Fig.8 Height-Time cross sections of ocean convective region averaged moisture budget terms (10-8
kg m-3 s-1) from 16 UTC 29 May to 08 UTC 30 May 2001 in the layer of surface-10 km for (a)
horizontal moisture flux convergence, (b) vertical moisture flux convergence, (c) turbulent vertical
moisture flux convergence and (d) Source/sink.

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Category and Session number: Ocean and Atmospheres, Session 8 (OA8)
Preferred Mode of Presentation: Oral


accumulate near cloud base. Whereas,                       Acknowledgments. This study was supported by
another downward moisture transport in                     National Science Council of ROC under Grants
mid-cloud level was found to be primary                    NSC92-2111-M-002-006.
associated with the resolvable vertical
moisture convergence, which was closely                                      References
related to the maximum vertical motion                     Chen, G.T.J., 1988: On the synoptic-climatological
appeared near 7000 m.                                           characteristics of the east Asian Mei-Yu
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                                                           Graf, J., C. Sasaki, C. Winn, W. T. Liu, W.
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The successful simulation allows detailed
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      Results     from     moisture     budgets
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mid cloud level lead to a downward moisture
transport in the convective cloud.


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