Features of dust devils in the urban area detected by a 3-D scanning Doppler lidar

15th Coherent Laser Radar ConferenceToulouse, June 22-26, 2009

in Bolivia（1Graduate School of Environmental Sci., Hokkaido Univ.; 2National Defense Academy; 3Institute of Low Temperature Sci., Hokkaido Univ.）

Chusei Fujiwara1, Kazuya Yamashita2, Mikio Nakanishi3,Yasushi Fujiyoshi4

OutlineMotivation

- What is dust devil?

- Possible formation mechanism of dust devils.

- Early studies issues

Specification of the Doppler Lidar

Results

- What is main organized airflow structure in urban area?

- Can we detect dust devils in urban area by Doppler lidar?

Summary and a future study

Doppler radar Observation of dust devil in Texas.：Bluestein et al. 2004; MWR

What is dust devil? ： small scale vortex Velocity：～20m/s, Diameter：Tens ～141m、Height：300～600m, Rotation Sense：Random

Transportation of much dust, heat and water vapor.

• Environment（Sinclair 1966,1969,1973;Hess and Spillane 1990）Weak wind, superadiabatic

Visible by tracers of dust in desert or relatively flat area..

Possible sources of vertical vorticity • Ambient horizontal shear. （Barcilon and Drazin 1972）

• Tilting of ambient horizontal vorticity associated with convective-cell circulation (“Fish nets”). (Kanak et al. 2000)

Kanak et al. 2000

U;V vector W

LES（Large Eddy Simulation）

Dust devil-like vortices

No study has reported observation of airflow structure in ABL scale and small scale dust devils simultaneously.

Invisible dust devil?Aircraft encounters with strong vortices over the boreal forest.

MacPherson and Betts 1997; JGR

East wind component

vertical wind component

BORES (Boreal Ecosystem/Atmosphere Study)

Pourpose1. What is main organized airflow structure in urban area? 2. Can we detect dust devils in urban area by Doppler lidar?- What are the characteristics of dust devils?-What airflow structures generate dust devils?-What environmental conditions favor dust devils?

IshikariBay

Mountainousarea

Urban

HokkaidoUniversity

Toyohirariver

4.4km

2km

Suburb

Specification of the Doppler Lidar

Site: Hokkaido University（28m ASL）Observation area

Wavelength ： 1.54μｍ（Eye Safe)

Scattering body : Aerosol, Cloud Particle

Instrument：3-D scanning cohrent Doppler Lidar (Mitsubishi Electric Corporation)

Measurement ： S/N, Doppler Velocity

Scan method : PPI (Elevation 2.2°)

Scanning speed: 4.5 deg/s（80s）

Instrument

Resolution：radial 50m，azimuthal 9～92m

High resolution

9m

92ｍ50m

4400mLidar

SoundingWind 59.5m

Analyzed data periods.2005.4～7,2006.6,2006.10～2007.6

Laser material : Er.Glass laserLaser pulse energy : 0.5mJ/Pluse

Pulse repetition : 4kHzLaser pulse width : 213±10ns

(Etling and Brown,1993)

NASDA,NASA

1. What is main organized airflow structure in urban area?

Bluestein et al. 2004 Hardy and Ottersten1969

Yamashita et al. 2005Etling and Brown,1993

Airflow structure in ABL scale.Fish nets by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m）

Zi（Boundary layer height）～850m

2006/10/1 11：35 Elevation 2.2°Obs:Doppler velocity

Weak wind（1.5ms-1）, large sensible flux

LES：u,v vector; w (Elevation 2.2°)

Airflow structure in ABL scale.Fish nets by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m）

The observational pattern is considered to show the existence of “Fish nets”.

2006/10/1 11：35 Elevation 2.2°Obs:Doppler velocity

Weak wind（1.5ms-1）, large sensible flux

LES：Doppler velocity

What is main organized airflow structure? wind velocity

Tenki，2005； J. Visualization，2006

Fish nets：weak wind in daytime Streaks：strong wind

5ｍ/ｓ

Obs. periods May of 2005～

2. Can we detect dust devils in urban area by Doppler lidar?- What are the characteristics of dust devils?-What airflow structures generate dust devils?-What environmental conditions favor dust devils?

Detection of vortex （Suzuki et al. 2006）• To detect large Azimuthal

derivation• To detect maximum andminimum Doppler velocity.

• To fit Rankine vortex.

rvs 20ζ=

ravs

12

20 ⋅=

ζ

)0( ar ≤≤

)( ra <

DVV )(2 m im a x−=δ

Calculation of vorticity（Bluestein et al. 2004）

RemoveNoise.

・raw data

)2( aD =

D=175m,V=6.32ms-1

D is the distance between maximum and minimum velocities.

Detection of dust devil (05/06/24)9:01-9:43

〜80ｓ/one scan

Dust devil

El2.2O

44-198m（AGL）

Detection of vortices (05/06/24)

110m,-0.11s-1

90m,0.15s-1

A: Cyclonic

B: Anticyclonic

Dust devils were observed in “fish nets”

Rotation is random

Characteristics and environment

Time Diameter Max Rotation Vectorial ziVPT Tota

l

Vorticity direction Mean U Lapse rate

(JST) (m) (s-1) ＋/－ (ms-1) (m) (x10-2Km-1)

2005/5/25 09:29-10:11 30-110 0.25 11/3 0.3 1150 2.0 142007/6/22 11:48-12:33 70-120 0.21 6/2 1.0 950 - 82005/7/17 10:58-11:41 30-120 0.26 7/3 1.1 950 - 102007/4/14 12:47-13:22 50-90 0.19 1/3 1.1 1150 - 42005/6/24 09:01-09:43 50-100 0.19 3/1 1.3 900 1.5 42005/4/24 14:04-14:46 50-90 0.15 1/3 2.0 650 - 42005/6/1 09:01-09:43 40-80 0.17 3/3 2.2 750 1.1 6

Weak wind, Large Zi, daytime,Superadiabatic

Similar feature in past studies.（e.g. Sinclair 1969; Hess and Spillane 1990）

Diameter: 30-120m, Maximum vorticity: 0.15-0.26s－1

Rotation direction cyclonic:anticyclonic=2:1

・Vorticity is absolute value, Zi estimated by RHI observation, Wind （59.5m ASL at SJMA),VPT lapse rate by using souding data from 1.5 to 100m, Net Radiation (ILTS observation)

・Only strong vortices (more than |0.1| s-1)

Summary

• In urban area, Sapporo, for the first time, we were able to detect “invisible dust devils” (phantom vortices) by using the Doppler lidar.

• When dust devils appeared, wind was weak, lapse rate was superadiabatic, and the “Fish nets” of wind fields was also detected by the Doppler lidar.

• Summary of dust devils’ characteristics: diameter (30-120m), maximum vorticity (0.15-0.26s-1), ratio of cyclonic to anticyclonic rotation (2:1).

Future study: other sources of vertical vorticityDust devils are seen to occur along Sea-breeze front.This fact suggests that horizontal shear associated with the sea-breeze front would play an important role in their formation.

07/6/9 18：12

S/N（EL2.2°） Doppler velocity（EL2.2°）

We plan to examine cases of airflow structures different from fish net and to discuss source of such vertical vortices.

2006/10/1 11：35 Elevation 2.2°Obs:Radial Convergence

Weak wind （1.5ms-1）, large sensible fluxLES: Radial Convergence

Airflow structure in ABL scale.Fish nets by LES （Nakanishi 2000）（50m grid）

X10--2 X10--2

What environmental conditions favor coherent structures ? Fish nets(16 cases)

Streaks(32 cases)

Mixed(16 cases)

Zi estimated by sounding VPT at 08:30JST, Wind （59.5m AGL at SJMA),VPT lapse rate by sounding data from 1.5 to ~100m.

Fish nets favor weak wind and large lapse rate.

Profile of virtual potential temperature from radiosonde

x10-2

Discussion: Occurrence time of dust devilsDeveloped “fish nets” appeared from 9-13JST.Superadiabatic & large zi & weak wind Large -zi/L(Hess and Spillane, 1990 )

1.Morning- “fish nets”, but weak updraft 2.Afternoon- strong wind (streak pattern) caused by convective mixing- “fish nets” are broken upby sea-breeze

Fish nets Streaks Mixed

Dust devils

Environment05/6/24

Superadiabatic, large Zi and weak wind

strongly unstable

MODISJAXA/EORC10:21JST

Fine day

9:51JST Zi=900m

Zi developedZi: height of the maximum negative gradient of S/N

Moderately Superadiabatic

8:30JST Zi=650m

Profile of virtual potential temperature from radiosonde

Zi（Boundary layer height）～700m

2006/10/17 11：07 Elevatio Obs:Doppler velocity

Strong wind（15ms-1）, small sensible fluxLES：w (Elevation 2.2°)

Airflow structure in ABL scale.Streaks by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m）

Zi（Boundary layer height）～700m

2006/10/17 11：07 Elevatio Obs:Doppler velocity

Strong wind（15ms-1）, small sensible fluxLES：Doppler velocity (Elevation 2.2°)

Airflow structure in ABL scale.Streaks by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m）

Rennno et al., 2004

Airflow structure: Fish nets

:Cyclonic :Anticyclonic

Difference between tornado and dust devilMax. wind ＜142ms－1 ＜20ms

－1

Diameter 100m～1000ｍ 10－141m

Rotation 85％(anticyclonic) randomWhere/when Cumlusnimbus in daytimeFormation Supercell Fish nets ?

Non supercell Local front?Developing updraft of upper layer Buoyancy near

surface

Boliviaにて

Tilting of ambient horizontal vorticity associated with convective-cell circulation (“Fish nets”). (Kanak et al. 2000)

Fish nets（Boundary layer scale）(No wind, large sensible flux)

U,V vector w

LES（Large Eddy Simulation）

①Horizontal vortex line

②Advection of horizontal vortex

③Formation of vortex pair by tilting of horitiontal vortex

Distribution of horizontal wind

（Kanak et al., 2000）

Vertical vorticesMicroscale

3-D airflow structueUpdraftregionDowndraft

regionDowndraft regionUpdraft region

Cyclonic Anti-cylonic

Vertical structure

Desert Sapporo Mars

Diameter (m) Tens-141(K2006)

30-120 5-1750(K2006)

Tangential vel. (ms-1)

5-20(K2006)

4-6 2-93(K2006)

Roughness Small Large Small

Dust particle ○ − ○

Superadiabatic Very large Relatively large Very large

Maximum Zi (m) 3000(H & S 1990)

1000 8000(F2005)

Wind speed (ms-1) <5(B2004)

<2 ?

LES experimental designLES :Nakanishi [2000] without moisture SGS: Sullivan et al. [1994]Boundary condition

• w=0 at the ground surface and top boundary. •Momentum flux :Monin-Obukhov similarity theory•Heat flux :constant•z0: 0.38 m •Lateral boundaries : Periodic conditions

Computational domain : 177x177 horizontally and 101 vertically. Grid spacing : Δx=Δy=50m、Δz=25 mInitial condition• neutral stratification (-600m), a strong capping inversion (0.01 K m-1) （600 - 800 m), and a moderately stable stratification (0.004Km-1) (800m-).• given a small random perturbations

Simulation

Ug

(ms-1)

Winddirection

Q0

(Kms-1)

Zi

(m)

Time

(h)

-zi/L

Fish nets 1.5 NNW 0.24 850 1 310Streaks 15 SE 0.04 700 4 12.5