Exporting Data from OASIS - Optical Array Shadow Imaging Software (OASIS) 1.0

Generated data products can be exported from OASIS to allow users to distribute data, or perform further analysis in a package of their choice. Currently OASIS can export data in the following formats:

 HDF5

 Tab-delimited text

To export data, enter the full path to the data which is to be exported in the Export Data Wave box. This can consist of a comma separated list of paths to output multiple parameters into a single file. Select the format for the output file from the Format dropdown box, then

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click on the Export button to export data to a file. This will bring up a dialog for you to specify the location for the outputted data file.

Figure 39: Export-Data Options in OASIS

The data product derived, NumPlot_z, is pointed to by the Waves string (root:OASIS:NumPlot_z). The file format is specified to be tab delimited text. Clicking the Export button brings up the dialog shown in Figure 40.

Figure 40: Dialog to Specify File Name and Destination for Exported Data

Warning: If you are exporting Time data, the exported data will have the IGOR Pro time format of seconds since midnight on 01/01/1904. For example, exporting the wave on the left in results in the .txt file shown on the right (opened here in Excel).

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Figure 41: A Time Wave in the IGOR Pro Data Browser (left) and the Corresponding Exported Data in a Tab-Delimited Text File (right).

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Appendix A: Image Processing Description

The operating principles of the CIP are described in the operations manual of this instrument and the details of how the image data is taken and encoded will not be described here. The DMT “Data Analysis Users’ Guide: Chapter II” (downloadable from the DMT website, www.dropletmeasurement.com) describes the CIP’s operating principles, as well, and in addition describes in detail how to analyze image data. Parts of that manual will be repeated here in order to help the user of OASIS choose which options to select when running the routine.

OASIS reads the compressed image files, decompresses them and calculates each individual image’s area, perimeter, maximum length, maximum width and projected length, as well as additional image properties and average statistical bulk properties. Figure 42 illustrates how the dimensions are determined.

Figure 42: Particle Size Derivations Generated by OASIS

Maximum Width This is the dimension labeled ‘Width’ in Fig. A.1. The width is the maximum number of diodes shadowed for any slice while the image is passing over the array, where a slice is a single measurement of the ON/OFF state of the 64 photodiodes in the array.

Maximum Length This is the dimension labeled ‘Length’ in Fig. A.1 and is measured by counting the number of slices and multiplying by the resolution of the probe.

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Projected Length This is also called ‘Projected Diameter’ (Heymsfield et al., 2002) and is calculated as the hypotenuse of the triangle formed by the maximum width and length, i.e.

Dproj = (W² + L²)^1/2

where W and L are the maximum width and length, respectively.

Particle Area If SA is set to “All-in” or “Centre-in,” the area of a particle is calculated by multiplying the number of shadowed pixels by the square of the resolution. If SA is set to

“Reconstructed,” the area is calculated using the method outlined in Heymsfield and Parrish (1978b as listed in the following references). See equations 13 and 16.

Perimeter The perimeter is estimated by summing all of the transitions from ‘ON’ to

‘OFF’ of the pixels, dividing by √2 and multiplying by the resolution. The calculated perimeter includes any gaps or ‘holes’ inside the image.

Minimum interarrival time (IAT), in microseconds for breakup detection

Ice crystals that strike the tips of the CIP will sometime shatter, depending on their size and habit, producing a small cloud of fragments. Some of these can be rejected if their time arrival, a measured parameter, is very short (Field et al., 2006). Selection of this parameter requires some trial and error, i.e. if selected too short, some shattered fragments will be erroneously accepted but if selected too long, some good particles will be rejected. One approach is to first run OASIS with this parameter set to zero. Then process the particle by particle file (PBP, see description below) and generate a frequency histogram of the elapsed time (see DMT Data Analysis Manual, Part II). Inspection of this histogram will usually allow optimum selection of the minimum arrival time to reject the shattered fragments.

Maximum allowable length to width ratio (streaker filter)

The tips of the CIP are designed to minimize the impact of water that sheds when flying in heavy liquid water; however, there can sometimes be this type of shedding that is unavoidable. The symptom is a very elongated image where the length is several times that of the width. The user can adjust this and see how the resulting size distributions are impacted.

Maximum allowable area ratio (shatter spray detection)

Another symptom of shattering or drop sprays when a large crystal or raindrop strikes the probe tip will be many small images within a single image frame (see DMT Data Analysis Manual, Part II). These can be rejected by looking at the ratio the maximum area to the measured area. The maximum area is that of the rectangle that encloses the maximum width and length and the measured area is just the sum of all the shadowed pixels, as defined previously. A spray of particles produces a lot of small, shadowed pixels, not necessarily connected to one another but creating what appears to the CIP processing electronics as a single image. The area ratio will usually be larger than about 4 under these conditions.

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Korolev correction flag

Particles that pass through the CIP laser beam at a distance farther from the center of focus than their optimum depth of field (DOF) will form an image that is usually larger than the particle size and will also look like they have a transparent center (Korolev, 2007). If the user selects “Korolev” in the DoF Corr. field, OASIS will correct images for water droplets that have these size and transparency issues.

Sample Area Flag: 0=All in, 1=Center in

The calculation of number concentration, liquid/ice water content, rain rate and reflectivity all require that the number and size of cloud particles are calculated as a function of a unit volume of air. This volume is the product of the area swept out per unit time. The area swept out is the product of the sample area, calculated as the DOF times the effective array width, the airspeed and the sample time (see DMT Data Analysis Manual, Part II). There are two ways to calculate the effective array width (Heymsfield, and Parrish, 1978a,b). One method rejects any particle that shadows one or both of the array end diodes (All-In method) while the other assumes that the center of the particle is within the array (Center-In). The All-in method assures that no partial images are incorrectly sized; however, this restricts the sample volume and requires longer sample times to acquire good statistics. The center-in technique provides a larger sample volume but might derive the size incorrectly. The default is 0, or all-in.

Diameter definition:

When measurements are made in liquid water clouds, the definition of the “diameter” of a particle is obvious; however, in ice or mixed phase clouds, the term “diameter” is no longer a correct definition yet to create a size distribution we need some description that is meaningful. The scientific community has not reached a consensus, but the most commonly used derivations of size are Maximum Diameter, calculated as the maximum width or length, Projected Diameter (see Figure 42) proposed by Heysmfield et al. (2002) and the Area Equivalent Diameter, the diameter of droplet with the measured area of the image (Baker and Lawson, 2006; Lawson and Baker, 2006).

Area to Perimeter^2 ratio tolerance

Numerous methods have been published for identifying cloud particle habits. The simplest is to distinguish round images from those that are not round. This is done by comparing the area to perimeter squared ratio of a circle (0.079) to that of the measured image. Due to the resolution of the probe and pixelizing of the image, the ratio computed from the measurements will not be exactly that for a circle when water droplets are measured. Hence, this parameter is used to provide a little flexibility when trying to separate water droplets from ice crystals. The default value is 0.9.

Ice Crystal Density to use

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The calculation of ice water content (IWC) requires knowledge of the density of ice crystals.

Various parameterizations have been developed; OASIS currently allows the user to select the Brown-Francis method.

References

 Baker, B. and R. Paul Lawson, 2006: Improvement in Determination of Ice Water Content from Two-Dimensional Particle Imagery. Part I: Image-to-Mass Relationships, Journal of Applied Meteorology and Climatology, 45, 1282-1290.

 Field, P. R., A. J. Heymsfield, and A. Bansemer, 2006: Shattering and Particle Interarrival Times Measured by Optical Array Probes in Ice Clouds, Journal of Atmospheric and Oceanic Technology, 23, 1357–1371.

 Gunn, R. and G. Kinzer, 1949: The terminal velocity for water droplets in stagnant air, J.Meteor. ,6, 243-248.

 Heymsfield, A.J. and J.L. Parrish, 1978a: Techniques employed in the processing of particle size spectra and state parameter data obtained with the T-28 aircraft platform.

NCAT Tech. Note NCAR/TN-137 + 1A, 78 pp.

 Heymsfield, A.J. and J.L. Parrish, 1978b: A computational technique for increasing the effective sampling volume of the PMS Two-Dimensional Particle Size Spectrometer, J.

Applied Meteor., 17, 1566-1572.

 Heymsfield, A. J., and D. Baumgardner, 1985: Summary of a workshop on processing 2D probe data. Bull. Amer. Meteor. Soc., 66, 437–440.

 Heymsfield, A.J. and J.L. Parrish, 1987: An interactive system for processing PMS Two-Dimensional Imaging Probe Data, Journal of Atmospheric and Oceanic Technology, 3, 734–

736.

 Heymsfield, A. J., S. Lewis, A. Bansemer, J. Iaquinta, L. M. Miloshevich, M. Kajikawa, C.

Twohy, M. R. Poellot, 2002: A General Approach for Deriving the Properties of Cirrus and Stratiform Ice Cloud Particles, J. Atmos. Sci., 59, 3-29.

 Korolev, A. V., 2007: Reconstruction of the Sizes of Spherical Particles from Their Shadow Images. Part I: Theoretical Considerations, Journal of Atmospheric and Oceanic Technology, 24, 376–389.

 Lawson, R. Paul and B.A. Baker, 2006: Improvement in Determination of Ice Water Content from Two-Dimensional Particle Imagery. Part II: Applications to Collected Data, Journal of Applied Meteorology and Climatology, 45, 1291–1303.

Appendix B: Structure of HDF Files

As noted in section 2.4, you will need an HDF file viewer to examine the HDF files created during raw file processing. Once you have installed the HDF viewer, you should be able to view the following datasets of interest.

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You can also view these waves in OASIS by selecting Data (from the IGOR Pro menu) > Data Browser, then clicking on the root directory (not the OASIS directory, which has similarly named but slightly different waves). See Figure 43.

Figure 43: Viewing Generated Waves after Raw File Processing

HeaderMatrixWv: Data from Data-Block Buffer Headers

Within the HDFn file is a dataset called HeaderMatrixWv. This data is obtained directly from the raw data files from the 16 byte buffer header. Table 6 shows the contents of this dataset.

Column Description

0 Year

1 Month

2 Day of month

3 Hours

4 Minutes

5 Seconds

6 Milli-seconds

7 Day of week

Table 6: Data Contained in HeaderWv

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ParticleTimesWv: Data from Single-Particle Headers

The HDF file also contains a dataset called ParticleTimesWv. It contains data is from the single particle timestamp and header which precedes each particle image.

Table 7 outlines the contents of ParticleTimesWv.

Column Description Comments

0 Seconds

1 Nano-seconds

2 Slices

3 Particle number Rolling 16-bit counter

4 DOF flag For acquisition system

5 True airspeed

6 Data block number For cross reference with HeaderWv to obtain year, month, etc.

7 Unallocated N/A

Table 7: Data Contained in ParticleTimesWv

ParticleStatsWv: Data from Images

The ParticleStatsWv stores statistics obtained from the image data. The format of ParticleStatsWv is given below.

Column Description

0 Number of particles

1 Number of pixels in largest particle 2 Number of pixels in secondary particles 3 Min diode triggered on array¹

4 Max diode triggered on array¹

5 Start slice¹

6 End slice¹

7 Particle perimeter¹

8 Image slices reported by Data Acquisition System

9 Min diode triggered on array² 10 Max diode triggered on array² 11 Start slice²

12 End slice²

13 Number of Edge(0) diodes triggered² 14 Number of Edge(63) diodes triggered² 15 Number of internal void pixels¹ 16 Number of Edge(0) diodes triggered¹ 17 Number of Edge(0) diodes triggered¹

18 -

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19 -

20 -

1For biggest particle

2For entire image event.

StrNamesWv, StrValuesWv, VarNamesWv, VarValuesWv

All of these HDF datasets have corresponding waves accessible through IGOR Pro’s data browser. StrNamesWv and StrValuesWv are used by OASIS for internal processing and are generally not of interest to the user. VarNamesWv and VarValuesWv contain the OASIS parameter names and parameter values, respectively, used during raw data file processing.

For instance, say the first data point in VarNamesWv is GzipGbVar, and the first data point in VarValuesWv is 1. This indicates that the Gzip field (in the File I/O section of the OASIS GUI) was set to 1 during raw file processing.

Appendix C: Revisions to Manual

Date Rev Changes Section

4/12/2013 B-3 Inserted diagram of updated OASIS panel Figure 1 Expanded information on Probe Config

settings

4.3.1 – 4.3.2 Added section on troubleshooting during

PADS .csv file uploads 4.4.1

Updated section on True Air Speed to reflect

changes to software 6.2 – 6.4

4/22/2013 B-4 Updated manual to reflect updated software

(Korolev correction now functional) 9.6.4, Appendix A

In document Optical Array Shadow Imaging Software (OASIS) 1.0 (Page 39-48)