Dependency of horizontal resolution on structure changes of atmospheric stratification

in the 2015 Hiroshima heavy rainfall

NHKスペシャル「Mega Disaster」から

Teruyuki KATO (Meteorological 

Research Institute,   JMA,Japan)

Some huge landslides killed 74 peoples.

2016.3.7: The 6th Research Meeting of Ultrahigh Precision Meso‐scale Weather Prediction

Heavy rainfall occurred ~300km south of stationary front.

Weather condition and Rainfall distribution

Surface weather map 3-h precipitation amounts

Similar to heavy rainfall events observed in the rainy season of Japan.

Max:238mm

0 50km

21JST19AUG

HiroshimaPrefecture

Yamaguchi

Shimane

0130-0430JST20AUG

Multi-cell Cluster

~50km

Single cell5~10km

> 100km

Band-shaped precipitation system5km

10km

15km

Cluster B Cluster A

Formation processes and structure of

Back-building type formation

Low-level winds

Midlevel winds

band-shaped precipitation system (cluster A&B)

250mNHM

1000km

800km

412km

330km

JMA’s Local Analysis18JST19 21 00JST20

2kmNHM

250mNHM

03 06Design of 250mNHM run

Model : JMANHM (Saito et al. 2006) Dynamics: Fully compressible equations

with a map factor Cloud physics: Bulk-type with six water species

(qv, qc, qr, qi, qs, qg)Convection: none Turbulence: MYNN scheme

(Nakanishi and Niino 2006)Surface flux: Beljaars and Holtslag (1991)Horizontal grid: 2km, 1km, 500m, 250 mInitial/boundary data: Hourly JMA-Local analysisadopting a 3DVAR assimilation system, but for 250mNumerical diffusion: 20min(linear), 10min(2D)Water vapor diffusion for grids with w > 10 m/s

Numerical model and experimental design

5kmNHM 2kmNHM

500mNHM 250mNHM

In this case, resolution of two kilometer can reproduce a rainband.

Max: 232mm Max: 122mm Max: 227mm

Max: 215mm Max: 231mm

1kmNHMMax: 210mm

AnalysisResults of 18JST19initial(3-houly accumulated rainfall at 04JST20)

Statistical domain

Reproductivity of band-shaped precipitation system 2kmNHM 250mNHM

SW NESW NE

SW

NE

SW

NE

Cell’s size is overestimated. Cluster’s structure is well reproduced.

A’

B’

250m resolution is necessary to reproduce the structure of multi-cell clusters.

2km-height 2km-height

(cluster A&B) in 2kmNHM and 250mNHM

Cluster B Cluster A

5km

10km

15km

Cluster B’ Cluster A’

2kmNHM 250mNHM

hourhourhour

(km) (km)

(km)(km)

Movement ofmulti-cell cluster

Movement ofmulti-cell cluster

Reproductibity of movement of convective cells（Mixing ratio of rain at a 466m height)

S-N d

irection

E-W

direction

Movements of multi-cell cluster are clear.Formations of convective cells located on the downstream side than 250mNHM.

1kmNHM 500mNHM

hourhourhour

(km) (km)

(km)(km)S-N d

irection

E-W

direction

Reproductibity of movement of convective cells（Mixing ratio of rain at a 466m height)

Movement ofmulti-cell cluster

Movement ofmulti-cell cluster

Movements of multi-cell cluster are not so clear.

Appearance frequency of atmospheric structure and e2kmNHM 250mNHM

Higher frequency for e > 350K is found at the middle level in 2kmNHM.

Black line: mean

HighLow

(23JST19~04JST20)

0.0015%

10%

0.001%

10%

Almost same frequency for e > 355K.High frequency for e > 350K.

Close tomean

Appearance frequency of atmospheric structure and e2kmNHM 250mNHM(2km-mean)

Black line: mean

HighLow

Frequency exceeding 355K disappears.

Distributions do not become close to 2kmNHM by averaging. Opposite effect

0.001%

10% 10%

1kmNHM 500mNHM

e is distributed close to mean in comparison with 2km/250mNHM.

Frequency exceeding 355K is very small.

Appearance frequency of atmospheric structure and e(23JST19~04JST20)

HighLow

Black line: mean

0.0003%

10%

0.0008%

10%

Closest tomean

23-24JST19 2-3JST20

250mNHM

Stratification is little changed by convective activities.

DevelopingStage

0.3% 0.6%0.001% 0.01%Increase of high e regionIs very small.

HighLow Black line: mean

MatureStage

Time change of atmospheric structure and e

2kmNHM 250mNHM

Max/mean distributions of e at 5km height

Maximum Maximum

Mean Mean

(23JST19~04JST20)

Downstreamside

Wind direction

Widerhigh e area

Largely shiftdownstream

High e areas largely shift downstream in mean.Shift of high e areas is not so large.

High correlations are found in distributions between e and updrafts.

2kmNHM 250mNHM

Max/mean distributions of vertical motions at 10km height(23JST19~04JST20)

Maximum Maximum

Mean Mean

Updraft peaks are found around tropopause.

Conversion from CAPE(~2000J/kg)

Lower peaks in strong updrafts

Differences are found in peak heights of upward motions, as well as intensity.

Peaks are also found in downdrafts.

HighLow

Black line: mean

2kmNHM 250mNHM

Appearance frequency of vertical/strong upward motions(23JST19~04JST20)

Compensation downdrafts

250mNHM

Total appearance

0.3%

Acceleration regions of updrafts with conserving e are very narrow.

0.03% 0.0015%

e >350K

e >355Ke >352.5K

HighLow

Black line: mean

Appearance frequency of vertical motions in high e regions(23JST19~04JST20)

frequency

0.0003%

0.0015%

Even low resolution can represent the acceleration of updrafts with conserving e.

0.0008%

250mNHM

2kmNHM

1kmNHM

500mNHM

0.001%HighLow

Black line: mean

Appearance frequency of vertical motions in grids with e>355K(23JST19~04JST20)

Total appearancefrequency

Even 2kmNHM can successfully reproduce a band-shapedrainfall area, but not structures of the precipitation system.For the reproduction, 250mNHM is necessary.Weaker updrafts shift rainfall areas downstream in 2kmNHM.Higher appearance frequency for e > 350K is found at themiddle level in 2kmNHM, but grids with e > 355K are foundalmost equal to that in 250mNHM.Acceleration regions of updrafts with conserving e are very narrow, which is found even in 2kmNHM, but their intensity.

Dependency of horizontal resolution on

structure changes of atmospheric stratification

Stratification is little changed by convective activities.