PVMdata.txt; 12/1/08 (H. Gerber, P.I.) 1. GENERAL DESCRIPTION The Particulate Volume Monitor (PVM) was deployed on the CIRPAS Twin Otter aircraft during the 17 flights for POST. The PVM used a dedicated laptop which recorded ElAPSED TIME, and voltage data for LWC, PSA, and SYNCH-pulse channels at ~1000 hz. The ~1000-hz data was also averaged to produce 100-hz, 10-hz data, and 1-hz files. The time- scaling- and offset-corrected data files are in text format, are comma deliminated, and are referenced to UTC at the start of each file. The file titles have the form: PVM1TO15.txt PVM10TO15.txt PVM100TO15.txt PVM1000TO15.txt where the number following PVM is the data rate, and TO15 is, for example, the identification for POST flight 15. 2. SCALING OF LWC AND PSA The voltage measured by the LWC channel of the PVM is nominally equal to liquid water content in g/m^3, thus the scaling factor is 1.00. The scaling factor for the PSA voltage is nominally equal to 3000 to yield the particle surface area in cm^2/m^3. 3. OFFSET, CALIBRATION, AND SCALING CORRECTION The PVM is set up to operate with a small offset voltage in both channels. These offsets were measured and are listed in the table below, and they were subtracted from the LWC and PSA voltages. The PVM was field calibrated for each flight just prior to takeoff of the Twin Otter by applying a standardized light-diffusing calibration disk to the PVM. The expected PVM responses to this disk are LWC = 0.220 V, and PSA = 1.070 V. The difference between the standard response of the disk and the field application of the disk yielded a scaling correction that is listed in the table below, and that was applied to the data. Offsets and scaling corrections differ markedly between flights TO1, TO2 and the rest of the flights. The former had the PVM outputs also connected to the main Twin- Otter data logger which caused interference. The latter show some variability in the scaling corrections which is due to the inability to position the calibration disk in exactly the same location each time. An average scaling correction of LWC = 0.941 and PSA = 0.929 was applied for the latter flights. TO Flt LWC PSA LWC PSA LWC cal PSA cal No. offset offset scaling scaling value value 1 0.000 0.025 1.913 1.382 0.115 0.774 2 0.003 0.020 1.290 1.010 0.170 1.057 3 0.017 0.019 0.940 0.931 0.234 1.149 4 0.017 0.021 0.940 0.905 0.234 1.182 5 0.017 0.024 0.950 0.951 0.232 1.125 6 0.017 0.021 0.936 0.908 0.235 1.179 7 0.017 0.022 0.928 0.911 0.237 1.174 8 0.016 0.022 0.940 0.922 0.234 1.160 9 0.018 0.021 0.961 0.973 0.229 1.100 10 0.016 0.021 0.936 0.930 0.235 1.150 11 0.017 0.021 0.932 0.905 0.236 1.182 12 0.017 0.021 0.948 0.951 0.232 1.125 13 0.016 0.021 0.940 0.930 0.234 1.150 14 0.016 0.022 0.948 0.934 0.232 1.145 15 0.017 0.022 0.921 0.892 0.239 1.200 16 0.017 0.023 0.940 0.927 0.234 1.154 17 0.017 0.024 0.956 0.964 0.230 1.110 4. TIME SYNCHRONIZATION CIRPAS provided a 1/2-hz square-wave synchronization pulse based on the GPS time standard. The synch pulses were recorded by the PVM laptop, which led to the finding (Steve Krueger) that the PVM laptop's clock was slow so that it did not yield exactly 1000-hz data. He found that this lag was ~7.3 x 10-5 s for each s of recorded data (mean value for all flights of 7.308995 x 10-5 s). The lag proved to be highly stable with time, so that it was possible to adjust each time of the 1000-hz data by adding a small time correction. The resulting data is thus slightly less than 1000 hz, but now follows closely the GPS time standard and UTC time. The 100-hz data has no time lag. It was constructed by averaging 10 values of the 1000-hz data, except very rarely by averaging 9 values of the 1000-hz data. The latter was necessitated by the slightly sub 1000-hz data rate of the logger. The 10-hz data was constructed by averaging 10 values of the 100-hz data. The times in the 100- hz and 10-hz data files are the times corresponding to the beginning of each averaging interval. 5. CALCULATION OF EFFECTIVE RADIUS The effective radius Re of the cloud droplets is given by Re(um) = 30,000 x LWC(g/m^3)/PSA(cm^2/m^3). As the values of LWC and PSA approach zero the noise in the calculated values for Re will approach infinity given Re is the ratio of the two quantities. Thus to get all meaningful values of Re, a conditional statement should be applied to the ratio to avoid excessive noise. The recommended approach is to chose a limiting value of LWC below which no value of Re is calculated. With additional care in removing any remaining small offsets in the LWC and PSA data, the limiting value of LWC can be chosen as small as 0.010 g/m^3 for this data set. 6. ACCURACY AND PRECISION OF THE PVM MEASUREMENTS The accuracy of the LWC and PSA measurements are based on calibrations that are traceable to comparisons with absolute measurements in the continuous cloud flow chamber in Petten, The Netherlands. Those calibrations are transferred to the field with a calibration disk. The absolute accuracy is approximately +/-10% for both channels over a limited size range of droplets. The precision of the PVM outputs is limited to a few mV consistent with the 16-bit digital recording of the logger. The lower size limit is ~ 4-um diameter, and the upper size limit is different for each channel, with the LWC channel rolling off in response to larger droplets faster than the PSA channel. The LWC channel starts deviating from a linear response for droplets ~>25-um diameter, and has a ~50% response for LWC at 50-um diameter. The effect of the LWC rolloff on the accuracy range of Re is to limit accurate Re values to ~< 25-um diameter. (Given the optical nature of the PVM, even drizzle drops will provide greatly reduced outputs for LWC, PSA, and Re.) These size limits for this PVM are based on numerous comparisons with co-located droplet spectrometers, and exposure to monodisperse glass beads. NOTE 4/6/09: After further data analysis of the PVM measurements on flights 1 and 2 it is concluded that the scaling factors for the LWC and PSA are suspect. It is recommended that for these two flights the PVM LWC data is re-scaled to the LWC measured by the CIRPAS Twin Otter hot-wire probe.