An Idealized Numerical Simulation of Meso--scale Low on

the Baiu Front

Hirotaka Tagami1, Hiroshi Niino2

1: Advance Soft corporation

2:Ocean Research Institute, Univ of Tokyo

1:Introduction and Purpose2:Setting3:Result4:Conclusion

An example of Meso--Low (MAL)

MAL on the Baiu Front (BF)often causes torrential rain in the Baiu season.

18UTC 4th July 2005

GMS IR imageWeather map

●Only a few hPa lower than the environment.●Active Cloud Cluster accompanies.

Past studies on the structure and dynamics of MAL

0 500km

400

500

600

700

800

900

1000

Vertical structure with sonde data Yoshizumi (1977)

Shade; potential temperature anomaly (θ’) (blue; negative, red; positive)Contour; geopotential anomaly

Cold air to the east of trough Trough tilts eastward with height　 (Ninomiya and Akiyama,1971, Akiyama1990b) Warm core above the low Meridionally different

Vertical structure

Theoretical study for vertical structure Adiabatic cooling associated with updraft enforced by diabatic heating (Yanase and Niino, 2004)

ProblemsAnalytical Study: Mechanism of 3-dimensional structure Developing process (Energy budget). The environment of MALs is different from case to case.

Theoretical Study: Meridionally uniform environment Crudely parameterized convective heating Non-linear effects

PurposesObtainment of realistic environment for the BF Idealized numerical simulation without cumulus convective parameterizationGeneral structure and developing process(Sensitivity to the baroclinicity)

Model and Setting• The anelastic equation system (for energy budget analysis) of Meteorol

ogical Research Institute / Numerical Prediction Division Non-Hydrostatic Model (Saito et al. 2000; NHM) is modified.

• Zonal BC ： cyclic• Meridional and Vertical BC ： free slip• 500x500x36 （ dx=dy=5km,dz=500m)• Cloud Physic ： Cold rain scheme (Lin et al. 1983)• No cumulus convective parameterization• Newtonian cooling (e-folding time = 5hour) is applied to the d

eviation of zonal mean of u,v,w, θ.• f-plane at 32.5 ゜ N

Anomaly : deviation from zonal mean. Horizontal smoothing over 100km square is applied to the model result.

Design of Environment Field• 726 cases distinct BF are sel

ected between 1958 and 2002

• 8-day low-pass filter• Superposed while keeping th

e south edge of BF• Geostrophic zonal wind• Distribution at 125°E is give

n uniformly in the x-direction

• qv is modified with modification of RH (≦95%)

• Thermal wind valanced weak vortex （ diameter :1000km height : 6km, max velocity : 2m/s ）

km

km

Meridionally vertical section of Basic Field for control run (CNTL)

Temperature (contour; every 5K), zonal wind(larger than 10m/s, shadowed every 5m/s)

Relative Humidity[%]

yz

↑: south edge of BF

Result of CNTLvertical integration of

condensational water [10-1g]& surface pressure [hPa] (2hPa)

GMS IR image 00UTC 21 June 2001& SLP of RANAL

·Round shaped Cloud Cluster appears in SE quadrant·Cloud zone such as cold front (trailing portion, Ninomiya et al. 1988)

Horizontal Structureu’ and v’ & updraft (shaded) at T=60hr

xy

z=1km

In the low-level ; Horizontal trough is oriented in SW-NE at north side of the LLJ. Barotropic energy conversion can occur.

[km]

[km]

0y

u

0y

u

[m/h]

[m/s]

Center of LLJ

Vertical Structure

Pressure trough tilts eastward Cold anomaly to the east

T=60hourのat the center

[km]

[km]

[km]

[km]

xz

T=60hourの150km north from the center

contour ： P’(every 0.2hPa), shadow ： positive ’[K] 　(meridionally averaged over 100km through the center)

slightly or westward tilting of the trough

Meridionally different vertical structure

Heat Budget Analysis around MALDistribution of each terms, averaged 25-30hrInterval is each 0.1K/hr in (a)～ (d), 0.5K/hr in (e)、 (f)

x=0 : Center of Low

Negative tendency

Cold area is induced by sum of adiabatic cooling and latent heat.

Cold area depends on the adiabatic cooling.

Condensational heating induces warm core at middle- or upper-level→updraft is enforced

xz

(a)tendency (b)zonal advection

( c ) meridional advection (d) (e) + (f)

(e)vertical advection (f)condensational heating

Developing Process -energy budget-Energy Diagram between 50 and 60hr.

Normalized with EKE.Dimension is [10-6s-1].

Mean Available Potential Energy

Eddy AvailablePotential Energy

(EPE)

Mean Kinetic Energty

EddyKinetic Energy(EKE)

Condensational heating

K→K’ is mainly barotropic conversion, and about 1/4 of the increase of K’,

Developing mainly depends on P’→K’ P’ mainly depends on Q, and P→P’ is much small.

Loading with precipitation

Conclusion

• Several observational features is well reproduced under an idealized environment without cumulus parameterization.

　　 (warm core, shape of cloud cluster, trailing portion)

• A vertical trough of MAL tilts eastward with heihgt, because adiabatic cooling induces cold area in low-level to the east of low.

• The vertical structure is meridionally different.

(Because of the difference of contribution of adiabatic cooling.)

• The development mainly depends on EPE induced by condensational heating. 　

Thank you for Listening!

Vertical Structure

Pressure trough tilts eastward Cold anomaly to the east

T=60hourのat the center

[km]

[km]

[km]

[km]

xz

T=60hourの150km north from the center

contour ： P’(every 0.2hPa), shadow ： positive ’[K] 　(meridionally averaged over 100km through the center)

slightly or westward tilting of the trough

Meridionally different vertical structure

[km]

[km]contour： P’(every 0.05hPa), shadow： positive ’[K]

When condensation is not concidered,

Vertical trough tilts westward with height.

Developing Process -energy budget-

Tendency of EKE averagedover whole region

[hour]

[J m-3]

Energy Diagram between 50 and 60hr.Normalized with TKE.Dimension is [10-6s-1].

Mean Available Potential Energy

Eddy AvailablePotential Energy

(EPE)

Mean Kinetic Energty

EddyKinetic Energy(EKE)

Condensational heating

K→K’ is mainly barotropic conversion, and about 1/4 of the increase of K’,

Developing mainly depends on P’→K’ P’ mainly depends on Q, and P→P’ is much small.

Loading with precipitation

Sensitivity to the Baroclinicity

CNTL

B15

Environment:●Baroclinicity is as same as 1.5 times of one of CNTL●qv becomes small amount in north side●Wind velocity becomes large

Environmental U and T of CNTL and B15

Overview

CNTL B15

Vertical integration of condensational water [10-1g] and SLP[hPa] at 70hr.

●Cloud Cluster exists to the east of low.

xy

Horizontal Structureu’,v’ and w at z=3km at 60hr.

CNTL B15

[m/h]

[m/s]

[m/h]

[m/s]

●Horizontal structure changed

Because of vertical shear becomes strong.

Center of Jet

Vertical StructureP’ (contour, 0.2hPa) and positive θ’(shadowed) at 70hr

B15

●Vertical trough still tilts eastward with increasing height

Y [km]

Z [km]

CNTL

Energetics

diffreswqgwg

y

uvu

z

uwupw

C

gp

t

E

s

k

''''

'')''(div

00

0

''0

''02

V

diffresQC

wg

yv

t

E

p

p

'''''' 0

0

00

Budget of Eddy Kinetic Energy :

Ek=ρ0(u’2+v’2+w’2)/2, Ep=αθ’2/2V : velocity (vector), p:pressure, Cs : sound velocity, Q : condensational heating q : condensational waterEk : Eddy Kinetic Energy, Ep: Eddy Available Potential Energyres: residual terms 、 diff : diffusion

1

0

z

g

[K,K’]z [K,K’]y

[P’,K’] dissipation

[P,P’] [K’,P’]

(￣ ): zonal mean,( )’ : deviation from zonal mean

Budget of Eddy Available Potential Energy :

[Q,P’]

redistributio

Difference of Developing Process Tendency of EKE[J m-3] averaged over whole region

Developing rate becomes small because qv becomes small amount

●mainly depends on Q●Contribution of Q becomes weak●Effect of Basic Field becomes strong

CNTL B15

Energy Diagram between 50-60hour

Mechanism of Trailing Portion 1

Mixing ratio of water vapor [g/kg] at z=0.5km

dry air advection from abovelarge gradient of qv

Mechanism of Trailing Portion 2Each Terms of Front Genesis

at 19hr(a) Condensation(b) Divergence(c) Deformation(d) Tilting Term

Deformation and Tilting aremuch strong!!

Mechanism:1:dry air advection from above with down draft 2:deformation and tilting strengthen