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Features of Dust Devils in the Urban Area Detected by a 3-D Scanning Doppler Lidar

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Features of Dust Devils in the Urban Area Detected by a 3-D Scanning Doppler Lidar Features of dust devils in the urban area detected by a 3-D scanning Doppler lidar 15th Coherent Laser Radar Conference Toulouse, June 22-26, 2009 Chusei Fujiwara1, Kazuya Yamashita2, Mikio Nakanishi3,Yasushi Fujiyoshi4 （1Graduate School of Environmental Sci., Hokkaido Univ.; 2National Defense Academy; 3Institute of Low Temperature Sci., Hokkaido Univ.） in Bolivia Outline Motivation - 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 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. Visible by tracers of dust in desert or relatively flat area.. • Environment （Sinclair 1966,1969,1973;Hess and Spillane 1990） Weak wind, superadiabatic Doppler radar Observation of dust devil in Texas.：Bluestein et al. 2004; MWR 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) LES（Large Eddy Simulation） U;V vector W Dust devil-like vortices Kanak et al. 2000 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. East wind component vertical wind component BORES (Boreal Ecosystem/Atmosphere Study) MacPherson and Betts 1997; JGR Pourpose 1. 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? Specification of the Doppler Lidar Instrument Observation area Instrument：3-D scanning cohrent Doppler Site: Hokkaido University（28m ASL） Lidar (Mitsubishi Electric Corporation) Wavelength ： 1.54μｍ（Eye Safe) Ishikari Laser material : Er.Glass laser Laser pulse energy : 0.5mJ/Pluse Bay Suburb Laser pulse width : 213±10ns Pulse repetition : 4kHz 4.4km Scattering body : Aerosol, Cloud Particle Sounding Hokkaido Measurement ： S/N, Doppler Velocity Wind 59.5m University Scan method : PPI (Elevation 2.2°) Mountainous Urban Scanning speed: 4.5 deg/s 80s （ ） area Toyohira Resolution：radial 50m， river azimuthal 9～92m High 50m 92ｍ resolution 2km 9m Analyzed data periods. Lidar 4400m 2005.4～7,2006.6,2006.10～2007.6 Bluestein et al. 2004 Hardy and Ottersten 1969 1. What is main organized airflow structure in urbanNASDA,NASA area? (Etling and Brown,1993) Etling and Brown,1993 Yamashita et al. 2005 Airflow structure in ABL scale. Fish nets by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m） LES：u,v vector; w (Elevation 2.2°) Obs:Doppler velocity Weak wind（1.5ms-1）, large sensible flux 2006/10/1 11：35 Elevation 2.2° Zi（Boundary layer height）～850m Airflow structure in ABL scale. Fish nets by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m） LES：Doppler velocity Obs:Doppler velocity Weak wind（1.5ms-1）, large sensible flux 2006/10/1 11：35 Elevation 2.2° The observational pattern is considered to show the existence of “Fish nets”. What is main organized airflow structure? wind velocity Obs. periods May of 2005～ Fish nets：weak wind in daytime Streaks：strong wind 5ｍ/ｓ Tenki，2005； J. Visualization， 2006 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 and minimum Doppler velocity. • To fit Rankine vortex. ζ v = 0 r (0 ≤ r ≤ a) s 2 ζ a2 1 v = 0 ⋅ (a < r) s 2 r Remove Noise. D=175m,V=6.32ms-1 Calculation of vorticity （Bluestein et al. 2004） 2(V −V ) ・raw data δ = m axmi D is the distance between maximum D (D = 2a) 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) Rotation is random A: Cyclonic 90m,0.15s-1 B: Anticyclonic Dust devils were observed in “fish nets” 110m,-0.11s-1 Characteristics and environment Diameter: 30-120m, Maximum vorticity: 0.15-0.26s－1 Rotation direction cyclonic:anticyclonic=2:1 Weak wind, Large Zi, daytime, Similar feature in past studies. Superadiabatic （e.g. Sinclair 1969; Hess and Spillane 1990） VPT Tota Time Diameter Max Rotation Vectorial z i 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 14 2007/6/22 11:48-12:33 70-120 0.21 6/2 1.0 950 - 8 2005/7/17 10:58-11:41 30-120 0.26 7/3 1.1 950 - 10 2007/4/14 12:47-13:22 50-90 0.19 1/3 1.1 1150 - 4 2005/6/24 09:01-09:43 50-100 0.19 3/1 1.3 900 1.5 4 2005/4/24 14:04-14:46 50-90 0.15 1/3 2.0 650 - 4 2005/6/1 09:01-09:43 40-80 0.17 3/3 2.2 750 1.1 6 ・Only strong vortices (more than |0.1| s-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) 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 vorticity Dust 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. Airflow structure in ABL scale. Fish nets by LES （Nakanishi 2000）（50m grid） LES: Radial Convergence Obs:Radial Convergence Weak wind （1.5ms-1）, large sensible flux 2006/10/1 11：35 Elevation 2.2° --2 X10--2 X10 What environmental conditions favor coherent structures ? Fish nets Streaks Mixed (16 cases) (32 cases) (16 cases) Profile of virtual potential temperature from radiosonde x10-2 Fish nets favor weak wind and large lapse rate. 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. Discussion: Occurrence time of dust devils Developed “fish nets” appeared from 9-13JST. Superadiabatic & large zi & weak wind Fish nets Streaks Mixed Large -zi/L(Hess and Spillane, 1990 ) Dust devils 1.Morning - “fish nets”, but weak updraft 2.Afternoon - strong wind (streak pattern) caused by convective mixing - “fish nets” are broken up by sea-breeze Profile of virtual potential Environment temperature from radiosonde 05/6/24 8:30JST Zi=650m Fine day Moderately MODIS 10:21JST JAXA/EORC Superadiabatic Zi developed Zi: height of the maximum negative gradient of S/N 9:51JST Zi=900m Superadiabatic, large Zi and weak wind strongly unstable Airflow structure in ABL scale. Streaks by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m） LES：w (Elevation 2.2°) Obs:Doppler velocity Strong wind（15ms-1）, small sensible2006/10/17 flux 11：07 Elevatio Zi（Boundary layer height）～700m Airflow structure in ABL scale. Streaks by LES （Nakanishi 2000）（Δx=Δy=50m,Δz=25m） LES：Doppler velocity (Elevation 2.2Obs:Doppler°) velocity Strong wind（15ms-1）, small sensible2006/10/17 flux 11：07 Elevatio Zi（Boundary layer height）～700m Rennno et al., 2004 Airflow structure: Fish nets :Cyclonic :Anticyclonic Difference between tornado and dust devil Max. wind ＜142ms－1 ＜20ms －1 Diameter 100m～1000ｍ 10 －141m Rotation 85％(anticyclonic) random Where/when Cumlusnimbus in daytime Formation 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 U,V vector w （Boundary layer scale） Vertical vortices (No wind, large sensible flux) Micro scale （Kanak et al., 2000） LES（Large Eddy Simulation） 3-D airflow structue Distribution of horizontal wind Updraft Downdraft region Downdraft Updraft region region region ②Advection of Cyclonic Anti-cylonic horizontal vortex ③Formation of vortex pair by ①Horizontal vortex line tilting of horitiontal vortex Vertical structure Desert Sapporo Mars Diameter (m) Tens-141 30-120 5-1750 (K2006) (K2006) Tangential vel. 5-20 4-6 2-93 (ms-1) (K2006) (K2006) Roughness Small Large Small Dust particle ○ − ○ Superadiabatic Very large Relatively large Very large Maximum Zi (m) 3000 1000 8000 (H & S 1990) (F2005) Wind speed (ms-1) <5 <2 ? (B2004) LES experimental design LES :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.
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