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Hodar - inline holography applied to raindrop and snowflake in situ measurements
Pergamon
Vol.28, Suppl. 1, pp. $375---$376,1997 ©1997ElsevierScienceLtd. All rightsreserved Printedin GreatBritain 0021-8502/97$17.00+0.00
J. Aerosol Sci.
PH:S0021-8502(97)00234-6
HODAR - INI,INE HOLOGRAPHY APPLIED TO RAINDROP AND SNOWFI,AKE IN SITU MEASUREMENTS H. VOssing, S. Borrmann, E. Uhlig, R. Jaenicke lnstitut for Physik der A/mosph~tre, Johannes Gutenberg-Universit~it Mainz, J.-J. Becherweg 21, D-55099 Mainz
KEYWORDS holography, precipitation, shape of raindrops, snowflakes, falling velocity INTRODUCTION Consideration of cloud evolution and understanding the related physical and chcmieal processes require the knowledge of the microsealc physical properties of hydrometcors. Ground based in situ measurements of low level stratus cloud microstructure has been performed with the HODAR (holographic droplet and aerosol recording) at the mountain Klciner Fcldberg (825m above NN) since several years. To investigate hydrometoors with low atmospheric frequency, we enlarged the sample volume of HODAR from approximately one litre up to 450 litres. The density of fog and cloud droplets is around 300 per cubiccentimetcr, but the density of raindrops, hailstones or snowflakes is only around one per litre. The size, shape and relative position of every individual hydrometcor can be measured. Based on these informations spatial and size distributions can be evaluated. By double exposure recording of holograms with time delay between (1 and 1000 IlS studies of the deformation and falling velocity of raindrops or snowflakes can be done. The recorded sample volume can be investigated several times using different methods, which is an advantage in comparison to other particle measurement methods. MEASUREMENT METHODS The HODAR device uses the inline or Fraunhofer holographic technique (Vikram 1995): A ruby laser illuminates the sample volume from the back side with coherent light. The light reaching the photoplate undefracted interferences with the light defracted by the objects and they together form a holographic image of the sample volume, see figure 1. The principal layout and hardware components are described in detail by Borrmann and Jaenicke (1993). dlwwgc~ insmltcam
ZD n - -
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camera
Figure 1: Principle setup of the HODAR device Measurements with the HODAR consists of two steps: l. In situ recording of a hologram : A sample volume of either 1 liter ( ~ 8 cm, 50 cm length) for small droplcts as fog and clouds (6 < d < 500 I,tm) or 450 liter ( 9 20 cm, 16 m length) for largcr hydromctcors (100 < d < 10000 pm) is recorded in the free atmosphere 1.50 m above the ground. The small sample volume is imaged directly onto the hologramplate (see also Borrmann el Jaenicke 1993). But to record (he large volume thc setup had to Ix: modified: In front of thc photoplatc a lens was iuserled to thc setup, producing an image with a dcmaguifieation of two. This resulted in a shrinking of image volume length by four times (Vikram 1995). Also a mirror had to be put into the beam path for practical reasons. The imaging properties of this modified setup was tested and thc outdoor persistence was proofed. Influences of the aparatus to the sample volume were taken in consideration. Especial thc air flow around the recording device was numerical modelled by Kiilzcr (1994).
$375
$376
Abstracts of the 1997 European Aerosol Conference
2, Analyzing of the image volume in the laboraloo: In Ibe laborato O' the hologram is oplieal reconslrncled Io a real 3 D image volume. This 31-) image conlaining the image of every individual parlicle is seanned wilh a TV videosvslenl. For high signal to noise ratio of the video image an anlomaled image processing algorilhm can be ulilized Io scan the volume and 1o recognize droplel images. This appliealion was sncccssfid for small hydromeleors (41m~ < r < 50 gin) (Uhlig 1995), Otherwise lhe image recognition has Io be done by an human observer mammlly. RESULTS The shown mcasurcmcnts wcrc pcrformcd with lhc modified sctnp for large volume during Ihe Fcldcx field campaign in autumn 1995 al Ihc lop of the mountain Klciner Feldberg in file Rhine Main area. Germany. The range of particle size, which can bc recorded in thc large volume setup is between 100 and 10iN)0 i~m. The shape of the bydromeleors is clearly seen. Thcrcfore various types of hydromcteors can Ix: distinguished Figure 2 (left) shows the size distribution of trec kinds of cocxisting particle: graupcl like. iccnccdles and snowflakes. Figure 2 (right) shows a raindrop size distribution of type 1.2 (Slrantz 1971). In figure 3 an example of a raindrop can be seen. Double exposure provides the measurement of falling velocities. The time delay of the exposures on figure 4 is 800 ps. the distance between them is 2,16 mm. The resulting velocity of the sno~41ake is 2.7 m/s.
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t
Figure 2: raindrop size dislribution (left) and iceparticle distribution(right) measured with the HODAR in the Felde× measurement campaign autumn 95.
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Figure 3: hnage of a raindrop, d = 3 mm
, .
Figure 4: Velocity measurement of a snowflakc The time delay is 8()0Its and the dislanee is 2.16 mm,. Thcrcforc wc gel a velocity of 2,7 m/s
ACKNOWLEDGEMENT This projecl is founded by the German Scicnce Foundation trough its Sonderforschungsbcreicb 233: Dynamics and Chemistry of hydrometcors REFERENCES Borrmann. S. et al. ,,On spatial dislributions and inter-droplet distances measured in stratus clouds wilh in-linc holography", Atmospheric Research, p. 229-245. Vol.29. 1993 Borrmann, S,,R. Jaenickc "Applieation of Microholography for Ground-based In Situ Measurements in Slratus cloud Layers: A Case Study". J. of Atmos. and Ocean Tcch. p.277 - 293. Vol. 10. 191~3 KOlzer, S. ctal., ,,Erwciterung des dreidimensionalen Stadtklimamodclls MISKAM auf Untcrstr0mungcn", Diplomarbcit, Univcrsit~t Mainz. 1994 Strantz. R. ,Typen der Rcgcntropfenspcktren", Mcteorologischc Rundschau 24. p. 19-24, 1971 Uhlig, E., "Holographische Untersuchungen der Wolkenmikrostruktur unlcr Anwendung cincs automalischcn Bildanalysesystems", Dissertation Univcrsit§t Mainz. 1995 Vikram, C. ,Particle Field Holography", Cambridge Uni. Press .1995
Vol.28, Suppl. 1, pp. $375---$376,1997 ©1997ElsevierScienceLtd. All rightsreserved Printedin GreatBritain 0021-8502/97$17.00+0.00
J. Aerosol Sci.
PH:S0021-8502(97)00234-6
HODAR - INI,INE HOLOGRAPHY APPLIED TO RAINDROP AND SNOWFI,AKE IN SITU MEASUREMENTS H. VOssing, S. Borrmann, E. Uhlig, R. Jaenicke lnstitut for Physik der A/mosph~tre, Johannes Gutenberg-Universit~it Mainz, J.-J. Becherweg 21, D-55099 Mainz
KEYWORDS holography, precipitation, shape of raindrops, snowflakes, falling velocity INTRODUCTION Consideration of cloud evolution and understanding the related physical and chcmieal processes require the knowledge of the microsealc physical properties of hydrometcors. Ground based in situ measurements of low level stratus cloud microstructure has been performed with the HODAR (holographic droplet and aerosol recording) at the mountain Klciner Fcldberg (825m above NN) since several years. To investigate hydrometoors with low atmospheric frequency, we enlarged the sample volume of HODAR from approximately one litre up to 450 litres. The density of fog and cloud droplets is around 300 per cubiccentimetcr, but the density of raindrops, hailstones or snowflakes is only around one per litre. The size, shape and relative position of every individual hydrometcor can be measured. Based on these informations spatial and size distributions can be evaluated. By double exposure recording of holograms with time delay between (1 and 1000 IlS studies of the deformation and falling velocity of raindrops or snowflakes can be done. The recorded sample volume can be investigated several times using different methods, which is an advantage in comparison to other particle measurement methods. MEASUREMENT METHODS The HODAR device uses the inline or Fraunhofer holographic technique (Vikram 1995): A ruby laser illuminates the sample volume from the back side with coherent light. The light reaching the photoplate undefracted interferences with the light defracted by the objects and they together form a holographic image of the sample volume, see figure 1. The principal layout and hardware components are described in detail by Borrmann and Jaenicke (1993). dlwwgc~ insmltcam
ZD n - -
/
camera
Figure 1: Principle setup of the HODAR device Measurements with the HODAR consists of two steps: l. In situ recording of a hologram : A sample volume of either 1 liter ( ~ 8 cm, 50 cm length) for small droplcts as fog and clouds (6 < d < 500 I,tm) or 450 liter ( 9 20 cm, 16 m length) for largcr hydromctcors (100 < d < 10000 pm) is recorded in the free atmosphere 1.50 m above the ground. The small sample volume is imaged directly onto the hologramplate (see also Borrmann el Jaenicke 1993). But to record (he large volume thc setup had to Ix: modified: In front of thc photoplatc a lens was iuserled to thc setup, producing an image with a dcmaguifieation of two. This resulted in a shrinking of image volume length by four times (Vikram 1995). Also a mirror had to be put into the beam path for practical reasons. The imaging properties of this modified setup was tested and thc outdoor persistence was proofed. Influences of the aparatus to the sample volume were taken in consideration. Especial thc air flow around the recording device was numerical modelled by Kiilzcr (1994).
$375
$376
Abstracts of the 1997 European Aerosol Conference
2, Analyzing of the image volume in the laboraloo: In Ibe laborato O' the hologram is oplieal reconslrncled Io a real 3 D image volume. This 31-) image conlaining the image of every individual parlicle is seanned wilh a TV videosvslenl. For high signal to noise ratio of the video image an anlomaled image processing algorilhm can be ulilized Io scan the volume and 1o recognize droplel images. This appliealion was sncccssfid for small hydromeleors (41m~ < r < 50 gin) (Uhlig 1995), Otherwise lhe image recognition has Io be done by an human observer mammlly. RESULTS The shown mcasurcmcnts wcrc pcrformcd with lhc modified sctnp for large volume during Ihe Fcldcx field campaign in autumn 1995 al Ihc lop of the mountain Klciner Feldberg in file Rhine Main area. Germany. The range of particle size, which can bc recorded in thc large volume setup is between 100 and 10iN)0 i~m. The shape of the bydromeleors is clearly seen. Thcrcfore various types of hydromcteors can Ix: distinguished Figure 2 (left) shows the size distribution of trec kinds of cocxisting particle: graupcl like. iccnccdles and snowflakes. Figure 2 (right) shows a raindrop size distribution of type 1.2 (Slrantz 1971). In figure 3 an example of a raindrop can be seen. Double exposure provides the measurement of falling velocities. The time delay of the exposures on figure 4 is 800 ps. the distance between them is 2,16 mm. The resulting velocity of the sno~41ake is 2.7 m/s.
i . . . .
t
Figure 2: raindrop size dislribution (left) and iceparticle distribution(right) measured with the HODAR in the Felde× measurement campaign autumn 95.
,~,,e
Figure 3: hnage of a raindrop, d = 3 mm
, .
Figure 4: Velocity measurement of a snowflakc The time delay is 8()0Its and the dislanee is 2.16 mm,. Thcrcforc wc gel a velocity of 2,7 m/s
ACKNOWLEDGEMENT This projecl is founded by the German Scicnce Foundation trough its Sonderforschungsbcreicb 233: Dynamics and Chemistry of hydrometcors REFERENCES Borrmann. S. et al. ,,On spatial dislributions and inter-droplet distances measured in stratus clouds wilh in-linc holography", Atmospheric Research, p. 229-245. Vol.29. 1993 Borrmann, S,,R. Jaenickc "Applieation of Microholography for Ground-based In Situ Measurements in Slratus cloud Layers: A Case Study". J. of Atmos. and Ocean Tcch. p.277 - 293. Vol. 10. 191~3 KOlzer, S. ctal., ,,Erwciterung des dreidimensionalen Stadtklimamodclls MISKAM auf Untcrstr0mungcn", Diplomarbcit, Univcrsit~t Mainz. 1994 Strantz. R. ,Typen der Rcgcntropfenspcktren", Mcteorologischc Rundschau 24. p. 19-24, 1971 Uhlig, E., "Holographische Untersuchungen der Wolkenmikrostruktur unlcr Anwendung cincs automalischcn Bildanalysesystems", Dissertation Univcrsit§t Mainz. 1995 Vikram, C. ,Particle Field Holography", Cambridge Uni. Press .1995